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Peri-Annulated Heterocyclic Systems. Part I

Peri-Annulated Heterocyclic Systems. Part I

Peri-Annulated Heterocyclic Systems. Part I VALERII V. MEZHERITSKII Research Institute of Physical and Organic Chemistry, Rostov-on-Don State Universi...

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Peri-Annulated Heterocyclic Systems. Part I VALERII V. MEZHERITSKII Research Institute of Physical and Organic Chemistry, Rostov-on-Don State University, 344090 Rostov-on-Don, Russia

I. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . II. Peri-Annulated Heterocyclic Naphthalene Derivatives with a Four-Membered Hetero Ring . . . . . . . . . . . . . . . . . . . . A. Naphth[1,8-bc]azete. . . . . . . . . . . . . . . . . . . . . . . . . . B. Naphtho[1,8-bc]phosphete . . . . . . . . . . . . . . . . . . . . . C. Naphth[1,8-bc]oxete . . . . . . . . . . . . . . . . . . . . . . . . . D. Naphtho[1,8-bc]thiete and Its S-Oxides . . . . . . . . . . . . E. Naphtho[1,8-bc]borete . . . . . . . . . . . . . . . . . . . . . . . . F. Naphtho[1,8-bc]silete. . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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1 4 5 6 8 10 18 20 23

I. Introduction The present-day chemistry of heterocyclic compounds with a closed aromatic (4n+2 electrons) and nonaromatic (4n electrons) p-system is based on heteromonocycles (e.g., I–III) and their ortho-fused derivatives (e.g., IV–VI) (Figure 1). The distinguishing feature of ortho-annulation is the possibility of formally extracting the parent heteromonocycle from the fused structure (IV–VI) (cf. formulas I–III and IV–VI). Moreover, the chemical characteristics of the ortho-fused compounds to a large extent reproduce the properties of their heteromonocyclic precursors. In particular, the fundamental qualities are retained that govern their properties, including p-excess and p-deficiency, which originate from the nature and hybridization character of the heteroatom (or atoms). With certain qualifications the so-called bridged heterocyclic systems where the heteroatom belongs simultaneously to two or three rings may also be assigned to this type. The specific feature of peri-annulation consists in the fact that the extraction of a heteromonocycle is impossible. Therefore, the minimum structural unit in this case is the tricyclic framework, for instance, (VII–IX). It is therefore obvious that periannulated heterocyclic systems possess qualitatively new structural features sufficient to separate these substances into an independent domain distinct from the array of the heteromonocyclic and ortho-fused heterocycles. Nonetheless no treatise or monograph on the chemistry of heterocycles contains a mention, let alone an entire chapter, on peri-fused heterocyclic compounds as independent objects worthy of special consideration, equal to those of heteromonocyclic and ortho-fused substances. The previously published chapter (90AHC(51)1) and present review attempts to remedy this situation. 1 ADVANCES IN HETEROCYCLIC CHEMISTRY VOLUME 95 ISSN: 0065-2725 DOI: 10.1016/S0065-2725(07)95001-1

r 2008 Elsevier Inc. All rights reserved

2

VALERII V. MEZHERITSKII

X

X

I

II

X

X

VII

X VI

V

X

X

X

X IV

III

[Sec. I

VIII

IX

Figure 1. Types of heterocyclic systems

The introduction to the first publication (90AHC(51)1) dedicated to syntheses of peri-annulated heterocyclic naphthalene derivatives outlined the problems of this new field. Nomenclature was considered, some basic concepts were introduced, a classification of all presumable structural types containing a heterocycle from fourmembered to seven-membered rings, and the principles of their building were discussed. We have not limited the present review to synthetic procedures but tried to consider all the aspects of the chemistry of peri-fused heterocyclic systems taking into account the new findings that have appeared since the first review (90AHC(51)1). The presentation follows the previously developed sequence, describing successively compounds with larger heterocycles, with increasing number of heteroatoms in the heteroatom order N, O, S, and occasionally other heteroatoms. The immense amount of information available induced us to divide it into separate chapters. This first chapter draws the attention to peri-fused heterocyclic naphthalene derivatives with a four-membered heterocyclic ring. Those peri-annulated heterocycles with a closed p-system that possess a double bond or another p- or p-electron ‘‘bridge’’ situated in the peri-position of the naphthalene ring opposite to the heterocycle should be set apart. Examples include the 16p-electron peri-fused heterocyclic acenaphthylene derivatives XII and XV, whose 14 p- and p-electrons are situated on the perimeter of the heteroaromatic skeleton forming an aromatic contour (according to Hu¨ckel), while two p-electrons occupy an internal orbital. Electronic structures of XII and XV should be compared with peri-annulated heterocyclic pair X and XI and pair XIII and XIV, respectively, each lacking a vinyl chain in the peri-position opposite to the heterocycle (Figure 2). The heterocyclic triad X, XI, and XII and triad XIII, XIV, and XV should be compared with their isoelectronic hydrocarbons XVI, XVII, and XVIII. In the hydrocarbon triad XVI, XVII, and XVIII the first two compounds, plyediene and aceplyediene, belong to the so-called unsaturated aromatic hydrocarbons possessing two 4p-electron nonbonding orbitals and a pronounced divinyl character of the seven-membered ring, which is prone to addition and not to substitution reactions (81MI1281, 73MI1240). They do not fit the Hu¨ckel (4n+2)p rule in spite of the presence of 14p-electrons.

Sec. I]

PERI-ANNULATED HETEROCYCLIC SYSTEMS. PART I .. Y X

.. Y X

3

.. Y X

X XI

X = O, NR; Y = CH, N

X

XII

X

X

XIII X = O+, S+, RN+, N

XVI

XIV

XV

XVII

XVIII

Figure 2. Examples of aromatic and nonaromatic peri-annulated carbocyclic and heterocyclic systems

In contrast, aceplyedilen (XVIII, Figure 2) is aromatic since it contains a closed 14p-electron (Hu¨ckel) external contour with alternating double bonds. The peripheral electrons are believed to play the dominant part in the formation of a stable 14p-electron ensemble (57JOC36), (64HCA1172). The internal double bond in XVIII may be regarded as having a proper importance but as part of the overall electronic system. The above principle is apparently also true for heterocyclic systems X, XI, and XII and also XIII, XIV, and XV. The comparison of the degree of aromaticity of heterocycles XII and XV with that of aceplyedilen (XVIII) is as appropriate as the likening of the aromaticity of 6p-electron five- and six-membered heterocycles to that of benzene or more generally of the aromaticity of aromatic hydrocarbons to heteroaromatic compounds (85MI280). Each of the above real and hypothetical heteroaromatic systems should possess a specific set of characteristics and its own chemistry whose study may be a subject of extensive fundamental research.

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VALERII V. MEZHERITSKII

[Sec. II

Not only is the investigation of the chemical reactions of novel heteroaromatic compounds of interest, but also the comparison of their physical properties (magnetic susceptibility, dipole moments, etc.), spectral characteristics (UV, IR, and NMR spectra), and the calculation criteria on going from nonaromatic structures XI and XIV to aromatic ones XII and XV, Figure 2.

II. Peri-Annulated Heterocyclic Naphthalene Derivatives with a Four-Membered Hetero Ring This section treats the hypothetical and real compounds with formula XIX whose positions 1 and 8 of the naphthalene framework are bonded to a single X heteroatom (Figure 3). In addition to article (90AHC(51)1) where the methods of preparation of these compounds were systematized, J. Nakayama published in 1981 (81MI2682) a review in Japanese, perhaps not available, dedicated to the chemistry of the peri-fused naphthalene derivatives with four-membered carbon and hetero rings. These strained structures are of interest for theoretical and experimental studies. The recent semiempirical (95MI1696) and ab initio (01JST287) calculations of the structural parameters of such molecules have provided information on their stability and made it possible to suggest the conditions for their existence and isolation. Roohi et al. (01JST287) suggested as stability criteria for the peri-fused naphthalenes XIX the quantities r (the value of the ratio of bond angles C4–C10– C5/C1–C9–C8) and p (the value of the torsion angle C1–C9–C10–C5). Based on these calculations a structure should have an r value not exceeding 1.4 in order to exist. Thus, structures with the following X could be relatively stable: CH2 (1.40); CQO (1.37); S (1.30); SO (1.26); SO2 (1.24); PH (1.25); PHO (1.23). In contrast the structures with X=NH or O having r 1.51 and 1.54, respectively, should be extremely unstable (cf. the values of naphthalene, r=1, and acenaphthene, r=1.14). The p value is related to the extent of coplanarity of the naphthalene skeleton. In the stable molecules (X=C, S, P) the p value falls into the range 0–2.81 meaning that the naphthalene framework is a virtually planar highly conjugated aromatic p-system. When nitrogen or oxygen is involved as heteroatom (X=N, O) the p values X 1

8 9

2

7 6

3 4

10

5

XIX (X = N, PR, O, S, SO, SO2, BR, SiR2, GeR2 )

Figure 3. General formula of peri-fused heterocycles with a four-memebered hetero ring

Sec. II.A]

PERI-ANNULATED HETEROCYCLIC SYSTEMS. PART I

5

are 18.71 and 22.61, respectively. To put it differently, in the latter case the naphthalene skeleton resembles a gable roof and suffer from significantly distorted conjugation of the aromatic p-system. Just these strong distortions in the aromatic skeleton and not the strain in the nitrogen or oxygen four-membered ring are apparently the main obstacle to the generation and existence of these molecules. Actually, a large number of stable representatives of four-membered heterocycles with a nitrogen or oxygen ring heteroatom are described, among them also their benzoannulated derivatives (84MI2237, 84MI1363).

A. NAPHTH[1,8-BC]AZETE Despite all attempts to synthesize naphth[1,8-bc]azete or its derivatives 2 not a single example has been prepared and no traces of such compounds as intermediates in chemical reactions have been found in keeping with the theoretical analysis (Figure 4). The plan for the preparation of naphth[1,8-bc]azete (2) was based on sulfur dioxide elimination from naphtho[1,8-de]thiazole S,S-dioxide (3, Scheme 1) or on nitrogen liberation from naphtho[1,8-bc]triazine (4) under pyrolysis (500–800 1C) or photolysis (68CC1026, 69JA1035, 70JCS(C)298, 72JOC2152). In all investigated instances biradical 6 formed as a key intermediate. Inasmuch as this biradical cannot close into a four-membered azetidine heterocycle as stated above, it suffers further transformations into a whole set of isolated and identified compounds 8, 9, 12, and 14–16, Scheme 1. For instance, biradical 6(R=H) undergoes fragmentation in two ways generating a pair of isomeric cyanoindenes 8 and 9 and naphthylamine 10. Presumably nitrene 7 as a reactive intermediate is a precursor to the formation of indenes 8 and 9. Three research groups (75TL3845, 77TL943, 80CC499) unsuccessfully later tried to obtain naphtho[1,8-bc]azetidine 2 via nitrene 7 generated by a photolysis of azide 5.

H N 85.6 91.2 92.1

138.7

1 r = 1.51; p = 18.7° (calculated values)

Figure 4. General formula, bond angles in the hetero ring, and other calculated parameters of naphth[1,8-bc]azete

6

VALERII V. MEZHERITSKII N3

N

[Sec. II.B CN CN

+ 5 N

RN

7

8

NH2

9

N

−N2

R=H

10

RN

4

HN

N

R=Ph SO2

RN

6

−SO2

R=Me 3

11

12

R N

Me N

MeNH

H

Ph

+ C6H6 2 Me

13

Ph

CH2

14

Me N Me Ph

CH2 15

16

Scheme 1

Biradical 6, R=Ph, converts into benzo derivative of naphtho[1,8-bc]pyridine (12), and 6, R=Me transforms depending on the reaction conditions and the presence of other reagents into 1-methylamine-8-phenylnaphthalene (14), 1-methylideneaminonaphthalene (15), or peri-fused azine (16).

B. NAPHTHO[1,8-BC]PHOSPHETE In conformity with the calculations the peri-fused heterocycles with a phosphorus atom proved to be quite stable. For instance, treating 1,8-dilithionaphthalene (18) with (N,N-diisopropylamino)dichlorophosphine afforded in a 75% yield a P-substituted naphtho[1,8-bc]phosphete (19) that took up tungsten pentacarbonyl to form a complex compound 20 (02AG(E)3897). The latter on further reaction with platinum tetratriphenylphosphinate and carbon monoxide suffers a recyclization of the four-membered ring into a five-membered one (20-21). The reaction of 1,8-dilithionaphthalene with a phenyl dichlorophosphine gave instead of an expected

Sec. II.B]

7

PERI-ANNULATED HETEROCYCLIC SYSTEMS. PART I i-Pr

Pr-i N

Li

i-Pr2N

P

Li

W(CO)5

+W(CO)5

i-Pr2NPCl2

18

P

19

20

PhPCL 2

Ph

Ph P

P

+Pt(PPh3)4 / CO (CO)5W

22

i-Pr2N

CO

P

Pt

PPh3

21

Scheme 2

H P 72.4 90.8 105.8

r = 1.25; p = 0.1° 132.4

17 Figure 5. General formula, bond angles in the hetero ring, and the other calculated parameters of naphtho[1,8-bc]phosphete

(P-phenyl)naphtho[1,8-bc]phosphete a product resulting from closure to a fivemembered heterocycle forming peri-annulated diphosphetol (22) (Scheme 2). The structures of compounds 19 and 22 were proved by X-ray diffraction analysis. The X-ray data for 19 provide bond lengths, bond angles, and the coplanar position of the hetero ring and the naphthalene skeleton, these and the other parameters have values close to those calculated for the simplest naphtho[1,8-bc]phosphetol (17) (Figure 5). Compound 19 is a white powder, very soluble in organic solvents (the melting point is not mentioned). The 1H and 13C NMR spectra show the symmetry of the molecule and confirm the structure of the compound with peri-phosphete 19 interesting opportunities arise for the preparation of unusual substances, e.g., 20 and 21, by uncommon reactions.

8

VALERII V. MEZHERITSKII

[Sec. II.C

C. NAPHTH[1,8-BC]OXETE A 1933 patent claimed the preparation of naphth[1,8-bc]oxete (23) along two routes: a-naphthol oxidation by iron(III) chloride, and 1,8-dihydroxynaphthalene dehydration (33BRP394,511) at 300 1C under a carbon dioxide atmosphere. Structure 23 (Figure 6) was assigned based on the elemental analysis, the cryoscopic measurement of the molecular weight, and its behavior in the chemical reactions. The compound formed in a good yield, was quite stable, and high melting (about 300 1C). These characteristics and its chemical behavior according to some chemists (66CRV593, 70JOC4261) hardly fit the assigned structure. A. Gordon (70JOC4261) in 1970 checked the results of the patent (33BRP394,511) carefully executing the oxidation of a-naphthol with iron(III) chloride precisely under the conditions described and thoroughly separating the products and succeeded in isolating and identifying only the initial a-naphthol (24) and the previously described (28CB362, 31JCS1265) product of its oxidative dimerization 25. In another experiment, the dark glassy residue obtained after a 15 min heating of 1,8-dihydroxynaphthalene (26) under a CO2 atmosphere was subjected to a number of procedures for reaction mixture separation (extraction, sublimation, chromatography, etc.). Some of initial dihydroxynaphthalene (26) was isolated, and the residue was an intractable tar. It was concluded (70JOC4261) based on these test experiments that the information in the patent (33BRP394,511) on the synthesis of naphth[1,8-bc]oxete (23) was erroneous (Scheme 3). Interesting results were obtained by pyrolysis of naphtho[1,8-bc]-1,2-oxathiol S,S-dioxide (26) with sulfur dioxide elimination. Depending on the addition to the pyrolyzed mixture of either methanol or carbon monoxide, two pairs of isomeric compounds, 28 and 29 or 30 and 31, respectively, were formed in high yield and in an B1:1 ratio (71TL4093) (Scheme 4). At first glance it would seem that the formation of isomer pairs provides conclusive evidence of naphth[1,8-bc]oxete intermediate (27) involvement. However, taking into consideration the drastic pyrolysis conditions (680 1C) and the predicted instability of the naphth[1,8-bc]oxete (23) structure, it is more likely that not a four-membered oxetane heterocycle, but a pair of interconverting biradical intermediates A and B takes part in the reaction where the formation of the new C1–O covalent bond occurs simultaneously with the rupture of the existing C8–O bond (A B).

O 84.3 92.4 90.6

r = 1.54; p = 22.6° 132.4

23 Figure 6. General formula, bond angles in the hetero ring, and other calculated parameters of naphth[1,8-bc]oxete

Sec. II.C]

9

PERI-ANNULATED HETEROCYCLIC SYSTEMS. PART I OH OH

OH FeCl3 / H2O reflux

+ 24

24

O

OH 25 OH

OH

OH

OH

23 300°C CO2 26

+

gum

26

Scheme 3

OH

OH +

O

O

SO2

MeOH

Me

Me 28

29

680°C − SO2

Me 26

O CO

Me

O

O

O

27

+ Me

Me 30

31

Scheme 4

The stabilization of ortho-fused benzoxete (32) occurred by valence isomerization into a quinoid isomer (33) (84MI1363). Such benzoid–quinoid processes are impossible for naphth[1,8-bc]oxete because of structural reasons. Therefore, the biradicals isomerization (A B) seems likely (Scheme 5). Apart from the above attempts, the failed efforts to prepare naphth[1,8-bc]azete and naphth[1,8-bc]oxete were mentioned in (74JA6532) but the experiments were not published.

10

VALERII V. MEZHERITSKII

O

.

.

.

Me A

[Sec. II.D

.O

Me B

O O 33

32

CH2

Scheme 5

D. NAPHTHO[1,8-BC]THIETE

AND

ITS S-OXIDES SO2

S

SO

72.7

71.6

74.2

92.0 103.4

91.1 105.9

89.3 107.3

134.3

r = 1.3; p = 0°

34

133.3

r = 1.26; p = 1.3°

35

132.9

r = 1.24; p = 0°

36

Figure 7. General formulas, bond angles in the hetero ring, and other calculated parameters of naphtho[1,8-dc]thiete and its S-oxides

The calculated r and p values (01JST287) predict that all three representatives of this heterocyclic system shown in Figure 7 [parent naphtho[1,8-bc]thiete (34) and its two S-oxides (35 and 36)] should be sufficiently stable to exist in a free state. Experimental data are consistent with theory. All the three compounds have been prepared by several teams. The first to be obtained was S,S-dioxide 36 (65AG(E)786, 67LA96). The parent naphtho[1,8-bc]thiete (34) and its sulfoxide 35 were prepared 10 years later (74JA6532, 76JA6643, 79JA7684, 81JCS(P1)413, 83TL821). The design strategy for this system is based either on the decomposition of other peri-annulated heterocycles with a five- or six-membered hetero ring capable of eliminating an atom or a group of X atoms under photolysis or thermolysis (37-38-39), or on generating a 1,8-dehydronaphthalene intermediate 40 followed by its reaction with certain sulfur-containing reagents (40-39) (Scheme 6).

Sec. II.D] X

SO n

SOn

S On [S]

hv or ∆ −X

37

11

PERI-ANNULATED HETEROCYCLIC SYSTEMS. PART I

38

39

40

Scheme 6

Naphtho[1,8-bc]thiete (34) or (39 n=0) can be prepared by the photolysis of naphtho[1,8-bc]-1,2-dithiol 1,1-dioxide (41), (74JA6532, 76JA6643), and also by the photolysis or thermolysis of naphtho [1,8-de]-1,2,3-thiadiazine (42) (79JA7684). The irradiation of dithiol 1,1-dioxide 41 for 9.5 h in dilute, dry, and oxygen-free benzene under a nitrogen atmosphere furnished naphtho[1,8-bc]thiete (34) after evaporation and chromatography on silica gel in a 97% yield. A more feasible preparative synthesis of this compound avoiding some of the special conditions of the above procedure is the photolysis of naphthothiadiazine 42 that is carried out by irradiating 1 mmol of 42 in 120 ml of a solvent (acetone, acetonitrile, benzene, or methanol) with a 120 W high-pressure mercury lamp in a Pyrex glass reactor at room temperature. This photolysis proceeds strikingly easily. After just 5 min irradiation the initial red solution turns light yellow with vigorous nitrogen liberation. After 15–20 min the irradiation is complete, and 34 is quantitatively isolated as in the previously described procedure. The formation of naphthothiete (34) from dithiol 41 and naphthothiadiazine (42) is preceded by biradical intermediate 43 as proven by the photolysis of naphthothiadiazine (42) in carbon disulfide. After 15 min irradiation alongside naphthothiete (34) obtained in a 52% yield there also formed in a 22% yield naphtho[1,8-de]-2,4-dihydro-1,3-dithiin-2-thione (44) resulting from the reaction of biradical 43 with carbon disulfide (Scheme 7). The behavior of naphthothiadiazine (42) on thermolysis was investigated. The compound when dissolved in 2-ethoxyethyl ether at 155 1C under a nitrogen atmosphere led to the formation of the expected naphthothiete (34) in 30% yield and three minor products whose structures were not rigorously determined; 12% of the initial unreacted compound was also recovered. A reactive specie of an outstanding interest, 1,8-dehydronaphthalene (40), can be generated by oxidation of 1-aminonaphtho[1,8-de]-1,2,3-triazine (45) (69JCS(C)756, 69JCS(C)760, 69JCS(C)765). The behavior of 1,8-dehydronaphthalene is essentially different from that of dehydrobenzene (benzyne), presumably because of the singlet diradical structure of the former, although it is also prone to cycloaddition to olefins and to radical reactions (69JCS(C)756, 69JCS(C)760, 69JCS(C)765, 71JA3802, 75JA681, 75BCJ932). The transformations of 1,8-dehydronaphthalene (40) in carbon disulfide studied by J. Nakayama et al. provided a complex mixture of products. From the mixture naphthothiete (34) was isolated in a 6–8% yield, and also in still lower yields (3–5%) the other peri-fused heterocycles (44, 47, and 48) were separated. The routes of formation are shown in Scheme 7. Usually the reactions of

12

VALERII V. MEZHERITSKII

S

SO2

[Sec. II.D

SPh

49

41 hv −SO2

−Ph

(PhS)2

S C

S

hv or ∆ −N2 N N S

40

46 hv or ∆

N

45 O

S

−CS

S

S S

CS2 42

N

Pb(OAc)4 CS2

+ CS2

− CS 34

H2NN

S

47 43

+CS2

48

S S

S

44

Scheme 7

1,8-dehydronaphthalene with organic compounds give a large number of intractable tars. In a still lower yield (1%) naphthothiete (34) was present in the mixture with the other products when 1,8-dehydronaphthalene (40) reacted with diphenyl disulfide (83TL821). Thus, the best laboratory procedure for preparation of naphtho[1,8-bc]thiete (34) is the photolysis of naphthothiadiazine (42). Inasmuch as naphtho[1,8-bc]thiete (34) was successively prepared by photolysis and thermolysis of naphthothiadiazine (42), Nakayama et al. (79JA7684) hoped to obtain in the same fashion naphtho[1,8-bc]thiete S-oxide (35) from naphtho[1,8-de]1,2,3-thiadiazine S-oxide (50, Scheme 8). They attempted to synthesize sulfoxide 50 by oxidation of thiadiazine (42) with m-chloroperbenzoic acid (m-CPBA). However, this reaction carried out at room temperature with an equivalent amount of m-CPBA occurred with nitrogen liberation and with the formation of naphtho[1,8-bc]thiete S-oxide (35) in 71% yield instead of the expected sulfoxide (50). Therefore, the preparation of naphtho[1,8-bc]thiete S-oxide (35) proved to be easier than expected due to the instability of sulfoxide 50 with S-oxide 35

Sec. II.D]

PERI-ANNULATED HETEROCYCLIC SYSTEMS. PART I

N

N

SO2

SO2 hv −N2

SO

51 N

N

13

36 S

N

N

SO

35 RCO3H

42

50 Scheme 8

naphtho[1,8-bc] thiete sulfone (36) was isolated in a 4% yield. However, 3 equiv of (m-CPBA) afforded naphtho[1,8-bc]thiete S,S-dioxide (36) in a 93% yield. Nakayama et al. (79JA7684) believed that under the mild conditions used sulfone 36 was the product of sulfoxide 35 oxidation and does not originate from thiadiazine (42) oxidation into sulfone 51 with its subsequent transformation into naphtho[1, 8-bc]thiete S,S-dioxide (36). Actually, naphthothiadiazine sulfone (51) is well known to be stable on heating and decomposes with nitrogen liberation only at harsh irradiation. Just the irradiation of naphthothiadiazine S,S-dioxide (51) furnished sulfone 36 as the first representative of peri-annulated heterocyclic naphthalene derivatives with a four-membered hetero ring (65AG(E)786, 67LA96). 1. Physical Properties and Spectral Characteristics of Naphtho[1,8-bc]thiete and Its S-Oxides (34–36) (65AG(E)786), (67LA966), (74JA6532), (76JA6643), (79JA7684), (81JCS(P1)413), (83TL821), (84MI3403). The main published sources containing the majority of information on their physical properties are printed in bold type. The parent naphtho[1,8-bc]thiete crystallized from hexane as a colorless substance, mp. 40–42 1C. Its two S-oxides 35 and 36 are also colorless crystalline compounds, mp. 105–106 1C (from hexane) and 183–184 1C (from methanol), respectively. All compounds are stable on melting and sublimation under reduced pressure. Being colorless, all compounds have no absorption band in their electronic spectra higher than 320 nm. In the IR spectrum of naphtho[1,8-bc]thiete (34) taken in CCl4 appeared an unexpected series of high-frequency bands at 3050, 1922, 1780, 1657, and 1615 cm 1 that totally disagreed with the structure of 34 and was not discussed in (76JA6643). These bands are apparently due to the presence of phosgene that readily formed from carbon tetrachloride on irradiation. At least in the IR spectrum of sulfoxide 35 recorded as a KBr pellet the band with the highest wave number was observed at 1467 cm 1. In each 1H NMR spectrum of compounds 34–36 appear two two-proton doublets and one two-proton triplet showing the presence of six aromatic protons and the symmetry of the structure. In the proton-decoupled 13C NMR

14

VALERII V. MEZHERITSKII

[Sec. II.D

spectra six signals of carbon atoms are observed excluding the possibility of dimer formation. The X-ray diffraction analysis of naphtho[1,8-bc]thiete S,S-dioxide (36) (76JA6643) revealed the coplanar position of the four-membered hetero ring and the naphthalene skeleton, and the r value [the ratio of bond angles C4–C10–C5/ C1–C9–C8 (132.5/1061)] equaled 1.25 in good agreement with the calculated data as were also many other parameters. 2. Reactions of Naphtho[1,8-bc]thiete (34) and Its S-Oxides (35, 36) The known transformations of naphtho[1,8-bc]thiete (34) and its S-oxides 35, 36 in the majority of instances consist of hetero ring opening due to irradiation, heating or to the action of chemical reagents. However, two types of reactions are known that are directed to the sulfur atom and do not cause ring opening: oxidation to S- oxides, and S-alkylation giving sulfonium salts. a. Photolysis and Thermolysis (67LA96, 79JA7684, 81MI2682). The photolysis or thermolysis of naphtho[1,8-bc]thiete (34) and its S-oxides 35 and 36 induces a homolytic cleavage of one or both sulfur bonds with the naphthalene core generating the corresponding biradical species 43, Scheme 7 and 52, Scheme 9. Further reactions of these biradicals depend on the presence or absence of other reagents. For instance, on heating or irradiating naphtho[1,8-bc]thiete (34) in a carbon disulfide environment the latter reacts with biradical 43 leading to naphtho[1,8-de]-2,4-dihydro-1,3-ditiin-2thione (44, Scheme 7), whereas on irradiating sulfone 36 in the absence of other reagents biradical 52 combines to a dimer (53, Scheme 9). Interestingly, if in place of OS

OS

O

O

∆ 52

55

SO2

SO2 hv

SO2

hv

SO2

52

36 ∆

53 MeO2C

CO2Me

MeO2C CO2Me 40

54

Scheme 9

Sec. II.D]

PERI-ANNULATED HETEROCYCLIC SYSTEMS. PART I

15

irradiation sulfone 36 is transferred into a gas phase on thermolysis biradical 52 instead of dimerizing is transformed into a five-membered hetero ring (52-55). Inasmuch as homolytic cleavage of the hetero ring is reversible, the final products 44 and 53 are present in the mixture with the initial compounds 34 and 36. Under severe thermolysis conditions both sulfur bonds to the naphthalene skeleton in sulfone 36 can suffer homolytic cleavage with SO2 liberation and formation of 1,8-dehydronaphthalene (40) capable of electrocyclic capture with dimethyl acetylenedicarboxylate (40-54). b. Reactions with Nucleophilic Reagents. The calculation of electron density distribution in 34 that we performed by the PM3 method for publication in this review showed that the largest negative charge is localized on the 1,8-atoms of the naphthalene skeleton ( 0.156), and the largest positive charge (0.197), on the sulfur atom. These results should permit prediction of the direction of attack by the positive and negative fragments of chemical reagents. Reactions were studied between naphtho[1,8-bc]thiete (34, Scheme 10) and nucleophiles including metal hydrides and organometallic compounds (76JA6643).

H S n SMe 62 (n =0, 28%) 63 (n =1, 10%) 64 (n =2, 7%)

M=Li Me S

M

SCl M=MgCl

+

S

61

60

SMe 63

MeM HS

S

Li

Li S LiAlH4 −H2

LiAlH4 56

34 (H or D) R S

H2O or D2O 57

R (H or D)

Me S

R(H or D) NaOH/MeJ 58

Li

59

Scheme 10

16

VALERII V. MEZHERITSKII O2SMe

Li

O2 SH

[Sec. II.D

1.H2O 2.MeI/NaOH 67

65 SO2

LiAlH4 O2SMe

Li

H2O

MeLi 36

66 LiAlH4 S

SMe 1. LiAlH4 2. H2O 3. MeI/NaOH

34

59(R=H)

Scheme 11

The nucleophilic reagent attacks the sulfur atom and the S–C1bond is ruptured. The exact mechanism is unknown. The reaction of naphthothiete (34) with the lithium aluminum hydride first gives 1,8-lithionaphthalenethiol (56) and then dilithium derivative (57), which on treating with water or deuterium oxide transformed into thiol or its deutero analog (58), respectively. The latter when alkylated with methyl iodide afforded a stable methyl naphthyl sulfide 59 whose structure was unambiguously proved. Thus, J. Meinwald et al. confirmed the assumed sequence of transformations. Naphthothiete (34) with methyllithium (MeM=MeLi, Scheme 10) formed 8-lithio-1-naphthyl methyl sulfide (60, M=Li) that on taking up 1 or 2 equiv of additional 34 was converted into dimer 63 or trimer 64. Sulfide 60 at treating with water formed methyl 1-naphthyl sulfide (62). Dimer 63 was also prepared by an independent synthesis between 60 (M=MgCl) and 1-naphthyl thiochloride (61). The reaction of naphthosulfone (36, Scheme 11) with the lithium aluminum hydride takes two directions. The first route consists in reduction to naphthothiete (34) with subsequent transformations represented in Scheme 10: (34-56-57-5859) on quenching dilithium intermediate 57 with water (R=H). The second path involves opening the hetero ring with the nucleophile without reduction (36-6567) also analogous to (34 - - - 59) for naphtho[1,8-bc]thiete proper (cf. Schemes 10 and 11). Methyl naphthyl sulfone (67) thus obtained was also prepared by an independent synthesis by treating naphtho[1,8-bc]sulfone (36) with methyllithium followed by quenching with water (36-66-67). The reaction of naphtho[1,8-bc]thiete S-oxide (35) with lithium aluminum hydride is more complex, and its main product is dinaphthyl disulfide (68, Scheme 12) arising as a result of a series of successive transformations where disproportionation of one

Sec. II.D]

17

PERI-ANNULATED HETEROCYCLIC SYSTEMS. PART I

SO

S

35

Me SO 2

S

1)LiAlH 4 2)H2O 3)MeI/NaOH

+

+

S O2

SMe

67

68

59(R=H)

from(35):

75%

4%

6%

from(36):

2%

65%

13%

YIELDS:

36

Scheme 12

SO3Na

SO2

NaOH 69 1).PhNHLi PhNHSO2

36

2).H2O 70 Scheme 13

of the intermediates plays a key role. The disproportionation and other reactions of the sulfur derivatives are treated in more detail in (73MI2). Thus, the reactions of naphtho[1,8-bc]thiete S-oxide (35) and S,S-dioxide (36) with the lithium aluminum hydride followed by methylation catalyzed by bases results in the formation of the same compounds but in different proportions (Scheme 12). Meinwald et al. (76JA6643) described reactions with sodium hydroxide and lithium phenylamide leading to the formation of sodium 1-naphthalenesulfonate (69) and 1-naphthalenesulfonamide (70), respectively. Again, the negatively charged fragment of the reagent attacks the sulfur atom, followed by protonation of the C1 atom of the naphthalene core (Scheme 13). c. Reactions at the Sulfur Atom Without Hetero Ring Opening. The known reactions of this type are rare; they include oxidation and alkylation (Scheme 14). Oxidation, already mentioned, is affected by peracids. Reaction of naphtho[1, 8-bc]thiete (34) with one equiv of a peracid affords sulfoxide 35, and with two equiv

18

VALERII V. MEZHERITSKII HO

Me

[Sec. II.E

Me SO

72

SO

73(86%) NaOH/H2O

Me S +

BF4−

S

35 RCO3H

Pyridine/∆ Me3O+ BF4−

2 RCO3H 34(45%)

71(64%)

RCO3H S O2

LiAlH4 MeS MeS

36

+ S 59 (54%)

74 (20%)

Scheme 14

sulfone 36. The latter also can be obtained by oxidation of sulfoxide 35 with a peracid. Naphtho[1,8-bc]thiete with trimethyloxonium tetrafluoroborate in dichloromethane led to the formation of sulfonium salt 71, as white needle crystals with mp 146–147 1C, stable on recrystallization from 2-propanol. The methyl group gives a sharp singlet at 3.82 ppm. Sulfonium salt 71 in boiling pyridine suffers demethylation to give naphthothiete (34), and alkali converts it into methyl naphthyl sulfoxide (73). Reductive opening of the hetero ring occurs in reaction with 71 and LiAlH4 leading to methyl naphthyl sulfide (59, R=H) and a dimeric sulfide 74. Dimer 74 presumably forms from dilithium derivative (57, Scheme 10) (originating from naphthothiete (34) and LiAlH4) and methylsulfonium cation of 71.

E. NAPHTHO[1,8-BC]BORETE The simplest naphtho[1,8-bc]borete (75) is unknown. However, the results of semiempirical calculations that we have performed by the AM1 method (Figure 8) and are first published in the present review suggest the compound to be stable. The first specimen in this series, N,N-di-iso-propylamino-naphtho[1,8-bc]borete (76), was obtained in 1994 (94AG(E)1247) by a reaction of 1,8-dilithionaphthalene (18) with di-iso-propylaminoboron dichloride (Scheme 15).

Sec. II.E]

19

PERI-ANNULATED HETEROCYCLIC SYSTEMS. PART I

H B

89.7 83.7 102.7

r = 1.3 p = 1.2° 133.7

75 Figure 8. General formula, bond angles and the other parameters of the simplest naphtho[1, 8-bc]borete calculated by the AM1 method

R Li

N

B

Li

R

R

R

N

X

Cl2BNR2

B

B

X X

BX3 76

18

77 −RX

HBX2 R X

B

+3 EtOH −3H2

R

R

N H B

X

79

X

B

N

B

X

78

R = i-Pr; 78, X = Cl (a), Br (b), OEt (c); 79, X = H (a), Et (b) Scheme 15

The reaction was carried out under mild conditions (hexane, 20 1C), and the product was purified by sublimation under reduced pressure. The yield of 76 was 89%, mp. 84 1C. The structure was confirmed by 1H, 13C, and 11B NMR spectra, by its mass spectrum and X-ray diffraction. The X-ray data show that the values of the bond and torsion angles indicating the molecule’s viability are close to the calculated values (Figure 8) neglecting the difference between the substituents at the boron atom (H or i-Pr2N). The hetero ring and the naphthalene framework are virtually coplanar, and the r is also about 1.3. Compound (76, Scheme 15) reacted under mild conditions (hexane, 20 1C) with boron trichloride or tribromide to form naphtho[1,8-cd][1,2,6]azadiborinine (78).

20

VALERII V. MEZHERITSKII

Mes Li

Li

Mes B

Mes2 B

BR2

Li(S)4 R2BBr

Mes2BF [S] 18

[Sec. II.F

80

81

Mes = 2,4,6-Me3C6H2; S = THF or Py; 81, R = Me (a), Ph (b) Scheme 16

This ring-expansion of the four-membered hetero ring into a six-membered one is preceded by rupture of the B–C bond and formation of nonisolable intermediate 77 followed by elimination of the iso-propyl chloride or bromide. Naphthoborete (76) with diborane and diethylborane (HBX2, X=H, Et) yielded m-di-iso-propylaminodiboranes (79a and 79b). In this case a B–H–B bridge forms preceding further processes. Compound (79a) was regarded by A. Hergel et al. as a stabilized isomer (77, X=H). They believed that this assumption was proved by the replacement of three hydrogen atoms attached to boron in compound (79a) by ethoxy groups at treatment with ethanol (79a-77, X=OEt). Intermediate (77, X=OEt) eliminates a molecule of ethyl isopropyl ether transforming into peri-annulated borodiazine (78c). The treatment of 1,8-dilithionaphthalene (18) with dimesitylboron fluoride gave heterocyclic anion 80 with counterion [Li(THF)4]+ or [Li(Py)4]+ (02MI1982). The counterion character depends on the solvent used for recrystallization of the primary product. Salts 80 were characterized by their NMR spectra proving the presence of a symmetrically substituted naphthalene core. While 80 with the [Li(THF)4]+ counterion is unstable and quickly looses THF at room temperature, crystals with the [Li(Py)4]+ counterion are sufficiently stable to be subjected to the X-ray diffraction analysis (Scheme 16). The treatment of 80 with dimethyl- or diphenylboron bromide induced hetero ring opening yielding 1,8-diboron-substituted naphthalenes (81). J. D. Hoefelmeyer et al. recently (04JCS(D1254)) studied more complex transformations of salts 80.

F. NAPHTHO[1,8-BC]SILETE The simplest naphtho[1,8-bc]silete (82) is unknown (Figure 9). Yet the results (r=1.19, p=0.1) of the semiempirical calculations that we have performed by the AM1 method for publication in the present review suggest the compound to be planar and capable of existence under common conditions. The first two specimens of these structures were Si,Si-dimethyl- and Si,Sidiethylnaphtho[1,8-bc]siletes (84a and 84b) (76CC775), (83CC866), and later (00MI15582), were more complex Si,Si-dialkyl derivatives 84c–84e. 1,8-Dilithionaphthalene (18) and dichlorodialkylsilanes reacted cleanly at 0 1C and at room temperature without needing oxygen removal and exclusion of atmospheric moisture. Yang and Shechter (76CC775) believed that the reaction proceeded

Sec. II.F]

PERI-ANNULATED HETEROCYCLIC SYSTEMS. PART I

H

21

H Si

76.8 87.3 108.8

r = 1.19 p = 0.1

129.1

82 Figure 9. General formula and some calculated parameters of naphtho[1,8-bc]silete

Li

Li

Si

SiR2

R2SiCl2 −LiCl 18

R

R

Cl Li

−LiCl 83

84

84, R = Me (a), Et (b), CH2CHMe2 (c), EtCMe2 (d), CH2SiMe3 (e).

Scheme 17

stepwise as shown in Scheme 17. The products were purified by high vacuum distillation and more thoroughly by preparative gas chromatography. The yields were in the range 25–70%. All the compounds exception (84e) were colorless oils. The structures of the compounds were unambiguously proved by 1H, 13C NMR, and mass spectra, and by elemental analyses. 1. Chemical Properties According to semiempirical calculations by the AM1 method the 1,8-atoms of the naphthalene skeleton linked to silicon are the most electron-rich. An electrophilic attack should take this direction and is well demonstrated by the reaction of silete (84a) in air that suffers a fast deliquescence due to reaction with water vapor resulting in dimer 85 (Scheme 18). Dimethyl derivative 84a is the most hygroscopic whereas naphthosiletes (84) with ethyl and larger alkyl substituents are stable in air. The reactions of Si,Si-dialkylnaphthosiletes (84) with various nucleophiles are shown on Scheme 19. A peri-bridged naphthalene compound was described (94JOM137) where the 1,8-positions of the naphthalene skeleton were connected to a germanium atom (94). This compound was obtained from dialkylgermanium dichloride and

22

VALERII V. MEZHERITSKII

Me

Me Si

Me

O H

[Sec. II.F

Me Si

Me2Si O

84a

85

H

SiMe2

+ 84a

Scheme 18

SiR2 R2SiOMe 91 R

R Si

Li

n

MeOH

86 H2C

MeLi

84

H

R2Si

Li

R2SiCH2Li Me3SiCl

Pd(PPh3)4 88

87

R2Si

R2SiCH2SiMe3

89

SiR2

90

Scheme 19

1,8-dilithionaphthalene (92, X=Li) or 1,8-naphthalenediylmagnesium (93). Organomagnesium compound 93 formed on treating with THF a 1,8-dimagnesium naphthalene derivative (92, X=MgBr2, MgI2) and 93 with dimethylgermanium dichloride furnished Ge,Ge-dimethyl(1,8-naphthalenediyl)germete as an only product. By contrast, with 1,8-dilithionaphthalene the product always contained as impurity dimers such as compound 90, (Scheme 19) with Ge atoms instead of Si. The freshly prepared Ge,Ge-dimethyl(1,8-naphthalenediyl)germete (94) is an oily fluid that on standing in a deuterobenzene solution forms a solid precipitate of polymer (95) with various units of number n (Scheme 20). A considerable number of naphtho[1,8-bc]cyclobutanes was synthesized where the bridging atom between the 1,8-positions of the naphthalene structure was carbon

Refs.]

23

PERI-ANNULATED HETEROCYCLIC SYSTEMS. PART I Mg +

MgX2

n

93

R2GeCl2 X=MgBr, MgI X

R R Ge

X

R2Ge

GeR2

GeR2



R2GeCl2 92

R2Ge

94

95

n

X = Li (a), MgBr (b), MgI (c)

Scheme 20

(Figure 3, X=CR2). Inasmuch as these compounds are hydrocarbons and not heterocycles they are outside the scope of this review. The interested reader should consult the following publications: (74JA8116), (77JA2371), (80CC190), (82TL2715), (83JA6096), (83JA6104), (83CC866) (83JA7786), (85JA7597), (91JOC1663).

REFERENCES 28CB362 31JCS1265 33BRP394,511 57JOC36 64HCA1172 65AG(E)786 66CRV593 67LA96 68CC1026 69JCS(C)756 69JCS(C)760 69JCS(C)765 69JA1035 70JCS(C)298 70JOC4261 71TL4093 71JA3802 72JOC2152 73MI1240

R. Willstater and L. Schuler, Chem. Ber., 61, 362 (1928). G. Clemo, J. Cocburn, and R. Spence, J. Chem. Soc., 1265 (1931). E. Ayers, Brit. Pat., 394, 511 (1933), [CA, 28, 181 (1934)]. P. D. Gardner and R. J. Thomson, J. Org. Chem., 22, 36 (1957). F. Gerson, E. Heilbronner, and V. Boekelheide, Helv. Chim. Acta, 47, 1172 (1964). R. W. Hoffman and W. Sieber, Angew. Chem., Int. Ed. Engl., 4, 786 (1965). V. Balasubramaniyan, Chem. Rev., 66, 593 (1966). R. W. Hoffman and W. Sieber, Justus Liebigs Ann. Chem., 703, 96 (1967). W. D. Crow and C. Wentrup, J. Chem. Soc., Chem. Commun., 1026 (1968). C. W. Rees and R. C. Storr, J. Chem. Soc.(C), 756 (1969). C. W. Rees and R. C. Storr, J. Chem. Soc.(C), 760 (1969). C. W. Rees and R. C. Storr, J. Chem. Soc(C), 765 (1969). P. Flowerday and M. J. Perkins, J. Am. Chem. Soc., 91, 1035 (1969). P. Flowerday and M. J. Perkins, J. Chem. Soc.(C), 298 (1970). A. J. Gordon, J. Org. Chem., 35, 4261 (1970). D. C. De Jongh and G. N. Evenson, Tetrahedron Lett., 4093 (1971). J. Meinwald and G. W. Gruber, J. Am. Chem. Soc., 93, 3802 (1971). D. C. De Jong and G. N. Evenson, J. Org. Chem., 37, 2152 (1972). O. E. Schelepin, Chemistry of Pericyclic Compound. (in Russia), RSU Press, Rostov-on-Don (1973).

24 73MI2 74JA6532 74JA8116 75JA681 75TL3845 75BCJ932 76JA6643 76CC775 77TL943 77JA2371 79JA7684 80CC499 80CC190 81MI1281 81MI2682 81JCS(P1)413 82TL2715 83JA6096 83JA6104 83CC866 83JA7786 83TL821 84MI1363 84MI2237 84MI3403 85MI280 85JA7597 90AHC(51)1

VALERII V. MEZHERITSKII

[Refs.

E. Kuhle, The Chemistry of Sulfenic Acids, Georg Thieme Verlag, Stuttgart (1973). J. Meinwald and S. Knapp, J. Am. Chem. Soc., 96, 6532 (1974). R. J. Bailey and H. Shechter, J. Am. Chem. Soc., 96, 8116 (1974). J. Meinwald, L. V. Dunkerton, and G. W. Gruber, J. Am. Chem. Soc., 97, 681 (1975). J. Rigaudy, C. Igier, and J. Barcelo, Tetrahedron Lett., 3845 (1975). M. Kato, S. Takaoka, and T. Miwa, Bull. Chem. Soc. Japan, 48, 932 (1975). J. Meinwald, S. Knapp, S. K. Obendorf, and R. E. Hughes, J. Am. Chem. Soc., 98, 6643 (1976). L. S. Yang, H. Shechter, and J. Chem. Soc., Chem. Commun., 775 (1976). S. E. Carrol, B. Nay, E. F. V. Scriven, and J. Suschitzky, Tetrahedron Lett., 943 (1977). M. Gessner, P. Card, H. Shechter, and G. Christoph, J. Am. Chem. Soc., 99, 2371 (1977). J. Nakayama, T. Fukushima, E. Seki, and M. Hoshino, J. Am. Chem. Soc., 101, 7684 (1979). I. R. Dunkin and P. C. P. Thomson, J. Chem. Soc., Chem. Commun., 499 (1980). J. Becker and C. Wentrup, J. Chem. Soc., Chem. Commun., 190 (1980). ‘‘The Common Organic Chemistry’’ (N. K. Kochetkov, ed.) (in Russia). Khimia, Moscov (1983). J. Nakayama, J. Synth. Org. Chem. Tokyo, 39(8), 682 (1981), [CA 95, 168589 (1981)]. J. Nakayama, S. Dan, and M. Hoshino, J. Chem. Soc. Perkin I, 413 (1981). T. A. Engler and H. Shechter, Tetrahedron Lett., 2715 (1982). R. J. Bailey, P. J. Card, and H. Shechter, J. Am. Chem. Soc., 105, 6096 (1983). P. J. Card, F. E. Friedli, and H. Shechter, J. Am. Chem. Soc., 105, 6104 (1983). L. S. Yang, T. A. Engler, and H. Shechter, J. Chem. Soc. Chem. Commun., 866 (1983). R. A. Hayes, T. C. Hess, R. J. McMahon, and O. L. Chapman, J. Am. Chem. Soc., 105, 7786 (1983). G. De Luca, A. Pizzabiocca, and G. Renzi, Tetrahedron Lett., 24, 821 (1983). S. Searles ‘‘Oxetanes and Oxetenes’’ in Comprehensive Heterocyclic Chemistry Vol. 7, p. 363, Pergamon Press (1984). D. E. Davies and R. S. Storr, ‘‘Azetidines, Azetines and Azetes’’ in Comprehensive Heterocyclic Chemistry Vol. 7, p. 237, Pergamon Press (1984). E. Block ‘‘Thietanes, Thietes and Fused-Ring Derivatives’’ in Comprehensive Heterocyclic Chemistry Vol. 7, p. 403, Pergamon Press (1984). A. F. Pozharskii ‘‘The Theoretical Bases of Chemistry of Heterocycles’’ (in Russia). Khimia, Moscov, 1985. R. J. McMahon, O. L. Chapman, R. A. Hayes, T. C. Hess, and H. P. Krimmer, J. Am. Chem. Soc., 107, 7597 (1985). V. V. Mezheritskii and V. V. Tkachenko, Adv. Heterocycl. Chem., 51, 1 (1990).

Refs.] 91JOC1663 94JOM137 94AG(E)1247 95MI1696 00MI15582 01JST287 02AG(E)3897 02MI1982 04JCS(D1254)

PERI-ANNULATED HETEROCYCLIC SYSTEMS. PART I

25

A. Kumar, F. E. Friedli, L. Hau, P. J. Card, N. Mathur, and H. Shechter, J. Org. Chem., 56, 1663 (1991). M. A. G. M. Tinga, G. J. H. Buisman, G. Schat, O. S. Akkerman, F. Bickhelhaupt, W. J. J. Smeets, and A. L. Spek, J. Organomet. Chem., 484, 137 (1994). A. Hergel, H. Pritzkow, and W. Siebert, Angew. Chem., Int. Ed. Engl., 33, 1247 (1994). I. Yavari and D. Nori-Shargh, J. Phys. Org. Chem., 8, 696 (1995), [CA 124, 116(1996)]. J. Ohshita, K. Matsushige, A. Kunai, A. Adachi, K. Sakamaki, and K. Okita, Organometallics, 19, 5582 (2000). H. Roohi, F. Deyhimi, and A. Ebrahimi, J. Mol. Struct. (THEOCHEM), 540, 287 (2001), [CA 135, 92216 (2001)]. M. Tsutomu, N. Tooru, and M. Katsuhiko, Angew. Chem., Int. Ed. Engl., 41, 3897 (2002). J. D. Hoefelmeyer and F. P. Gabbai, Organometallics, 21, 982 (2002). J. D. Hoefelmeyer, S. Sole, and F. P. Gabbai, Dalton Transaction, 1254 (2004).