0 Ebevier Sequoia Sk,
259-263 Printed in The Netherlands
PALLADIUM CATALYZED SYNTHESIS OF ARYL, HETEROCYCLIC VINYLIC ACETYLENE DERIVATIVES
H.A. DIECK and F.R. HECK* University
of Delaware. Newark,
Monosubstituted acetylenes are converted into disubstituted acetylenes hy reaction with aryl, heterocyclic or vinylic bromides or iodides at 100°C in the presence of a basic amine and diacetatobis(triphenylphosphine)palladium(II) catalyst.
Introduction Previously we reported that pbenyl, metbyl, and carbomethoxyl groups could be substituted for the acetylenic hydrogen in phenylacetylene by reaction of the compound at O”-25°C with phenyl-, methyl-, and carbomethoxy-palladium acetates, respectively. The organopalladium acetate6 were prepared in situ from the corresponding organomercuric acetates and palladium acetate Cl]- The reaction with carbomethoxypalladium acetate was not improved by the addition of tri-n-but&mine [l]. The reaction was believed to proceed according to the following mechanism, based on better understood similar reactions which occurred with olefins [2,3]. [AcOPdCOOCH3]
i- Pd k HOAc
The trans elimination of the hydridopalladium acetate was unusual; partic.I ularly since it did not appear to be base catalyzed. However, there was a piece- .-m dent in olefin chemistry- When the preferred c&-hydride elimination canriot .-:. occur for steric reasons, a trans elimination can occur, at least in the reaction&.- .- _.: -_.-; ~_
” : --. __.. _.I__ -: ._-: .. -.- ._: ,:‘. .:.1.__.-_. .-..- ._...:I
of 4methoxy-3-nitrophenyl143 or phenyl- [ 5 J palladium chlorides with indeneThe cis addition of the arylpalladium group places the palladium on the benzylic carbon with no ck-fl-hydrogens available for the subsequent elimination. The [email protected]
is apparently then lost since the 2-arylindenes are one of the products formed.
More recently we have found it much more practical to generate organopalladium complexes by oxidative addition reactions from organic halides and palladium(O)phosphine complexes rather than from mercurials, for use in ok fink substitution reactions 163 _ We thought it possible that substituted acetylenes might be prepared more practically also by use of the oxidative addition reaction. This paper reports experiments showing that this can be done very readily, but that the mechanism of the reaction may be different from that believed to operate in the above examples. Results and discussion
-4 variety of organic halides and acetylenes were reacted at 100°C with basic amines and catalytic amounts of Pd(OAc)t[P(C,H5),]2. These conditions were generally about the same as those used previously in the palladium catalyzed olefin substitution reactions 163 except that a much larger excess of the amine was used as solvent. Successful reactions producing disubstituted acetyE enes are summarized in Table 1. Four aryl halides (iodobenzene, bromobenzene, Qbromobenzaldehyde and 4bromonitrobenzene); four vinylic halides (E- and Z-Zbromostyrene, 2-bromopropene, and methyl E-3-bromo-2-methylpropenoate) and one heterocychc halide (2-bromothiophene) were employed_ These halides were reacted with one or more of the following four acetylenes: phenylacetylene-l-hexyne,2-methyl-1-buten-3-yne andt-butylacetylene- Reactions were carried out at 100°C for 0.5 to 2.5 h and yields of products varied from 53 to 88%. When Pd(OAc), without a phosphine was used as catalyst no diphenylacetylene was obtained from the reaction of phenylacetylene with bro-
mobenzene, in the presence of triethylamine, and only a 40% yield was obtained with iodobenzeneIt is clear that the reacting acetylenes were partly being used up in reactions other than those forming the disubstituted acetylene, since increasing the amount of the acetylene relative to the halide present generally led to a higher yield of disubstituted acetylene product. Diluting the reaction mixture with a large excess of amine also decreased the amount of side reaction, presumably polymerization, relative to the substitution. _The yields of products were influenced by the structure of the organic haiide; the more reactive the halide was in the oxidative addition, the better the reaction generally proceeded. Thus, iodobenzene gave higher yields than bromo-
benzene, with phenylacetylene and triethylamine it gave 73% diphenylacetylene iu l-5 h, while bromobenzene in 2.5 h (with a larger excess of phenylacetylene) produced only 51% of that product. Electron-withdrawing~substituents in the halide also generally improved the reactions. p-Bromobenzaldehyde and phenylacetylene with triethylamine gave 66% of 4-phenylethynylbenzaldehyde in 1 h compared with bromobenzene which required 2.5 h to react to form only 51% diphenylacetylene. The alkylacetylenes were less reactive than phenylacetylene. With tsiethylamine as the base only the more reactive halides will add $0 alkylacetylenes in significant yield. Iodobenzene, for example, does not react with I-hexyne or t-butylacetylene under these conditions. The alkylacetylenes, however, became more reactive when more basic secondary amines were employed rather than triethylamiue- Thus, with piperidine as the base, 1-hexyne and iodobenzene reacted readily forming 1-phenyl-1-hexyne in 62% yield. The data suggest that under the new reaction conditions, either the palladium hydride elimination from the reaction intermediate becomes base catalyzed or another mechanism is operating. A possible alternative mechanism could involve an attack of an incipient acetylide anion on the organopalladium intermediate followed by reductive elimination of the disubstituted acetylene. The effect of the base then would be to assist in removing the acetylenic proton and since monoalkylacetylenes are less acidic than monoarylacetylenes, the more basic amine would be required in the reactions of the monoalkylacetylenes, particularly when they were being reacted with the less reactive halides. Cassar has demonstrated the reductive elimination reaction with presumably analogous arylnickel complexes 171. If the acetylide intermediate was not formed rapidly then acetylene polymerization would probably occur instead by a multiple insertion mechanism. The initial reduction of the catalyst presumably is caused by the acetylene perhaps by forming a diacetylene. Triethylamiue does not reduce the catalyst at 100°C.
+ 2 RC=C
+ 2 OAc-)
+ R’X + R’Pd(X)[P(C&),],
+ R’C=CR + Pd[P(C6&)3]2 Thus, the major limitation of this disubstituted acetylene and enyne preparation is that halides with strongly electron-donating substituents, or halides which are otherwise made relatively unreactive in the oxidative reaction with the palladium(O) catalyst, do not react well. Otherwise, the reaction appears widely applicable and it will provide a convenient new method for obtaining a variety of acetylene derivatives.
Tricthyl. amlnc (40)
4rBromonltrobcnzcne t~Butylacctylcnc (30) (20)
2~Phcnylcthynyl~ thiophcnc (63.0)
2.Mctlryl.4.pbo~ryl. l.butcn.3.ync (38)b d.(Phcnylcthynyl). benznldchydc (6&O)
Mel, wt, found
3% 184,036 (184,036) % 136.040 (186,034) 3(1s 186,034 (186,030) 203,ODl (203,096)
(roportcd)% (cnlcd,) --.--_.-,l-.-_l-
B,p, or m.p,
[(CD3)2301 0,088 (lH), 2,OOd(2H), 2,20d (2H) J = 8 Hz, 2,20_2,7Om (6H) (CCl4) 2,32-2,86m (7H), 3.00,3,06,3.09, 3,14m (1H)
Idcntlcnl to thnt previously reported ‘d (Neat) 2,34.2,36m (GH), 4,GGm(lH), 4.74m (lH), 8,07s(br) (3H)
(CDC13)2,Old (2H), 2JIGd (2H) J = D Hz, 8,679 (DH) ‘I’rlcthyl. C Methyl &2,6,G~trlnrcthyl~ 47~48/ 130,116 2,2G (Neut) 3,41q (lH), Mothyl &Smbromo= t.Butylacctylonc ilmlnc (40) hcpt-2.cn.4-ynootc (68.6) 0,45 Torr (180,116) 3,02d (3H) J < 2 Hie, Bmcthylpropcnontc (30) 6,308 (3H), 8,736 (WI) j c-w ,_....._ I”...,-_..-I .,.-......_ .._“,. ....(“.“..I,__.,..I....,_. “.“” ,-.I,.“.....,.,” .. .... ........“.._.._ oh = 1 mol ‘x,Pd(OAc)l, 2 mol % Ph3P: B = 2 mol Z (IJh3P)2Pd(OAc)2:C m 1 mol% (Ph3P)2Pd(OAc)z,bYicld determined by VPC, ‘Rcf, 10. dRef. 11, ‘Ret, 12,
Trlcthyl. C umlnc (20) Triothyl. C nmlnc (100)
Phcnylncctylcnc (1286) Phcnylncctylcnc (76)
2~IIromoproponc (10) bIIromobcnxnldchydc (60)
TAIILE 1. ACETYLENIO SUBSTITUTION REACTIONS (REACTION TPMP, = 100°C) ---I_.---_-..Acotylanlo rcnctnnt CntaReaction Product hmlna Orwnlo halldc lysta (mmol) tlmo (h) (yield Ib) (mmol) (ml) __--..l.-..I-.---L1.--.------__-“. Phenylncotylcno Triothyl~ A Dlphcnylncctylone 116 Iodobonzcne (67, 73 “) (12,G) nmlno (20) W-N Dlphcnylaoetylcno Phonylncctylcnc Tricthyl. A 2,G Bromobcnxeno ” (10) nmlno (20) (GlP (20) Plpcrldlno A Phcnylncctylcnc Dlphonylacatylcnc 2,G Bromobcnzeno (64)” (20) (20) (10) l.Hcxync Plpcrldlno II l~Phcnyl.l.hcxync 1 Iodobcnzcne (10) (10) (62)b (6) E-B-Bromostyronc Phonylncctylonc 1 TriothylA &l,l.Dlphcnyb (20) nmlno (60) (10) l.buton.S.yncd (7o)b C2~Rromostyrcnc Phcnylucctylcno 1 Trlothyl. C Z.l,l.Dlphcnyl. nmlno (60) l.buton4wwd (60,9) (20) (40) Iodobcnzono IL.Mcthyl.l-butcnTrlothyk C 2.Mcthyl.l.pbcnyl. 3wc (200) l*butcw3.ync (71) amino (200) (100)
Experimental Methyl E-3-bromo-Z-methylpropenoate _ This material was prepared by the method of Canbere [S]. Z-2-Bromostyiene This was prepared by the method of Cristol and Norris 191. General method for acetylenic substitution reactions The indicated quantities of organic halide, acetylenic reactant, amine, and catalyst were placed in a heavy-walled Pyrex reaction bottle which was then flushed with argon and capped with a self-sealing rubber-lined cap. The mixture’ was stirred magnetically and heated on a steam bath. In reactions where yield was determined by VPC analysis, an internaI standard was added initially and the reactions were run until the yields of products no longer increased. In cases where products were isolated, the reactions were run until VPC analysis showed that all of the starting halide had reacted. The bottles were then cooled and opened. The reaction mixtures were diluted with ether, filtered, and the filtrates were washed several times with ether. Volatile materials were removed under reduced pressure and the residues were then either distilled or sublimed. Sublimed materials were subsequently recrystallized. Z-1,4Diphenyl-l-buten3-yne was isolated by chromatography of the residue, eluting with hexane. 4(t-Butyiethynyl)nitrobenzene CBromonitrobenzene (4.04 g, 20 mmol), t-butylacetylene (2.46 g, 30 mmol) and 40 ml triethylamine were placed in a heavy-walled Pyrex reaction bottle containing a magnetic stirring bar. (Ph3 P&Pd(OAc), (0.298 g, 0.4 mmol) was added and the bottle was flushed with argon and capped with a self-sealing rubber-lined cap. The mixture was then heated with stirring on a Steam bath. After 30 mm, VPC analysis showed that essentially all of the starting bromide had reacted. The reaction mixture was diluted with 100 ml ether and filtered and the residue on the filter was washed several times with ether. The ether and other volatile materials in the filtrate were removed under reduced pressure leaving a brown solid which was sublimed at lOO”C/O.2 Torr. Recrystallization
of the sublimate from hexane afforded 3.59 g (88.4%) nitrobenzene, m-p. 103-104°C.
References 1 2 3 4 5 6 7 8 9 10 11 12
R-F. Heck, J. Amer. Chem. Sot-. 94 (1972) 2712. R.F. Heck. J. Amer. Che SOG 91(1969) 6707. R-F. Heck. J. Amer. Chem. Sot. 93 (1971) 6896. R-F_ Heck. J. Amer. Chem. Sot.. SO (1968) 5518. H. Horino. M_ Arai end N. Inoue. Bull Chem. Sot. Japan. 47 (1974) 1683. H-A. Dieck and R-F. Heck. J. Amer. Chem. Sot.. 96 (1974) 1133. L. Cassar. J. Organometal. Chem.. 93 (1975) 253. P. Canbere. BulL Sot Chim. France. <1964) 144. S.J. Cristol and W-P_ Norrir J. Amer. Chem- Sot-. 75 (1953) 2645 A.C. Cope. D-S. Smith and R-J- Cotter. Org. Syath_. COIL VoL 4. (1963) 377. A. Schoenberg and R.F. Heck. J- Org. Chem_. 39 (1974) 3327. D. Brown. J-C. Craig. N-H. Dyson and W.C. Westlery. J. Chem. Sot_. (1966) 89.