J. Electroanal. Chem., 86 (1978) 241--244 © Elsevier Sequoia S.A., Lausanne -- Printed in The Netherlands
Preliminary n o t e ELECTROCHEMICAL SYNTHESIS OF HETEROCYCLIC COMPOUNDS PART V. ANODIC SYNTHESIS O F SOME N-HETEROCYCLES*
I. TABAKOVIC, M. TRKOVNIK and D. GALIJA~ Faculty of Technology, University of Banjaluka, 78000 Banjtduka (Yugoslavia) (Received 5th September 1977; in revised form 18th October 1977)
INTRODUCTION Controlled potential electrolysis is a valuable tool to induce ring closure reactions [1, 2] b y bringing the reaction centers into a suitable oxidation state. The aim of the present paper is an investigation of the anodic oxidation of several compounds ~having an azomethine function which makes new preparations of heterocyclic compounds possible. EXPERIMENTAL The apparatus and cells for voltammetry, coulometry and preparative electrolysis have been described earlier [ 3]. Acetonitrile was purified b y refluxing over potassium permanganate for l h , followed b y distillation over P2Os. All oxidations were carried o u t in a divided electrolytic cell previously described  using acetonitrile and tetraethylammonium or lithium perchlorate as a solvent and supporting electrolyte respectively.
Preparation o f 3-phenyl-s-triazolo [4,3-a]pyridine (Ia ) To 100 ml of the anolyte (CH3CN--0.1 M Et4NC104) benzylidene 2-pyridylhydrazone (I) (0.4 g; 0.002 mol) was added. The potential was maintained at 1.6 V vs. SCE and the electrolysis was discontinued when the current decayed to the background level ( ~ 6 mA) at t h e end of electrolysis after complete conversion of the substrate. The anolyte was evaporated to a b o u t 10 ml volume and 100 ml of saturated aqueous sodium hydrogen carbonate was added. The precipitated product was collected b y filtration and was further purified b y column chromatography over a short column of silica gel (ethylacetate--benzene mixture as eluant) and recrystallised from benzene. Yield: 0.280 g (74%); m.p. 172--173 ° (ref. , m.p. 172--1736). *Part IV: Z. Gruji~, I. Tabakovi~ and M. Trkovnik, Tetrahedron Lett., 4823 (1976).
Preparation of 3, 4-dipheny lfuroxan (IIa ) : To 100 ml of anolyte (CHsCN--0.1 M Et4NCIO4) benzildioxim (IT) (0.5 g, 0.002 tool) was added. After electrolysis at controlled potential (1.2 V vs. SCE) most of the acetonitrile was removed by distillation and 70 ml of saturated aqueous sodium hydrogen carbonate was added. The precipitated product was filtered off and recrystallised from ethanol. Yield: 0.45 g (92%); m.p. 114--116°C (ref. , m.p. 116°C).
Preparation of 2, 4,5-triphenyb l,2,3-triazole-l-oxide (Illa ) To 100 ml of anolyte (CHsCN--0.1 M Et4NC104) benzilmonoxime phenylhydrazone (III) (0.2g, 0.00063 mol) was added. After electrolysis at controlled potential (1.4 V vs. SCE) most of the acetonitrile was removed by distillation and 70 ml of saturated aqueous sodium hydrogen carbonate was added. The precipitated product was filtered off and recrystallised from 80% aqueous ethanol. Yield: 0.165 g (83%); m.p. 167--169°C (ref. , m.p. 169°C).
Preparation of 6-chloro-s-triazolo.[1,5.b]pyridazine-3-oxide (IVa) To 100 ml of anolyte (CHsCN--0.1 M LiC104) 3-hydroxyiminomethyleneamino-6-chloropyridazine (IV) (0.2g; 0.00108 mol) was added. At the end of electrolysis at controlled potential (1.6 V vs. SCE) the current was 50 mA. Acetonitrile was evaporated to a volume of about 10 ml and 100 m l of water was added. The mixture was neutralized with sodium hydrogen carbonate and extracted with chloroform. After evaporation of solvent the residue was subjected to column chromatography over column of silica gel using chloroformmethanol mixture (10 : 1) as an eluant. 6-Chloro-s-triazolo [ 1,5-b ] pyridazine-3oxide (IVa) was isolated as a second fraction. Three additional products were also isolated whose structure is unknown at a present time'. Yield: 0.055 g (28%); m.p. 217--218 ° (ref. , m.p. 218). RESULTS AND DISCUSSION
In this paper we report a convenient new synthesis of 3-phenyl-s-triazolo [4,3-a]pyridine (Ia), 3,4-diphenylfuroxan (IIa), 2,4,5-triphenyl-l,2,3-triazole-l,0xide (IIIa) and 6-chloro-s-triazolo [ 1,5-b] pyridazine-3-oxide (IVa) initiated by anodic intramolecular oxidative cyclisations according to the following scheme. ~N
ct ~ " % , ~ '
All conversions, w i t h e x c e p t i o n I V -~ I V a , o c c u r r e d s m o o t h l y a t a g r a p h i t e a n o d e (1.5 x 5 c m ) w i t h o u t t h e i n h i b i t i o n o f t h e e l e c t r o d e . I n t h e r e a c t i o n I V -~ I V a a Pt-gauze (3 x 5 c m ) was used. T h e y i e l d o f p r o d u c t s , d a t a derived f r o m cyclic v o l t a m m e t r y a n d c o n t r o l l e d p o t e n t i a l c o u l o m e t r y are s u m m a r i s e d in T a b l e 1. Cyclic v o l t a m m o g r a m s o f all f o u r c o m p o u n d s i n v e s t i g a t e d e x h i b i t irreversible waves in t h e r a n g e o f s w e e p r a t e s f r o m 0.01 t o 1 Vs -1 . I t f o l l o w s f r o m t h e linear p l o t o f t h e p e a k c u r r e n t ip vs. v 1~, i.e., t h e s q u a r e r o o t o f t h e s w e e p r a t e , t h a t t h e limiting c u r r e n t s f o r t h e c o m p o u n d s I, I I a n d I I I are p a r t l y c o n t r o l l e d b y diffusion. T h e c o u l o m e t r i c n values f o r all t h r e e c o m p o u n d s w e r e v e r y close t o 2 and p r e p a r a t i v e electrolysis gave g o o d yields o f t h e h e t e r o c y c l i c p r o d u c t s l a , I I a a n d I I I a . T h e cyclic v o l t a m m o g r a m o f 3 - h y d r o x y i m i n o m e t h y l e n a m i n o - 6 - c h l o r o p y r i d a z i n e , IV, s h o w s t w o irreversible waves w h o s e c u r r e n t f u n c t i o n s , ip/V lh, decrease w i t h s w e e p rate. T h e h e i g h t o f t h e first wave is o n l y h a l f o f t h a t o f t h e TABLE 1 Electroanalytical and preparative results Conversion
I~ I a
Cyclic voltammetry results a Ep/2/V (vs. SCE)
Applied potential/ V (vs. SCE)
Yield c of product/ %
II --, II a
III --, III a
IV ~ IV a
a Sweep rate, 0.1 V/s; substrate concentration, 10 -3 M; CH3CN--0.1 M Et,NCIO4; electrode, pyrolitic graphite for substrates I, II, III and Pt for substrate IV. Determined by coulometry at controlled potential. c All products gave spectra consistent with their assigned structure. d In the reaction IV ~ IVa the current was switched off when 2.3 electrons per molecule of IV were transferred. The final current was 50 mA.
second wave. The coulometry at the first potential gave an n value of about 2, but in the preparative electrolysis the heterocyclic product IVa was obtained. Electrolysis at the second potential (1.6 V vs. SCE) gave the expected 6-chloro-striazolo[1,5-b]pyridazine-3-oxide in 28% yield and three additional products were identified in reaction mixture. The nature of all products in reaction mixture after oxidation of the compound IV at the first and the second potential will be investigated. The chemical synthesis of 3-phenyl-s-triazolo[4,3-a]pyridine [4, 5], 3,4
We thank Prof. B. Stanovnik for providing the sample of 3-hydroxyiminomethyleneamino-6-chloropyridazine. We wish to thank the Scientific Foundation of SR Bosnia and Hercegovina for financial support.
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