Electrochemical reactions. Part 20. Intramolecular cyclisation during the reduction of 2-chloro-NN-diphenylbenzamides

摘要

2456 J.C.S. Perkin IElectrochemical Reactions. Part 20.l Intramolecular Cyclisation duringthe Reduction of 2-Chloro-NN-diphenylbenzamidesBy James Grimshaw and David Mannus, Department of Chemistry, Queen's University, Belfast BT9 5AGThe aryl radical formed by reductive cleavage of the carbon-chlorine bond in 2-chloro-NN-diphenylbenzamideundergoes exclusive capture by the netghbouring N-phenyl group to yield N-phenylphenanthridone and biphenyl-2 - carboxa n i I ide. 2 - C hloro-NN-d i -4- to1 yl benzamide undergoes a similar reaction.STUDIES on the electrochemical reduction of 2-halo-geno-N-methylbenzanilides showed that the initiallyformed phenyl radicals can be trapped in good yield byreaction with the adjacent benzene ring. Hey2 has0 X 'GR 6- R c1-R(4 1 ( 5 )demonstrated that thermal decomposition of the diazo-nium chloride from 2-amino-NN-diphenylbenzamidegives 10-phenylphenanthridone (4; R = H) and 2-hydroxy-NN-diphenylbenzamide (1 ; R = H, X = OH),but decomposition at room temperature on addition ofcopper powder gives 10-phenylphenanthridone (4 ;R = H) and biphenyl-2-carboxanilide ( 5 ; R = H).The first reaction proceeds by a carbocation mechanismand the second by a free radical mechanism.We wishedto compare these reactions with the electrochemicalreduction of 2-chloro-NN-diphenylbenzamide.Reduction of 2-chloro-NN-diphenylbenzamide at thepotential of the first polarographic wave yielded twoproducts, which were separated by preparative t.1.c. andidentified as the N-phenylphenanthridone (4; R = H)and the anilide ( 5 ; R = H).NN-Diphenylbenzamidewas not present. Chloro-NN-di-4-tolylbenzamide gavesimilar results on electroreduction.shaw, preceding paper.1 Part 19, J. Grimshaw, R . J . Haslett, and J . Trocha-Grim-These results indicate very efficient intramoleculartrapping of the aryl radical (2) formed by reduction ofthe carbon-chlorine bond. The intermediate (3) isreduced both electrochemically and also under Hey'sconditions with copper power to yield the biphenyl-carboxanilide. The yields (see Table) of this anilideand the N-phenylphenanthridone from both our ownand Hey's experiments are closely similar. In both setsof experiments, yields were determined after isolation ofthe products by chromatography.EXPERIMENTALStarting Materia2s.-2-Chlorobenzoyl chloride and theappropriate diphenylamine were heated for 1 h in refluxingether containing an excess of pyridine.Evaporation andaddition of water precipitated the product. 2-Chloro-NN-diphenylbenzamide, n1.p. 141-142" (from ethanol) (lit.,3142-143"), showed El - 1.98 V us. s.c.e. on polarographyin dimethylformamide (0.1 M in tetrapropylammoniumperchlorate). 2-Chloro- NN-di-4-to2ylbenzamide crystallisedfrom ethanol as needles, m.p. 133-134" (Found: C, 75.2;H, 5.4; C1, 10.7; N, 4.2. C,lH,,CINO requires C, 75.1;H, 5.4; C1, 10.6; N, 4.20/,), 7 2.62-2.70 (12 H, aromatic)and 7.77 (6 H, s, CH,), M f 337/335, Eh -1.98 V vs. s.c.e.The polarograms showed other waves with E4 more negativethan - 2.6 V.EEectrochentical Reduction.-An H-type electrochemicalcell was used with a mercury cathode, platinum anode, andsaturated calomel reference electrode.The electrolyte was0.lM-tetrapropylammonium perchlorate in anhydrous di-methylformamide. A solution of the 2-chloro-NN-diphenyl-benzamide (2 g) in the electrolyte (15 ml) was reduced at acontrolled cathode potential of - 2.0 V for 48 h ; the currenthad then fallen to a low value. The catholyte was evapor-ated under reduced pressure and the residue dissolved inchloroform; the solution was washed with water, dried(Na,SO,), and evaporated. The products (total recovery97) were separated by chromatography on silica gel platesand elution with dichloromethane. Yields are calculatedon the basis of replacement of C1 by H.Product yields from the electroreduction of 2-chloro-NN-diphenylbenzamides compared with yields from de-composition of diazonium salts with Cu (from ref. 2)Starting material Product( 1 ; R = H, X = C1) (4; R = H)(5; R = H)(1; R = H, X = N,+Cl-) (4; R = H)(5; R = H)( 1 ; R = Me, X = C1) ( 4 ; R = Me)( 5 ; R = Me)( 1 ; R = Me, X = N,+Cl-) (4; R = Me)( 5 ; R = Me)M.p.("C)226-227107-109225109-1 10176-177132-133175-1 76134-135Yield4231404536592858(04)7/773 Received, 6th May, 197712 D. H. Hey and T. M. Moynehan, J . Chetn. SOC., 1959, 1563.A. W. C.hapman. .I. Chew. SOC., 1927, 1748