Benzamides trapping

The all-d.v benzamide 12, readily obtainable from 8 at low temperature with methyllithium,16 is cleanly transformed to the mono-trans isomer 13 above O C. Both anions 12, 13 can be trapped at low temperature with methyl chloroformate to yield the corresponding methyl azepinc-A -carboxylates 14 and 15, respectively, with retention of the configuration.17... 

The sole adduct formed from thermolysis of 163e was A-butoxy-A-(hex-5-en-l-yl)benzamide (168e) rather than the methylcyclopentyl isomer. The rate constant for trapping must at least exceed that for ring closure of hexenyl at these temperatures (ca 5 X 10 8 ) ° . 

When benzylbromide is allowed to react with 4-(3-mercapto-5-phenyl-[l,2,4]triazol-4-ylmethyl)-benzamide (32) in order to synthesize the corresponding thiobenzyl adduct 4-(3-benzylsulfanyl-5-phenyl-[l,2,4]tria-zol-4-ylmethyl)-benzamide (33), a nucleophilic sulfur-based resin (34) is employed to trap excess benzylbromide from the reaction mixture affording a final clean product (33).28 In addition, a basic Amberlite (OH-) is added to assist in the efficient deprotonation of the mercapto functionality in (34) as depicted in Fig. 13. 

Various additives can scavenge these intermediates. Added hydrogen traps the diphenyl diradical, while acetylene combines with the dehydrobenzene. Ammonia forms addition compounds with all three intermediates to give aminobenzaldehyde, benzamide, aniline and carbazol. 

The reaction was discovered in 1923 in interesting circumstances. Products from the decomposition of hydrazoic acid in various solvents were investigated and the decomposition catalysed by sulphuric acid in benzene was found to lead to aniline Schmidt considered an imine radical NH to be the species responsible and on attempting to trap this with benzophenone he obtained benzamide. The reacdon was generalized and rapidly exploited, mainly by its discoverer. The currently accepted mechanism was proposed in outline shortly after-wards and is shown in its modern form in scheme (19) for carboxylic... 

Apparently, benzoylnitrene is formed in the thermolysis (120 C) of 0,jV-bis(trimethyl-silyljbenzocarboximidicacid and trapped with cyclohexene to give, V-(3-cyclohexenyl)benzamide (10/ ,) together with phenyl isocyanate.168 Benzoylnitrene is apparently also formed by sul-filimine photolysis and trapped with cyclohexene to give A/-benzoyl-7-azabicyclo[4.1.0]heptane (15.6%). 55 ... 

In contrast to the case where aniline is used as the nucleophile, the benzamide reaction can be improved by utilizing dioxygen in the reaction mixture since 11 is resistent to autoxidation. Under aerobic conditions the nitrobenzene radical anion is readily trapped by O2 generating superoxide and nitrobenzene (Figure 10) (11). This reaction pathway inhibits the formation of azoxybenzene by diverting the electron transfer cascade and ultimately utilizing dioxygen as the terminal oxidant. Thus, under aerobic reaction conditions 12 is the only observed reaction product. 

The benzamide bisdiene 134 proved to be a particularly good substrate for the carbocyclization with 1-pyrrolidino-l-cyclohexene. Its cyclization using 2 equiv of triph-enylphosphine or tris(o-tolyl)phosphine or 1 equiv of diphenylphosphinoethane (dppe) in conjunction with Pd(OAc)2 proceeds in about 90% overall yield to the substituted N-acylpyrrolidine 135 (Scheme 43). Notably, indole also proves to be an effective trapping reagent in the reaction of 134. The adduct 136 is obtained in 91% yield from the palladium acetate-catalyzed reaction in the presence of lris(( -tolyl)phosphine. 

---