Steric Factors in the Single-Electron Transfer Carbolithiation and Transannular Reduction of 6,12-Diphenyldibenzo[b,f]­[1,5]­diazocine by Organolithium Reagents

The puckered tricyclic heterocycle 6,12‐diphenyldibenzo[b,f][1,5]diazocine, a novel probe for revealing single‐electron transfer (SET) reactions, is here employed as a model substrate for investigating the carbolithiation of imines and the stereochemistry, the regiochemistry, and the electronic nature (polar or SET) of such R–Li additions. The chemical behavior of the following organolithium reagents (mainly in THF) toward the diazocine was studied: benzyl‐, methyl‐, phenyl‐, n‐butyl‐, and phenylethynyllithiums, as well as the reactive allylmagnesium chloride and the typical tert‐butylmagnesium chloride. The only reagent that failed to carbometalate or to reduce the diazocine transannularly was phenylethynyllithium. The tert‐butyl Grignard reagent could not carbometalate but did reduce the diazocine with photocatalysis. The other five organometallics yielded both carbometalation of the diazocine and, upon hydrolysis, transannular SET reduction to 4b,9b‐diphenyl‐4b,5,9b,10‐tetrahydroindolo[3,2‐b]indole (“indoloindole”) in varying proportions. Three types of carbometalation products were observed in such hydrolyzed reactions: (1) exo‐1,2‐addition to a C=N bond by C6H5CH2Li, H2C=CHCH2MgCl, and CH3Li, as established by spectral and XRD data; (2) 1,4‐addition to the ortho‐phenyl carbon Co=C‐C=N linkage by phenyllithium with the subsequent elimination of LiH; and (3) 1,4‐addition, as in type 2, by n‐butyllithium but without the elimination of LiH. The foregoing modes of reactivity, the concomitant formation of the telltale SET reduction product, “indoloindole”, and the failure of PhC≡CLi to react at all with the diazocine are marshaled to support the conclusion that the organometallics reacting with the diazocine, by carbometalation and by transannular reduction, do so by SET processes and not by straightforward nucleophilic attack. The relative stabilities of sp3‐, sp2‐, and sp‐hybridized carbon radicals possibly involved were assessed in concluding that PhC≡CLi is incapable of reacting through any SET pathway. The reaction of 6,12‐diphenyldibenzo[b,f][1,5]diazocine with organolithium and Grignard reagents produces 1,2‐ or 1,4‐carbometalation and transannular single‐electron transfer (SET) reduction, except with phenylethynyllithium. The ease of the reaction, the 1,2‐syn,exo stereochemistry, and the 1,4‐regiochemistry point to SET processes governed by the steric demands of the associated R–M.