Electrocyclic aromatic substitution by the diazo-group. Part 3. Studies on substituent directive effects and on the mechanism of benzo-1,2-diazepine formation by the cyclisation of 1-aryl-3-diazoalkenes
Abstract
The 1,7-cyclisation of diazo-compounds of type (4) which have an unsymmetrically placed substituent X gives both benzodiazepine isomers (6) and (8). The majority of substituents studied (X = R, OR, Cl) exert a directing effect which favours the formation of the less thermodynamically stable isomer (6) while only t-butyl and trifluoromethyl favour the formation of the para-isomer (8). A mechanistic study of the cyclisations of (6; X = Me and H) using 2H-labelled substrates has shown that the electrocyclisation step is reversible (Scheme 1; k–1≠ 0). However for other substituents (X = OR, CF3, Cl)k–2(o)≠ 0 and the ortho-isomers undergo slow thermal isomerisation at 80 °C to give the more stable para-isomers (8). The observed directing effects of the substituents X are exerted via their effects on k1, k–1, and k2 and although it is not possible to separate these effects for the majority of the substituents it was shown from the labelling study that the k2/k–1 ratio for the ortho-intermediate (5; X = Me) was markedly higher than that for the para-intermediate (7; X = Me).