Anaerobic photolysis of dimethoxynitrobiphenyls IIIa-VIa in aqueous alkaline tert-butanol gave products of nucleophilic photosubstitution of methoxyl by hydroxide anion, while the dimethoxybiphenyls Ia and IIa were found unreactive. Regioselectivity of the reaction was examined in view of a possible “extended meta activation” by the nitro group. The most reactive substrate IIIa gives both C-3 and C-4 substitution products with an unsubstantial preference for the latter, which opposes the “extended meta selectivity” rule. All of the other compounds obey the rule, and 3,4-dimethoxy-3'-nitrobiphenyl (IVa) even displayed absolute selectivity by yielding C-3 substituted compound as the only product. 2,5-Dimethoxy substituted compounds underwent photosubstitution which much lower quantum yields than their 3,4-substituted counterparts, most probably due to some steric hindrance of conjugation. Similarly, 3-nitro-substituted biphenyls exhibited much lower overall reactivity than 4-nitro derivatives.
London Dispersion Rather than Steric Hindrance Determines the Enantioselectivity of the Corey‐Bakshi‐Shibata Reduction
