The observation of remarkable effects of remotely connected but spatially proximate hydroxy-groups on the rates and regiochemistry of the birch reduction of aromatic rings and double bonds

Abstract

Product and competitive-rate studies of Birch reduction (Li, Bu^tOH, NH₃) of a series of alicyclic compounds, (6)–(13), are described. The hydrocarbons (6a)–(8a) and the syn-methoxy derivative (6c) are slowly reduced to give the unconjugated dienes such as (16a). In contrast the syn-alcohols (6b)–(8b)were rapidly reduced to give the monoenes (17),(19), and (22), respectively. Reduction of the syn-alcohol (10b) was also extremely rapid but that of the norbornenols (11a) and (12a) only showed moderate rate enhancements. Each of the above alcohols displays intramolecular OH ⋯π bonding. Birch reductions of (1), (6a), (10a), (11b), and (12b), none of which possess intramolecular OH ⋯πbonds, appear to obey third-order kinetics[equation(3)] whereas those of the OH ⋯π-bonded alcohols (6b), (10b), (11a), and (12a) followed a combination of second- and third-order kinetics[equation (5)]. The rate and product data for the reduction of the alcohols (6b)–(8b), (10b), (11a), and (12a) are explained in terms of the presence of intramolecular OH ⋯π bonding in these substrates. The observed second-order kinetics are explained in terms of intramolecular protonation of the anion-radical intermediate and is supported by the data for reduction of (6c) and (9). Geometric features which affect the efficacy of intramolecular proton transfer are discussed. Full STO-3G geometry optimisations on the anion radicals of (11a and b) reveal the presence of strong OH ⋯π bonding in the anion-radical of (11a) amounting to some 27 kJ mol^–1.