Orbital interactions. XII. Product studies and competition kinetic measurements of the Birch reduction of a series of hexahydrodimethanonaphthalenes and their interpretation in terms of orbital interactions through space and through bonds

Abstract

Relative rate constants for the Birch reduction (Li/liq. NH₃/Bu^tOH) of the three isomeric hexahydrodimethanonaphthalenes (3)-(5) and the octahydro analogues (10)-(13) were obtained and compared with those obtained for the reduction of norbornadiene and norbornene from an earlier study. Diene (5) was reduced almost 2000 times more rapidly than norbornene and 20000 times more rapidly than the monoene (13). Rate-enhancement factors for dienes (3) and (4) were less substantial but meaningful: 19 for (3) [compared with (10)] and 35 for (4) [compared with (12)]. These rate enhancements were attributed to the operation of π* orbital interactions through space in diene (5) and to the presence of π* orbital interactions through four bonds in dienes (3) and (4). The existence of a linear relationship between the natural logarithm of the rate of reduction of a substrate and its LUMO energy (obtained from either gas-phase electron affinities or ab initio MO calculations) supports this conclusion. The only-fair correlation of the above relationship was attributed to the neglect of other factors, such as the electronic structure and the geometry of the anion radical, which contribute to the overall rate of the Birch reduction. These two factors were explored by using PMO theory and ab initio MO calculations. In particular, full geometry optimizations (UHF, STO-3G basis set) on the anion radicals of norbornadiene (1) (C_2v symmetry constraint) and norbornene (22) (C_s symmetry constraint) were carried out, and their geometries reported. Noteworthy is the strong pyramidalization of the olehic centres of (1) and (22) in the endo direction. These pyramidalizations explain the observed stereoselective exo protonation of the anion radical of (1), and also the much faster rate of reduction of (1) compared with (5), since the pyramidalization in the anion radical of (5) is such as to hinder protonation. The geometries of anion radicals appear to have a profound effect on rates, on stereoselectivity of protonation, and on the structures of the final products, and this is discussed in detail. The synthesis of the diene (3) is also described.