Empirical Force Field Calculations XIII. The Ring Interconversion Modes of cis‐1, 2, 3, 4, 4a, 5, 8, 8a‐Octahydronaphthalene and its 1α, 4α‐Dimethyl Derivative

Abstract

AbstractThe inversion of the (half‐chair) chair conformers of cis‐1, 2, 3, 4, 4a, 5, 8, 8a‐octahydronaphthalene (1) and of its 1α, 4α‐dimethyl derivative (2) are studied with molecular mechanics using the white and Bovill empirical force field. It is a multi‐stage process: first the chair form of the saturated ring transforms into a twist conformation, then the (half‐chair) twist form can follow several pseudorotational paths on which the one half‐chair form of the unsaturated ring passes into the other via a boat form, and finally the twisted saturated ring converts into the inverted chair conformation. In 1 the first part of this process is of higher energy (ΔH≠ = 34.4 kJ mol−1) than the lowest pseudorotational path (30.8 kJ mol−1), in 2 it is the pseudorotational part of the inversion that is of somewhat higher energy (34.0 kJ mol−1) than the chair‐to‐twist stage (32.9 kJ mol−1). The methyl substituents cut off low energy pseudorotational paths available to 1 and direct the pseudorotation in 2 to regions of higher energy. The experimental determination of the inversion barrier of 1 yields ΔG≠ = 41.4 kJ mol−1.