Application of empirical potential energy calculations to organic chemistry. Part 22. Restricted internal rotation in substituted 1,1′-bipiperidines, 1-cyclohexylpiperidines, and related molecules due to 1,5-interactions across the pivot bond

Abstract

The reported high barriers (75–80 kJ mol^–1) for a conformational process of 2,2′-dimethyl-4,4′-dialkyl-1,1′-bipiperidines (1) do not originate from the claimed eclipsing conformation but are attributed to a staggered barrier appearing in the course of the rotation of N–N bond, wherein two sets of g^Pg^M arrangements in Me–C(2)–N(1)–N(1′)–C(6′) and C(6)–N(1)–N(1′)–C(2′)–Me units are inescapably locked into a strained S₂ symmetric disposition. This proposal is based on molecular mechanics calculations of model molecules including 1-(2-methylcyclohexyl)-2-methylpiperidine (4; R¹= R²= Me). Bond drive calculations of bicyclohexyls variously substituted at 2,2′,6, or 6′ positions also predict a number of examples wherein the rotation around the pivot bond should be restricted.