Preparation, characterization and rearrangement pathways of 10‐alkyl‐9‐decalyl carbocations

The preparation and characterization of 10‐methyl‐ and 10‐ethyl‐9‐decalyl cations is described. Both cations undergo a rapid, degenerate 10, 9‐alkyl shift, but the ethyl group migration is about 700 times slower at −120°C than that of the methyl group. This preference is opposite to that usually found for alkyl migration rates and, on the basis of MO calculations, it is suggested that a conformational interchange may be the rate‐determining step. Both cations irreversibly rearrange at about −100°C, giving in the methyl case the rearrangement cascade 10‐methyl‐9‐decalyl → cis‐1‐methyldecalyl → trans‐1‐methyldecalyl → trans‐2‐methyldecalyl. Each of these rearrangement ions was independently prepared and characterized by NMR spectroscopy. One can rationalize the initial formation of the less stable cis ring junction on the basis of suprafacial hydride and methyl shifts. Attempts to add molecular hydrogen to the 10‐methyl‐9‐decalyl cation were unsuccessful.