Degenerate rearrangements of long-lived 9,10-dimethyl-9-(1-methyl-1-vinyl)-phenanthrenium and 9,10-dimethyl-9-( trans-1-methyl-1-propenyl)phenanthrenium ions: unexpected strong retardation of vinyl migrations

As found by dynamic NMR, the rates of degenerate rearrangements of long-lived 9,10-dimethyl-9-(1-methyl-1-vinyl)phenanthrenium and 9,10-dimethyl-9-( trans-1-methyl-1-propenyl)phenanthrenium ions proceeding by 1,2-vinyl shifts are unexpectedly much lower than those of the 9,10-dimethyl-9-vinylphenanthrenium and 9,10-dimethyl-9-( cis-1-methyl-1-propenyl)phenanthrenium ions.