MEMORY EFFECTS IN MULTIPLE CARBONIUM ION REARRANGEMENTS .I. RING-EXPANSION ROUTE TO CATIONS OF BICYCLO[2.2.2]OCTENYL SERIES

Ring expansions of 2-norbornenyl-syn-7-carbinyl substrates in a variety of carbonium ion reactions lead to twice-rearranged products via intermediates different from those generated from the anti isomers. The major products from the syn system are in the G series previously encountered by Goering in solvolyses of bicyclo[2.2.2]-oct-5-en-2-endo-yl derivatives, whereas those from the anti system give mainly products (L series) observed by LeBel from bicyclo[2.2.2]oct-5-en-2-exo-yl derivatives. The specificity of the second rearrangement step is not perfect, however, and some “memory” is lost by crossover from the syn system to the L series and from the anti system to the G series. The product-forming twice-rearranged cations show all the characteristics of the typical G and L intermediates, but the latter two cations when generated from bicyclo[2.2.2]oct-5-en-2-yl substrates do not cross over appreciably. An extra intermediate or reaction path permitting crossover therefore must be inse ted in the mechanism for the ring expansions. This intermediate is not efficiently trapped by external nucleophiles. Environmental factors such as solvent, ionic concentration, electrophilic catalysts, and variation of the leaving counterion have remarkably little effect on the selectivities of the second rearrangement step. © 1969, American Chemical Society. All rights reserved.