THE REACTION OF ALPHA-DIAZO-BETA-HYDROXY ESTERS WITH BORON-TRIFLUORIDE ETHERATE - GENERATION AND REARRANGEMENT OF DESTABILIZED VINYL CATIONS - A DETAILED EXPERIMENTAL AND THEORETICAL-STUDY

Cyclic ethyl 2-diazo-3-hydroxy carboxylates were prepared by treating ethyl diazoacetate with LDA followed by reaction with a series of cycl ic ketones. Further treatment of these alpha-diazo-beta-hydroxy esters with boron trifluoride etherate in various solvents affords an unusua l array of products. Product types and ratios were found to be strongl y dependent on ring size and the solvent used. The reaction proceeds b y Lewis acid complexation of the alcohol functionality of the diazo hy droxy ester with BF3 etherate followed by neighboring-group participat ion of the diazo moiety to generate a cycloalkylidene diazonium salt. Loss of nitrogen produces a highly reactive, destabilized, linear viny l cation. Ring expansion via a 1,2-methylene shift leads to the format ion of a more stable, bent cycloalkenyl vinyl cation. A subsequent 1,2 -methylene shift results in ring contraction ultimately leading to a s table allylic cation. This cation is either trapped by the solvent or else undergoes cyclization with the adjacent ester group to give a lac tone. Computational studies at the 6-31G level were performed to dete rmine the geometry of the optimized vinyl cations. Relative energies s uggest a moderate energy gain for isomerization of the initial vinyl c ation V-1 to the rearranged vinyl cation V-2 followed by a large stabi lization in energy for subsequent conversion to the allyl cation A(1). Compared with isolated product distributions, the energy profiles sug gest kinetically-controlled V-1 --> V-2 --> A(1) migrations. Finally, the calculations suggest that in diethyl ether the carbocations may be coordinated to a molecule of solvent resulting in ''protected'' catio nic intermediates with nonlinear geometries.