The Reaction of α-Diazo-β-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 cyclic ketones. Further treatment of
these α-diazo-β-hydroxy esters with boron trifluoride
etherate in various solvents affords an unusual array of products.
Product types and ratios were found to be strongly
dependent on ring size and the solvent used. The reaction proceeds
by Lewis acid complexation of the alcohol
functionality of the diazo hydroxy ester with BF3 etherate
followed by neighboring-group participation of the diazo
moiety to generate a cycloalkylidene diazonium salt. Loss of
nitrogen produces a highly reactive, destabilized,
linear vinyl cation. Ring expansion via a 1,2-methylene
shift leads to the formation of a more stable, bent
cycloalkenyl
vinyl cation. A subsequent 1,2-methylene shift results in ring
contraction ultimately leading to a stable allylic cation.
This cation is either trapped by the solvent or else undergoes
cyclization with the adjacent ester group to give a
lactone. Computational studies at the 6-31G* level were performed
to determine the geometry of the optimized
vinyl cations. Relative energies suggest a moderate energy gain
for isomerization of the initial vinyl cation V1
to
the rearranged vinyl cation V2 followed by a large
stabilization in energy for subsequent conversion to the
allyl
cation A1. Compared with isolated product
distributions, the energy profiles suggest kinetically-controlled
V1 → V2
→ A1 migrations. Finally, the calculations
suggest that in diethyl ether the carbocations may be coordinated to
a
molecule of solvent resulting in “protected” cationic intermediates
with nonlinear geometries.