An investigation of the cycloaddition reactions of a
nitrogen-substituted oxyallyl cation is presented.
The nitrogen-substituted oxyallyl cation,
M+-1 (M+ = H+ or
Li+), can be generated from the
dibromide 2 using either
CF3CH2OH/Et3N or
LiClO4/CH3CN/Et3N. These
oxyallyl cations were
found to undergo [4 + 2] cycloaddition reactions with furan,
cyclopentadiene, 2-methylfuran, and
2-methoxyfuran. The diastereo- and regioselectivities in these
reactions were found to be positively
influenced by the presence of Li+. Reaction of
M+-1 with the 2-substituted furans led to
primarily
those adducts arising from endo addition (as was observed with the
unsubstituted dienes) and
which had the 2-substituent syn to the bromine. Consideration of
the frontier molecular orbitals
of the reacting species (Li+-1 and
2-substituted furans calculated at the PM3 semiempirical
level
of theory) led to the conclusion that FMO theory does not explain the
regiochemistry observed in
this process, although the relative electrophilicity of
M+-1 (M+ = H+ or
Li+) could be rationalized.
Transition structure modeling was consistent with empirical
observations in that it predicted an
endo addition of furan to Li+-1 via a stepwise
reaction. Calculation of the reaction coordinate for
this nonconcerted process predicted a ΔH
rxn
∼ −50 kcals/mol and ΔH
act ∼ +11
kcal/mol, with the
second bond-forming process having a ΔH
act ∼
+1.7 kcal/mol.
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