A family of β,β-disubstituted
α-(p-methoxyphenyl)vinyl cations has been generated
by the laser
flash photolysis of the corresponding vinyl bromides, and the rates of
reactions of the cations with
ethanol in acetonitrile have been measured at 25 °C. The
observed rate constants differ greatly
depending on the substituents, ranging from 3.05 × 105 L
mol-1 s-1 to 8.18
× 107 L mol-1
s-1. The
thermodynamic stabilities of the vinyl cations have been estimated by
means of ab initio MO
calculations for model compounds, which reveals that their stability is
almost unaffected by β,β-dialkyl substituents. In the present system, therefore, the
stability−reactivity relation, in which
a less stable cation is expected to show higher reactivity, breaks
down. Several model transition
structures have been considered in the MO calculations, and the results
indicate that the β
substituents can move away from the incoming nucleophile to avoid
steric congestion but that there
still exists a large steric repulsion in the transition state. The
calculations indicate that the
reactivity of the vinyl cations is primarily controlled by this steric
effect, which is the reason for
the breakdown of the stability−reactivity relationship.
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