Page 294 - Mechanism and Theory in Organic Chemistry

1,2-Shifts in Carbenium Ions 281

NHz HNOa
(in 33% yield) (6.18)
86-88% retained
23
racemization involves the cyclopropanol 24 as an intermediate. Their proposed
mechanism is abbreviated in Equation 6.19.38 The intermediacy of 24 is sup-
ported by the fact that deuterium is incorporated into 23 when the deamination
is carried out in D,O.

racemized
24
The stereochemistry at the migration terminus depends on the relative
timing of the leaving group's departure and the 1,2-shift. In the

system, if Z begins to migrate before X has completely departed, the migration
terminus, C,, will be inverted. We have already seen in Cram's work that phenyl
migration with neighboring-group participation leads to an inverted migration
terminus. If, however, X departs before Z begins to move, either retention or
inversion can occur at C,. If the lifetime of the carbocation is very short, retention
will result if Z was cis to the leaving group in the unreacted starting material
(Equation 6.20) and inversion will result if Z was trans (Equation 6.21).

i If the C, carbocation has a long lifetime and there is free rotation about the
! C,C, bond, the relative amounts of retention and inversion will depend on
1
38 Kirrnse and co-workers suggest that 24 is formed and destroyed via a protonated cyclopropane
(see Section 6.2, p. 310).