The PinacoljPinacolone Rearrangement/Me\_ Me Me
Me—C—C; -> Me—C—C-R -> Me—C—C—R
I cHvR' K v II >
OHTfeN O-J-H R' O R'
eattack taking place 'from the back' in an internal SN 2 type displace
ment reaction. That the migrating group prefers to move in from the
side opposite to that of the leaving group may be demonstrated in
cyclic systems where there is restricted rotation about the Ct—C 2
bond; it is then found that compounds in which migrating and leav
ing groups are trans to each other rearrange very much more readily
than do those in which the groups are cis. It is noteworthy that the
migrating alkyl group in this and other cases is migrating with its
bonding electrons and so can obviously act as a powerfully nucleo
philic reagent. Where the migrating group is asymmetric, it has in
certain other cases, though not in this particular one, been shown to
retain its configuration as it migrates, indicating that it never actually
becomes wholly free from the rest of the molecule; other evidence is
also against the migrating group ever becoming free, e.g. no 'crossed
product' when two different but very similar piqgcols (that undergo
rearrangement at approximately the same rate)^re rearranged^njhe
same solution: thus the reaction is said to be a typical in/romolecular,
as opposed to /n/ermolecular, rearrangement. Indeed, it is probable
that the migrating group begins to be attached to the carbonium ion
carbon before becoming separated from the carbon atom that it is
leaving. A state such as
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probably intervenes between the initial and the rearranged carbon
ium ions (cf. bromonium ion structures encountered in the addition
of bromine to blefines, p. 138)
As the migrating group migrates with its electron pair i.e. as a
nucleophile, it might be expected that where the groups on the non-
carbonium ion carbon are different, it would be the more nucleo
philic of them, i.e. the more powerful electron donor, that would
actually migrate. Thus in the example