A Guidebook to Mechanism in Organic Chemistry

Carbonium Ions, Electron-deficient N and O Atoms

it is the p-MeO-C 6 H 4 that migrates in preference to C 8 HB owing to
the electron-donating effect of the MeO group in the /7-position.
Steric factors also play a part, however, and it is found that
o-MeO-CgHa migrates more, than a thousand times less readily than
the correspondingjjp-substituted group—less readily indeed than
phenyl itself—due to^its interference in the transition state with the
non-migrating groups.
In pinacols of the form
Ph Ph
II
R—C—C—R
I I
OHOH

Ph will migrate in preference to R because of the greater stabilisation
it can, by delocalisation, confer on the intervening bridged inter­
mediate (cf. p. 83).
(d) The Wolff rearrangement: This involves the loss of nitrogen
from a-diazoketones (XVI) and their rearrangement to highly
reactive ketenes (XVII):

o © Ag.o s—-*x »

R—C—CH-r-N^N >(R-5C-T-CH-« > 0=*=C=CH—R

II " ^> -

N

w

llly

o o

(XVI) (XVIII) (XVII)