Stereospecificity in E2 Eliminations/\ B:H
B:rH R R R
R^Ck-R' -> C=C
\
R Br R' R'
; £j
Br^0
(VI) (VII)leaving group should all be coplanar in the transition state in an E2
elimination, but other factors may well play a part in securing
stereospecific,?/ms elimination.
In the case of benzene hexachloride (V) considered above, restricted
rotation about a single bond prevents the leaving groups from getting
into this preferred orientation and elimination is thus inhibited: a
similar phenomenon is also encountered in the conversion of suitably
substituted defines to acetylenes for the same reason. Thus bast
induced elimination of HQ to yield acetylene dicarboxylic acid (VIII)
proceeds much more rapidly from chlorofumaric acid (IX) than from
chloromaleic acid (X),
CI CO.H CO.HC -HO
II •
c «•»
/ \
HOjC H
(IX) (VIII)
as does the elimination of acetic acid from anti- as compared with
syn-benzaldoxime acetate (XI and XII, respectively) to yield benzo-
nitrile(XIII):
Ph H Ph Ph H
\ / I \ /
C -Me CO,!I C -Me CO.H C
II • III « I!
N easy » difficult N
/ \
MeCOO * OOCMe
(XI) • (XIII) (XII)
A fact that may be made use of in assigning configurations to a pair
of stereoisomeric aldoximes.