A Guidebook to Mechanism in Organic Chemistry

Addition to Carbon-Oxygen Double Bonds

R X R R R' R X R' R X R'
\i/ \ / \i/ \i/
C trans C cis C C
I ^— II — I <±>
c c c o
/A\ / \ /IV
R H R R R' R H R' H

R R' R X R

V /

trans C cis

i

H

C

cb

o

A new asymmetric centre has been introduced but, as always, a race-
mate will be produced. If, however, an asymmetric centre is already
present, e.g. RCHMeCOMe, and if the addition is carried out on
one of the pure optical isomers, the addition is takhf^pffle in an
asymmetric environment and different quantities of the two possible
products are often formed. This is due to the preferential formation
of that isomer whose production involves a transition state in which
steric interaction is at a minimum. Thus where R is a large group and
the reaction is, for example, addition of a Grignard reagent, the initial
attack of the reagent, as a Lewis acid on oxygen, will yield a complex
(XLVTII) in which the now complexed oxygen atom will be as far
away from the bulky R group as possible. As the nucleophile now
attacks the carbon atom of the carbonyl complex it will tend to move
in preferentially from the side on which its approach is hindered only
by hydrogen rathSWhan by the bulkier methyl group:

R Me

V

H C—Me

II

O

R Me

(i)R'MgBr V /

> C Me

(ii)H®/H.O y \

H C

' V

R' OM

(XLIX)

R Me

R'MgBr \ /

> C Me

• V./
  H C
  \ 0
  OMgR
  I
  Br
  (XLVIII)
  R Me
  C R'

H'-V

Me* OH

(L)

The overall result being the formation of (XLIX) rather than (L)

as the major product. The above argument is essentially the working