_P.-Powell-auth.-Principles-of-Organometallic-Chemistry-Springer-Netherlands-1988

Zinc, cadmium and mercury
situations where they are especially useful. Allyl Grignard reagents are not easy
to prepare in high yields. Good yields of allyl and benzylzinc bromides, however,
result from activated zinc and the appropriate bromide in tetrahydrofuran. With
aldehydes and ketones they afford, after hydrolysis, secondary or tertiary alcohols
respectively. The allyl compounds react by allylic transposition, probably via a
six-membered transition state.
KrZnBr
R R' J ~
r
R'
R''
Allyl and benzylzinc bromides are also useful for preparing hydrocarbons by
crossed Wurtz coupling with reactive organic halides.
The Reformatsky reaction provides a method of converting o:-bromoesters via
the zinc reagent into /:1-hydroxy- or {:1-keto-esters. The reactions may be carried
out either in situ, where zinc, the bromo ester and the carbonyl compound are ali
warmed together in a solvent (ether, benzene or 1, 2-dimethoxyethane), or in two
stages by forming the zinc intermediate first. The actual zinc reagent is probably
an enolate rather than an organozinc species:
R"
1 ~o
BrC-C
1 "'-oR
R'
R"
+ Zn-- BrZn-C-C
1 go
Y "'-oR
R'
i)Zn 1
RR'CO + BrCH2C02Et ....:..:=:.:.__- HOC-CH2C02Et
iil H3 o+ 1
R'
O
OZnBr
,11 Zn , 1
RCOR + BrCH2C02Et - R C- CH2C02Et
1
OR
R
H o+
3
o
,li
RCCH 2C02Et
59