Page 323 - Advanced Organic Chemistry Part B - Reactions & Synthesis

The reductive replacement of mercury using sodium borohydride is a free radical 295
chain reaction involving a mercuric hydride intermediate. 18
SECTION 4.1
II
II
RHg X + NaBH 4 RHg H Electrophilic Addition to
Alkenes
.
II
In + RHg H In-H + RHg I
RHg I R . + Hg 0
II
R . + RHg H RH + RHg I
The evidence for the free radical mechanism includes the fact that the course of the
reaction can be diverted by oxygen, an efficient radical scavenger. In the presence
of oxygen, the mercury is replaced by a hydroxy group. Also consistent with a
free radical intermediate is the formation of cyclic products when 5-hexenylmercury
19
compounds are reduced with sodium borohydride. This cyclization reaction is highly
characteristic of reactions involving 5-hexenyl radicals (see Part A, Section 11.2.3.3).
In the presence of oxygen, no cyclic product is formed, indicating that O traps the
2
radical faster than cyclization occurs.

NaBH 4 CH 2 CH(CH ) CH + CH 3
2 3
3
CH 2 CH(CH ) HgBr THF, H O
2
2 4
, O
NaBH 4 2
CH CH(CH ) CH OH
THF, H O 2 2 3 2
2
Tri-n-butyltin hydride can also be used for reductive demercuration. 20 An alternative
reagent for demercuration is sodium amalgam in a protic solvent. Here the evidence
is that free radicals are not involved and the mercury is replaced with retention of
configuration. 21

OCH 3 Na – Hg OCH 3
D O
2
HgCl D

The stereochemistry of oxymercuration has been examined in a number of
systems. Conformationally biased cyclic alkenes such as 4-t-butylcyclohexene and
4-t-butyl-1-methylcyclohexene give exclusively the product of anti addition, which is
consistent with a mercurinium ion intermediate. 17 22
OH
OH
CH 3 CH 3 NaBH 4
(CH ) C Hg(OAc) (CH ) C (CH ) C CH 3
2
3 3
3 3
3 3
HgOAc
18 C. L. Hill and G. M. Whitesides, J. Am. Chem. Soc., 96, 870 (1974).
19 R. P. Quirk and R. E. Lea, J. Am. Chem. Soc., 98, 5973 (1976).
20
G. M. Whiteside and J. San Fillipo, Jr., J. Am. Chem. Soc., 92, 6611 (1970).
21 F. R. Jensen, J. J. Miller, S. J. Cristol, and R. S. Beckley, J. Org. Chem., 37, 434 (1972); R. P. Quirk,
J. Org. Chem., 37, 3554 (1972); W. Kitching, A. R. Atkins, G. Wickham, and V. Alberts, J. Org.
Chem., 46, 563 (1981).
22
H. C. Brown, G. J. Lynch, W. J. Hammar, and L. C. Liu, J. Org. Chem., 44, 1910 (1979).