Page 981 - Advanced Organic Chemistry Part B - Reactions & Synthesis

There is a good deal of information available about the absolute rates of free radical 957
reactions. A selection from these data is given in Table 11.3 of Part A. If the steps
in a projected reaction sequence correspond to reactions for which absolute rates are SECTION 10.3
known, this information can allow evaluation of the kinetic feasibility of the reaction Reactions Involving Free
Radical Intermediates
sequence.

10.3.1. Sources of Radical Intermediates

There is a discussion of some of the sources of radicals for mechanistic studies in
Section 11.1.4 of Part A. Some of the reactions discussed there, particularly the use of
azo compounds and peroxides as initiators, are also important in synthetic chemistry.
One of the most useful sources of free radicals in preparative chemistry is the reaction
of halides with stannyl radicals. Stannanes undergo hydrogen abstraction reactions and
the stannyl radical can then abstract halogen from the alkyl group. For example, net
addition of an alkyl group to a reactive double bond can follow halogen abstraction
by a stannyl radical.

.
.
initiation In + R′ 3 SnH R′ 3 Sn + In H
propagation R X + R′ Sn. R . + R′ Sn X
3
3
R . + X Y R X Y .
R X Y . + R′ Sn H R X Y H + R′ Sn .
3
3
This generalized reaction sequence consumes the halide, the stannane, and the reactant
X=Y, and effects addition to the organic radical and a hydrogen atom to the X=Y
bond. The order of reactivity of organic halides toward stannyl radicals is iodides >
bromides > chlorides.
Esters of N-hydroxypyridine-2-thione are another versatile source of radicals, 286
where the radical is formed by decarboxylation of an adduct formed by attack at sulfur
by the chain-carrying radical. 287 The generalized chain sequence is as follows.

X . + CO + R .
. 2
S N X S N X S N
OCR O C R
O O
.
R + X Y R Y + X .

When X−YisR Sn−H the net reaction is decarboxylation and reduction of the
3
original acyloxy group. Halogen atom donors can also participate in such reactions.

286 D. Crich, Aldrichimica Acta, 20, 35 (1987); D. H. R. Barton, Aldrichimica Acta, 23, 3 (1990).
287
D. H. R. Barton, D. Crich, and W. B. Motherwell, Tetrahedron, 41, 3901 (1985); D. H. R. Barton,
D. Crich, and G. Kretzschmar, J. Chem. Soc., Perkin Trans. 1, 39 (1986); D. H. R. Barton, D. Bridson,
I. Fernandez-Picot, and S. Z. Zard, Tetrahedron, 43, 2733 (1987).