Page 460 - Advanced Organic Chemistry Part B - Reactions & Synthesis

Tri-n-butyltin hydride also serves as a hydrogen atom donor in radical-mediated 433
methods for reductive deoxygenation of alcohols via thiono esters. 203 The alcohol is
converted to a thiocarbonyl derivative. These thiono esters undergo a radical reaction SECTION 5.5
with tri-n-butyltin hydride. The resulting radicals fragment to give the alkyl radical, Reduction Reactions
Involving Hydrogen
and the chain is propagated by hydrogen atom abstraction. Atom Donors
S S SnBu 3 O
R OCX + Bu Sn· ROCX R· + XCS SnBu 3
3
·
R· + Bu 3 SnH R H + Bu Sn·
3
This procedure gives good yields from secondary alcohols and by appropriate
adjustment of conditions can also be adapted to primary alcohols. 204
Owing to the expense, toxicity, and purification problems associated with use
of stoichiometric amounts of tin hydrides, there has been interest in finding other
hydrogen atom donors. 205 The trialkylboron-oxygen system for radical generation (see
Part A, Section 11.1.4) has been used with tris-(trimethylsilyl)silane or diphenylsilane
as a hydrogen donor. 206

S
Et B, O 2
3
c-C H OCO F c-C H
12 24
12 23
(Ph) SiH 2
2
96%
Chain reaction mechanism
H ) B + O C H ·
(C 2 5 3 2 2 5
SiH C H + R Si·
C H · +R 3 2 6 3
2 5
·
R Si· + R′OCOR′ R′OCOR′
3
S SSiR 3
·
R′OCOR′
R′· + R′O CSSiR 3
2
SSiR 3
R′· + R 3 SiH R′ H+ R Si·
3
The alcohol derivatives that have been successfully deoxygenated include thionocar-
bonates and xanthates. 207 Peroxides can be used as initiators. 208
Scheme 5.9 illustrates some of the conditions that have been developed for the
reductive deoxygenation of alcohols. Entries 1 to 4 illustrate the most commonly used
methods for generation of thiono esters and their reduction by tri-n-butylstannane.
These include formation of thiono carbonates (Entry 1), xanthates (Entry 2), and thiono
imidazolides (Entries 3 and 4). Entry 5 is an example of use of dimethyl phosphite
as the hydrogen donor. Entry 6 uses tris-(trimethylsilyl)silane as the hydrogen atom
donor.

203
D. H. R. Barton and S. W. McCombie, J. Chem. Soc., Perkin Trans. 1, 1574 (1975).For reviews of
this method, see W. Hartwig, Tetrahedron, 39, 2609 (1983); D. Crich and L. Quintero, Chem. Rev., 89,
1413 (1989).
204 D. H. R. Barton, W. B. Motherwell, and A. Stange, Synthesis, 743 (1981).
205
A. Studer and S. Amrein, Synthesis, 835 (2002).
206
D. H. R. Barton, D. O. Jang, and J. C. Jaszberenyi, Tetrahedron Lett., 31, 4681 (1990).
207 J. N. Kirwan, B. P. Roberts, and C. R. Willis, Tetrahedron Lett., 31, 5093 (1990).
208
D. H. Barton, D. O. Jang, and J. C. Jaszberenyi, Tetrahedron Lett., 33, 7187 (1991).