132 J. CHEM. RESEARCH (S), 1998
J. Chem. Research (S),
1998, 132±133$
Thiol-catalysed Radical-chain Reduction of Organic
Halides by Hexabutylditin in the Presence of
Malonic Acid$
Kyoung-Mahn Kim and Brian P. Roberts*
Christopher Ingold Laboratories, Department of Chemistry, University College London,
20 Gordon Street, London WC1H 0AJ, UK
In the presence of a thiol catalyst and a suitable initiator, a mixture of hexabutylditin and malonic acid brings about
protodehalogenation of organic bromides and iodides by a radical-chain mechanism.
Tributyltin hydride (TBTH) is a key reagent in radical-
based organic synthesis.1 The Sn0H bond is relatively
weak and hydrogen-atom abstraction from TBTH by a
carbon-centred radical is rapid.2 The resulting tributyl-
stannyl radical reacts readily at electronegative elements in a
wide variety of compounds to generate new radicals and
lead to chain processes. For example, alkyl bromides are
readily reduced to the corresponding alkanes by TBTH, via
the sequence of propagation reactions shown in eqns. (1)
and (2). Rather than the simple reduction product R1H,
the compound R2H resulting from the trapping of the
rearranged radical R2ꢀ is often desired [eqns. (3) and (4)].
Now the high rate of reaction (2) can become a dis-
advantage, because of trapping of R1ꢀ prior to its rearrange-
ment, and very dilute solutions have been employed or the
TBTH has been added slowly to the reaction mixture using
a syringe pump, in order to increase the yield of R2H
relative to R1H. Other approaches to alleviate this diculty
have included the replacement of the TBTH with less
reactive hydrogen-atom donors such as trialkylgermanes3 or
tris(trimethylsilyl)silane.4 Alternatively, photolysis of hexa-
alkyldistannanes (free or polymer-supported and usually
in the presence of a ketone sensitiser) have been used
as sources of stannyl radicals, in conjunction with poor
hydrogen-atom donors such as propan-2-ol, for non-chain
reduction with rearrangement.5,6
conditions and the dodecane is evidently formed via the
sequence of propagation reactions shown in eqns. (5)±(7).
When the amount of benzenethiol was reduced to
0.05 mmol, under otherwise identical conditions, the yield of
dodecane fell correspondingly to 4% (entry 2). However,
when malonic acid (1.2 mmol) was also present the yield
of dodecane rose again to 84%, presumably because the
carboxylic acid reacts with the Bu3SnSPh according to
eqn. (8) to regenerate the thiol,8 which now functions as a
catalyst for the reduction (entry 3). Negligible reduction
takes place in the absence of benzenethiol or in the
absence of AIBN (entries 4 and 5). Other thiols behaved in
a generally similar way, but benzenethiol and dodecane-1-
thiol were most eective. Yields of dodecane were somewhat
higher in re¯uxing benzene solvent than in DME, although
malonic acid is only very sparingly soluble in the former
and the reaction mixture remains heterogeneous throughout.
Bu3Snꢀ RHal 4 Bu3SnHal Rꢀ
Rꢀ PhSH 4 RH PhSꢀ
ꢁ5
ꢁ6
ꢁ7
ꢁ8
PhSꢀ Bu3SnSnBu3 4 Bu3Snꢀ Bu3SnSPh
Bu3SnSPh RCO2H 4 Bu3SnO2CR PhSH
A number of representative types of organic halides were
reduced successfully in benzene solutions using benzenethiol
as a catalyst, under the conditions speci®ed for entry 9 of
Table 1, and the results are given in Table 2. It was thought
conceivable that a-bromoacetophenone (entry 4) might be
reduced by a heterolytic mechanism, but in the absence of
AIBN almost no acetophenone (0.4%) was formed.
Bu3Snꢀ R1Br 4 Bu3SnBr R1ꢀ
R1ꢀ Bu3SnH 4 R1H Bu3Snꢀ
R1ꢀ 4 R2ꢀ
ꢁ1
ꢁ2
ꢁ3
ꢁ4
The benzenethiol-catalysed reduction of 1-bromododecane
by HBDT and malonic acid was also eectively initiated
R2ꢀ Bu3SnH 4 R2H Bu3Snꢀ
Here we report the use of hexabutylditin (HBDT),
together with malonic acid and a thiol catalyst, as an eec-
tive replacement for TBTH for the radical-chain reduction
of organic halides. In this system the actual hydrogen-atom
donor is the thiol, the concentration of which can be
controlled so as to promote the formation of rearranged
products without the need to work under conditions of high
dilution.
After a solution in 1,2-dimethoxyethane (DME; 4 cm3)
containing 1-bromododecane (1.0 mmol), HBDT (1.1 mmol),
benzenethiol (1.0 mmol), decane (ca. 1 mmol, as an internal
reference) and azoisobutyronitrile (AIBN; 0.05 mmol) had
been heated under re¯ux under argon for 3 h, GLC analysis
showed that dodecane had been formed in 82% yield
(Table 1, entry 1). The phenylthiyl radical is known7 to
displace tributylstannyl radicals from HBDT under these
Table 1 Reduction of 1-bromododecane by HBDT and malonic
acid in the presence of thiols and AIBNa
Yield dodecane
(%)c
Entry
Solvent
Thiol (mol%)b
1d
2d
3
DME
DME
DME
DME
DME
DME
DME
DME
PhSH (100)
PhSH (5)
PhSH (5)
None
PhSH (5)
82
4
84
0.5
2
34
74
81
93
89
4
5e
6
tert-C12H25SHf (5)
MeO2CCH2SH (5)
n-C12H25SH (5)
PhSH (5)
7
8
9
10
Benzene
Benzene
n-C12H25SH (5)
aReaction mixtures consisted of 1-bromododecane (1.0 mmol),
HBDT (1.1 mmol), malonic acid (1.2 mmol), decane (ca. 1 mmol)
and AIBN (0.05 mmol) in solvent (4 cm3) and were heated under
reflux under argon for 3 h before analysis by GLC. bBased on
1-bromododecane. cBy GLC using decane as internal reference.
dMalonic acid was absent in these experiments. eAIBN was
*To receive any correspondence (e-mail: b.p.roberts@ucl.ac.uk).
$This is a Short Paper as de®ned in the Instructions for Authors,
Section 5.0 [see J. Chem. Research (S), 1998, Issue 1]; there is there-
fore no corresponding material in J. Chem. Research (M).
f
absent in this experiment. This is the isomeric mixture of tert-
dodecanethiols as obtained from the Aldrich Chemical Co.