"Front Matter". In: Organosilanes in Radical Chemistry - Index of

Tris(trimethylsilyl)silane 55
4.3.1 DEHALOGENATIONS
The reductive removal of bromine and iodine atoms by (TMS) 3SiH is straightforward.
Generally, equimolar or a slight excess of reducing agent is employed.
A variety of solvents can be used although aromatic solvents such as benzene
or toluene are the most common. The reactions are complete after a short time.
A few examples are given in Reactions (4.8) – (4.11) [28–31]. Multiple dehalogenations
are possible in a one-pot procedure by using the corresponding
equivalent of reducing agent (Reaction 4.11). The efficiency depends to a
limited extent on the substituent; in alkyl bromides the rate constants decrease
along the series benzyl > tertiary alkyl > secondary alkyl > primary alkyl >
phenyl (see Table 4.1) [32]. Relative rate constants for the reaction of
(TMS) 3Si: with 13 substituted benzyl bromides have been measured at 80 8C,
and analysis of the data in terms of a Hammett-type plot shows this silyl radical
to be strongly nucleophilic [33].
Table 4.1 Rate constants at 20 8C for the reaction of (TMS) 3Si:
radicals with some bromides [32]
O
RO
I
N
RO
O
Br
Bromide k=M 1 s 1
PhCH2Br 9:6 108 (CH3) 3CBr 1:2 108 CH3CH2CH(CH3)Br 4:6 107 CH3(CH2) 4Br 2:0 107 C6H5Br 4:6 106 O
O
O
N
H
N
NH
CN
COOMe
O
(TMS) 3 SiH
55 �C
(TMS) 3 SiH
AIBN, 80 �C
R = t-BuMe 2Si
O
RO
N
RO
O
O
90%
O
H
N
94%
O
N
COOMe
NH
CN
O
(4.8)
(4.9)