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

Tris(trimethylsilyl)silane 57
A good diastereocontrol is obtained for the debromination of Reaction (4.14)
and it is attributed to the bulky reducing agent, which approaches the radical
intermediate from the less hindered face anti to the two vicinal substituents [35].
EtO
O
Br
(TMS) 3 SiH
Et 3B, O 2, −78 �C
EtO
O
+
4:1
EtO
O
(4.14)
The reduction of bromides bearing a b-sulfinyl group by (TMS) 3SiH is an
emerging method for the preparation of substituted allenes. Reaction (4.15) was
tested with different a-bromo vinyl sulfoxides and yields ranged from satisfactory
to good ones [36]. The reaction involves a bromine atom removal followed
by b-elimination of a sulfinyl radical. The use of large excess of initiator suggests
that these transformations do not involve a properly maintained radical chain.
R 1
R
S(O)Ar
Br
(TMS) 3SiH
AIBN (1 equiv), 80 �C
R 1
R
30-80%
(4.15)
(TMS) 3SiH can be also used as a reagent for driving the reduction of iodides
and bromides through a radical mechanism together with sodium borohydride,
the reductant that is consumed [37]. For example, 1-bromonaphthalene is
treated with an excess of NaBH4 (50 equiv) and a small amount of
(TMS) 3SiH (0.1 equiv), under photochemical initiation conditions, to give the
reduced product in 88 % yield.
For tertiary, secondary, and primary chlorides the reduction becomes increasingly
difficult due to shorter chain lengths. On the other hand, the replacement
of a chlorine atom by hydrogen in polychlorinated substrates is much
easier. Table 4.2 shows the rate constants for the reaction of (TMS) 3Si: radical
with some chlorides [32]. The comparison with the analogous data of Table 4.1
shows that for benzyl and tertiary alkyl substituents the chlorine atom abstraction
is 2–3 orders of magnitude slower than for the analogous bromides.
Table 4.2 Rate constants at 20 8C for the reaction of (TMS) 3Si:
radicals with some chlorides [32]
Chloride k=M 1 s 1
CCl4 1:7 10 8
CHCl3 6:8 10 6
PhCH2Cl 4:6 10 6
CH3(CH2) 5C(CH3) 2Cl 4:0 10 5
RCH2C(O)Cl 7 10 5a
a At 80 8C.