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

Other Silicon Hydrides 73
The decrease in reactivity for monohalogenated compounds along the series I
> Br > Cl is also due in part to a decrease of the preexponential factor, which
has been attributed to the diminished importance of the charge-transfer interactions
along this series. Rate constants for the reaction of the Et3Si: radical
with a series of ring-substituted benzyl chlorides were also obtained. A
Hammett plot gives a r ¼þ0:64, one of the largest positive r values reported
for a free-radical reaction that is indicative of the strong nucleophilic character
of silyl radicals [93].
The reactions of trichloro- and tetrachloroethylene with Et3Si: have been
studied in detail [94]. With the former, Et3Si: radicals undergo addition as well
as Cl abstraction, and at 65 8C these reactions proceed at almost equal rates.
With tetrachloroethylene, Et3Si: radicals proceed more that 95 % via Reaction
(4.56) with a rate constant of 1:0 10 7 M 1 s 1 at 27 8C [95].
Et 3Si
Cl Cl
Cl
+ Et3SiCl + (4.56)
Cl Cl
Cl Cl
Absolute rate constants for the reaction of Et3Si: with a variety of sulfonyl
halides (Reaction 4.57) have been measured by laser flash photolysis [96,97].
For sulfonyl chlorides, the rate constants are very high, approaching the diffusion-controlled
limit and are almost independent of the substituent, i.e., the rate
constants at ca 25 8C are 3:2 10 9 ,4:6 10 9 ,5:4 10 9 and 7:5 10 9 M 1 s 1
for MeSO2Cl, PhSO2Cl, Me2NSO2Cl and EtOSO2Cl, respectively. Polar contributions,
similar to those in Equation (4.55) have also been proposed for such
fast processes. Sulfonyl fluorides also react with Et3Si: radicals in a similar
fashion. Rate constants for MeSO2F and p-Me-C6H4SO2F are found to be
1:3 10 7 and 0:9 10 7 M 1 s 1 , respectively, at room temperature.
Et3Si: þ RSO2X !Et3SiX þ RSO2: ð4:57Þ
4.4.2 PHENYL SUBSTITUTED SILICON HYDRIDES
Phenyl or mixed alkyl/phenyl substituted silicon hydrides show similar reactivities
to trialkylsilanes. Indeed, by replacing one alkyl by a phenyl group the
effect on the hydrogen donating ability of SiH moiety increases, only slightly
(see Section 3.1). As an example, the rate constant for hydrogen abstraction by
primary alkyl radical from Ph3SiH is about one order of magnitude higher than
from Et3SiH. The rate constants for halogen abstraction by the phenyl substituted
silyl radicals are collected in Table 4.5. The reactivity trends of
t-BuPh2Si: and Ph3Si: are similar and for a particular RX the rate constants
range between Et3Si: (smaller) and (TMS) 3Si: (higher).
Although under canonical radical chain conditions these silanes are poor
reducing agents, there are some interesting applications, in particular, using
Ph2SiH2 [100]. For example, the reaction of a variety of substituted adamantanes