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

44 Hydrogen Donor Abilities of Silicon Hydrides
Table 3.6 Rate constants for reactions of benzophenone n p* triplet with silicon
hydrides at room temperature [23, 25]
Silane k=M 1 s 1a
k=M 1 s 1b
n-C5H11SiH3 8:8 106 PhSiH3 5:0 106 Et3SiH 9:6 106 2:4 107 n-Bu2MeSiH 3:0 107 (Me3SiCH2) 2MeSiH 2:7 107 PhMe2SiH 8:8 106 (Me3SiO) 2MeSiH 3:4 106c (EtO) 3SiH 1:7 106 (Me3Si) 3SiH 3:4 108 a Laser flash photolysis; in benzene [23].
b Phosphorescence; 9:1 (v/v) acetonitrile/acetone [25].
c A value of 2:5 10 6 M 1 s 1 was obtained in benzene.
photolysis in benzene (Table 3.6) [23]. Rate constants of phosphorescence quenching
of benzophenone with a variety of silicon hydrides have also been measured in
9:1 (v/v) mixture of acetonitrile–acetone as the solvent and are also collected in
Table 3.6 [35]. Similarly, the rate constants of benzophenone triplet with Et4Si and
(EtO) 4Si are found to be < 1 10 5 and 1 10 6 M 1 s 1 , respectively [35], which
suggests that the reaction with Et3SiH involves only the SiH moiety whereas with
(EtO) 3SiH involves H atom abstraction from both the SiH moiety and EtO
groups, with about 55:45 regioselectivity favoring the SiH abstraction. The Arrhenius
parameters obtained from laser flash photolysis experiments for the reaction
of benzophenone triplet with Et3SiH are log A=M 1 s 1 ¼ 8:9 and
Ea ¼ 10:9kJ=mol [23]. Comparing the data of n-C5H11SiH3, PhSiH3,Et3SiH
and (Me3Si) 3SiH from Table 3.6 with the corresponding kinetic data of t-BuO:
radicals (Table 3.4), it can be seen that these two transient species have a rather
similar reactivity towards silanes. In particular, the rate constants for benzophenone
triplet with n-C5H11SiH3 and PhSiH3 are slightly slower, whereas with
Et3SiH and (Me3Si) 3SiH are 5 and 3 times faster, respectively.
Excited triplets of other ketones have been studied to a lesser extent [23,35].
The xanthate and the p-methoxyacetophenone triplets were found to be more
(6:0 10 7 M 1 s 1 ) and less ( 8 10 5 M 1 s 1 ) reactive, respectively, than the
benzophenone triplet with Et3SiH. The p-methylbenzophenone triplet behaves
similarly to benzophenone triplet toward (Me3SiO) 2MeSiH, whereas the
p-phenylbenzophenone triplet shows no reaction. Also no reaction was observed
for the benzil triplet with Et3SiH and (Me3SiO) 2MeSiH.
3.6 HYDROGEN ATOM: AN EXAMPLE OF GAS-PHASE KINETICS
The substituent effect on the reactivity of the Si w H bond in Me4 nSiHn, where
n ¼ 0–3, towards different atoms has been the subject of several experimental