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

36 Hydrogen Donor Abilities of Silicon Hydrides
3.1.2 OTHER TYPES OF CARBON-CENTRED RADICALS
In Table 3.2 are collected the rate constants of some silicon hydrides with a
variety of radicals like the p-type secondary or tertiary alkyl radicals, the s-type
phenyl or acyl radicals, and the halogenated carbon-centred radicals. A few
Arrhenius parameters are also available and reported here below.
The number of available rate constants or secondary and tertiary alkyl
radicals is limited with respect to the primary ones. Their reactions with
(Me3Si) 3SiH have been studied in detail by free-radical clock methods analogous
to Reaction (3.3). The Arrhenius parameters are log A=M 1 s 1 ¼ 8:3 and
Ea ¼ 18:0 kJ/mol for secondary and log A=M 1 s 1 ¼ 7:9 and Ea ¼ 14:2 kJ/mol
for tertiary alkyl radicals [10]. It is worth mentioning that the rate constants for
the reaction of primary, secondary, and tertiary alkyl radicals with (Me3Si) 3SiH
are very similar in the range of temperatures that are useful for chemical
transformation in the liquid phase. This is due to compensation of entropic
and enthalpic effects through this series of alkyl radicals.
The Arrhenius expression for the reaction of the o-(allyloxy)phenyl radical
(15) with (Me3Si) 3SiH relative to this unimolecular rearrangement (Reaction 3.8)
has been established [10]. A rate constant kc ¼ 8 10 9 s 1 has been estimated for
Table 3.2 Rate constants for the reactions of radicals with silicon hydrides
Radical Silane Solvent T (8C) k=M 1 s 1 Reference
RCH2C : HCH2 (Me3Si) 3SiH a
RCH 2
CH 3
C
CH 3
n-tetradecane 27 1:4 10 5 [10]
Et3SiH benzene 50 3 103b [14]
Ph3SiH benzene 50 9 103b [14]
(Me3Si) 3SiH a
n-tetradecane 27 2:6 105 [10]
Ph2SiH2 CCl4 60 3:4 107 [1]
Ph3SiH CCl4 60 2:1 107 [1]
8 c
(Me3Si) 3SiH n-octane 20 3 10
O
(Me3Si) 3SiH
R
a
toluene 27 1:8 104 [15]
RfCF2CF2: Et3SiH Et3SiH 30 7:5 105 [16]
t-BuMe2SiH 1, 3-(CF3) 2-C6H4 25 4:9 107 [17]
(Me3Si)Me2SiH 1, 3-(CF3) 2-C6H4 25 3:1 106 [17]
(Me3Si) 2MeSiH benzene-d6 30 1:6 107 [16]
(Me3Si) 3SiH benzene-d6 30 5:1 107 [16]
(CF3) 2CF: Et3SiH Et3SiH 26 3:6 106 [18]
(CF3) 3C: Et3SiH Et3SiH 26 2:4 108 [18]
Cl3C: Et3SiH a
CCl4=c-C6H12 27 5:0 102 [19]
a For Arrhenius parameters, see text.
b Approximate values.
c See text.