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

42 Hydrogen Donor Abilities of Silicon Hydrides
Absolute rate constants for the reaction of Et3SiH with aroyloxyl radicals 26
(Reaction 3.17) were measured by laser flash photolysis and were found to be in
the range of (3:8–7:4) 10 6 M 1 s 1 at 24 8C [32]. The rate constants of t-BuO:
and PhC(O)O: radicals with Et3SiH are identical within experimental error.
Furthermore, the rate constants were correlated by the Hammett equation
using s þ substituent constants, and a r value of þ0:68 was obtained.
X
O
O
26
X = H, Me, Cl, OMe
+
Et 3SiH
3.4 SULFUR-CENTRED RADICALS
X
O
OH
+
Et 3 Si
(3.17)
The reaction of thiyl radicals with silicon hydrides (Reaction 3.18) is the key
step of the so called polarity-reversal catalysis in the radical-chain reduction of
alkyl halides as well as in the hydrosilylation of olefins using silane–thiol couple
(see Sections 4.5 and 5.1) [33]. The reaction is strongly endothermic and reversible
(Reaction 3.18).
RS: þ R3SiHÐ kSH
RSH þ R3Si: (3:18= 3:18)
kSiH
The rate constants kSH and kSiH were determined in cyclohexane at 60 8C
relative to 2kt for the self-termination of the thiyl radicals, using the kinetic
analysis of the thiol-catalysed reduction of 1-bromooctane and 1-chlorooctane
by silane, respectively [34]. The kSH values are collected in Table 3.5 (third
column) the lowest value being 3:2 10 4 M 1 s 1 for the reaction of 1-adamantanethiyl
radical (1-AdS:) withEt3SiH. The reverse rate constants (kSiH) were
obtained for the reaction of Et3Si: with 1-AdSH and (t-BuO) 3SiSH and found to
be 5 10 7 M 1 s 1 for both of them. The equilibrium constants Keq ¼ kSH=kSiH
are 6:2 10 4 and 2:6 10 3 and hence DGr are þ20:4 and þ16:5 kJ/mol,
respectively. Unfortunately, DSr is unknown, which would have allowed one to
determine DHr, i.e., the difference in bond dissociation enthalpies of the corresponding
silane and thiol in solution. The difference of the simpler silane and
thiol, DH(Me3Si w H) and DH(MeS w H), in the gas phase corresponds to
DHr ¼þ31:9 kJ/mol (cf. Chapter 2).
Rate constants for the reaction of thiyl radicals with the t-BuMePhSiH were
also extracted from the kinetic analysis of the thiol-catalysed radical-chain
racemization of enantiomerically pure (S)-isomer [34]. Scheme 3.2 shows the
reaction mechanism that involves the rapid inversion of silyl radicals together
with reactions of interest. The values in cyclohexane solvent at 60 8C are
collected in the last column of Table 3.5.