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

104 Addition to Unsaturated Bonds
R
(TMS) 3 Si
Ph
O
33
Me
1) LiAlH 4
2) H 2 O, 30 �C
R
Ph
OH
Me
R
Ph
OH
34
R 34:35
35
Me 2.9:1
i-Pr 13.3:1
t-Bu 8.3:1
R
OSi(TMS) 3
Me
Ph
C12H25SH 30 �C
R
OSi(TMS) 3
Me +
Ph
R
OSi(TMS) 3
Me
Ph
36 37
R
Me
37:38
2.9:1
38
i-Pr 12.6:1
t-Bu 5.3:1
Scheme 5.6 Cram’s rule for radical reactions
products derived from a fast ring opening of the intermediate a-silyloxyl
radicals prior to hydrogen abstractions. Carbaldehyde 39 has been used as a
probe for discrimination between radical and cationic mechanisms [57]. Indeed
the ring-opening reaction depended on whether a radical or cationic intermediate
develops at the carbonyl carbon, with consequent cleavage at the
C1 w C2 or C1 w C3 bond, respectively [58]. For example, the reaction with
(TMS) 3SiH under standard radical conditions afforded a mixture of (Z)- and
(E)-alkene 40 from regioselective ring opening of the C1 w C2 bond (Scheme
5.7). Interestingly, the reaction of carbaldehyde 39 with tetramesityldisilene 41
at 60 8C yielded the cyclic compound 42 as the only product providing
unequivocal evidence for the presence of radical intermediates in the reaction
of carbonyl compounds with disilene [57].
Me
O
p-MeOC 6H 4
O
1) (TMS) 3SiH
AIBN, 80 �C
2) Bu 4NF
Cl3SiH hν, 23 �C
+
Me
O
75%
p-MeOC 6H 4
O
87%
Me
(5.27)
(5.28)