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

146 Consecutive Radical Reactions
O
O
R R�
+
BuI
(TMS) 3 SiH
AIBN, 80 �C
O
O
R R�
R = Ph, R� = H
60%, cis:trans = 98:2
R = H, R� = Ph 60%, cis:trans = 94:6
Bu
+
O
O
R R�
Stereoelectronic effects can be invoked for the radical reaction at anomeric
centre of carbohydrates. The high stereoselective preparation of a-substituted
C-glycosyl phosphonates in a a: b ratio of 98:2 was achieved by reductive
addition of bromide 2 to a-phosphonoacrylate (Reaction 7.5) [10]. Yields (in
parentheses) depend on the sugar configuration: d-galacto (80 %), d-manno
(47 %), d-gluco (30 %) and l-fuco (62 %).
(AcO) n
O
2
Br
+
MeO
O
P
OMe
O
OMe (TMS) 3SiH (AcO) n
O
Et 2 O reflux,
hν, 20h
MeO
Bu
O
P OMe
OMe
O
30 - 80%
Other silanes have been used as mediators in the intermolecular C w C bond
formation. They can be used alone, as in the following example of the disilane
(Ph2SiH) 2 shown in Reaction (7.6), for the reductive addition of a bromide or a
xanthate to phenyl vinyl sulfone [11,12].
X
+
X = Br
X = OC(S)SMe
SO 2 Ph
(Ph2SiH) 2
AIBN, 77 �C
88%
71%
SO 2 Ph
Alternatively, the couple Ph3SiH=RSH has been used for the reductive alkylation
of electron-rich terminal alkenes as shown in Reaction (7.7) [13]. In this
example, 5 mol% of MeOC(O)CH2SH or Ph3SiSH is used as the catalyst with a
slight excess of Ph3SiH. Silyl radicals obtain the electrophilic carbon-centred
radical through the halogen removal. Subsequently, the thiol effects the hydrogen
atom transfer to the adduct radical and the derived RS: radical regenerates
the catalyst and the silyl radical (cf. Section 4.5). The enantioselective version of
this reaction utilized prochiral alkenes, with Ph3SiH in the presence of homochiral
thiol catalysts (such as 1-thio-b-d-glucopyranose tetraacetate), and gave
products of moderate enantiomeric purity.
(7.4)
(7.5)
(7.6)