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

Carbon–Carbon Double Bonds 93
Depending upon the choice of substrates, the hydrosilylation of alkenes with
(TMS) 3SiH can also be highly stereoselective. The reaction of (TMS) 3SiH with
methylmaleic anhydride, proceeded regiospecifically to the less substituted side,
but also diastereoselectively to afford the thermodynamically less stable cis
isomer (Reaction 5.6) [25]. Stereoselectivity increased by decreasing the reaction
temperature, indicating the difference in enthalpy of activation for syn vs anti
attack.
O
O
Me
O
(TMS) 3SiH
AIBN, 90 �C
Et 3B, O 2, −15 �C
(TMS) 3 Si
O
O
Me
O
(TMS) 3Si
O
O
89%, cis:trans = 16:1
60%, cis:trans = 99:1
Me
+ (5.6)
The addition of (TMS) 3SiH to prochiral diethyl methyl fumarate (5)
gave both diastereoisomers with preferential formation of the threo isomer
(Reaction 5.7) [25]. This suggests that the intermediate adduct 6 adopts a
preferred conformation due to the allylic strain effect, in which the silyl
moiety shields one face of the prochiral radical center, favouring hydrogen
transfer to the opposite face, and therefore the threo product is predominantly
formed.
EtO 2 C
CO 2 Et (TMS) 3SiH
Me
Et 3B, O 2, −15 �C
Me
EtO2C (TMS) 3 Si
Si(TMS) 3
EtO 2C
CO2Et H
Me
CO 2Et
+
(TMS) 3 Si
EtO 2 C
5 50%, threo:erythro = 45:1
6
Me
O
CO 2Et
Addition of (TMS) 3SiH to a-chiral (E)-alkene 7 was found to take place with
a complete Michael-type regioselectivity (Reaction 5.8) [26]. A complete syn
stereoselectivity was observed for R ¼ Me, and it was rationalized in terms of
Felkin–Ahn transition state 8, which favours the syn product similar to nucleophilic
addition.
(5.7)