Page 848 - Advanced Organic Chemistry Part B - Reactions & Synthesis

824 be induced by fluoride ion. Fluoride adds at silicon to form a hypervalent anion having
enhanced nucleophilicity. 114
CHAPTER 9
Carbon-Carbon CH CHCH SiR + F – –
Bond-Forming Reactions 2 2 3 CH 2 CHCH 2 Si F
of Compounds of Boron,
Silicon, and Tin
The THF-soluble salt tetrabutylammonium fluoride (TBF) is a common source
of fluoride. An alternative reagent is tetrabutylammonium triphenyldifluorosilicate
(TBAF). 115 Unsymmetrical allylic anions generated in this way react with ketones at
their less-substituted terminus.
F OH
–
CH 2 CHCH Si(CH ) + F – CH 2 CHCH Si(CH ) RCH O RCHCH 2 CH CH 2
3 3
2
2
3 3
TBAF H O
2
(CH ) C CHCH Si(CH ) + Ph C O (CH 3 2 CHCH 2 CPh 2
) C
3 2
2
2
3
87%
OH
An allylic silane of this type serves as a reagent for the introduction of isoprenoid
structures. 116
+ –
N F
(CH ) C CHCH O + (CH ) SiCH CCH CH 2 R 4 (CH ) C CHCHCH 2 CCH CH 2
2
3 2
3 3
3 2
CH 2 OH CH
2
70%
Fluoride-induced desilylation has also been used to effect ring closures. 117
CH
H O 2 H HO CH 2
CCH Si(CH )
+ –
CH 2 2 3 3 R N F
4
SO Ph SO 2 Ph
2
H H 94%
Allylic trimethoxysilanes are activated by a catalytic combination of CuCl and
TBAF. 118 The mechanism of this reaction is not entirely clear, but it seems to involve
fluoride activation of the silane. These reactions are stereoconvergent for the isomeric
2-butenyl silanes, indicating that reaction occurs through an acyclic TS.
OH OH
Si(OCH )
CH 3 Si(OCH ) 10 mol % CuCl + 10 mol % CuCl 3 3
3 3
Ph Ph
10 mol % TBAF 10 mol % TBAF CH 3
+ PhCH O CH 3 + PhCH O
CH 3
2.6:1 syn:anti
114 A. Hosomi, A. Shirahata, and H. Sakurai, Tetrahedron Lett., 3043 (1978); G. G. Furin, O. A. Vyazankina,
B. A. Gostevsky, and N. S. Vyazankin, Tetrahedron, 44, 2675 (1988).
115
A. S. Pilcher and P. De Shong, J. Org. Chem., 61, 6901 (1996).
116
A. Hosomi, Y. Araki, and H. Sakurai, J. Org. Chem., 48, 3122 (1983).
117 B. M. Trost and J. E. Vincent, J. Am. Chem. Soc., 102, 5680 (1980); B. M. Trost and D. P. Curran, J.
Am. Chem. Soc., 103, 7380 (1981).
118
S. Yamasaki, K. Fujii, R. Wada, M. Kanai, and M. Shibasaki, J. Am. Chem. Soc., 124, 6536 (2002).