Page 111 - Advanced Organic Chemistry Part B - Reactions & Synthesis

LiClO . 55 Cerium, samarium, and other lanthanide halides promote addition of silyl 83
4
ketene acetals to aldehydes. 56 Triaryl perchlorate salts are also very active catalysts. 57
SECTION 2.1
In general terms, there are at least three possible mechanisms for catalysis. One is
Aldol Addition and
through Lewis acid activation of the electrophilic carbonyl component, similar to that
Condensation Reactions
discussed for BF , TiCl , and SnCl . Another is by exchange with the enolate equiv-
3
4
4
alent to generate a more nucleophilic species. A third is activation of a catalytic cycle
that generates trimethylsilyl cation as the active catalysts.
Aldol additions of silyl enol ethers and silyl ketene acetals can be catalyzed by
+
Cp Zr 2+ species including
Cp ZrO-t-Bu and Cp Zr O SCF . 58
2
2
2
3 2
3
O (Cp) Zr(O SCF ) O CH 3 OTMS
3 2
2
3
TMSO 5 mol % CH 3
C CH + CH CCH CH 3 Ph
3
2
2
Ph
The catalytic cycle involves transfer of the silyl group to the adduct.
Zr Zr TMS
+O OTMS O O TMS O O
+
+ CH 2 + Zr +
R R Ph R Ph R Ph
R R
Trialkylsilyl cations may play a key role in other Lewis acid–catalyzed reactions. 59
For example, trimethylsilyl triflate can be formed by intermolecular transfer of the
silyl group. When this occurs, the trimethylsilyl triflate can initiate a catalytic cycle
that does not directly involve the Lewis acid.
LA LA LA
+
OTMS O TMS CF SO - O
O + O 3 3 O
+ (CH ) SiOSO CF 3
2
3 3
R′ R R′ R′
H R R
TMS
O O +
+ (CH ) SiOSO CF 3 + CF SO 3 -
3
2
3 3
H R H R
TMS TMS
TMS O TMS O
+
O + OTMS O O + (CH ) SiOSO CF
R′ R R′ R R′ 3 3 2 3
H R
55 M. T. Reetz and D. N. A. Fox, Tetrahedron Lett., 34, 1119 (1993).
56
P. Van de Weghe and J. Colin, Tetrahedron Lett., 34, 3881 (1993); A. E. Vougioukas and H. B. Kagan,
Tetrahedron Lett., 28, 5513 (1987).
57 T. Mukaiyama, S. Kobayashi, and M. Murakami, Chem. Lett., 447 (1985); T. Mukaiyama, S. Kobayashi,
and M. Murakami, Chem. Lett., 1759 (1984); S. E. Denmark and C.-T. Chen, Tetrahedron Lett., 35,
4327 (1994).
58 (a) T. K. Hollis, N. P. Robinson, and B. Bosnich, Tetrahedron Lett., 33, 6423 (1992); (b) Y. Hong,
D. J. Norris, and S. Collins, J. Org. Chem., 58, 3591 (1993).
59
E. M. Carreira and R. A. Singer, Tetrahedron Lett., 35, 4323 (1994); T. K. Hollis and B. Bosnich,
J. Am. Chem. Soc., 117, 4570 (1995).