Page 102 - Advanced Organic Chemistry Part B - Reactions & Synthesis

74 Table 2.2. Diastereoselectivity of Boron Enolates toward Aldehydes a

CHAPTER 2 OBL OH O OH O
O OBL 2
2
L BX 2 R CH O
Reactions of Carbon CH 3 2 CH + 1 2 R 1 + R 2 R 1
Nucleophiles with R 1 R 3 N 3 R 1 R R
Carbonyl Compounds CH anti
Z 3 E syn CH 3 CH 3
R 1 L X R 2 Z E syn:anti
b
C 2 H 5 n-C 4 H 9 OTf Ph >97 3 >97 3
b
C 2 H 5 n-C 4 H 9 OTf Ph 69:31 72:28
b
C 2 H 5 n-C 4 H 9 OTf n-C 3 H 7 >97 3 >97 3
b
C 2 H 5 n-C 4 H 9 OTf t-C 4 H 9 >97 3 >97 3
b
C 2 H 5 n-C 4 H 9 OTf CH 2 =CHCH 3 >97 3 92:8
b
C 2 H 5 n-C 4 H 9 OTf E-C 4 H 7 >97 3 93:7
b OTf Ph 45:55 44:56
i-C 3 H 7 n-C 4 H 9
b OTf Ph >99 1 >97 3
i-C 4 H 9 n-C 4 H 9
b OTf Ph >99 1 >97 3
t-C 4 H 9 n-C 4 H 9
c OTf Ph 95:5 94:6
n-C 5 H 11 n-C 4 H 9
c OTf Ph 91:9 91:9
n-C 9 H 19 n-C 4 H 9
c OTf Ph 95:5 94:6
c-C 6 H 11 n-C 4 H 9
c OTf Ph 98:2 >99 1
PhCH 2 n-C 4 H 9
Ph b n-C 4 H 9 OTf Ph 96:4 95:5
d
C 2 H 5 c-C 6 H 11 Cl Ph 21:79
d Cl Ph <3 97
i-C 3 H 7 c-C 6 H 11
d Cl Ph <1 99
c-C 6 H 11 c-C 6 H 11
d Cl Ph <3 97
t-C 4 H 9 c-C 6 H 11
a. From a more complete compilation, see C. H. Heathcock, in Asymmetric Synthesis, Vol. 3, J. D. Morrison, ed., Academic
Press, New York, 1984, Chap. 3.
b. D. A. Evans, J. V. Nelson, E. Vogel, and T. R. Taber, J. Am. Chem. Soc., 103, 3099 (1981).
c. I. Kuwajima, M. Kato, and A. Mori, Tetrahedron Lett., 21, 4291 (1980).
d. H. C. Brown, R. K. Dhar, R. K. Bakshi, P. K. Pandiarajan, and P. Singaram, J. Am. Chem. Soc., 111, 3441 (1989);
H. C. Brown, K. Ganesan, and R. K. Dhar, J. Org. Chem., 58, 147 (1993).
Titanium enolates can be prepared from lithium enolates by reaction with a
trialkoxytitanium(IV) chloride, such as tris-(isopropoxy)titanium chloride. 21 Titanium
enolates are usually prepared directly from ketones by reaction with TiCl and a tertiary
4
amine. 22 Under these conditions, the Z-enolate is formed and the aldol adducts have
syn stereochemistry. The addition step proceeds through a cyclic TS assembled around
titanium.
OTiCl O OH
O 3
1) TiCl 4 CH 3 CH (CH 3 ) 2 CHCH O
CH 3 CH 3 3 CH 3 CH(CH )
2) i-Pr NEt 3 2
2
CH 3
R R
Ti O R′ E Ti O R′
O R O R E
R Z R Z
E
Z-enolate, R = H, syn
Z
E-enolate, R = H, anti
Entry 8 in Scheme 2.1 is an example of this method. Titanium enolates are frequently
employed in the synthesis of complex molecules and with other carbonyl derivatives,
21
C. Siegel and E. R. Thornton, J. Am. Chem. Soc., 111, 5722 (1989).
22
D. A. Evans, D. L. Rieger, M. T. Bilodeau, and F. Urpi, J. Am. Chem. Soc., 113, 1047 (1991).