Page 45 - Advanced Organic Chemistry Part B - Reactions & Synthesis

O R' SiH OSiR' 3 17
3
Si SECTION 1.1
O Pt Generation and
R R Properties of Enolates
Si
and Other
SiR' , = Si(Et) , Si(i-Pr) , Si(Ph) , Si(Me) C(Me) 3 Stabilized Carbanions
3
2
3
3
3
Excellent yields of silyl enol have also been obtained from enones using B C F as
6 5 3
a catalyst. 40 t-Butyldimethylsilyl, triethylsilyl, and other silyl enol ethers can also be
made under these conditions.
CH (Ph) Si
2
3
O O
CH 3 CH 3
F )
B(C 6 5 3
+ CH Si(Ph) H
2
3
CH 2 CH 2
CH 3 CH 3
These and other reductive methods for generating enolates from enones are discussed
more fully in Chapter 5.
Another very important method for specific enolate generation is the conjugate
addition of organometallic reagents to enones. This reaction, which not only generates
a specific enolate, but also adds a carbon substituent, is discussed in Section 8.1.2.3.
R R' β − R R'
+ [(R ) Cu]
2
β –
O R O

1.1.4. Solvent Effects on Enolate Structure and Reactivity

The rate of alkylation of enolate ions is strongly dependent on the solvent in
41
which the reaction is carried out. The relative rates of reaction of the sodium enolate
of diethyl n-butylmalonate with n-butyl bromide are shown in Table 1.3. Dimethyl
sulfoxide (DMSO) and N,N-dimethylformamide (DMF) are particularly effective in
enhancing the reactivity of enolate ions. Both of these are polar aprotic solvents. Other

Table 1.3. Relative Alkylation Rates of Sodium Diethyl
n-Butylmalonate in Various Solvents a
Solvent Dielectric constant
Relative rate
Benzene 2.3 1
Tetrahydrofuran 7.3 14
Dimethoxyethane 6.8 80
N,N-Dimethylformamide 37 970
Dimethyl sulfoxide 47 1420
a. From H. E. Zaugg, J. Am. Chem. Soc., 83, 837 (1961).

40 J. M. Blackwell, D. J. Morrison, and W. E. Piers, Tetrahedron, 58, 8247 (2002).
41
For reviews, see (a) A. J. Parker, Chem. Rev., 69, 1 (1969); (b) L. M. Jackmamn and B. C. Lange,
Tetrahedron, 33, 2737 (1977).