Substrate Selective Reactions in the Presence of Lewis Acids
15
complex 12 (Figure 3.1). In both cases the most stable conformers have been
compared.
H 3 C
H
O
BF 3
F 3 C
H
O
BF 3
less stable
11
12
Figure 3.1
2. The
13 C-NMR signals of the carbonyl carbons (–78°C in CD 2 Cl 2 ) of a mixture
of acetophenone 13 and trifluoroacetophenone 14 appear at 197.94 and 179.59
ppm, respectively. When the mixture was treated with BF 3 ·OEt 2 (up to two
equivalents) signals at 214.65 and 179.59 ppm were observed (Figure 3.2).
Ph
Me
O
CF
Ph
3
O
G 197.94 ppm
G 179.59 ppm
13
1 4
Figure 3.2
Discussion
The carbonyl carbon is electron deficient due to the electronegativity of the oxygen atom. In consequence, the nucleophilic attack to a C=O group strongly depends on the electrophilicity of the carbon atom in the carbonyl function. Electron-withdrawing substituents (R
1
, R
2 = EWG) will make this position more
electrophilic and hence, will favor the nucleophilic addition as indicated in
Scheme 3.4.
R 1 R 2
O
R 1 R 2
O
R 1
R 2
O
Nu
Nu
products
Nucleophilic addition to a carbonyl group
Scheme 3.4
These affirmations could explain the results obtained in the reaction between
benzaldehydes 3 and 4 and allyltributylstannane. Thus, p-trifluoromethyl benz-