Mechanism

Scheme 14

3.1.2.7 Reactions with α,β-Unsaturated Carbonyl Compounds

In the case of Grignard reagents, α,β-unsaturated carbonyl compounds undergo reaction either at 1,2- or 1,4-addition depending on the structure of the carbonyl compound. The main reason is steric hinderance. While the organolithium reagents undergo reaction exclusively to give 1,2-addition products (Scheme 15). Exclusive formation of 1,4-addition product, however, can be achieved using lithium dialkylcuprates.

Scheme 15

3.1.2.8 Deprotonation

The basic nature of organolithiums can also be put to good use in achieving umpolang at the carbonyl centre of an aldehyde. In this protocol a C=O function is first protected by 1, 3-dithiane and then the proton is removed by an organolithium (Scheme 17).

Scheme 16

The stereochemical outcome of the nucleophillic addition of organolithiums is similar to that of Grignard reaction. It can be predicted on the basis of Cram’s rule.

3.1.2.9 Ortholithiation

The ortholithiation is useful reaction because the starting material does not need to have a halogen atom. For examples, in the case of benzyldimethylamine, the nitrogen atom directs attack of the butyllithium (Scheme 18). Likewise, N-cumylbenzamide with excess secondary butyllithium in the presence of TMEDA gives orotholithiated intermediate that could readily reacted with benzaldehyde with 80% yield.

Scheme 17