Deprotonation dimethylphosphine

Enantioselective deprotonation of tertiary dimethylphosphines 143 can be achieved with s-BuLi in the presence of accessible and cheap derivatives of alkaloid ( )-cytisine 148. The derivatives of cytisine 148 are useful sparteine... 

Scheme 46 Enantioselective deprotonation of tertiary dimethylphosphines with (—)-sparteine or...

Despite its versatility, the ephedrine methodology clearly suffers from some limitations. Exceedingly bulky phosphines (bearing mesityl or 9-anthryl groups for example) can only be prepared in low yields if at all. This is because the method is based on nucleophilic substitutions at the phosphorus atom, which become progressively more difficult with increased steric shielding. Another limitation is that all the phosphines prepared bear at least one aryl group (almost exclusively phenyl) and are therefore very basic, trialkylphosphines are not accessible via the ephedrine method. This limitation has been partially overcome by another complementary and extremely important method the enantioselective deprotonation of dimethylphosphine derivatives described in detail in the next chapter. 

This method is not only restricted to dimethylphosphine boranes and sulfides. It has been also applied to cyclic diphosphine derivatives by deprotonation of one of the enantiotopic methylene groups a to the phosphorus atom (Scheme 5.3). In this case, a second stereogenic centre is created and therefore it is necessary to control also the diastereoselectivity of the reactions. 

In 1995 Evans and co-workers reported the first synthetically useful enantioselective deprotonation lectrophilic quenching of dimethylphosphine boranes (Scheme 5.4). 

Scheme 5.2 Overview of compounds obtainable by enantioselective deprotonation of dimethylphosphine derivatives.

The enantioselective deprotonation protocol has also been applied with certain success to dimethylphosphine sulfides (Scheme 5.43). 

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