The Schlosser Modification of the Wittig Reaction allows the selective formation of E-alkenes through the use of excess lithium salts during the addition step of the ylide and subsequent deprotonation/protonation steps.
Mechanism of the Schlosser Modification
Lithium salts effect, that the intermediate betaines (see Wittig Reaction) do not react further. These lithiobetaines which are quite stable may be deprotonated. Deprotonation is equal to the lost of one stereogenic center. Use of a steric hindered proton donator then leads to the trans lithiobetaine. The reaction takes "normal" course, if lithium is exchanged by potassium.