Lithium salts, role carbonylation

Addition of the ylide to the carbonyl is postulated to lead first to the zwitterionic intermediate betaine, which would then close to form a four-membered cyclic intermediate, an oxaphosphetane. The existence of the betaine hasn t been fully established, although its intermediacy plays an important role in the Schlosser Modification. Betaines may be stabilized by lithium salts leading to side products therefore, suitable bases in the Wittig Reaction are for example NaH, NaOMe, NEt3). 

Among monoamines, both enantiomers of 1-phenylethylamine and their derivatives play a prominent role. They are commercially available, but can also be prepared by resolution of the racemate, obtainable by Leuckart- Wallach reaction of acetophenone1, with malic acid2 or, more conveniently, with tartaric acid in methanol3. They are used as chiral additives for the addition of zinc alkyls to aldehydes in Section D. 1.3.1.4., as copper complexes for the synthesis of biaryls in Section B.2., as lithium salts for enantioselective deprotonation in Section C., and as imines in Sections D.1.1.1.3.1., D.1.1.1,4.. D.1.4.4., D.1.5.2., D.1.5.8., D. 1.6.1.2.1., D.2.3.I., and D.8. A general procedure for the synthesis of imines from carbonyl compounds and primary amines, with many examples of both chiral carbonyl compounds and chiral amines is given in reference 4. 

Addition of lithium salts enhances the stereoselectivity of the attack by MeLi on 4-t-butylcyclohexanone (87—94% equatorial attack)." Other work has shown that free Li ions are crucial for the addition of alkyl-lithium to a carbonyl function to take place, and that the cation plays a role in determining the preferred direction of the addition." The reaction of 2-phenyl-2-lithio-l,3-dithian and 4-t-butylcyclohexanone in tetrahedrofuran is thermodynamically controlled, giving the axial alcohol only. ... 

The presence of a lithium salt (Lil) as cocatalyst and hydrogen is very important for efficient production of acetic anhydride. The proposed reaction mechanism is shown in Figure 5 [42,43,47]. In this mechanism, there are two catalytic cycles for the formation of methyl acetate a rhodium-catalyzed cycle and a lithium-catalyzed cycle. The rhodium-catafyzed cycle is similar to the Monsanto process of methanol carbonylation (Fig. 1). The participation of the second cycle was discovered when it was found that the reaction rate was much enhanced when hydrogen and a lithium salt were added [43,44]. The role of hydrogen is to reduce the catalytically inactive Rh(CO)2l4 to the active Rh(CO)2l2. In the anhydrous medium used in the reaction, the formation of hydrogen by the reaction of carbon monoxide with water as in the water-gas shift reaction is not possible. Thus hydrogen must be added. 

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