Magnesium carbenoids

SCHEME 4. Stmctures of lithium carbenoid 3 and the magnesium carbenoid 4... 

A. Cyclopropanation of Allylic Alcohols with Magnesium Carbenoids. . . 722... 

Reaction of magnesium carbenoids with Grignard reagents... 723... 

Reaction of magnesium carbenoids with A-lithio arylamines. 728... 

A. Generation of Magnesium Carbenoids by Halogen-Magnesium Exchange Reaction... 

Magnesium carbenoids (5 and 6) were generated from a geminal diiodoalkane by diastereoselective iodine-magnesium exchange reaction with isopropylmagnesium halide in THF at —78°C for 45 min (Scheme 1). Subsequent reaction of the magnesium... 

The Simmons-Smith-type cyclopropanation of olefins is one of the most well-known reactions of carbenes and carbenoids. However, cyclopropanation of simple olefins with magnesium carbenoids is usually very difficult and only cyclopropanation of allylic alcohols was reported. Thus, treatment of allylic alcohols (23) in CH2CI2 at —70°C with i -PrMgCl and diiodomethane for 48 to 60 h afforded cyclopropanes in up to 82% yield as a mixture of syn- and and-isomers. In this reaction, 5yn-isomers were mainly or exclusively obtained (synianti = 5 1-400 1) (equation 10). 

Carbenoids have both a nucleophilic and an electrophilic nature. This is one of the most striking characteristics of carbenoids. Especially, electrophilic reaction of magnesium carbenoids with carbon and nitrogen nucleophiles has recently received much attention and various new interesting synthetic methods have appeared. 

In 1990, Hahn and Tompkins reported an interesting multi-carbon homologation of alkyl halides by the reaction of magnesium carbenoid with a Grignard reagent (Scheme 2) . 

As mentioned above, the configuration of the magnesium carbenoid is rather stable at low temperature chiral Grignard reagents having over 90% ee could be generated from optically active 1-chloroalkyl aryl sulfoxides (Scheme 4)20-22... 

The olefin formation described above can be conducted with magnesium carbenoids derived from 1-chloroalkyl phenyl sulfoxides. An example is shown in equation 16. 

Thus, magnesium carbenoid 38, generated from 1-chloroalkyl phenyl sulfoxide (37) in THF at —65°C with 2.8 eq of i-PrMgCl, reacts with a-sulfonyl lithium carbanion to lead to 1,2-di- and 1,1,2-trisubstituted olefins (60). Yields are better in such conditions as compared to the reaction described in equation 14. 

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