Simmons-Smith reagent structure

These early studies on zinc carbenoids provide an excellent foundation for the development of an asymmetric process. The subsequent appearance of chiral auxiliary and reagent-based methods for the selective formation of cyclopropanes was an outgrowth of a clear understanding of the achiral process. However, the next important stage in the development of catalytic enantioselective cyclopropanations was elucidation of the structure of the Simmons-Smith reagent. 

Scheme 6.26. Auxiliary-based asymmetric cyclopropanations (addition of CH2 ) of a, 3-unsaturated aldehydes and ketones, (a) [98] (b) [99,100] (c) [101-104] (d) Proposed transition structures [104]. Only one zinc and the transfer methylene are shown other atoms associated with the Simmons-Smith reagent are deleted for clarity.

Also, the structure of the Simmons-Smith reagent is unknown, and aggregates may be involved. Thus, this explanation must still be regarded as tentative. 

Draw a structural formula for the product of treating each alkene with the Simmons-Smith reagent. 

Reaction of ( )-3-penten-2-ol with the Simmons-Smith reagent gave a mixture of two isomers having the molecular formula C6H12O. Suggest reasonable structures for these products. 

Although the Simmons-Smith cyclopropanation has attracted increased attention during recent years, the exact structure of the cyclopropanating reagent is still uncertain. NMR-spectroscopic investigations revealed a Schlenk equilibrium between IZnCH2l and ICH2ZnCH2l [Eq. (2)] [15. ... 

Synthesis of model compounds and structural units are being investigated. A double Simmons-Smith reaction on the l,3-dioxolane-4,5-diylbis(alkene) 107 afforded the product 108 with excellent stereoselectivity. The required asymmetry in the double cyclopropanation was the result of coordination of the zinc carbenoid reagent by the Lewis basic dioxolane ring oxygen prior to each cyclopropanation event. The cyclopropanated product was converted to ( )-l,2-bis[(l 5,25)-2-methylcyclopropyl]ethene, a relevant model for the complete structural assignment of FR-900848. 

Although methylene prepared from diazomethane itself is not synthetically useful, addition of methylene to an alkene can be accomplished using a reaction first reported by the American chemists Howard Simmons and Ronald Smith. The Simmons-Smith reaction uses diiodomethane and zinc dust activated by a small amount of copper (a so-called "zinc-copper couple") to produce iodomethylzinc iodide, in a reaction reminiscent of a Grignard reaction. Even though we show the Sinunons-Smith reagent here as ICH2ZnI, its structure is considerably more complex and not fully imderstocxl. 

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