Epoxidations Simmons-Smith reagent

A cis addition mechanism is generally accepted for the reaction, because cis addition to an olefinic bond generally occurs with predominant attack at trans bonds, and the Simmons-Smith reagent attacks preferentially one of the trans olefinic bonds of trans,trans,cis-1,5,9-cyclodode-catriene and then the cis double bond of the monoadduct (378). The close correspondence in relative rates of olefins for the cyclopropane formation by the Simmons-Smith reaction with those for diimide reduction and peroxide epoxidation supports the concept 409). The latter two reactions are generally considered to proceed via cis addition. 

Reaction of a-hydroxy-ketones (e.g. 275) with the Simmons-Smith reagent replaces the carbonyl group by a methylene (276) or ethano-group (277), depending upon the conditions employed. Bromomethyl-lithium in THF reacts with saturated aldehydes and ketones to give epoxides. Several steroid examples are given. 

Cyclopropanation of alkenes with the Simmons-Smith reagent bears some similarity to epoxidation. Both reactions are stereospecific cycloadditions, and iodomethylzinc iodide behaves, like peroxy acids, as a weak electrophile. Both cycloadditions take place faster with more highly substituted double bonds than less substituted ones, but are sensitive to steric hindrance in the alkene. These similarities are reflected in the mechanisms proposed for the two reactions shown in Mechanism 14.2. Both are believed to be concerted. 

Many epoxidising agents, notably mCPBA and f-BuOOH/VO(acac)2, are capable of bonding to the OH group of an allylic alcohol. Epoxidation then occurs more readily on allylic alcohols and on the same side of the alkene as the OH groups, rather in the style of the Simmons-Smith reaction. The stereochemical outcome is easiest to see in cyclic allylic alcohols. Thus the cyclohexenol 120 gives the syn epoxide 121 cleanly with mCPBA via the conformation 122 in which the axial OH delivers the reagent to the same face of the alkene. 

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