Chiral Amino Alcohol Modified Halomethylzinc Reagents

A novel class of tunable cyclopropanation reagents (RXZnCH2Y) and their synthetic applications

The Simmons-Smith cyclopropanation is a widely used method to synthesize cyclopropanes from alkenes using methylene iodide and a zinc reagent. A novel class of organozinc species, RXZnCH(2)Y, has been found to efficiently cyclopropanate alkenes, including traditionally unreactive unfunctionalized alkenes. The reactivity and selectivity of this class of organozinc reagents can be regulated by tuning the electronic and/or steric nature of the RX group attached to Zn. During recent years, this class of organozinc reagent has been widely used in organic synthesis as a reagent for cyclopropanation and other useful synthetic transformations. Catalytic, asymmetric versions of this reaction have been developed providing high enantiomeric excess for unfunctionalized olefins.

Enantioselective Cyclopropanation of Allylic Alcohols. The Effect of Zinc Iodide

The effect of zinc iodide on the catalytic, enantioselective cyclopropanation of allylic alcohols is examined with bis(iodomethyl)zinc as the reagent and bis-methanesulfonamide 7 as the catalyst. Significant rate enhancement was observed when 1 equiv of zinc iodide was present, but more importantly, the enantiomeric excess of the product cyclopropane increased from 80% to 89% for the substrate cinnamyl alcohol. Reaction studies and spectroscopic investigations show that this remarkable influence is the result of reagent modification via a Schlenk equilibrium that produces the more reactive and selective species (iodomethyl)zinc iodide.

Catalytic, Enantioselective Cyclopropanation of Allylic Alcohols. Substrate Generality

Catalytic, enantioselective cyclopropanation of a broad range of allylic alcohols and one homoallylic alcohol was carried out. The cyclopropanation reagent employed was bis(iodomethyl)zinc generated by the method of Furukawa, and the chiral promoter used (10 mol %) was the N,N-bis(methanesulfonyl) derivative of (R,R)-1,2-diaminocyclohexane. Three experimental features were found to be critical for the rapid and selective cyclopropanation: (1) use of the ethylzinc alkoxide of the allylic alcohol as the substrate by prior deprotonation of the allylic alcohols by diethylzinc, (2) the formation of the zinc complex of the promoter by prior deprotonation of the bis-sulfonamide with diethylzinc, and (3) the use of added zinc iodide generated in situ from diethylzinc and iodine. The stereoselectivity of cyclopropanation was found to be independent of olefin geometry and worked well for substrates bearing both aliphatic and aromatic substituents at either or both 3-positions of the allylic alcohol. However, a methyl substituent on the 2-position of the allyl alcohol was not well tolerated and led to slow reactions and poor enantioselectivities. A rationale for the observed selectivities is proposed.