Zinc diethyl-: Simmons-Smith reaction with

Other methods used to improve the cyclopropanation in Simmons Smith reactions are ultrasonic cavitation and the use of catalytic amounts of titanium(IV) chloride to promote the reaction. A much better method is to use 1 mol% of acetyl chloride (based on zinc) and dibromo-methane in the presence of zinc dust and copper(I) chloride in diethyl ether. This system not only strongly accelerates alkene cyclopropanation, but also causes no special problems with Lewis acid sensitive substrates. Acetyl chloride works as a promoter by reacting with... 

This sequence has been used for the synthesis of the sesquiterpenes ( —)- and ( + )-thujop-sene. The allylic /1-hydroxysulfoximines 47 and 49 were separated by column chromatography on silica gel and treated separately with excess of Simmons Smith reagent prepared from zinc/ silver amalgam and diiodomethane in diethyl ether giving 48 and 50, respectively. The Simmons-Smith reaction took place stereospecifically cis to the hydroxy group of the allylic ff-hydroxysulfoximines. Cyclopropanes 48 and 50 were converted to ( —)- and (- -)-thujopsene. 

Normally the chiral auxiliaries are introduced and removed in the asymmetric synthesis of Simmons-Smith reactions of allylic alcohols to provide mostly /rani-disubstituted cyclopropanes. Stereoselective syntheses of c -disubstituted cyclopropanes are difficult to achieve. Starting from (Z)-3-phenylprop-2-en-l-ol (80a) and (Z)-6-phenylhex-2-en-l-ol (80b), the corresponding c -disubstituted cyclopropanes 81a and 81b were prepared by first treating them with diethylzinc followed by diethyl (- -)-(/ ,7 )-tartrate (DET). A zinc-bridged intermediate is assumed to be formed first. This is subsequently treated with diethylzine and diiodomethane to give the products 81. The reaction conducted at — 12 "C gave the cyclopropanated products 81a and 81b with 70 and 81% ee, respectively.This method has the advantages that the introduction of the chiral auxiliary to the substrate and its removal are not neccessary and that both cis- and trans-disubstituted cyclopropanes could be prepared from (Z)- and ( )-allylic alcohols, repectively. 

A valuable cyclopropanation reaction is the Simmons-Smith reaction of alkenes with diiodomethane and zinc-copper couple or diethyl zinc. This is a versatile reaction and has been applied with success to a wide variety of alkenes. Many... 

Simmons-Smith reaction Areactionin which a cyclopropane ring is produced from an alkene. It uses the Simmons-Smlth recent, which was originally diiodo-methane (CH2I2) with aZn/Cu couple. Usually, diethyl zinc is used rather than Zn/Cu. The mechanism involves the formation of H2C(I) (Znl) and carbene transfer from the zinc to the double bond of the alkene. 

The more recently reported route to diethyl 3-oxoalkylhosphonates uses a zinc carbenoid-mediated approach, which is believed to proceed through the intermediacy of a cyclopropylzinc alkoxide. Thus, treatment of simple diethyl 2-oxoaIkylphosphonates with the Furukawa-modifled Simmons-Smith reagent provides a rapid and efficient preparation of 3-oxoalkyIphosphonates. The chain extension of simple, unfunctionalized P-ketophosphonates requires an excess (6 eq) of both Et2Zn and CH2I2 at room temperature. The presence of a-substitution on the 2-oxoalkylphosphonate does not diminish the efficiency of the reaction (see Section 7.2.3.7). 

Pathway A). Indeed, it was this method that Simmons and Smith first used in their seminal study, with many methods of zinc activation available (see 1.4.5.1). Furukawa s modification of the reaction uses diethyl zinc, and proceeds via alkyl ligand exchange to give 8 (Pathway B). Finally, Wittig reported the insertion of diazomethane (7) into zinc iodide to give 6, although this method is not widely used. 

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