REETZ Titanium alkylation

The highly regio- and diastereoselective addition of an alkyl and an arylthio group to an olefinic double bond ( carbosulfenylation ) is achieved with arenesulfenyl chlorides and alkyl-chloro-titanium(IV) species (Reetz reagent, from R2Zn/TiCU 5 1 M. T. Reetz, 1987, 1989), Use of the more bulky 2,4,6-triisopropylbenzenesulfenyl chloride improves the yield of the highly versatile alkyl aryl sulfide products. 

Three reactions, which were known from the literature to be catalyzed by Lewis acids were selected as test reactions. A, was the Reetz alkylation of silyl enol ethers with -butyl chloride for which titanium tetrachloride is known to be useful [52]. B, was the Diels-Alder reaction between furan and acetylenedicarboxylic ester for which aluminium trichloride is a good catalyst [53]. C, was a Friedel-Crafts acylation for which aluminium trichloride is the preferred catalyst [54]. The reactions are summarized in Scheme 6. 

Studies pertaining to diastereoselectivity in Lewis acid catalyzed alkylations of enol derivatives have been limited. Reetz has reported that r-butylation of l-trimethylsiloxy-4-f-butylcyclohex-l-ene gave an 8S 1S mixture of cis- and frafu-2,4-di-f-butylcyclohexanone, which could result from kinetic equatorial and axial alkylation, respectively. However, equilibration of the products, which would favor formation of the former isomer, was not ruled out. Titanium tetrachloride promoted phenylthiomethylation of the more-substituted TMS enol ether of 1-decalone gave a 4 1 mixture of cis- and rranj-fused 1-deca-lones. In this case, where equilibration of the product could not occur, the diastereoselectivity was similar to that of methylation of the corresponding lithium enolate (49). ... 

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