Boron trifluoride diastereofacial selectivity

In addition to the boron trifluoride-diethyl ether complex, chlorotrimcthylsilanc also shows a rate accelerating effect on cuprate addition reactions this effect emerges only if tetrahydrofuran is used as the reaction solvent. No significant difference in rate and diastereoselectivity is observed in diethyl ether as reaction solvent when addition of the cuprate, prepared from butyllithium and copper(I) bromide-dimethylsulfide complex, is performed in the presence or absence of chlorotrimethylsilane17. If, however, the reaction is performed in tetrahydrofuran, the reaction rate is accelerated in the presence of chlorotrimethylsilane and the diastereofacial selectivity increases to a ratio of 88 12 17. In contrast to the reaction in diethyl ether, the O-silylated product is predominantly formed in tetrahydrofuran. The alcohol product is only formed to a low extent and showed a diastereomeric ratio of 55 45, which is similar to the result obtained in the absence of chlorotrimethylsilane. This discrepancy indicates that the selective pathway leading to the O-silylated product is totally different and several times faster than the unselective pathway" which leads to the unsilylated alcohol adduct. A slight further increase in the Cram selectivity was achieved when 18-crown-6 was used in order to increase the steric bulk of the reagent. 

The diastereofacial selectivity of Lewis acid promoted reactions of allylsilancs with chiral aldehydes has been thoroughly investigated58. Aldehydes with alkyl substituted a-stereogenic centers react with a mild preference for the formation of Cram products, this preference being enhanced by the use of boron trifluoride-diethyl ether complex as catalyst58. 

Nucleophilic Additions. MgBr2 has been shown to form discrete bidentate chelates with various species, particularly a-and/or a,3-alkoxy carbonyl compounds, and thus functions as a diastereofacial control element in many nucleophilic addition reactions. In many cases, its inclusion completely reverses the nonchelation-controlled stereochemistry observed with nonchelating Lewis acids such as Boron Trifluoride Etherate. Highest diastereoselectivity is observed with a-substituted aldehydes (eq 1). High selectivities are observed for p-alkoxy aldehydes as well, including cases where three contiguous chiral centers are defined during the reaction (eq 2). ... 

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