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d1- and d3-Selective Reactions of Sulfur-substituted Allylic Titanium Derivatives with Aldehydes and Ketones
Widler, Leo,Weber, Theodor,Seebach, Dieter
, p. 1329 - 1344 (2007/10/02)
As previously reported, double deprotonation of allylmercaptane with butyllithium produces the moderately to poorly d3-selective dilithium derivative 1a of thioacrolein (or 1-thiabutadiene).Addition of an equimolar amount of chlorotriisopropoxytitanium generates a reagent 1c, which combines with aldehydes and ketones almost exclusively in the γ-position (d3-reactivity, products 2, Table 1, 18 examples).The 2-methyl analogue 4b exhibits a similar d3-selectivity, with somewhat lower chemical yields (products 5, Table 2, 7 examples).In contrast to the dianion deri vatives, methyl- and phenyl-allyllithium combine with chlorotriisopropoxytitanium to give reagents (6a, 6b) which add to the same electrophiles practically only in the α-RS-position (d1-reactivity, products 7 and 8, Table 3, 12 examples).Possible conversions of the selectively produced α- and γ-adducts to vinyloxiranes, to γ-hydroxy- and to ε-keto aldehyde derivatives are discussed (Scheme 1).Some examples of Hg(II)-assisted alcoholyses (CH3OH, C2H5OH) of γ-adducts 2 to 2-alkoxytetrahydrofurans (9, 10) are described (Scheme 2, Table 4). "Titanation" of the Li derivatives not only causes high regioselectivity, but also excellent chemical selectivity of the nucleophilic reagents: they preferentially add to aldehydes vs. ketones and are compatible with other functional groups, such as CN, NO2, I, in the substrate.
