silicon and allylsilane

Three gamma-(amino)silyl-substituted allylsilanes 14a-c have been prepared in three steps from the corresponding dialkyldichlorosilane. The aminosilyl group has been used to link this allylsilane nucleophile to a series of beta-hydroxy aldehydes through a silyl ether temporary connection. The size of the alkyl substituents at the silyl ether tether governs the outcome of the reaction on exposure to acid. Thus, treatment of aldehyde (E)-9aa, which contains a dimethylsilyl ether connection between the aldehyde and allylsilane, with a range of Lewis and Brønsted acid activators provides an (E)-diene product. The mechanism of formation of this undesired product is discussed. Systems containing a sterically more bulky diethylsilyl ether connection react differently: thus in the presence of TMSOTf and a Brønsted acid scavenger, intramolecular allylation proceeds smoothly to provide two out of the possible four diastereoisomeric oxasilacycles, 23 (major) and 21 (minor). A diene product again accounts for the remaining mass balance in the reaction. This side product can be completely suppressed by using a sterically even more bulky diisopropylsilyl ether connection in the cyclization precursor, although this is now at the expense of a slight erosion in the 1,3-stereoinduction in the allylation products. The sense of 1,3-stereoinduction observed in these intramolecular allylations has been rationalized by using an electrostatic argument, which can also explain the stereochemical outcome of a number of related reactions. Levels of 1,4-stereoinduction in the intramolecular allylation are more modest but can be significantly improved in some cases by using a tethered (Z)-allylsilane in place of its (E)-stereoisomer. Oxidation of the major diastereoisomeric allylation product 23 under Tamao-Kumada conditions provides an entry into stereodefined 1,2-anti-2,4-syn triols 28.

Topics: Aldehydes; Alkenes; Cyclization; Ether; Ketones; Magnetic Resonance Spectroscopy; Models, Chemical; Molecular Structure; Silanes; Silicon; Static Electricity; Stereoisomerism

[reaction: see text] Chiral N-acylhydrazones derived from commercially available 4-benzyl-2-oxazolidinone provide a rigid, conformationally restricted template to impart facial selectivity in additions to C=N bonds. In the presence of indium(III) trifluoromethanesulfonate [In(OTf)3], N-acylhydrazones undergo highly diastereoselective fluoride-initiated additions of allylsilanes (aza-Sakurai reaction). Mechanistic studies including control experiments and comparisons with allyltributylstannane, allylmagnesium bromide, and allylindium species implicate a dual activation mechanism involving addition of an allylfluorosilicate species to a chelate formed from In(OTf)3 and the chiral N-acylhydrazone. The N-N bonds of the adducts are readily cleaved in a two-step protocol to provide synthetically useful homoallylic N-trifluoroacetamides. Further elaboration of the latter compounds through Wacker oxidation and olefin metathesis provides diversely functionalized building blocks and expands the potential applications of this C-C bond construction approach to asymmetric amine synthesis.

Topics: Acetamides; Acylation; Aldehydes; Alkenes; Amines; Electrons; Fluoroacetates; Hydrazones; Ketones; Magnetic Resonance Spectroscopy; Molecular Structure; Nitrogen; Silanes; Silicon; Stereoisomerism; Trifluoroacetic Acid