Enophiles, allylsilane

Ene reactions with allylsilanes. This reactive enophile adds to allylsilanes to afford vinyl- and alkynylsilanes. ... 

Coverage in this chapter is restricted to the use of alkenes or alkynes as enophiles (equation 1 X = Y = C) and to the use of ene components in which a hydrogen is transferred. Coverage in Sections 1.2 and 1.3 is restricted to ene components in which all three heavy atoms are carbon (equation 1 Z = C). Thermal intramolecular ene reactions of enols (equation 1 Z = O) with unactivated alkenes are presented in Section 1.4. Metallo-ene reactions are covered in the following chapter. Use of carbonyl compounds as enophiles, which can be considered as a subset of the Prins reaction, is covered in depth in Volume 2, Chtqiter 2.1. Addition of enophiles to vinylsilanes and allylsilanes is covered in Volume 2, Chapter 2.2, while addition of enophiles to enol ethers is covered in Volume 2, Chapters 2.3-2.S. Addition of imines and iminium compounds to alkenes is presented in Volume 2, Part 4. Use of alkenes, aldehydes and acetals as initiators for polyene cyclizations is covered in Volume 3, Chapter 1.9. Coverage of singlet oxygen, azo, nitroso, S=N, S=0, Se=N or Se=0 enophiles are excluded since these reactions do not result in the formation of a carbon-carbon bond. 

Knoevenagel products are highly reactive compounds because of their low energy LUMO. They can act as dienophiles in the normal Diels-Alder reaction, as heterodienes in the hetero Diels-Alder reaction with inverse electron demand,- as dipolarophiles in 1,3-dipolar cycloadditions, as enophiles in the ene reaction and as acceptors for the addition of allylsilanes. Sigmatropic rearrangements and photochemical reactions have been described. 

The alkylidene-1,3-dicarbonyl moiety is also a highly reactive enophile. Thus, the Knoevenagel adduct (223) obtained from aldehyde (222) and dimethyl malonate in the presence of piperidinium acetate, cy-clizes by treatment with Lewis acids such as FeCb on AI2O3 exclusively to the rranr-substituted cyclohexane (224 trans.cis > 99.5 0.5 Scheme 44).7 °3 The sesquiterpene veticadinol (225) has been synthesized in enantiomerically pure form by this method.Contrary to the expectation, rranr-sub-stituted cyclopentanes can also be obtained with excellent induced and noninduced dia-stereoselectivities. Similarly, aldehydes containing an allylsilane moiety can be used for a highly stereoselective formation of rran.r-l,2-disubstituted cyclopentanes and cyclohexanes by a Knoevenagel... 

The ene and Prins reactions are not mechanistically distinct. Coverage will therefore be organized by the nature of the carbonyl compound, with intermolecular reactions presented first, followed by intramolecular reactions. The emphasis will be on material published since the field has been reviewed " and on examples demonstrating the stereo-, regio- and chemo-selectivity of these reactions. Coverage is restricted to the addition of carbonyl and thiocarbonyl compounds to simple alkenes. Addition of carbonyl compounds to vinylsilanes, allylsilanes and enol ethers is covered in the following chapters. Addition of imines and iminium compounds to alkenes is presented in Part 4 of this volume. Ene reactions with alkenes and alkynes as enophiles are covered in Volume 5, Chapter 1.1. Use of aldehydes and acetals as initiators for polyene cyclizations is covered in Volume 3, Chapter 1.6. 

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