Synthesis of Novel Silicon-Containing Compounds via Lewis Acid-Catalyzed Reactions

Abstract

This review will describe Lewis acid-catalyzed reactions such as allylsilylation, intramolecular allyl-migration, Friedel-Crafts alkylation, and hydrosilylation reactions commonly used in organosilicon chemistry. The carbenium and silylenium ion intermediates are generated by the interactions of organosilicon compounds such as allyltriorganosilanes,1a,2 alkenylchlorosilanes,1b,3 (chloroalkyl)silanes,1b,4 and triorganohydrosilanes5 with Lewis acid catalysts, which then react with simple unsaturated electrophilic organic compounds.1 The isolation and properties of silylenium ions is discussed by Mu？ller in this volume (p. 155). This review will particularly focus on the Lewis acid-catalyzed reactions with unsaturated hydrocarbon compounds. It is well recognized that the reactivity of such electrophiles varies depending upon the electronic nature of substituents and the cation position in the organosilicon compounds.1b,3,4. It is useful to compare the reactivity and reaction patterns of various cationic organosilicon species in the Lewis acid-catalyzed reactions with those of their organic analogs. Positive charges on the carbon β to silicon are stabilized by the electron-donating silyl group through σ- π conjugation, commonly known as β-stabilization.6-8 The reactivity and reaction modes of allylsilanes are different from one another, depending on the number (n) of chlorine substituent on the silicon atom of allylchlorosilanes (CH2{double bond, long}CHCH2SiMe3-nCln). Allylsilanes containing two or more chlorine substituents on silicon react readily with aromatic compounds to give alkylation products, 2-aryl-1-silylpropanes [Eq. (1)].3 However, allyltrimethylsilane, having three methyl groups on the silicon (1a), in benzene solvent is dimerized to give the allylsilylation product 5-(trimethylsilyl)-4-(trimethylsilylmethyl)-1-pentene without any alkylation products [Eq. (2)].2{A formula is presented}{A formula is presented}. In aluminum chloride-catalyzed reactions, a small amount of hydrogen chloride, resulting from the reaction of anhydrous aluminum chloride with moisture inevitably present in the reactants, initiates the reaction.9 The proton from the hydrogen chloride interacts with the π-bond of allylsilanes to give a carbenium ion on the carbon β to silicon, because the secondary silylpropyl cation can be stabilized by the electron-donating silyl group through β-stabilization as described above.1b,3 The stabilization effects can be more significant for allyltrimethylsilane than for allylchlorosilanes because of the electron-donating effects from the methyl groups on silicon.1b,2 This facilitates the protodesilylation1a,2,10 of allyltrimethylsilane by hydrogen chloride in the presence of aluminum chloride, which gives propene and a Me3SiCl-AlCl3 complex. Me3SiCl has been reported to be an excellent promoter of allylsilylation reactions.1a,11-13 In contrast, the collapse of the β-silyl cation intermediates of allylchlorosilanes is largely retarded, primarily due to less effective σ- π conjugation brought on by the presence of the electronegative chlorine atom(s) on silicon.1b,3 The cation intermediates undergo alkylation reactions with electron-rich aromatic systems faster than protodesilylation.1b,3. ？ 2005 Elsevier Inc. All rights reserved.