Intramolecular hydrosilations

Asymmetric Intramolecular Hydrosilation. Intramolecular hydrosilation of allylic alcohols followed by oxidation is a convenient method for the stereoselective preparation of 1,3-diols. An enantioselective version is achieved by use of diene-free BINAP-Rh+ (eq 6). Both silyl ethers derived from cinnamyl alcohol and its cis isomer give (iJ)-l-phenylpropane-l,3-diol in high ee regardless of alkene geometry. 

Intramolecular hydrosilation. Tamao et al. have extended their hydrosily-lation-oxidation sequence (12,243-245) to allyl and homoallyl alcohols as an approach to 1,3-diols. When applied to cyclic homoallylic alcohols, only a cis-1,3-diol is obtained, presumably by way of a cyclic intermediate (equation I). Acyclic homoallylic alcohols can be converted by this approach to either syn- or anti-1,3-... 

Regio- and Stereoselective Intramolecular Hydrosilation of a-Hydroxy Enol Ethers 2,3-syn-2-Methoxymetnoxy-1,3-nonanediol. 

Silathietanes can be readily prepared from the appropriate bis(chloromethyl)silane and KSH or by intramolecular hydrosilation in the presence of Wilkinson s catalyst (Scheme 97) (81JOM(204)13). Electron impact and photoionization mass spectrometry support the loss of silathione ions (R2Si=S) (R = Me, Et) indicating a transannular interaction, though decomposition by the loss of silenes and thioaldehyde also readily occurs (81JOM(214)145). 

The intramolecular hydrosilation presents a convenient route to silacyclopentanes. It is catalyzed by chloroplatinic acid and gives only small quantities of the isomeric cyclohexane... 

Rhodium(II) acetate catalyzes C—H insertion, olefin addition, heteroatom-H insertion, and ylide formation of a-diazocarbonyls via a rhodium carbenoid species (144—147). Intramolecular cyclopentane formation via C—H insertion occurs with retention of stereochemistry (143). Chiral rhodium (TT) carboxamides catalyze enantioselective cyclopropanation and intramolecular C—N insertions of CC-diazoketones (148). Other reactions catalyzed by rhodium complexes include double-bond migration (140), hydrogenation of aromatic aldehydes and ketones to hydrocarbons (150), homologation of esters (151), carbonylation of formaldehyde (152) and amines (140), reductive carbonylation of dimethyl ether or methyl acetate to 1,1-diacetoxy ethane (153), decarbonylation of aldehydes (140), water gas shift reaction (69,154), C—C skeletal rearrangements (132,140), oxidation of olefins to ketones (155) and aldehydes (156), and oxidation of substituted anthracenes to anthraquinones (157). Rhodium-catalyzed hydrosilation of olefins, alkynes, carbonyls, alcohols, and imines is facile and may also be accomplished enantioselectively (140). Rhodium complexes are moderately active alkene and alkyne polymerization catalysts (140). In some cases polymer-supported versions of homogeneous rhodium catalysts have improved activity, compared to their homogenous counterparts. This is the case for the conversion of alkenes direcdy to alcohols under oxo conditions by rhodium—amine polymer catalysts... 

Hydrosilation of 1,5- and 1,6-dienes, catalyzed by a chiral neodymium complex, leads to intramolecular bond formation and results in the formation of silylated methylcy-clopentanes (equation 147)538. This reaction is in stark contrast to the reaction catalyzed by group VHI complexes, which gives a range of silanes but no carbocyclic compounds539,540. 

The C—Si bond formed by the hydrosilation of alkene is a stable bond. Although it is difficult to convert the C—Si bond to other functional groups, it can be converted to alcohols by oxidation with MCPBA or H2O2. This reaction enhances the usefulness of hydrosilylation of alkenes [219], Combination of intramolecular hydrosilylation of allylic or homoallylic alcohols and the oxidation offers regio- and stereoselective preparation of diols [220], Internal alkenes are difficult to hydrosilylate without isomerization to terminal alkenes. However, intramolecular hydrosilation of internal alkenes can be carried out without isomerization. Intramolecular hydrosilylation of the silyl ether 572 of the homoallylic alcohol 571 afforded 573 regio- and stereoselectively, and the Prelog-Djerassi lactone 574 was prepared by applying this method. 

Intramolecular hydrosilation has been extensively used to control regio-and enantioselectivity.149 This strategy has been applied by Bergens et a/.150... 

Intramolecular hydrosilation was found to be a nsefiil method for 1,2- or 1,3-chirality transfer. The hydrosUyl moiety is easily attached to a heteroatom snch as oxygen or nitrogen and subsequently an intramolecnlar hydrosilation... 

Also norbomene and related systems, as well as dihydrofuranes, have been hydrosilated with good enantioselectivities with similar MOP ligands. Intramolecular rhodium catalyzed asymmetric hydrosUations have been achieved with good enantioselectivities, for example with meso- or other allylsilylethers. ... 

Analogously, intramolecular hydrosilation reactions have been carried out with alkenes cyclization of alkenyloxy-silanes catalyzed by thiols <1998J(P1)467> and by Pt <1996TL827> has been described. In addition, intramolecular temporary silicon-tethered rhodium-catalyzed [4- -2- -2]-cycloisomerization reactions have been carried out by Evans and Bawn (Equation 50) <2004JA11150>. 

Dimethyl- and 3,3-diethyl-l,3-thiasilacyclobutanes are obtained by intramolecular cyclization of the bis(chloromethyl)silane with hydrosulfide ion, as exemplified by the synthesis of 522 or by an intramolecular hydro-silation, as exemplified by the synthesis of 523. A chloroplatinic acid catalyst (Speiers catalyst) also has been used for the hydrosilation... 

Co2Rh2(CO)i2] and [Co3Rh(CO)i2] catalyze the hydrosilation of isoprene, cyclohexanone, cyclohexenone, and 1-alkynes, the regioseleetive inter- or intramolecular (Eq. 5) silylformylation of alkynes and 1-alkynals, and the silylcarbo-cyclization of enynes, diynes, and alkynals. These synthetically important reactions have been reviewed recently. 

Curtis and Holmes have examined the intramolecular hydrosilation of the enol ether (111). 

One common class of catalytic reactions involves 1,2-addition of a CH bond across a multiple bond (eq. 23). Such is the case for alkene hydrogenation (XY = H2), hydrosilation (XY = RsSi—H), hydroboration (XY = R2B—H), and disilylation (XY = RsSi—SiRs). An alkene isomerization like equation 10 can be considered as intramolecular C—H addition across the 2,3—C=C bond. The Heck reaction (eq. 24) is an example of an addition of a C—Hal bond combined with an elimination of a H—Hal group. Similarly, the Wacker process is effectively an addition of H—OH across the ethylene C—C bond, followed by an elimination of H2. 

Intramolecular Hydrosilation of Allyl and Homoallyl Alcohols. For a detailed description of the intramolecular hydro-silation-oxidation sequence, see 1,1,3,3-tetramethyldisilazane. 1,1,3,3-Tetramethyldisilazane and AA -diethylaminodimethyl-silane are frequently employed for preparation of hydrodimethyl-silyl ethers for the intramolecular hydrosilation of allyl and homoallyl alcohols. Chlorodimethylsilane in combination with a tertiary amine such as triethylamine is another useful reagent for the synthesis of hydrodimethylsilyl ethers, especially for large scale preparations. ... 

Intramolecular Hydrosilation of Allyl Amines. The amino group of an allyl amine is silylated with chlorodimethylsilane, and then the resulting silazane is subjected to standard platinum-catalyzed cyclization and oxidation steps to form a syn-2-amino alcohol (eq 1). Exclusive formation of the four-membered cyclic product and the high stereoselectivity should be noted. ... 

---