Enones hydrosilylation

Chiral diamino carbene complexes of rhodium have been merely used in asymmetric hydrosilylations of prochiral ketones but also in asymmetric addition of aryl boron reagents to enones. 

The 1,4-hydrosilylation of enones can be used as a method for the introduction of the silyl enol ether functionality, and may be accomplished with the combination of PhMe2SiH/RhH(PPh3)4 (Eq. 278),374 Et3SiH/RhCl(PPh3)3 411 (HMe2 Si)20/[CuH(PPh3) ], 455 Et3 SiH/Pt-complex,456 chloromethy ldimethy 1-... 

To date, the reductive cyclization of allenic alkenes remains undeveloped. However, the reductive cyclization of activated alkene partners in the form of 1,3-dienes and conjugated enones has been achieved using late transition metal catalysts. Indeed, the hydrosilylative dimerization of 1,3-dienes reported in 1969 appears to be the first reductive... 

Dinh is illustrative of the methods and their potential (Figure 4).1261 Hydrosilylation of enones like cydo-hexenone was conducted in two different ways. Under biphasic reaction conditions, an organic solvent like toluene containing the enone and phenyl-dimethyl silane was heated with a fluorocarbon solvent (perfluoromethylcyclohexane) containing a fluorous rhodium catalyst. The hydrosilylation reaction occurred over 10 h, and the reaction mixture was cooled and the phases were separated. Distillation of the residue from the organic phase gave the hydrosilylated products in excellent isolated yields... 

L. V. Dinh, J. Gladysz, Transition Metal Catalysis in Fluorous Media Extension of a New Immobilization Principle to Biphasic and Monophasic Rhodium-Catalyzed Hydrosilylations of Ketones and Enones , Tetrahedron Lett. 1999, 40,8995. 

In total, over the past six years, the chelating P,N-ligands have shown considerable promise in a variety of enantioselective processes, including transfer-hydrogenation and hydrosilylation of ketones, hydroboration of alkenes, conjugate addition to enones and Lewis-acid catalysed Diels-Alder reactions, in addition to those described above.128,341 It is anticipated that this list will continue to grow, and... 

Titanium-catalyzed cyclization/hydrosilylation of 6-hepten-2-one was proposed to occur via / -migratory insertion of the G=G bond into the titanium-carbon bond of the 77 -ketone olefin complex c/iatr-lj to form titanacycle cis-ll] (Scheme 16). cr-Bond metathesis of the Ti-O bond of cis- iij with the Si-H bond of the silane followed by G-H reductive elimination would release the silylated cyclopentanol and regenerate the Ti(0) catalyst. Under stoichiometric conditions, each of the steps that converts the enone to the titanacycle is reversible, leading to selective formation of the more stable m-fused metallacycle." For this reason, the diastereoselective cyclization of 6-hepten-2-one under catalytic conditions was proposed to occur via non-selective, reversible formation of 77 -ketotitanium olefin complexes chair-1) and boat-1), followed by preferential cyclization of chair-1) to form cis-11) (Scheme 16). 

Growe and co-workers independently reported a procedure for the cyclization/hydrosilylation of enones and enals similar to that reported by Buchwald." As an example, reaction of a 1 1 mixture of 3,3-dimethyl-5-hexenal and triethoxysilane catalyzed by Gp2Ti(PMe3)2 (20 mol%) in pentane at room temperature for 3 h gave m-l,l,3-trimethyl-4-triethoxysilyloxycyclopentane in 88% yield as a single diastereomer (Equation (38)). 

Hydrosilylation of various carbonyl compounds, enones and related functional groups catalyzed by Group VIII transition metal complexes, especially phosphine-rhodium complexes, have been extensively studied1,3, and the reactions continue to serve as useful methods in organic syntheses. 

Hydrosilylation of carbonyl compounds. The definitive report on reduction of carbonyl compounds by hydrosilylation catalyzed by Wilkinson"scatalyst is available.1 Hydrosilylation can be used to effect regioselective 1,2- or 1,4-reduction of a,/ -enals or -enones by proper choice of the hydrosilane. In general, monohydrosilanes favor 1,4-adducts, whereas dihydrosilanes favor 1,2-adducts. The regioselectivity is also influenced by the substituents on silicon and on the substrates. The presence of a phenyl group on the enone system can effect dramatic changes in the selectivity. Examples ... 

CH3P(C6H5)2. The reaction involves hydrosilylation of the enone to furnish an intermediate enol ether (a), which then undergoes coupling with an aldehyde. 

Metal/carbene complexes (30-34) have also proved fruitful in the hydrosilylation of alkynes, olefins, enones, and ketones. Lappert and Maskell showed that ketones and alkynes could be reduced to silyl ethers and vinylsilanes, respectively, using complexes 30 and 31 (Eqs. 33 and 35) [75]. The yields tended to be high (92 and 98%, respectively), but the reactions needed to be rim neat. Poor selectivity was observed in the hydrosilylation of alkynes producing a mixture of the -trans, (i-c/ s, and a products. 

Rhodium complexes of various types, e.g. IRhCI(PPh3)3], [RhCl(CO)(PPh,3)2], [lRhCl(CO)2 2l, [RhH(CO)(PPh3)3], [(RhCl(CgHi4)212] and [ RhCl(C2H4)2 2], are available for the hydrosilylation of alkenes and alkynes as well as enones and ketones. Under strictly deoxygenated conditions with the pure rhodium complex, the reaction is extremely slow, and a trace amount of oxygen or peroxide is necessary to activate the catalyst. ... 

These fluorous Rh catalysts are also effective in the hydrosilylation of enones, which employs a similar binary solvent system (Scheme 6). Hexanes and ether can be used in place of toluene. Since these organic solvents are miscible with PFMC at room temperature, effective phase separation requires cooling to -30°C. 

Representative procedure for the asymmetric hydrosilylation of cyclic enones isophorone (S/L = l,000 lfr ... 

In addition to molecules containing prochiral sites, racemic molecules can participate in catalytic asymmetric transformations. In some transformations, the stereocenter is destroyed in the course of the reaction, and equilibrating prochiral intermediates are formed. An example of such a process is the asymmetric arylation of ketones (Figure 14.8C). In other cases, one of the enantiomers of the substrate reacts with the asymmetric catalyst significantly faster than the other enantiomer. In this case, an enantioenriched product is formed, and the opposite enantiomer of the reactant remains. This last process is called a kinetic resolution and is illustrated for the conjugate reduction of enones via hydrosilylation (Figure 14.8D). In this case, the top and bottom faces of the C-C double bond are diastereotopic because reaction at each face of the enone generates diastereomeric products. This section of the chapter first presents the principles that relate to reactions at prochiral centers of achiral substrates and then presents the principles that relate to reactions of racemic or meso compounds. 

Hydroxyapatite, a solid, insoluble base of the formula Caio(P04)6(OH)2 is a very efficient catalyst for the hydrosilylation of aldehydes, ketones, esters and enones by the usually ineffective reductant triethoxysilane [109]. 

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