Ketones with diphenylsilane

New N-heterocyclic carbene rhodium and iridium complexes derived from 2,2 -diaminobiphenyl were successfully synthesized and their structures unambiguously characterized by X-ray diffraction (XRD) analysis. These are cata-lytically active for the hydrosilylation of ketones with diphenylsilane, although an NHC—rhodium complex was found to be the best among those investigated [45]. 

Enantioselective hydrosilylation of ketones.1 The complex formed from RhCl3 and la when activated by a silver salt, AgBF4 or AgOTf, is an effective catalyst for enantioselective hydrosilylation of ketones with diphenylsilane to provide, after acidic hydrolysis, (S)-secondary alcohols. In all cases, addition of free ligand (4-6 mole %) improves the enantioselectivity markedly. The highest enantioselectivity... 

The optically active (A)-(+)- and (i )-(—)-2,2 -bis[diarylstibano]-l,l -binaphthyls (BINASb) have been prepared and used as chiral auxiliaries in the Rh-catalyzed asymmetric hydrosilylation of ketones with diphenylsilane.49,49a When acetophenone is reduced using 0.25 mol% of [Rh(COD)Cl]2 as the catalyst and 0.5 mol% of (i )-BINASb (aryl = />-tolyl) as the ligand, (i )-l-phenylethanol is formed in 78% yield and in 32% ee (Equation (15)). When (i )-BINAP is used as the chiral ligand instead of (i )-BINASb, the yield and enantiomeric excess of (7 )-l-phenylethanol are 42% and 0.6%, respectively. 

The rhodium(I) complex was found to be a moderately effective catalyst for the hydrosi-lylation of ketones with diphenylsilane. 

It was described earlier that the diselenide 2 acted as chiral ligand for the Rh(I)-catalyzed asymmetric hydrosilylation of unfunctionalized alkyl aryl ketones with diphenylsilane in tetrahydrofuran [6]. When the reaction was carried out in methanol as solvent, it gave directly a chiral alcohol, not a hydro-silylated product [7] (Scheme 5). 

The Cl symmetrical bis(oxazolinylpyridine), PYBOX (90), also serves as an excellent ligand. Treatment of (90) with [RhCh(H20)3] in ethanol affords the complex (95), which has been characterized by single crystal X-ray analysis. Hydrosilylation of ketones with diphenylsilane (1.5 equiv.) in the presence of AgBP4 (0.2 equiv.), (90) (O.CKS equiv.) and (95) (0.01 equiv.) at 0 °C gives (5)-alcohols of high enanti-... 

The monodentate phosphonite ligands L18a-c (Fig. 8) modified by TADDOL ligands, which were derived from tartarates, were examined to show relatively high values of % ee 62-87% ees for acetophenone and 2-naphthyl methyl ketone with diphenylsilane [38]. 2-Naphthyl methyl ketone (K9) was reduced in 92% yield with 87% ee (R) by using a large excess of L18c (10-fold based on Rh) and [Rh(COD)Cl]2. 

New chiral oxazolinylcarbene-rhodium complexes 149 proved to be efficient catalysts for asymmetric hydrosilylation of dialkyl ketones with diphenylsilane (77-95% ee) <04AG(E)1014>. 

In 2007, Tsuji and co-workers showed NHC-Rh complexes 107 with dendrimer substituents were more active than Rh analogues in the hydrosilylation of 2-cyclohexenone (Figure 13.18). Furthermore, 107 gave the 1,4-adduct as major product, whereas Rh complexes 100 led preferentially to the 1,2-adduct instead. Alternatively, related complex 108 was fairly elfective for the hydrosilylation of ketones with diphenylsilane to give the corresponding alcohols in moderate to good yields (67-93... 

The combination of [Rh(Cl(NBD)]2 and ligands 89, 90, 91, or 92 with diphenylsilane asymmetrically reduces aryl alkyl ketones, including acetophenones, in excellent yields and in 81 to 90% ee (Eq. 346).574 The best results are with ferrocene 91 and acetophenone in toluene.575 Other phosphine-substituted ferrocenes do not give comparable results. Rhodium(I) complexes of TADDOL-derived... 

A cyclic ketone, 1-tetralone, was reduced with diphenylsilane in the presence of the Rh/(S,R)-2 complex (substrate Rh ligand=50 l 2) in toluene at room temperature followed by hydrolysis to give the S alcohol in 92% optical yield (Scheme 2) [8]. Use of the Rh/4 complex (substrate Rh ligand=100 l 1.2) at 0 °C yielded the S product in 80% optical yield [17]. The reduction of 4-chromanone with the same complex gave an optical yield of 87%. 

Enantioselective reduction of simple aliphatic ketones is one of the most challenging of the currently unresolved problems in this field. The Rh/4 complex catalyzed the hydrosilylation of 2-butanone with diphenylsilane at 0 °C, which after hydrolysis gave (S)-2-butanol in 56% ee (Scheme 3) [17]. 2-Octanone and 4-phenyl-2-butanone were reduced with diphenylsilane in the presence of the cationic Rh/EtTRAP-H at -50 °C and gave optical yields of 77% and 81%, respectively [12], The cationic Rh/(R,R)-t-Bu-MiniPHOS was also effective for the reduction of 4-phenyl-2-butanone with 1-naphthylphenylsilane at -20 °C, affording the R product in 80% ee [13]. 3-Methyl-2-butanone was reduced using the Rh/4 complex with 76% optical yield [17]. Hydrosilylation of cyclohexyl methyl ketone with the Rh/(R,S)-2 complex followed by hydrolysis afforded the R alcohol in 87% ee [8]. Highly enantioselective hydrosilylation of pinacolone with diphenylsilane at -20 °C was achieved by means of the Rh/4 complex and yielded the desired R product in 95% ee [17]. 

The reduction of a,p-unsaturated ketones is equally fast and remarkably selective for 1,2-addition of the silane. A variety of both cyclic and acyclic a,p-unsaturated ketones were reduced with diphenylsilane under the standard conditions with yields ranging from 66-99% for 1,2-addition. Cyclic enones were reduced with a higher degree of regioselectivity than the acyclic enone, mesityl oxide. 

Table 1 Hydrosilylative Reduction of Methyl Ketones with [Pybox-(5,5)-ip]RhCl3 and Diphenylsilane...

The catalytic hydrosilylation of acetophenone or t-butyl methyl ketone with diethyl-, methylphenyl- or diphenylsilane in the presence of rhodium(I) catalysts containing (R,i )-(-l-)- or (S,S)-(—)-170, followed by acid cleavage of the intermediate silyl ethers, affords the respective alcohols with optical yields of 10—42% . The synthesis of (ff)-(-l-)-l-phenylethanol from acetophenone and diethylsilane in conjunction with the catalyst derived from (S,S)-( —)-170 was the most effective reaction (equation 28). In... 

Aminomethyl)silanes (E) can be metallated by lithiumalkyls and characterized by trapping with disulfides (Scheme 3). The (phenylthiomethyl)(aminomethyl)silanes G can be metallated again in toluene or THF In reactions of (lithiomethyl)(aminomethyl)diphenylsilanes with aldehydes or ketones we observed no clear addition/deprotonation profile. The (lithiomethyl)silanes H add to ketones with de up... 

The carbon-nitrogen double bonds of nitrones N1-N3 (Fig. 14) were catalytical-ly reduced with diphenylsilane in the presence of Ru2Cl4(Tol-BINAP, L24)2(NEt3) to give hydroxylamines in high % ees [56]. The hydroxylamine HI was obtained in 63% yield with 86% ee (S) and the hydroxylamine H3 was formed in 91% ee. It was also proposed that this process opened a new access to optically active amines from racemic amines, via nitrones and hydroxylamines. The iron complex [(Cp)2Fe2(HPMen2> L25)(CO)2] was reported to be a catalyst in the asymmetric hydrosilylation of ketones under irradiation, where acetophenone was reduced in up to 33% ee [57]. 

In a side reaction enolizable ketones may give rise to varying amounts of silyl enol ethers. On hydrolysis, these silyl enol ethers are reconverted into the starting material. Thus, even on complete hydrosilylation, product formation is not quantitative and the secondary alcohol is accompanied by the ketone to the extent in which the silyl enol ether was formed. Thus, the hydrosilylation of 1-phenylethanone and its enol form with diphenylsilane affords the corresponding silyl ether and silyl enol ether and subsequent hydrolysis gives 1-phenylethanol and 1-phenylethanone13. 0 0H... 

Alkyl alkyl ketones have also been enantioselectively hydrosilylated with rhodium catalysts containing phosphorus-based ligands. The results were similar to those from the reactions of 1-phenylethanone3-5. The highest value was 72% ee for the hydrosilylation of 3,3-dimethyl-2-butanone to (S)-3,3-dimethyl-2-butanol with diphenylsilane using a rhodium/Amphos system, based on the optically active aminophosphane ligand Amphos22. 

Enones can be reduced to the saturated ketones with triethylsilane and Wilkinson s catalysis62 (equation 54). Interestingly, the same product was prepared via a Tiffeneau-Demjanov ring expansion wherein trimethylsilyl cyanide was used (equation 55). The selective reduction of the double bond of enones can also be carried out with diphenylsilane in the presence of palladium(O) or palladium(II) and zinc chloride63 (equation 56), or more effectively with phenylsilane and molybdenum hexacarbonyl64 (equation 57). This latter reagent was also used to reduce the double bond of a,/ -unsaturated esters, amides and nitriles. 

LFP studies of the N-H insertion of silylenes have shown that the reaction proceeds by formation of an acid - base complex at almost a diffusion rate with a noticeable dependence of the steric hindrance of the amine. Rhodium-catalysed hydrosilyation of ketones by diphenylsilane has been found to occur via the intermediacy of the rhodium silylene (158). ... 

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