Ketones using trichlorosilane

Next, a few examples of asymmetric reductions with trichlorosilane are presented. An asymmetric reduction of ketones and imines was reported by Matsumura and coworkers by using trichlorosilane as reductant and A-formyl pyrrolidine derivative 36 as ligand (Scheme 28) [101, 102]. 

Although the reduction of imines has been widely explored, as described above, the reduction of carbonyl groups has been less studied until now. Specifically, the reduction of ketones is more limited due to the low reactivity shown by these compounds. In this field, the pioneering works using trichlorosilane as reducing agent and a chiral Lewis base were reported by Matsumura and co-workers in 1999,... 

A well-defined iron hydride complex FeH(CO)(NO)(Ph3P)2 is highly active as a catalyst for selective hydrosilylation of internal alkynes to vinylsilanes. Depending on the silane employed, either E- or Z-selective hydrosilylation products are formed in excellent yields and good to excellent stereoselectivities. The stereochemical course of this transformation is dependent on the steric demand of the substituents on the silane. " A new 0 family of Lewis-basic 2-pyridyloxazolines catalyses the enantioselective reduction of prochiral aromatic ketones and ketimines using trichlorosilane. 1-Isoquinolyloxazoline (20) derivative was identified as the most efficient catalyst of the series capable of delivering high enantioselectivities in the reduction of both ketones (up to 94% ee) and ketimines (up to 89% (p)... 

A similar reducing system is created by combining dilithium catecholate and trichlorosilane at —78° in tetrahydrofuran. It is speculated that the relatively unstable pentacoordinate bis(l,2-benzenediolato)hydridosilicate (61) is formed in situ and that it is this species that can reduce aldehydes and ketones, but not esters, to alcohols when they are added to the reaction mixture at 0° (Eq. 168).93 In a like manner, the dilithium salt of 2,2/-dihydroxybiphenyl, which forms a pentacoordinate intermediate that is stable enough to react at room temperature, can also be used to promote the reduction reaction. The alkoxides of aliphatic diols... 

METIL TRICLOROSILANO (Spanish) (75-79-6) see methyl trichlorosilane. METIL VINIL CETONA (Spanish) (78-94-4) see methyl vinyl ketone. METOLACHLOR (51218-45-0) CijHjjClNOj Combustible liquid (flash point 374°F/190°C cc). Incompatible with strong acids, nitrates, oxidizers azo and diazo confounds. Moisture may cause slow decomposition. Thermal decomposition releases toxic nitrogen oxides. On small fires, use dry chemical powder (such as Purple-K-Powder), alcohol-resistant foam, or sand extinguishers. 

A clearly innovative catalytic system was reported by Nakajima and co-workers [48], who introduced chiral phosphine oxides as suitable Lewis bases for activating trichlorosilane in stereoselective transformations. Indeed trichlorosilane has been used in the conjugate reduction of a,p-unsaturated ketones in the presence of a catalytic amount of a chiral Lewis base. The reduction of 1,3-diphenylbutenone promoted by catalytic amounts of 2,2 -bis(diphenylphosphanyl)-l,l -binaphthyl dioxide (Cat. 15, BINAPO) at 0°C led to the corresponding saturated compound in 97% yield and a somehow surprising, but very good, 97% ee (Scheme 15.19). 

Reductive amination of a-branched ketones and p-anisidine using Hantzsch ester as a hydride source and chiral Bronsted acid, TRIP, as a catalyst gave chiral p-branched amine (Scheme 5.17) [58]. This catalyst system was extended to three-component Kabachnik-Eields reaction, which uses phosphite as nucleophile instead of hydride, to give p-branched a-amino phosphonate [59]. Reductive ami-nation of p-keto ester or p-keto nitrile with trichlorosilane as a hydride source... 

Ketones (e. g. octanone [159] and cyclohexanone [160]) can also be used as solvents for the chloroplatinic acid. A continuous process for the hydrosilation of olefins with trichlorosilane in the presence of chloroplatinic acid/isopropanol has been patented [161]. 

In 2009, the imidazole-derived Lewis base catalyst 92, which was prepared by Jones et al., was employed for the reduction of ketimines with trichlorosilane as the reducing agent Interestingly, low catalyst loading (as low as 1% mol) works well for this reduction (entry 13, Table 32.1) [57]. During their studies, the authors found that 92 is able to selectively reduce the imine while it is inactive for ketones. Therefore, in 2011, the same group developed an asymmetric reductive amination of ketones 16 catalyzed by 92 with trichlorosilane as the hydride donor (Scheme 32.19). However, the yields for the threealkyl ketimines, a two-step, one-pot procedure plus microwave irradiation is needed to secure a useful synthetic yield [60]. 

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