Silicon hydrides carbonyl compounds

It is well known that strong electrophiles such as carbocations are reduced by organosilicon hydrides (Eq. 1).3,70,71 On the other hand, simple mixtures of organosilicon hydrides and compounds with weakly electrophilic carbon centers such as ketones and aldehydes are normally unreactive unless the electrophilicity of the carbon center is enhanced by complexation of the carbonyl oxygen with Brpnsted acids3,70 73 or certain Lewis acids (Eq. 2).1,70,71,74,75 Using these acids, hydride transfer from the silicon center to carbon may then occur to give either alcohol-related or hydrocarbon products. 

Alternatively, unreactive mixtures of organosilicon hydrides and carbonyl compounds react by hydride transfer from the silicon center to the carbon center when certain nucleophilic species with a high affinity for silicon are added to the mixture.76 94 This outcome likely results from the formation of valence-expanded, pentacoordinate hydrosilanide anion reaction intermediates that have stronger hydride-donating capabilities than their tetravalent precursors (Eq. 6).22,95 101... 

Rhodium(i) complexes are excellent catalysts for the 1,4-addition of aryl- or 1-alkenylboron, -silicon, and -tin compounds to a,/3-unsaturated carbonyl compounds. In contrast, there are few reports on the palladium(n) complex-catalyzed 1,4-addition to enones126,126a for the easy formation of C-bound enolate, which will result in /3-hydride elimination product of Heck reaction. Previously, Cacchi et al. described the palladium(n)-catalyzed Michael addition of ArHgCl or SnAr4 to enones in acidic water.127 Recently, Miyaura and co-workers reported the 1,4-addition of arylboronic acids and boroxines to a,/3-unsaturated carbonyl compounds. A cationic palladium(n) complex [Pd(dppe)(PhCN)2](SbF6)2 was found to be an excellent catalyst for this reaction (dppe = l,2-bis(diphenyl-phosphine)ethane Scheme 42).128... 

Most examples in the literature on hydrosilylation with iron complexes as catalyst concern Fe(CO)5 or related iron carbonyl compounds [41]. The first use of iron pentacarbonyl was reported for the reaction of silicon hydrides with alkenes at 100-140 °C to form saturated and unsaturated silanes according to Scheme 4.20 [42, 43]. 

Silicon hydrides are an interesting class of carbonyl reducing agents as they are reasonably stable under normal conditions requiring activation with a transition metal complex,fluoride ion or Lewis acid. The correct choice of reaction conditions allows highly chemo- and stereo-selective reduction of particular classes of carbonyl compounds with these convenient reagents. 

Silicon being more electropositive than hydrogen (Pauling electronegativities Si 1.90, H 2.20), silanes with Si-H bonds can reduce a variety of functional groups such as carbonyl compounds, imines, and even alcohols by transferring a hydride anion. Triethylsilane is a common reagent, and typical reaction conditions also include either a Brpnsted or a Lewis acid, as shown below for the reduction of a ketone ... 

Transition metal-free hydrosilylation of carbonyl compounds can be realized with the use of Brpnsted or Lewis acids as well as Lewis bases. Alkali or ammonium fiuorides (CsF, KF, TBAF, and TSAF) are highly effective catalysts for the reduction of aldehydes, ketones, esters, and carboxylic acids with H2SiPh2 or PMHS. Lithium methoxide promotes reduction of esters and ketones with trimethoxysilane. A generally accepted mechanism of Lewis base-catalyzed hydrosilylation of carbonyl compovmds involves the coordination of the nucleophile to the silicon atom to give a more reactive pentacoordinate species that is attacked by the carbonyl compound giving hexacoordinate silicon intermediates (or transition states), in which the hydride transfer takes place (Scheme 30) (235). 

On the other hand, trialkylhydrosilanes are capable of reducing carbonyl compounds and some activated olefins in the presence of Brc nsted or Lewis acids. Apparently, coordination of carbonyl oxygen to acid is important to activate electrophilic carbons. We thought that pentacoordinate hydridosilicates should be a good hydride transfer reagent because of the intrinsic nucleophilicity, while the significant Lewis acid character of the silicon center should activate the substrate carbonyl compounds. Reduction of carbonyl compounds with pentacoordinate hydridosilicates are expected to proceed without any additives and thus very interesting from both mechanistic and practical point of view. 

Peterson Alkenation. Trimethylsilylmethylmagnesium chloride (1) reacts with carbonyl compounds to give /3-hydroxysilanes (2).2>4,s silanes can then be eliminated to provide an alkene under acidic or basic conditions, such as with sodium hydride or potassium hydride (eq l).ia>ib,s,6 -pj g giinunation can also be accomplished by acetyl chloride or thionyl chloride. For the introduction of ex o-methylene groups, reagent (1) has been found to be superior to a Wittig approach the silicon reagent reacts rapidly and the byproduct is simple to remove. ... 

Silicon and germanium hydrides react with cobalt, manganese and rhenium carbonyls affording complexes having a silicon (or germanium)-metal bond. These reactions, described previously for inactive compounds have been used in the synthesis of optically active silyl and germyl-transition metals ... 

The relative reactivities of the carbonyls M3(CO)i2, where M = Os, Ru, or Fe, towards trialkylsilicon or trialkyltin hydrides can be related to the relative strengths of the respective metal-silicon or metal-tin bonds, The compounds Ma(PF8)8, where M = Rh or Ir, react with silanes and germanes RH, where R = Ph3Si, (EtO)3Si, ClaSi, or PhaGe, according to the equations ... 

---