Pentacoordinate hydridosilicate

An analogous pentacoordinate hydridosilicate reagent (91), prepared from trichlorosilane and catechol or 2,2 -dihydroxybiphenyl with butyllithium (equation 23), reduced aldehydes and ketones in THF under mild conditions in high yield to the corresponding alcohols without any added catalyst (equation 24). ... 

ABSTRACT. Pentacoordinate organosilicon species behave as Lewis acid as well as nucleophile. This unique character provides several interesting applications in selective reactions useful for organic synthesis. Accordingly, new reduction of carbonyl compounds with pentacoordinate hydridosilicates, new regio- and enantioselective allylation of aldehydes with pentacoordinate allylsilanes, and the new cleavage reaction of C-Si bonds are discussed. In these reactions, it is demonstrated that unique characters of pentacoordinate silicon species as a Lewis acid play an important role. 

Reduction of Carbonyl Compounds with Pentacoordinate Hydridosilicates... 

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. 

From these data, we have proposed a mechanism shown below, which involves coordination of carbonyl oxygen to the Lewis acidic silicon center followed by hydride transfer. Combination of the Lewis acidity and the ability of hydride transfer in pentacoordinate hydridosilicates opens a new versatile reducing systems. 

The author discussed an interesting new reduction of carbonyl compounds with pentacoordinate hydridosilicates in the previous section. However, anionic hydridosilicates have never been characterized until now. Many attempts to isolate bis(l,2-benzenediolato)hydridos icates by our hands have also failed. However, very recently, we have succeeded the first isolation of the bis(triphenylphosporanylidene)ammonium (PPN) salt of the hydridosilicates by low temperature manipulation. ... 

Whereas further purification of 6 was unsuccessful because of the lability in air and moisture, the crystals gave the satisfactory spectroscopic data to indicate the structure. Although the Si-H coupling constants for anionic pentacoordinate hydridosilicates have never been reported, the value ( Jsi.nl = 253.5 Hz) is quite reasonable in comparison with the value reported for the (tetrafluoroethyl)silane-trimethylamine complex (246 Hz), indicative of the equatorial location of the hydrido ligand which is bonded to the sp hybridized orbital on silicon. 

Table II. Reduction of Carbonyl Compounds by Pentacoordinate Hydridosilicates 5 and 6...

The similar method to the synthesis of pentacoordinate hydridosilicates was applied to the preparation of the lithium salts of pentacoordinate allylsilicates (7 and 8). 

HSi(OEt)3 is easily converted into a more reactive form, namely a pentacoordinate hydridosilicate (eq 7). ... 

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... 

Usual aldehydes and ketones were reduced very smoothly under mild conditions by 5. The reactivity of 5 was similar to that of bis(l,2-benzenediolato)hydridosilicates. Whereas Corriu et al. have recently reported the high reactivity of a neutral pentacoordinate hydrosilane towards carbonyl compounds, the anionic hydridosilicates should be much more activated as reducing reagents than the neutral analogs. 

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