Silylation, hydrogenative

Recently, the crystal structure of a nickel(II) complex with a tridentate silyl ligand has been reported [20]. The structure in the solid state shows an //2-(Si-H) binding to nickel, with a Ni-H distance of 1.47 A NMR spectra of the complex in solution at -80 °C suggest the formation of a nickel(IV) hydride species through oxidative addition of the silyl-hydrogen to nickel [20]. 

The reaction of triethoxyvinylsilane, CH2=CHSi(OEt)3, with HSi(OEt)3 catalyzed by Ni(acac)2 gives a complex mixture of products, arising from dehydrogenative silylation, hydrogenation, disproportionation, and dimerization besides the normal hydrosilylation product, (EtO)3SiCH2CH2Si(OEt)356. The dimerization product is a mixture of (EtO)3Si(CH2)4Si(OEt)3 and (EtO)3SiCH(Me)(CH2)2Si(OEt)3. A possible mechanism that can accommodate the formation of these products is proposed, which includes key intermediates such as (EtO)3Si(CH2)2Ni—CH[Si(OEt)3]CH2Si(OEt)3 and Ni[(CH2)2Si(OEt)3]56. 

Hydrosilanes, RsSiH, can be viewed as silylated hydrogen and used as a reducing reagent. Hydrosilation (equation 20) resembles hydrogenation in this sense. Reduction of prochiral ketones through hydrosilation has been investigated extensively with platinum and rhodium complexes having... 

Thermally stable tris(trimethylsilyl)silyl hydrogen telluride, (Me>,Si)>,SiTeH, is isolated according to Scheme 15.17. On heahng at 130°C for 1 day in hydrocarbon soluhon (Me jSi) jSi l( l I is stable without decomposihon it readily decomposes on irradiahon with sunlight, however, forming tellurium mirror [53]. 

Dolgov, O.N., and Voronkov, M.G., Telomerization of dialkyl hydrogen phosphites and hw(trialkyl-silyl) hydrogen phosphites with vinylsilane derivatives, Zh. Obshch. Khim., 40, 1668, 1970 J. Gen. Chem. USSR (Engl. Transl.), 40, 1660, 1970. 

Fig. 10.22 Comparison of the INS spectrum of (a) PDMS and that of a model oligomer (b) H3Si-[0-Si(CH3)2]3-0-SiH3 calculated by DFT. The silyl hydrogens are assigned zero cross section and a constant bandwidth of 7 cm" is used.

Catalytic reduction of secondary functionaUties in sulfonates, in which the sulfonate moiety is unchanged, is accompHshed using standard hydrogenation techniques (21). Sulfonic acids may be converted to the corresponding silyl esters in very high yields (22). 

Trimethylsilyloxazolidinone. This reagent can be used to silylate most active hydrogen compounds. 

Silylated cyanohydrins have also been prepared via silylation of cyanohydrins themselves and by the addition of hydrogen cyanide to silyl enol ethers. Silylated cyanohydrins have proved to be quite useful in a variety of synthetic transformations, including the regiospecific protection of p-quinones, as intermediates in an efficient synthesis of a-aminomethyl alcohols, and for the preparation of ketone cyanohydrins themselves.The silylated cyanohydrins of heteroaromatic aldehydes have found extensive use as... 

Selective fluonnation in polar solvents has proved commercially successful in the synthesis of 5 fluorouracil and its pyrimidine relatives, an extensive subject that will be discussed in another section Selective fluonnation of enolates [47], enols [48], and silyl enol ethers [49] resulted in preparation of a/phn-fluoro ketones, fieto-diketones, heta-ketoesters, and aldehydes The reactions of fluorine with these functionalities is most probably an addition to the ene followed by elimination of fluonde ion or hydrogen fluoride rather than a simple substitution In a similar vein, selective fluonnation of pyridmes to give 2-fluoropyridines was shown to proceed through pyridine difluondes [50]... 

Systems usually fluonnated by electropositive fluorine reagents include acti-vated alkenes (enol ethers, enol acetates, silyl enol ethers, and enamines), activated aromatic systems, certain slightly activated carbon-hydrogen bonds, and selected organometallics. 

The reaction of silylated phosphorus(V) itnides with anhydrous hydrogen fluoride gives high yields of alkylenebis(ditluorophosphoranes) [Sf] (equation 19). 

Hydrogenation of the product 147 removes the benzyl protecting groups and at the same time reduces the triazine to its dihydro derivative 148. A roundabout scheme is required for dehydrogenation due to the sensitivity of the intermediates. The product is thus converted to its silyl ether 149 exposure to air results in oxidation and desilylation. There is thus obtained the antineoplastic agent fazarabine (150), also known as ara-A C. 

Merck s thienamycin synthesis commences with mono (V-silylation of dibenzyl aspartate (13, Scheme 2), the bis(benzyl) ester of aspartic acid (12). Thus, treatment of a cooled (0°C) solution of 13 in ether with trimethylsilyl chloride and triethylamine, followed by filtration to remove the triethylamine hydrochloride by-product, provides 11. When 11 is exposed to the action of one equivalent of tm-butylmagnesium chloride, the active hydrogen attached to nitrogen is removed, and the resultant anion spontaneously condenses with the electrophilic ester carbonyl four atoms away. After hydrolysis of the reaction mixture with 2 n HC1 saturated with ammonium chloride, enantiomerically pure azetidinone ester 10 is formed in 65-70% yield from 13. Although it is conceivable that... 

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