Amides silicon hydrides

Combination of silicon hydrides with catalytic amounts of a ruthenium(II) complex in tetrahydrofuran, chloroform or benzene has afforded a new reducing system capable of efficient reduction of a,p-unsatu-rated carboxylic acids, esters, amides, etc. Addition of a weak proton source, such as a sterically hindered phenol significantly increases reaction rates. The ruthenium mixture was found to exhibit the same regioselectivity observed with the above-described palladium systems. 

Liquid injection molding, for silicone rubbers, 3, 674—675 Liquid ligands, in metal vapor synthesis, 1, 229 Liquid-phase catalysis, supported, for green olefin hydroformylation, 12, 855 Lithiacarbaboranes, preparation, 3, 114 Lithiation, arene chromium tricarbonyls, 5, 236 Lithium aluminum amides, reactions, 3, 282 Lithium aluminum hydride, for alcohol reductions, 3, 279 Lithium borohydride, in hydroborations, 9, 158 Lithium gallium hydride, in reduction reactions, 9, 738 Lithium indium hydride, in carbonyl reductions, 9, 713—714... 

A further method for the synthesis of the title compounds with only hydrogen as byproduct is the base-catalyzed dehydrogenative coupling (index D) of ammonia and tris(hydridosilylethyl)boranes, B[C2H4Si(R)H2]3 (R = H, CH3). Initially, the strong base, e.g. n-butyl lithium, deprotonates ammonia. The highly nucleophilic amide replaces a silicon-bonded hydride to form a silylamine and lithium hydride, which then deprotonates ammonia, resuming the catalytic cycle. Under the conditions used, silylamines are not stable and by elimination of ammonia, polysilazane frameworks form. In addition, compounds B[C2l-L Si(R)H2]3 can be obtained from vinylsilanes, H2C=CHSi(R)H2 (R - H, CH3), and borane dimethylsulfide. 

AMIDES N-Ethoxycarbonyl-2-ethoxy-l,2-dihydroquinoline (EEDQ). Silicon tetrachloride. Sodium hydride-Dimethylsulf-oxide. Triphenyl phosphite. 

SCHEME 18.30 Proposed mechanism of the n-BuLi catalyzed dehydrocoupling of tris(hydridosilyl-ethylene)boranes and ammonia. (1) deprotonation with formation of an amide, (2) substitution of silicon-bonded hydride with amide, (3) polymerization through condensation. 

It was observed that ammonolysis of B(C2H,Si(R)H2)3 (Scheme 2, route A) requires basic catalysts such as n-butyl lithium. The reaction is performed in analogy to the potassium hydride-catalyzed cross-linking of cyclic silazanes described by Seyferth et al. [8]. Most probably, n-BuLi initially deprotonates the weak nucleophile ammonia with the formation of lithiiun amide and evaporation of n-butane. The stronger nucleophilic amide then replaces a silicon-bonded hydride, which subsequently deprotonates ammonia, leading to the evolution of molecular hydrogen. The silylamines that arise are not stable under the reaction conditions applied (refluxing solvent), and by fast condensation of ammonia the polymeric precursors form [6]. 

Acidic transition-metal hydrides have been used to prepare metal-silicon bonded compounds by an interesting condensation reaction that proceeds with amine elimination (equation 30)66. An attempt to carry out a similar reaction between Ph3SiH and the zirconium amide Cp2Zr(NMe2)2 failed67. 

---