Base catalyst, hydrosilylation with

Ojima reported on the reduction of R R C=NR type Schiff bases by hydrosilylation with several catalysts. The reaction was also affected by the quality of hydrosilane. Using dihydrosilanes, which were found to be more active than monohydrosilanes and trihydrosilanes, the activity of the catalysts used decreased in the order RhCl(PPh3)3 Rh(CO)Cl-(PPh3)2 > Py2RhCl(dmf)BH4 > [Rh(HD)Cl]2 > [Rh(COD)Cl]2 > PdCb > PdCl2(PPh3)2. 

Thermal cure system. The thermal cure system is based on a hydrosilylation addition reaction between vinyl-functionalized and silicon-hydrido functionalized polysiloxanes [32,33,35], Unsaturated organic groups react with a Si-H functionality in the presence of a platinum-based catalyst (Scheme 10). 

A single example of the reductive cyclization of allenic carbonyl compounds is reported, which employs a rhodium-based catalyst in conjunction with Et3SiH as terminal reductant.113 This protocol promotes hydrosilylation-cyclization to form both five- and six-membered rings with exceptional levels of yy -diastereocontrol. As revealed... 

A hydrosilylation/cyclization process forming a vinylsilane product need not begin with a diyne, and other unsaturation has been examined in a similar reaction. Alkynyl olefins and dienes have been employed,97 and since unlike diynes, enyne substrates generally produce a chiral center, these substrates have recently proved amenable to asymmetric synthesis (Scheme 27). The BINAP-based catalyst employed in the diyne work did not function in enyne systems, but the close relative 6,6 -dimethylbiphenyl-2,2 -diyl-bis(diphenylphosphine) (BIPHEMP) afforded modest yields of enantio-enriched methylene cyclopentane products.104 Other reported catalysts for silylative cyclization include cationic palladium complexes.105 10511 A report has also appeared employing cobalt-rhodium nanoparticles for a similar reaction to produce racemic product.46... 

Iridium complexes are known to be generally less active in hydrosilylation reactions when compared to rhodium derivatives, although iridium-based catalysts with bonded chiral carbene ligands have been used successfully in the synthesis of chiral alcohols and amines via hydrosilylation/protodesilylation of ketones [46-52] and imines [53-55], The iridium-catalyzed reaction of acetophenone derivatives with organosubstituted silanes often gives two products (Equation 14.3) ... 

Hydroboration of a variety of alkenes and terminal alkynes with catecholborane in the fluorous solvent perfluoromethylcyclohexane was performed using fluorous analogs of the Wilkinson catalyst.135 136 Recycling of a rhodium-based alkene hydrosilylation catalyst was also successful.137 Activated aromatics and naphthalene showed satisfactory reactivity in Friedel-Crafts acylation with acid anhydrides in the presence of Yb tris(perfluoroalkanesulfonyl)methide catalysts.138... 

Since 1957 and the discovery of the Speir s catalyst H2PtCl6/ PrOH, considerable efforts have been made to find new catalysts with high activity and selectivity. Along with the platinum-based catalysts, the Wilkinson s complex [103] Rh(Ph3P)3Cl is one of the most popular hydrosilylation catalysts. Ruthenium catalysts are also able to promote the addition of silanes to unsaturated carbon-carbon bonds, and several reports have shown during the past decade that the well-defined ruthenium complexes of type Ru(H)(Cl)(CO)L can provide excellent activity and selectivity [104—... 

Although 187-189 were not active catalysts for polymerization process, 187 and 189 proved to be active olefin hydrosilylation catalysts, presumably 187 first reacted with a silane to form a reactive metal hydride species. They are the first examples of d° metal complexes with non-Cp ligands in the catalytic hydrosilylation of olefins. The mechanism was believed to be consistent with that of other d° metallocene-based catalysts and included two steps 1) fast olefin insertion into the metal hydride bond and 2) a slow metathesis reaction with the silane. The catalysts exhibited a high regioselective preference for terminal addition in the case of aliphatic olefins... 

Alkyl alkyl ketones have also been enantioselectively hydrosilylated with rhodium catalysts containing phosphorus-based ligands. The results were similar to those from the reactions of 1-phenylethanone3-5. The highest value was 72% ee for the hydrosilylation of 3,3-dimethyl-2-butanone to (S)-3,3-dimethyl-2-butanol with diphenylsilane using a rhodium/Amphos system, based on the optically active aminophosphane ligand Amphos22. 

The synthesis of bifimctional silicone polymers was performed by a step by step hydrosilylation process using a platinum catalyst and the corresponding monomers (Scheme 4). We synthesized new hydrosilylable bisperfluoroalkyl monomers (e.g. monomer a) by trans-esterification of bis-alkyl malonate with perfluoroalcohols using specific Ti(IV)-based catalyst complexes. 

The asymmetric hydrosilylation of synthesized a-acetoxy-y3-enamino esters proceeded smoothly in the presence of a chiral Lewis base catalyst, (282), to provide a wide range of chiral a-acetoxy jS-amino acid derivatives in high yields with good diastereoselectivities and enantioselectivities." ° ... 

Carbonyl compounds can be reduced efficiently in hydrosilylation reactions with an inorganic solid acid or base catalyst present [108, 109]. Iron montmorillonite catalyses hydrosilylation reactions most effectively (e.g. equation 4.21) [108], while sodium montmorillonite is completely inactive. 

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). 

Hydrosilylation of C N bond. Efficient hydrosilylation of a wide variety of aromatic nitriles with HSiMea to give the corresponding JY AT-disilylamines at high yield is catalyzed by [Co2(CO)8] (247). The same complex, when combined with PPha, efficiently catalyzes the hydrosilylation of aliphatic nitriles. Copper-based catalysts [IPrCu(OAc)2], Cu(OAc)2/XANTPHOS, or Cu(OAc)2/DPEPHOS in the presence of stoichiometric amovmts of tert-butanol as an additive catalyze efficient 1,4-hydrosilylation of a, -unsaturated nitriles with PMHS (248). 

Platinum and rhodium catalysts have been the most frequently used catalysts among all the metal-based catalysts for the hydrosilylation of alkenes to date. In particular, a variety of rhodium catalysts have been extensively studied , while the development of other Group Vni transition metal catalysts such as those of palladium and ruthenium continues. In general, the hydrosilylation of an alkene gives the corresponding silylalkanes with varying regioselectivity (equation 1). 

This Chapter is devoted to the development of cobalt-, rhodium- and iridium-based catalysts that contain N-heterocyclic carbene (NHC) ligands. It will cover their most relevant catalytic applications, along with stoichiometric model reactions, except for catalytic oxidations and reductions such as hydrogenations, hydrosilylations and hydroborations, which are treated in detail in Chapters 12 and 13. Since the NHC chemistry of Group 9 metals is one of the most developed areas in this field, this overview will only cover the chemistry of classical NHCs, namely cyclic diaminocarbenes. Chapter 5 reviews the chemistry of the non-classical NHCs, to which the reader is referred. Finally, this Chapter does not pretend to be exhaustive and further details may be found in previous overviews. ... 

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