Nickel complexes hydrosilylation

Nickel complexes formed in situ by the reaction of NiCl.S-COD) with the iini-dazolium salts IMesHCl or IPrHCl in the presence KO Bu catalyse the hydrosilylation of internal or terminal alkynes with EtjSiH. Interestingly, Ni tri-butylphosphine complexes are inactive in this hydrosilylation reaction. The monosilylated addition products were obtained with slow addition rates of the alkyne in the reaction mixture and were formed with variable degree of stereoselectivity, depending on the type of the alkyne, the silane and the ligand on Ni [50],... 

Their advantage over other types of dendrimers is their straightforward synthesis and, most importantly, their chemical and thermal stabilities. Two distinct steps characterize their synthesis a) an alkenylation reaction of a chlorosilane compound with an alkenyl Grignard reagent, and b) a Pt-cata-lyzed hydrosilylation reaction of a peripheral alkenyl moiety with an appropriate hydrosilane species. Scheme 2 shows the synthesis of catalysts Go-1 and Gi-1 via this methodology. In this case, the carbosilane synthesis was followed by the introduction of diamino-bromo-aryl groupings as the precursor for the arylnickel catalysts at the dendrimer periphery. The nickel centers of the so-called NCN-pincer nickel complexes were introduced by multiple oxidative addition reactions with Ni(PPh3)4. 

Phenylacetylene did not form an adduct under similar conditions. The complex between 1,2-bis(dimethylphosphino)-1,2-dicarbaclosododeca-borane with nickel(II) chloride effectively catalyzed the hydrosilylation of olefins (209). Catalysis by this nickel complex differed, however, in that considerable amounts of internal adducts were formed ... 

Nickel exhibits lower catalytic activity than platinum and rhodium catalysts , and in many cases, phosphine-nickel complexes cause the disproportionation of chlorohy-drosilanes giving complex results. However, in some cases, the regioselectivity in the hydrosilylation of styrene with trichlorosilane catalyzed by nickel complexes is quite different from that achieved by platinum or rhodium catalysts. For instance, a-adduct is exclusively formed by the catalysis of [Ni(CO)(ir-C5H5)]2 ... 

Nickel complexes also exhibit a high catalytic activity toward the hydrosilylation of conjugated dienes under mild conditions. The reaction usually occurs in the manner of a 1,4-addition, but the regio- and stereoselectivity is rather low compared with that achieved by palladium catalysts. 

Representative procedure general conditions for nickel complex-mediated hydrosilylation of carbonyl-containing substrates ... 

Numerous complexes of nickel(II) and nickel(O) catalyze the addition of the Si—H bond to olefins. Among such catalysts are nickel-phosphine complexes, such as [Ni(PR3)2X2] (where X = Cl, I, and NO3 R = alkyl and aryl), [Ni(PPh3)4], and [Ni(CO)2(PPh3)2], as well as bidentate complexes [NiCl2-(chelate)], [Ni(acac)2L] (L = phosphine), and [Ni(cod)2(PR3)2l (3,6,10,64). Ni(0)-phosphine complexes were used for the hydrosilylation of alkenes and butadienes (65,66). Cationic nickel complexes, such as [(indenyl)Ni(PPh3)]+ (67) and [Ni( r-allyl)PR2(CH2CH=CH2)]+ (68), were reported as novel effective catalysts of regioselective hydrosilylation of styrene with PhSiHa. 

Regarding the use of well-defined nickel complexes as catalysts for reduction of carbonyl groups, only three examples are described in the literature. In 2009, Guan and coworkers [77] described the efficiency of a nickel PCP-pincer complex performing the hydrosilylation of aldehydes. In the same year, the catalytic hydrosilylation of ketones via a transient Ni-H complex supported by a monoanionic bidentate amidophosphine ligand was reported by Mindiola [78]. Later, Jones investigated well-defined PNP nickel pincer complexes, which catalyzed the hydrosilylation of aldehydes [79] (Fig. 10.16). 

It was observed that Si-H/Si-Cl exchange sometimes occurred in the hydrosilylation of simple terminal olefins using tertiary phosphine-nickel complexes, depending on the electronic nature of the substrate. Nickel complexes containing electron-accepting ligands suppressed this reaction and therefore only traces of interchange products were observed. On the other hand, a tendency for... 

Kumada has extended hydrosilylation by phosphine-stabilized complexes into nickel chemistry (173). The most effective catalyst is (XXX). This... 

Other dichloro(ditertiary phosphine)nickel(II) complexes (see Table VI) catalyze both hydrosilylation and H/Cl exchange, but analogous complexes containing monodentate phosphine ligands or bidentate amine groups are essentially inactive (173). 

Platinum(II) complexes such as [PtCl2(PhMePR)]2 (R = benzyl or propyl) have been used for asymmetric reduction of phenylketones to alcohols with up to 19% ee via the consecutive hydrosilylation-hydrolysis process (Section III,A,4) (2//, 307). A nickel(II) complex with the ben-zylphosphine, and palladium(II) phosphine complexes did not catalyze the hydrosilylation (211). 

Beyond palladium, it has recently been shown that isoelectronic metal complexes based on nickel and platinum are active catalysts for diyne reductive cyclization. While the stoichiometric reaction of nickel(O) complexes with non-conjugated diynes represents a robust area of research,8 only one example of nickel-catalyzed diyne reductive cyclization, which involves the hydrosilylative cyclization of 1,7-diynes to afford 1,2-dialkylidenecyclohexanes appears in the literature.7 The reductive cyclization of unsubstituted 1,7-diyne 53a illustrates the ability of this catalyst system to deliver cyclic Z-vinylsilanes in good yield with excellent control of alkene geometry. Cationic platinum catalysts, generated in situ from (phen)Pt(Me)2 and B(C6F5)3, are also excellent catalysts for highly Z-selective reductive cyclization of 1,6-diynes, as demonstrated by the cyclization of 1,6-diyne 54a.72 The related platinum bis(imine) complex [PhN=C(Me)C(Me)N=Ph]2Pt(Me)2 also catalyzes diyne hydrosilylation-cyclization (Scheme 35).72a... 

Tamao and Ito proposed a mechanism for the nickel-catalyzed cyclization/hydrosilylation of 1,7-diynes initiated by oxidative addition of the silane to an Ni(0) species to form an Ni(ii) silyl hydride complex. Gomplexation of the diyne could then form the nickel(ii) diyne complex la (Scheme 1). Silylmetallation of the less-substituted G=C bond of la, followed by intramolecular / -migratory insertion of the coordinated G=G bond into the Ni-G bond of alkenyl alkyne intermediate Ila, could form dienylnickel hydride intermediate Ilia. Sequential G-H reductive elimination and Si-H oxidative addition would release the silylated dialkylidene cyclohexane and regenerate the silylnickel hydride catalyst (Scheme 1). 

Mori has reported the nickel-catalyzed cyclization/hydrosilylation of dienals to form protected alkenylcycloalk-anols." For example, reaction of 4-benzyloxymethyl-5,7-octadienal 48a and triethylsilane catalyzed by a 1 2 mixture of Ni(GOD)2 and PPhs in toluene at room temperature gave the silyloxycyclopentane 49a in 70% yield with exclusive formation of the m,//7 //i -diastereomer (Scheme 14). In a similar manner, the 6,8-nonadienal 48b underwent nickel-catalyzed reaction to form silyloxycyclohexane 49b in 71% yield with exclusive formation of the // /i ,// /i -diastereomer, and the 7,9-decadienal 48c underwent reaction to form silyloxycycloheptane 49c in 66% yield with undetermined stereochemistry (Scheme 14). On the basis of related stoichiometric experiments, Mori proposed a mechanism for the nickel-catalyzed cyclization/hydrosilylation of dienals involving initial insertion of the diene moiety into the Ni-H bond of a silylnickel hydride complex to form the (7r-allyl)nickel silyl complex li (Scheme 15). Intramolecular carbometallation followed by O-Si reductive elimination and H-Si oxidative addition would release the silyloxycycloalkane with regeneration of the active silylnickel hydride catalyst. 

Migration of organosilicon groups from metal to ligand is believed to be a key step in the mechanism of olefin hydrosilylation catalyzed by nickel-phosphine complexes (59), as shown in Scheme 1. (Substituents are omitted for clarity.)... 

Alkenes. Most Group VIII metals, metal salts, and complexes may be used as catalyst in hydrosilylation of alkenes. Platinum and its derivatives show the highest activity. Rhodium, nickel, and palladium complexes, although less active, may exhibit unique selectivities. The addition is exothermic and it is usually performed without a solvent. Transition-metal complexes with chiral ligands may be employed in asymmetric hydrosilylation 406,422... 

Nickel catalysts exhibit similar activity but are usually less selective than palladium 435-438 In addition to chlorosilanes, however, alkylsilanes also add in the presence of nickel catalysts. Nickel vapor showed an exceptionally high activity and selectivity in hydrosilylation.435 The photocatalytic hydrosilylation of 1,3-dienes with a chromium(O) complex occurs at room temperature to afford regioisomeric 1,4-adducts in quantitative yield.439... 

Dimethyl bis-benzamidinato complex, with Ti(IV), 4, 344 Dimethyl carboranes, with tantalum, 5, 163 Dimethyl complexes with cadmium, 2, 464 with nickel, 8, 58-59 with platinum(II), 8, 460 with titanium(IV), 4, 331, 4, 343 Dimethyl diallylmalonate, cyclization-hydrosilylation, 11, 385 Dimethyl diamido complexes, with Ti(IV), 4, 332... 

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