Reductive silylation complexes

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. 

The reaction mechanism commonly accepted to account for the double silylation of unsaturated substrates involves three key steps. First, the disli-lane undergoes oxidative addition to the metal center, forming a transition metal-bis(silyl) complex. The unsaturated moiety inserts into the metal-silyl bond, followed by Si-C reductive elimination to give the double sily-... 

An alternative mechanism for catalysis of Si-Si bond formation by later transition metal complexes was originally proposed by Curtis and Epstein.33 This mechanism, illustrated in Scheme 3, also invokes a sequence of oxidative addition/reductive elimination steps but does not resort to a silylene intermediate. A serious weakness in this scheme is that, as far as we know, no example of the spontaneous reductive elimination of a disilane from a bis(silyl) complex has yet been observed. 

Organometallic complexes of PdIV were almost unknown until a few years ago. They are however accessible by the oxidative addition of Mel or PhCH2Br to palladium dialkyl complexes stabilized by nitrogen ligands such as bipy. Most are thermally sensitive and reductively eliminate R—Me at room temperature.49 The first PdIV silyl complex was obtained in a clean methyl exchange reaction between (dmpe)PdMe2 and l -QH SiFI the compound is thermally remarkably stable.50... 

Palladium-catalyzed reductive silylation has also been reported. The cathodic reduction of allylic acetates in the presence of silylating agents and a catalytic amount of (Ph3P)4Pd [53] gives the corresponding allylsilanes. The initially formed rr-allyl-palladium(II) complex seems to be reduced at the cathode to generate the allyl anion intermediate, which reacts with chlorosilane to give the final product. 

Keywords metal silyl complexes, oxidative addition, reductive elimination... 

These reactivities encouraged the investigation of reduction into NH3 and spectacular results were obtained with triphosphine-borane Fe complexes. Various V-bound Fe complexes 65 relevant to the catalytic reduction of into NH were prepared. Most significant are the protonation of the [FeJ-NH complex into [Fe]-NHj and the reductive displacement of NH by N, which are the toal steps proposed to account for the fixation/reduction of at Fe (Scheme 25). This stoichiometric reaction sequence was adapted into a catalytic transformation. Treatment of the anionic complex 65d with excess and KCj at -78°C results in NHj production. Under these conditions, up to 7 NH3 molecules were generated per Fe complex and more than 40% of the furnished protons were delivered to Nj. The borane moiety plays a major role in this process, being capable of accommodating the various [FelN H, )] species involved in the catalytic cycle due to its versatile coordination properties. This is further supported by the inability of the related triphosphine-silyl complex to promote the reduction of Nj under similar conditions. 

When l,l,2,2-tetramethyl-l,2-disilacyclopentane 2 was added to 15a in CgD in a sealed NMR tube, nearly quantitative formation of a bis(silyl)complex 16 was observed after 24 h at room temperature, together with 13a (97%) arising from reductive elimination (Scheme 9). 

The mechanism of the Si(OMe)4 formation is still unclear. Scrambling reactions of silicon substituents in metal-silyl complexes are not uncommon and are usually explained by intramediate silylene complexes [5]. Formation of an L Pt[=Si(OMe)2][Si(OMe)3](OMe) intermediate by migration of one methoxy group from silicon to the metal is possibly facilitated by the hemilabUe PnN ligand. Si(OMe)4 is then formed by reductive elimination of the methoxy and the Si(OMe)3 ligands. 

Transition-metal-silyl complexes are also formed by the reactions of metal-alkyl complexes with silanes to form free alkane and a metal-silyl complex. Two examples are shown in Equations 4.114 and 4.115. ° The synthesis of silyl complexes by this method has been accomplished with both early and late transition metal complexes. The formation of metal-silyl complexes from late-metal-alkyl complexes resembles the hydrogenolysis of metal-alkyl complexes to form metal hydrides and an alkane. The mechanisms of these reactions are discussed in Chapter 6. In brief, these reactions with late transition metal complexes to form silyl complexes typically occur by a sequence of oxidative addition of the silane, followed by reductive elimination of alkane. An example of this is shown in the coupling of 1,2-bis-dimethylsilyl benzene with a dimethyl platinum(II) complex (Equation 4.114). Similar reactions occur with d° early metal complexes by a a-bond metathesis process that avoids these redox events. For example, the reaction of Cp ScPh with MesSiH, has been shown to proceed through this pathway (Equation 4.115). ... 

Several observations led to the proposal that some of the catalysts containing metals other than platinum do not react by the Chalk-Harrod mechanism. First, carbon-silicon bond-forming reductive elimination occurs with a sufficiently small number of complexes to suggest that formation of the C-Si bond by insertion of olefin into the metal-silicon bond could be faster than formation of the C-Si by reductive elimination. Second, the formation of vinylsilane as side products - or as the major products in some reactions of silanes with alkenes cannot be explained by the Chalk-Harrod mechanism. Instead, insertion of olefin into the M-Si bond, followed by p-hydrogen elimination from the resulting p-silylalkyl complex, would lead to vinylsilane products. This sequence is shown in Equation 16.39. Third, computational studies have indicated that the barrier for insertion of ethylene into the Rh-Si bond of the intermediate generated from a model of Wilkinson s catalyst is much lower than the barrier for reductive elimination to form a C-Si bond from the alkylrhodium-silyl complex. ... 

Five-coordinate Pt(TV) species with silyl ligands are poised to perform Si-C or Si-H reductive elimination from Pt(IV). Note that the microscopic reverse of the latter reaction, Si-H oxidative addition, was used to synthesize the first five-coordinate Pt(TV) complexes with silyl ligands (2a-c) [84]. Complex 6, which has Pt-Me groups and a Pt-SiMe3 group, was observed to react over time at room temperamre to form tetramethylsilane, the product of Si-C reductive elimination, and intractable Pt products [91]. The five-coordinate complex ( pypyr)Pt(H>2SiEt3, 7a, was found to react with HSiMeaEt to form product 7b. Study of this reaction showed that Si-H reductive eliminaticm from 7a was rate-determining and it occurred directly from the five-coordinate complex [97]. Reaction of 7a with phosphines at room temperature led to the formation of a Pt(II)H(PR3) complex and free silane, the product of Si-H reductive elimination. Complex 7a was observed to be an active catalyst for the hydrosilylation of ethylene, tert-butylethylene, and alkynes [97]. 

MeOH and Si react directly on a fluid bed to give (MeO)3SiH and (MeO>4Si, M(CO)2(dmpe)2Cl on reductive silylation gives the bissiloxy alkyne derivative which acidifies to the first dihydroxyacetylene complex, and silyl acetylenes prepared regioselective. The relative stability of ketene and silaketene radical cations are compared and ketene thermally eliminated from ethyl silyl acetates. ... 

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