Oxidative addition silyl compounds

Oxidation of Silyl Compounds. MTO-catalyzed oxidization of silyl enol ethers with hydrogen peroxide yields a-hydroxyketones in high yield. The reactions were conducted with pyridine as the amine additive in acetic acid (eq 22). 

This synthetic approach is known from the synthesis of L M(alkene)H compounds from LnM(CO)alkane precursors and can easily be applied to the analogous silyl complexes. The Si—H bond even shows an increased activity for oxidative addition reactions [42, 43]. 

The sesquiterpene skeleton has also been assembled by the intramolecular nitrile oxide cycloaddition sequence. Oxime 238 (obtained from epoxy silyl ether 237), on treatment with sodium hypochlorite gave isoxazoline 239, which was sequentially hydrolyzed and then subjected to the reductive hydrolysis conditions-cyclization sequence to give the furan derivative 240 (330) (Scheme 6.93). In three additional steps, compound 240 was converted to 241. This structure contains the C11-C21 segment of the furanoterpene ent-242, that could be obtained after several more steps (330). 

Diyne cyclization/hydrosilylation catalyzed by 4 was proposed to occur via a mechanism analogous to that proposed for nickel-catalyzed diyne cyclization/hydrosilylation (Scheme 4). It was worth noting that experimental evidence pointed to a silane-promoted reductive elimination pathway. In particular, reaction of dimethyl dipropargylmalonate with HSiMc2Et (3 equiv.) catalyzed by 4 led to predominant formation of the disilylated uncyclized compound 5 in 51% yield, whereas slow addition of HSiMe2Et to a mixture of the diyne and 4 led to predominant formation of silylated 1,2-dialkylidene cyclopentane 6 (Scheme 5). This and related observations were consistent with a mechanism involving silane-promoted G-H reductive elimination from alkenylrhodium hydride species Id to form silylated uncyclized products in competition with intramolecular carbometallation of Id to form cyclization/hydrosilylation products (Scheme 4). Silane-promoted reductive elimination could occur either via an oxidative addition/reductive elimination sequence involving an Rh(v) intermediate, or via a cr-bond metathesis pathway. 

As shown in Scheme 9.10, the silylene 3 underwent oxidative additions with alkali metal (M) bis(trimethylsilyl)amides to give new alkali metal amides, rather than metal fais(amino)silyls M[SifN. ) N(SL Ie3)2 ](thf), which, however, were suggested to have been intermediates.Whereas M[N Si(NN)fSi. Vie ,) (Si. Vlc ) (thfwith R = Me underwent a further reaction with 3 to yield M[N Si(NN)(SiMe3) 2], for R = Ph the double addition compound was the sole product in the 3/Li[N(SiMe3)(SiMe2R)] system. ... 

Both amido and pinacol derivatives of B-Si compounds 125 and 126 added to terminal and internal alkynes in the presence of a palladium244-246 or platinum(O) catalyst247 by a mechanism involving an oxidative addition-insertion process (Equation (39)).248 On the other hand, phosphine-free nickel(O) catalyst resulted in the dimerization of alkynes giving a Z,Z-isomer of l-silyl-4-borylbutadiene derivatives.249 Since the palladium-catalyzed cross-coupling at the C-B bond is faster than the G-Si bond of 137, a silylboration-cross-coupling sequence provided a method for the synthesis of 1-alkenylsilanes.246... 

A mechanism for catalysis by platinum compounds was proposed in 1965 by Chalk58) and has since been supported by increasing knowledge about silyl-metal systems and by the direct detection of Pt-Si211) and Rh-Si61,18s) complexes in the reaction mixtures. The suggested mechanism requires olefin coordination to the Pt(II) species (in the case of H2PtCl6 formed by reduction by the silicon hydride), oxidative addition of the silane, formation of an intermediate in which silicon and alkyl are both bonded to the platinum center, and reductive elimination of alkylsilane, probably assisted by coordination of more olefin ... 

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... 

The discovery of the hydrosilation reaction has greatly stimulated research on transition metal-silyl complexes. Currently, there are examples of silyl complexes with almost every transition rnetal. " Silyl hydride complexes are of special interest with respect to the chemistry of the hydrosilation reactions. Such compounds can be seen as the products of oxidative addition into the Si-H bond and thus are intermediates in hydrosilation reactions following the ChaUc-Harrod mechanism. 

This class is illustrated by some silylated derivatives, Me3SiN=PPh2-NR -PPh2 with R = Et, -Pr, -Bu, prepared from Ph2PNR PPh2 with trimethylsilylazide (Scheme 42). Theses compounds can be readily undergo oxidative addition of snlfur and selenium to the P(III) site and react with... 

The formation of compound 109 can be explained in terms of oxidative addition of an sp-hybridized C-Si bond of the coordinated bis(trimethylsilyl)acetylene, in complex 113, to the nickel atom, followed by a shift of the trimethyl-silyl group from the nickel atom to the trimethylsilyl-substituted sp carbon, giving vinylidenenickel complex 114 (Scheme 14). 

In the nickel(ll)-catalyzed NHK reaction, the first step is the reduction of Ni " to Ni that inserts into the halogen-carbon bond via an oxidative addition. The organonickel species transmetallates with Cr " to form the organochromium(lll) nucleophile, which then reacts with the carbonyl compound. To make the process environmentally benign, a chromium-catalyzed version was developed where a chlorosilane was used as an additive to silylate the chromium alkoxide species in order to release the metal salt from the product. The released Cr " is reduced to Cr " with manganese powder. 

Oxidative addition of Si-Si bonds onto palladium(O) has long been presumed to be involved in a number of palladium-catalyzed bis-silylation reactions of unsaturated carbon compounds. The oxidative addition and its reverse reaction, i.e., reductive elimination, may be in rapid equilibrium, whose direction is influenced by the structure of disilanes and ligands on the palladium atom. In spite of early reports on the formation of bis(organosilyl)palladium(II) complexes [14,15], a well-characterized complex was first synthesized in 1992 by reaction of hydro disilanes with hydridepalladium complex, probably through initial activation of Si-H bond followed by silylene migration (see Sect. 2.3) [16]. Since... 

Early transition-metal silyl compounds have received much less attention, largely due to the fact that general, straightforward synthetic routes have not been available. Syntheses based on oxidative additions are less applicable, given the more electropositive character of the early metals. Particularly for d° silyl complexes, most syntheses are based on nucleophilic displacement of halide by a silyl anion reagent, usually an alkali metal derivative. 

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