Hydrometallation, metal-catalyzed

Metal-catalyzed hydrometallations in formation of heterocycles 98PAC1059. 

In this review, an attempt is made to exhaustively catalog the rapidly growing subset of metal-catalyzed reductive C-C bond formations comprising the hydrometallative and hydrogenative carbocyclization of 7t-unsaturated substrates, and application of these methods in target-oriented synthesis. Content is organized on the basis of reaction... 

Among the hydrometallations reported to date, hydroboration and hydrosilylation have been the most widely investigated while hydroalumination has received less attention. The resulting organoalanes from hydroalumination are often more reactive than the organoboranes or organosilanes. While the first metal-catalyzed hydroalumination... 

The hydroacylation of olefins with aldehydes is one of the most promising transformations using a transition metal-catalyzed C-H bond activation process [1-4]. It is, furthermore, a potentially environmentally-friendly reaction because the resulting ketones are made from the whole atoms of reactants (aldehydes and olefins), i.e. it is atom-economic [5]. A key intermediate in hydroacylation is a acyl metal hydride generated from the oxidative addition of a transition metal into the C-H bond of the aldehyde. This intermediate can undergo the hydrometalation ofthe olefin followed by reductive elimination to give a ketone or the undesired decarbonyla-tion, driven by the stability of a metal carbonyl complex as outlined in Scheme 1. 

Considering the mechanistic rationales of the transition metal-catalyzed enyne cycloisomerization, different catalytic pathways have been proposed, depending on the reaction conditions and the choice of metal catalyst [3-5, 45], Complexation of the transition metal to alkene or alkyne moieties can activate one or both of them. Depending on the manner of formation of the intermediates, three major mechanisms have been proposed. The simultaneous coordination of both unsaturated bonds to the transition metal led to the formation of metallacydes, which is the most common pathway in transition metal-catalyzed cycloisomerization reactions. Hydrometalation of the alkyne led to the corresponding vinylmetal species, which reacts in turn with olefins via carbometalation. The last possible pathway involves the formation of a Jt-allyl complex which could further react with the alkyne moiety. The Jt-allyl complex could be formed either with a functional group at the allylic position or via direct C-H activation. Here the three major pathways will be discussed in a generalized form to illustrate the mechanisms (Scheme 8). 

Most of the transition metal catalyzed reactions are explained in terms of the hydrometallation mechanism (Harrod-Chalk mechanism) summarized in Scheme 2. The alkene coordinates to the metal (M) to give an alkene complex (5), which oxidatively adds to the Si—H bond of the hydrosilane to give (6). 

Metal catalyzed hydrometalations, specifically hydroboration and hydrosi-lylation, are particularly attractive due to the potential for enantiocontrol by the use of chiral ligands coordinated to the metal center. While the first metal catalyzed hydroalumination was described more than 40 years ago, the first synthetically useful enantioselective hydroalumination was described in 1995 and the scope and synthetic utility of this reaction are only just emerging. 

Mechanistic hypotheses play an important role in developing new catalytic and selective heterofunctionalizations of alkenes. Two basic reaction cycles for metal-catalyzed hydroalkoxylation (and hydration, for R = H) of alkenes can be postulated (Scheme 2). One pathway leads to Markovnikov products via activation of the nucleophile, oxy-metallation, and protonolysis (hydro-de-metallation) (Scheme 2a). Alternatively to the inner sphere syn-oxymetallation depicted in Scheme 2a, external anti-attack of the nucleophUe to coordinated olefin is plausible. The oxidation state of the metal remains constant in this cycle. The alternative hydrometallation pathway (Scheme 2b) proceeds via oxidative addition of the H-OR bond, hydrometallation of the olefin, and reductive elimination to the anti-Markovnikov addition product [3,4]. 

Metal-catalyzed hydrometallations in formation of heterocycles 98PAC1059. Microwave activation in phase-transfer catalysis of formation of heterocycles 99T10851. 

A variety of alternate methods for the reductive coupling of aldehydes and alkynes have been developed. A number of important hydrometallative strategies have been developed, although most of these methods require the stoichiometric formation of a vinyl metal species or metallacycle. A very attractive hydrogenative coupling method has recently been developed, and its scope is largely complementary to the nickel-catalyzed methods. A very brief overview of these methods is provided below. 

The initially expected (75) cis-hydrometallation or olefin-insertion step with fumarate (R = C02Me) yields the threo isomer 8, which then undergoes the k2 step with retention to give racemic 1,2-dideuterosuccinate. Such retention is necessary to give the usually observed (7, p. 407) overall cis addition of H2 to olefinic bonds, but this study provided the first direct experimental proof, the difficulty being the scarcity of stable metal alkyl-hydride intermediates. The Cp2MoH2 complex also catalyzes hydrogenation of 1,3- or 1,4-dienes to monoenes (197). 

In addition to transition metals, recent work has demonstrated that strong Lewis acids will catalyze the addition of silanes to alkynes in both an intra- and an intermolecular fashion.14,14a-14c The formation of vinylsilanes from alkynes is possible by other means as well, such as the synthetically important and useful silylcupration15,15a of alkynes followed by cuprate protonation to afford vinylsilanes. These reactions provide products which can be complementary in nature to direct hydrometallation. Alternatively, modern metathesis catalysts have made possible direct vinylsilane synthesis from terminal olefins.16,16a... 

Recently, another type of catalytic cycle for the hydrosilylation has been reported, which does not involve the oxidative addition of a hydrosilane to a low-valent metal. Instead, it involves bond metathesis step to release the hydrosilylation product from the catalyst (Scheme 2). In the cycle C, alkylmetal intermediate generated by hydrometallation of alkene undergoes the metathesis with hydrosilane to give the hydrosilylation product and to regenerate the metal hydride. This catalytic cycle is proposed for the reaction catalyzed by lanthanide or a group 3 metal.20 In the hydrosilylation with a trialkylsilane and a cationic palladium complex, the catalytic cycle involves silylmetallation of an alkene and metathesis between the resulting /3-silylalkyl intermediate and hydrosilane (cycle D).21... 

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