Nickel hydrometalation

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. 

It is postulated that the mechanism of the silane-mediated reaction involves silane oxidative addition to nickel(O) followed by diene hydrometallation to afford the nickel -jr-allyl complex A-16. Insertion of the appendant aldehyde provides the nickel alkoxide B-12, which upon oxygen-silicon reductive elimination affords the silyl protected product 71c along with nickel(O). Silane oxidative addition to nickel(O) closes the catalytic cycle. In contrast, the Bu 2Al(acac)-mediated reaction is believed to involve a pathway initiated by oxidative coupling of the diene and... 

Keywords Nickel catalysts, BINAP, Hydrometalation, Reduction, Cyclohexenols, Cyclohepten-ols, DIBALH, Carbon-metal bond, Organoalane... 

Many classes of nickel-catalyzed processes involving simple 1,2-additions to alkenes and alkynes or 1,4-additions to 1,3-dienes have been reported. Among these methods, the hydrometallation of simple tt-components has been demonstrated with a wide range of main group and transition metal species. Nickel-catalyzed processes... 

The basic features of the reaction may be summarized as follows (Scheme 3-92). Nickel 7t-allyl dimers are often used as pre-catalysts. Treatment with monodentate phosphines and Lewis acids likely generates a cationic nickel hydride species, which serves as the active catalyst for the reaction. Regioselective hydrometallation of the styrene generates intermediate 26, with the regiochemistry being driven by benzylic stabilization. Insertion of ethylene followed by p-hydride elimination produces the hydrovinylation product and regenerates the active cationic nickel hydride species. 

Studies by Mori demonstrate that triethylsilane and dienals undergo reductive cyclization in the presence of bis(T] -cycloocta-l,5-diene)nickel(0) (2) and triphenylphosphine (1 2) to produce the silyl ether of cycloalkanols P l in this instance, y.b-misaturated products are obtained. However, if the reaction is carried out in the presence of cyclohexa-l,3-di-ene, an analogous reaction proceeds to give 8,e-unsaturated products. This effect is reported to be derived from selective diene hydrometalation followed by addition of the or-ganonickel intermediate to the tethered aldehyde. The reaction proceeds with five-, six-, and seven-membered ring formation and with heterocyclic substrates. Several synthetic applications of this cyclization methodology are reported (Scheme 8). Intermolecular processes with simple dienes and aldehydes to afford y.S-unsaturated silyl ethers are also possible. 

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