Metal hydrides, reaction with alkenes

A number of transition metal complexes react with alkenes, alkynes and dienes to afford insertion products (see Volume 4, Part 3). A general problem is that the newly formed carbon-metal bond is usually quite reactive and can undergo a variety of transformations, such as -hydride elimination or another insertion reaction, before being trapped by an electrophile.200 Usually, a better stability and lower reactivity is observed if the first carbometallation step leads to a metallacycle. It is worthy to note that the carbometallation of perfluorinated alkenes and alkynes constitutes a large fraction of the substrates investigated with transition metal complexes.20015... 

Most of the reported reactions between tetranuclear clusters and alkynes involve mixed-metal cluster species. In these systems hydride and carbon monoxide substitution generally occurs [Eq. (11)] (194-200), although in some cases Me3NO has been used to activate the starting material (201, 202), and in still others cluster breakdown takes place even under mild reaction conditions (203). Rh4(CO)12 (204) and Ir4(CO)12 (205) retain their nuclearity in reactions with alkynes, but in the latter case the metal framework geometry is altered (Fig. 7). The use of [Ir4(CO)11Br] instead of Ir4(CO)12 in reactions with alkenes produces alkene-substituted tetranuclear complexes (189), as shown in Fig. 7. Few other homonuclear clusters have been found to react with alkynes (206-208). In the reaction between the tetranuclear cluster Cp2W2Ir2(CO) 0 and diphenylacetylene two independent processes... 

The utility of metal hydride-transition metal halide reductions is limited only to alkenes and alkynes which do not contain functional groups which can react with the metal hydride, or with potential low-valent transition metal species. This severely limits the use of this reaction. 

The addition of a metal hydride to an alkene or an alkyne is a useful way of simultaneously introducing stereochemistry and a reactive carbon-metal bond into a molecule. Carbon-metal bonds are among the most versatile reactive species available, due to their reactions with a variety of functional groups and easy transformation into a C-O, C-N, or C-C bond. Transmetalation from one carbon-metal bond (C-M) to a carbon-metal bond which has a different reactivity profile (C-M ), further expands the range of possibilities. 

Primary dialkylboranes react readily with most alkenes at ambient temperatures and dihydroborate terminal acetylenes. However, these unhindered dialkylboranes exist in equiUbtium with mono- and ttialkylboranes and cannot be prepared in a state of high purity by the reaction of two equivalents of an alkene with borane (35—38). Nevertheless, such mixtures can be used for hydroboration if the products are acceptable for further transformations or can be separated (90). When pure primary dialkylboranes are required they are best prepared by the reduction of dialkylhalogenoboranes with metal hydrides (91—93). To avoid redistribution they must be used immediately or be stabilized as amine complexes or converted into dialkylborohydtides. 

There is no clear reason to prefer either of these mechanisms, since stereochemical and kinetic data are lacking. Solvent effects also give no suggestion about the problem. It is possible that the carbon-carbon bond is weakened by an increasing number of phenyl substituents, resulting in more carbon-carbon bond cleavage products, as is indeed found experimentally. All these reductive reactions of thiirane dioxides with metal hydrides are accompanied by the formation of the corresponding alkenes via the usual elimination of sulfur dioxide. 

The mechanism of homogeneous hydrogenation catalyzed by RhCl(Ph3P)3 ° involves reaction of the catalyst with hydrogen to form a metal hydride (PPh3)2RhH2Cl (43), which rapidly transfers two hydrogen atoms to the alkene. 

In Scheme 1 is represented an idealized picture of the two possibilities for the hydrogenation of alkenes by metal complexes not containing an M—11 bond. One possibility involves initial coordination of the alkene followed by activation of H2 (alkene route). The other (more general) possibility is the hydride route, which involves initial reaction with H2 followed by coordination of the alkene. The second general mechanism, usually adopted by catalysts containing an M—H bond, is shown in Scheme 2. 

Another approach is based on the palladium-catalyzed intramolecular carbocyclization of the allylic acetate moiety with the alkene moiety (Scheme 96). After the formation of a 7t-allylpalladium complex, with the first double bond the intramolecular carbometallation of the second double bond occurs to form a new C-C bond. The fate of the resulting alkylpalladium complex 393 depends on the possiblity of /3-elimination. If /3-elimination is possible, it generates a metallated hydride and furnishes the cycloadduct 394. This cyclization could be viewed as a pallada-ene reaction, in which palladium replaces the hydrogen atom of the allylic moiety.231... 

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... 

Pettit and coworkers—metal hydride intermediates by weak base attack over Fe carbonyl catalysts. Pettit et al.ls approached the use of metal carbonyl catalysts for the homogeneous water-gas shift reaction from the standpoint of hydroformyla-tion by the Reppe modification.7 In the typical hydroformylation reaction, an alkene is converted to the next higher aldehyde or alcohol through reaction of CO and H2 with the use of a cobalt or rhodium carbonyl catalyst. However, in the Reppe modification, the reduction is carried out with CO and H20 in lieu of H2 (Scheme 6) ... 

Phase-transfer catalytic conditions provide an extremely powerful alternative to the use of alkali metal hydrides for the synthesis of cyclopropanes via the reaction of dimethyloxosulphonium methylides with electron-deficient alkenes [e.g. 54-56] reaction rates are increased ca. 20-fold, while retaining high yields (86-95%). Dimethylphenacylsulphonium bromide reacts in an analogous manner with vinyl-sulphones [57] and with chalcones [58] and trimethylsulphonium iodide reacts with Schiff bases and hydrazones producing aziridines [59]. 

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