Alkylmetal-hydride complexes

With few exceptions, transition metal complexes of terminal alkyl groups are more stable than those of isomeric secondary or tertiary alkyl groups. Consistent with this trend, the terminal alkylmetal hydride complexes of systems that undergo oxidative addition of C-H bonds are thermodynamically more stable than the branched alkylmetal hydride complexes. "- The oxidative addition of alkanes by the Cp Ir(PMe3) fragment is reversible, and this reversibility allowed for an evaluation of the thermodynamic stability of alkyl hydride complexes formed from a series of alkanes, as weU as from arenes. These data showed that the alkyl hydride complex formed from oxidative addition at the primary C-H bond leads to more stable products than those formed from... 

Much information has been gained on the mechanism of C-H bond-forming reductive elimination (see Equation 8.9). In addition to creating an understanding of C-H bond formation, this information has been used to understand the mechanism of the opposite reaction, the oxidative addition of C-H bonds. Because reductive eliminations of alkanes are faster from three- and five-coordinate species than from four- and six-coordinate species, square planar and octahedral complexes often dissociate or associate a dative ligand prior to reductive elimination. However, elimination to form a C-H bond from a four- or six-coordinate complex can also be fast enough that it occurs directly from the alkylmetal-hydride complexes prior to ligand dissociation. 

Equation (1) depicts an early example of an intermolecular addition of an alkane C-H bond to a low valent transition metal complex [12], Mechanistic investigations provided strong evidence that these reactions occur via concerted oxidative addition wherein the metal activates the C-H bond directly by formation of the dative bond, followed by formation of an alkylmetal hydride as the product (Boxl). Considering the overall low reactivity of alkanes, transition metals were able to make the C-H bonds more reactive or activate them via a new process. Many in the modern organometallic community equated C-H bond activation with the concerted oxidative addition mechanism [10b,c]. 

CH proton 2) the ability of the adduct to undergo oxidative addition and 3) the preference, probably of steric origin, for binding at the least-hindered CH bond. The first allows facile deprotonation or H/D exchange in an alkane complex. The second leads to alkylmetal hydrides that can evolve to give alkene or a functionalized product. The third may be a factor in the selectivity (primary > secondary > tertiary) shown by many of the systems described in this article, a selectivity that contrasts with that seen for conventional radical and electrophilic CH reactions (tertiary > secondary > primary). 

Protonolysis of electron-rich alkylmetals may proceed via initial electrophilic attack at metal, i.e. hydride complex formation. Different from the case of the halogenolysis, the subsequent C-H bond formation occurred via internal reductive elimination with overall retention of configuration (Eq. 8.24) [129]. 

In the second type of process the metal acts as a carbenoid and inserts into the C—H bond, a process generally termed oxidative addition in organometallic chemistry (equation 1 b). This reaction is believed to go via the same sort of alkane complex as in the Shilov system, but, instead of losing a proton, it goes instead to an alkylmetal hydride. This may be stable, in which case it is observed as the final product, or it may react further. 

All of the metal atoms of the first transition series undergo photoinsertion into the C—H bond of methane in matrices (equation 38) . In some cases (e.g. Fe, Co), a precursor to the insertion product, identified as an alkane complex, was also detected. Subsequent irradiation at a longer wavelength generally caused reductive elimination (e.g. Fe, Co). In other cases (e.g. Cu), the alkylmetal hydride fragments to an alkyl radical and an M—H group. Longer-chain alkanes also react with Fe . 

The reactions of a variety of alkylmetal carbonyl complexes, including [Fe(Cp)(00)2(Me)] and [Os(CO) (Me)2 , with various metal hydrides (such as [ReH(CO) ], [OsH2(CO) ], [MnH(CO)g], and [WH(CO)2(Cp)]) have been shown to lead to dinuclear complexes or polynuclear hydrides with the organic products eliminated usually being aldehydes although alkane elimination is also seen. ... 

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... 

ALKYLALUMINIUM DERIVATIVES, ALKYLBORANES, ALKYLHALOBORANES ALKYLHALOPHOSPHINES, ALKYLHALOSILANES, ALKYLMETALS ALKYLNON-METAL HYDRIDES, ALKYLPHOSPHINES, ALKYLSILANES ARYLMETALS, BORANES, CARBONYLMETALS, COMPLEX ACETYLIDES COMPLEX HYDRIDES, HALOACETYLENE DERIVATIVES HEXAMETHYLNITRATODIALUMINATE SALTS, METAL HYDRIDES NON-METAL HYDRIDES, ORGANOMETALLICS, PYROPHORIC ALLOYS PYROPHORIC CATALYSTS, PYROPHORIC IRON-SULFUR COMPOUNDS PYROPHORIC METALS... 

The insertion of alkene to metal hydride (hydrometallation of alkene) affords the alkylmetal complex 34, and insertion of alkyne to an M—R (R = alkyl) bond forms the vinyl metal complex 35. The reaction can be understood as the cis carbometallation of alkenes and alkynes. 

The addition of nucleophiles to alkenes is mediated by Hg(II) salts and catalyzed by Pd(II) salts. The difference between the two reactions is the fate of the alkylmetal(II) intermediate obtained after addition of the nucleophile to the tt complex. The alkylmer-cury(II) intermediate is stable and isolable, whereas the alkylpalladium(II) intermediate undergoes rapid /3-hydride elimination. 

The reaction of a C-H bond at the carbon atom p to the metal in an alkylmetal complex leads to a facile elimination of an alkene, that is, P-hydride eUmination [Eq. (6.80)], which is the main decomposition pathway for metal alkyls. Requirements are a two-electron vacant site at the metal and a near-coplanar arrangement of the M-C-C-H moiety to bring the P-H close to the metal. ... 

The -alkylmetal complexes discussed above are undoubtedly stable because they are coordinatively saturated and, lacking a vacant coordination site, are unable to undergo a facile /S-hybride elimination reaction. Since 8-hydride elimination is the most common way for > -alkylmetal complexes to decompose, coordinatively unsaturated complexes are often considerably less stable. 

Reactions of mtemal olefins are more complex than reactions of terminal olefins (Scheme 16.3). As mentioned previously, terminal nitriles are often formed from reactions of internal olefins. The formation of terminal nitriles results from insertion of the internal olefin to form a branched alkylmetal intermediate (A in Scheme 16.3) that undergoes isomerization to the terminal alkyl intermediate B prior to reductive elimination of the final linear nitrile faster than it undergoes reductive elimination to form the branched nitrile. Internal olefins react more slowly than terminal olefins, and this relative rate can be traced to the slower insertion of internal olefins into metal hydrides. Lewis acids, such as ZnClj and AlClj, promote these reactions of isolated alkenes. 

Examples of inserting species are carbenes, olefins, acetylenes, and epoxides. Reaction of diazomethane with metal hydrides or halides has given several alkylmetal complexes, for example. 

The elimination of a hydrogen atom positioned on a carbon to the central metal constimtes an important reaction in transition metal catalysis. In the classical example, an alkylmetal intermediate is reversibly converted to an alkene and a metaUiydride (scheme 1.12). Despite the fact, that the resulting hydridometal complex can be exploited in various catalytic processes including polymerization reactions, [57] cycloisomerizations, [58] annulations, [59] etc., the ]S-hydride elimination is often considered undesired in transition metal catalyzed cross couplings. Thus, efforts have often been concentrated towards the prohibition of this fundamental reaction [60]. Nevertheless, the ]S-hydride elimination is a vital transformation in a number of catalytic processes including the ene-yne coupling reported by Trost [61] and Skrydstrup, [62] oxidation of alcohols, [63] the Heck reaction etc [64]. 

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