Alkylmetals, alkyl groups

My question to Dr. J. Kochi is whether it is possible to correlate the steric factor in his equations describing the oxidation of alkylmetal compounds to some measure of the bulkiness of the alkyl groups such as cone angles similar to ones suggested by Dr. C.A. Tolman for tertiary phosphine ligands. 

The stable alkylmetal(III) porphyrins are accessible from the hydrides via their corresponding bases (i.e. Rh(I) or Ir(I) porphyrins) by action of alkyl halides (paths 1, m) or from the respective chlorometal(III) porphyrins by action of lithium alkyls (path c,n). They form a very important class of compounds in view of their close relation to the alkylcobalt corrinoids. Of course, the Rh or Ir alkyl group is much more stable than the corresponding Co alkyl group which, in the case of the porphyrins, is photolabile. A variety of axial ligands have been added to the iridium alkyl porphyrins (path o). 

Alkyl halides other than aryl and vinyl halides do not readily undergo oxidative addition with Pd and must, in general, first be converted to alkylmetals. In this connection, however, a recent report on the Ni catalysed selective alkyl-alkyl cross-coupling with Ni(acac)2 and Lil in THF represents a significant breakthrough.34 In cases where the alkyl group contains a p-H atom, reduction of the organic halide via p-dehydropalladation reductive elimination can be a serious side-reaction (Scheme 11.6). 

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... 

Nucleophilic attack of transition metal centers on a-bound ligands occurs as readily as reaction with the common nucleophiles of organic chemistry. Degenerate exchange of alkyl groups from one metal to another, as exemplified by the reactions in Equations 11.11 and 11.12, is proposed to occur by nucleophilic attack. This exchange of alkyl groups has been proposed as a mechanism to racemize chiral alkylmetal complexes. - ... 

Belonging to group (i) are alkylmetal carbonyls and cyclopentadienylmetal alkyl carbonyls of formula RMn(CO)5, CpFe(CO)2R, and CpMo(CO)3R. Solvent dependence of the reaction of MeMn(CO)5 with CjHi,NH2 is illustrated also in Table I. The rate varies markedly with the dielectric constant and with the nucleophilic power of the solvent. For example, on going from dimethylformamide to mesitylene, the rate of insertion is reduced by 10. Similarly, the sequence MeCN > MejCO > THF > CHCI3 > CjHj was reported for the reaction of MeMn(CO)5 with P(0CH2)3CR (R = Me and Et) in various solvents (97). Analogous trends were observed for the insertion reactions of CpFe(CO)2R and CpMo(CO)3R (48, 80, 98). 

Furthermore, alkyl- and aryl-metals of the third and fifth main group react very readily with other electron-donors or -acceptors, respectively thus alkylmetals of the third group (electron acceptors) react with those of the fifth group (electron donors) to form addition compounds, for example, (CH3)3B As(CH3)3. A detailed review of this aspect has been published by Stone.20... 

Li. Despite some early successful results with alkyllithiums, they have scarcely been used in Pd-catalyzed alkylation. This must, in part, be due to their inability to tolerate many conventional polar functional groups. As discussed earlier, however, the high intrinsic reactivity of alkyllithiums appears to be responsible for some of the difficulties observed with alkyllithiums. In any event, it may be stated that the current scope of Pd-catalyzed alkylation with alkyllithiums is rather limited. In cases where alkylmetals containing other metals are prepared via alkyllithiums, however, it would be desirable and advisable to examine the reaction of alkyllithiums themselves to justify the use of other metals. Some of the favorable results observed with alkyllithiums are summarized in Scheme 2. 

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 . 

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