Oxidative addition and insertion

Alkyl compounds can be synthesized by substitution, oxidative addition and insertion reactions... 

In 1976, Lappert and co-workers " reported the first stable dialkylstan-nylene (154, Scheme 14.71) in solution. They found that 154 exists as a monomer in the gas phase and as a dimer 153 in the solid state, whereas it exists as a monomer-dimer equilibrium mixture in solution. Extensive studies on the reactions of 154 were carried out, especially on oxidative addition and insertion reactions leading to a variety of new organotin compounds such as 165 and 166 (Scheme... 

The red crystals which form in this reaction melt at 30 °C and are reasonably stable. Two other important routes to transition metal alkyls are oxidative addition and insertion, topics that are discussed more in the section on organometallic reactions. 

Halpern et al. had already stressed the importance of radical mechanisms in the oxidative addition and insertion reactions of both [Rh(OEP)]2 and Rh(OEP) [338]. Thus, [Rh(OEP)]2 reacted with trimethylphosphite according to sequence (37), forming a rhodiophosphonate Rh(PO OMe 2) (OEP) and methyl radicals which were subject to further reactions [339]. In the presence of excess P(OMe)3, they were trapped by formation of MePO(OMe)2 in more than stoichiometric quantities, indicating a radical chain process. 

Both Mg and transition metal complexes similarly undergo oxidative addition and insertion. Whereas the main reaction path of Grignard reagents is the insertion of a... 

The Grignard reaction proceeds via oxidative addition and insertion. The reaction product 93 is isolated after hydrolysis of the insertion product 92 with dilute aqueous... 

The diboration of allenes afforded another series of allylboron compounds that each have a boryl group at the vinyl carbon. The addition had a strong tendency to occur at the internal double bond of terminal allenes such as 1,2-heptadiene (Equation (33)).237 However, steric hindrance in both allenes and phosphine ligands forced the addition toward the terminal double bond of dimethylallene (Equation (34)).237 On the other hand, addition to the terminal double bond occurred selectively for both monosubstituted and 1,1-disubstituted allenes when Pd(dba)2 was used in the presence of a co-catalyst (RI) such as I2, Arl, and iodoalkenes (Equation (35)).238 The role of co-catalyst was attributed to in situ formation of I-Bpin intermediate, which undergoes oxidative addition and insertion leading to 2-boryl-7r-allylpalladium intermediate. 

The arylation of activated alkenes with aryl halides in the presence of base was discovered by R. F. Heck in 1971 and is now one of the standard methods for C—C bond formation. The catalysts are mostly palladium or nickel phosphine complexes, which react via a succession of oxidative addition and insertion reactions, as shown in the following simplified cycle ... 

There are a few reports of the catalysis of the hydrostannation of active alkenes by soluble palladium catalysts,105- 106 but, with less active alkenes, the major product is the hexaalkyldistannane. Good yields of hydrostannation products, however, can be obtained with heterogeneous catalysts, and Table 4-2 shows examples of the hydrostannation of alkenes in the presence of a PdjOHVC catalyst.107 A recent, thorough, survey in Chemical Reviews is available.108 The mechanism of the palladium-catalysed hydrostannation is not known in detail, but presumably it involves oxidative addition and insertion of the alkene by stannylmetallation (or hydrometallation) as shown, followed by reductive elimination (Scheme 4-4). 

Rhodium-catalyzed-carbonylation of o-alkynyl phenols has resulted in the formation of 3-substituted coumarins and 3-substituted benzofuran-2-ones, under the water-gas shift reaction conditions (Takashi et al. 1998). Rhg(CO)ig was employed as the catalyst in the carbonylation. The reaction has been shown to involve oxidative addition and insertion of carbon monoxide followed by reductive elimination. Coumarins were obtained in low yield (20%-40%) (Scheme 10.4). 

B. C. Soderberg, Comp. Organomet. Chem. II, 1995,12, 241-297. Transition Metal Alkyl Complexes Oxidative Addition and Insertion. 

Trisubstituted alkenes are usually poor substrates for Heck chemistry, and must almost always be used in an intramolecular way. Treatment of the phenolic ether 5.177 with a palladium catalyst resulted in oxidative addition and insertion, leading to a stable ti -intermediate 5.179 as it has no available p-hydrogen... 

Transition-metal-alkyl bonds can be formed by a variety of reactions that include metathetical replacement of a halide ion, oxidative addition, and insertion of an alkene into a metal-hydride bond. " A similar set of reactions is available for the synthesis of transition-metal-aryl bonds, although the analogous insertion of a benzyne intermediate into a metal-hydride bond is not particularly viable as a synthetic route. For alkyl complexes that have longer chains than methyl, thermal decomposition to give the metal-hydride complex by a j5-hydrogen transfer reaction is frequently observed at ambient temperature. 

Elementary steps. The reactivity of the anchored complexes has been studied in only a few cases. Evidence for a few elementary steps on surfaces has been obtained, including ligand association and dissociation reactions, oxidative addition, and insertion. Understanding of these will be a prerequisite to the fundamental understanding of the elementary steps of catalysis. At this point, these can at best be inferred by analogy to molecular chemistry in solution. Therefore, a systematic approach to the chemistry of these elementary steps on surfaces is recommended. 

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