Organometallic coupling reaction mechanism

The general mechanism of coupling reactions of aryl-alkenyl halides with organometallic reagents and nucleophiles is shown in Fig. 9.4. It contains (a) oxidative addition of aryl-alkenyl halides to zero-valent transition metal catalysts such as Pd(0), (b) transmetallation of organometallic reagents to transition metal complexes, and (c) reductive elimination of coupled product with the regeneration of the zero-valent transition metal catalyst. 

The first examples of the use of palladium as a catalyst for carbon-carbon coupling reactions were reported almost thirty years ago [14], and over recent decades a massive effort has been devoted to the extension of the scope of palladium-catalyzed reactions. Organic and organometallic chemists have received extensive input from palladium-coordination chemistry in the task of understanding the mechanisms behind these efficient synthetic procedures [14]. 

Less attention has been paid, however, to C02 organometallic chemistry during the past decade. Whilst many reduction or coupling reactions are known to proceed in the presence of stoichiometric or catalytic amounts of transition metal complexes, very few examples remain where the formation of a metal-C02 complex has led to an effective, catalytic reduction reaction of C02. Carbon dioxide complex photoactivation also represents an attractive route to CO bond cleavage, coupled with O-atom transfer. However, progress in the area of C02 utilization requires a better understanding of the reaction mechanisms, of the thermodynamics of reaction intermediates, and of structure-reactivity relationships. 

Iodo dimethyl amido complexes, with Ti(IV), 4, 331-332 Iodonium salts, cross-coupling with lead reagents, 9, 413 Ionic addition reactions, mechanisms, 1, 101 Ionic bis(isonitrile) complexes, liquid crystals, 12, 280 Ionic character, organometallic compound dn configuration,... 

An aryl halide such as chlorobenzene is relatively unreactive towards nucleophilic substitution. The S l and Sj. 2 pathways involve mechanisms that are not open to aryl halides. The greater s character of the sp bond makes it more difficult to cleave the bond to generate a carbo-cation. However, these restrictions do not apply to radical or carbanion chemistry. Hence, aryl halides undergo radical coupling reactions and metal insertion reactions, leading to organometallic compounds. 

The unique antagonistic features of the (butadiene)zirconocene isomers 3a/5a have been used as a probe for the elucidation of organometallic reaction mechanisms. In some cases it was possible to distinguish between mechanistic alternatives by simply allowing the isomeric substrates 3 and 5 to compete for a reagent. An example is as follows. Transition metal-induced C—C coupling between a conjugated diene and an olefin can occur by two basically different types of reaction sequence. Either a new C— C bond can be formed by olefin insertion into a metal-carbon bond of a ( T-allyl)M-type intermediate (24) (95), or, alternatively, the alkene may... 

Apart from (3-hydride elimination, another important pathway by which palladium(II) intermediates can lead to neutral organic fragments is reductive elimination. This forms the basis of the mechanism for cross-coupling reactions between organometallic reagents and an organic halide or triflate. 

Ricci, A., Lo Sterzo, C. A new frontier in the metal-catalyzed cross-coupling reaction field. The palladium-promoted metal-carbon bond formation. Scope and mechanism of a new tool in organometallic synthesis. Journal of Organometallic Chemistry 2002, 653, 177-194. 

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