Kinetics hydride complexes

The corresponding reactions of transient Co(OEP)H with alkyl halides and epoxides in DMF has been proposed to proceed by an ionic rather than a radical mechanism, with loss of from Co(OEP)H to give [Co(TAP), and products arising from nucleophilic attack on the substrates. " " Overall, a general kinetic model for the reaction of cobalt porphyrins with alkenes under free radical conditions has been developed." Cobalt porphyrin hydride complexes are also important as intermediates in the cobalt porphyrin-catalyzed chain transfer polymerization of alkenes (see below). 

The mechanism proposed for aromatic C-H borylation of aromatic compounds 1 by B2pin2 3 catalyzed by the Ir-bpy complex is depicted in Scheme 3 [6-9]. A tris(boryl)Ir (III) species [5, 6, 11] 6 generated by reaction of an Ir(I) complex 5 with 3 is chemically and kinetically suitable to be an intermediate in the catalytic process. Oxidative addition of 1 to 6 yields an Ir(V) species 7 that reductively eliminates an aromatic boron compound 4 to give a bis(boryl)Ir(III) hydride complex 8. Oxidative addition of 3 to 8 can be followed by reductive elimination of HBpin 2 from 9 to regenerate 6. 2 also participates in the catalytic cycle via a sequence of oxidative addition to 8 and reductive elimination of H2 from an 18-electron Ir(V) intermediate 10. Borylation of 1 by 2 may occur after consumption of 3, because the catalytic reaction is a two-step process - fast borylation by 3 then slow borylation by 2 [6],... 

Anionic chromium hydride complexes proved to be efficient hydrogen atom donors. Newcomb determined PPN+ HCr(CO)5 to be an efficient radical initiator and reducing agent for radicals and determined the kinetics of the hydrogen abstraction reaction [214]. In line with the observation that 3d metal complexes are much more prone to radical pathways than the corresponding 4d and 5d complexes, an increase of the extent of competing S -pathways for the bromide abstraction was found for molybdenum and tungsten complexes compared to the chromium complex. 

A system kinetically very similar to the phosphine-free rhodium carbonyl catalyst is obtained with bulky phosphites (Fig 6.3). At temperatures from 50 to 80°C, and CO and H2 partial pressures ranging from 10 to 70 bar, the rate of aldehyde formation is first order in H2 and approximately minus one order in CO. The reaction rate is independent of the concentration of 1-octene at conversions below 30%. The reaction was found to be first order in rhodium concentration and insensitive to the phosphite/rhodium ratio, provided that the absolute concentration was sufficiently high to generate a hydride complex from the pentanedionate precursor (reaction 9). 

Enolates can also be prepared by rhodinm-catalyzed isomerization of allylic Uthinm alcoholates, such as 14 (equation 5)". Subsequent treatment of the intermediately formed rhodium hydride complexes (15 and 16) with an electrophile led to the formation of various products. For example, alkyl halides gave a-alkylated ketones (17) in good yields, as shown in Table 4. Interestingly, addition of benzaldehyde under kinetically controlled... 

Kinetic and spectroscopic evidence and calculational studies support the formation of the seven-membered cyclometallated ring Pt hydride complex through insertion of one of the phenyl ligands into the C-Pt(IV) bond (104) followed by the reductive elimination of benzene with the involvement of transition state 105. ... 

Kinetic studies made on [Pd(PP2)(PEt )[(BF )2> and reported elsewhere (45), indicate that the rate of catalysis is first order in CO2, first order in catalyst, and first order in acid at low acid concentrations. These results are consistent with the mechanism shown in Scheme 2. In comparison with Scheme 1, two important features should be noted. First in Scheme 2, the formation of a coordinatively unsaturated metal hydride complex is necessary for CO2 insertion to occur. A priori there is no way of knowing whether or not the generation of a coordinatively unsaturated metal hydride will be required for catalysis since evidence exists for both associative and dissociative pathways for CO2 insertion into metal hydride and metal carbon bonds (20-25). This is the reason that complexes of the types... 

One of the intriguing attributes of many systems that initiate C—H oxidative addition is the commonly observed selectivity for stronger C—H bonds, which can be divided into kinetic and thermodynamic selectivity. For metal-mediated C—H activation, kinetic and thermodynamic selectivities are often identical. For example, arenes often undergo reaction more rapidly than alkanes that possess weaker C—H bonds, and aryl hydride complexes (plus free alkane) are commonly favored thermodynamically over alkyl hydride systems (plus free aromatic substrate). Assuming that... 

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