Homogeneous catalytic activation of carbon-hydrogen bonds

Abstract

Very recently it has become apparent that some soluble transition-metal complexes can activate the C-H bonds of hydrocarbons in much the same way that the H-H bond of hydrogen is activated in catalytic hydrogenation reactions. This discovery points the way to a new type of homogeneous catalysis for reactions such as aromatic substitution1 and opens some interesting opportunities for applications in synthesis.The classical methods for substitution of arenes and alkanes involve attack of the 'hydrocarbon by a highly reactive reagent. The role of the catalyst has been to make the attacking reagent more reactive. In halogenation, for example, the catalyst generally promotes dissociation of the X-X bond, either by heterolytic cleavage $0 produce a highly reactive species with characteristics approaching those of X+ or by homolytic cleavage to give a X . species. In either case, it is the halogen reagent that is activated rather than the hydrocarbon.Recent work in several laboratories has shown that the C-H bonds in benzene are activated toward processes such as H-D exchange by a variety of transition-metal complexes including high-valent species such as TaVH3(C5H&, low-valent species like RuO-( d m p e )~ (dmpe = ( C H~) Z P C H~C H~P ( C H~)~) , and electrophiles (Pd2+ and Pt2+). The activation is generally agreed to involve cleavage of a C-H bond by the metal and formation of a C-M bond. The C-M bond then undergoes reaction with a reagent such as D2 or C12. The process is similar to the metal-catalyzed activation of H2 as illustrated by H-D exchange with a metal deuteride C-H

A Theoretical Model for the Orientation of 16-Electron [CpML] Insertion into the C-H Bond of Propane and Cyclopropane and Its Regio- and Stereoselectivity

“…This is a typical example for the oxidative addition reaction of a transition metal complex into the C ± H bond. [1] It should be pointed out here that this concept of an insertion mechanism was expressed for the first time by Bach et al [7] With the above analysis in hand, we would like to extend the methane case to the larger hydrocarbon systems in this work, that is propane and cyclopropane. The requisite canonical MOs for the primary (18) and secondary (28) carbon of propane are given in Figure 2.…”

“…Thus, the activation of C ± H bonds in saturated hydrocarbons has been a topic of much interest in recent years. [1] Recently, Bergman and co-workers have examined a wide range of alkane substrates with the iridium-and rhodium ± P(CH 3 ) 3 complexes. Irradiation of [{h 5 -C 5 (CH 3 ) 5 }M{P(CH 3 ) 3 }H 2 ] in various mixtures of alkane solvents allows the generation of product mixtures which contain alkyl hydrides.…”

“…The reactant configuration describes a situation in which the two electrons on the [CpML] fragment are spin-paired to form the lone pair, while the two electrons on the alkane moiety are spin-paired to form a C ± H s bond. Its valence bond (VB) configuration is labeled 1 [CpML] 1 [CH] (6). On the other hand, the product configuration corresponds to a situation in which the electron pairs are coupled to allow both M ± H and M ± C bond formation and simultaneous C ± H bond breaking.…”

Biochemical catalysis involving coenzyme B-12: a rational stepwise mechanistic interpretation of vicinal interchange rearrangements.

“…It seems reasonable to start with two points that seem solidly based: (i) by some process the bond between the 5'-deoxyadenosyl ligand and cobalt is replaced by a bond between the 5'-deoxyadenosyl group and a hydrogen that was originally bonded to a carbon in the substrate, and (ii) the carbon of the substrate that gives up the hydrogen becomes bonded to cobalt. Given these two requirements, one process from the body of knowledge within organo-transition-metal chemistry stands out as preeminently suited to provide an effective mechanism: the oxidative addition-reductive elimination reaction (10)(11)(12) …”

Charge‐Transfer Effects on Arene Structure and Reactivity

“…Many more examples of metal-activated cleavage of CaH bonds are known for aromatic compounds than for alkanes [37]. To account for this difference, arene/metal coordination is proposed [38], although experimental evidence for such intermediate complexes and their structural features is lacking.…”