ACCOUNTS OF CHEXICAL, RESEARCH V O L U M E 8
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Homogeneous Catalytic Activation of C-H Bonds George W. Parshall Central Research Department, Experimental Station, E. I. d u Pont de Nemours and Company, Wilmington, Delaware 19898 Received J u l y 12,1974
Very recently it has become apparent that some Although many of the studies of hydrocarbon actisoluble transition-metal complexes can activate the vation involve the trivial reaction of hydrogen-deuteC-H bonds of hydrocarbons in much the same way rium exchange, more substantial synthetic applicathat the H-H bond of hydrogen is activated in catations are emerging. As described below, C-H activalytic hydrogenation reactions. This discovery points tion can be utilized to introduce substituents such as the way to a new type of homogeneous catalysis for -Al(&H& directly onto an aromatic ring. Chelate reactions such as aromatic substitution1 and opens effects can produce specific ortho substitution in arosome interesting opportunities for applications in matic rings, while enhanced steric effects can direct synthesis. substituents into meta and para positions. The classical methods for substitution of arenes This Account relates several lines of research in and alkanes involve attack of the 'hydrocarbon by a our laboratories which bear on the problem of how a highly reactive reagent. The role of the catalyst has metal atom interacts with a C-H bond, either arobeen to make the attacking reagent more reactive. In matic or aliphatic. halogenation, for example, the catalyst generally proBackground motes dissociation of the X-X bond, either by hetTo put the situation in its proper context, the actierolytic cleavage $0 produce a highly reactive species with characteristics approaching those of X+ or by vation of benzene C-H bonds by heterogeneous catalysts has been known for almost 40 years. Farkas and homolytic cleavage to give a X . species. In either Farkas2 discovered that a platinum film catalyzes the case, it is the halogen reagent that is activated rather exchange between gaseous benzene and D2 even a t than the hydrocarbon. Recent work in several laboratories has shown that room temperature. Subsequent work has provided a reasonable picture of the m e ~ h a n i s mAs . ~ discussed ~~ the C-H bonds in benzene are activated toward prolater, there seems to be a close analogy between the cesses such as H-D exchange by a variety of transition-metal complexes including high-valent species processes occurring a t the active sites of a metal sursuch as TaVH3(C5H&, low-valent species like RuOface and those occurring at the central metal atom in ( d m p e ) ~(dmpe = ( C H ~ ) Z P C H ~ C H ~ P ( C H and ~ ) ~ ) soluble , complex. electrophiles (Pd2+and Pt2+).The activation is genThe first observation of interaction between benerally agreed to involve cleavage of a C-H bond by zene and a soluble transition-metal compound was the metal and formation of a C-M bond. The C-M made by Chatt and f l a ~ i d s o n They .~ found that a rebond then undergoes reaction with a reagent such as duction product of RuClz(dmpe)z reacts with benD2 or C12. The process is similar to the metal-catazene, naphthalene, and other aromatic hydrocarbons lyzed activation of H2 as illustrated by H-D exchange to give arylruthenium hydride complexes such as 1.6 with a metal deuteride Catalytic activation of aromatic C-H bonds by soluble species was discovered by Garnett and coworkC-H
+
M-D
==
=F= C-D -t M-H
C-M
