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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 new aromatic substitution reaction, possibly related to classical electrophilic substitution, has recently emerged. In this reaction, an aryl C-H bond in a donor ligand of a transition metal complex reacts with the central metal atom to form a metal-carbon bond. The hydrogen originally attached to carbon adds to the metal to form a metal-hydrogen bond (eq 1)' or is eliminated as H+ (eq 2).2 This reaction is generally intramolecular and involves an ortho hydrogen bond of an aromatic N-or P-donor ligand.CH2NR2 I 1 In the following discussion we shall first consider individual examples of this reaction with N-donor ligands and P-donor ligands and then attempt to relate these very diverse reactions. Finally, we shall consider extensions of this substitution reaction to intermolecular systems including metal-catalyzed substitution of benzene. Nitrogen Donor LigandsThe substitution of an aryl group which is part of an N-donor ligand molecule has been studied most extensively with palladium. The simplest example is that of eq 2, in which a benzyldialkylamine is treated with methanolic LizPdC14 to give an o-palladiobenzene (1) complex.2 The reaction of a benzyldialkylamine with K2PtC14 proceeds similarly a1 though more slowly to give the platinum analog of 1. Benzylamine and (1) G. Hata, H . Kondo, and A. Miyake, J . Amer. Chem. Soc., 90, (2) A. C. Cope and E. C. Friedrich, ibid., 90, 909 (1968). 2278 (1968).monoalkylbenzylamines do not meta1at.e with Pd or P t salts, but instead form simple coordination complexes such as (CeH6CH2NH2)2PdClz. The orthosubstitution reaction involves proton elimination (eq 2), and it is promoted by proton acceptors such as tributylamine or excess benzyldialkylamine.2 However, the effect is probably kinetic rather than thermodynamic because l-phenylpyrazole undergoes metalation to give 2 even in dilute HC1 or HC104.3 2 3The ortho-palladation reaction shows a strong tendency to form five-membered rings.2 In the series C6Hs(CH2)nN(CH3)2, in which n may be 0, 1, 2, or 3, only the benzylamine (n = 1) undergoes metalation. The other amines, which would form four-, six-, or seven-membered rings by ortho substitution, give only the complexes [C~H~(CHZ)~N(CH~)Z]ZP~C~Z. Similarly, l-naphthyldimethylamine, which could be attacked at either the 2 or 8 position, forms the five-membered structure 3 by reaction with LitPdC14. Azobenzene4 reacts with divalent Pd and Pt salts to give dimeric ortho-metalation products (4) similar to those obtained from the benzylamines. These dimeric products (1-4) have been difficult to characterize structurally because they are insoluble in most common organic solvents. However, tractable monomeric derivatives are formed by treatment with amines4 and phosphine^.^ The phosphines cleave not only the chloride-bridge bonds as do amines, but also the N-Pd coordinate bond. A crystal structure determination5 on 5 demonstrated that a C-Pd bond had formed in the ortho position. This result is in accord with the earlier finding4 that LiAlD4 cleavage of the C-Pd bond in 4 gives o-d...

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