Aryl hydride complexes

Both Ni and Pd reactions are proposed to proceed via the general catalytic pathway shown in Scheme 8.1. Following the oxidative addition of a carbon-halogen bond to a coordinatively unsaturated zero valent metal centre (invariably formed in situ), displacement of the halide ligand by alkoxide and subsequent P-hydride elimination affords a Ni(II)/Pd(ll) aryl-hydride complex, which reductively eliminates the dehalogenated product and regenerates M(0)(NHC). ... 

This work was extended to consider the analogous toluene (179+) and p-xylene (180+) systems, and [K2-(HTp )Pt(Ph)(r 2-C6H6)]+ (181+, Scheme 14). Each of these were obtained by protonation of the respective aryl hydride complex Tp Pt(R)H(Ar) (R = H, Ar = Tol 182, 2,5-Xyl 183 R = Ar = Ph 153).72 As with 178+, no evidence for arene dissociation is observed for either 179+ or 180+ below 0 °C, though both lose the arene above this temperature to afford the same dihydride-bridged dimer. In contrast, 181+ slowly exchanges coordinated benzene with the solvent at -100 °C. 

Arene complexes may also occur as intermediates in the formation of other aryl hydride complexes such as (7r-C5H5)2Mo(H)(aryl) (aryl = phenyl, />-tolyl) 171). 

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... 

M—H bond energies are approximately constant and negligible difference in AS, the thermodynamic preference of metal aryl hydride complexes and free alkane over metal alkyl hydride complexes and free aromatic substrate suggests that the ABDE for M Ar versus M—R is greater than the ABDE for Ar—H versus R—H (Chart 11.6). 

When there is a single substituent on the benzene ring, there are six possible isomeric aryl hydride complexes the metal center can be cis or trans and the attack on the ring can... 

Arene elimination from Cp - or Tp -coordinated aryl-hydride complexes follows a similar reaction process, while this reaction involves an r -arene complex as a relatively stable intermediate [69,70]. The mechanism proposed for Tp RhH(Ph)(CNCH2CMe3) is given in Scheme 9.29 [70]. The first step is the formation of a benzene-coordinated intermediate via C-H reductive ehmination involving k -k rearrangement of the Tp ligand. The benzene ligand is subsequently displaced by an external L (CNCH2CMe3) to afford the final products. 

The reductive elimination of arenes from aryl hydride complexes draws many parallels with the reductive elimination from alkyl hydrides. For example, arene complexes are believed to form from ttie reductive elimination prior to release of the free arene. Studies by Feher and Jones provided compelling evidence for the presence of arene complexes, - and complementary synthetic studies have led to the isolation of closely related "q -arene complexes. " As shown in Equation 8.24, the arylrhodium deuteride complex scrambles deuterium into the aryl ring, just as alkyhnetal deuteride complexes scramble deuterium into the alkyl group. This isotopic exchange presumably occurs by formation of the arene complex shown at the center of Equation 8.24... 

Treatment of the octahedral ruthenium(Il) chloride (P— P)2RuQ2, where P—P = Me2PCH2CH2PMe2, with arylsodium gives complexes of stoicheiometry Ru(arene)(P— P)2, where arene = benzene, naphthalene, anthracene and phenanthrene [22]. Detailed chemical infrared and proton magnetic resonance studies have shown that the compound in solution is an equilibrium mixture of the arene complex and the a-aryl hydride complex, e.g. 

The discussion has focused so far on activation of alkanes, where formation of the a-complex seems to precede oxidative addition. For arenes, formation of the analogous a(cH)-arene complex is thought to occur before oxidative addition to form an aryl hydride. These a-com-plexes have never been observed, presumably because they are unstable with respect to the 71-complexes. Both types of arene complexes are, for the case of benzene, shown in Scheme 25 the a(CH)-arene complex as A and... 

The proposed mechanism for a standard Heck reaction is depicted in Scheme 6.5. Generally, a haloalkene or haloarene undergoes oxidative addition to an in situ generated, coordinatively unsaturated 14-electron palladium(O) complex, but other substrates such as tosylates, triflates or diazonium salts can also be applied. Subsequent, sy -insertion into the C=C double bond of a complexed olefin yields a t7-(j -alkenyl) or (j- aryl)alkylpalladium complex. If no hydrogen atom in a pseudo cis-position relative to the palladium is present, an internal rotation step is required prior to syw-elimination of the olefin to afford the traws-olefin product and a palladium(II) hydride complex. The latter is restored to the initial Pd(0) species by base-induced reductive elimination.137"401... 

Hydride complexes of palladium and platinum are almost invariably stabilized by phosphine ligands and play an important role in catalytic processes such as hydrogenation. Examples are Pt(H)ClL2 and Pt(H)2L2, as well as hydrido alkyls and aryls, trans,-Pt(H)(R)L2. There are cis and trans isomers. A typical reaction is the insertion of alkenes and alkynes into the Pt—H bond 33... 

The latter is outside the scope of organometallic chemistry, but within the first two topics the work involved three main themes olefin and acetylene complexes, alkyl and aryl complexes, and hydride complexes. As continuous subsidiary themes throughout ran the complex chemistry of tertiary phosphines and such ligands, the nature of the trans effect, and the nature of the coordinate bond. All the work from 1947 to 1969 was carried out in the Butterwick Research Laboratories, later renamed Akers Research Laboratories, of Imperial Chemical Industries Ltd., and I am indebted to that Company and particularly to Mr. R. M. Winter, the Company s Controller of Research, and Sir Wallace Akers, its Director of Research, who in 1947, made available to me the opportunity to develop my research in my own way, in those laboratories. 

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