Hydrogenation monohydrido complexes

A common, quite general route for activation of hydrogen is the oxidative addition by homolytic cleavage to form a dihydride complex24,124,125 [Eq. (11.21)]. It involves an increase of 2 in the formal oxidation state of the metal. In turn, monohydrido complexes can be formed through both homolytic [Eq. (11.22)] and heterolytic [Eq. (11.23)] cleavages of hydrogen 109... 

The dihydrido complex [RhH2Cl(PPh3)2] is a very important intermediate in the homogeneous catalytic hydrogenation of alkenes.20 The monohydrido complexes (Table 63) can be made by the oxidative addition of HY species to rhodium(I) complexes (equation 187). Similar complexes can be obtained when bulky tertiary phosphines are allowed to react with alcoholic solutions of hydrated rhodium trichloride.268 269... 

There are two monohydrido complexes which have been prepared by the oxidative addition of hydrogen halides to [Rh P(OMe)3 5][BPh4] (equation 223). Two other monohydrido complexes having hydrogen-bonded anions have been prepared by simultaneous substitution of and oxidative addition to dinuclear rhodium(I) complexes (equation 224). The anions give resonances at very low fields in the H NMR spectra of the complexes.1050 The physical properties of the monohydrido complexes are listed in Table 77. 

The other hydrogen halides add oxidatively to rhodium(I) complexes of ditertiary phosphines or arsines giving rise to numerous monohydrido complexes, whose physical properties are also listed in Table 79. However, it is possible to prepare certain monohydrido complexes from rhodium(III) halides. One interesting reaction, carried out under an atmosphere of CO, gives rise to dicar-bonyldichlororhodate(I) salts (equation 241).226... 

Chlorohydridotris(triphenylphosphine)ruthenium(ll) was the first complex in which homogeneous hydrogenation of alkenes was shown to follow the alkyl route." It can be prepared from dihydrogen and [RuCl2(PPh3)3] in the presence of base (equation 41). Most other alkyl route catalysts are also monohydrido complexes. They are usually specific for terminal alkenes. The behavior of several exo-nirfo-dicarbaborane complexes of rhodium has been reviewed." ... 

Another important subgroup of monohydrido complexes can be prepared by the oxidative addition of nido-heteroborane anions to [RhCl(PPh3)3] (Scheme 17). Further, the c/oTO-dicarborane complex is the starting material for the preparation of other rhodium(III) borane complexes (Scheme 18). They are also weakly active hydrogenation catalysts. 

The overall reaction from 10.25 to 10.26 is an insertion into a metal-hydrogen bond. It is, however, only an apparent insertion, as the Rh — H bond dissociates in the diazene diazenido equilibrium (10-9), as already emphasized by Sutton in 1975. Other interesting cases are the reactions of tungsten mono- and bis-hydrido complexes with diazonium salts. The monohydrido complex 10.27 yields the aryldiazene complex 10.28 (Smith and Hillhouse, 1988) in an 1,1-insertion (10-11). The bishydrido complex 10.29 (10-12), however, adds one of the two H-atoms at the... 

From all the above observations, it was concluded that, for diphosphine chelate complexes, the hydrogenation stage occurs after alkene association thus, the unsaturated pathway depicted in Scheme 1.21 was proposed [31 a, c, 74]. The monohydrido-alkyl complex is formed by addition of dihydrogen to the en-amide complex, followed by transfer of a single hydride. Reductive elimination of the product regenerates the active catalysts and restarts the cycle. The monohydrido-alkyl intermediate was also observed and characterized spectroscopically [31c, 75], but the catalyst-substrate-dihydrido complex was not detected. 

Fig. 17.79. Key intermediates in the enantioselective hydrogenations of Figure 17.76 (left). The BINAP ligand is shown schematically as U-shaped, with two PPh2 substituents. Ru-phosphine complexes undergo hydrogenation via Ru(II)/Ru(IV) intermediates, and the double bond of the substrate is hydrometatated in a monohydrido metal complex. Here, HeOH oxidatively adds to Ru(II) (I —> G), white it completes the ligand sphere of the metal Rh(I) in Figure 17.78.

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