Exchange of hydride

FIGURE 10.11 Degenerate exchange of hydride ligands in an osmium complex.29... 

Bis(dimethylamino)borane may be prepared by the reduction of CIB[N(CH3)2]21 or by gas-phase exchange of hydride and dimethylamino groups on boron.2,3 The latter method, using diborane(6) and excess B[N(CH3)2]3 and described here, is the most convenient for the synthesis of a few millimoles of product. The basic vacuum-line techniques necessary to carry out this synthesis are detailed in reference 4. 

Analogous situations exist with allyl hydrides and propene complexes, agostic propene, propene hydrides, and isopropyl complexes. Exchange of hydride with methylene carbene is relevant to the Fischer-Tropsch reaction,... 

The exchange of hydride ligand with ethylenic protons in the complex [NbH-(C2H4)a(dmpe)2l is not observed on the n.m.r. time-scale at 95 and hence the dynamic insertion-deinsertion process is considerably slower than in the isoelec-tronic cation [MoH(C2H4)2(dmpe)2]. ... 

Transfer hydrogenation of aldehydes with isopropanol without addition of external base has been achieved using the electronically and coordinatively unsaturated Os complex 43 as catalyst. High turnover frequencies have been observed with aldehyde substrates, however the catalyst was very poor for the hydrogenation of ketones. The stoichiometric conversion of 43 to the spectroscopically identifiable in solution ketone complex 45, via the non-isolable complex 44 (Scheme 2.4), provides evidence for two steps of the operating mechanism (alkoxide exchange, p-hydride elimination to form ketone hydride complex) of the transfer hydrogenation reaction [43]. 

The proposed mechanism of the catalytic reaction involves the formation of the Cu(l) alkoxide 68 by displacement of either the chloride or the NHC from 65-67, followed by conversion to the hydride 69 by metathetical exchange of the tert-butoxide by the H of the silane (Fig. 2.10). 

B2) Metathetical exchange of a nickel(II(-bonded anionic ligand by an anion of a stronger Brdnsted acid. The nickel II) component can be either an organonickel complex or a nickel hydride. 

Insertion of the alkyne into the Pd-H bond is the first step in the proposed catalytic cycle (Scheme 8), followed by insertion of the alkene and /3-hydride elimination to yield either the 1,4-diene (Alder-ene) or 1,3-diene product. The results of a deuterium-labeling experiment performed by Trost et al.46 support this mechanism. 1H NMR studies revealed 13% deuterium incorporation in the place of Ha, presumably due to exchange of the acetylenic proton, and 32% deuterium incorporation in the place of Hb (Scheme 9). An alternative Pd(n)-Pd(iv) mechanism involving palladocycle 47 (Scheme 10) has been suggested for Alder-ene processes not involving a hydridopalladium species.47 While the palladium acetate and hydridopalladium acetate systems both lead to comparable products, support for the existence of a unique mechanism for each catalyst is derived from the observation that in some cases the efficacies of the catalysts differ dramatically.46... 

A detailed combined experimental computational mechanistic study, performed for isotope exchange in 2-dimethylamino pyridine, showed how the presence of hydrides in the Ir(III) intermediates helps to flatten the potential energy surface, accounting for the extremely high rates of exchange. In this case, carbene intermediates were also involved as a result of double C-H activation. 

Hydride transfer reactions from [Cp2MoH2] were discussed above in studies by Ito et al. [38], where this molybdenum dihydride was used in conjunction with acids for stoichiometric ionic hydrogenations of ketones. Tyler and coworkers have extensively developed the chemistry of related molybdenocene complexes in aqueous solution [52-54]. The dimeric bis-hydroxide bridged dication dissolves in water to produce the monomeric complex shown in Eq. (32) [53]. In D20 solution at 80 °C, this bimetallic complex catalyzes the H/D exchange of the a-protons of alcohols such as benzyl alcohol and ethanol [52, 54]. 

The mechanism of alkyl hydrogen exchange was not clarified, but a possible mechanism was postulated. Partial hydride abstraction by a Lewis acid site may have occured forming a carbocation-like species followed by exchange of a proton at a R-carbon. Such a mechanism predicts exchange to occur preferentially at methyl groups adjacent to the most stable carbocations (benzylic > 3° > 2° > 1°). This is consistent with the observed relative rates of epimerization of steranes during thermal maturation of sediments (83). 

There is ample evidence that the reductive elimination of alkanes (and the reverse) is a not single-step process, but involves a o-alkane complex as the intermediate. Thus, looking at the kinetics, reductive elimination and oxidative addition do not correspond to the elementary steps. These terms were introduced at a point in time when o-alkane complexes were unknown, and therefore new terms have been introduced by Jones to describe the mechanism and the kinetics of the reaction [5], The reaction of the o-alkane complex to the hydride-alkyl metal complex is called reductive cleavage and its reverse is called oxidative coupling. The second part of the scheme involves the association of alkane and metal and the dissociation of the o-alkane complex to unsaturated metal and free alkane. The intermediacy of o-alkane complexes can be seen for instance from the intramolecular exchange of isotopes in D-M-CH3 to the more stable H-M-CH2D prior to loss of CH3D. 

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