Hydride heterolysis

In contrast to the thermal solvolysis, a rearranged enol ether 45 (and also the hydrolysis product, acetophenone) is formed in addition to the unrearranged product 44. The rearrangement is more apparent in less nucleophilic TFE. The results are best accounted for by heterolysis to give the open primary styryl cation 46 (Scheme 8). This cation gives products of substitution 44 and elimination 30 by reaction with the solvent. Alternatively, 46 can rearrange to the a-phenyl vinyl cation 47 via 1,2-hydride shift, which gives rise to 45 and 30. 

A. Heterolysis of the Metal-Carbon a-Bond Homolysis of the Metal-Carbon a-Bond Oxidation of Lm iM +1-R Followed by Homolysis P-Hydride Shift Reactions P-Elimination Reactions P-Elimination of Carboxylates CO Insertion/Methyl Migration... 

For the Ru system the thiol hydride could not be detected, while for the Rh system and also [IrH2(HS(CH2)3SH)(PCy3)2]+(which similarly catalyzes D2/H + exchange (79)), the H2 complex could not be seen but is a transient. A related system, Ni(NHP Pr3)(S3) clearly shows that heterolysis of D2 can also occur at nickel sites, which may be relevant to H2 activation in [FeNi] hydrogenases (78). 

A mononuclear hydride results here as the final organometallic product (unlike unobserved 14 in Scheme 5). Another possible scenario in Scheme 5 is intermolecular heterolysis of H2, e.g., protonation of the Me group in equilibrium quantities of 11 by the acidic H2 in 13 to give CH4,12, and 14. 

The silane substrate would then displace H2 to give back the starting silane complex for further alcoholysis, and this was determined to be the rate-limiting step. These are all known reactions, and this mechanism and rate-determining step were recently supported by theoretical calculations that showed the heterolysis to be a highly concerted process, i.e., transformation of the a-silane complex to the H2 complex could even take place in a single step, thus circumventing the transient hydride complex (124). It is noteworthy that the mechanism of this reaction involves two different a complexes M(r 2 -Si-H) and M(r 2 -H2). 

The D2/H+ exchange reaction requires a heterolytic cleavage of the D2 molecule into D+ and D species and, vice versa, of H2 into H+ and H-. When such a H2 heterolysis takes place at [MS] sites, plausible intermediates are metal hydride thiol species forming according to Eq. 46. 

Nickel-sulfur hydride complexes were a primary research target. However, the few which were found did not catalyze the H2 heterolysis according to Eqs. 45b or 46 (142). In the quest for other metal-sulfur complexes exhibiting the [M(H)(SH)] motif, the Rh and Ru hydride complexes [Rh(H)(CO)(S4)] (51, 143) and [Ru(H)(PCy3 HS4) (144) were found. They proved to catalyze a D2/ H+ exchange according to Eq. 47. In order to do so, the Rh complex required the addition of catalytic amounts of Brpnsted acids such as aqueous HC1 or HBF4. 

The interconversion of zirconaaziridine enantiomers is slower when there is not a primary or secondary alkyl on the zirconaaziridine carbon and isomerization to an azaallyl hydride is not possible. A mechanism that remains available involves the isomerization of each enantiomer to a planar rf complex (Eq. 43), such as that known to interconvert the enantiomers of aromatic aldehydes [16]. For the chelated zirconaaziridine 2d, high-level density functional theory (DFT) methods and a continuum solvation model have shown that enantiomer interconversion occurs through an rj1-imine intermediate (A) rather than through homolysis (B) or heterolysis (C) of the Zr-C bond [71] (Fig. 13). 

Molecular binding and heterolysis of H2 on metal surfaces and small metal clusters is rarely observed since formation of hydrides is favored. H2 binding to... 

A 1,2-hydride shift As formulated (Scheme 12.2), the reaction could be considered an aza-analogous pinacol-pinacolone rearrangement. However, it seems doubtful that the C=N + < heterolysis on its own is efficient enough to pull off this self-oxidation/reduction. 

Intramolecular heterolysis involves proton transfer to a cis ligand L (e. g. H or Cl) or to the counteranion of a cationic complex. This can occur via the intermediacy of a so-called cis-interaction, which essentially is a hydrogen-bonding like interaction of H2 with a cis ligand, such as a hydride, that has a partial negative charge (S-) [2, 5aj. 

Crabtree first demonstrated heterolysis of H2 by showing that the H2 in [IrIH(H2) (LL)(PPh3)2]+ is deprotonated by LiR in preference to the hydride ligand.61 A milder base, NEt3, was shown by Heinekey62 to more rapidly deprotonate the tj2-H2 tautomer... 

Intramolecular heterolysis of L MX(H2) to give HX (X = halide, usually Cl) is common and is useful for preparative and catalytic chemistry, e.g a metal halide (including bridging X) can be converted to a metal hydride in the presence of base or under phase-transfer or high-pressure conditions 41 64 89 90... 

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