Hydrides, metal protonation

Advances can be found in references [35-44], A model sequence of reactions for iridium is shown in Figure 2.42. Crucial to most mechanisms is the oxidative addition of the C-S moiety to the metal centre, for which many examples have been reported. The model reaction of 2.42 involves stepwise reactions with hydride and protons and is as yet stoichiometric [45],... 

CHC13 CeHs (CH3)2C0 CH4 metal hydrides, or protons in molecules with shift reagents added... 

This effect is the result of selective protonation of the hydride ligand of 3t because, according to Papich et al., there is no plausible reason for metal protonation to be faster for 3t than 3c. ... 

Quantitative treatment of rate constants for the hydride attack, / hp, the metal protonation, A fpm. and the exchange process in the framework of Scheme 10.7 have resulted in A hp = 2.7 x 10 M Vs and A fpm = 2.8 x 10 M /s. Thus, the hydride protonation occurs faster by a factor of 10. In earlier chapters we have shown that transition metal hydrides form dihydrogen bonds in the presence of proton donors. Now, based on the principle of microscopic reversibility, one can suggest that proton transfer to a hydridic hydrogen actually occurs via a dihydrogen bond. 

Hydrides of Pt(II) are the most numerous of any transition metal hydride group. In addition to the presence of the hydride ligand, the complexes invariably have a coordinated phosphine, and synthetic routes to these compounds using both hydridic and protonic reagents have been reported (I). The pure complexes are usually both air stable and kinetically inert. The purpose of this chapter is to show the diversity of hydrides that can be obtained from protonation reactions on zero-valent and di-valent triphenylphosphine platinum compounds, and to rationalize the type and nature of the product formed from the character of the acid HX. 

The above results are consistent with a steric specific syn 1,2-addition-elimination of metal hydride intermediate which is formed fast in a pre-equilibrium [MH] [MD] and adds to the olefinic substrate to form the metal alkyl intermediate (equation 261). The /1-hydride elimination of the most stable rotamer (equation 262) is the RDS in the rearrangement, leading to a metal hydride-product complex, which starts a new cycle faster than uncoordinated metal hydride. The protonated catalyst, 434, produces a precursor... 

Protonation can take place on a ligand in preference to the metal proton transfer to a hydride occurs in CpFe(Ph2CH2CH2 PPh2)H to give an H2 complex as the kinetic product that only slowly rearranges to the final thermodynamic product, a cationic Fe(IV) dihydride. ... 

Table 1. Formation of Cationic Hydrides by Protonation OF Neutral Transition-Metal Complexes ...

Most transition-metal anions give hydrides by protonation at the metal, but acyl anions generally give hydroxycarbene complexes instead of acyl hydrides " ... 

This chapter is devoted to the discussion of the reduction of carbon-carbon double and triple bonds by noncatalytic methods, These methods include reductions by diimide, by dissolving metals in the presence or absence of proton donors, by low-valent metal ions, by metal hydride-metal halide combinations and by so-called ionic hydrogenation procedures. Of these widely diverse methods of reduction of carbon-carbon double and triple bonds, the reduction by diimide appears to be the most versatile. The reduction of carbon-carbon double and triple bonds by diimide occurs with complete stereoselectivity and stereo-specificity, and can be effected in the presence of a variety of other, very chemically reactive functional... 

The hydride-metal-nitrogen-proton motif and chiral ligand elements for enantioselectivity... 

Figure 1 The hydride-metal-nitrogen-proton motif in the catalyst attacking a ketone in a bifunctional manner (a) and the source of enantioselectivity in the ADH of ketones catalyzed by trans-RuHJ(R)-binap) (diamine) complexes (b) ...

Since many organometallics behave as Lewis bases due to electron-rich metals, protonation is a common reaction. For example, the tungsten hydride 3 undergoes reversible protonation at the metal, forming the water-soluble cationic hydride 4 (Eq. 5). In nickelocene 5, a 20e complex, protonation occurs at the jr-bonded cyclo-pentadienyl ligand the intermediate 6 has a stable, isolable counterpart in the fully methylated derivative. Consecutive loss of cyclopentadiene forms the cation 7, which adds to unchanged nickelocene forming the tripledecker sandwich 8 (Scheme 3). 

E. The Formation of Transition Metal Hydrides by Protonation in Strong... 

Angelici and co-workers have ranked the acidity and the bond dissociation energies of about 50 cationic hydrides by protonating neutral metal complexes in CH2CICH2CI (DCE) with triflic acid (HOSO2CF3) and measuring the enthalpy of the reaction [53, 54]. These fall in the range of 10 to 40 kcal/mol. These results will be anion-dependent because of the low dielectric constant of this solvent. 

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