Metals hydride-metal-nitrogen-proton

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) ...

In the case of HDN, an additional interesting possibility also consistent with the heterolytic mechanism arises, since substrates like the pyridines -or intermediate alkyl or aryl amines- are sufficiently basic to promote the activation of hydrogen so as to form a metal hydride plus a protonated base (e.g. a pyridinium or an alkylammonium cation). Furthermore, some of the most widely accepted amine HDN mechanisms include the initial protonation of the amine nitrogen, followed by elimination of ammonia from the ammonium cation. Therefore, it is very easy to combine the idea of a heterolytic hydrogen activation promoted by, say n-pentylamine, with a subsequent degradation by a Hoffmann mechanism, to conform a reasonable HDN catalytic cycle. A simplified representation of this idea is given in Fig. E4. 

The first site of protonation in a dinitrogen complex, based on mechanistic studies of complexes, must be the dinitrogen itself, yielding the diazenido species (N2H). If protonation occurs at the metal then reaction proceeds no further, or results in the loss of coordinated dinitro-gen. The formal insertion of dinitrogen into a metal-hydride bond (a popular proposal in the early chemical nitrogen-fixation literature) is unknown. 

Figure 14. Proposed transition state for the hydrogenation of ketones by the transfer of proton and hydride from nitrogen and the metal, respectively.

In this system some of the dipole-dipole interactions are substantially reduced by the rapid intramolecular rotation of the NH3 ligand around the C3-axis of the metal-nitrogen bond. In fact the static solid-state spectrum of the compound shows just a broad line (half height width of about 2.1 kHz) centered at the same chemical shift as that found for the NH3 protons in the MAS spectrum, but the hydride ligand resonances cannot be detected because the mutual dipolar interaction, not moderated by any intramolecular motion, results in much broader lines. 

The rates at which protons can be removed from transition metal hydrides (their "kinetic acidities") generally parallel their thermodynamic acidities pK values). However, the removal of a proton from a metal is much slower than the removal of a proton from an electronegative atom like nitrogen or oxygen. The reverse is also true the protonation of a metal (to form a hydride) is slower than the protonation of a nitrogen or an oxygen examples are shown in Equations 3.113 and 3.114. The low kinetic acidity of transition metal hydrides is much like that of carbon acids in organic chemistry. - - ... 

Anion displacement of halides with azolyl anions is one common route to azolyl complexes. One example of such a synthesis is shown in Equation 4.21. In other cases, azolyl complexes have been prepared by proton transfer between the free azole and a metal alkox-ide or hydroxide. An example involving the synthesis of palladium-azolyl complexes is shown in Equation 4.22. In some rare cases, reactions of pyrrole and d early metal alkyls also lead to the formation of a metal-nitrogen bond via o-bond metathesis, as shown in Equation 4.23. Finally, several late-transition-metal-azolyl complexes possessing accompanying hydride Hgands have been prepared by N-H activation of pyrrole and other azoles. 

A second route to dithiocarbamates that utilizes organic isothiocyanates involves the transfer of a sulfur atom between two of them, and also the addition of a proton to nitrogen, generally from a metal hydride (Eq. 37). 

According to a detailed mechanistic study, the first step is the abstraction of the relatively acidic hydrazone proton (93- 97). This is followed by hydride attack on the trigonal carbon of the C=N bond, mainly from the a-side at C-3, together with the concomitant loss of the tosylate anion (97 -> 98). Expulsion of nitrogen from the resulting intermediate (98) yields a fairly insoluble anion-metal complex (99) which upon decomposition with water provides the methylene derivative (100). 

The suggested catalytic cycle for the diamine catalysts indicates that the NH group of the diamine plays a direct role in the hydride transfer through a six-membered TS.53 A feature of this mechanism is the absence of direct contact between the ketone and the metal. Rather, the reaction is pictured as a nucleophilic delivery of hydride from ruthenium, concerted with a proton transfer from nitrogen. 

In the transition metal-catalyzed reactions described above, the addition of a small quantity of base dramatically increases the reaction rate [17-21]. A more elegant approach is to include a basic site into the catalysts, as is depicted in Scheme 20.13. Noyori and others proposed a mechanism for reactions catalyzed with these 16-electron ruthenium complexes (30) that involves a six-membered transition state (31) [48-50]. The basic nitrogen atom of the ligand abstracts the hydroxyl proton from the hydrogen donor (16) and, in a concerted manner, a hydride shift takes place from the a-position of the alcohol to ruthenium (a), re-... 

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