Hydride transfer processes from metal

Triazasilatranes 179 and 180 react with various nucleophiles such as organometallic reagents (equation 176), metal alkoxides (equation 177) and amides (equations 178 and 179) to give the substitution products 172, 181-184 as well as hydride transfer products 169, 170. The relative ratios of these products depend on stereoelectronic factors, the nature of the nucleophilic reagents and the reaction conditions312. Thus, the reaction of triazasilatrane 180 with /i-butyllithiurn affords 181a, which is the product of substitution, while only 1-hydrotriazasilatrane (170) is formed from 180 and /e/t-butyllithiurn in a hydride transfer process. 

In fact, the C-H bond activation by the zirconium or tantalum hydride on 2,2-dimethylbutane can occur in three different positions (Scheme 3.5) from which only isobutane and isopentane can be obtained via a P-alkyl transfer process the formation of neopentane from these various metal-alkyl structures necessarily requires a one-carbon-atom transfer process like an a-alkyl transfer or carbene deinsertion. This one-carbon-atom process does not preclude the formation of isopentane but neopentane is largely preferred in the case of tantalum hydride. 

Reaction (64) demonstrates the production of a metal formyl complex by intermolecular hydride transfer from a metal hydride which is expected to be regenerable from H2 under catalytic conditions. Further, it provides a plausible model for the interaction of [HRu(CO)4] with Ru(CO)4I2 during catalysis, and suggests a possible role for the second equivalent of [HRu(CO)4]- which the kinetics indicate to be involved in the process (see Fig. 23). Since the Ru(CO)4 fragment which would remain after hydride transfer (perhaps reversible) from [HRu(CO)4] is eventually converted to [HRu3(CO)),] [as in (64)] by reaction with further [HRu(CO)4], the second [HRu(CO)4]- ion may be involved in a kinetically significant trapping reaction. 

The catalytic effect of metal ions such as Mg2+ and Zn2+ on the reduction of carbonyl compounds has extensively been studied in connection with the involvement of metal ions in the oxidation-reduction reactions of nicotinamide coenzymes [144-149]. Acceleration effects of Mg2+ on hydride transfer from NADH model compounds to carbonyl compounds have been shown to be ascribed to the catalysis on the initial electron transfer process, which is the rate-determining step of the overall hydride transfer reactions [16,87,149]. The Mg2+ ion has also been shown to accelerate electron transfer from cis-dialkylcobalt(III) complexes to p-ben-zoquinone derivatives [150,151]. In this context, a remarkable catalytic effect of Mg2+ was also found on photoinduced electron transfer reactions from various electron donors to flavin analogs in 1984 [152], The Mg2+ (or Zn2+) ion forms complexes with a flavin analog la and 5-deazaflavins 2a-c with a 1 1 stoichiometry in dry MeCN at 298 K [153] ... 

In other work cyclic operation of a metal hydride refrigerator [3] was analyzed. With the help of mathematical model for calculation of transfer processes to HHP the algorithm of definition of optimum switching of heat flows in system has been developed at given cycle time. The algorithm is constructed from a condition of achievement of the maximal refrigerating effect (real COP) on unit of brought heat. 

From the characterization of the hydrolytically solubilized coal, the following observations can be made (1) the polycyclic aromatic rings are being reduced, and a possible mechanism is hydride transfer from the solvent (2) oxygen heteroaromatics are destroyed (3) aryl ethers are being cleaved by this process (4) the ability of the glycols to chelate the positive alkali metal ions could contribute to the enhanced yields of soluble coal compared to other protic solvents. 

Both the oxidant carbonyl compound (acetone) and the substrate alcohol are bound to the metal ion (aluminum). The alcohol is bound as the alkoxide, whereas the acetone is coordinated to the aluminum which activates it for the hydride transfer from the alkoxide. The hydride transfer occurs via a six-membered chairlike transition state. The alkoxide product may leave the coordination sphere of the aluminum via alcoholysis, but if the product alkoxide has a strong affinity to the metal, it results in a slow ligand exchange, so a catalytic process is not possible. That is why often stoichiometric amounts of aluminum alkoxide is used in these oxidations. 

There are, however, numerous organic precendents. The Cannizzaro reaction, in which two equivalents of a nonenolizable aldehyde such as bezaldehyde are reacted with hydroxide to form a primary alcohol and the salt of a carboxylic acid, is thought to involve hydride transfer to one aldehyde carbonyl from the carbonyl-addition product of the other aldehyde and hydroxide. The Leuckart reaction, formation of a tertiary amine from formic acid, a primary amine and either a ketone or an aldehyde, seems to procede via hydride transfer from formate to an iminium ion. And the Meervein-Ponndorf-Verley-Oppenauer reaction, the reversible transfer of hydrogen between ketones and secondary alcohols in the presence of excess aluminum isopropoxide, is almost certainly a hydride-transfer reaction. This latter process is of particular interest to us because it requires a metal, just as GI does. The aluminum acts as a Lewis acid, coordinating the carbonyl oxygen and... 

As demonstrated in this chapter, there have always been the fundamental mechanistic questions in oxidation of C-H bonds whether the rate-determining step is ET, PCET, one-step HAT, or one-step hydride transfer. When the ET step is thermodynamically feasible, ET occurs first, followed by proton transfer for the overall HAT reactions, and the HAT step is followed by subsequent rapid ET for the overall hydride transfer reactions. In such a case, ET products, that is, radical cations of electron donors and radical anions of electron acceptors, can be detected as the intermediates in the overall HAT and hydride transfer reactions. The ET process can be coupled by proton transfer and also by hydrogen bonding or by binding of metal ions to the radical anions produced by ET to control the ET process. The borderline between a sequential PCET pathway and a one-step HAT pathway has been related to the borderline between the outer-sphere and inner-sphere ET pathways. In HAT reactions, the proton is provided by radical cations of electron donors because the acidity is significantly enhanced by the one-electron oxidation of electron donors. An electron and a proton are transferred by a one-step pathway or a sequential pathway depending on the types of electron donors and acceptors. When proton is provided externally, ET from an electron donor that has no proton to be transferred to an electron acceptor (A) is coupled with protonation of A -, when the one-electron reduction and protonation of A occur simultaneously. The mechanistic discussion described in this chapter will provide useful guide to control oxidation of C-H bonds. 

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