Organometallics hydride transfer

Reduction by hydride transfer from the organometallic reagent can become predominant if bulky organometallics are employed. 

Kaim, W. Thermal and Light Induced Electron Transfer Reactions of Main Group Metal Hydrides and Organometallics. 169, 231-252 (1994). 

CO formation on copper electrodes appears to be accompanied by hydride formation as well [103]. In Sch. 3, the surface bound CO is reduced by a hydride transfer reaction to form a formyl species as shown in step 2. There are precedents in organometallic chemistry for late transition metal hydrides reducing bound CO [105-109]. Protonation of the adsorbed formyl in step 3 results in the formation of a hydroxy carbene species [110, 111]. This hydroxycarbene species could be considered to be an adsorbed and rearranged form of formaldehyde, and the reduction of formaldehyde at a copper electrode has been reported to form hydrocarbons [102]. However, reduction of... 

A common motif in organometallic chemistry is the agostic interaction, which can act to stabilize low-coordination low-e-count complexes. The requirement is an alkyl group with a / - or a y-C—H bond attached to the metal within reach of (i.e., cis to) an empty coordination site. An attractive interaction occurs with the C—H bond acting as a 2e donor into the low-lying metal valence orbital that occupies that site. In the case of a / -C—H bond, hydride transfer may occur with little activation, resulting in an M—H sigma bond and complex with an alkene as discussed above. 

The hydride transfer is analogous to the transfer of R e from organometallic compounds to carbonyl groups (Section 14-12A). 

Although the Mukaiyama oxidation is not in the top list of the most frequently used alcohol oxidants, the authors of this book have decided to pay full attention to this procedure because it succeeds in very sensitive organometallic compounds, where most other oxidants fail. The Mukaiyama oxidation operates via a somehow unique mechanism involving a hydride transfer from a metal alkoxide to a very good hydride acceptor, which resembles the Oppenauer oxidation. In variance with the Oppenauer oxidation, the Mukaiyama protocol involves much milder conditions and it does not promote as easily base-induced side reactions. 

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. 

The heterolytic activation of H2 in the above system is particularly interesting in that it may be applicable to reactions in which ionic hydrogenation of hindered substrates from a metal catalyst and H2 is desired. In 1989, Bullock reported the first examples of ionic hydrogenation wherein a mixture of an organometallic hydride such as CpMoH(CO)3 and a strong acid like HO3SCF3 reduces sterically hindered olefins to alkanes via protonation to carbocations followed by hydride transfer from the metal hydride [Eq. (10)] (49). 

When a nucleophile containing a heteroatom reacts at a carboxyl carbon SN, reactions occur that convert carboxylic acid derivatives into other carboxylic acid derivatives, or they convert carbonic acid derivatives into other carbonic acid derivatives. When an organometallic compound is used as the nucleophile, SN reactions at the carboxyl carbon make it possible to synthesize aldehydes (from derivatives of formic acid), ketones (from derivatives of higher carboxylic acids), or—starting from carbonic acid derivatives—carboxylic acid derivatives. Similarly, when using a hydride transfer agent as the nucleophile, SN reactions at a carboxyl carbon allow the conversion of carboxylic acid derivatives into aldehydes. 

BC is reasonably stable at neutral-to-alkaline pH but decomposes rapidly in acidic media. The reaction of [99mTc04]" with BC at pH 11-12 yields quantitatively the organometallic aqua ion [99mTc(OH2)3(CO)3]+. Mechanistically, it is possible that BC acts as a ligand which binds to the technetium centre. Hydride transfer followed or paralleled by reduction occurs concomitantly with CO coordination. The X-ray structure of a model with K+ as the metal is shown in Fig. 1. 

In spite of their practical importance for organic synthesis and industrial catalysis, main group metal hydrides and organometallic compounds without active d orbitals at the metal center have received relatively little attention with regard to their obvious potential for electron transfer reactivity. One reason for this neglect lies in frequent complications arising from solvent-dependent self-association... 

Cyclic Organohydroborate Anions as Hydride Transfer Agents in Reactions with Organic and Organometallic Compounds. In Advances in Boron Chemistry,... 

Despite such intensive studies on MPV reduction chemistry, the corresponding alkylation, i.e., MPV alkylation had never been realized, mainly because of the inertness of alkyl transfer [C] compared with the facile hydride transfer [B] in the MPV reduction, until Maruoka presented the first example of MPV alkynylations for various aldehydes [34]. This truly represents a non-organometallic way of effecting carbonyl alkylation of aldehydes. The success of the approach relies heavily on the discovery of a ligand-accelerated mode for the MPV alkynylations, which has a beneficial effect on the rate of alkynyl transfer. 

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