By Hydride Transfer

Chromate oxidations are further complicated by the ability of other valencies of chromium to oxidise alcohols. Cr, in acidic aqueous solution, probably as CrO, oxidises alcohols probably by hydride transfer, the chromous (Cr ) ion product reacting with Cr to give two equivalents of Cr with a near-diffusional rate constant of 2 x Cr will also oxidise alcohols  

2 By Hydride Transfer. An important pathway of oxidation is the removal of hydride attached to an oxygen atom if the oxygenated centre is OH, the reaction is accelerated by partial or complete removal of the hydroxyl proton. 

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Approach of borohydnde to the top face of the carbonyl group is sterically hindered by one of the methyl groups The bottom face of the carbonyl group is less congested and the major product is formed by hydride transfer from this direction... 

FIGURE 24.12 The mechanism of acyl-CoA dehydrogenase. Removal of a proton from the u-C is followed by hydride transfer from the /3-carbon to FAD. 

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

Trinitrophenol is degraded in a reaction involving ring reduction by hydride transfer from an NADPH-dependent F420 reductase (Hofmann et al. 2004). 

Details of the mechanism have revealed the role of Meisenheimer-type hydride complexes, which is consistent with the presence of the strongly electron-withdrawing nitro groups that facilitate what are formally hydride reductions involving reduced coenzyme F420 (Ebert et al. 1999). For example, 2,4,6-trinitrophenol is reduced successively by hydride transfers catalyzed by F420, followed by loss of nitrite, further reduction, and ring fission to 4,6-dinitrohexanoate (Hofmann et al. 2004). 

The diborane is generated (in situ, or separately, from NaBH4 and Et2Offi—BF3e), and probably complexes, as the monomeric BH3, with the ethereal solvent used for the reaction. BH3 is a Lewis acid and adds to the least substituted carbon atom of the alkene (Markownikov addition), overall addition is completed by hydride transfer to the adjacent, positively polarised carbon atom ... 

Alternatively, unreactive mixtures of organosilicon hydrides and carbonyl compounds react by hydride transfer from the silicon center to the carbon center when certain nucleophilic species with a high affinity for silicon are added to the mixture.76 94 This outcome likely results from the formation of valence-expanded, pentacoordinate hydrosilanide anion reaction intermediates that have stronger hydride-donating capabilities than their tetravalent precursors (Eq. 6).22,95 101... 

Addition of a second molecule of BH3 at the oxygen atom of 5, followed by hydride transfer and loss of an oxyborane moiety, is proposed as a route to the metal ethyl complex 6 ... 

Hydride transfer from [(bipy)2(CO)RuH]+ occurs in the hydrogenation of acetone when the reaction is carried out in buffered aqueous solutions (Eq. (21)) [39]. The kinetics of the reaction showed that it was a first-order in [(bipy)2(CO)RuH]+ and also first-order in acetone. The reaction proceeds faster at lower pH. The proposed mechanism involved general acid catalysis, with a fast pre-equilibrium protonation of the ketone followed by hydride transfer from [(biPy)2(CO)RuH]+. 

No intermediate tungsten complexes were observed in this reaction. The alcohol, sec-phenethylalcohol, is consumed at a rate which is much faster than that of its formation. It was shown separately to be converted to ethylbenzene (Eq. (23)) by HOTf and [Cp(CO)3WH]. This reaction presumably proceeds through loss of water from the protonated alcohol, followed by hydride transfer from [Cp(CO)3WH] to give ethylbenzene. 

The cationic tantalum dihydride Cp2(CO)Ta(H)2]+ reacts at room temperature with acetone to generate the alcohol complex [Cp2(C0)Ta(H01Pr)]+, which was isolated and characterized [45]. The mechanism appears to involve protonation of the ketone by the dihydride, followed by hydride transfer from the neutral hydride. The OH of the coordinated alcohol in the cationic tantalum alcohol complex can be deprotonated to produce the tantalum alkoxide complex [Cp2(C0)Ta(01Pr)]. Attempts to make the reaction catalytic by carrying out the reaction under H2 at 60 °C were unsuccessful. The strong bond between oxygen and an early transition metal such as Ta appears to preclude catalytic reactivity in this example. 

In a series of investigations of the cracking of alkanes and alkenes on Y zeolites (74,75), the effect of coke formation on the conversion was examined. The coke that formed was found to exhibit considerable hydride transfer activity. For some time, this activity can compensate for the deactivating effect of the coke. On the basis of dimerization and cracking experiments with labeled 1-butene on zeolite Y (76), it is known that substantial amounts of alkanes are formed, which are saturated by hydride transfer from surface polymers. In both liquid and solid acid catalysts, hydride transfer from isoalkanes larger than... 

A viable iron carbonyl-mediated reduction process converts acid chlorides and bromoalkanes into aldehydes [3, 6]. Yields are high, with the exception of nitro-benzoyl chloride, and the procedure is generally applicable for the synthesis of alkyl, aryl and a,(i-unsaturated aldehydes from the acid chlorides. The reduction proceeds via the initial formation of the acyl iron complex, followed by hydride transfer and extrusion of the aldehyde (cf. Chapter 8). 

The reactions of the species H3O+, NO+, and O2+ with a range of aldehydes and ketones have been studied by the selected ion flow tube (SIFT) method. H3O+ protonates ketones and aldehydes, with the latter eliminating water under the conditions of measurement. Similarly, NO+ associates with ketones, but this is followed by hydride transfer for the aldehydes. O2+ reactions typically produce several ionic products. 

Trapping the Norbornyl Ion by Hydride Transfer from Methylcyclopentane in 2M SbFs/H(T)S03F. ([MCP]/[norbornyl ion] = 40)... 

According to further papers from Gronowitz etal. [70ZC389 73ACS2257, 73CS(3)165 78JHC285], the reduction of other tricyclic tropones does not require the presence of a Lewis acid. This might be due to the stability (see Section III,B,3,b) of the carbenium ions (formed from the intermediate tropols), which are further reduced to the tropilidenes by hydride transfer. For example, tropylium ions 237 and 238 can be prepared by this method. 

This reaction may account in part for the oligomers obtained in the polymerization of pro-pene, 1-butene, and other 1-alkenes where the propagation reaction is not highly favorable (due to the low stability of the propagating carbocation). Unreactive 1-alkenes and 2-alkenes have been used to control polymer molecular weight in cationic polymerization of reactive monomers, presumably by hydride transfer to the unreactive monomer. The importance of hydride ion transfer from monomer is not established for the more reactive monomers. For example, hydride transfer by monomer is less likely a mode of chain termination compared to proton transfer to monomer for isobutylene polymerization since the tertiary carbocation formed by proton transfer is more stable than the allyl carbocation formed by hydride transfer. Similar considerations apply to the polymerizations of other reactive monomers. Hydride transfer is not a possibility for those monomers without easily transferable hydrogens, such as A-vinylcarbazole, styrene, vinyl ethers, and coumarone. 

Pyridine is difficult to reduce (as is benzene ), but pyridinium salts, e.g. alkylpyridinium halides, are partly reduced by hydride transfer reagents such as lithium aluminium hydride (LiAlH ) and sodium borohydride (NaBH4). LiAlH, which must be used in anhydrous conditions, only gives the 1,2-dihydro derivative, but the less vigorous reductant NaBH in aqueous ethanol yields the 1,2,5,6-tetrahydro derivative (Scheme 2.30)1... 

Table 3. Chiral Allenes from Propynyl Compounds by Hydride Transfer from Aluminum Hydride Reagents in THF...

Termination by hydride transfer can be achieved using silane to give aldehydes (Equation (35)). ... 

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