Positive ions hydride transfer

On the basis of these results we embarked on a systematic study on the synthesis of vinyl cations by intramolecular addition of transient silylium ions to C=C-triple bonds using alkynyl substituted disila alkanes 6 as precursors.(35-37) In a hydride transfer reaction with trityl cation the alkynes 6 are transformed into the reactive silylium ions 7. Under essentially nonHnucleophilic reaction conditions, i.e. in the presence of only weakly coordinating anions and using aromatic hydrocarbons as solvents, the preferred reaction channel for cations 7 is the intramolecular addition of the positively charged silicon atom to the C=C triple bond which results in the formation of vinyl cations 8-10 (Scheme 1). 

Disproportionation has been observed frequently with thiopyrans and rarely with 4//-pyrans, and all cases involve a tetragonal carbon center (position 2 or 4) bearing at least one C—H bond. Some molecules of the substrate are aromatized to corresponding thiopyrylium or pyrylium ions and others reduced to dihydro or tetrahydro products. The relative abilities of pyrans and thiopyrans to disproportionate were interpreted within a proposed hydride transfer mechanism by a CNDO/2 method.45... 

The chemistry of the reduction of NAD+ has been solved most elegantly (Chapter 8, section Bi).2 Oxidation of the alcohol involves the removal of two hydrogen atoms. One is transferred directly to the 4 position of the nicotinamide ring of the NAD+, and the other is released as a proton (equation 16.1).3,4 It is generally thought that the hydrogen is transferred as a hydride ion H , but a radical intermediate cannot be ruled out. For convenience, we shall assume that the mechanism is the hydride transfer. 

Product formation was interpreted in terms of transalkylation of substituted triphenylmethanes. Protonation at the ipso position of the substituted phenyl ring to form arenium ion 64 followed by the C—C bond breaking yields the diphenylmethyl cation, which alkylates benzene or is stabilized by hydride transfer (Scheme 5.30). The protonated intermediate 64 is highly unstable when the ring has an electron-withdrawing substituent. Consequently, its transformation is extremely slow and the primary product triphenylmethane can be isolated. 

The experimental evidence obtained (116, 117) indicates that the central carbon of the acetal function becomes positively charged during the oxidation step. Furthermore, a rather high primary isotope effect (k /k = 6.5) has been measured (121). These results indicate that the reaction mechanism proceeds either via a direct hydride transfer yielding a dialkoxycarbonium ion and a hydrotrioxide ion which would collapse to the hydrotrioxide intermediate (162 - 163 165) (Fig. 20), or via an insertion of ozone in a 1,3-... 

Superelectrophilic activation has also been proposed to be involved, based upon the reactivity of carbocations with molecular hydrogen (a a-donor).16 This chemistry is probably even involved in an enzymatic system that converts CO2 to methane. It was found that A. A -menthyl tetrahy-dromethanopterin (11) undergoes an enzyme-catalyzed reaction with H2 by hydride transfer to the pro-R position and releases a proton to give the reduced product 12 (eq 15). Despite the low nucleophilicity of H2, cations like the tert-butyl cation (13) are sufficiently electrophilic to react with H2 via 2 electron-3 center bond interaction (eq 16). However, due to stabilization (and thus delocalization) by adjacent nitrogen atoms, cations like the guanidinium ion system (14) do not react with H2 (eq 17). 

The reaction catalysed by alcohol dehydrogenases is a transfer of hydride ion from the alcohol to the 4-position of the pyridinium ring of the coenzyme NAD+ (Scheme 6), [For a review of hydride transfer in model systems, see Watt (1988).] The two hydrogen atoms at the 4-position of the dihydro-pyridine ring of NADH are diastereotopic, and over the years it has become apparent that some alcohol dehydrogenases transfer the pro-/ ... 

Carbocation thermochemical data are usually obtained from measurements of proton transfer or hydride transfer equilibria. In the first case, alkyl cations are produced by protonation of the corresponding olefin. However, some species, like, for example, the benzyl cation, cannot be obtained by this route. Moreover, measurements of proton-transfer equilibria are often complicated by side reactions like addition of the cations to the double bonds. With respect to H transfer, Cl transfer reactions offer the advantage that the position of reactive attack is well defined. Moreover, they are usually much faster than H" transfers and lead to a deeper well in the reaction coordinate because the intermediate adduct [R—Cl—R ]+, a chloronium ion, is more stable than [R—H—R ]+, the corresponding proton bound species4. On the other hand, one disadvantage of the CF transfer measurements is the much greater scarcity of A//°(RC1) data as compared to AH°(RH). 

In inosine monophosphate dehydrogenase, the monovalent metal ion accelerates the hydride transfer step of the reaction with apparently few other effects on the enzyme structure. Probably the monovalent cation is involved in helping position the nicotinamide cofactor. The active site and location of the potassium ion are shown in Figure 2. Mycophenolic acid in this diagram is an inhibitor that is thought to lock inosine monophosphate into the active site, as shown. Note the large distance between the inhibitor (in the active site) and the K+. 

Despite what appeared to be conclusive evidence that the mobile positive ion in cyclohexane was the radical cation diffusing via an electron transfer mechanism, Trifunac et al. disputed this for many years, proposing alternative mechanisms of proton and hydride ion transfer. These mechanisms were however eventually retracted in the mid 1990s and the hole mechanism could then be considered to be universally accepted. 

Cycloheptatriene and derivatives thereof donate hydride readily to a variety of carbonium ion acceptors. The position of the end equilibrium depends on the thermodynamics of the exchange. " These reactions are prototypes of a broad area of carbonium ion chemistry wherein carbonium ions equilibrate via intra- and inter-molecular hydride shifts between a donor C—H bond, usually jp hybridized, and a carbonium ion acceptor. This chemistry is often achieved with heterogeneous catalysts and is of great industrial significance it lies outside the emphasis of this review, however. Excellent treatises are available, and a review has appeared on the use of carriers like adamantane to promote hydride transfer in hydrocarbons under strongly acidic conditions. ... 

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. 

NADH (nicotinamide adenine dinucleotide) is a biochemical source of hydride. In the following example NADH reduces acetaldehyde to ethanol via minor pathway H t., hydride transfer to a cationic center. A Zn ion acts as a Lewis acid to polarize the acetaldehyde carbonyl (similar to protonating the carbonyl). The Lewis acid makes the carbonyl a better electron sink by increasing the partial positive charge on carbon. In fact, the electrophilic catalysis by 2+ and 3+ metal ions can accelerate additions to carbonyls by over a million times. The formation of the aromatic pyridinium ring in the NAD" product helps balance the energetics of this easily reversible reaction. 

In this system, planar chain-end C-H bonds in pentane radical cations from which proton donation takes place only come into close contact with secondary C-H bonds at the irmer (C5) position in decane, as well as with primary C-H bonds the latter have a much lower protonation energy than secondary C-H bonds, however, and thus cannot compete effectively as acceptor in the protonation process. Experiments show a marked predominance of 5-chlorodecane over more lateral secondary chlorodecanes, in accordance with the restricted accessibility of secondary C-H bonds in decane to planar chain-end C-H bonds in pentane radical cations. Perhaps even more importantly, no substantial preference is observed for the penultimate position relative to the C3 and C4 positions. This shows unequivocally that the preference for the penultimate position in the experiments with other systems described above is not due to the transformation of alkyl carbenium ions by hydride transfer, i.e., reactions such as... 

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