Hydride transfer equilibria

Gas-phase mass spectrometric studies891-894 also indicate exceptional stability of the 2-norbomyl cation relative to other potentially related secondary cations. A study by Kebarle and co-workers895 also suggests that the 2-norbornyl cation is more stable than the tert-butyl cation in the gas phase (based on hydride transfer equilibria from their respective hydrocarbons). 

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). 

Solutions of aluminium bromide in dichloromethane used as a catalyst in hydride-transfer equilibrium experiments should be kept cold, as a potentially dangerous exothermic halide exchange reaction occurs on warming. 

The hydricity data established using the thermochemical cycles have been utilized in conjunction with hydride-transfer equilibrium measurements between various organometallic and organic molecules. 

There are also reactions in which hydride is transferred from carbon. The carbon-hydrogen bond has little intrinsic tendency to act as a hydride donor, so especially favorable circumstances are required to promote this reactivity. Frequently these reactions proceed through a cyclic TS in which a new C—H bond is formed simultaneously with the C-H cleavage. Hydride transfer is facilitated by high electron density at the carbon atom. Aluminum alkoxides catalyze transfer of hydride from an alcohol to a ketone. This is generally an equilibrium process and the reaction can be driven to completion if the ketone is removed from the system, by, e.g., distillation, in a process known as the Meerwein-Pondorff-Verley reduction,189 The reverse reaction in which the ketone is used in excess is called the Oppenauer oxidation. 

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]+. 

The kinetics of the ionic hydrogenation of isobutyraldehyde were studied using [CpMo(CO)3H] as the hydride and CF3C02H as the acid [41]. The apparent rate decreases as the reaction proceeds, since the acid is consumed. However, when the acidity is held constant by a buffered solution in the presence of excess metal hydride, the reaction is first-order in acid. The reaction is also first-order in metal hydride concentration. A mechanism consistent with these kinetics results is shown in Scheme 7.8. Pre-equilibrium protonation of the aldehyde is followed by rate-determining hydride transfer. 

Of particular interest is the dinuclear Ru complex 34, the so-called Shvo catalyst [55, 56]. It has been established that, under the reaction conditions, this complex is in equilibrium with two monometal complexes (35 and 36) [57-59]. Both of these resemble catalytic intermediates in the concerted proton-hydride transfer pathway (Scheme 20.13), and will react in a similar way (Scheme 20.15) involving the six-membered transition state 37 and the reduction of the substrate via 38. 

Butene as the feed alkene would thus—after hydride transfer—give 2,2,3-TMP as the primary product. However, with nearly all the examined acids, this isomer has been observed only in very small amounts. Usually the main components of the TMP-fraction are 2,3,3-, 2,3,4-, and 2,2,4-TMP, with the selectivity depending on the catalyst and reaction conditions. Consequently, a fast isomerization of the primary TMP-cation has to occur. Isomerization through hydride- and methyl-shifts is a facile reaction. Although the equilibrium composition is not reached, long residence times favor these rearrangements (47). The isomerization pathways for the TMP isomers are shown schematically in Fig. 3. 

Fast hydride transfer reduces the lifetime of the isooctyl cations. The molecules have less time to isomerize and, consequently, the observed product spectmm should be closer to the primary products and further from equilibrium. This has indeed been observed when adamantane, an efficient hydride donor, was mixed with zeolite H-BEA as the catalyst (78). When 2-butene/isobutane was used as the feed, the increased hydride transfer activity led to considerably higher 2,2,3-TMP and lower 2,2,4-TMP selectivities, as shown in Fig. 5. 

If secondary isotope effects arise strictly from changes in force constants at the position of substitution, with none of the vibrations of the isotopic atom being coupled into the reaction coordinate, then a secondary isotope effect will vary from 1.00 when the transition state exactly resembles the reactant state (thus no change in force constants when reactant state is converted to transition state) to the value of the equilibrium isotope effect when the transition state exactly resembles the product state (so that conversion of reactant state to transition state produces the same change in force constants as conversion of reactant state to product state). For example in the hydride-transfer reaction shown under point 1 above, the equilibrium secondary isotope effect on conversion of NADH to NAD is 1.13. The kinetic secondary isotope effect is expected to vary from 1.00 (reactant-like transition state), through (1.13)° when the stmctural changes from reactant state to transition state are 50% advanced toward the product state, to 1.13 (product-like transition state). That this naive expectation... 

Kurz, L.C. and Erieden, C. (1980). Anomalous equilibrium and kinetic alpha-deuterium secondary isotope effects accompanying hydride transfer from reduced nicotinamide adenine dinucleotide. J. Am. Chem. Soc. 102, 4198-4203... 

Alkyl cations are thus not directly observed in sulphuric acid systems, because they are transient intermediates present in low concentrations and react with the olefins present in equilibrium. From observations of solvolysis rates for allylic halides (Vernon, 1954), the direct observation of allylic cation equilibria, and the equilibrium constant for the t-butyl alcohol/2-methylpropene system (Taft and Riesz, 1955), the ratio of t-butyl cation to 2-methylpropene in 96% H2SO4 has been calculated to be 10 . Thus, it is evident that sulphuric acid is not a suitable system for the observation of stable alkyl cations. In other acid systems, such as BFj-CHsCOOH in ethylene dichloride, olefins, such as butene, alkylate and undergo hydride transfer producing hydrocarbons and alkylated alkenyl cations as the end products (Roberts, 1965). This behaviour is expected to be quite general in conventional strong acids. 

That specific hydride transfer from carbon to carbon does occur, was established by showing that use of labelled (Me2CDO)3Al led to the formation of RjCDOH. The reaction probably proceeds via a cyclic T.S. such as (47), though some cases have been observed in which two moles of alkoxide are involved—one to transfer hydride ion, while the other complexes with the carbonyl oxygen atom. The reaction has now been essentially superseded by MH reductions, but can sometimes be made to operate in the reverse direction (oxidation) by use of Al(OCMc3)3 catalyst, and with a large excess of propanone to drive the equilibrium over to the left. This reverse (oxidation) process is generally referred to as the Oppenauer reaction. 

A hydride transfer from the methylene group to the copper ion followed by the oxidation of the latter by the oxidizing agent is also unlikely, in view of the fact that the deamination reaction is not affected by the addition of nucleophiles like chloride ions, which would be expected to interfere with a pre-equilibrium involving hydride ions. 

Participation of the hydride-formyl equilibrium in (16) is also plausible in light of an apparent inverse kinetic deuterium isotope effect for the catalytic process. Use of deuterium gas instead of hydrogen (cf. Expts. 6 and 4 in Table II) causes an increased rate, with kH/k = 0.73 (37). The existence of an isotope effect implies that hydrogen atom transfer occurs before or during the rate-determining step, and an inverse kinetic isotope effect may be possible in the case of a highly endothermic, product-like transition state (73). On the other hand, Bell has concluded that inverse kinetic isotope... 

The reverse reaction corresponds to intramolecular C—H bond formation. It requires that the alkene rotate from its equilibrium perpendicular orientation toward the less favored parallel orientation. The barrier hindering the rotation of the alkene may be partially or totally offset by the incipient agostic interaction. If the alkene is unsym-metrical, the question of regioselectivity of hydride transfer arises. In the case of an unsymmetrical alkene with an X or C substituent, the donor orbital is polarized away from the substituent (Figure 13.9) and the metal lies closer to that end. Conversely,... 

Positional Isomerization. A different type of isomerization, substituent migration, takes place when di- and polyalkylbenzenes (naphthalenes, etc.) are treated with acidic catalysts. Similar to the isomerization of alkanes, thermodynamic equilibria of neutral arylalkanes and the corresponding carbocations are different. This difference permits the synthesis of isomers in amounts exceeding thermodynamic equilibrium when appropriate reaction conditions (excess acid, fast hydride transfer) are applied. Most of these studies were carried out in connection with the alkylation of aromatic hydrocarbons, and further details are found in Section 5.1.4. 

It is possible to hydrogenate aromatics in the superacids HF-TaF5, HF-SbF5, or HBr-AlBr3 in the presence of hydrogen. The reduction of benzene was shown to give an equilibrium mixture of cyclohexane and methylcyclopentane.243 244 Reduction was postulated to proceed via initial protonation of benzene followed by hydride transfer ... 

Oxidative-reductive disproportionation is a rather typical property of some pseudo bases. Thus, l,3-dimethyl-2-hydroxybenzimidazoline (229), which exists in the solid state in the open-chain form (228), on heating at 165-185°C, is converted to mixture of 1,3-dimethylbenzimidazolone (230) (49%) and 1,3-dimethylbenzimidazoline (231) (46%) (85KGS1694). Evidently, the process proceeds via an equilibrium amount of (229) undergoing hydride transfer. 

---