Hydride transfer reaction, transition states

Many rhodium(II) complexes are excellent catalysts for metal-carbenoid-mediated enantioselective C-H insertion reactions [101]. In 2002, computational studies by Nakamura and co-workers suggested the dirhodium tetracarboxylate catalyzed diazo compounds insertion reaction to alkanes C-H bonds proceed through a three-centered hydride-transfer-like transition state (Fig. 25) [102]. Only one rhodium atom of the catalyst is involved in the formation of rhodium carbene intermediate, while the other rhodium atom served as a mobile ligand, which enhanced the electrophilicity of the first one and facilitate the cleavage of rhodium-carbon bond. In this case, the metal-metal bond constitutes a special example of Lewis acid activation of Lewis acidic transition-metal catalyst. 

The data for the four compounds [83]—[86] show a good linear relationship (correlation coefficient r = 0.995) between the (C-)H" C( = 0) distance and the activation energy for hydride transfer reaction of the alkoxide anion (Fig. 16). Here also there is a simple and strong correlation between geometry and reactivity ground state structures closer to the presumed transition state structure give faster reactions. 

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... 

The structural studies have given a clear and chemically satisfying description of the stereochemical and catalytic requirements of the hydride transfer reaction. In three of the examples, there is an acid-base catalyst that forms a hydrogen bond with the carbonyl or alcohol group of the substrate, helps orient it correctly, and stabilizes the transition state for the reaction (equation 16.18). 

Even the alkylation of isobutane by the ferf-butyl cation 4 despite the highly unfavorable steric interaction has been demonstrated142 by the formation of small amounts of 2,2,3,3-tetramethylbutane 36. This result also indicates that the related five-coordinate carbocationic transition state (or high-lying intermediate) 35 of the degenerate isobutylene-terf-butyl cation hydride transfer reaction is not entirely linear, despite the highly crowded nature of the system (Scheme 5.21). 

First, as discussed earlier in connection with the aluminum halide catalyzed rearrangements of hydrocarbons (Section II. A. 2), intermolecular hydride transfer reactions appear to be fairly unselective processes. Apparently, charge development in the transition states of these reactions is minimized a penta-coordinate carbon intermediate may be involved. As a result, the strong preference for the bridgehead positions exhibited by most ionic substitution reactions is partially overcome. 

Transition states of the hydride transfer reaction have been studied quantum chemically using the cluster approach [132,133]. In the transition state, the hydrogen atom becomes positioned between the two carbon atoms, in between which the hydrogen atom changes its position. In Fig. 14, the schematic... 

Fig. 15. Energy contribution to the transition-state energy due to sterical effects for hydride transfer reactions in three zeolites [74].

Chart 4.2. The transition-state descriptors for hydride-transfer reactions as developed by Kreevoy, Han Lee, and their coworkers [12, 40-43][ l... 

Transition state has no bonding to H which is therefore in a hydride-like (H ) circumstance. The electron density available for bonding (2 electrons in a formal hydride-transfer reaction) is thus sequestered on H and shared neither by the donor moiety nor the acceptor moiety in the critical state for the identity reactions (rate constant ku). For the acceptor moiety, this corresponds to no change in electron density and for the donor moiety a decrease of unit electron density, as in the equilibrium (Kio) reaction. Thus ku and KiQ respond equally to donor/acceptor structure (d[ln(feii)]/d[ln(Kio)] — +1) and t — 1 — 1 = 0. 

Figure 17.3. Three-dimensional vibrational wavefunctions representing the transferring hydride for reactant, transition state, and product configurations obtained from hybrid quantum/classical molecular dynamics simulations of the hydride transfer reaction...

The cover picture is derived artistically from the potential-energy profile for the dynamic equilibrium of water molecules in the hydration layer of a protein see A. Douhal s chapter in volume 1) and the three-dimensional vibrational wavefunctions for reactants, transition state, and products in a hydride-transfer reaction (see the chapter by S.J. Benkovic and S. Hammes-Schiffer in volume 4). 

Meerwein-Ponndorf-Verley-Oppenauer (MPVO) reactions are usually mediated by metal alkoxides such as Al(0/-Pr)3. The activity of these catalysts is related to their Lewis-acidic character in combination with ligand exchangeability. The mechanism of these homogeneous MPVO reactions proceeds via a cyclic six-membered transition state in which both the reductant and the oxidant are co-ordinated to the metal center of the metal alkoxide catalyst (Scheme 1). The alcohol reactant is co-ordinated as alkoxide. Activation of the carbonyl by co-ordination to Al(III)-alkoxide initiates the hydride-transfer reaction from the alcoho-late to the carbonyl. The alkoxide formed leaves the catalyst via an alcoholysis reaction with another alcohol molecule, usually present in excess [Ij. 

On the basis of the kinetic and thermodynamic data, a plausible mechanism for the Tishchenko reaction is presented in Scheme 15. In the first step of the reaction, the precatalyst 1 reacts with two equivalents of the aldehyde to give exothermically the alkoxo complex 42 (Step i in Scheme 15 AHcaic = —68 kcal/mol). A second insertion of an aldehyde into the thorium-alkoxide bond yields complex 43 (step ii in Scheme 15). The concomitant hydride transfer from complex 43 to an additional aldehyde releases the ester 44 and produces the active catalytic species 45 (step iii in Scheme 15). The insertion of an aldehyde into complex 45 (step iv, AHcaic = —25 kcal/mol) gives complex 46, and its hydride transfer reaction (step v, rate determining step, AHcaic = —22 kcal/mol) with an additional aldehyde via a plausible six-centered chair-like transition state (47) produces the ester 38 and regenerates the active complex 45. 

There is also a group of reactions in which hydride is transferred from carbon. The carbon-hydrogen bond has little intrinsic polarity or tendency to break in the way required for hydride transfer. These reactions usually proceed via cyclic transition states in which new C-H bonds are formed simultaneously with the cleavage. Hydride transfer is also facilitated by high charge density on the donor carbon atom. The Cannizzaro reaction, the base-catalyzed disproportionation of aldehydes, is one example of a hydride-transfer reaction. A general mechanism is outlined below. The hydride transfer is believed to occur from a species bearing two... 

Asymmetric [l,5]-hydride transfer reactions have recently emerged as being an important method to realize the asymmetric functionalization of C(sp )—H bonds. Both transition metal complexes and small molecule organocatalysts are found to be capable of catalyzing these reactions. This chapter aims at providing a concise description of the state-of-the-art of this promising area. 

Olah s investigation of norbomyl cation provided him with an example illustrating a concept he had advanced in an independent line of research, namely, formation of pentacoordinate carbocation intermediates by attack of electrophiles on C-C and C-H bonds. Space does not permit discussion of the impressive experimental work associated with this concept, but it established that species analogous to the well-known CHs, so common in electron impact mass spectrometry, could be generated in solution. Attack of an electrophile on a C-H bond probably involves a triangular transition state, and results in hydride abstraction if a stable tricoordinate carbocation can be produced. Hydride transfer reactions between carbonium ions are quite common in media of low nucleophilicity ... 

The intercept in Figure 5 leads to n 0.492 and X=86 kJ The transition state bond order is extremely close to 0.5, implying that there is conservation of chemical bond order and resonance effects at the transition state are negligible. The study of other hydride transfer reactions present values of n between 0.50 and 0.485, weakly dependent on the ionization energy, I, of the aromatic molecules nt decreases slightly with an increase in I [34]. 

The last two contributions show the controversy about the mechanism and involvement of acid sites in the hydride transfer reactions over zeohtes. If the hydride transfer activity depends on the hydrophobicity, the rate should correlate to the acid strength of the sites, but not to the acid site density, i.e., the concentration of adjacent active sites. If the reaction is expected to proceed via two carbenium ions adsorbed on adjacent sites or one adsorbed carbenium ion on one site and a feed molecule influenced by the second adjacent site, the acid site density would be probed, but the information about the acid strength would be less obvious. However, in both cases it does not seem that these test reactions can be applied to compare large, medium, and small pore zeolite structures, due to the large (bimolecular) transition state proposed for hydride transfer reactions (unless very small molecules are used [203]). 

A new kinetic equation to estimate activation energies of various hydride transfer reactions has been developed according to transition state theory by using the Morse-type free energy curves of hydride donors and acceptors to model hydride ion release and capture, respectively. " A perfect unity of the kinetic equation and thermodynamic equation for hydride transfer reactions has been achieved. 

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