H-transfers in Complex Systems

Although the model systems of the previous section already involved major molecular motions, the latter can be even much more complex in living systems. Here, only two extreme examples are considered, i.e. hydride transfer in an enzyme and proton transfer in pure methanol. 

Firstly, let us discuss the example of a thermophilic alcohol dehydrogenase from Bacillus stearothermophilus (bsADH) studied by Kohen et al. [91, 92]. This enzyme catalyzes the abstraction of a hydride to the nicotinamide cofactor NAD+ as depicted in Fig. 6.54. The Arrhenius diagram is depicted in Fig. 6.54(a) a sudden decrease in the apparent slope and the apparent intercept of the Arrhenius curves is observed around room temperature (Fig. 6.54(b)). The puzzling observation is that the kinetic isotope effects are independent of temperature in the high-temper-ature regime but dependent on temperature in the low-temperature regime. 

The solid lines in Fig. 6.54 were calculated recently [54] assuming the simple reaction network of Fig. 6.55. It is assumed that the enzyme adopts two different 

Xi and X2 correspond to the mole fractions of states 1 and 2 and K is again the equilibrium constant of the formation of state 2 from state 1. According to Table 

2 The Case of H-transfer in Pure Methanol and Calix[4]arene 

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These reactions of electrophiles with alkene complexes to abstract a hydride from the saturated carbon adjoining the coordinated ir-system can create a sequence for the functionalization of dienes that are derived from the Birch reduction of arenes. A commonly used example of this reaction is the abstraction of a hydride from (cyclohexadiene)Fe(CO)3 (Equation 12.71) by trityl cation as described by Fischer. Such hydride abstractions have been shown to occur by initial electron transfer, followed by H transfer in some cases. ... 

Finally, concurrently with addition, reduction of tri- or dihalomethyl groups in the adduct can occur under conditions of initiating by metal-complex systems in the presence of hydrogen donor chain transfer at C-H bond, at C-Br one, is also possible to form compounds containing one bromine atom less than adducts. 

Cyclic chain termination with aromatic amines also occurs in the oxidation of tertiary aliphatic amines (see Table 16.1). To explain this fact, a mechanism of the conversion of the aminyl radical into AmH involving the (3-C—H bonds was suggested [30]. However, its realization is hampered because this reaction due to high triplet repulsion should have high activation energy and low rate constant. Since tertiary amines have low ionization potentials and readily participate in electron transfer reactions, the cyclic mechanism in systems of this type is realized apparently as a sequence of such reactions, similar to that occurring in the systems containing transition metal complexes (see below). 

Electrocyclization of 1,4-dienes is an efficient process when a heteroatom with a lone pair of electrons is placed in the 3-position, as in 77 (Scheme 20)38. Photoexcitation of these systems generally results in efficient formation of a C—C bond via 6e conrotatory cyclization to afford the ylide 78. These reactive intermediates can undergo a variety of processes, including H-transfer (via a suprafacial 1,4-H transfer) to 79 or oxidation to 80. In a spectacular example of reaction, and the potential it holds for complex molecule synthesis, Dittami and coworkers found that the zwitterion formed by photolysis of divinyl ether 81 could be efficiently trapped in an intramolecular [3 + 2] cycloaddition by the... 

This system fulfills the four above-mentioned conditions, as the active species is a rhodium hydride which acts as efficient hydride transfer agent towards NAD+ and also NADP+. The regioselectivity of the NAD(P)+ reduction by these rhodium-hydride complexes to form almost exclusively the enzymatically active, 1,4-isomer has been explained in the case of the [Rh(III)H(terpy)2]2+ system by a complex formation with the cofactor[65]. The reduction potentials of the complexes mentioned here are less negative than - 900 mV vs SCE. The hydride transfer directly to the carbonyl compounds acting as substrates for the enzymes is always much slower than the transfer to the oxidized cofactors. Therefore, by proper selection of the concentrations of the mediator, the cofactor, the substrate, and the enzyme it is usually no problem to transfer the hydride to the cofactor selectively when the substrate is also present [66]. This is especially the case when the work is performed in the electrochemical enzyme membrane reactor. 

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