Hydride transfer between NAD+ analogues

Abstract

Rate (k) and equilibrium constants ( K ) have been measured for 17 non-degenerate hydridetransfer reactions, the donors and acceptors generally being variously substituted and elaborated pyridinium ions. Rate constants have also been measured for 6 degenerate reactions of the same type. These results have been used to test the Marcus theory of atom transfer. The Brsnsted plot shows a good deal of scatter, but around K = 1.0 it has about the predicted slope (a) of 0.5. Using the measured values of / c for degenerate reactions and assuming intrinsic reactivity to be constant within structural families, values of k for 12 non-degenerate reactions were calculated without introducing adjustable parameters. They reproduce 99 % of the variation in the measured values, the measured values generally being larger than the calculated values. The data do not permit a test of the predicted variation of a with K .A proposed mechanism involving electron transfer followed by proton transfer followed by another electron transfer, with metastable intermediates between the steps, can be excluded. Such a mechanism would lead to values of k below those predicted by the theory.* We thank Mr M. A, Mahmud for this measurement.

Charge transfer complexation of N,N-dimethyl-p-toluidine with nitrogen heteroaromatic cations as a model for association complexes in hydride transfer reactions

“…In fact, the formation of a charge-transfer complex between 1-benzyl-1,4-dihydronicotinamide (1) and the 10-methylacridinium cation (2) has been spectroscopically observed (5). This complex has A, , , in the vicinity of 550 nm in methanolic solution, which is quite similar to that reported (9) (A,,, = 555 nm) for the complex formed between N,N-dimethylaniline and this same cation.…”

“…This concentration dependence of the absorbance (A) is consistent with eq. [2], which is derived for 1:l complexation between donor and acceptor, with the donor (D) being present in large excess (>lo-fold) over the acceptor cation (total concentration [A+IT of complexed and free cation), and the complex being the only species with a significant extinction coefficient (E,) at the wavelength under study. Fitting eq.…”

“…In general, one would expect (2)(3)(4) that the reaction of eq. [ l ] would be preceded by the formation of an association complex between A+ and DH, and a minimal description of such reactions would be that shown in Scheme From the observation of saturation kinetics for pseudo-firstorder rate constants for reactions of the type shown in eq.…”

Kinetics of the reduction of the tropylium and xanthylium cations by 1,4-dihydropyridine derivatives

“…One of these exceptions (the 2-methyl-5-nitroisoquinolinium cation with kyl = 120) is not unexpected since the reduction of this cation by BNH is known to be complicated by the presence of nonproductive complexation between these two reactants (2,22). The other exception involves the 10-methylacridinium cation for which ky' appears to be "normal" in acetonitrile solution (4) but is reported to be threefold larger in 80% 2-propanol -20% water (13). Thus, aside from these two exceptions, ky' varies by only twofold despite the fact that the individual reactivities of these oxidants vary by 10"-fold with either of these reducing agents individually.…”

“…Reactions of this type originally attracted interest as mechanistic probes of the chemistry of enzymic reactions employing the nicotinamide coenzyme redox couple (NAD(P)H-NAD(P)+) (1 1, 12). More recently, rate-equilibrium correlations of such reactions have been vigorously explored in connection with the application of Marcus theory to the hydride transfer process (1,(13)(14)(15). There is now a significant body of literature relating to the reduction of a variety of pyridinium, quinolinium, isoquinolinium, acridinium, and phenanthridinium cations (2).…”