R. Daniel Libby

Nicotinamide coenzymes are involved in enzymatic reactions that interconvert alcohols, amines or activated methylene compounds with ketones, imines or olefins respectively. The chemistry of these interconversions tends to be very rapid. In fact, in some cases the rates of hydrogen transfer surpass those of product release from the enzyme.

Thus far, three types of mechanisms have been proposed for hydrogen transfer in these reactions:¹

1. Direct bimolecular hydride transfer, in which the hydrogen nucleus and both electrons are transferred as a single unit.²
2. Free radical mechanisms in which the hydrogen is transferred as two hydrogen atoms or as electrons and protons in separate steps.
3. Mechanisms involving covalent intermediates in which the coenzyme and substrate become covalently linked and the electrons are transferred through the covalent bonds while the hydrogen is transferred as a proton.

Pathways involving covalent intermediates solve the problem of high activation energies by employing equilibrium controlled addition-elimination and proton transfer reactions. The mechanism proposed by Hamilton is shown in Eqn. 2.³

The first step of this process is a nucleophilic addition of the substrate to the iminium function of the nicotintium ion. The second step is a cyclic intramolecular proton or hydride transfer. Such a reaction can also be classified as a retro-ene reaction which involves a cyclic aromatic transition state and should be a relatively low energy process. The proposed mechanism is very similar to retro-ene reactions that are known to occur with allyl ethers (Eqn. 3).⁴

If the reaction is viewed in the direction opposite to that discussed by Hamilton, it can be considered as an aromatization of the dihydronicotinamide ring. Similar aromatizations of 1,4-cyclohexadienes can be accomplished by tetracyanoethylene or various quinones.⁵ Evidence has been presented indicating that aromatizations of 1,4-cyclohexadienes by tetracyanoethylene involve an ene type mechanism.⁶ The quinone reactions were believed to occur by a hydride transfer mechanism, however, other evidence introduces some doubt about this conclusion.⁷

Sulzbach and Iqbal reported that 1,4-bis(trimethylsilyl)-1,4-dihydropyridine reacts with acrylonitrile yielding a stable ene adduct.⁸ (Eqn. 4) Wallenfels et. al. found that dihydronicotinamides react with alkylidenemalononitriles and arylmethylenemalononitriles with direct hydrogen transfer to yield nicotinium ions and reduced cyano compounds.⁹ Their study considered only final products, thus, no attempt was made to detect intermediates in these reactions.
 
 

Progress: Early work at Barnard (Natalia Chestnoy) and Colby (William Jenkins & Mishudu Tshamano) with N-benzyl-1,4-dihydropyridine-3-sulfonate as a nicotinamide model showed that it is rapidly oxidized (aromatized) by either tetracyanoethylene or benzylidenemalononitrile. The reaction of the dihydropyridine with benzylidenemalononitrile appears to involve an intermediate. At low concentrations, the disappearance of the uv-vis spectrum characteristic of the dihydropyridine is nearly complete before the spectrum of the pyridinium ion begins to appear. The rapidity of this reaction at high concentration has made structural studies by nmr difficult and less electron deficient alkene substrates such as acrylonitrile and cinnamonitrile, as well as simple aldehydes do not react with this model compound. However, ethyl vinyl ketone and acrolein did yield as yet uncharacterized products whose NMR spectra do not show characteristics of the dihydropyridine ring or the pyridinium ion. The complexity of the spectra could be the result of formation of two different intermediates as indicated in reaction (2).

At Moravian we attempted to reduce the spectral complexity by using a symmetric dihydropyridine. Ryan Mehl synthesized the compound used by Sulzbach and Iqbal, 4-bis(trimethylsilyl)-1,4-dihydropyridine and studied several of its reactions. ⁸ He was able to demonstrate that in the presence of a protonic solvent, the ene-adduct product of equation 4 slowly decomposes to yield oxidized pyridine and propanenitrile thus completing the overall reduction of acrylonitrile. Further work has provided direct NMR evidence for an ene adduct intermediate in the reaction of 1,4-bis(trimethylsilyl)-1,4-dihydropyridine with a variety of substituted acrylonitriles, acroleins and a,b unsaturated ketones.¹⁰

Future Research Plans: Most recently, two Moravian students, Adam Stahler and Catherine Huegler, synthesized N-benzyl-1,4-dihydropyridine and N-benzyl-1,4-dihydroquinoline and began a study of their reactions with acrolein and methyl acrylate. Based on our earlier studies we expect these two new model compounds to be intermediate in reactivity between the two models previously studied. If the ene mechanism does operate, these models should produce relatively stable ene adducts which can be characterized by nmr. Preliminary nmr results with N-benzyl-1,4-dihydroquinoline indicate that reactions did occur. Product spectra are complex, but there are no indications of that oxidized quilnolinium ion was produced. Further analysis should allow the identification of the products of the reactions. This year Jennifer Meitzler is continuing the study focussing on reaction of N-benzyl-1,4-dihydropyridine. The symmetry of this model reduces the number of possible ene adducts to one and should yield reaction spectra that are more easily analyzed. The study of the reactions of this model compound will be expanded to survey a number of the many substrates used in previous model studies.² Beside a,b-unsaturated nitriles, aldehydes and ketones, two other inviting substrates are trifluoroacetophenone and N-methylacridinium ion. Kinetic isotope studies with the latter two substrates have yielded evidence for intermediates in their reactions with model dihydronicotinamides.²

Student Involvement: Eleven undergraduate students have worked on this project. Because of the theory and instrumentation involved in this project, it requires more background experience than my chloroperoxidase project. Students who have worked on this project had completed physical chemistry.
 

Moravian  1992 - Present	Colby  1985 - 1992	Barnard  1982 - 1985	Skidmore  1977-80
Catherine Huegler Julie Jones Ryan Mehl  Jennifer Meitzler Keith Moore Susan Schiavo Adam Stahler Rachel Toroney	William Jenkins  Mishudu Tshamano	Natalia Chestnoy	Karen Schuman

Referencs:

1. (a.)  Cook, P.F. and Cleland, W.W., Biochemistry 20, 1790-1796 (1981);  (b.)  McFarland, J.T., J. Biol. Chem. 252, 3493  (1977);  (c.)  Holbrook, J.J. and Gutfreund, H., FEBS Lett. 31, 157 (1973)
2. Sigman, D.S., Hajdn, J. and Creighton, D.J., Bioorg. Chem. Supp. 4, 385 (1978) and references therein.
3. Hamilton, G.A., Prog. Biorg. Chem. 1, 83-157  (1971).
4. Viola, A., Collins, J.J. and Filipp, N., (1981) Tetrahedron 37, 3765-3811 and references therein.
5. (a.)  Longone, D.T. and Smith, G.L., (1962) Tet. Lett. 1962 205;  (b.) Jackman. L.M., (1960) Adv. in Org. Chem. 2, 239 and references therein;  (c.)  Walker, D. and Hiebert, J.D., (1967) Chem. Rev. 67, 153.
6. Jacobson, B.M., (1980) J. Am. Chem. Soc.102, 886.
7. (a.)  Muller, P. and Joly, D., (1980) Tet. Lett. 1980, 3033;  (b.)  Hasish, S.M. and Hoodless, I.M., (1976) Can. J. Chem. 54, 2261;  (c.)  Muller, P., (1973) Helv. Chim. Acta 56, 1243.  (d.)  Stoos, F. and Rocek, J. (1972) J. Am. Chem. Soc. 94, 2719;  (e.)  Muller, P. and Rocek, J., (1972) J. Am. Chem. Soc. 94, 2719.
8. Sulzbach, R.A. and Iqbal, A.F.M., (1971) Angew. Chem. 83, 758.
9. Wallenfels, K., Ertel, W. and Freidrich, (1973) Justus Liebigs Ann. Chem. 1973, 1663.
10. R. Daniel Libby and Ryan A. Mehl*,  (2001) Manuscript in preparation for submission to J. Am. Chem. Soc.