Metal Ion-Catalyzed Cycloaddition vs Hydride Transfer Reactions of NADH Analogues with p-Benzoquinones

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Abstract
1-Benzyl-4-tert-butyl-1,4-dihydronicotinamide (t-BuBNAH) reacts efficiently with p-benzoquinone (Q) to yield a [2+3] cycloadduct (1) in the presence of Sc(OTf)3 (OTf = OSO2CF3) in deaerated acetonitrile (MeCN) at room temperature, while no reaction occurs in the absence of Sc3+. The crystal structure of 1 has been determined by the X-ray crystal analysis. When t-BuBNAH is replaced by 1-benzyl-1,4-dihydronicotinamide (BNAH), the Sc3+-catalyzed cycloaddition reaction of BNAH with Q also occurs to yield the [2+3] cycloadduct. Sc3+ forms 1:4 complexes with t-BuBNAH and BNAH in MeCN, whereas there is no interaction between Sc3+ and Q. The observed second-order rate constant (kobs) shows a first-order dependence on [Sc3+] at low concentrations and a second-order dependence at higher concentrations. The first-order and the second-order dependence of the rate constant (ket) on [Sc3+] was also observed for the Sc3+-promoted electron transfer from CoTPP (TPP = tetraphenylporphyrin dianion) to Q. Such dependence of ket on [Sc3+] is ascribed to formation of 1:1 and 1:2 complexes between Q•- and Sc3+ at the low and high concentrations of Sc3+, respectively, which results in acceleration of the rate of electron transfer. The formation constants for the 1:2 complex (K2) between the radical anions of a series of p-benzoquinone derivatives (X−Q•-) and Sc3+ are determined from the dependence of ket on [Sc3+]. The K2 values agree well with those determined from the dependence of kobs on [Sc3+] for the Sc3+-catalyzed addition reaction of t-BuBNAH and BNAH with X−Q. Such an agreement together with the absence of the deuterium kinetic isotope effects indicates that the addition proceeds via the Sc3+-promoted electron transfer from t-BuBNAH and BNAH to Q. When Sc(OTf)3 is replaced by weaker Lewis acids such as Lu(OTf)3, Y(OTf)3, and Mg(ClO4)2, the hydride transfer reaction from BNAH to Q also occurs besides the cycloaddition reaction and the kobs value decreases with decreasing the Lewis acidity of the metal ion. Such a change in the type of reaction from a cycloaddition to a hydride transfer depending on the Lewis acidity of metal ions employed as a catalyst is well accommodated by the common reaction mechanism featuring the metal-ion promoted electron transfer from BNAH to Q.