Mechanistic Models of Asymmetric Reductions

Ohno, Atsuyoshi; Ushida, Satoshi

doi:10.1007/978-1-4899-3123-8_1

Cited by 4 publications

(5 citation statements)

References 163 publications

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“…Dihydronicotinamide adenine dinucleotide (NADH) and analogues act as the source of two electrons and a proton, thus formally transferring a hydride ion to a suitable substrate . The mechanism of the hydride transfer has so far been extensively studied by using NADH analogues in the reactions with various substrates. − In these investigations, the mechanism has been discussed concerning two main possibilities, i.e., concerted hydride transfer or sequential electron−proton−electron (equivalent to a hydride ion) transfer. Since both processes involve the formation of a formal positive charge in the transition state, it has been difficult to differentiate between the mechanisms based on the classical approach of electronic and substitution effects. − We have previously reported that the distinction between the two mechanisms can be made by comparing the reactivities of different types of NADH analogues which have different donor abilities in the initial and second electron transfer in the electron−proton−electron sequence .…”

Section: Introductionmentioning

confidence: 99%

Hydride Transfer from 9-Substituted 10-Methyl-9,10-dihydroacridines to Hydride Acceptors via Charge-Transfer Complexes and Sequential Electron−Proton−Electron Transfer. A Negative Temperature Dependence of the Rates

et al. 2000

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The reactivity of 9-substituted 10-methyl-9,10-dihydroacridine (AcrHR) in the reactions with hydride acceptors (A) such as p-benzoquinone derivatives and tetracyanoethylene (TCNE) in acetonitrile varies significantly spanning a range of 10 7 starting from R ) H to Bu t and CMe 2 COOMe. Comparison of the large variation in the reactivity of the hydride transfer reaction with that of the deprotonation of the radical cation (AcrHR •+ ) determined independently indicates that the large variation in the reactivity is attributed mainly to that of proton transfer from AcrHR •+ to A •following the initial electron transfer from AcrHR to A. The overall hydride transfer reaction from AcrHR to A therefore proceeds via sequential electron-proton-electron transfer in which the initial electron transfer to give the radical ion pair (AcrHR •+ A •-) is in equilibrium and the proton transfer from AcrHR •+ to A •is the rate-determining step. Charge-transfer complexes are shown to be formed in the course of the hydride transfer reactions from AcrHR to p-benzoquinone derivatives. A negative temperature dependence was observed for the rates of hydride transfer reactions from AcrHR (R ) H, Me, and CH 2 Ph) to 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ) in chloroform (the lower the temperature, the faster the rate) to afford the negative activation enthalpy (∆H q obs ) -32, -4, and -13 kJ mol -1 , respectively). Such a negative ∆H q obs value indicates clearly that the CT complex lies along the reaction pathway of the hydride transfer reaction via sequential electron-proton-electron transfer and does not enter merely through a side reaction that is indifferent to the hydride transfer reaction. The ∆H q obs value increases with increasing solvent polarity from a negative value (-13 kJ mol -1 ) in chloroform to a positive value (13 kJ mol -1 ) in benzonitrile as the proton-transfer rate from AcrHR •+ to DDQ •may be slower.

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Section: Introductionmentioning

confidence: 99%

Hydride Transfer from 9-Substituted 10-Methyl-9,10-dihydroacridines to Hydride Acceptors via Charge-Transfer Complexes and Sequential Electron−Proton−Electron Transfer. A Negative Temperature Dependence of the Rates

et al. 2000

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“…Nicotinamide adenine dinucleotide (NADH) is the most important electron source in biological redox reactions, providing a hydride ion that is equivalent to two electrons and a proton. Mechanisms of hydride transfer reactions of NADH analogues with hydride acceptors such as carbonyl compounds and organic cations have been extensively studied chemically − or electrochemically. , The effects of metal ions on hydride transfer reactions from NADH analogues to substrates have particularly attracted considerable interest in relation to the catalytic role of metal ions in the redox reactions of nicotinamide coenzymes in the native enzymatic system. − ,, Metal ions acting as Lewis acids are known to promote electron-transfer reactions, where metal ions bind to the product radical anions produced in the electron-transfer reactions. − Both thermal and photochemical redox reactions that would otherwise be unlikely to occur are made possible to proceed efficiently by the catalysis of metal ions on the electron-transfer steps. − Among metal ions, rare-earth metal ions have particularly attracted considerable attention as much more effective Lewis acids than divalent metal ions such as Mg 2+ and Zn 2+ in various carbon−carbon bond-forming reactions due to the strong affinity to carbonyl oxygen. − In particular, scandium ion (Sc 3+ ) has recently been reported to accelerate electron-transfer reduction of p -benzoquinone derivatives much more efficiently than any other metal ion, including lanthanide ions . The reactions of NADH analogues with p -benzoquinone derivatives, which are normally good hydride acceptors, are also accelerated most remarkably by Sc 3+ .…”

Section: Introductionmentioning

confidence: 99%

Scandium Ion-Promoted Reduction of Heterocyclic NN Double Bond. Hydride Transfer vs Electron Transfer

2002

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Hydride transfer from 10-methyl-9,10-dihydroacridine (AcrH(2)) to 3,6-diphenyl-1,2,4,5-tetrazine (Ph(2)Tz), which contains a N=N double bond, occurs efficiently in the presence of Sc(OTf)(3) (OTf = OSO(2)CF(3)) in deaerated acetonitrile (MeCN) at 298 K, whereas no reaction occurs in the absence of Sc(3+). The observed second-order rate constant (k(obs)) increases with increasing Sc(3+) concentration to approach a limited value. When AcrH(2) is replaced by the dideuterated compound (AcrD(2)), the rate of Sc(3+)-promoted hydride transfer exhibits the same primary kinetic isotope effect (k(H)/k(D) = 5.2+/-0.2), irrespective of Sc(3+) concentration. Scandium ion also promotes an electron transfer from CoTPP (TPP(2)(-) = tetraphenylporphyrin dianion) and 10,10'-dimethyl-9,9'-biacridine [(AcrH)(2)] to Ph(2)Tz, whereas no electron transfer from CoTPP or (AcrH)(2) to Ph(2)Tz occurs in the absence of Sc(3+). In each case, the observed second-order rate constant of electron transfer (k(et)) shows a first-order dependence on [Sc(3+)] at low concentrations and a second-order dependence at higher concentrations. Such dependence of k(et) on [Sc(3+)] is ascribed to formation of 1:1 and 1:2 complexes between Ph(2)Tz(*)(-) and Sc(3+) at the low and high concentrations of Sc(3+), respectively, which results in acceleration of the rate of electron transfer. The formation of 1:2 complex has been confirmed by the ESR spectrum in which the hyperfine structure is different from that of free Ph(2)Tz(*)(-). The 1:2 complex formation results in the saturated kinetic dependence of k(obs) on [Sc(3+)] for the Sc(3+)-promoted hydride transfer, which proceeds via Sc(3+)-promoted electron transfer from AcrH(2) to Ph(2)Tz, followed by proton transfer from AcrH(2)(*)(+) to the 1:1 Ph(2)Tz(*)(-)-Sc(3+) complex and the subsequent facile electron transfer from AcrH(*) to Ph(2)TzH(*). The effects of counteranions on the Sc(3+)-promoted electron transfer and hydride transfer reactions are also reported.

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“…Mechanisms of hydride transfer reactions of nicotinamide adenine dinucleotide (NADH) analogues with hydride acceptors such as carbonyl compounds and organic cations have been extensively studied chemically − or electrochemically. , The effects of metal ions on hydride transfer reactions from NADH analogues to substrates have particularly attracted considerable interest in relation to the essential role of metal ions in the redox reactions of nicotinamide coenzymes in the native enzymatic system. − ,, Metal ions acting as Lewis acids have been reported to accelerate electron-transfer reactions, where metal ions bind to the product radical anions produced in the electron- transfer reactions. − Both thermal and photochemical redox reactions which would otherwise be unlikely to occur have been made possible efficiently by the catalysis of metal ions on the rate-determining electron-transfer steps. − Among metal ions, rare-earth metal ions have attracted much attention as much more effective Lewis acids than divalent metal ions such as Mg 2+ and Zn 2+ in various carbon−carbon bond forming reactions due to the strong affinity to carbonyl oxygen. − However, there has been no report on rare-earth metal ion-catalyzed reactions of NADH analogues.…”

Section: Introductionmentioning

confidence: 99%

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

2001

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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 = OSO(2)CF(3)) in deaerated acetonitrile (MeCN) at room temperature, while no reaction occurs in the absence of Sc(3+). 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 Sc(3+)-catalyzed cycloaddition reaction of BNAH with Q also occurs to yield the [2+3] cycloadduct. Sc(3+) forms 1:4 complexes with t-BuBNAH and BNAH in MeCN, whereas there is no interaction between Sc(3+) and Q. The observed second-order rate constant (k(obs)) shows a first-order dependence on [Sc(3+)] at low concentrations and a second-order dependence at higher concentrations. The first-order and the second-order dependence of the rate constant (k(et)) on [Sc(3+)] was also observed for the Sc(3+)-promoted electron transfer from CoTPP (TPP = tetraphenylporphyrin dianion) to Q. Such dependence of k(et) on [Sc(3+)] is ascribed to formation of 1:1 and 1:2 complexes between Q(*)(-) and Sc(3+) at the low and high concentrations of Sc(3+), respectively, which results in acceleration of the rate of electron transfer. The formation constants for the 1:2 complex (K(2)) between the radical anions of a series of p-benzoquinone derivatives (X-Q(*)(-)) and Sc(3+) are determined from the dependence of k(et) on [Sc(3+)]. The K(2) values agree well with those determined from the dependence of k(obs) on [Sc(3+)] for the Sc(3+)-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 Sc(3+)-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(ClO(4))(2), the hydride transfer reaction from BNAH to Q also occurs besides the cycloaddition reaction and the k(obs) 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.

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Mechanistic Models of Asymmetric Reductions

Cited by 4 publications

References 163 publications

Hydride Transfer from 9-Substituted 10-Methyl-9,10-dihydroacridines to Hydride Acceptors via Charge-Transfer Complexes and Sequential Electron−Proton−Electron Transfer. A Negative Temperature Dependence of the Rates

Hydride Transfer from 9-Substituted 10-Methyl-9,10-dihydroacridines to Hydride Acceptors via Charge-Transfer Complexes and Sequential Electron−Proton−Electron Transfer. A Negative Temperature Dependence of the Rates

Scandium Ion-Promoted Reduction of Heterocyclic NN Double Bond. Hydride Transfer vs Electron Transfer

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

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