Base-catalyzed hydrogen isotope

Base-catalyzed hydrogen isotope exchange in inactivated selenophenes occurs only in highly basic media. The relative rates of exchange of deuterium at the a-position in furan, thiophene, and selenophene in 0.4 M t-BuOK/DMSO... 

Benzannelation results in an attenuation of aromaticity effects in 4n + 2rc-systems.255 The base-catalyzed hydrogen isotope exchange and isomerization reactions of isomeric dihydrobenzazocines 135 and 136 show these compounds have moderately enhanced kinetic acidity as compared with dihydroquinoline models and is attributable to a small degree of aromatic stabilization in the incipient 1()n electron benzazocinyl anions.255... 

Base-catalyzed hydrogen isotopic exchange in heteroaromatic compounds 74KGS1587. 

J.A. Rabi and J J. Fox, Facile base-catalyzed hydrogen isotope labeling at position 6 of pyrimidine... 

The base-catalyzed hydrogen exchange of pyrazoles has been studied by Vaughan and conveniently summarized in <74AHC(16)l) and (B-76MI40402). The method has been used to prepare the isotopic species necessary to assign the fundamental vibration of pyrazole (Sections 4.04.1.3.7(i) and 4.04.3.3.3). 

The azo coupling reaction proceeds by the electrophilic aromatic substitution mechanism. In the case of 4-chlorobenzenediazonium compound with l-naphthol-4-sulfonic acid [84-87-7] the reaction is not base-catalyzed, but that with l-naphthol-3-sulfonic acid and 2-naphthol-8-sulfonic acid [92-40-0] is moderately and strongly base-catalyzed, respectively. The different rates of reaction agree with kinetic studies of hydrogen isotope effects in coupling components. The magnitude of the isotope effect increases with increased steric hindrance at the coupler reaction site. The addition of bases, even if pH is not changed, can affect the reaction rate. In polar aprotic media, reaction rate is different with alkyl-ammonium ions. Cationic, anionic, and nonionic surfactants can also influence the reaction rate (27). 

Unlike the Rh-based hydrogenation of a-(acylamino)acrylates, the corresponding Ru chemistry has not been studied extensively. Ru complexes of (S)-BINAP and (S,S)-CHIRAPHOS catalyze the hydrogenation of (Z)-a-(acylamino)cinnamates to give the protected ( -phenylalanine with 92% ee [74] and 97% ee [75], respectively. It is interesting that the Rh and Ru complexes with the same chiral diphosphines exhibit an opposite sense of asymmetric induction (Scheme 1.6) [13,15,56,74,75]. This condition is due primarily to the difference in the mechanisms the Rh-catalyzed hydrogenation proceeds via Rh dihydride species [76], whereas the Ru-catalyzed reaction takes place via Ru monohydride intermediate [77]. The Rh-catalyzed reaction has been studied in more detail by kinetic measurement [78], isotope tracer experiments [79], NMR studies [80], and MO calculations [81]. The stereochemical outcome is understandable by considering the thermodynamic stability and reactivity of the catalyst-enamide complexes. 

Okazaki O, Guengerich FP. Evidence for specific base catalysis in M-dealkylation reactions catalyzed by cytochrome P450 and chloroperoxidase. Differences in rates of deprotonation of aminium radicals as an explanation for high kinetic hydrogen isotope effects observed with peroxidases. / Biol Chem 1993 268 1546-52. 

The hydrogen-deuterium solvent isotope effect for the base-catalyzed... 

The mechanisms in Figures 7.8 and 7.9 are supported further by two observations (i) chiral ketones undergo both acid- and base-catalyzed race-mization, and (ii) exchange of hydrogen isotopes on the a carbon atoms is also observed. The rate constants for acid-catalyzed iodination and racemization are identical for phenyl sec-butyl ketone. The same is true for acid-catalyzed bromination and racemization of (-H )-2-(2-carboxybenzyl)indanone as... 

NiFe] and [NiFeSeJ-Hydrogenasas. The hydrogen-isotope exchange reactions show that the enzyme catalyzes the heterolytic cleavage of H2 into a hydride,, and a proton, H+. To hold a hydride we need a suitable electron-deficient metal center, M, and to hold a proton we need a base, B ... 

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