Hydrogenation Mechanism Studies

Peroxyoxalate. The chemical activation of a fluorescer by the reactions of hydrogen peroxide, a catalyst, and an oxalate ester has been the object of several mechanism studies. It was first proposed in 1967 that peroxyoxalate (26) was converted to dioxetanedione (27), a highly unstable intermediate which served as the chemical activator of the fluorescer (fir) (6,9). 

BW Beck, IB Koerner, RB Yelle, T Ichiye. Unusual hydrogen bonding ability of sulfurs m Fe-S redox sites Ah initio quantum and classical mechanical studies. I Phys Chem B, submitted. 

The traditional place to begin a quantum-mechanical study of molecules is with the hydrogen molecule ion H2+. Apart from being a prototype molecule, it reminds us that molecules consist of nuclei and electrons. We often have to be aware of the nuclear motion in order to understand the electronic ones. The two are linked. 

ASTM B695, Coatings of Zinc Mechanically Deposited on Iron and Zinc, ASTM (1985) James, D. G., Mechanical Deposition —Hydrogen Embrittlement Study . Paper to IMF Annual Conf., UK (1985)... 

It is important to point out one of the limitations of mechanism studies. Usually more than one mechanism is compatible with the same experimentally obtained rate expression. To make a choice between alternative mechanisms, other evidence must be considered. A classic example of this situation is the reaction between hydrogen and iodine... 

Reduction of acetophenone by PrOH/H has been studied with the ruthenium complexes [Ru(H)(ri2-BH )(CO)L(NHC)], (L = NHC, PPh3, NHC = IMes, IPr, SIPr). The activity of the system is dependent on the nature of the NHC and requires the presence of both PrOH and H, implying that transfer and direct hydrogenation mechanisms may be operating in parallel [15]. 

Tri-rc-butylstannane is able to reductively replace halogen by hydrogen. Mechanistic studies indicate a free radical chain mechanism.199 The order of reactivity for the halides is RI > RBr > RC1 > RF, which reflects the relative ease of the halogen atom abstraction.200... 

The three prototype mixed p agonist/S antagonists described in this chapter have excellent potential as analgesics with low propensity to produce tolerance and dependence. The pseudotetrapeptide DIPP-NH2[ ] has already been shown to produce a potent analgesic effect, less tolerance than morphine, and no physical dependence upon chronic administration. In preliminary experiments, the tetrapeptides DIPP-NH2 and DIPP-NH2[T] were shown to cross the BBB to some extent, but further structural modifications need to be performed in order to improve the BBB penetration of these compounds. The Tyr-Tic dipeptide derivatives can also be expected to penetrate into the central nervous system because they are relatively small, lipophilic molecules. In this context, it is of interest to point out that the structurally related dipeptide H-Dmt-D-Ala-NH-(CH2)3-Ph (SC-39566), a plain p-opioid agonist, produced antinociception in the rat by subcutaneous and oral administration [72], As indicated by the results of the NMR and molecular mechanics studies, the conformation of the cyclic p-casomorphin analogue H-Tyr-c[-D-Orn-2-Nal-D-Pro-Gly-] is stabilized by intramolecular hydrogen bonds. There-... 

A Tour Guide to Mass Spectrometric Studies of Hydrogenation Mechanisms... 

Ionic hydrogenation mechanisms involve the sequential transfer of hydride and proton to the substrate [67]. This was suggested by the Leitner group for the hydrogenation of C02 with the catalyst precursor RhH(dppp)2 (Scheme 17.7) [50]. Spectroscopic evidence for each of the three intermediates was obtained by studying the steps as stoichiometric reactions. However, catalyst precursors that generate the highly active RhH (diphosphine) species in solution were subsequently found to operate by a more conventional insertion mechanism [20]. 

For all catalysts, the hydrogenation mechanism performed under our conditions (liquid phase and low hydrogen pressure) is consistent with an Horiuti-Polanyi mechanism. We found that the phenylacetylene (PhAc) hydrogenation reaction was zero order with respect to hydrocarbon reactants up to 80% of conversion, in agreement with several other studies devoted to the hydrogenation of polyunsaturated compounds." ... 

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