Transition states ethylene oxide reactions

Modifying the selectivity for a particular product is a more challenging task. To understand why Ag is the most selective catalyst for ethylene epoxidation, an highly important reaction practiced industrially for decades, Linic et al. performed detailed spectroscopic and kinetic isotope experiments and DFT calculations, and they concluded that the selectivity between the partial and total oxidation of ethylene on Ag(l 11) is controlled by the relative stability of two different transition states (TS s) that are both accessible to a common oxametallacycle intermediate One results in the closure of the epoxide ring and ethylene oxide (EO), while the other leads to acetaldehyde (AC) via intra-molecular H shift and eventually combustion. The authors... 

The detection of 1,2-propylene oxide in the products from methyl ethyl ketone combustion is particularly interesting. It parallels the formation of ethylene oxide in acetone combustion (8) and of 1,2-butylene oxide in the combustion of diethyl ketone. Thus, there is apparently a group of isomerization reactions in which carbon monoxide is ejected from the transition state with subsequent closing of the C—C bond. Examination of scale molecular models shows that reactions of this type are, at any rate, plausible geometrically. 

The products of the thermolysis of 3-phenyl-5-(arylamino)-l,2,4-oxadiazoles and thiazoles have been accounted for by a radical mechanism.266 Flash vacuum pyrolysis of 1,3-dithiolane-1-oxides has led to thiocarbonyl compounds, but the transformation is not general.267 hi an ongoing study of silacyclobutane pyrolysis, CASSF(4,4), MR-CI and CASSCF(4,4)+MP2 calculations using the 3-21G and 6-31G basis sets have modelled the reaction between silenes and ethylene, suggesting a cyclic transition state from which silacyclobutane or a trcins-biradical are formed.268 An AMI study of the thermolysis of 1,3,3-trinitroazacyclobutane and its derivatives has identified gem-dinitro C—N bond homolysis as the initial reaction.269 Similar AMI analysis has determined the activation energy of die formation of NCh from methyl nitrate.270 Thermal decomposition of nitromethane in a shock tube (1050-1400 K, 0.2-40 atm) was studied spectrophotometrically, allowing determination of rate constants.271... 

The base-catalysed hydrolysis of ethylene oxide has been studied by the MNDO method.28 The structures of tire reactant, product, and transition state were optimized and a reaction mechanism was proposed. 

Deactivation processes competing with fluorescence are mainly nonradiative deactivation to the S0 state (IC) and nonradiative transition to a triplet state (intersystem crossing, ISC). Photochemical products are often formed from this triplet state. Important photochemical reactions are the E—yZ isomerization of ethylene, the oxidation of pyrazoline to pyrazole, and the dimerization of cou-marins. 

The process of catalyst oxidation and reduction can be treated as a reversible phase transition [136]. It is to this process that the authors of recent investigations [37, 47-49, 85] ascribe critical effects. When studying kinetic self-oscillations in the oxidation of hydrogen over nickel [37] and measuring CPD, the authors established that the reaction performance oscillates between the states in which oxygen is adsorbed either on the reduced or on the oxidized nickel surface. Vayenas et al. [47-49], by using direct measurements of the electrochemical activity of 02 adsorbed on Pt, showed that the isothermal self-oscillations of the ethylene oxidation rate over Pt are due to the periodic formation and decomposition of subsurface Pt oxides. A mathemati-... 

Metal oxides are widely used as catalyst supports. For instance, a-Al203 is employed as a support for catalysts in the partial oxidation of ethylene to ethylene oxide, because a non-reactive material is essential for such applications [141]. However, aluminas are also important catalysts in their own right. Transition aluminas are known to catalyze the isomerization of alkenes, the dehydration of alcohols, H/D exchange reactions and C—H bond activation [142]. Consequently, the development of an understanding of both their bulk and their surface structure has been a key goal in catalysis, with solid-state NMR being widely employed to this end. 

Cluster DFT calculations were also used to identify a transition state for the formation of ethylene oxide. In this transition state, the Ag-0 bonds are elongated relative to the oxametallacycle. The product of this reaction is gaseous EO. The activation energy for this step had previously been determined by Linic and Barteau experimentally.62 The predicted activation barrier from DFT calculations, 16 kcal/mol, is in very good agreement with the experimental result of 17 kcal/mol. 

A summary of bimolecular rate constants for the acid-catalyzed hydrolysis of a series of alkyl-, vinyl- and phenyl-substituted epoxides is given in Table 1. Propylene oxide (7) is 6.6 times more reactive than ethylene oxide, and from a study of its reaction in H2018, it was shown that 70% of the glycol product results from addition of solvent to the secondary carbon and 30% from addition of solvent to the primary carbon. The reactivity per primary carbon of ethylene oxide is one-half of the observed reactivity of ethylene oxide, and thus the introduction of a methyl group on ethylene oxide results in an increase in reactivity at the primary carbon by a factor of 4 and an increase in reactivity at the secondary carbon by a factor of 9. These results are consistent with A-2 mechanisms for the acid-catalyzed hydrolyses of ethylene oxide and propylene oxide, in which some amount of positive charge generated on carbon at the transition state is stabilized by a methyl group. 

Acid-catalyzed hydrolysis of isobutylene oxide (8) is >750 times faster than that of ethylene oxide (6), and > 99% of the glycol product is from addition of solvent at the tertiary carbon.23 These results are consistent with a mechanism in which there is significant positive charge on the tertiary carbon at the transition state, as discussed in the previous section. Butadiene monoepoxide (10) is slightly less reactive than isobutylene oxide,36 and its acid-catalyzed hydrolysis can potentially proceed via a resonance-stabilized allyl cation (Scheme 6). However, the acid-catalyzed hydrolysis of 10 yields 96% of 3-buten-l,2-diol (15) and only 4% of 2-butene-1,4-diol (16),36 and the acid-catalyzed methanolysis of 10 is reported to yield only 2-methoxy-3-buten-l-ol.37 An A-2 mechanism proceeding via transition state 17 may account for the observation that 1,2-diol 15 is the predominant product from acid-catalyzed hydrolysis of 10. The minor yield of the 1,4-diol 16 may be formed from reaction of... 

Another catalytic cycle studied by Matsubara, Morokuma, and coworkers [77] is the hydroformylation of olefin by an Rh(I) complex. Hydroformylation of olefin by the rhodium complex [78-80] is one of the most well known homogeneous catalytic reactions. Despite extensive studies made for this industrially worthwhile reaction [81, 82], the mechanism is still a point of issue. The active catalyst is considered to be RhH(CO)(PPh3)2, 47, as presented in Fig. 25. The most probable reaction cycle undergoes CO addition and phosphine dissociation to generate an active intermediate 41. The intramolecular ethylene insertion, CO insertion, H2 oxidative addition, and aldehyde reductive elimination are followed as shown with the surrounding dashed line. Authors have optimized the structures of nearly all the relevant transition states as well as the intermediates to determine the full potential-... 

Dehydration of the optically active relay (178) derived from enmein (62) gave a 1 2 mixture of 5,6-ene and 6,7-ene. Separation could be achieved by means of the ethylene acetal (187), whose ozonolysis product was subjected to successive Jones oxidation, methylation, Wittig reaction, and treatment with dilute hydrochloric acid to afford the 3-on-16-ol derivative (188). Bromination of (188) followed by dehydrobromination and subsequent dehydration afforded (189). The purified compound (189), after conversion into the acetal, was hydrolyzed to carboxylic acid (190), which was transformed into the desired lactone (191) by treatment with boron trifluoride etherate. The reaction produced a single product uncontaminated by the C-1 epimer, because of easy formation of a favored transition state which satisfied the stereoelectronic requirements. 

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