Previously proposed new mechanism

Investigation of a Previous Proposed New Mechanism by Addition of Alcohols to the Electrolyte... 

Cationic ruthenium complexes of the type [Cp Ru(MeCN)3]PF6 have been shown to provide unique selectivities for inter- and intramolecular reactions that are difficult to reconcile with previously proposed mechanistic routes.29-31 These observations led to a computational study and a new mechanistic proposal based on concerted oxidative addition and alkyne insertion to a stable ruthenacyclopropene intermediate.32 This proposal seems to best explain the unique selectivities. A similar mechanism in the context of C-H activation has recently been proposed from a computational study of a related ruthenium(ll) catalyst.33... 

Biosynthesis of Digalloylglucose. Besides the above mentioned acyltransferase, oak leaves also contained a completely different type of acyltransferase that catalyzed the formation of digalloylglucose (41). It became evident that this ester was synthesized by a new reaction mechanism in which / -glucogallin was utilized as both acyl donor and acceptor this conclusion was supported by the isolation of analogous acyltransferases related to other metabolic pathways (cf. Table III). Recent studies (54) have shown, in accordance with previous proposals (5,7,8), that 1,6-O-digalloylglucose was produced by the enzyme, and that the stoichiometry of the reaction... 

A previously proposed mechanism for the acid-catalysed cyclization of 5-cyclodecynone (268) to 272 that involved hydration of the intermediate vinyl cation (269 — 270) has now been ruled out, since no significant incorporation of 180 could be observed when the reaction was carried out in H2180. Instead, a new mechanism has been proposed, involving the cyclization of 269 to 271 followed by fragmentation of the C—O bond396. 

The structure of the polymer obtained by the polymerization of butadiene, isoprene, and other dienes with organometallic catalysts depends on the nature of the monomer, the catalyst system, and reaction conditions. In this article, the authors review and propose a mechanism for some of the previously reported results on the polymerization of butadiene and isoprene and present some new data on the polymerization of cyclopentadiene. 

A new mechanism for the formation of hydrocarbons from methanol over proton-ated zeolites invoking adsorbed carbenium ions as an essential part of the catalytical ly active site is proposed. Ihe mechanism is based on new experimental results which can not be explained from previously proposed mechanisms. 

The amination of ethanol over a 10% Ni/silica catalyst was investigated using [ H], [ H], [ C]-tracers. The kinetics of the reaction was also studied. Analysis of the results suggested that the accepted mechanism did not explain the observed behaviour. Other previously proposed mechanisms were also found to be unable to describe the results. A new mechanism is proposed. 

We are left therefore with a situation that none of the mechanisms proposed can accurately describe the results obtained. In developing a new mechanism that can accommodate the results it is important to use as much of previous mechanisms as possible. As we have shown above, much of what has been proposed is in agreement with the results. However there has been much greater understanding of molecular adsorption/desorption since the publication of the Schwegler-Adkins mechanism. The adsorption of alcohols on Ni(l 11) [12] and our own exchange results show that the 0-H bond is the easiest to break in this system. Therefore the initial adsorption under reaction temperatures will result in the formation of an ethoxy species on the surfaee. Similarly the adsorption of... 

In this case, the Tj translocation (Section 1.2.1.1) is often accepted as the likely mechanism. In 1982, however, a new mechanism called the vehicle mechanism was proposed . At first sight, it resembles the previous proton-carrying process (Fig, 1.3a), It differs, however, in that the proton migrates in one direction as etc, bonded to a vehicle such... 

A new molecular-associative mechanism has been proposed for the formation of cyanoethylcellulose by means of reactive complexes having hydroxy- and anhydro-D-glucose hydroxy-groups and including polarized molecules of acrylonitrile in the sphere of elementary interaction. Experimental evidence against the previously accepted ionic mechanisms for cyanoethylation is presented. 

The acid-catalysed rearrangement of thevinols has been shown to proceed with retention of configuration at C(7) and therefore not via an -hybridised centre at C(7) as had been previously proposed. A proposed mechanism for the rearrangement is shown in Scheme 100. Cathovaline-like compounds have been prepared from the rearrangement of vindoline derivatives in superacidic media,and the observed formation of the bridged 16-spiro compound (350) upon acetic anhydride-pyridine treatment of the hydroxyindolenine (349) represents a new rearrangement in iboga alkaloids see Scheme 101. 

A statistical evaluation If a statistical evaluation of the fitting results demonstrate a good description of the experiment by the model, a quantitative reaction mechanism is obtained. If, on the other hand, no good agreement between experiment and model is achieved, a new mechanism has to be proposed and the previous steps should be repeated. 

To discuss these arguments we will have a look at the structures of the transition states for the three mechanisms previously proposed. Mechanism 1 involves two steps and in consequence two transition states (10 and 11) should be considered, but none of them is particularly crowded. The first 10, involves the C2-C1 bond breaking of triazine 5 (dissociative process) leading to triazine cation 7. The amine is involved in the second step, consisting on the formation of a new bond between an unsubstituted position of the triazine ring (C2) and the morpholine nitrogen atom (Scheme 26.6). 

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