Stepwise mechanism Subject

Mechanistic and theoretical studies of the Diels-Alder reaction have resulted in the characterization of this reaction as a concerted, although not necessarily synchronous, single-step process28-31 45. The parent reaction, the addition of 1,3-butadiene to ethylene yielding cyclohexene, has been the subject of an ongoing mechanistic debate. Experimental results supported a concerted mechanism, whereas results from calculations seemed to be dependent on the method used. Semi-empirical calculations predicted a stepwise mechanism, whereas ab initio calculations were in favor of a concerted pathway. At the end of the 80s experimental and theoretical evidence converged on the synchronous mechanism29-31. 

While the concerted pathway has been preferred in early publications on the subject, evidence for a stepwise mechanism involving distonic ion intermediates is presented in more recent work taking kinetic isotope effects into account. [90] This is also in agreement with the postulation that reactions involving multiple bonds are generally stepwise processes. [37,91] Nevertheless, this question is still a matter of debate. [90]... 

Taken together, the above data suggest a stepwise mechanism of Cu(I) transfer from Atxl to Ccc2a as illustrated in Fig. 3 (Brown et al., 1991 Pufahl et al., 1997 Huffman and O Halloran, 2000). When the Cu(I) center can adopt a two-coordinate ligand geometry as in Atxl, the Cu(I) ion is subject to attack by the sulfur ligands of the MXCXXC motif... 

The mechanism of benzene hydrogenation has been studied with the aid of [14-C]labelled hydrocarbons.The reverse process, cyclohexane dehydrogenation, has been the subject of twelve papers by Tetenyi et al., published between 1961 and 1974. The results of these have been summarized in a review by Tetenyi.In both systems the point of interest was whether the intermediates cyclohexene and cyclohexadiene were formed during the reaction. From experiments in which a 1 1 mixture of labelled [14-C]cyclohexane and inactive cyclohexene was allowed to react in the presence of Ni, Pt, and Rh catalysts, it was deduced that [14-C]-cyclohexene was produced from cyclohexane. Thus, a stepwise mechanism was proposed, involving both cyclohexene and cyclohexadiene as intermediates. 

The oxidative functionalization of olefins mediated by transition metal oxides leads to a variety of products including epoxides, 1,2-diols, 1,2-aminoalcohols, and 1,2-diamines [1]. Also the formation of tetrahydrofurans (THF) from 1,5-dienes has been observed, and enantioselective versions of the different reactions have been developed. Although a lot of experimental data has been available, the reaction mechanisms have been a subject of controversial discussion. Especially, osmium (VIII) complexes play an important role there, as the proposal of a stepwise mechanism [2] for the dihydroxylation (DH) of olefins by osmium tetroxide (OSO4) had started an intense discussion about the mechanism [2—11],... 

The [4-1-2] cycloaddition of dienes to double bonds, the Diels-Alder reaction (see Pericycttc Reactions The Diels-Alder Reaction), is one of the most useful synthetic reactions in organic chemistry. The mechanism of the Diels-Alder reaction has been the subject of intensive study using a large number of different methods. This is at least in part due to the fact that, based on thermochemical estimates, there is a reasonable possibility for a stepwise mechanism. The energy of concert , i.e., the difference between the concerted and a diradicaloid mechanism, is small enough to be potentially overcome in some cases. The comparison of the results for the two different pathways suffers, however, from the difficulties of comparing closed-shell specie.s and diradicals within a single-determinant MO approach. Only recently, the use of multireference and DFT methods in combination with experimental kinetic isotope effect studies allow an unequivocal assessment of the reaction mechanism of the Diels-Alder reaction. ... 

Reaction Mechanism. The reaction mechanism of the anionic-solution polymerization of styrene monomer using n-butyllithium initiator has been the subject of considerable experimental and theoretical investigation (1-8). The polymerization process occurs as the alkyllithium attacks monomeric styrene to initiate active species, which, in turn, grow by a stepwise propagation reaction. This polymerization reaction is characterized by the production of straight chain active polymer molecules ("living" polymer) without termination, branching, or transfer reactions. 

The reaction mechanisms of these transition metal mediated oxidations have been the subject of several computational studies, especially in the case of osmium tetraoxide [7-10], where the controversy about the mechanism of the oxidation reaction with olefins could not be solved experimentally [11-20]. Based on the early proposal of Sharpless [12], that metallaoxetanes should be involved in alkene oxidation reactions of metal-oxo compounds like Cr02Cl2, 0s04 and Mn04" the question arose whether the reaction proceeds via a concerted [3+2] route as originally proposed by Criegee [11] or via a stepwise [2+2] process with a metallaoxetane intermediate [12] (Figure 2). 

Concerted mechanisms have also been considered to justify the high degree of stereoselectivity observed in many instances as, for example, in the cases shown in Scheme 3 [13,18-21], However, the high stereochemical control often observed in many ODPM rearrangements does not necessary imply that the reaction is taking place via concerted mechanisms. A stepwise process is also consistent with the stereochemical outcome of the reaction, where there are conformational or configurational restrictions to rapid C—C rotation. This subject has been extensively discussed and reviewed by Schuster [16]. 

In the traditional surface science approach the surface chemistry and physics are examined in a UHV chamber at reactant pressures (and sometimes surface temperatures) that are normally far from the actual conditions of the process being investigated (catalysis, CVD, etching, etc.). This so-called pressure gap has been the subject of much discussion and debate for surface science studies of heterogeneous catalysis, and most of the critical issues are also relevant to the study of microelectronic systems. By going to lower pressures and temperatures, it is sometimes possible to isolate reaction intermediates and perform a stepwise study of a surface chemical mechanism. Reaction kinetics (particularly unimolecular kinetics) measured at low pressures often extrapolate very well to real-world conditions. 

At The American University, Isbell s major interest in research turned to the study of the oxidation of saccharides with hydrogen peroxide. In collaboration with Dr. Frush, he published some forty papers on the subject. A number of major discoveries were made, including that of a stepwise degradative peroxidation, which is catalyzed by base or by such metals as iron(II). It starts at the anomeric carbon of an aldose, either in the acyclic or the cyclic form, and affords the lower aldose and formic acid (see Fig. 8). Two mechanisms were recognized an ionic one prevalent in strong alkali, and a free-radical process catalyzed by Fe(II) (see Fig. 9). 

The smaller-sized cyclohexene and cycloheptene have also been subjected to enantiodifferentiating photoisomerization, although the corresponding ( )-isomers are short-lived transient species. Photosensitization of (Z)-cyclohexene 31Z with chiral benzene(poly)carboxylates affords trans-anti-trans-, cis-trans-, and cis-anti-cis-cyclodimers 34 (Sch. 32). Interestingly, of the former two chiral products, only the trans-anti-trans isomer is optically active and its ee reaches up to 68%, whilst the cis-trans isomer is totally racemic under a variety of irradiation conditions, for which two competing, concerted, and stepwise cyclodimerization mechanisms are responsible. Thus, the enantiodifferentiating photoisomerization of 31Z to the optically... 

The 1,2-acyloxy migration in /3-acyloxyalkyl radicals has been the subject of many experimental3 as well as some theoretical studies.4,17 While a broad spectrum of mechanisms is conceivable for this kind of process,3 only three will be discussed here in detail (Scheme 4) (i) stepwise heterolysis/recombination involving... 

---