Unified Mechanistic Scheme

Until recendy it was difficult to envision a single, unified mechanistic scheme that would satisfactorily explain the formation of the many different products isolated from vinyl azide decomposidon reactions. 

More recently, Bouwman carried out a detailed study on the carbonylation of nitrobenzene in methanol with palladium bidentate phosphane complexes as catalysts [29-31]. After a careful analysis of the reaction, mixtures revealed that besides the frequently reported reduction products of nitrobenzene [methyl phenyl carbamate (MFC), A, 7/-diphenylurea (DPU), aniline, azobenzene (Azo) and azoxyben-zene (Azoxy)], large quantities of oxidation products of methanol were co-produced (dimethyl carbonate (DMC), dimethyl oxalate (DMO), methyl formate (MF), H2O, and CO). They proposed the Pd-imido species P2Pd = NPh, which is the central key intermediate that can link together all the reduction products of nitrobenzene and all the oxidation products of methanol into one unified mechanistic scheme. 

The formulation of a unified mechanistic scheme for Lewis base-catalyzed-Lewis acid-mediated reactions brings our journey fuU circle, a journey that began with attempts to imderstand aberrations in our study of the Lewis base-catalyzed... 

The electron-transfer paradigm for chemical reactivity in Scheme 1 (equation 8) provides a unifying mechanistic basis for various bimolecular reactions via the identification of nucleophiles as electron donors and electrophiles as electron acceptors according to Chart 1. Such a reclassification of either a nucleophile/ electrophile, an anion/cation, a base/acid, or a reductant/oxidant pair under a single donor/acceptor rubric offers a number of advantages previously unavailable, foremost of which is the quantitative prediction of reaction rates by invoking the FERET in equation (104). 

Unifying Mechanistic Redox Scheme for Standard and Fast SCR Reactions... 

Cyclization of an allylic pyrophosphate is a key step in the biosynthesis of most monoterpenes. Early hypotheses concerning the nature of the acyclic precursor and the cyclization process are first described, and chemical models for the cyclization presented. Following a review of several representative cyclase enzymes and the reactions that they catalyze, a series of stereochemical and mechanistic experiments with partially purified cyclases are reported. The results of these studies have allowed a detailed description of events at the active site and the formulation of a unified stereochemical scheme for the multistep isomerization-cyclization reaction by which the universal precursor geranyl pyrophosphate is transformed to cyclic monoterpenes. 

This chapter centers on the mobility of molecules between supramolecular structures and the homogeneous phase. Mobility can be observed by following the relocation of the excited state or by measuring the reaction efficiency of the complexed excited state with quencher molecules. Figure 1 shows a scheme covering the simplest mechanistic outline to describe the dynamics of probe and/ or quencher mobility. Many reports in the literature have employed only a subset of this mechanistic scheme. The objective of presenting Fig. 1 is to provide a unified view of the dynamic processes, to qualitatively describe under which conditions different rate constants can be experimentally determined, and to state the important underlying assumptions. 

Although conclusive evidence is still pending, based on the available experimental data, the authors proposed the unified mechanistic picture depicted in Scheme 7.49 [120]. Both catalyses are considered to originate from the common o-alkyl palladium(II) intermediate 199, which arises from regioselective electrophilic pal-ladation at C-3 (196 199). It is hypothesized that the kinetics of the deprotonation... 

One of the most intriguing features of olefin metathesis, and an aspect which continues to receive increasing attention, is the stereochemistry of the reaction. Rationalizations of stereochemical peculiarities have undergone a metamorphosis which parallels advances of mechanistic theories from the pairwise scrambling schemes to the now-popular car-bene-to-metallocycle scheme. As yet there exists no unified stereochemical model which can adequately account for all the observed results. 

In a series of papers, we have proposed the torsional mechanism of energy transduction and ATP synthesis, the only unified and detailed molecular mechanism of ATP synthesis to date [16-20,56] which addresses the issues of ion translocation in Fq [16, 20, 56], ionmotive torque generation in Fq [16, 20, 56], torque transmission from Fq to Fj [17,18], energy storage in the enzyme [17], conformational changes in Fj [18], and the catalytic cycle of ATP synthesis [18, 19]. We have also studied the thermodynamic and kinetic aspects of ATP synthesis [19,20,41,42,56]. A kinetic scheme has been developed and mathematically analyzed to obtain a kinetic model relating the rate of ATP synthesis to pHjn and pH m in the Fq portion and the adenine nucleotide concentrations in the Fj portion of ATP synthase. Analysis of these kinetic models reveals a wealth of mechanistic details such as the absence of cooperativity in the Fj portion of ATP synthase, order of substrate binding and product release events, and kinetic inequivalence of ApH and Aip. 

Now, if one wants to tie together all the mechanistic data available for indirect carbonylation reactions on group VIII catalysts, no unifying picture currently emerges and, depending on the nature of the catalytic system used, the mechanism according to either Scheme 5 (b) or (c) appears very likely to be operative. 

Substrate probes have aided mechanistic understanding of the key C— H activation step in the MMOH reaction cycle. Chiral alkanes and radical-clock substrate probes " " were used to discriminate between radical recoil/rebound and nonsynchronous concerted insertion pathways. A short lifetime (< 150 fs) estimated for the putative radical species derived from cyclopropane-based radical-clock substrates favors the latter process,whereas partial racemization of chiral ethane substrate is consistent with the former scenario. A unifying model was proposed, in which both recoil/rebound and concerted reaction channels are available for a bound radical intermediate and the partitioning between each trajectory is dependent on the substrate. Formation of carboca-tion-derived products from certain probes implicates yet another route involving a formal OH+ insertion.Participation of multiple species capable of oxygen transfer is an emerging mechanistic view in both heme and nonheme systems, as exemplified by the studies of cP450s and their synthetic models.Scheme 3 depicts various density functional theory (DFT) models of MMOHq and their computed reaction pathways, which are reviewed in detail elsewhere. 

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