Mechanistic concerted pathway

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. 

The exact pathway of the Peterson reaction is still not clear despite the intensive research effort. Most of the mechanistic studies suggest that both the stepwise and concerted pathways are feasible under basic conditions. In the concerted pathway a pentacoordinate 1,2-oxasiletanide is formed. The stepwise pathway is expected when chelation control operates in the reaction. The driving force is the formation of a very strong Si-0 bond. Under acidic conditions the 3-hydroxysilane undergoes an E2 elimination to afford the other alkene isomer. 

These aspects introduce different mechanistic patterns expected for the 1,3-DC reactions, as compared with DA cycloadditions (concerted vs stepwise with some zwitterionic character). This result may again be traced to the electrophilicity difference at the ground states of the reacting pairs.39 These results suggest that the description of the reactivity and the reaction mechanism involved in the 1,3-DC processes can be systematized as in the case of the DA cycloadditions. Such a model should be able to determine the charge transfer pattern and to decide which of the partners is acting as nucleophile/electrophile in a polar process, or even to anticipate a concerted pathway in those cases where the electrophilicity/nucleophilicity difference is small. 

The thermal cycloaddition of two ethylenes is one of the textbook examples used in the illustration of the Woodward-Hoffmann rules of orbital symmetry control in concerted reactions. Therefore, the related potential energy surface can provide various types of information of chemical and theoretical interest. A first question is associated with the mechanistic question of whether this reaction proceeds via diradical or concerted pathways. Since this reaction is an example of a concerted thermally forbidden process, it can be expected that the flavoured path be the diradical one. However, it is important to have a detailed description of the structural and energetic features of these different pathways. A second question is associated... 

Fig. 2. The dissociative and associative mechanistic extremes, and the concerted pathway for phosphoryl transfer. The concerted pathway is drawn to indicate the dissociative transition state, which is typical of phosphate monoester reactions, but in principle could have a transition state at any point between the mechanistic extremes. The sum of the bond orders to the nucleophile and leaving group is less than in the substrate in a dissociative transition state and is greater in an associative transition state.

The mechanistic cycle of the transfer hydrogenation protocol is depicted (Scheme 39.29). It proceeds through the formation of an 18-electron Ru-hydride species that transfers hydride to the in situ generated imine in a concerted pathway. Here the formic acid serves as the terminal hydrogen donor that helps to regenerate the active Ru-hydride species during the reaction. 

For the mechanistic course of that reaction two pathways are discussed " a concerted [l,3]-sigmatropic rearrangement, and a pathway via an intermediate diradical species. Experimental findings suggest that both pathways are possible. The actual pathway followed strongly depends on substrate structure the diradical pathway appears to be the more important. 

With respect to reaction mechanism, it is likely that CpCo(CO)2-mediated alkyne cyclotrimerizations proceed through discrete orga-nometallic intermediates and are therefore not concerted.12 A plausible mechanistic pathway for the CpCo(CO)2-catalyzed cyclotri-... 

Although the nucleophilic addition of secondary amines to thiirene dioxides can be interpreted as following the same mechanistic pathway, the reaction was found to be second order in amine119 (which is typical for the addition of amines to olefins in appropriate solvents13 2 133), and the addition is syn. As a result, mechanisms with a cyclic-concerted addition across the carbon-carbon bond, or a stepwise addition involving two molecules of amine per one molecule of thiirene dioxide, have been proposed. 

Fig. 8. Mechanistic pathways involved in the chlorine replacement reaction of chloro-cyclophosphazenes (A) = S 2 (non-polar) (B) = S 2 (concerted) (C) = S l (D) = SnI(CB).

A detailed examination of the kinetics of dimethylaminolysis of N3P3C16 by Krishnamurthy and co-workers has revealed that there is a gradual and subtle mechanistic change that occurs as the degree of replacement of chlorines increases (92). While the first chlorine replacement follows an Sn2 pathway involving the formation of a neutral five-coordinate intermediate [Fig. 8(A)], at the second stage the mechanism can be induced to follow a concerted path [Fig. 8(B)] by using acetonitrile as the solvent. The polar transition state of the concerted path reaction pathway is stabilized in acetonitrile. This postulate has sup-... 

From a mechanistic point of view, two very general pathways can be envisaged for hydrogen transfer direct hydrogenation transfer, consisting of a concerted process that involves a six-membered cyclic transition state in which both the hydrogen donor and the acceptor are coordinated to the metal (1 in Scheme 22) and a hydridic route (2 in Scheme 22).116... 

DFT calculations have been used to study the mechanistic pathway of the intramolecular Diels-Alder cycloaddition involved in the biosynthesis of natural products paraherquamide A and VM55599. The cycloaddition involves a dihydropyrolo[l,2- ]pyrazine as the azadiene and a standard alkene as the dienophile (Scheme 1). Analysis of the results reveals that these cycloadditions take place through concerted transition structures associated with [4+2]... 

FIGURE 4.78 Mechanistic pathways for aromatic hydroxylation by concerted addition of oxene, pathway 1, or by stepwise addition of oxene, pathway 2. Pathways 2, 3, and 4 describe the formation of phenol that bypasses the arene oxide intermediate. 

As with an isolated double bond, epoxide formation in an aromatic ring, i.e., arene oxide formation, can occur mechanistically either by a concerted addition of oxene to form the arene oxide in a single step, pathway 1, or by a stepwise process, pathway 2 (Fig. 4.78). The stepwise process, pathway 2, would involve the initial addition of enzyme-bound Fe03+ to a specific carbon to form a tetrahedral intermediate, electron transfer from the aryl group to heme to form a carbonium ion adjacent to the oxygen adduct followed by... 

---