Of concerted

C=N, and O2 can also act as dienopbiles to give heterocycHc products. These types of concerted reactions have been the subject of extensive orbital symmetry studies (118,119). 

Lithium aluminum hydride reduction of 2,3,4-triphenylisoxazolin-5-one yielded 1,2,3-triphenylaziridine and dibenzylaniline. The reaction was considered to proceed by a concerted [l,3]-sigmatropic migration of the N to a C atom. HOMO-LUMO calculations show this type of concerted reaction is possible (Scheme 68) (80JA1372). 

Fuke and Kaya [1989] studied the vibrational selectivity of concerted two-proton transfer in 7-azoindole in the excited electronic state... 

The rate constants and ko were equal to 3 x 10 and lO s respectively [Shian et al. 1980 Bratan and Strohbusch 1980]. There are two equivalent ways of stepwise transfer, and, therefore, the transition state and MEP are two-fold, if the stepwise transfer is energetically preferable. On the other hand, there is only one way of concerted transfer, which lies between the saddle points. Based on this reasoning, de la Vega et al. [1982] have found that the barrier for stepwise transfer (25kcal/mol) is 3.1 kcal/mol lower than that for concerted transfer. These authors have proposed a model two-dimensional PES,... 

A straightforward calculation of A has been performed by Fraser [1989] within the framework of the one-dimensional model of concerted interconversion. The diabatic terms were taken in the form... 

Entries 1 and 2 in Scheme 2.9 are typical of concerted syn addition to alkene double bonds. On treatment with peroxyacetic acid, the Z-alkene affords the cis-oxirane, whereas the -alkene affords only the iraws-oxirane. Similarly, addition of dibromocarbene to Z-2-butene yields exclusively l,l-dibromo-cw-2,3-dimethylcyclopropane, whereas only 1,1-dibromo-/ra 5-2,3-dimethylcyclopropane is formed from -2-butene. There are also numerous stereospecific anti additions. Entiy 3 shows the anti stereochemistry typical of bromination of simple alkenes. 

In solvents containing low concentrations of water in acetic acid, dioxane, or sulfolane, most of the alcohol is formed by capture of water with retention of configuradon. This result has been explained as involving a solvent-separated ion pair which would arise as a result of concerted protonation and nitrogen elimination. ... 

There is another usefiil viewpoint of concerted reactions that is based on the idea that transition states can be classified as aromatic or antiaromatic, just as is the case for ground-state molecules. A stabilized aromatic transition state will lead to a low activation energy, i.e., an allowed reaction. An antiaromatic transition state will result in a high energy barrier and correspond to a forbidden process. The analysis of concerted reactions by this process consists of examining the array of orbitals that would be present in the transition state and classifying the system as aromatic or antiaromatic. 

In a definitive study of butadiene s reaction with l,l-dichloro-2,2-difluoio-ethylene, Bartlett concluded that [2+4] adducts of acyclic dienes with fluorinated ethylenes are formed through a mixture of concerted and nonconcerted, diradical pathways [67] The degree of observed [2+4] cycloaddition of fluorinated ethylenes IS related to the relative amounts of transoid and cisoid conformers of the diene, with very considerable (i.e., 30%) Diels-Alder adduct being observed in competition with [2+2] reaction, for example, in the reaction of 1,1 -dichloro-2,2-difluoro-ethylene with cyclopentadiene [9, 68]... 

Azepines arc particularly versatile in that they display both dienic and dienophilic character, and function as the 2 k-, 4k-, and 67r-components in a variety of concerted cycloadditions, as exemplified in the following sections. 

Most Diels-Alder reactions, particularly the thermal ones and those involving apolar dienes and dienophiles, are described by a concerted mechanism [17]. The reaction between 1,3-butadiene and ethene is a prototype of concerted synchronous reactions that have been investigated both experimentally and theoretically [18]. A concerted unsymmetrical transition state has been invoked to justify the stereochemistry of AICI3-catalyzed cycloadditions of alkylcyclohexenones with methyl-butadienes [12]. The high syn stereospecificity of the reaction, the low solvent effect on the reaction rate, and the large negative values of both activation entropy and activation volume comprise the chemical evidence usually given in favor of a pericyclic Diels-Alder reaction. 

The transition state of concerted Diels-Alder reactions has stringent regio- and stereochemical requirements and can assume settled configurations if the reaction is carried out in a molecular cavity. Cyclodextrins, porphyrin derivatives and cyclophanes are the supramolecular systems that have been most investigated. 

More mechanistic work undoubtedly will be done on the stereochemistry of solvolysis, particularly of simple alkyl-substituted acyclic vinyl systems. The questions of concerted anchimerically assisted or nonconcerted rearrangements both to the double bond and across the double bond need further development. 

As a first step of this work, we wanted to verily if the predictions of the previous theoretical study on the prototype system (PC) [24] were vahd for BMPC, as well. For this aim we theoretically analysed the pathways leading from tram to (2-3 and 3-4) mono-cA isomers and explored, in addition, the possibility of concerted isomerizations at two C-C bonds (taking the 2-3, 4-5 double isomerization as an example). 

This chapter will deal mainly with tetrahedral intermediates from carbonyl derivatives, with some discussion on the much less-studied analogs for phosphorus and sulfur. It will also address the issue of concerted mechanisms which can sometimes bypass these intermediates. 

For acyl transfer, phosphoryl transfer, and sulfonyl transfer, the primary kind of evidence in favor of concerted mechanisms for some reactions is a linear Bronsted plot of log k versus for a range of nucleophiles, spanning p/ir/ = - p r s = 0, coupled... 

J. P. Guthrie, Multidimensional Marcus Theory an Analysis of Concerted Reactions, J. Am. Chem. Soc. 1996,118,12878. 

Goldstein, E., Beno, B., Houk, K. N., 1999, Transition Structures and Exo/Endo Stereoselectivities of Concerted [6+4] Cycloadditions with Density Functional Theory, Theor. Chem. Acc., 103, 81. 

Fig. 6. A summary illustration of concerted recruitment of mDCs and pDCs. K, CTL LF, lymph follicle mDCpre, myeloid dendritic cell precursor pDCpre, plasmacytoid dendritic cell precursor. 

The object of this paper is to draw attention to the possible importance of concerted molecular reactions, of the type termed pericyclic by Woodward and Hoffman (1), in the mechanism of coal liquefaction. 

C5Me5) was stable up to room temperature (108,109). A surprising feature of the matrix photochemistry (6,7) was that only the trans isomer of [CpFe(CO)2]2 was photolyzed, and it may be that this was the consequence of concerted CO loss from the trans isomer and isomerization to form the triply bridged product (6). 

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