Reaction nonconcerted cycloaddition

In terms of the approach used, the authors drew a general conclusion that the [3+ 2]-cycloaddition reactions of nitronates with dipolarophiles under study should be considered as either nonconcerted or as those, which substantially differ in the positions of transition states on the reaction coordinate (337). 

Cycloadditions may proceed via concerted or nonconcerted mechanisms. Photochemical [2 + 2] cycloadditions take place via triplet intermediates [20]. Photochemical cycloaddition reactions of olefins to carbonyl compounds known as Paterno-Biichi reactions [21]... 

It should be noted that certain doubts have been thrown upon the conclusions as to the nonconcerted character of [4 4- 2]-cycloaddition reactions. It was, for example, pointed out [32,33] that the semi-empirical methods based on the NDO approximation (Sect. 2.4) cannot account for the role of the four-electron destabilization in Eq. (4.10), which for the nonsymmetrical structures of the XIV, XVIII type is greater than in the case of the symmetrical structures XV. 

The conclusion as to the nonconcerted character of the [4 -h 2]-cycloaddition reactions and sharp asynchronization of the process of formation of the -bonds in the course of the reaction has been borne out by a careful kinetical analysis, performed by Dewar, of reactions of typical dienophile maleic anhydride with a number of furane derivatives as well as by critical examination of other experimental data on the Diels-Alder reactions [29,35]. A similar conclusion has also been drawn for the [4 + l]-cycloaddition reaction [31], for some other... 

A cycloaddition reaction is one in which two unsaturated molecules add to one another to yield a cyclic product. As with electrocyclic reactions, cycloadditions are controlled by the orbital symmetry of the reactants. Symmetry-allowed processes often take place readily, but symmetry-disallowed processes take place with difhculty, if at all, and then only by nonconcerted pathways. Let s look at two examples to see how they differ. 

Concerted ene reactions are allowed by the Woodward-Hoffmann rules (Figure 7.5) but occur relatively slowly. For example, an electrophilic alkene will undergo Diels-Alder cycloaddition to a diene which bears allylic hydrogens rather than an ene reaction. Nonconcerted free-radical formation of ene products can sometimes compete with their formation by the concerted process. 

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]... 

The synthetic potential of such transformations for the preparation of medium-size heterocycles172 has been discussed elsewhere2. It is generally accepted that the reaction between thiirene dioxides and enamines is a stepwise (nonconcerted) thermal [2 -I- 2] cycloaddition. However, a concerted [4 + 2] cycloaddition, in which the lone pair of the enamine nitrogen atom participates, cannot be excluded. 

The behavior of complexes of nitroalkenes (42) with LA toward conjugated dienes is yet another factor underlying the role of these complexes. Conjugated nitroalkenes (42) are considered as active dienophiles in classical Diels-Alder reactions (104, 105). On the contrary, in the presence of SnCl4, nitroalkenes (42) react with cyclopentadiene and 1,3-cyclohexadiene exclusively at one double bond (103). Therefore, it is highly probable that the 42 + 43 cycloaddition proceeds by a nonconcerted mechanism in the presence of LA (see Scheme 3.40). 

Initially, a complex of nitroalkene (42) with LA (A) is reversibly formed. The efficient concentration of the latter is determined by the reaction conditions and the nature of heterodiene (42) and LA. This complex acts as a Michael substrate and adds alkene (43) to give bipolar adduct B, which undergoes cycliza-tion to give cationic intermediate C. The latter eliminates LA to yield target nitronate (35). In the case of nonconcerted cycloaddition, ionic intermediate B can undergo different isomerization reactions, some of which are considered below. The stereoselectivity of the process depends on the reactive conformation... 

Schindler and coworkers verified the formation of hydroxyl radicals kinetically and further RRKM calculations by Cremer and coworkers placed the overall concept on a more quantitative basis by verifying the measured amount of OH radical. An extensive series of calculations on substituted alkenes placed this overall decomposition mechanism and the involvement of carbonyl oxides in the ozonolysis of alkenes on a firm theoretical basis. The prodnction of OH radicals in solution phase was also snggested on the basis of a series of DFT calculations . Interestingly, both experiment and theory support a concerted [4 4- 2] cycloaddition for the ozone-acetylene reaction rather than a nonconcerted reaction involving biradical intermediates . 

Cycloaddition of stericaUy crowded 2,2,4,4-tetramethyl-3-thioxocyclobutanone (5)-methylide (69) (Scheme 5.25) with extremely electron-poor dipolarophUes are of special interest with respect to the theory of dipolar cycloaddition chemistry. In these reactions, a nonconcerted, two-step mechanism was proposed (96,97). 

Diphenylketene and more reactive haloketenes will undergo cycloaddition with allenes such as 1,1-dimethylallene giving regioisomeric mixtures of cyclobutanones. Yields of these reactions are only modest5-7 and distributions of isomers are solvent dependent, which is indicative of a nonconcerted process involving ionic intermediates.8... 

Although the ketene iminium salt route has advantages over the ketene route, limitations of this method include the nonstereospecificity of these cycloadditions in some instances because of the nonconcerted nature of these reactions. Of greater concern is the fact that the major product from alkenes which can form a tertiary carbocation is often the Friedel-Crafts product. This method is therefore largely limited to mono- and 1,2-disubstituted alkenes. 

Addition of DMAD to the carbonyl-stabilized nitrogen ylid 329 gave the furan 330 and the isoindolenine 331. This and similar reactions201 could involve either a symmetry allowed concerted cycloaddition or a nonconcerted reaction followed by elimination of the tertiary amine. 

Lewis acids play a dominant role on the chemoselectivity of the cycloaddition of allylsilanes and 3-butyn-2-ones. AICI3 and EtAlCl2 promote [2+2] cycloaddition predominantly, whereas TiCLt-mediated reaction gives significant amount of the [3+2] adduct288,289. The reaction of 2,2-bis(trifluoromethyl)ethylene-l,l-dicarbonitrile with allylsilane without any Lewis acid catalysts yields the cyclobutane derivative via a nonconcerted fashion290. 

The continuing accumulation of experimental material made it, however, still more apparent that the above criteria, both kinetic and stereochemical [61-64], cannot be regarded as absolute [65], since there are probably both concerted reactions which are forbidden as well as nonconcerted ones which are allowed. An example of the first type of reaction is some 2 + 2 retro-cycloadditions on sterically strained systems [62,66] the second type of processes is then represented by the so-called multibond reactions [67]. This, of course, stimulates to formulate a new and more universal criterion of concertedness as well as a deeper understanding of the very concept of concertedness. 

Choi and Gordon19 showed that the low energy [2 + 2] cycloaddition pathway on the Si(100) surface constituting a nonsymmetric, nonconcerted reaction path exists. There are at least two reaction paths to the [2 + 2] product depending on which Si—C bond forms first. Both transition states have small barriers of about 5 kcal mol 1 suggesting that [2 + 2] cycloaddition is facile. 

Fig. 2. Plots of bond indices for two new bonds formed in a cycloaddition for synchronous and simultaneous concerted reactions, and for a nonconcerted process...

Vinyl thiophenes, for example, 68, can be used as 47t-components in [4+2] cycloaddition processes. Reaction with DMAD proceeds via a nonconcerted, zwitterionic reaction pathway to give a mixture of products including 69-72 (Scheme 6) <2004TL2189>. 

The catalysis of symmetry-forbidden reactions is currently of broad interest and a number of workers have expressed different points of view on the metal s role. Katz, for example, seems to take the position that these catalytic processes are very likely stepwise in nature, and, therefore, for-bidden-to-allowed arguments do not apply i5,40). This criticism is perhaps more broadly directed toward the concerted, "n complex multicenter reaction mechanism first postulated by Schrauzer to explain metal-assisted cycloaddition chemistry 54). A preference for nonconcerted mechanisms in this chemistry is also shared by Cassar, Eaton, and Halpem... 

Some Diels-Alder reactions have only modest rate responses to changes in solvent,and when contrasted with a [2 + 2] addition exhibiting a strong dependence on solvent polarity, a neat mechanistic demarcation may be drawn distinctions between concerted versus nonconcerted reactions, and between early and late transition states, may be advanced. The large solvent effect in the [2 + 2] addition may be ascribed to solvent stabilization of a zwitterionic intermediate. However, when similar reactants, one set giving Diels-Alder product, the other [2 + 2] product, are examined for solvent effects on reaction rates, the dependencies may be virtually identical. A case in point is provided by the reactions of TCNE with 9,10-dimethylanthracene (Diels-Alder reaction) and with 2,5-dimethylhexa-2,4-diene ([2 + 2] cycloaddition). 

The iron carbonyl-mediated cycloaddition is also highly stereospecific despite a nonconcerted mechanism. In the reaction of dibromo ketone (57) with m-p-deuteriostyrene, only the cis stereoisomer of the adduct is detected at low conversion (equation 50). - This stereospecificity is attributed to the formation of a U-shaped intermediate, in which the bonds are fixed rigidly by charge-transfer or coulombic attraction between the iron enolate and the cationic center. The result can also be explained by rapid bond formation between the two termini before bond rotation can occur. Traru-anethole also couples stereos-pecifically with (57) (equation 51). ... 

Sensitized irradiation of cyclohexenes and cycloheptenes in protic media results in protonation. This phenomenon, which is not shared by other acyclic or cyclic olefins, has been attributed to ground-state protonation of a highly strained tran -cycloalkene intermediate. In aprotic media, either direct or triplet-sensitized irradiation of cyclohexene produces a stereoisomeric mixture of (2 -I- 2] dimers 49-51 as the primary products, with 50 predominating. The reaction apparently involves an initial cis-trans photoisomerization of cyclohexene followed by a nonstereospecific nonconcerted ground-state cycloaddition, promoted by the high degree of strain involved. In contrast, cycloheptene undergoes only a slow addition to the p-xylene used as sensitizer. 

Cycloadditions may be pericyclic reactions or (nonconcerted) stepwise reactions. The term dipolar cycloaddition is used for cycloadditions of 1,3-dipolar compounds. 

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