Nonconcerted

Potential of mean force for the Cr+MeCl reaction in various solvents. (Figure redrawn from Chandrasekhar ] and W L Jorgensen 1985. Energy Profile for a Nonconcerted S>]2 Reaction in Solution. Journal of tho American Chemical Society 107 2974-2975.)... 

The Flory and Mayo proposals can be combined by the common diradical -D-, which collapses to either DH or 1,2-diphenylcyclobutane. Nonconcerted Diels-Alder reactions are permissible for two nonpolar reactants (122). 

This particular reaction has quite a high activation energy, which suggests that a nonconcerted mechanism might be involved. An alternative explanation for the observed stereochemistry has been advanced. 

Is the reaction concerted As was emphasized in Chapter 11, orbital symmetry considerations apply only to concerted reactions. The possible involvement of triplet excited states and, as a result, a nonconcerted process is much more common in photochemical reactions than in the thermal processes. A concerted mechanism must be established before the orbital symmetry rules can be applied. 

Fluorinaied dienophiles. Although ethylene reacts with butadiene to give a 99 98% yield of a Diels-Alder adduct [63], tetrattuoroethylene and 1,1-dichloro-2,2-difluoroethylene prefer to react with 1,3-butadiene via a [2+2] pathway to form almost exclusively cyclobutane adducts [61, 64] (equation 61). This obvious difference in the behavior of hydrocarbon ethylenes and fluorocarbon ethylenes is believed to result not from a lack of reactivity of the latter species toward [2+4] cycloadditions but rather from the fact that the rate of nonconcerted cyclobutane formation is greatly enhanced [65]... 

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]... 

Symmetry-allowed reactions often occur under relatively mild conditions, but symmetry-disallowed reactions can t occur by conceited paths. Either they take place by nonconcerted, high-energy pathways, or they don t take place at all. 

Cassar and Halpern (85) provided evidence that the Rh -catalyzed valence isomerization of quadricyclene to norbornadiene also proceeds through a nonconcerted mechanism ... 

The most plausible inference from these studies is to consider for the metathesis reaction a nonconcerted reaction mechanism involving a five-membered metallocyclic intermediate. In the case of the metathesis of propene, this can be visualized as follows ... 

It is clear that a detailed mechanism for the metathesis reaction of alkenes cannot yet be given with certainty. In view of the fact that, for similar reactions which are formally cyclobutane-dialkene transformations, a nonconcerted reaction pathway has been demonstrated, a concerted fusion of two alkenes to form a cyclobutane complex and its decomposition in the same way with a change in the symmetry plane is less probable. On the basis of the information on the two other mechanisms to date, the mechanism involving a metallocyclic intermediate is more plausible than a mechanism involving carbene complexes. 

Several mechanistic explanations98—including both concerted symmetry-allowed nonlinear chelatropic paths96, and nonconcerted stepwise mechanisms (such as that in which a diradical species is involved99)—have been advanced to accommodate the stereospecific experimental results2,173,73,99. 

The higher strain energy in thiirene dioxides (19) compared to thiirane dioxides (17) is obvious. Yet, the elimination of sulfur dioxide from the latter is significantly faster than one would expect for a thermally allowed concerted process. Consequently, either aromatic-type conjugative stabilization effects are operative in thiirene dioxides2,12 or the relative ease of S02 elimination reflects the relative thermodynamic stability of the (diradical )99 intermediates involved in the nonconcerted stepwise elimination process. 

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. 

It has been suggested that another type of diradical two-step mechanism may be preferred by some substrates. Indeed, a nonconcerted Cope rearrangement has been reported. In this pathway, the 1,6 bond is formed before the 3,4 bond breaks ... 

Diaminobiphenyl is formed by a completely different mechanism, though the details are not known. There is rate-determining breaking of the N—N bond, but the C—C bond is not formed during this step. The formation of the o-semidine also takes place by a nonconcerted pathway. Under certain conditions, benzidine rearrangements have been found to go through radical cations. 

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. 

Cyclobutanes can also be formed by nonconcerted processes involving zwitter-ionic intermediates. The combination of an electron-rich alkene (enamine, enol ether) and an electrophilic one (nitro- or polycyanoalkene) is required for such processes. 

Although the concerted mechanism described in the preceding paragraph is available only to those azo compounds with appropriate orbital arrangements, the nonconcerted mechanism occurs at low enough temperatures to be synthetically useful. The elimination can also be carried out photochemically. These reactions presumably occur by stepwise elimination of nitrogen, and the ease of decomposition depends on the stability of the radical R ... 

The stereochemistry of the nonconcerted reaction has been a topic of considerable study. Frequently, there is partial stereorandomization, indicating a short-lived diradical intermediate. The details vary from case to case, and both preferential inversion and retention of relative stereochemistry have been observed. 

The metal-carbenoid intermediates, especially ones derived from a-diazocarbonyl compounds, are electrophilic, and electron-rich olefins in general react more easily with the carbenoid intermediates than electron-deficient olefins. For the interaction of metal carbenoid and olefin, three different mechanisms have been proposed, based on the stereochemistry of the reactions and the reactivity of the substrates (Figure 12) 21 (i) a nonconcerted, two-step process via a metallacyclobutane 226,264... 

A theoretical study of degenerate Boulton-Katritzky rearrangements concerning the anions of the 3-hydroxyimi-nomethyl-l,2,5-oxadiazole has been carried out by using semi-empirical modified neglect of diatomic overlap (MNDO) and ab initio Hartree-Fock procedures. Different transition structures and reactive pathways were obtained in the two cases. Semi-empirical treatment shows asymmetrical transition states and nonconcerted processes via symmetrical intermediates. By contrast, ab initio procedures describe concerted and synchronous processes involving symmetrically located transition states <1998JMT(452)67>. 

The nonconcerted mechanism of decomposition was observed by Szwarc et al. [67], They studied the photolytic decomposition of cyclopropyltrifluoromethyl diazene in the gas phase... 

It is considered that the decomposition of symmetrical azo-compounds proceeds via the concerted mechanism, and some unsymmetrical azo-compounds are decomposed by the concerted and nonconcerted mechanisms simultaneously. Phenyl-substituted azo-compounds are decomposed by the nonconcerted mechanism [3]. 

This equation helps to distinguish between the concerted and the nonconcerted decomposition of the initiator (for concerted decomposition, kfl = 0). 

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... 

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