Nonconcerted mechanism

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. 

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

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 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]. 

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). 

Formation of 2//-azirines by thermal decomposition of vinyl azides has been shown to exhibit small entropy of activation and insensitivity to solvent polarity acyclic vinyl azides decompose more readily than analogous cyclic ones and it is advantageous to have a hydrogen atom cis to the azido group ( -are more reactive than Z-isomers). These results and the linear correlation found for ring-substiment effects on decomposition of a-styryl azides are consistent with a nonconcerted mechanism in which elimination of nitrogen and cyclization into a three-membered ring proceeds synchronously. 

Among the catalysts used are Lewis acids991 and phosphine-nickel complexes.992 Certain of the reverse cyclobutane ring openings can also be catalytically induced (8-40). The role of the catalyst is not certain and may be different in each case. One possibility is that the presence of the catalyst causes a forbidden reaction to become allowed, through coordination of the catalyst to the -it or a bonds of the substrate.993 In such a case the reaction would of course be a concerted 2S + 2S process. However, the available evidence is more consistent with nonconcerted mechanisms involving metal-carbon a-bonded intermediates, at least in most cases.994 For example, such an intermediate was isolated in the dimerization of nor-bornadiene, catalyzed by iridium complexes.995... 

P-450-catalyzed hydroxylations of aliphatic C—H bonds most often involve a nonconcerted mechanism (Figure 8), which occurs in two steps (1) an abstraction of the hydrogen atom by the P-450 active oxygen complex, which exhibits a free radical-like reactivity, and (2) an oxidation of the substrate-derived free radical formed in this step by the Fe(IV)—OH intermediate [34,37,38],... 

If now the same analysis is applied to allowed conrotatory reaction then the corresponding More O Ferrall diagram (Fig. 7) is in this case compatible with both concerted and nonconcerted mechanisms and the discrimination between them requires a knowledge of the corresponding reaction path. 

The available evidence indicates a nonconcerted mechanism which is depicted in Scheme 14. Oxidation of a Pd(0) species by the carbon-nitrogen bond of the allyle-nammonium ion gives cleavage to the 7r-allylpalladium complex and an enamine. Nucleophilic reaction by the enamine on the Pd salt then forms resultant imines after a loss of a proton. The role of co-catalyst CF3C02H is to form the N-allylenammonium ion, which reacts readily with the Pd(0) species. 

A nonconcerted mechanism of the 3-aza-Cope rearrangement of the enammonium salts has also been proposed by Mariano and coworkers 116a,ft, although the charge-accelerated rearrangements so far discussed proceed via the pericyclic route. The zwitterionic 7V-vinylisoquinuclidines formed from substrates 173 and a propiolate ester rearrange to fused heterocyclic compounds 174 through a stepwise pathway (Scheme... 

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... 

A nonconcerted mechanism, involving a least-hindeted approach of the cycloaddends and a stepwise addition of the palladium complex to the p-caibon of the a.p-unsaturated alkene, followed by cycliza-tion without loss of alkene geometry and/or competitive rotation about the Ca-Cp bond prior to cycliza tion with loss of stereochemistry, provides a reasonable account of the cycloaddition (Scheme 5). ... 

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). ... 

This is an example of a symmetry-forbidden reaction that, nonetheless, actually takes place. Failure to conform to the selection rules usually is taken to mean that the reaction proceeds by a nonconcerted mechanism. 

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]... 

The copper or copper(I) iodide-catalyzed insertion of diazomethane into the acyl-selenium linkage of selenol esters was reported to afford the corresponding ketones as the chief products. - A nonconcerted mechanism involving a tetrahedral intermediate (42 Scheme 11) has been suggested. 

Many stereochemical results like these, and the consistent observation of nonthermodynamic products that would not be expected from two-step mechanisms, have accumulated to support the operation of the pericyclic mechanism in most cases. Recent advances in experimental and theoretical techniques have allowed chemists to probe the mechanisms of pericyclic reactions at time scales far shorter than the time required to rotate about a C—C cr bond, and these experiments have supported concerted mechanisms for some pericyclic reactions and nonconcerted mechanisms for others. In practical terms, though, if no experimental or theoretical evidence rules it out, Occam s razor compels one to propose a concerted mechanism in favor of a nonconcerted one. 

The Claisen rearrangement is driven in the forward direction by formation of a carbonyl tt bond. The Claisen rearrangement originally referred only to the isomerization of an O-allylphenol to a 2-allylphenol. One might expect that in this particular case, the equilibrium would lie on the side of the aromatic compound, not the carbonyl. However, the carbonyl quickly tautomerizes (by a nonconcerted mechanism ) to the aromatic 2-allylphenol, which can t undergo the reaction in the reverse direction. 

Thermal [1,3] H shifts such as the conceited rearrangement of enols to carbonyl compounds are disallowed. The allylic C-C-O unit itself can only react suprafa-cially, as it is geometrically impossible for the H(ls) orbital to bond simultaneously to a top lobe on one terminus and a bottom lobe at the other terminus, and the H atom itself must also react suprafacially, as the H(ls) orbital has only one lobe. The Woodward-Hoffmann rules, though, say that one of the two components of this four-electron rearrangement must react antarafacially for it to be allowed. Therefore this rearrangement reaction always proceeds through a nonconcerted mechanism and requires acidic or basic catalysis. 

The factors determining which of the two product types 10 or 11 results are not fully understood. It seems clear that a nonconcerted mechanism is operating in the case of dichloro- and di-bromocarbene adducts, and in the formation of a-difluoro ketones from difluorocarbene adducts. 

A nonconcerted mechanism of the 3-aza-Cope rearrangement of the enammonium salts has also been proposed by Mariano and coworkers although the charge-... 

Hori et al. reported the ring expansion of 3-acetylbenzothiazoline 1-oxide (75) in refluxing acetic anhydride118-120 to give 4-acetylbenzothiazines (76) via a sulfenic anhydride intermediate this reaction is nonstereospecific, supporting the proposed nonconcerted mechanism. 

Four different types of (2 + 2)-cycloaddition and -cycloreversion reactions of heterocyclic compounds are known intermolecular and intramolecular reactions, both thermal and photochemical. Three of these have already been discussed in the previous sections on (2 + 2)-cycloadditions, and as far as the mechanism is concerned both the forward and the reverse reaction suffer from the same ambiguity Do they proceed via a concerted or a nonconcerted mechanism do they involve an ionic or a diradical intermediate are they symmetry-allowed or forbidden So far only one reaction type is known to be limited to the reverse reaction, viz., the thermal intramolecular reaction [Eq. (10)], in which one o-bond is broken and a conjugated -electron system is... 

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