Transition structures asynchronous

Nitrones are a rather polarized 1,3-dipoles so that the transition structure of their cydoaddition reactions to alkenes activated by an electron-withdrawing substituent would involve some asynchronous nature with respect to the newly forming bonds, especially so in the Lewis acid-catalyzed reactions. Therefore, the transition structures for the catalyzed nitrone cydoaddition reactions were estimated on the basis of ab-initio calculations using the 3-21G basis set. A model reaction indudes the interaction between CH2=NH(0) and acrolein in the presence or absence of BH3 as an acid catalyst (Scheme 7.30). Both the catalyzed and uncatalyzed reactions have only one transition state in each case, indicating that the reactions are both concerted. However, the synchronous nature between the newly forming 01-C5 and C3-C4 bonds in the transition structure TS-J of the catalyzed reaction is rather different from that in the uncatalyzed reaction TS-K. For example, the bond lengths and bond orders in the uncatalyzed reaction are 1.93 A and 0.37 for the 01-C5 bond and 2.47 A and 0.19 for the C3-C4 bond, while those in... 

The theoretical investigations of Lewis acid-catalyzed 1,3-dipolar cycloaddition reactions are also very limited and only papers dealing with cycloaddition reactions of nitrones with alkenes have been investigated. The Influence of the Lewis acid catalyst on these reactions are very similar to what has been calculated for the carbo- and hetero-Diels-Alder reactions. The FMOs are perturbed by the coordination of the substrate to the Lewis acid giving a more favorable reaction with a lower transition-state energy. Furthermore, a more asynchronous transition-structure for the cycloaddition step, compared to the uncatalyzed reaction, has also been found for this class of reactions. 

TABLE 2.5 Solvent Effect on the Asynchronicity (Ad, in A) and Energies (AE, in kcal/mol) of MEV-TSexo Transition Structure at B3LYP/6-31G(d,p) Level"... 

We have carried out DFT level investigations to explain the observed stereo and regioselectivities. Concerted nature of the mechanism has been confirmed by the involvement of aromatic but asynchronous transition structures as confirmed by their NICS values in each case [101, 102],... 

Additionally, investigations into imidazolidinone catalysed Diels-Alder reactions (Schemes 2 and 6) [234] have shown that iminium ions of a,P-unsaturated aldehydes and ketones have lower activation barriers for the Diels-Alder reaction with cyclopentadiene than the parent compound (13 and 11 kCal mol", respectively). It was also noted that transition structures show the formation of the bonds is concerted but highly asynchronous. 

A systematic examination of the transition structures for the peroxyformic acid epoxidation at various levels of theory confirms the above suggestion that the MP2 method does provide adequate geometries for a conjugated carbon-carbon double bond in 1,3-butadiene (Figure 23). However, the inverse KIEs measured by HanzUk and Shearer " for 4-vinylbiphenyl are specific to conjugated alkenes and should not be used as evidence of an asynchronous epoxidation TS for simple nonconjugated aUcenes. ... 

Several computational studies have addressed whether the dipolar cycloaddition of nitronates is a concerted or stepwise process (93,100). Natural population analysis reveals that their is very little zwitterionic character in the transition state. The formation of the C C bond marginally precedes the C—O bond on the basis of calculated bond lengths and orders in the transition structure. These calculations also show that the reaction is a concerted process that is shghtly asynchronous. In addition, the cycloaddition likely proceeds through an early transition state and is overall an exothermic process. 

The regiochemistry of the reaction has been studied computationally (Scheme 34) [109], It was found that the asynchronicity and the polar nature of the transition structures favor the formation of the 4-substituted cycloadducts (136) via transition structures (135) when electron-releasing groups are present. The alternative transition structures (137) were calculated to be of much higher energy. 

Calculations on the experimental system and a model reaction were unable to locate a transition structure for formation of the diradical intermediate of a stepwise mechanism. The stepwise mechanism and the asynchronous concerted mechanism start out geometrically similar, and the two pathways appear to have merged at a highly asynchronous concerted transition structure. 

A recent DFT study has shown that retention of configuration observed for the above reactions is in agreement with the insertion of singlet nitrene and a concerted product formation [150 is the transition state for the reaction shown in Equation (6.135)]. Du Bois and coworkers have performed detailed mechanistic investigations of the intramolecular sulfamate ester C-H amination reaction catalyzed by a dirhodium complex. Reactivity patterns, Hammett analysis, and kinetic isotope effect studies have provided support for the concerted, asynchronous transition structure 151. A similar conclusion was arrived at for an analogous intermolecular process. ... 

The former reaction is unsymmetrical, like many Diels-Alder reactions, and the transition structure will also be unsymmetrical. This will make f3, which is distance-dependent, different for each of the CC bonds being formed, and will effectively increase the difference between the Zc2-valucs calculated above. Asynchronicity involving initial overlap of the two largest lobes will also be enhanced by the stabilisation of whatever radical character the transition structure has, and hence will augment the frontier-orbital explanation for the site selectivity in several other conjugated systems. 

Fig. 10.9. Asynchronous transition structures for Diels-Alder reactions of butadiene with maleic acid and 1,2,4-triazoline using B3LYP/6-31G calculations. Reproduced from Tetrahedron, 57, 5149 (2001) and J. Am. Chem. Soc., 120, 12303 (1998), by permission of Elsevier and the American Chemical Society, respectively.

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