Computational cyclization reaction

The qualitative predictions and experimental findings in the case of cyano substituents have been analyzed computationally by performing (8/8)CASSCF and CASPT2/6-31G ab initio calculations. Table 7 summarizes the results for the cyclization reactions of ortho-, meta-, para-, and... 

Singleton and coworkers took up the ene cyclization reaction of ene-allene (Scheme 4) and carried out combined experimental-computational investigation.43 The ene reaction had been known to show mechanistic uncertainty, in particular whether it proceeds via a concerted or stepwise route, and therefore provided a challenge for dynamics study. KIE measurement for the reaction of 22 (Ri = R.2 = TMS) in toluene at 50°C gave kcus/kcm of 1.43, which was smaller than what was normally observed in concerted ene reactions. However, the isotope effect was too large to support a stepwise ene reaction. Thus, this was in line with the idea that the mechanism is near the concerted-stepwise borderline. 

The advent of faster, more powerful computing facilities over the past five years has seen the increased use of ab initio and other molecular orbital techniques applied to more complex problems, including radical cyclization reactions. 

A theoretical study using the density functional theory (DFT) method has been performed to rationalize the mechanism of the reactions between 1,3-dialkynes and hydroxylamine or hydrazine for the formation of 3,5-disubstituted isoxazoles or pyrazoles, respectively The computational results support a bimolecular proton transfer as the rate-determining step providing valuable clues for the use of Bronsted acid/base catalysts to promote the cyclization reaction (14OBC7503). 

Extensive mechanistic studies of this cyclization reaction were carried out by Myers et al. and extended with theoretical work by Squire s et al. It is known that, in contrast to the Bergman cyclization of the ene-diyne (Chapter 4.2), this transformation proceeds as an exothermic process determined by the increased stability of a benzyl radical versus a phenyl radical. The barrier for cyclization from substrate to a diradical product is low and can further be reduced by an appropriate substitution at the allenic terminus of the substrate. The dichotomous (polar and free radical) reactivity is observed on pyrolysis in the presence of polar reactants. Both radical and polar products arise from a common intermediate, which is described as a polar diradical, a linear combination of limiting structure 7 and zwitterion 11. According to Squires, polar diradical singlet species are involved. Based on computational studies supported by experimental product distribution studies, it has been proposed that both the diradical 7 and... 

With this validated methodology at hand, our research group has systematically computed new reaction modes of polyunsaturated carbon-ridi systems. For instance, we evaluated alternatives to the experimentally known cyclization modes... 

Many anodic oxidations involve an ECE pathway. For example, the neurotransmitter epinephrine can be oxidized to its quinone, which proceeds via cyclization to leukoadrenochrome. The latter can rapidly undergo electron transfer to form adrenochrome (5). The electrochemical oxidation of aniline is another classical example of an ECE pathway (6). The cation radical thus formed rapidly undergoes a dimerization reaction to yield an easily oxidized p-aminodiphenylamine product. Another example (of industrial relevance) is the reductive coupling of activated olefins to yield a radical anion, which reacts with the parent olefin to give a reducible dimer (7). If the chemical step is very fast (in comparison to the electron-transfer process), the system will behave as an EE mechanism (of two successive charge-transfer steps). Table 2-1 summarizes common electrochemical mechanisms involving coupled chemical reactions. Powerful cyclic voltammetric computational simulators, exploring the behavior of virtually any user-specific mechanism, have... 

The previously outlined mechanistic scheme, postulating reversible propagation and cyclization, was simplified by neglecting the de-cyclization because in the very short time of the studied reaction the extent of de-cyclization is negligible. The rate constants appearing in the appropriate differential equations were computer adjusted until the calculated conversion curves, shown in Fig. 7, fit the experimental points. The results seem to be reliable inspite of the stiffness of the differential equations. 

The detailed mechanism of this enantioselective transformation remains under investigation.178 It is known that the acidic carboxylic group is crucial, and the cyclization is believed to occur via the enamine derived from the catalyst and the exocyclic ketone. A computational study suggested that the proton transfer occurs through a TS very similar to that described for the proline-catalyzed aldol reaction (see page 132).179... 

Olefination Reactions Involving Phosphonium Ylides. The synthetic potential of phosphonium ylides was developed initially by G. Wittig and his associates at the University of Heidelberg. The reaction of a phosphonium ylide with an aldehyde or ketone introduces a carbon-carbon double bond in place of the carbonyl bond. The mechanism originally proposed involves an addition of the nucleophilic ylide carbon to the carbonyl group to form a dipolar intermediate (a betaine), followed by elimination of a phosphine oxide. The elimination is presumed to occur after formation of a four-membered oxaphosphetane intermediate. An alternative mechanism proposes direct formation of the oxaphosphetane by a cycloaddition reaction.236 There have been several computational studies that find the oxaphosphetane structure to be an intermediate.237 Oxaphosphetane intermediates have been observed by NMR studies at low temperature.238 Betaine intermediates have been observed only under special conditions that retard the cyclization and elimination steps.239... 

Fig. 7 Internal reaction coordinate (IRC) computations for the Bergman cyclization of model enediynes.

Fig. 19 The reaction energy profiles for thermal (on the left) and radical-anionic (on the right) C1-C6 and C1-C5 cyclizations of the parent enediyne computed at the B3LYP/ 6-31G level.

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