11  Reaction mechanisms : Part (iii) Pericyclic reactions

Abstract

This review is a collection of the highlights in the field of pericyclic reactions published in the year 2004. Research in this fascinating topic continues to be centred on the understanding of the mechanisms of the three major classes of pericyclic reactions: cycloaddition reactions, electrocyclic reactions, and sigmatropic rearrangements, as well as into those of more unusual pseudopericyclic processes. HighlightsThese featured papers published in 2004 involve fundamental questions in pericyclic reaction mechanisms. Baldwin and co-workers studied the forbidden disrotatory 4-electron ring opening of cis-bicyclo[4.2.0]oct-7-ene, and using secondary kinetic isotope effect measurements the authors conclude that the rearrangement occurs through a concerted disrotatory process; conrotation almost certainly occurs, but the strained product reverts to the reactant. 1,2 Doering et al. studied the sigmatropic [1,5] hydrogen migration in phenyl-substituted 1,3-pentadienes to determine whether this reaction, like the Cope rearrangement, has a chameleonic transition state, dependent on the position and nature of substituents. 3 Although substituent effects are small, the authors conclude that the results are consistent with the chameleonic transition state. Calculations of the same reaction were reported by Borden and co-workers and also show small substituent effects; also there are few structural differences between the contributing structures of the transition state but large energy differences. 4 Although the computational data do not support a chameleonic transition state, perhaps a different [1,5] rearrangement might. Gajewski published a second, updated edition of ''Hydrocarbon Thermal Isomerizations'', which includes many pericyclic rearrangements. 5 Two computational benchmarking studies of pericyclic reactions were published. Houk, Pulay and co-workers compared the use of the Handy/Cohen OLYP and O3LYP functionals in pericyclic reactions to B3LYP and CBS-QB3 methods. 6 Although the OLYP functional was found in a previous benchmarking study to be superior to BLYP in predicting thermochemistry and molecular geometry, it is comparable to BLYP for pericyclic reactions and O3LYP is comparable to B3LYP. Also, in a collaborative effort, Goumans et al. presented a new computational method for the inclusion of electron correlation. 7 This spin-component-scaled MP2 (SCS-MP2) method was tested on a variety of Diels-Alder reactions, [3,3] sigmatropic rearrangements and electrocyclic reactions, cases in which MP2 calculations fail. The results are in excellent agreement with experiment, are closer to G3 values than MP2 or B3LYP, and the results improve with the basis set size, unlike MP2 and B3LYP.

The catalytic power of enzymes: Conformational selection or transition state stabilization?

“…The reaction is a crucial step in the biosynthesis of aromatic amino acids in microorganisms and plants. Chemically speaking, the isomerization is a Claisen rearrangement [9], which proceeds, as demonstrated by Knowles and coworkers [10,11], through a “chair‐like” transition state for the atoms of the [3,3]‐pericyclic region, both in solution and in the enzyme. This implies that the enolpyruvyl side chain must be positioned over the cyclohexadienyl for the isomerization reaction (see Fig.…”

Quantum chemical study of reaction mechanism of [4+2]-cycloaddition between 2,3-dimethylbuta-1,3-diene and methyl acrylate

“…[1][2][3] Sometimes chemical evidences are not enough to set reaction path. The problems concerning the timing of forming and breaking bonds are principally difficult to solve by experiment.…”

“…The reaction of 2,3-dimethylbuta-1,3-diene with alkyl methacrylates belongs to the pericyclic reactions 3 and can proceeds according to the concerted mechanism in a one-step or two-step interaction between atoms. The one-step pathway can occur through synchronous or asynchronous transition states.…”

“…Theoretical calculations can be important in situations of this kind, and a number of calculations for Diels-Alder reactions have been reported. [2][3][4][5][6][7][8] Computer technologies advances in quantum chemistry provide an opportunity to explore the theoretical aspects of some chemical reactions. [6][7][8] And use of these kind technologies provides a significant aid in the experimental studies to elucidate the mechanism of DielsAlder reaction.…”

“…There are many studies 2,3,5,12 where authors by different methods of calculations obtain information about the mechanism of these kind reactions. In the article, 5 authors suggest that Diels-Alder reactions in general proceed via very unsymmetrical transition states, close to biradicals in structure and with energies differing little from those of the corresponding biradicals.…”