Evidence for non-biradicaloid transition states in Diels–Alder reactions

Hancock, R. A.; Wood, B. F.

doi:10.1039/c39880000351

Cited by 13 publications

(7 citation statements)

References 3 publications

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“…[11, 23] Singleton et al, was interested if this would apply to doubly activated dienophiles, [23] due to the observation of attenuated activation by a second activating group. [11, 24] Reaction of 1,3-butadiene with bis(boryl)-acetylene is predicted by density functional theory to undergo cycloaddition through a highly unsymmetrical transition state. [23] Becke3LYP calculations with a 6-31G* basis set predicted the symmetrical structure, forced C s symmetry, to be 6.5 kcal/mol higher in energy than the unsymmetrical transition structure.…”

Section: Diels Alder Reaction Mechanismmentioning

confidence: 99%

“…While recent studies reveal that Diels–Alder reactions with unsymmetrical addends undergo cycloaddition through asynchronous transition states, it was accepted that reactions with symmetrical addends would undergo cycloaddition through synchronous or nearly synchronous transition states. , Singleton was interested in knowing if this would apply to doubly activated dienophiles due to the observation of attenuated activation by a second activating group. , Reaction of 1,3-butadiene with bis(boryl)acetylene is predicted by density functional theory to undergo cycloaddition through a highly unsymmetrical transition state . Becke3LYP calculations with a 6-31G* basis set predicted the symmetrical structure, forced C s symmetry, to be 6.5 kcal/mol higher in energy than the unsymmetrical transition structure .…”

Section: Diels–alder Reaction Mechanismmentioning

confidence: 99%

“…40 Figure 13. Biosynthesis of spinosyn A (24) where SpnF has been proposed as the putative cyclase. Recent X-ray crystal structure elucidation of the enzyme by Keatinge-Clay et al was obtained to probe further the putative mechanism of the SpnF-coded PKS-catalyzed cyclization.…”

Section: Spinosyn a Biosynthesismentioning

confidence: 99%

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Natural Diels–Alderases: Elusive and Irresistable

et al. 2015

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Eight examples of biosynthetic pathways wherein a natural enzyme has been identified and claimed to function as a catalyst for the [4+2] cycloaddition reaction, namely, Diels-Alderases, are briefly reviewed. These are discussed in the context of the mechanistic challenges associated with the technical difficulty of proving that the net formal [4+2] cycloaddition under study, indeed proceeds through a synchronous, mechanism and that the putative biosynthetic enzyme deploys the pericyclic transition state required for a Diels-Alder cycloaddition reaction.

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Section: Diels Alder Reaction Mechanismmentioning

confidence: 99%

Section: Diels–alder Reaction Mechanismmentioning

confidence: 99%

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Natural Diels–Alderases: Elusive and Irresistable

et al. 2015

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“…The experimentally observed stereospecificity and the uncertain existence of a biradical intermediate suggest that the D‐A reaction occurs by a concerted mechanism. Substituent effects on the reaction rates support this interpretation . Corroboration also comes from computational studies of a large number of D‐A reactions that favor concerted mechanism over the stepwise one [6g, .…”

Section: Introductionmentioning

confidence: 94%

“…Substituent effects on the reaction rates support this interpretation. [8,9] Corroboration also comes from computational studies of a large number of D-A reactions that favor concerted mechanism over the stepwise one. [6g,9,10] Nevertheless, intermediates of retro Diels-Alder reactions involving norbornadiene, cyclohexene, norbornene and bicyclo [2,2,2]oct-2-ene, have been detected in femtosecond-resolved mass spectrometry experiments.…”

Section: Introductionmentioning

confidence: 99%

Revisiting the concept of the (a)synchronicity of diels‐alder reactions based on the dynamics of quasiclassical trajectories

et al. 2015

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A number of model Diels-Alder (D-A) cycloaddition reactions (H2C=CH2 + cyclopentadiene and H2C=CHX + 1,3-butadiene, with X = H, F, CH3, OH, CN, NH2, and NO) were studied by static (transition state - TS and IRC) and dynamics (quasiclassical trajectories) approaches to establish the (a)synchronous character of the concerted mechanism. The use of static criteria, such as the asymmetry of the TS geometry, for classifying and quantifying the (a)synchronicity of the concerted D-A reaction mechanism is shown to be severely limited and to provide contradictory results and conclusions when compared to the dynamics approach. The time elapsed between the events is shown to be a more reliable and unbiased criterion and all the studied D-A reactions, except for the case of H2C=CHNO, are classified as synchronous, despite the gradual and quite distinct degrees of (a)symmetry of the TS structures.

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The utility of the PM3 method for predicting the reactivities of cyanoethenes in diels‐alder reactions with pyrrole

Jursic

Здравковски

1994

Journal of Heterocyclic Chem

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The reliability of PM3 for predicting the reactivity of cyanoethenes in Diels‐Alder reactions with pyrrole was tested. Using frontier orbital theory it was predicted that the reaction is LUMO dienophile controlled, and the reaction should be facilitated in comparison to cyclopentadiene additions (pyrrole has higher HOMO energy). The transition structures were generated and compared with similar ones generated with both ab initio and PM3 calculations for cyclopentadiene additions: The geometries of the transition structures are similar, although the new CC forming bonds are 2–6% shorter, and the degree of asynchronicity is higher. The prediction of the activation barriers with the PM3 method failed totally. The preferred exo isomer in each case, indicated that the predicted stereoselectivity was incorrect. The prediction of the relative reactivity was also unreliable. Qualitative analyses of similar series of compounds can be obtained by frontier orbital energy correlations. All conclusions were made on the assumption that the reaction is concerted, which does not exclude the possibility that PM3 might be able to predict reliable reaction barriers if some other mechanisms are considered.

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Evidence for non-biradicaloid transition states in Diels–Alder reactions

Cited by 13 publications

References 3 publications

Natural Diels–Alderases: Elusive and Irresistable

Natural Diels–Alderases: Elusive and Irresistable

Revisiting the concept of the (a)synchronicity of diels‐alder reactions based on the dynamics of quasiclassical trajectories

The utility of the PM3 method for predicting the reactivities of cyanoethenes in diels‐alder reactions with pyrrole

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