High-level Computational Study of the Site-, Facial- and Stereoselectivities for the Diels-Alder Reaction Between o-Benzoquinone and Norbornadiene

Abstract

Ab initio and DFT quantum chemical calculations have been applied to a study of the Diels-Alder reaction of o-benzoquinone as diene and norbornadiene as dienophile. Transition states for the different reactions are located and activation energies estimated. The prefered exo-π-facial selectivity and exo,endo-stereoselectivity exhibited in this cycloaddition are readily predicted using RHF/3-21G or higher levels of calculations. Differences between experimentally observed results and calculations may be explained by the postulation of a second, nonconcerted biradical mechanism leading to formation of hetero Diels-Alder products.

Biogenesis of Triterpene Dimers from Orthoquinones Related to Quinonemethides: Theoretical Study on the Reaction Mechanism

“…Therefore, in both cases, the HDA reactions may proceed through inverse electron demand (IED) and then the charge transfer (CT) of these cycloaddition reactions will take place from tingenone 1 to oq-6-oxotingenol 2 to form the A-A dimers and from isopristimerol 7 to oq-isopristemerol 8 to form the B-A dimers. These results are in agreement with those reported in previous works on orthoquinones systems [46,47,48,49]. Within the scale of electrophilicity and nucleophilicity power introduced by Domingo et al [50] the isopristimerol is a strong nucleophile, the tingenone is a moderate nucleophile and the orthoquinones studied in this work are strong electrophiles and the latter can be classified as highly reactive molecules.…”

Reaction of 2,5-bis-trifluoromethyl-1,3,4-oxadiazole with 7-oxanorbornenes revisited: Experimental and quantum-chemical study of reaction stereoselectivity

“…This prediction is in good accordance with previously published experimental and theoretical results on norbornene p-facial selectivity [11,28,[32][33][34]. Furthermore, dipolarophile favorably approaches 1,3-dipole 20 from its endo-p-face.…”

Tandem [4+2]/[3+2] Cycloadditions of 1,3,4-Oxadiazoles with Alkenes

“…This prediction is in good accordance with published results on norbornene -facial selectivity [5,[51][52][53][54]. It is evident from calculations that the cycloaddition reaction on the exo--face of the approaching 42 is greatly favored over the endo--face attack.…”

“…The second cycloaddition is the 1,3dipolar addition, [3 +2 ] of a second alkene (same or different) with the 1,3-dipolar intermediate 88 (or epoxide 171) and this is a regular electron-demand process. [51][52][53][54] In addition, 169 is thermodynamically more stable, indicating that the stereospecificity of the [4+2] addition of OD is kinetically controlled process. Looking in detail to this cycloaddition cascade, it is evident that the first cycloaddition proceeds via a transition state which could be either of the exo,exo-or exo,endo-orientation (167 and 168).…”