title: [2 + 4] and [4 + 2] cycloadditions of o-thioquinones with 1,3-dienes: a computational study.
abstract: Cycloadditions of o-thioquinones (o-TQs) with 1,3-dienes could proceed via either a [2 + 4] or a [4 + 2] mechanism. Under kinetic control and with acyclic dienes the reaction affords the spiro cycloadducts 5deriving from the [2 + 4] path as the main products. Under thermodynamic control, or with cyclic dienes, the o-TQs behave as heterodienes to give the benzoxathiin derivatives 4, in most cases with complete regioselectivity. In the present computational study, DFT calculations were performed in order to achieve a deep understanding of both [2 + 4] and [4 + 2] paths. The reactions of three o-TQs with six 1,3-dienes were thoroughly investigated at the B3LYP/TZVP//B3LYP6-31G level, and the two reaction mechanisms were then compared, evidencing that [2 + 4] cycloadditions are kinetically favored, strongly asynchronous, or even unconcerted, while [4 + 2] reactions are thermodynamically favored, quite asynchronous, but undoubtedly concerted. Moreover, the observed regioselectivity was rationalized by mean of the FMO theory and by comparison of the activation energies for different pathways.
authors.affiliation: Istituto di Chimica Organica "Alessandro Marchesini", Facoltà di Farmacia, Università degli Studi di Milano, via Venezian 21, 20133 Milano, Italy. alessandro.contini@unimi.it
authors.forename: Alessandro
authors.initials: A
authors.lastname: Contini
authors.forename: Samantha
authors.initials: S
authors.lastname: Leone
authors.forename: Stefano
authors.initials: S
authors.lastname: Menichetti
authors.forename: Caterina
authors.initials: C
authors.lastname: Viglianisi
authors.forename: Pasqualina
authors.initials: P
authors.lastname: Trimarco
mesh_terms: D000466:Alkadienes / Q000737:chemistry*; D019295:Computational Biology; D003500:Cyclization; D007700:Kinetics; D008956:Models, Chemical; D008958:Models, Molecular; D008968:Molecular Conformation; D015394:Molecular Structure; D011809:Quinones / Q000737:chemistry*; D013237:Stereoisomerism; D013438:Sulfhydryl Compounds / Q000737:chemistry*
chemical_list: D000466:Alkadienes; D011809:Quinones; D013438:Sulfhydryl Compounds