title: Why nature eschews the concerted [2 + 2 + 2] cycloaddition of a nonconjugated cyanodiyne. Computational study of a pyridine synthesis involving an ene-Diels-Alder-bimolecular hydrogen-transfer mechanism.
abstract: An intramolecular formal metal-free intramolecular [2 + 2 + 2] cycloaddition for the formation of pyridines has been investigated with M06-2X and B3LYP density functional methods, and compared to the experimentally established three-step mechanism that involves ene reaction-Diels-Alder reaction-hydrogen transfer. The ene reaction of two alkynes is the rate-determining step. This is considerably easier than other possible mechanisms, such as those involving an ene reaction of an alkyne with a nitrile, a one-step [2 + 2 + 2] cycloaddition, or a 1,4-diradical mechanism. The relative facilities of these processes are analyzed with the distortion-interaction model. A bimolecular hydrogen-transfer mechanism involving a radical-pair intermediate is proposed rather than a concerted intramolecular 1,5-hydrogen shift for the last step in the mechanism.
authors.affiliation: Department of Chemistry and Biochemistry, University of California, Los Angeles, California 90095-1569, USA.
authors.forename: Yu
authors.initials: Y
authors.lastname: Lan
authors.forename: Rick L
authors.initials: RL
authors.lastname: Danheiser
authors.forename: K N
authors.initials: KN
authors.lastname: Houk
mesh_terms: D000480:Alkynes / Q000737:chemistry*; D060326:Chemistry Techniques, Synthetic / Q000379:methods*; D006859:Hydrogen / Q000737:chemistry*; D008958:Models, Molecular; D008968:Molecular Conformation; D009570:Nitriles / Q000737:chemistry*; D011725:Pyridines / Q000138:chemical synthesis* / Q000737:chemistry*; D013816:Thermodynamics
chemical_list: D000480:Alkynes; D009570:Nitriles; D011725:Pyridines; D006859:Hydrogen; C023666:pyridine