title: Computational studies of tungsten-catalyzed endo-selective cycloisomerization of 4-pentyn-1-ol.
abstract: Endo- and exo-cycloisomerizations of 4-pentyn-1-ol have been studied computationally with density functional theory, in conjunction with double-zeta and triple-zeta basis sets, both in the absence and in the presence of tungsten carbonyl catalyst. In the absence of the catalyst, both endo- and exo-cycloisomerizations have been calculated to have a very high activation barrier of approximately 50-55 kcal/mol and cannot take place. With tungsten pentacarbonyl catalyst, endo-cycloisomerization becomes a complex multiple-step reaction and proceeds with a rate-determining barrier of 26 kcal/mol at the C(alpha) --> C(beta) hydride migration step to form a vinylidene intermediate. The primary role of the tungsten catalyst is to stabilize the vinylidene intermediate, thus lowering the rate-determining barrier. The second important role of the tungsten catalyst in endo-cycloisomerization is to assist the OH hydride migration to C(alpha) by making it a multistep process with small activation barriers. The exo-cycloisomerization with the catalyst still has a high rate-determining barrier of 47 kcal/mol. These findings clearly explain the experimentally observed endo-selectivity in the cycloisomerization of 4-pentyn-1-ol derivatives and support the experimentally proposed mechanism.
authors.affiliation: Cherry L. Emerson Center for Scientific Computation and Department of Chemistry, Emory University, Atlanta, Georgia 30322, USA.
authors.forename: Yinghong
authors.initials: Y
authors.lastname: Sheng
authors.forename: Djamaladdin G
authors.initials: DG
authors.lastname: Musaev
authors.forename: K Subba
authors.initials: KS
authors.lastname: Reddy
authors.forename: Frank E
authors.initials: FE
authors.lastname: McDonald
authors.forename: Keiji
authors.initials: K
authors.lastname: Morokuma
mesh_terms: D000480:Alkynes / Q000737:chemistry*; D002384:Catalysis; D006844:Hydrocarbons, Cyclic / Q000138:chemical synthesis*; D015394:Molecular Structure; D009942:Organometallic Compounds / Q000737:chemistry*; D000439:Pentanols / Q000737:chemistry*; D014414:Tungsten / Q000737:chemistry*
chemical_list: C456418:4-pentyn-1-ol; D000480:Alkynes; D006844:Hydrocarbons, Cyclic; D009942:Organometallic Compounds; D000439:Pentanols; D014414:Tungsten