title: 3-bromozinc propenyl esters: an experimental and theoretical study of the unique stereocrossover observed in their addition to aromatic and aliphatic aldehydes.
abstract: We report the results of a combined experimental and theoretical study on the reaction of 3-bromopropenyl acetate in the presence of zinc with three different aldehydes (i.e., benzaldehyde, 2-methylpropanal, and cyclohexanecarboxaldehyde). A 80% de in favor of the anti product has been experimentally observed with both saturated aldehydes, while for benzaldehyde, a 1:1 syn/anti ratio has been found. DFT computations show the existence of three eta1-allylic organozinc complexes [gamma-(Z)-5a, gamma-(E)-5a, and alpha-5a], very close in energy. Only gamma-(Z)-5a and gamma-(E)-5a lead to the observed product. The computational investigation of the reaction of these allylic organozinc complexes with benzaldehyde and 2-methylpropanal demonstrates in both cases the existence of two competitive reaction paths leading to the syn and anti adducts, respectively. An anti preference has been found for 2-methylpropanal with both gamma-(Z)-5a and gamma-(E)-5a species (a diastereoselectivity larger than 80% is predicted), in agreement with the experiment. With benzaldehyde, while the reaction of gamma-(Z)-5a retains an anti-stereopreference (de = 70%), that involving gamma-(E)-5a is characterized by two degenerate transition states. In this case, the agreement between computations and experiments would be satisfactory under the assumption that the initial oxidative addition affords the gamma-(E)-5a zinc complex only. Additional MP2 computations have demonstrated that pi-stacking interactions can play a significant role in determining the relative energy of the transition states leading to the syn and anti products.
authors.affiliation: Dipartimento di Chimica G. Ciamician, Università di Bologna, via Selmi 2, I-40126 Bologna, Italy. andrea.bottoni@unibo.it
authors.forename: A
authors.initials: A
authors.lastname: Bottoni
authors.forename: M
authors.initials: M
authors.lastname: Lombardo
authors.forename: G P
authors.initials: GP
authors.lastname: Miscione
authors.forename: J B Pujol
authors.initials: JB
authors.lastname: Algué
authors.forename: C
authors.initials: C
authors.lastname: Trombini
mesh_terms: D000438:Alcohols / Q000138:chemical synthesis / Q000737:chemistry; D000447:Aldehydes / Q000737:chemistry*; D003198:Computer Simulation; D004952:Esters / Q000737:chemistry*; D006842:Hydrocarbons, Brominated / Q000737:chemistry; D008956:Models, Chemical; D015394:Molecular Structure; D009942:Organometallic Compounds / Q000737:chemistry*; D013237:Stereoisomerism; D015032:Zinc / Q000737:chemistry*
chemical_list: D000438:Alcohols; D000447:Aldehydes; D004952:Esters; D006842:Hydrocarbons, Brominated; D009942:Organometallic Compounds; D015032:Zinc