title: Ti-catalyzed Barbier-type allylations and related reactions.
abstract: Titanocene(III) complexes, easily generated in situ from commercial Ti(IV) precursors, catalyze Barbier-type allylations, intramolecular crotylations (cyclizations), and prenylations of a wide range of aldehydes and ketones. The reaction displays surprising and unprecedented mechanistic subtleties. In cyclizations a fast and irreversible addition of an allyl radical to a Ti(III)-coordinated carbonyl group seems to occur. Intermolecular additions to conjugated aldehydes proceed through a coupling of a Ti(IV)-bound ketyl radical with an allyl radical. Reactions of ketones with allylic halides take place by the classical addition of an allylic organometallic reagent. The radical coupling processes enable transformations such as the highly regioselective alpha-prenylation that are otherwise difficult to achieve. The mild reaction conditions and the possibility to employ titanocene complexes in only catalytic quantities are highly attractive features of our protocol. These unusual properties have been taken advantage of for the straightforward synthesis of the natural products rosiridol, shikalkin, and 12-hydroxysqualene.
authors.affiliation: Department of Organic Chemistry, University of Granada, Faculty of Sciences, C. U. Fuentenueva s/n 18071 Granada, Spain.
authors.forename: Rosa E
authors.initials: RE
authors.lastname: Estévez
authors.forename: José
authors.initials: J
authors.lastname: Justicia
authors.forename: Btissam
authors.initials: B
authors.lastname: Bazdi
authors.forename: Noelia
authors.initials: N
authors.lastname: Fuentes
authors.forename: Miguel
authors.initials: M
authors.lastname: Paradas
authors.forename: Duane
authors.initials: D
authors.lastname: Choquesillo-Lazarte
authors.forename: Juan M
authors.initials: JM
authors.lastname: García-Ruiz
authors.forename: Rafael
authors.initials: R
authors.lastname: Robles
authors.forename: Andreas
authors.initials: A
authors.lastname: Gansäuer
authors.forename: Juan M
authors.initials: JM
authors.lastname: Cuerva
authors.forename: J Enrique
authors.initials: JE
authors.lastname: Oltra