title: Cyclic 1,3-dipoles or acyclic phosphonium ylides? Electronic characterization of "Montréalones".
abstract: The structural features of a recently introduced class of 1,3-dipolar reagents have been computed by density functional theory and ab initio methods. The reagents are formally derived from Münchnones by replacement of the C O group with a PR3 unit. The parent species (PR3 = PH3) shows a long P...O interaction (2.55 A at the B3LYP/6-31+G(d) level), together with a nonplanar ring, and is best described as a weakly chelated acylamino-phosphonium ylide. The corresponding acyclic form, in which the P...O interaction is absent, is predicted to be 2-3 kcal mol-1 higher in enthalpy. Variation of the phosphorus substituents exerts a marked effect on the P...O distance, with electron-withdrawing groups favoring a covalent interaction [P...O 1.97 A for PR3 = PPh(catechyl)] and electron-donating groups favoring a weak interaction [P...O 3.92 A for PR3 = PPh3]. However, this variation has little effect on the relative energies of the cyclic and acyclic forms. The barriers for concerted cycloadditions with ethylene are 22.8 kcal mol-1 (PH3), 31.7 kcal mol-1 (PPh3), and 16.2 kcal mol-1 [PPh(catechyl) with axial O], which correspond with experimental observations and follow the same trend as the energies required to distort the dipole to the TS geometry.
authors.affiliation: Department of Chemistry and Biochemistry, University of California, Los Angeles, California 90095-1569, USA.
authors.forename: Elizabeth H
authors.initials: EH
authors.lastname: Krenske
authors.forename: K N
authors.initials: KN
authors.lastname: Houk
authors.forename: Bruce A
authors.initials: BA
authors.lastname: Arndtsen
authors.forename: Daniel J
authors.initials: DJ
authors.lastname: St Cyr