Jose Cabral

Comparative investigation of kinetic consequences associated with long-range electronic effects on catalytic ruthenium-promoted ring-closing metathesis

Abstract When exposed to the Grubbs ruthenium catalyst, trienyl substrates of type 4 undergo highly regioselective ring closure to give the common product 5 . These reactions proceed invariably under pseudo-first-order kinetics, considerably faster when X=H ( t 1/2 =9.1), followed by X=CO 2 Et ( t 1/2 =26.5).

Comparison of an internally coordinated 2-pentenyllithium with its 4-sila analog. Structure and dynamic behavior: unexpected 13C7Li spin coupling.

Four allylic lithium compounds have been prepared with a tethered ligand, (CH(3)OCH(2)CH(2))(2)NCH(2)C(CH(3))(2)-L, attached to a terminal allyl carbon. They are equimolar equilibrium mixtures of 3-endo-L-allyllithium with 3-exo-L-allyllithium, 9en and 9ex, respectively, and (separately) 1-exo-TMS-3-endo-L-allyllithium with 1-exo-TMS-3-exo-L-allyllithium, 13en and 13ex, respectively. Carbon-13 NMR analysis shows that all four compounds are monomeric and internally coordinated, the allyl moiety in each one is partially localized, and (7)Li is spin coupled to both (31)C(1) and (13)C(2) of allyl. NMR line shape changes show that 9en, 9ex, 13en, and 13ex all undergo fast transfer of the lithium-coordinated ligand between faces of the allyl planes at low temperatures. Within each equilibrium system the compound with the exo-tethered ligand inverts faster than its endo analog. All four compounds invert faster in THF-d(10) compared to in diethyl ether-d(10) as solvent. Corresponding Eyring activation parameters are reported. Also, inversion in THF-d(8) for all four compounds is characterized by large negative DeltaS(double dagger) values, whereas in diethyl ether-d(10) DeltaS(double dagger) values for inversion are near neutral. NMR line shape changes due to the dynamics of bimolecular C,Li exchange and rotation around the C(2)-C(3) bonds are just detected via selective line broadening effects above 290 K. These processes are much slower than inversion.

Rearrangement of 3-Acyl Derivatives of L-Ascorbic Acid

Abstract Evidence of rearrangement of 3-acyl derivatives of ascorbic acid to 2-acyl derivatives has been found for carbon and phosphorus acyl groups. The observations are consistent with intramolecular rearrangement through a cyclic intermediate in which the acyl group is bonded to both the 2- and 3-oxygen atoms of ascorbic acid. A rate of rearrangement has been measured for the 3-diphenylphosphinate ester. Calculated results indicate an increase in negative atomic charge at O-2 in the 3-acyl esters but a decrease in the charge of O-3 in the 2-acyl esters.