The aminoxyl radical BTNO (benzotriazole-N-oxyl; >N-O*) is generated from HBT (1-hydroxybenzotriazole; >N-OH) by oxidation with a Ce(IV) salt. BTNO presents a broad absorption band with lambda(max) 474 nm that lends itself to investigate the kinetics of H-abstraction from H-donor substrates by spectrophotometry. Thus, rate constants (k(H)) of H-abstraction by BTNO from CH(2)-groups alpha to the nitrogen atom in X-substituted-(N-acetyl)benzylamines (X-C(6)H(4)CH(2)NHCOCH(3)) have been determined in MeCN solution at 25 degrees C. Correlation of the k(H)(X) data with the Hammett sigma(+) parameters gives a small value for rho (-0.65) that is compatible with a radical H-abstraction step. The sizeable value (k(H)/k(D)=8.8) of the kinetic isotope effect from a suitably deuteriated amide substrate further confirms H-abstraction as rate-determining. Evidence is acquired for the relevance of stereoelectronic effects that speed up the H-abstraction whenever the scissile C-H bond is co-linear with either the nitrogen lone-pair of the amide moiety or an adjacent aromatic group. An assessment of the dissociation energy value of the benzylic C-H bond in ArCH(2)NHCOMe is accordingly reported.

No abstract

The hexamethylbenzene ruthenium(II) dimer [{RuCl(μ‐Cl)(η6‐C6Me6)}]2 (5 mol%), tested among a series of ruthenium(II) and ruthenium(IV) complexes, represents an efficient precatalyst source for the dimerization of terminal arylalkynes ArCCH [Ar=C6H5, 3,4,5‐(OMe)3C6H2, 4‐MeOC6H4, 2‐MeOC6H4, 4‐MeC6H4, 2,4,5‐Me3C6H2, 4‐BrC6H4, 4‐ClC6H4, 4‐FC6H4, 4‐HC(O)C6H4, 4‐CH2CHC6H4, 3‐NCC6H4, 4‐O2NC6H4, 4‐EtO2C‐(CH2)3OC6H4, 4‐HO(CH2CH2O)3C6H4, 3‐HO(CH2CH2O)3‐C6H4] in acetic acid/water mixture (1:1, v/v). The reactions proceed for 24 h at room temperature under heterogeneous conditions and afford the dimeric enyne derivatives (E)‐ArCHCHCCAr in high yields and stereoselectivity. The preformed acetato complex [RuCl(η6‐C6Me6)(κ2‐OAc)] catalyzes the dimerization of phenylacetylene under analogous conditions, with rapid substrate conversion. The presence of cosolvents of acetic acid different from water reduces dramatically the efficiency and selectivity of the reaction. The aqueous medium facilitates the activation stage of the precatalyst by assisting the splitting of the ruthenium dimer. The addition or generation in situ of acetate salts results in shorter reactions times (0.5–3 h) and excellent yields, due to the rapid formation of active acetato complexes. Circumstantial evidence indicates that the π‐bound alkyne molecule is activated by intramolecular proton abstraction. This is currently the most efficient, E‐selective and wide‐scope catalytic system for the alkyne dimerization reaction in protic aqueous media.

No abstract