No abstract

Aerobic alcohol oxidation catalyzed by the newly synthesized dimeric [(neocuproine)Pd(µ-OAc)] 2 -(OTf) 2 (1; neocuproine ) 2,9-dimethyl-1,10-phenanthroline) proceeds under exceptionally mild conditions (room temperature, ambient air) compared to those required for the previously reported monomeric diacetate analogue (neocuproine)Pd(OAc) 2 (2) and the monomeric ditriflate analogue (neocuproine)Pd-(MeCN) 2 (OTf) 2 (3). An unprecedented initial turnover frequency (TOF i ) for such mild conditions (78 (Pd atom) -1 h -1 ) is observed with catalyst 1; however, competitive oxidation of a methyl group on the ligand to a carboxylate group results in catalyst inactivation. During the study of 1, we isolated for the first time [(neocuproine)Pd(µ-OH)] 2 (OTf) 2 (4), a possible intermediate in the catalytic cycle, and we report its crystal structure and catalytic activity herein.

No abstract

The regio- and chemoselective oxidation of unprotected vicinal polyols with [(neocuproine)Pd(OAc)]2(OTf)2 (1) (neocuproine = 2,9-dimethyl-1,10-phenanthroline) occurs readily under mild reaction conditions to generate α-hydroxy ketones. The oxidation of vicinal diols is both faster and more selective than the oxidation of primary and secondary alcohols; vicinal 1,2-diols are oxidized selectively to hydroxy ketones, whereas primary alcohols are oxidized in preference to secondary alcohols. Oxidative lactonization of 1,5-diols yields cyclic lactones. Catalyst loadings as low as 0.12 mol % in oxidation reactions on a 10 g scale can be used. The exquisite selectivity of this catalyst system is evident in the chemoselective and stereospecific oxidation of the polyol (S,S)-1,2,3,4-tetrahydroxybutane [(S,S)-threitol] to (S)-erythrulose. Mechanistic, kinetic, and theoretical studies revealed that the rate laws for the oxidation of primary and secondary alcohols differ from those of diols. Density functional theory calculations support the conclusion that β-hydride elimination to give hydroxy ketones is product-determining for the oxidation of vicinal diols, whereas for primary and secondary alcohols, pre-equilibria favoring primary alkoxides are product-determining. In situ desorption electrospray ionization mass spectrometry (DESI-MS) revealed several key intermediates in the proposed catalytic cycle.

The retroarc fold-and-thrust belt of the Central Andes exhibits major along-strike variations in its pre-Cenozoic tectonic confi guration. These variations have been proposed to explain the considerable southward decrease in the observed magnitude of Cenozoic shortening. Regional mapping, a cross section, and U-Pb and (U-Th)/He age dating of apatite and zircon presented here build upon the preexisting geological framework for the region. At the latitude of the regional transect (24-25°S), results demonstrate that the thrust belt propagated in an overall eastward direction in three distinct pulses during Cenozoic time. Each eastward jump in the deformation front was apparently followed by local westward deformation migration, likely refl ecting a subcritically tapered orogenic wedge. The fi rst eastward jump was at ca. 40 Ma, when deformation and exhumation were restricted to the western margin of the Eastern Cordillera and eastern margin of the Puna Plateau. At 12-10 Ma, the thrust front jumped ~75 km toward the east to bypass the central portion of a horst block of the Cretaceous Salta rift system, followed by initiation of new faults in a subsystem that propagated toward the west into this preexisting structural high. During Pliocene time, deformation again migrated >100 km eastward to a Cretaceous synrift depocenter in the Santa Bárbara Ranges. The sporadic foreland-ward propagation documented here may be common in basement-involved thrust systems where inherited weaknesses due to previous crustal deformation are preferentially reactivated during later shortening. The minimum estimate for the magnitude of shortening at this latitude is ~142 km, which is moderate in magnitude compared to the 250-350 km of shortening accommodated in the retroarc thrust belt of southern Bolivia to the north. This work supports previous hypotheses that the magnitude of shortening decreases signifi cantly along strike away from a maximum in southern Bolivia, largely as a result of the distribution of pre-Cenozoic basins that are able to accommodate a large magnitude of thinskinned shortening. A major implication is that variations in the pre-orogenic upper-crustal architecture can infl uence the behavior of the continental lithosphere during later orogenesis, a result that challenges geodynamic models that neglect upper-plate heterogeneities.