Action planning and the timescale of evidence accumulation

Perceptual decisions are based on the temporal integration of sensory evidence for different states of the outside world. The timescale of this integration process varies widely across behavioral contexts and individuals, and it is diagnostic for the underlying neural mechanisms. In many situations, the decision-maker knows the required mapping between perceptual evidence and motor response (henceforth termed “sensory-motor contingency”) before decision formation. Here, the integrated evidence can be directly translated into a motor plan and, indeed, neural signatures of the integration process are evident as build-up activity in premotor brain regions. In other situations, however, the sensory-motor contingencies are unknown at the time of decision formation. We used behavioral psychophysics and computational modeling to test if knowledge about sensory-motor contingencies affects the timescale of perceptual evidence integration. We asked human observers to perform the same motion discrimination task, with or without trial-to-trial variations of the mapping between perceptual choice and motor response. When the mapping varied, it was either instructed before or after the stimulus presentation. We quantified the timescale of evidence integration under these different sensory-motor mapping conditions by means of two approaches. First, we analyzed subjects’ discrimination threshold as a function of stimulus duration. Second, we fitted a dynamical decision-making model to subjects’ choice behavior. The results from both approaches indicated that observers (i) integrated motion information for several hundred ms, (ii) used a shorter than optimal integration timescale, and (iii) used the same integration timescale under all sensory-motor mappings. We conclude that the mechanisms limiting the timescale of perceptual decisions are largely independent from long-term learning (under fixed mapping) or rapid acquisition (under variable mapping) of sensory-motor contingencies. This conclusion has implications for neurophysiological and neuroimaging studies of perceptual decision-making

Oxidation of ethylbenzene over "neat" and zeolite-Y-encapsulated copper tri- and tetraaza macrocyclic complexes

Copper tri- and tetraaza macrocyclic complexes were synthesized, encapsulated in zeolite-Y and characterized. The "neat" and encapsulated complexes exhibited good catalytic activity in the oxidation of ethylbenzene at 333 K, using tert-butyl hydroperoxide as the oxidant. Acetophenone was the major product though small amounts of o- and p-hydroxyacetophenones were also formed revealing that C---H bond activation takes place both at benzylic and aromatic ring carbon atoms. Ring hydroxylation was more over the "neat" complexes than over the encapsulated complexes. The differences in selectivity are attributed to the formation of different types of "active" copper-oxygen intermediates, such as side-on peroxide, bis-μ-oxo complexes and Cu-hydroperoxo species, in different proportions over the "neat" and encapsulated complexes. Oxidation of ethylbenzene over "neat" and zeolite-Y-encapsulated copper tri- and tetraaza macrocyclic complexes using tert-butyl hydroperoxide as oxidant is reported. C---H bond activation takes place both at benzylic and aromatic ring carbon atoms. Ring hydroxylation was more over the "neat" than the encapsulated complexes. The differences in the selectivity are attributed to different types of "active" copper-oxygen intermediates formed in different proportions

Alkali cleavage of α,α-disubstituted β-ketoesters, nitriles and β-diketones

670-671Nucleophilic cleavage of several α,α-disubstituted β-ketoesters, nitriles and β-diketones has been examined. Only alkali hydroxides participated in the reaction yielding substituted esters, nitriles and ketones

EPR spectroscopy of copper and manganese complexes encapsulated in zeolites

The structural basis for the enhanced catalytic activities of copper and manganese Schiff base complexes encapsulated in zeolite-Y is investigated by EPR spectroscopy. The study provides an unequivocal evidence for the encapsulation of complexes inside the supercages of zeolite-Y. The EPR spectroscopy distinguishes the encapsulated complexes from the "neat" and surface-adsorbed metal complexes. Neat complexes showed broad EPR spectra corresponding to nearest neighbour spin–spin interactions whereas the zeolite-encapsulated metal complexes showed well resolved metal hyperfine features similar to the spectra in dilute frozen solutions. The spin Hamiltonian parameters reveal a distorted square pyramidal geometry and an increase in the in-plane covalency of metal–ligand bond as a consequence of encapsulation. The observed changes in the molecular electronic structure are correlated to the enhanced catalytic activity of the encapsulated metal complexes

Two competing mechanisms for the copper-free Sonogashira cross-coupling reaction

The mechanism of the copper-free Sonogashira cross-coupling was investigated using a model reaction with differently para-substituted phenylacetylenes and 4-iodobenzotrifluoride as coupling partners and a Pd 2 (dba) 3 -CHCl 3 -AsPh 3 catalyst system in methanol. A carbopalladation mechanism was ruled out through a series of experiments in which the equivalent of a carbopalladation reaction intermediate was synthesized by an alternate route, and its conversion to product was monitored. A Hammett correlation study revealed a possible mechanistic changeover when going from electron-rich to electron-poor alkynes in the model reaction. It is advocated that the reaction mechanism changes from a pathway involving a fast proton transfer from a slowly forming cationic Pd complex to a pathway involving a slow proton transfer from a neutral Pd complex on going from electron-rich to electron-poor alkynes. The amine base is believed to act as a base in both pathways and as a nucleophile promoting the formation of the cationic complex in the reactions involving electron-rich alkynes. This was substantiated by the observation of a primary isotope effect (K Alkyne-H /K Aikyne-D ≈ 2) for the electron-poor alkyne and a pronounced base dependence for the electron-rich one. \ua9 2008 American Chemical Society

Catalytic activities of oxo-Mn-triazacyclononane complexes: spectral studies and single crystal X-ray structure of [Mn<SUB>4</SUB>O<SUB>6</SUB>(1,4,7-triazacyclononane)<SUB>4</SUB>](ClO<SUB>4</SUB>)<SUB>4</SUB>·H<SUB>2</SUB>O

Monomeric terminal-oxo- and tetrameric bridged-oxo–Mn complexes viz., [(tmtacn)Mn(O)(H2O)]SO4, and [Mn4O6(tacn)4](ClO4)4&#183;H2O, respectively, were isolated and characterized by FT-IR, UV–vis, EPR and magnetic susceptibility. Here, TMTACN = N,N',N"-trimethyl-1,4,7-triazacyclononane and TACN = 1,4,7-triazacyclononane. Single crystal X-ray structure of the tetrameric bridged-oxo-Mn complex is reported. The catalytic oxidation activities of these complexes for benzylic C---H bond oxidation of ethylbenzene with aqueous H2O2 and tert-butyl hydroperoxide (TBHP) are reported. The studies reveal that nuclearity and type of oxo–Mn speciation influence the catalytic activity. While the monomeric terminal-oxo-Mn complex exhibits efficient C---H bond oxidation activity, the tetrameric bridged-oxo-Mn complex shows the catalase activity

Palladium(II) containing hydrotalcite as an efficient heterogeneous catalyst for Heck reaction

Palladium(II) containing hydrotalcite (Pd-HT) has been found to be an efficient and reusable catalyst in Heck reaction between aryl halides (X = Br, I) and olefins to give carbon-carbon coupled products in good to moderate yields

Benzylic oxidation with HO catalyzed by Mn complexes of N,N',N"-trimethyl-1,4,7-triazacyclononane: spectroscopic investigations of the active Mn species

Mn complexes of N,N',N"-trimethyl-1,4,7-triazacyclononane (Mn–tmtacn) exhibit good catalytic activity, at ambient temperatures, for the benzylic oxidation of aromatics selectively to the corresponding alcohol and carbonyl compounds with H2O2 as oxidant in the presence of carboxylate buffers. The active Mn species in the reaction medium was investigated by UV-visible (UV-Vis), FT-IR–attenuated total reflectance (FT-IR-ATR) and electron spin resonance (ESR) spectroscopic techniques. The studies revealed the formation of terminal oxo- and &#956;-oxo-Mn(IV)-tmtacn complexes during the reaction. The oxo-manganese complexes and the nature of the carboxylic acid play an important role in the activation of the benzylic C-H bond