Copper(I)-catalyzed substitution of propargylic carbonates with diboron: selective synthesis of multisubstituted allenylboronates

A versatile synthetic method for the preparation of the allenylboronates, Cu(I)-catalyzed reaction of propargylic carbonates with a diboron, is described. A Cu(O-t-Bu)-Xantphos catalyst system was effective for the preparation of various allenylboron compounds, allenylboronates with different substitution patterns, those with functional groups, and an axially chiral one. The Lewis acid promoted stereoselective addition to aldehydes leading to homopropargylic alcohols showed the usefulness of the new allenylboronates.     

Use of Sm(III)-{1,2-propanediamine-N,N,N',N'-tetra(alpha,alpha-dideuterioacetate)} complex for NMR determination of absolute configuration of each alpha-amino acid in peptide hydrolysate mixtures

(1)H NMR analyses of individual alpha-amino acids in their mixture were simultaneously conducted in the presence of Sm-(pdta-d(8)) in water: high regularity, promising for direct simultaneous determination of absolute configurations of each alpha-amino acids in peptide hydrolysate mixtures, was observed between absolute configuration and the induced shifts.     

Pt monolayer electrocatalysts for O2 reduction: PdCo/C substrate-induced activity in alkaline media

We measured the activity of electrocatalysts, comprising Pt monolayers deposited on PdCo/C substrates with several Pd/Co atomic ratios, in the oxygen reduction reaction in alkaline solutions. The PdCo/C substrates have a core-shell structure wherein the Pd atoms are segregated at the particle’s surface. The electrochemical measurements were carried out using an ultrathin film rotating disk-ring electrode. Electrocatalytic activity for the O₂ reduction evaluated from the Tafel plots or mass activities was higher for Pt monolayers on PdCo/C compared to Pt/C for all atomic Pd/Co ratios we used. We ascribed the enhanced activity of these Pt monolayers to a lowering of the bond strength of oxygenated intermediates on Pt atoms facilitated by changes in the 5d-band reactivity of Pt. Density functional theory calculations also revealed a decline in the strength of PtOH adsorption due to electronic interaction between the Pt and Pd atoms. We demonstrated that very active O₂ reduction electrocatalysts can be devised containing only a monolayer Pt and a very small amount of Pd alloyed with Co in the substrate. Dedicated to Professor Oleg Petrii on the occasion of his 70th birthday on August 24, 2007.     

Excitation energy flow control in {Ru(2,2'-bipyridine)2}-{pyridylporphyrin}2 systems

The direction of excitation energy migration is reversed in a system composed of {Ru(bpy)(2)}-{pyridylporphyrin}(2) by the addition of a Zn(2+) ion. The Zn(2+) system shows an excitation-wavelength dependent emission.     

Interactions between spiropyrans and room-temperature ionic liquids: photochromism and solvatochromism

A series of imidazolium-based room-temperature ionic liquids (RTILs) containing anions from organic carboxylic acids were prepared. A set of dye probes, including Reichardt's dye (30), 4-nitrioaniline, and N, N-diethyl-4-nitroaniline, were used to determine the ET(30) scales and the Kamlet-Taft parameters (pi*, alpha, and beta) of the RTILs. On the basis of the polarity properties, these RTILs were categorized into three groups: group A with beta >0.9, alpha <0.9; group B with beta <0.9, alpha <0.9; and group C with beta <0.9, alpha >0.9. Interactions of these RTILs with four photochromic spiropyran derivatives (SP-I, SP-II, SP-III, and SP-IV) were investigated. It was found that the spiropyrans could present photochromism (positive or negative) or not, depending mainly on the polarity properties of the RTILs and also on the structure itself. A new spectroscopic method based on the molecular transition energy of the spiropyran probes (ESP) was proposed to determine the polarity of those protic or fluorine-containing RTILs, which were failed with the Reichardt's dye (30) probe.     

Diastereoselective Brook rearrangement-mediated [3+4] annulation: application to a formal synthesis of (+)-laurallene

The formal synthesis of (+)-laurallene, a halogenated eight-membered ring ether, was accomplished. The synthesis involves construction of a trans alpha,alpha'-disubstituted oxocene structure 16 through a Brook rearrangement-mediated [3+4] annulation using acryloylsilane 10 and 6-oxa-2-cycloheptenone 9 and its conversion into 2, which has been transformed into (+)-laurallene by Crimmins and co-workers.     

Stereocontrolled synthesis of the A/B-ring fragment of gambieric acid B: reassignment of the absolute configuration of the polycyclic ether region

Stereocontrolled synthesis of the A/B-ring fragment of the originally assigned structure of gambieric acid B and its possible diastereomers has been accomplished. Detailed comparison of their 1H and 13C NMR data with those of the corresponding moiety of the natural product culminated in a stereochemical reassignment of the absolute configuration of the polycyclic ether region of gambieric acid B.     

Cyclophane-type imidazolium salts with planar chirality as a new class of N-heterocyclic carbene precursors

Cyclophane-type imidazolium salts with planar chirality were synthesized from enantiopure 2-amino alcohols, of which the N(1) and N(3) positions were connected with a bridge. The structural profiles of the imidazolium salts and their derivative N-heterocyclic carbenes (NHCs) were investigated by means of several analyses. The chiral NHCs derived from these imidazolium salts were found to catalyze the asymmetric cross-annulation of an alpha,beta-unsaturated aldehyde with a ketone by means of the "conjugated" umpolung of the enal to give the target gamma-lactone with good to excellent enantioselectivity (up to 94% ee). Based on the expected structure of the NHCs and their intermediates, together with the absolute configuration of the products, a plausible mechanism for the stereocontrol was proposed.