Adsorption of procaine at the mercury/electrolyte solution interface

The adsorption of the local anaesthetic procaine hydrochloride at the mercury/electrolyte interface solution is followed using capacitance measurements. The adsorption is studied at various procaine concentrations, in potassium chloride, potassium bromide or potassium fluoride used as supporting electrolytes, and at various pH values and temperatures. Procaine has basic properties with two acidity constants K. The results indicate the way the procaine molecules orientate at the interface. In all cases studied no hemimicelles or condensed film are observed.     

Insights into CO/styrene copolymerization by using Pd(II) catalysts containing modular pyridine-imidazoline ligands

Continuing our studies into the effect that N-N' ligands have on CO/styrene copolymerization, we prepared new C(1)-symmetrical pyridine-imidazoline ligands with 4',5'-cis stereochemistry in the imidazoline ring (5) and 4',5'-trans stereochemistry (6-10) and compared them with our previously reported ligands (1-4). Their coordination to neutral methylpalladium(II) (5 a-10 a) and cationic complexes (5 b-10 b), investigated in solution by NMR spectroscopy, indicates that both the electronic and steric properties of the imidazolines determine the stereochemistry of the palladium complexes. The crystal structures of two neutral palladium precursors [Pd(Me)(2-n)Cl(n)(N-N')] (n=1 for 8 a; n=2 for 9 a') show that the Pd-N coordination distances and the geometrical distortions in the imidazoline ring depend on the electronic nature of the substituents in the imidazoline fragment. Density functional calculations performed on selected neutral and cationic palladium complexes compare well with NMR and X-ray data. The calculations also account for the formation of only one or two stereoisomers of the cationic complexes. The performance of the cationic complexes as catalyst precursors in CO/4-tert-butylstyrene copolymerization under mild pressures and temperatures was analyzed in terms of the productivity and degree of stereoregularity of the polyketones obtained. Insertion of CO into the Pd-Me bond, which was monitored by multinuclear NMR spectroscopy, shows that the N ligand influences the stereochemistry of the acyl species formed.     

Chemistry and biology of diazonamide A: second total synthesis and biological investigations

As an especially unique target for chemical synthesis, diazonamide A has the potential to be constructed through a plethora of synthetic routes, each attended by different challenges and opportunities for discovery. In this article, we detail our second total synthesis of diazonamide A through a sequence entirely distinct from that employed in our first campaign, one whose success required the development of several special strategies and tactics. We also disclose our complete studies regarding the chemical biology of diazonamide A and its structural congeners, and more fully delineate the scope of our protocol for Robinson-Gabriel cyclodehydration using pyridine-buffered POCl(3).     

Asymmetric Allylic Substitution Reactions with a Xylophos-Pd Catalyst

Summary.  The chiral diphosphine ligand xylophos (1) was tested as an auxiliary in palladium catalyzed allylic substitution reactions. Whereas its activity was found to be generally good only in the case of 1,3-diphenylprop-2-en-1-yl acetate, a fair level of asymmetric induction was achieved with sodium dimethyl malonate (83%ee) and benzylamine (66%ee) as nucleophiles. Received June 13, 2000. Accepted July 3, 2000     

Fluorinated acrylates via alkoxycarbonylation of 1-alkynes with fluorinated alcohols

The system formed by combining in situ Pd(OAc)₂ with (2-pyridyl)diphenylphosphine (PyPPh₂) and CH₃SO₃H catalyzes efficiently the carbonylation of terminal alkynes (phenylacetylene or 1-hexyne) with alcohols having perfluorinated segments of the type CF₃(CF₂)_m(CH₂)_n–OH (m=1 or 3, n=1, 2 or 3) or with pentafluorophenol. Good carbonylation rates accompanied by high regioselectivity towards acrylate ester formation are obtained under mild reaction conditions (T=60–80 °C, P(CO)=30 atm). The influence of the CO pressure, the catalyst composition, the temperature and the number (n) of protonated methylene groups on the catalysis has been studied.     

Detection of early stage precursor during formation of plastic crystal ethanol from the supercooled liquid state: a simultaneous dielectric spectroscopy with neutron diffraction study

Transformation of deuterated ethanol from supercooled liquid into a plastic crystal or rotator phase is investigated by means of a particular experimental setup combining simultaneously dielectric spectroscopy with neutron diffraction techniques. We demonstrate that, previous to the growth of the bcc lattice of the plastic crystal phase, the formation of a precursor or intermediate phase through a liquid-liquid phase separation takes place. Once this precursor phase is formed, subsequent (plastic) crystalline nucleation and growth is expected to develop.     

Suzuki-Miyaura cross-coupling of aryl carbamates and sulfamates: experimental and computational studies

The first Suzuki-Miyaura cross-coupling reactions of the synthetically versatile aryl O-carbamate and O-sulfamate groups are described. The transformations utilize the inexpensive, bench-stable catalyst NiCl(2)(PCy(3))(2) to furnish biaryls in good to excellent yields. A broad scope for this methodology has been demonstrated. Substrates with electron-donating and electron-withdrawing groups are tolerated, in addition to those that possess ortho substituents. Furthermore, heteroaryl substrates may be employed as coupling partners. A computational study providing the full catalytic cycles for these cross-coupling reactions is described. The oxidative addition with carbamates or sulfamates occurs via a five-centered transition state, resulting in the exclusive cleavage of the aryl C-O bond. Water is found to stabilize the Ni-carbamate catalyst resting state, which thus provides rationalization of the relative decreased rate of coupling of carbamates. Several synthetic applications are presented to showcase the utility of the methodology in the synthesis of polysubstituted aromatic compounds of natural product and bioactive molecule interest.