Distinct mechanisms for the oxidative addition of chloro-, bromo-, and iodoarenes to a bisphosphine palladium(0) complex with hindered ligands

We report an example of a bisphosphine palladium(0) complex with hindered ligands that undergoes oxidative addition of chloro-, bromo-, and iodoarenes in high yield. Addition of PhX (X = I, Br, Cl) to [Pd(Q-phos-tol)2] produced [Pd(Q-phos-tol)(Ph)(I)], [Pd(Q-phos-tol)(Ph)(Br)], and [Pd(Q-phos-tol)(Ph)(Cl)]2. To study the mechanisms of the oxidative addition of the three haloarenes to [Pd(Q-phos-tol)2], we determined the order of the reaction on the concentration of ligand and haloarene. The different haloarenes reacted through different mechanistic pathways. Addition of iodobenzene occurred by irreversible associative displacement of a phosphine. Addition of bromobenzene occurred by rate-limiting dissociation of phosphine. Addition of chlorobenzene occurred by reversible dissociation of phosphine, followed by rate-limiting oxidative addition. The mechanism of exchange of ligands from the Pd(0)L2 was also studied. The rate constant value for dissociation of ligand calculated from ligand exchange experiments is in agreement with the value calculated through experiments on oxidative addition.     

Über den sterischen Verlauf der Umsetzung von Enaminen aus offenkettigen Aldehyden und Ketonen mit Nitroolefinen zu 2,3-disubstituierten 4-Nitroketonen

On the Steric Course of the Reaction of Enamines Derived from Open-Chain Aldehydes and Ketones with Nitro-olefins Yielding 2,3-Disubstituted 4-Nitroketones The relative configuration of the nitro-ketone 12 obtained by the title reaction has been proved by a Baeyer-Villiger degradation and subsequent reduction to the amino-alcohol (±)-(u)- 20 , the relative configuration of which is identical with a sample prepared from 2-methyl-3-hydroxybutyrate of known configuration (Scheme 1). The stereochemical course of the title reaction is thus as predicted by our topological rule. It is assumed that the other nitro-ketones 13–18 described here are also formed by (lk)-combination of the trigonal centers of the educts (diastereoselectivity > 90%, see Formulae and Fig. 1). It is not possible to reverse this relative topicity by employing (Z)- instead of (E)-components of the starting materials, due to the reversibility of the primary step of these Michael additions (Schemes 2 and 3, and Fig. 2).     

Further investigations of the interaction of trimethylsilyl azide with substituted maleic anhydrides. Synthesis of 4- and 5-aryl substituted 1,3-(3H)oxazine-2,6-diones

The reactions of phenyl-, o-chlorophenyl-, p-chlorophenyl-, 3,4-dichlorophenyl-, p-fluoro- and p-anisylmaleic anhydrides with trimethylsilyl azide are described. In all cases mixtures of isomeric 4- and 5-aryl-2H-1,3-(3H)oxazine-2,6-diones are obtained after hydrolysis with the 4-isomer predominating. The yield of 5-isomer is greatest for o-chlorophenyl maleic anhydride, and substantial for other arylmaleic anhydrides, indicating increased importance of steric effects in these reactions, in contrast to previously reported syntheses of methyl and halo-substituted oxazine-diones, where electronic factors appeared dominant.     

The proton affinity of proline analogs using the kinetic method with full entropy analysis

The proton affinity of proline analogs, L-azetidine-2-carboxylic acid (Aze), L-proline (Pro), and L-pipecolic acid (Pip), have been measured using the Armentrout modification of the extended kinetic method in a quadrupole ion trap instrument. Experimental values of 223.0 +/- 1.5, 224.9 +/- 1.6, and 225.6 +/- 1.6 kcal/mol have been determined for the 298K proton affinities of Aze, Pro, and Pip respectively. High level theoretical calculations using both MP2 and B3LYP methods at a variety of basis sets were carried out in order to give theoretical predictions for the 298 K proton affinity and gas phase basicity of all three analogs. Recommended values for the gas phase basicity and proton affinity for proline based on our work and other recent determinations are 216 +/- 2 and 224 +/- 2 kcal/mol.     

Purification by HPLC and the UV/Vis absorption spectra of the nitrogen-containing incar-fullerenes iNC60, and iNC70

We report the purification of the nitrogen-containing incar-fullerenes iNC(60) and iNC(70), and their characterisation by UV-Vis absorption spectroscopy.     

Phase-vanishing methodology for efficient bromination, alkylation, epoxidation, and oxidation reactions of organic substrates

[reaction: see text] In cases where both reactants in a phase-vanishing reaction are less dense than the fluorous phase, an alternative to the U-tube method is to employ a solvent with greater density than the fluorous phase, such as 1,2-dibromoethane. This modification has been successfully applied to the methylation of a phenol derivative with dimethyl sulfate and to the m-CPBA-induced epoxidation of alkenes, N-oxide formation from nitrogen-containing compounds, and S-oxide or sulfone formation from organic sulfides.     

Stereoselective synthesis of the octahydroisobenzofuran skeleton of the eunicellins

A de novo asymmetric synthesis of the octahydroisobenzofuran skeleton contained within the eunicellin family of natural products has been completed. The key transformations involve the convergent assembly of a tetrasubstituted tetrahydrofuran by condensation of a functionalised allylsiloxane with an aldehyde; controlled epimerisation of a C4 aldehyde by intramolecular trapping; installation of the isopropyl substituent by stereoselective Michael addition to a 5,5-bicyclic enone; and ring expansion of the 5,5-system to the target structure by radical-mediated cyclopropane fragmentation.