J. Lukas

Platinum(0) complexes of cyclopropenes

Abstract Cyclopropene and its 3-methyl, 3,3-dimethyl, 1,2-dimethyl, 1,3,3-trimethyl and 1,2,3-trimethyl derivatives have been coordinated to platinum by replacement of ethylene in Pt(C2H4)(PPh3)2. The complexes have been identified by NMR spectroscopy and X-ray analysis.

The stereospecific addition of dienes to palladium chloride

Abstract Using 2,4-hexadiene as a model compound we have shown that dienes add to palladium chloride in a stereospecific way. The three diene isomers react to give a pair of epimeric π-allylpalladium diastereomers, one of which is formed from the cis,trans-diene, the other from either the trans,trans or the cis,cis-isomer. The mechanisms of the epimerization and solvolysis of the products have been studied, and the reactions found to proceed via a diene-palladium complex.

syn- and anti-(1-Alkyl-π-Allyl)palladium chlorides

Abstract π-Allylpalladium chlorides with bulky substituents on the terminal carbon atoms are often formed with these substituents in the less stable anti configuration. The activation energy of the π-σ-π rearrangement to the syn configuration increases with the bulkiness of the substituent. The difference in energy between the anti and syn configurations decreases with increasing size of the substituents on the terminal and meso carbon atoms. The phenomena can best be explained by assuming that there is steric interaction between the substituents on the terminal carbons of the π-allyl ligand and either other ligands on the metal or the substituent on the mso carbon of the π-allyl ligand.

Carbonium ion derived from substituted π-allylpalladium chloride complexes

Abstract Cationic butadienylpalladium chloride complexes, assumed to be intermediates in the nucleophilic substitutions at [1-(1-chloroalkyl)π-allyl]palladium chlorides, can be prepared by abstraction of chloride from [1-(1-chloroalkyl)-π-allyl]- or hydride from (1-alkyl-π-allyl)palladium chlorides by strong acids or SbF5. NMR data indicate that the butadienyl group is bonded as a cisoid, tetrahapto ligand. Solutions of a cationic (trimethylenemethane palladium complex can be obtained analogously by chloride abstraction from [2-(dichloromethyl)-π-allyl]palladium chloride.

The formation of allenes via PdII and PtII complexes

Abstract 4-Methoxy-1,2-butadiene has been prepared from chloroprene via a 2-chloro-π-allylpalladium complex. Attempts to synthesize a similar platinum complex from 2-chloroallyl chloride led to insertion of the metal into the vinylic carbon-chlorine bond.

Low-temperature NMR studies of dimeric palladium salts

Abstract The preparation and properties of a number of allylic palladium carboxylates and chlorides are reported. The NMR spectra show that the allyl signals of both the carboxylate and the chloride dimers split at low temperature (−80° to 40°) in solution, behaviour which is attributed to the freezing out of the transition between several isomers. The temperature dependence of the spectra of the complexes containing dicarboxylates confirms the mechanism proposed previously. In spite of the long distance the chemical shifts of the two allyl groups at both ends of the PdCl 2 Pd unit of the dimeric chlorides are influenced by their relatives orientation, as is apparent from the NMR spectra. The explanation of the spectral behaviour is based on the assumption that in many allyl complexes the structure known from the solid is not the only one and sometimes not even the most stable one in solution.

Preparation and crystal structure of bis(triphenylphosphine)-(1,2-dimethylcyclopropene)platinum

The formation and crystal structure are reported of a thermally stable 1,2-dimethylcyclopropene–platinum(0) complex.

Chirality in transition-metal chemistry. The formation of diastereomers in the reaction of the stereoisomers of hexa-2,4-diene with palladium chloride

Diastereomeric pairs of enantiomers of 1-(1-chloroethyl)- 3-methyl-π-allylpalladium chloride are formed when either trans,trans-, cis,cis-, or ci,strans-hexa-2,4-diene are allowed to react with palladium chloride at –40° in an aprotic solvent: these reactions can be employed to effect the conversions, cis,cis→trans,trans⇄cis, trans-hexa-2,4-diene.