Atropisomerism in palladacycles derived from the chloropalladation of heterosubstituted alkynes

Abstract

The chloropalladation of 2-substituted phenyl N, N-propargylamines, Y-2-C 6H 4CCCH 2NMe 2 ( 1a, Y=H; 2a, Y=CF 3; 3a, Y=OMe; 4a, Y=SMe and 5a, Y=NH 2) affords palladacycles in different ratios of isomers (geometric and atropisomers). In solution, the parent alkyne (Y=H) and the CF 3 substituted derivative generate a mixture of cisoid and transoid chloro-bridged dimer palladacycles of the type {Pd[κ 1- C, κ 1- N–C(Y-2-C 6H 4)C(Cl)CH 2NMe 2](μ-Cl)} 2. Moreover, in the case of Y=CF 3 palladacyclic derivative each of the geometric isomers comprises a mixture of two diastereoisomers due to the restricted rotation of the C(vinyl)C(aryl) sigma bond (atropisomers). Palladacycles 1a (Y=H) and 2a (Y=CF 3) crystallize as the single transoid and cisoid-anti isomer, respectively. The OMe substituted alkyne yields a similar dimeric compound that crystallizes as a single cisoid-anti isomer. In solution this dimeric compound is in fast equilibrium with a monomeric pincer compound of the type Pd[κ 1- C, κ 1- N, κ 1- OC(MeO-2-C 6H 4)C(Cl)CH 2NMe 2](Cl) assisted through the weak coordination of the OMe group. Pincer palladacycles Pd[κ 1- C, κ 1- N, κ 1- YC(Y-2-C 6H 4)C(Cl)CH 2NMe 2](Cl) (Y=SMe and NH 2) were the sole products obtained in the chloropalladation of alkynes 4a and 5a. The bridge splitting reaction of the dimeric palladacycles 1b– 3b with pyridine is highly selective, affording exclusively the corresponding monomeric compounds 1c– 3c. The monomeric palladacycle 2d, which comprises a mixture of two atropisomers (2:1 ratio of anti/ syn) was obtained from the reaction of 2b with 2-methylpyridine. Theoretical calculations indicated that the anti isomer of 2d is 5.42 kJ mol −1 more stable than its syn isomer.