Phosphines with Tethered Electron-Withdrawing Olefins as Ligands for Efficient Pd-Catalyzed Aryl-Alkyl Coupling

• Published in: Organometallics 2013-08, Vol.32 (15), p.4255-4261
• Main Authors: Gioria, Estefanía, Martínez-Ilarduya, Jesús M, García-Cuadrado, Domingo, Miguel, Jesús A, Genov, Miroslav, Espinet, Pablo
• Format: Article
• Language: English
• ISSN: 0276-7333; 1520-6041
• DOI: 10.1021/om4004303

A group of phosphine/alkene ligands L = Ph2P(2-RC6F4) (R = CHCHMe, CHCHPh, CHCHCOPh) or Ph2P(CH2)2CFCF2 and their [PdCl2L] complexes have been prepared. These phosphines are easy to prepare and fairly stable toward oxidation. Their palladium complexes feature chelated L structures with the double bond coordinated as the Z isomer, except for Ph2P(CH2)2CFCF2, where the double bond is not coordinated and the complex is a dimer with Cl bridges. The ligands have been tested for their activity in the Negishi palladium-catalyzed aryl-alkyl coupling, where there is a competition of coupling and reduction products. Only the ligands forming chelated PdII complexes rise the coupling/reduction ratio to values 42/58 or higher. Of these, it is the ligand bearing the more electron-withdrawing substituent (R = CHCHCOPh) that is the one producing remarkably high selectivity toward coupling: 90/10 under the usual Negishi conditions, and noticeably higher (97/3) if the proportion of ZnEt2 in the reaction is lowered. These results fit well with the hypothesis that chelating phosphines with tethered electron-acceptor olefins improve the selectivity toward coupling products mostly because they reduce the activation barrier for C–C coupling, and not because they protect the complex from β-H elimination.