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

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.