The reaction of (E)-1,3-diphenyl-3-acetoxyprop-1-ene, PhCHCH−CH(Ph)-OAc, with
palladium(0) complexes Pd0L2, generated from Pd0(PPh3)4 or Pd0(dba)2 + 2L (L = PPh3 or
L2 = dppb), gives cationic [(η
3-PhCH-CH-CHPh)PdL2]+ complexes with AcO- as the
counteranion in DMF. It is established that this reaction proceeds through two successive
equilibria via neutral intermediate complexes (η
2-PhCHCH-CH(Ph)-OAc)Pd0L2, characterized from the kinetics and by UV and 31P NMR spectroscopy. The rate constants and
equilibrium constants of the successive steps have been determined in DMF. They depend
on the ligand and the Pd0 precursor. In all cases, for the concentration range investigated
here, the complexation is considerably faster than the ionization, which is the rate-determining step of the overall process. Under similar experimental conditions, the formation
of the cationic complex [(η
3-PhCH-CH-CHPh)Pd(dppb)]+ is considerably slower than the
formation of the complex [(η
3-CH2-CH-CH2)Pd(dppb)]+ in DMF.
In chloroform, the reaction of cis-5-phenylcyclohex-2-enyl 4-Z-benzoate (cis-1 Z , Z = NO 2 , Cl, H, Me, MeO) with Pd 0 complexes ligated to PPh 3 is reversible and proceeds with isomerization at the allylic position. The rate of isomerization of cis-1 Z to trans-1 Z depends on the catalytic precursor: Pd 0 (PPh 3 ) 4 Ͼ {Pd 0 (dba) 2 + 2PPh 3 } in agreement with an S N 2 mechanism in the rate-determining isomerization of the cationic (η 3 -allyl)palladium complexes formed in the oxidative addition. For a given precursor, the rate of isomerization of cis-1 Z to trans-1 Z also depends on the substituent Z, i.e., on the leaving group. The isomerization rate follows the same order as the leaving group properties:
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