Mechanism of the oxidative addition of Pd0 complexes generated from Pd0(dba)2 and a phosphole ligand DBP: a special case where dba does not play any inhibiting role

The reactivity of the palladium(0) complexes generated from Pd0(dba)2 (dba, trans,trans-dibenzylideneacetone) and n equivalents 1-phenyl-dibenzophosphole (DBP) has been investigated in the oxidative addition with PhI. Pd0(dba)(DBP)2 is generated as the major complex when n=2. When n≥4, the dba of Pd0(dba)(DBP)2 is substituted by the ligand to generate Pd0(DBP)4, which is the major complex in solution. Such a situation considerably differs from PPh3, since at least 100 equivalents PPh3 are required to completely displace dba from Pd0(dba)(PPh3)2 to form Pd0(PPh3)3. Consequently, dba has no decelerating effect on the kinetics of the oxidative addition of PhI to the Pd0 complex formed from Pd0(dba)2 and n equivalents DBP (n≥4). Pd0(DBP)4 is considerably less reactive than Pd0(PPh3)4 despite the fact that DBP is considered to be less hindered and more basic than PPh3. Pd0(DBP)2 is the reactive species. The reaction order in the ligand DBP is −2 compared with −1 for PPh3. The dissociation of one DBP from Pd0(DBP)4 is considerably more endergonic than that of one PPh3 from Pd0(PPh3)4 and is at the origin of the lower reactivity of Pd0(DBP)4. The oxidative addition of PhI generates the trans-PhPdI(DBP)2 complex via the reversible formation of an intermediate complex assigned to cis-PhPdI(DBP)2.