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 Pd-0(dba)(2) (dba. trans. trans-dibenzylideneacetone) and n equivalents 1-phenyl-dibenzophosphole (DBP) has been investigated in the oxidative addition with PhI. Pd-0(dba)(DBP)(2) is generated as the major complex when n = 2. When n greater than or equal to 4. the dba of Pd-0(dba)(DBP)(2) is substituted by the ligand to generate Pd-0(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 Pd-0(dba)(PPh3)(2) to form Pd-0(PPh3)(3). Consequently. dba has no decelerating effect on the kinetics of the oxidative addition of PhI to the Pd-0 complex formed from Pd-0(dba)(2) and n equivalents DBP (n greater than or equal to 4). Pd-0(DBP)(4) is considerably less reactive than Pd-0(PPh3)(4) despite the fact that DBP is considered to be less hindered and more basic than PPh3. Pd-0(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 Pd-0(DBP)(4) is considerably more endergonic than that of one PPh3 from PdI(PPh3)(4) and is at the origin of the lower reactivity of Pd-0(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). (C) 2002 Elsevier Science B.V. All rights reserved.