Mechanistic aspects of the alternating copolymerization of propene with carbon monoxide catalyzed by Pd(II) complexes of unsymmetrical phosphine-phosphite ligands

The reaction steps responsible for the highly enantioselective asymmetric copolymerization of propene with carbon monoxide catalyzed by a cationic Pd(II) complex bearing an unsymmetrical chiral bidentate phosphine-phosphite, (R,S)-BINAPHOS [(R,S)-2-(diphenylphosphino)-1,1'-binaphthalen-2'-yl 1,1'-binaphthalene-2,2'-diyl phosphite = L-1], have been studied. Stepwise identification and characterization were carried out for catalyst precursors (SP-4-2)- and (SP-4-3)-Pd(CH3)Cl(L-1) (1a and 1b) and (SP-4-3)-[Pd(CH3)(CH3CN)(L-1)].X-1 (X-1 = B(3,5-(CF3)(2)C6H3)(4)) (2), and complexes related to the reaction steps, (SP-4-3)-[Pd(COCH3)(CH3CN)(L-1)].X-1 (3), (SP-4-3)- and (SP-4-4)-[Pd(CH2CH(CH3)COCH3)(L-1)].X-1 (4a and 4b), (SP-4-3)-[Pd(COCH2CH(CH3)COCH3)(CH3CN)-(L-1)].X-1 (5), and (SP-4-3)-[Pd{CH2CH(CH3)COCH2CH(CH3)COCH3}(L-1)].X-1 (6). An X-ray structure of alkyl complex 4a has been obtained. Studies on [Pt(CH3)(2)(L-1)] (8) reveal that the methyl group is more stabilized at a position trans to the phosphine than at the cis position. This is consistent with the structures of 1-6 in which all carbon substituents are trans to the phosphine moiety in their major forms, On the basis of analogous studies using platinum complexes, an isomerization from (SP-4-3)-[Pd(CH3)(CO)(L-1)].X-1 (13a) to the (SP-4-4) isomer (13b) is suggested to occur for the GO-insertion process 2 --> 3, which results in the activation of the methyl group for the migration to the coordinated CO. Rapid equilibrium was observed between the two isomers 4a and 4b during the CO insertion process to give 5. Theoretical studies have been carried out on the transformation of 3 to 4a and 4b. The B3LYP and MPn calculations indicated that the alkene insertion into the Pd-acyl bond trans to a phosphine is more favorable than that into the Pd-acyl bond trans to a phosphite. The MM3 calculations demonstrated that one specific transition structure is more favorable than the other possible transition structures for the transformation of (SP-4-4)-[Pd(COCH3)(propene)(L-1)].X-1 (14b) to 4b. The difference originates from the steric effects of the BINAPHOS ligand, and the results account for high enantio-and regioselectivities experimentally observed. The two key steps, propene insertion into 3 and CO insertion into 4, were monitored by H-1 NMR spectroscopy. The activation energies for these two steps were estimated to be 19.0-19.6 kcal/mol at -20 to 0 degrees C, their difference being insignificant. The living nature of the copolymerization was proved, Some related chiral ligands were examined for the copolymerization. Copolymerization of other olefins with CO was also investigated.