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 asymmet ric copolymerization of propene with carbon monoxide catalyzed by a ca tionic Pd(II) complex bearing an unsymmetrical chiral bidentate phosph ine-phosphite, (R,S)-BINAPHOS R,S)-2-(diphenylphosphino)-1,1'-binaphth alen-2'-yl 1,1'-binaphthalene-2,2'-diyl phosphite = L-1], have been st udied. Stepwise identification and characterization were carried out f or catalyst precursors (SP-4-2)- and (SP-4-3)-Pd(CH3)Cl(L-1) (1a and 1 b) 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(COCH 3)(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 con sistent with the structures of 1-6 in which all carbon substituents ar e 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 sugg ested to occur for the GO-insertion process 2 --> 3, which results in the activation of the methyl group for the migration to the coordinate d CO. Rapid equilibrium was observed between the two isomers 4a and 4b during the CO insertion process to give 5. Theoretical studies have b een 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-acy l bond trans to a phosphite. The MM3 calculations demonstrated that on e specific transition structure is more favorable than the other possi ble transition structures for the transformation of (SP-4-4)-[Pd(COCH3 )(propene)(L-1)].X-1 (14b) to 4b. The difference originates from the s teric effects of the BINAPHOS ligand, and the results account for high enantio-and regioselectivities experimentally observed. The two key s teps, 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 d ifference being insignificant. The living nature of the copolymerizati on was proved, Some related chiral ligands were examined for the copol ymerization. Copolymerization of other olefins with CO was also invest igated.