Mechanistic Aspects of the Alternating Copolymerization of Propene with Carbon Monoxide Catalyzed by Pd(II) Complexes of Unsymmetrical Phosphine−Phosphite Ligands

Abstract

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 = L1], have been studied. Stepwise identification and characterization were carried out for catalyst precursors (SP-4-2)- and (SP-4-3)-Pd(CH3)Cl(L1) (1a and 1b) and (SP-4-3)-[Pd(CH3)(CH3CN)(L1)]·X1 (X1 = B{3,5-(CF3)2C6H3}4) (2), and complexes related to the reaction steps, (SP-4-3)-[Pd(COCH3)(CH3CN)(L1)]·X1 (3), (SP-4-3)- and (SP-4-4)-[Pd{CH2CH(CH3)COCH3}(L1)]·X1 (4a and 4b), (SP-4-3)-[Pd{COCH2CH(CH3)COCH3}(CH3CN)(L1)]·X1 (5), and (SP-4-3)-[Pd{CH2CH(CH3)COCH2CH(CH3)COCH3}(L1)]·X1 (6). An X-ray structure of alkyl complex 4a has been obtained. Studies on [Pt(CH3)2(L1)] (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)(L1)]·X1 (13a) to the (SP-4-4) isomer (13b) is suggested to occur for the CO-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)(L1)]·X1 (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 1H NMR spectroscopy. The activation energies for these two steps were estimated to be 19.0−19.6 kcal/mol at −20 to 0 °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.

Catalyst activity or stability: the dilemma in Pd-catalyzed polyketone synthesis

“…66 In the case of CO/alkene copolymerization the site selective coordination of the alkene was observed for both palladium complexes with amine-imine ligands, 27 with hybrid P-N ligands 48 and with the phosphino-phosphite (P-PO) molecule BINAPHOS. 67 A mechanistic hypothesis involving cis/trans isomerization was invoked.…”

Asymmetric polymerization using C1 resources

“…4(b)]. 26,43 Jiang and Sen 25 reported that poly(propene-alt-carbon monoxide) made with an enantiomerically pure catalyst showed a single 13 C NMR peak for the carbonyl carbon in the presence of a chiral europium shift reagent and that poly(propenealt-carbon monoxide) made from a racemic mixture of the catalyst showed two carbonyl peaks. Because the polycarbonate poly(cyclohexene oxide-altcarbon monoxide) can be hydrolyzed into trans-1,2-diol and CO 2 under basic aqueous conditions under reflux, the degree of asymmetric induction, 80% ee, could be deter- mined unambiguously by chiral gas chromatography analysis of the isolated trans-cyclohexane-1,2-diol.…”

Methylenation of Optically Active γ-Polyketones

“…8 Potassium tert-butoxide (0.19 g, 1.7 mmol) was added to the solution of CH 3 PPh 3 I (0.35 g, 0.86 mmol) in THF (15 mL) at 0 • C. A solution of the polyketone 1 (0.030 g, 0.43 mmol) in CH 2 Cl 2 (1.8 mL) was added to the mixture at the same temperature, and the resulting mixture was stirred for 20 h at room temperature. The reaction mixture was filtered through a Celite pad and the filter cake was washed with chloroform (100 mL) but no polymeric material was extracted.…”

“…These are γ-polyketones which have an asymmetric carbon at the α-position of each carbonyl group. [8][9][10][11] The carbonyl groups can be transformed into various functional groups, and therefore chemical modification of the carbonyl groups would provide new classes of chiral polymers. 9,12 Although many examples were reported for the functionalization of polyketones, mainly poly(ethene-co-CO) or achiral poly(propene-alt-CO), no report has ever appeared for the nucleophilic addition of carbanions to the carbonyl groups.…”