A Highly Efficient Chiral-Bridged Diphosphine Ligand Modified Cationic Palladium(II) Catalyst System for Asymmetric Alternating Copolymerization of Propene and Carbon Monoxide

Abstract

A highly efficient chiral-bridged biphenyl diphosphine ligand [(R _ax)-BuP] modified cationic palladium(II) catalyst system for the synthesis of optically active polyketone by stereoselective alternating copolymerization of propene and carbon monoxide is reported for the first time. The results show that [Pd(MeCN)₄][OTf]₂ is an excellent catalyst precursor in a mixed solvent of MeNO₂-MeOH. The highest catalytic activity was found to be 221 g polymer/(g Pd˙h). A chiral polyketone with molecular weight M_n = 2.9 × 10⁴, polydispersity = 1.4, and molar optical rotation = +37˚ was afforded under optimized reaction conditions.

Key words

palladium - chiral-bridged biphenyl diphosphine - propene and CO - asymmetric alternating copolymerization - chiral polyketone

References and Notes

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8

General Procedure for the Copolymerization of Propene and Carbon Monoxide: A mixture of Pd(MeCN)₄][OTf]₂ (0.01 mmol), chiral-bridged ligand (R _ax)-BuP (0.012 mmol), and BF₃×OEt₂ (0.04 mmol) in MeNO₂-MeOH (12.5 mL/1 mL) was stirred magnetically under nitrogen for 1 h at r.t. A 100-mL stainless steel reactor was dried, purged with N₂, the above catalyst solution was introduced into it, and then propylene (15 g) was introduced first at 0 ˚C followed by carbon monoxide (3.5 MPa). The reaction mixture was heated to 55 ˚C in an oil bath and stirred for 17 h. At the end of a copolymerization the unreacted monomers were vented off after cooling to r.t., and the copolymer was purified by flash column chromatography eluting with CH₂Cl₂ and acetone consecutively and dried under vacuum at 40 ˚C overnight, resulting in a light yellow solid (3.24 g). ^¹³C NMR (125 MHz, CDCl₃): δ = 215.6, 212.4, 209.1 (C=O), 44.8 (CH₂), 40.1 (CH), 16.5 (Me). ^¹H NMR (400 MHz, CDCl₃): δ = 2.95-3.06 (CH₂), 2.50 (CH), 1.06 (Me). IR (KBr): 1707 (C=O, carbonyl), 825, 1041-1084 (C-O-C, spiroketal). Molar optical rotation: [Φ]_D ^²0 = +37˚ (c = 5 mg/mL, CHCl₃). Number average molecular weight: 2.9 ↔10⁴ versus polystyrene standards, M_w/M_n = 1.4.