Stereocontrolled anionic alternating copolymerization of ethylphenylketene with benzaldehyde by a bisoxazoline ligand

Abstract

Anionic copolymerization of ethylphenylketene with benzaldehyde with butyllithium or diethylzinc as the initiator proceeded in a perfect 1:1 alternating manner to produce the corresponding polyester, whose repeating unit had two adjacent chiral centers. The relative stereochemistry between these two chiral centers was successfully controlled by the addition of (S,S)-(-)-2,2Ј-isopropylidenebis(4-tert-butyl-2oxazoline), producing the corresponding polyester that had excellent diastereoselectivity (erythro-configuration : threo-configuration ϭ 4:96). The diastereomeric ratio was determined by high-performance liquid chromatography analysis of the diol, which was obtained by reductive degradation of the polyester while maintaining the configuration of the repeating unit.

100th Anniversary of Macromolecular Science Viewpoint: The Past, Present, and Future of Stereocontrolled Vinyl Polymerization

“…The use of chiral ligands as additives in combination with Li-based initiators has also been explored. For example, lithiation in the presence of chiral ligands (i.e., (−)-sparteine, (−)-, or (+)-1,4-bis­(dimethylamino)-2,3-dimethoxy-butane) enabled the highly stereoselective (≥99% mm ) polymerization of trityl methacrylate (TrMA). , Chiral initiators were later utilized to prepare a variety of optically active polymers derived from vinyl monomers. − …”

100th Anniversary of Macromolecular Science Viewpoint: Polymerization of Cumulated Bonds: Isocyanates, Allenes, and Ketenes as Monomers

“…An intriguing opportunity presented by ketene monomers is the ability to access stereoregular polymers, provided adequate substitution of the ketene and suitable reaction conditions. Nagai and co-workers demonstrated enantioselective control of a ketene-aldehyde alternating copolymerization via a transition metal-mediated pathway. , Ethylphenylketene was copolymerized with benzaldehyde using diethylzinc (ZnEt 2 ) as initiator and the optically active tridentate 2,6-bis­(4-isopropyl-2-oxazolin-2-yl)­pyridine (Pybox) as ligand, forming the active initiator in situ (Figure B) . Of the conditions studied, the highest molecular weight was observed in toluene at −40 °C (26 kDa), but with a relatively low specific optical rotation ([α D ] = −58°).…”

Anionic Alternating Copolymerization of Ketene and Aldehyde: Control of Enantioselectivity by Bisoxazoline-Type Ligand for Synthesis of Optically Active Polyesters

“…The main chain structure of the resulting polymer was confirmed to be polyester by its characteristic IR absorption at 1737 cm -1 . [3][4][5] The obtained polymer showed optical activity {specific optical rotation ([R] D 25 ) ) -16°}, indicating that our design of tridentate Pybox is appropriate for stereocontrol. Accordingly, Pybox was selected for the optimization of stereocontrol.…”

“…However, this copolymerization system using the bidentate coordinating t Bubox did not provide an optically active polymer under a variety of conditions. 5 Because the bidentate t Bubox would be insufficient to fill the vacant orbitals of the zinc, we next chose tridentate (S,S)-(-)-2,6-bis(4-isopropyl-2-oxazolin-2-yl)pyridine (Pybox, Scheme 1). 7 Table 1 summarizes the results of the polymerization in the presence of Pybox.…”

“…We recently reported an attempt to control the enantioselectivity in the copolymerization of ethylphenylketene and benzaldehyde using diethylzinc (Et 2 Zn) as the initiator in the presence of optically active bis(oxazoline) ligand, ( S , S )-(−)-2,2‘-isopropylidenebis(4- tert -butyl-2-oxazoline) ( t Bubox) . In this copolymerization, Et 2 Zn did not initiate the polymerization, but it was activated through bidentate coordination with t Bubox to initiate the polymerization, affording the corresponding polymers quantitatively.…”