Phenolate Substituent Effects on Ring-Opening Polymerization of ε-Caprolactone by Aluminum Complexes Bearing 2-(Phenyl-2-olate)-6-(1-amidoalkyl)pyridine Pincers

Abstract

Interaction of the 2-(phenyl-2-ol)-6-ketiminopyridines 2-(4′-R 1 -C 6 H 3 -2′-OH)-6-{CMeN(2″,6″-i-Pr 2 C 6 H 3 )}C 5 H 3 N (R 1 = H (L1 a -H), Bu t (L1 b -H), Cl (L1 c -H), F (L1 d -H)) with AlMe 3 at elevated temperature and subsequent crystallization from acetonitrile affords the five-coordinate 2-(phenyl-2-olate)-6-(2-amidoprop-2-yl)pyridine aluminum− methyl complexes [2-(4′-R 1 C 6 H 3 -2′-O)-6-{CMe 2 N(2″,6″-i-Pr 2 C 6 H 3 )}C 5 H 3 N]AlMe(NCMe) (R 1 = H (1a), Bu t (1b), Cl (1c), F (1d)), as their acetonitrile adducts, in good yield. In each case, complexation results in concomitant C−C bond formation via methyl migration from aluminum to the corresponding imino unit in L1-H. On the other hand, reactions of the aldimine-containing compounds 2-, in which the migrated methyl group and aluminum−methyl are disposed mutually cis; evidence for the minor trans isomers 2a′−c′ is presented. The ring-opening polymerization of ε-caprolactone employing 1 and 2 in the presence of PhCH 2 OH proceeded efficiently, producing polymers of narrow molecular weight distribution with the catalytic activities highly dependent on the nature of the phenolate-containing 4-R 1 substituent with the F and Bu t initiators showing the highest activities (1b ≈ 1d > 1a ≈ 1c and 2b > 2a > 2c); in general the CMe 2 -containing series 1 were more active than CH(Me)-containing 2 at 30 °C. The bimetallic complex [{2-(4′-Bu t -C 6 H 3 -2′-O)-6-{CHMeN(2″,6″-i-Pr 2 C 6 H 3 )}C 5 H 3 N}AlMe(μ-OMe)AlMe 2 ] (3), the result of adventitious oxygenation, is also reported. The single-crystal X-ray structures are reported for L1 d -H, L1 f -H, 1a−d, 2a, 2b/2b′, 2c, and 3.

Controlled ring‐opening homo‐ and copolymerization of ɛ‐caprolactone and d,l‐lactide by iminophenolate aluminum complexes: An efficient approach toward well‐defined macromonomers

“…S14). These data are consistent with known low efficiency of aluminium complexes with bidentate ligands towards stereocontrolled ROP of d,l ‐lactide …”

“…According to Supporting Information Figure S14, almost no dependence of the microstructure of synthesized polymers on the substituents in the imine nitrogen was observed: slightly prevailing isotactic poly(lactide)s were obtained with both of catalysts studied here. We supposed that conducting the polymerization at lower temperature can improve the stereoregulation during the ROP of d,l ‐lactide . However, no significant differences were observed in homonuclear decoupled 1 H NMR spectra of poly(lactide)s synthesized at 130 °C in bulk and at 70 °C in toluene (Supporting Information Fig.…”

“…Thus, a large number of metal complexes including Mg, Zn, Fe, Ti, rare‐earth elements, Al and others has been used as initiators/catalysts for ROP of cyclic esters. Among them, aluminum complexes with different multidentate ligands such as salen, salan, dialkoxy‐diimino, and others have attracted much attention due to their high activity, effectiveness in controlling the molecular weight, and polymer tacticity (for ROP of lactide) as well as biocompatibility. Although substituted iminophenols are synthetically easily available in a cost‐effective approach, aluminum complexes with bidentate iminophenolate {ON}– or related ligands are considerably less studied in the polymerization of CL and d,l ‐lactide …”

Dimethyl-Aluminium Complexes Bearing Naphthyl-Substituted Pyridine-Alkylamides as Pro-Initiators for the Efficient ROP of ε-Caprolactone

“…It has been proposed previously that the overall rate of the polymerization depends on a combination of factors including the Lewis acidity of the metal center and the alkoxide nucleophilicity [17]. For the systems described in this work it would appear that there is a delicate balance between these factors with the superior alkoxide nucleophilicity in 1b/PhCH2OH likely to more influential in this case; similar electron donating rate enhancements have been noted elsewhere [17,32,67]. Undoubtedly the rearrangements that ensue in the final ring opening of the monomer further contribute to the overall polymerization rate.…”

Molecular Catalysis for Precise Olefin Polymerization and ROP 2015

“…Upon activation with benzyl alcohol, the system employing the electron donating iPr group at the para position proved to be the most active, for example at 100% conversion was achieved at 50˝C over 1 h. However, given the bromide derivative was move active that that containing a para-hydrogen, the rate of polymerization is thought to be determined by a number of factors including alkoxide nucleophilicity and the Lewis acidity at the metal [3]. The related 2-(Phenyl-2-olate)-6-(1-amidoalkyl)pyridine dimethylaluminium complexes have also shown promise in the ROP of ε-caprolactone [9].…”