Abstract
Three sterically-enhanced 2-imino-6-(1-naphthyl)pyridines, 2-{CMe=N(Ar)}-6-(1-C10H7)C5H3N [Ar = 2,6-i-Pr2C6H3 (L1dipp), 2,4,6-i-Pr3C6H2 (L1tripp), 4-Br-2,6-i-Pr2C6H2 (L1Brdipp)], differing only in the electronic properties of the N-aryl group, have been prepared in high yield by the condensation reaction of 2-{CMe=O}-6-(1-C10H7)C5H3N with the corresponding aniline. Treatment of L1dipp, L1tripp and L1Brdipp with two equivalents of AlMe3 at elevated temperature affords the distorted tetrahedral 2-(amido-prop-2-yl)-6-(1-naphthyl)pyridine aluminum dimethyl complexes, [2-{CMe2N(Ar)}-6-(1-C10H7)C5H3N]AlMe2 [Ar = 2,6-i-Pr2C6H3 (1a), 2,4,6-i-Pr3C6H2 (1b), 4-Br-2,6-i-Pr2C6H2 (1c)], in good yield. The X-ray structures of 1a–1c reveal that complexation has resulted in concomitant C–C bond formation via methyl migration from aluminum to the corresponding imino carbon in L1aryl; in solution, the restricted rotation of the pendant naphthyl group in 1 confers inequivalent methyl ligand environments. The ring opening polymerization of ε-caprolactone employing 1, in the presence of benzyl alcohol, proceeded efficiently at 30 °C producing polymers of narrow molecular weight distribution with the catalytic activities dependent on the nature of the substituent located at the 4-position of the N-aryl group with the most electron donating i-Pr derivative exhibiting the highest activity (1b > 1a > 1c); at 50 °C 1b mediates 100% conversion of the monomer to polycaprolactone (poly(CL)) in one hour. In addition to 1a, 1b and 1c, the single crystal X-ray structures are reported for L1dipp and L1tripp.
Highlights
The controlled ring-opening polymerization (ROP) of cyclic esters to give biodegradable polymers (e.g., poly, poly) mediated by well-defined organo-aluminum (III)precursors and their alkoxide derivatives has been the subject of extensive research over the last two decades, or so [1,2,3,4,5]
As the catalytic properties of the aluminum species employed are greatly influenced by the ancillary ligand, a wide variety of multidentate ligand architectures have been developed and investigated in this field
1a–1c were treated with one equivalent of benzyl alcohol in toluene prior to the addition of the ε-CL (250 equivalents) and the start of the run; all systems were evaluated at 30 °C and selected examples at 50 °C (Scheme 2)
Summary
The controlled ring-opening polymerization (ROP) of cyclic esters to give biodegradable polymers (e.g., poly (lactic acid), poly (caprolactone)) mediated by well-defined organo-aluminum (III)precursors and their alkoxide derivatives has been the subject of extensive research over the last two decades, or so [1,2,3,4,5]. We have been attracted by the intriguing properties displayed by pyridylimine-containing ligands including their redox activity [43,44] and their capacity to undergo nucleophilic attack on the imine carbon [45,46,47,48,49,50] and the pyridine ring [51]. N,N-pyridyl-alkylamides and their 6-aryl and -naphthyl derivatives have been attracting attention due, in large measure, to their emergence as supports for exceptionally active N,N- [45,55,56] and N,N,C-bound group 4 olefin polymerization catalysts [57,58,59,60]
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