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Bis(8-quinolinolato)aluminum ethyl complexes: Iso-Selective Initiators for rac-Lactide Polymerization Clare Bakewell,† Rachel H. Platel,† Samantha K. Cary,‡ Steven M. Hubbard,‡ Joshua M. Roaf,‡ Alex C. Levine,‡ Andrew J. P. White,† Nicholas J. Long,† Michael Haaf,*,‡ and Charlotte K. Williams*,† †

Department of Chemistry, Imperial College London, London SW7 2AZ, U.K. Center for Natural Sciences, Ithaca College, Ithaca, New York 14850, United States

‡

S Supporting Information *

ABSTRACT: The synthesis and characterization of a series of bis(8quinolinato)aluminum ethyl complexes, substituted at the 2-, 5-, and 7positions on the 8-quinolinol ligand, are presented. These complexes are viable initiators for the ring-opening polymerization of rac-lactide in the presence of 1 equiv of isopropyl alcohol. The polymerization control is good, it shows a linear evolution of molecular weight as the polymerization progresses, the polylactide molecular weights are in close agreement with those determined on the basis of the reaction stoichiometry, and the polydispersity indices are narrow. The polymerization kinetics have been monitored, and the influence of the site of ligand substitution has been related to the rates. Some of the initiators show stereocontrol, producing PLA with a good probability of isotactic enchainment (Pi = 0.76).

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INTRODUCTION Polylactide (PLA) is the leading bioderived polymer; it is produced by the fermentation of corn/sugar beet and can be degraded to metabolites.1 It is currently applied in commodity applications, including packaging and fiber technology, as well as in a range of biomedical applications, such as wound healing, controlled release, and regenerative medicine. 2 PLA is produced by the ring-opening polymerization (ROP) of lactide (LA) (Scheme 1), a process initiated by various metal alkoxide/

far fewer initiators that produce isotactic PLA from rac-LA. However, iso-selective rac-LA ROP is particularly important, because it yields stereoblock/gradient PLA which has superior thermal and mechanical properties (Scheme 1).8a The best isoselective initiators are pentacoordinate aluminum complexes, ligated by tetradentate salicylaldehyde (Salen) Schiff base ligands. Spassky and co-workers first reported that A (Figure 1,

Scheme 1. Ring-Opening Polymerization of rac-LA To Give Rise to Isotactic PLA as either a Stereoblock/Gradient or Complex Polymer

Figure 1. Structures of leading iso-selective initiators for rac-LA ROP.

amide complexes.3 Controlling the stereochemistry of raclactide polymerization is of high importance, as it determines the properties of the polymer, including its thermal resistance, melting temperature, mechanical strength, and degradation rate.4 Stereocontrolled ROP of rac-LA can yield either heterotactic (a disyndiotactic enchainment of LA) or isotactic PLA (Scheme 1). There are multiple examples of highly heteroselective initiators using a very wide range of metal centers, including (but not limited to) complexes of Zn,5 Ca,6 Sc,7 Y,8 Zr,9 Al,8a,10 In,11 Ge,12 and lanthanides.8b,13 There are © 2012 American Chemical Society

as the R enantiomer) showed a 20:1 selectivity for the polymerization of (R,R)-LA compared to (S,S)-LA, leading to a stereogradient PLA (Scheme 1).14 Since this important report, various other achiral, chiral, and racemic Schiff base aluminum complexes have been reported, showing Pi values as high as Received: April 16, 2012 Published: June 20, 2012 4729

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0.98.15 Recent examples include aluminum complexes ligated by fluorinated dialkoxy-diimino ligands, which produce PLA with Pi values up to 0.81.10b,15g,16 Feijen and co-workers have also reported bis(pyrrolidene) Schiff base aluminum isopropoxide complexes, which have been shown to produce isotactically enriched PLA (Pi = 0.74).15f Various studies have addressed the steric and electronic features of the Schiff base ligands and the structure−activity influences on LA ROP.8a,10b,d,15b,f,g,16,17 There remain very few examples of isoselective initiators using other metal centers.15f,18 One interesting example is complex B (Figure 1), which showed excellent iso-selectivity (Pi = 0.83 at −18 °C) and remains a rare example of a bidentate ligand system capable of exerting high degrees of stereocontrol.18 We targeted new bidentate ligands, designed for coordination to aluminum, in order to exert isotactic stereocontrol. Here, we report the potential for aluminum bis(8-quinolinolato) complexes to produce isotactic PLA. Aluminum tris(8quinolinolato) complexes are widely used in electronics, in particular AlQ3 (tris(8-quinolinolato)aluminum), which is an efficient electron-transporting material for organic lightemitting devices.19 The homoleptic AlQ3 complexes are unsuitable polymerization initiators, as they lack an initiating group; however, it is known that substitution at the 2quinolinate position enables the isolation of complexes [AlQ2R], where R = alkyl.20 These heteroleptic complexes are expected to be suitable initiators, particularly in the presence of exogenous alcohol. We were encouraged to note that Redshaw and co-workers reported various aluminum complexes of 2-(arylimino)quinolinates and quinolone amides; these complexes are viable initiators for cyclic ester polymerizations, although without any notable stereocontrol.21

Complexes 1−10 were synthesized by reaction of 2 equiv of the respective 8-hydroxyquinoline pro-ligand 1a−10a with 1 equiv of AlEt3, in toluene at 298 K. On addition of triethylaluminum, ethane gas evolved and the reaction mixture was stirred, at 298 K, for approximately 12 h, after which time the solvent was removed in vacuo. Compounds 1−10 were isolated as yellow solids in 60−80% yields, after washing the residues with hexane. For all of the new compounds, the 1H NMR spectra showed a characteristic shift in the quinolinolate protons, indicative of coordination to the Lewis acidic aluminum center resulting in deshielding of the nuclei. The aluminum ethyl groups show a triplet at 0.5−1 ppm, assigned to the methyl protons. The methylene protons each resonate as a doublet of quartets at 0−0.5 ppm, indicating that the complexes are chiral and the methylene protons are diastereotopic. This is particularly relevant in the context of stereocontrolled polymerization, as the growing polymer chain will occupy this coordination site during polymerization. X-ray Crystallography. Crystals suitable for X-ray diffraction experiments were grown of complexes 5 and 6 from toluene and pentane, respectively. The structures of the two complexes are closely related, and both crystallized as racemic mixtures. Additionally, complex 6 crystallized with two independent molecules; Figure 2 illustrates the structure of one

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RESULTS AND DISCUSSION Synthesis of Initiators 1−10. A series of new initiators were prepared by reaction of the 8-hydroxyquinoline proligands with triethylaluminum, as illustrated in Scheme 2. Scheme 2. General Synthesis of Initiators 1−10, with the Initiator Numbering Schemea

Figure 2. Crystal structure of one (6-A) of the two crystallographically independent complexes present in the crystals of 6 (60% probability ellipsoids). a

Reagents and conditions: (i) toluene, 298 K, 12 h, 60−80%.

of these (6-A), while the other (6-B) is illustrated in Figure S3 (Supporting Information). All three complexes have a pentacoordinate aluminum center with similar distortedtrigonal-bipyramidal coordination geometries, the τ values being 0.74, 0.73, and 0.73 for 5, 6-A, and 6-B, respectively. The 7-iodo substituent in complex 6 leads to a slight widening of the N(29)−Al−N(9) angle. In both complexes the N atoms in the quinolinolate rings are disposed trans to one another and occupy the axial coordination sites. The O donors from the

The 8-hydroxyquinoline pro-ligands 1a and 3a are commercially available, while pro-ligands 2a and 4a−10a were synthesized according to literature methods and were obtained in moderate to good yields (Scheme 2).22 The ligand series was targeted so as to investigate the steric and electronic influences of the substituents; for example, R2 was expected to exert a more substantial electronic influence, while R1 and R3 are closer to the active site. 4730

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Table 1. Comparative Selected Bond Lengths (Å) and Angles (deg) for Both Independent Complexes Present in the Crystals of 6 bond length

6-A

6-B

bond length

6-A

6-B

Al(1)−O(1) Al(1)−N(9) Al(1)−O(21) bond angle

1.807(6) 2.093(7) 1.808(6) 6-A

1.802(6) 2.087(7) 1.810(6) 6-B

Al(1)−N(29) Al(1)−C(40)

2.087(7) 1.984(9)

2.102(7) 1.969(8)

O(1)−Al(1)−N(9) O(1)−Al(1)−O(21) O(1)−Al(1)−N(29) O(1)−Al(1)−C(40) N(9)−Al(1)−O(21)

82.8(3) 119.8(3) 90.6(3) 118.8(4) 89.3(3)

82.8(3) 121.2(3) 90.7(3) 119.9(4) 89.9(3)

bond angle

6-A

6-B

N(9)−Al(1)−N(29) N(9)−Al(1)−C(40) O(21)−Al(1)−N(29) O(21)−Al(1)−C(40) N(29)−Al(1)−C(40)

165.5(3) 96.8(4) 82.7(3) 121.4(4) 97.7(4)

165.1(3) 98.0(3) 82.0(3) 118.9(3) 96.9(3)

Table 2. Polymerization Data Using Initiators 1−10 initiator (I)

R1, R2, R3 (Scheme 2)

time, h

conversn,b %

kobs, 10−6 s−1 c

Mn(exp)d

Mn(calcd)

PDId

Pie

1 2 3 4 5 6 7 8 9 10

H, H, Me H, Me, Me Cl, Cl, Me Br, Br, Me H, Cl, Me Cl, I, Me I, I, Me Me, H, Me H, H, tBu H, OMe, Me

168 391 137 168 180 168.5 354 336 16 290

94 98 91 90 88 80 94 85 96 90

4.3 2.1 5.0 4.2 3.2 2.5 2.4 1.4 58 2.2

9 300 9 200 9 900 12 400 9 600 7 000 9 800 8 500 13 800 10 600

13 500 14 100 13 100 12 900 12 700 11 500 13 500 12 200 13 800 12 900

1.19 1.14 1.11 1.07 1.11 1.04 1.09 1.06 1.08 1.07

0.62 0.64 0.72 0.75 0.65 0.75 0.76 0.70 0.57 0.64

a

Polymerization conditions: toluene, 348 K, 1/1/100 [I]/[iPrOH]/[LA], 1 M [LA]. bDetermined by integration of the methine region of the 1H NMR spectrum (LA, 4.98−5.04 ppm; PLA, 5.08−5.22 ppm). cDetermined from the gradients of the plots of ln{[LA]0/[LA]t} versus time, where the average errors (determined using initiator 3) are 1−15% (Figure 4). dDetermined by GPC in THF, using multiangle laser light scattering (GPCMALLS). eDetermined by analysis of all the tetrad signals in the methine region of the homonuclear decoupled 1H NMR spectrum.25

of isopropyl alcohol gave very little conversion. Furthermore, experiments conducted in THF or methylene chloride at 298 K failed, likely due to the failure to form any aluminum alkoxide initiator at lower temperatures. In order to better understand the alcoholysis reactions, an experiment to monitor the reaction between compound 3 and 1.2 equiv of isopropyl alcohol was conducted. Complete conversion to the alkoxide species was only observed, in toluene, at 348 K after 72 h. However, for the polymerization experiments, complete conversion to the alkoxide is unnecessary, as the unreacted isopropyl alcohol will undergo rapid and reversible chain transfer/exchange reactions with the aluminum alkoxide species. It is well-known in lactide polymerization chemistry that the rates of chain transfer exceed those of propagation, thereby obviating the need for full conversion to the aluminum alkoxide complex.3b,5c,8a,10a,d,14,15,15d,f,17a−d,23,24 The high degrees of polymerization control (vide supra, Table 2) indicates that rapid and reversible chain transfer is also occurring for these initiating systems. The polymerizations are, however, highly sensitive to any contamination by water, which is a chain-terminating agent; it is therefore necessary to carry out all polymerizations under an argon atmosphere. The polymerizations were monitored by taking regular aliquots which were analyzed by NMR spectroscopy to determine the conversion and by GPC-MALLS to determine the evolution of the number-averaged molecular weight. The tacticity of the resulting PLA, in particular the probability of isotactic enchainment at the tetrad level, Pi, was assessed by integration of the methyne region of the homonuclear decoupled NMR spectrum and by using Bernoullian statistics to predict tetrad probabilities according to the method

quinolinolate rings are coordinated at equatorial sites, with the remaining site being occupied by the C of the ethyl group. The bond lengths in 5 and 6 are within the range expected for pentacoordinate aluminum complexes (Table 1).20a,c rac-Lactide Polymerization. The new compounds were tested as initiators for rac-lactide ring-opening polymerization (Table 2). The polymerizations were all conducted in toluene at 348 K and with 1 equiv of isopropyl alcohol (Figure 3). It is, perhaps,

Figure 3. General polymerization procedure. Reagents and conditions: (i) [1−10]/[iPrOH]/[LA] 1/1/100, toluene, 348 K.

relevant to note that a series of VT-NMR experiments using the bis(2-methyl-8-quinolinolato)aluminum ethyl complex showed that exchange of the diastereotopic methylene proton signals only occurred at temperatures exceeding 373 K.20b Thus, the chirality at the aluminum center might be expected to be maintained under the experimental conditions. All experiments were run at a standard concentration of rac-lactide (1 M) and using 1 mM concentration of initiator (i.e., 1/100 loading of initiator/lactide). During the catalysis, the isopropyl alcohol reacts with Al-Et to generate an aluminum isopropoxide complex, which is the true initiating species; such a protocol is quite typical for polymerizations using aluminum initiators.3b,5c,8a,10a,d,14,15,15d,f,17a−d,23 The complexes in the absence 4731

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Figure 4. Plots of ln([LA]0/[LA]t) vs time (h): (i) initiators 1−3, 5, and 6; (ii) initiators 4, 7, 8, and 10; (iii) initiator 9. Conditions: [LA]0 = 1 M, 1/1/100 [initiator]/[iPrOH]/[LA], toluene, 348 K.

described by Coudane et al.25 In particular, all the tetrad signals were analyzed and used to determine an average Pi value (Figure S5, Supporting Information). The polymerization kinetics were monitored for each initiator, showing a firstorder dependence on lactide concentration in every case; the pseudo-first-order rate constants, kobs, were obtained as the gradient of the linear fits to plots of ln([LA]0/[LA]t) versus time (Figure 4). Complexes 1−10 were all active initiators in the polymerization of rac-LA. Polymerizations using complexes 1−8 and 10 were very slow, with full conversion being achieved in excess of 135 h; it should be noted that such rates of polymerizations are, however, quite typical of aluminum initiators.10c,23b,26 On the other hand, complex 9 was significantly faster; it showed a kobs value which was 1 order of magnitude higher. Complex 9 has a tert-butyl substituent at the R3 position instead of the methyl substituent in all the other complexes. This increase in activity is rather surprising, given the increase in steric shielding of the active site which would usually be expected to slow the rate of polymerization; investigations into the influence of this site will be the focus of future studies. The initiators all give rise to PLA which has some degree of isotactic enrichment. This finding is interesting because, as already mentioned, the great majority of PLA initiators give rise to heterotactic enrichment. Furthermore, there are clear differences in the degree of iso-selectivity of the complexes depending on the substituents. The iso-selectivity increases with the steric hindrance of the substituent at the R1 position. Initiators that give rise to the most isotactic PLA have either a halide or a methyl substituent at R1, with 3, 4, and 6−8 giving

the highest Pi values of 0.70−0.76 (for a representative NMR spectrum, see Figure S5 in the Supporting Information). Additionally, an analysis of the Pi values against conversion was undertaken using initiator 3. This showed that the Pi values remain constant and high throughout the polymerization (Table S2, Supporting Information). These complexes are markedly more iso-selective than 1, 2, 5, and and 10, where R1 = H, which give Pi values of 0.62, 0.64, 0.65, and 0.64, respectively. These findings suggest that steric hindrance at R1 increases the degree of iso-selectivity at the metal center. Analysis of the methine region of the homonuclear decoupled 1 H NMR spectrum also allows determination of the mechanism of stereocontrol: i.e., enantiomorphic site control vs chain end control.8a This is achieved by analysis of the tetrads resulting from stereoerrors (i.e., tetrads other than iii). The analysis of the isotactic PLA produced by the most selective initiators (i.e., 3, 4, and 6−8) indicates that an enantiomorphic site control mechanism is dominant ([sis]/[sii]/[iis]/[isi] = 1/1/1/2 ratio (Figure S5 (Supporting Information); note that the sis resonance is of slightly lower intensity than would be expected for a purely enantiomorphic site control mechanism).8a There is also a high degree of polymerization control, with all initiators showing a linear evolution of Mn with percent conversion (Figure 5) and Mn values being close to those predicted on the basis of the initiator concentration (Table 2). PDIs are narrow for all polymerizations and are below 1.2 in all cases. A sample of PLA, prepared using initiator 6, was analyzed using MALDI-TOF mass spectrometry, which showed that the major series were chains end-capped with isopropyl ester 4732

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commercial sources. Triethylaluminum was obtained from Strem. Toluene was distilled from sodium, degassed, and stored under nitrogen. Isopropyl alcohol was heated to reflux over CaH2, distilled onto fresh CaH2, and further refluxed and then distilled, degassed, and stored under nitrogen. Benzene-d6 was distilled from sodium, and toluene-d8 and CDCl3 were dried over CaH2; all were then degassed and stored under nitrogen. Rac-lactide was obtained from Purac Plc and was recrystallized from dry toluene and sublimed at 323 K three times under vacuum. Nuclear magnetic resonance (NMR) spectra were recorded on a Bruker AV400 spectrometer operating at 400 MHz for 1H and 100 MHz for 13C{1H} spectra. Solvent peaks were used as internal references for 1H and 13C chemical shifts (ppm). Higher resolution 1 H{1H} NMR (homodecoupled spectroscopy) experiments were performed on a Bruker AV500 spectrometer and also a DRX 400 spectrometer. Spectra were processed and analyzed using Mestrenova software. MALDI-TOF mass spectra were performed on a Waters/ Micromass MALDI micro MX, using potassium or sodium salts for ionization. Elemental analyses were determined by Mr. Stephen Boyer at London Metropolitan University, Science Centre, 29 Hornsey Road, London N7 7DD, U.K.. GPC-MALLS measurements were conducted on a Polymer Laboratories PL GPC-50 instrument at 35 °C, using two Polymer Laboratories Mixed D columns in series and THF as the eluent, at a flow rate of 1 mL min−1. The light scattering detector was a Dawn 8 instrument from Wyatt Technology, and data were analyzed using Astra V version 5.3.4.18. X-ray Crystallographic Data for 6: C22H17AlCl2I2N2O2, Mr = 693.06, monoclinic, P21/n (No. 14), a = 10.3112(3) Å, b = 36.5740(9) Å, c = 13.3312(4) Å, β = 107.432(3)°, V = 4796.6(2) Å3, Z = 8 (two independent complexes), Dc = 1.919 g cm−3, μ(Mo Kα) = 2.904 mm−1, T = 173 K, yellow blocks, Oxford Diffraction Xcalibur 3 diffractometer, 15 981 independent measured reflections (Rint = 0.0370), F2 refinement, R1(obs) = 0.0985, wR2(all) = 0.2015, 12 613 independent observed absorption-corrected reflections (|Fo| > 4σ(|Fo|), 2θmax = 65°), 563 parameters, CCDC 886471. Bis(2-methyl-8-quinolinolato)ethylaluminum (1). Triethylaluminum (71.60 mg, 0.60 mmol) in toluene (3 mL) was added dropwise, with stirring, to a solution of 8-hydroxy-2-methylquinoline (0.20 g, 1.20 mmol) in toluene (10 mL). The solution was stirred for 12 h, after which time a small amount of white precipitate was observed. The precipitate was isolated and the filtrate removed in vacuo. The product was washed with hexane, filtered, and dried in vacuo to yield a pale yellow powder (0.15 g, 0.40 mmol, 64%). 1 H NMR (400 MHz, toluene-d8; δ (ppm)): 0.38 (dq, 1H, CH2, 2JHH = 14.4 Hz, 3JHH = 8.4 Hz), 0.57 (dq, 1H, CH2, 2JHH = 14.4 Hz, 3JHH = 8.4 Hz), 1.07 (t, 3H, CH3, 3JHH = 8 Hz), 3.00 (s, 6H, CH3), 6.66 (d, 2H, CH, 3JHH = 8.4 Hz), 6.86 (dd, 2H, CH, 3JHH = 8.4 Hz, 4JHH = 1 Hz), 7.17 (dd, 2H, CH, 3JHH = 8.4 Hz, 4JHH = 1 Hz), 7.28 (t, 2H, CH, 3 JHH = 8.4 Hz), 7.50 (d, 2H, CH, 3JHH = 8.4 Hz). 13C{1H} NMR (100 MHz, CDCl3; δ (ppm)): 2.93 (CH2CH3), 9.9 (CH2CH3), 22.9 (CH3), 112.7, 113.29, 123.6, 127.3, 129.2, 138.3, 140.4, 140.4, 156.2, 158.5. Anal. Calcd for AlC22H21N2O2: C, 70.96; H, 5.68; N, 7.52. Found: C, 70.91; H, 5.70; N, 7.46. Bis(2,5-dimethyl-8-quinolinolato)ethylaluminum (2). Triethylaluminum (0.60 mL, 4.50 mmol) in toluene (10 mL) was added dropwise, with stirring, to a solution of 8-hydroxy-2,5-dimethylquinoline (1.54 g, 8.89 mmol) in toluene (15 mL). The solution was stirred for 12 h. The solvent was removed in vacuo, and the product was washed with hexane, filtered, and dried in vacuo to yield a yellow powder (1.22 g, 3.30 mmol, 75%). 1 H NMR (400 MHz, toluene-d8; δ (ppm)): 0.38 (dq, 1H, CH2, 2JHH = 14.2 Hz, 3JHH = 7.79 Hz), 0.57 (dq, 1H, CH2, 2JHH = 14.2 Hz, 3JHH = 7.79 Hz), 1.05 (t, 3H, CH3, 3JHH = 7.79 Hz), 2.21 (s, 6H, CH3), 3.02 (s, 6H, CH3), 6.71 (d, 2H, CH, 3JHH = 8.70 Hz), 7.00 (m, 4H, CHCH), 7.62 (d, 2H, CH, 3JHH = 8.70 Hz). 13C{1H} NMR (100 MHz, toluene-d8; δ (ppm)): 2.7 (CH2CH3), 9.7 (CH2CH3), 22.6 (CH3), 112.0 (CH), 119.1 (CIV), 122.8 (CH), 126.2 (CIV), 129.1 (CH), 135.1 (CH), 140.6 (CIV), 155.3 (CIV), 156.6 (CIV). Anal. Calcd forAlC24H25N2O2: C, 71.98; H, 6.29; N, 7.00. Found: C, 72.03; H, 6.37; N, 6.84.

Figure 5. Evolution of Mn versus percent conversion for the polymerization using initiator 3 (circles) and PDI versus percent conversion (squares). Conditions: [LA]0 = 1 M, 1/1/100 [3]/ [iPrOH]/[LA], toluene, 348 K.

groups (Figure S6, Supporting Information). The peaks are separated by 144 amu, consistent with only limited intermolecular transesterification occurring. Extended reaction times led to the appearance of signals from cyclic PLA due to intermolecular transesterification. Interestingly, initiator 9 showed slightly different behavior; it had peaks separated by 72 amu as the major series, implying a high degree of intermolecular transesterification (Figure S7, Supporting Information).

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CONCLUSIONS A series of bis(8-quinolinolato)ethylaluminum complexes have been synthesized by reaction of 8-hydroxyquinoline ligands with triethylaluminum. All complexes were fully characterized, and X-ray crystallographic data were obtained in some cases. All of the complexes were active initiators for rac-lactide ringopening polymerizations. The polymerizations were slow, with reactions proceeding over the course of several days, but were very well controlled throughout. The PLA produced was isotactically enriched, with Pi values reaching 0.76, in the best case. This is in contrast to the majority of PLA initiators, which show heterotactic selectivity. It was found that increasing the steric bulk, particularly at site R1, on the ancillary ligand increases the stereocontrol. Lactide polymerization using initiator 9 was 1 order of magnitude faster than for all other initiators. This complex had increased steric hindrance at site R3; however, the iso-selectivity was reduced. The initiators all gave PLA with Mn close to the expected values, on the basis of initiator concentration, and with narrow polydispersity indices. The MALDI-TOF mass spectra indicate that most of these initiators show very few transesterification side reactions, giving rise to series of chains separated by the mass of a lactide unit (144) rather than that for a lactic acid (72). In conclusion, iso-selectivity has been achieved from a bidentate ligand system that has a limited amount of steric bulk. This result is significant in understanding ligand design in both these compounds and other related aluminum species.

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EXPERIMENTAL SECTION

Materials and Methods. All reactions were conducted under an inert nitrogen atmosphere, using a nitrogen-filled glovebox or standard Schlenk techniques. All solvents and reagents were obtained from 4733

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the product was washed with hexane, filtered, and dried in vacuo to yield an orange powder (0.37 g, 0.42 mmol, 57%). 1 H NMR (400 MHz, benzene-d6; δ (ppm)): 0.26 (dq, 1H, CH2, 2 JHH = 14.6 Hz, 3JHH = 8.2 Hz), 0.44 (dq, 1H, CH2, 2JHH = 14.6 Hz, 3 JHH = 8.2 Hz), 0.90 (t, 3H, CH3, 3JHH = 8.4 Hz), 3.01 (s, 6H, CH3), 6.51 (d, 2H, CH, 3JHH = 8.8 Hz), 7.68 (d, 2H, CH, 3JHH = 8.8 Hz), 8.20 (s, 2H, CH). 13C{1H} NMR (100 MHz, benzene-d6): 1.7 (CH2CH3), 10.0 (CH2CH3), 23.6 (CH3), 79.8 (CIV), 82.1 (CIV), 125.7 (CH), 128.9 (CIV), 139.1 (CIV), 143.0(CH), 145.9 (CH), 159.0 (CIV), 159.4 (CIV). Anal. Calcd for AlC22H17N2O2I4: C, 30.16; H, 1.96; N, 3.20. Found: C, 30.22; H, 2.05; N, 3.18. Bis(2,7-dimethyl-8-quinolinolato)ethylaluminum (8). To a solution of triethylaluminum (0.25 g, 2.19 mmol) in toluene (20 mL) was added 2,7-dimethyl-8-hydroxyquinoline (0.77 g, 4.38 mmol) in toluene (20 mL), dropwise over 1 h. The reaction mixture was stirred in a capped Schlenk tube for 1 week. The solvent was removed in vacuo, affording a pale yellow solid. Recrystallization from toluene afforded small yellow crystals (0.16 g, 0.41 mmol, 19%). 1 H NMR (400 MHz, toluene-d8; δ (ppm)): 0.32 (dq, 1H, CH2, 2JHH = 14.4 Hz, 3JHH = 8.4 Hz), 0.49 (dq, 1H, CH2, 2JHH = 14.4 Hz, 3JHH = 8.4 Hz), 0.98 (t, 3H, CH3, 3JHH = 8.4 Hz), 2.46 (s, 6H, CH3), 3.03 (s, 6H, CH3), 6.66 (d, 2H, CH, 3JHH = 8.3 Hz), 6.81 (d, 2H, CH, 3JHH = 8.3 Hz), 7.19 (d, 2H, CH, 3JHH = 8.24 Hz), 7.49 (d, 2H, CH, 3JHH = 8.24 Hz). 13C{1H} NMR (100 MHz, benzene-d6; δ (ppm)): 1.75 (CH2CH3) 10.5 (CH2CH3), 16.5 (CH3), 23.3 (CH3), 113.0 (CH), 122.3 (CIV), 123.0 (CH), 126.2 (CIV), 132.0 (CH), 138.6 (CH), 140.1 (CIV), 156.1 (CIV), 156.5 (CIV). Anal. Calcd for AlC24H25N2O2: C, 71.98; H, 6.29; N, 7.00. Found: C, 71.98; H, 6.37; N, 6.91. Bis(2-tert-butyl-8-quinolinolato)ethylaluminum (9). A solution of triethylaluminum (85 mg, 0.74 mmol) in toluene (5 mL) was added dropwise to a solution of 8-hydroxy-2-tert-butylquinoline (0.30 g, 1.48 mmol) in toluene (15 mL), and the solution was stirred for 12 h. The solvent was removed in vacuo to reveal a yellow oil. Pentane (3 mL) was added, and a yellow powder precipitated out. The powder was isolated and dried in vacuo (0.20 g, 0.44 mmol, 59%). 1 H NMR (400 MHz, toluene-d8; δ (ppm)): 0.37 (dq, 1H, CH2, 2JHH = 15.6 Hz, 3JHH = 8.6 Hz), 0.51 (dq, 1H, CH2, 2JHH = 15.6 Hz, 3JHH = 8.6 Hz), 0.77 (t, 3H, CH3, 3JHH = 8.0 Hz), 1.72 (s, 18 H, C(CH3)3), 7.22 (d, 2H, CH, 3JHH = 8.8 Hz), 7.32 (m, 4H, CHCH), 7.64 (d, 3JHH = 8.8 Hz, 2H, CH). 13C{1H} NMR (100 MHz, benzene-d6; δ (ppm)): 1.8 (CH2CH3), 9.1 (CH2CH3), 31.1 (CIV), 31.6 (tBu), 114.7 (CH), 115.4 (CH), 120.7 (CH), 128.5 (CIV), 129.1 (CH), 139.1 (CH), 141.4 (CIV), 157.7 (CIV), 169.2 (CIV). Anal. Calcd for AlC28H33N2O2: C, 73.66; H, 7.29; N, 6.14. Found: C, 73.50; H, 7.21; N, 6.11. Bis(5-methoxy-2-methyl-8-quinolinolato)ethylaluminum (10). A solution of triethylaluminum (388 mg, 3.4 mmol) in toluene (5 mL) was added dropwise to a solution of 8-hydroxy-5-methoxy-2methylquinoline (1.27 g, 6.7 mmol) in toluene (20 mL) over the period of 1 h. The solution was stirred for 12 h. The precipitate was filtered, dried in vacuo, and recrystallized from toluene to afford bright yellow crystals (1.02 g, 2.35 mmol, 69%). 1 H NMR (400 MHz, CDCl3; δ (ppm)): 0.38 (dq, CH2, 2JHH = 14.7 Hz, 3JHH = 8.2 Hz, 1H), 0.58 (dq, CH2, 2JHH = 14.7 Hz, 3JHH = 8.2 Hz, 1H), 1.06 (t, 3H, CH3, 3JHH = 8.2 Hz), 2.99 (s, 3H, CH3), 3.43 (s, 3H, CH3), 6.39 (d, 2H, CH, 3JHH = 8.2 Hz), 6.68 (d, 2H, CH, 3JHH = 8.2 Hz), 6.99 (d, 2H, CH, 3JHH = 8.2 Hz), 8.28 (d, 2H, CH, 3JHH = 8.2 Hz). 13C{1H} NMR (100 MHz, benzene-d6; δ (ppm)): 1.8 (CH2CH3), 10.4 (CH2CH3), 23.4 (CH3), 55.8 (OCH3), 107.5, (CH) 111.5 (CIV), 120.0 (CH), 123.2 (CH), 134.2 (CH), 140.9 (CIV), 145.4 (CIV), 152.6 (CIV), 157.2 (CIV). Anal. Calcd for AlC24H25N2O4: C, 66.66; H, 5.83; N, 6.48. Found: C, 66.84; H, 5.95; N, 6.39. General Polymerization Procedure. In a glovebox, a Young’s tap ampule was loaded with rac-lactide (432 mg, 3 mmol) and bis(5,7dichloro-2-methyl-8-quinolinolato)ethylaluminum (3; 15.7 mg, 0.03 mmol). Toluene (2.9 mL) and iPrOH (0.03 mmol) were injected into the reaction mixture, such that the overall concentration of lactide was 1 M and that of initiator was 10 mM. The ampule was removed from the glovebox and placed in an oil bath at 348 K. Aliquots were taken from the reaction under an argon atmosphere. Aliquots were quenched

Bis(5,7-dichloro-2-methyl-8-quinolinolato)ethylaluminum (3). Triethylaluminum (0.50 g, 4.40 mmol) in toluene (5 mL) was added dropwise, with stirring, to a solution of 5,7-dichloro-8-hydroxy2-methylquinoline (2.00 g, 8.77 mmol) in toluene (15 mL). The solution was stirred for 12 h. The solvent was removed in vacuo, and the product was washed with hexane, filtered, and dried in vacuo to yield a yellow powder (1.73 g, 3.40 mmol, 77%). 1 H NMR (400 MHz, toluene-d8; δ (ppm)): 0.23 (dq, 1H, CH2, 2JHH = 14.5 Hz, 3JHH = 8.4 Hz), 0.39 (dq, 1H, CH2, 2JHH = 14.5 Hz, 3JHH = 8.4 Hz), 0.87 (t, 3H, CH3, 3JHH = 8 Hz), 2.95 (s, 6H, CH3), 6.53(d, 2H, CH, 3JHH = 8 Hz), 7.33 (s, 2H, CH), 7.81 (d, 2H, CH, 3JHH = 8 Hz). 13C{1H} NMR (100 MHz, CDCl3; δ (ppm)): 1.6 (CH2CH3), 9.32 (CH2CH3) 23.3 (CH3), 116.6 (CIV), 117.2 (CIV), 123.8 (CIV), 125.0 (CH), 129.2 (CH), 136.5 (CH), 140.3 (CIV), 152.5 (CIV), 159.3 (CIV). Anal. Calcd for AlC22H17N2O2Cl4: C, 51.79; H, 3.36; N, 5.49. Found: C, 51.93; H, 3.26; N, 5.56. Bis(5,7-dibromo-2-methyl-8-quinolinolato)ethylaluminum (4). Triethylaluminum (26 mg, 0.23 mmol) in toluene (3 mL) was added dropwise, with stirring, to a solution of 5,7-dibromo-8-hydroxy2-methylquinoline (0.15 g, 0.47 mmol) in toluene (10 mL). The solution was stirred for 12 h. The solvent was removed in vacuo, and the product was washed with hexane, filtered, and dried in vacuo to yield a yellow powder (0.16 g, 0.14 mmol, 62%). 1 H NMR (400 MHz, CDCl3; δ (ppm)): 0.23 (dq, 1H, CH2, 2JHH = 14.6 Hz, 3JHH = 8.4 Hz), 0.41 (dq, 1H, CH2, 2JHH = 14.6 Hz, 3JHH = 8.4 Hz) 1.01 (t, 3H, CH3, 3JHH = 8.2 Hz), 3.06 (s, 6H, CH3), 6.59 (d, 2H, CH, 3JHH = 8.4 Hz), 7.82 (s, 2H, CH), 7.86 (d, 2H, CH, 3JHH = 8.4 Hz). 13C{1H} NMR (100 MHz, toluene-d8; δ (ppm)): 1.8 (CH2CH3), 10.0 (CH2CH3), 23.3 (CH3), 106.0 (CIV), 107.2 (CIV), 124.3 (CH), 135.0 (CH), 138.6 (CH), 140.5 (CIV), 155.3 (CIV), 159.2 (CIV). Anal. Calcd for AlC22H17N2O2Br4: C, 38.41; H, 2.49; N, 4.07. Found: C, 38.26; H, 2.62; N, 3.98. Bis(5-chloro-2-methyl-8-quinolinolato)ethylaluminum (5). Triethylaluminum (48 mg, 0.42 mmol) in toluene (5 mL) was added dropwise, with stirring, to a solution of 5-chloro-8-hydroxy-2methylquinoline (0.16 g, 0.84 mmol) in toluene (8 mL). The solution was stirred for 12 h. The solvent was removed in vacuo, and the product was washed with hexane, filtered, and dried in vacuo to yield a yellow powder (0.11 g, 0.25 mmol, 60%). 1 H NMR (400 MHz, benzene-d6; δ (ppm)): 0.48 (dq, 2H, CH2, 2 JHH = 14.7 Hz, 3JHH = 8.2 Hz), 1.05 (t, 3H, CH3, 3JHH = 8.2 Hz), 2.85 (s, 6H, CH3), 6.58 (d, 2H, CH, 3JHH = 8.6 Hz), 6.88 (d, 2H, CH, 3JHH = 8.6 Hz), 7.21 (d, 2H, CH, 3JHH = 8.6 Hz), 8.03 (d, 2H, CH, 3JHH = 8.6 Hz). 13C{1H} NMR (100 MHz, benzene-d6; δ (ppm)): 2.7 (CH2CH3), 10.1 (CH2CH3), 23.2 (CH3), 113.1 (CH), 116.5 (CIV), 124.9 (CH), 125.4 (CIV), 129.4 (CH), 136.1 (CH), 141.0 (CIV), 157.5 (CIV), 157.7 (CIV). Anal. Calcd for AlC22H19N2O2Cl2: C, 59.88; H, 4.34; N, 6.35. Found: C, 59.73; H, 4.24; N, 6.42. Bis(5-chloro-7-iodo-2-methyl-8-quinolinolato)ethylaluminum (6). Triethylaluminum (78 mg, 0.68 mmol) in toluene (2 mL) was added dropwise, with stirring, to a solution of 5chloro-7-iodo-8-hydroxy-2-methylquinoline (0.44 g, 1.36 mmol) in toluene (10 mL). The solution was stirred for 12 h. The solvent was removed in vacuo, and the product was washed with hexane, filtered, and dried in vacuo to yield a yellow powder (0.34 g, 0.49 mmol, 72%). 1 H NMR (400 MHz, benzene-d6; δ (ppm)): 0.27 (dq, 1H, CH2, 2 JHH = 14.4 Hz, 3JHH = 8.4 Hz), 0.43 (dq, 1H, CH2, 2JHH = 14.4 Hz, 3 JHH = 8.4 Hz), 0.89 (t, 3H, CH3, 3JHH = 8 Hz), 3.03 (s, 6H, CH3), 6.57 (d, 2H, CH, 3JHH = 8.4 Hz), 7.72 (s, 2H, CH), 7.80 (d, 2H, CH, 3 JHH = 8.4 Hz). 13C{1H} NMR (100 MHz, benzene-d6; δ (ppm)): 2.6 (CH2CH3), 10.1 (CH2CH3), 23.9 (CH3), 80.1 (CIV), 117.8 (CIV), 125.2 (CH), 125.3 (CIV), 136.4 (CH), 136.5 (CH), 138.6 (CIV), 158.1 (CIV), 159.1 (CIV). Anal. Calcd for AlC22H17N2O2Cl2I2: C, 38.12; H, 2.47; N, 4.04. Found: C, 38.20; H, 2.41; N, 3.93. Bis(5,7-diiodo-2-methyl-8-quinolinolato)ethylaluminum (7). Triethylaluminum (83 mg, 0.73 mmol) in toluene (10 mL) was added dropwise, with stirring, to a solution of 5,7-diiodo-8-hydroxy-2methylquinoline (0.60 g, 1.46 mmol) in toluene (15 mL). The solution was stirred for 12 h. The solvent was removed in vacuo, and 4734

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with hexane (1−2 mL), and the solvent was allowed to evaporate. The crude product was analyzed by 1H NMR and homonuclear decoupled 1 H NMR spectroscopy, GPC, and MALDI-TOF mass spectrometry. The conversion of LA to PLA was determined by integration of the methine proton peaks of the 1H NMR spectra: δ 5.00−5.30. The Pi value was determined by integration of the methine region of the homonuclear decoupled 1H NMR spectrum: δ 5.1−5.24.25 The PLA number-averaged molecular weight (Mn) and polydispersity index (Mw/Mn; PDI) were determined using gel permeation chromatography equipped with multiangle laser light scattering (GPC-MALLS). The refractive angle increment for polylactide (dn/dc) in THF was 0.042 mL g−1.27

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ASSOCIATED CONTENT

S Supporting Information *

Figures, tables, and CIF files giving additional characterization data and crystal data. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (M.H.); [email protected]. uk (C.K.W.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Yuta Naro, Kaylee Underkofler, Matthew Zeitler, and Stephanie Thompson are acknowledged for their synthetic contribution to the project. Emil Lobkovsky is acknowledged for the X-ray structural determination of compound 5. The research was supported by funding from the EPSRC (EP/H046380, EP/ C544846/1, EP/C544838/1).

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REFERENCES

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Organometallics

Article

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dx.doi.org/10.1021/om300307t | Organometallics 2012, 31, 4729−4736