Highly efficient metal(iii) porphyrin and salen complexes for the polymerization of rac-lactide under ambient conditions.

Ligated metal(iii) complexes, Al(iii), In(iii), Cr(iii), and Co(iii), containing tetraphenylporphyrin (TPP) and cyclohexylsalen (Cy-salen) ligands were investigated for the polymerization of rac-lactide (rac-LA). A combination of metal complexes, co-catalyst, and co-initiator was shown to be highly efficient for the ring-opening polymerization of rac-LA at room temperature. Influences of metal centers, ligands, and co-catalysts were investigated. Higher Lewis acidity of metal centers and ligands enhanced catalytic activity. Ionic co-catalysts showed increased polymerization rates. Polymerization of rac-LA catalyzed by tetraphenylporphyrin aluminum chloride ((TPP)AlCl) and bis(triphenylphosphine)iminium chloride (PPN+Cl-) in cyclohexene oxide (CHO) as a solvent produced isotactic-enriched polylactide. The order of reactivities based on the metals supported by the TPP ligand is Cr(iii) > Al(iii) > In(iii) > Co(iii) while the reactivities of the catalysts supported by Cy-salen ligands are in the order In(iii) > Cr(iii) > Al(iii) > Co(iii).

Catalytic Enantioselective Hetero-dimerization of Acrylates and 1,3-Dienes.

1,3-Dienes are ubiquitous and easily synthesized starting materials for organic synthesis, and alkyl acrylates are among the most abundant and cheapest feedstock carbon sources. A practical, highly enantioselective union of these two readily available precursors giving valuable, enantio-pure skipped 1,4-diene esters (with two configurationally defined double bonds) is reported. The process uses commercially available cobalt salts and chiral ligands. As illustrated by the use of 20 different substrates, including 17 prochiral 1,3-dienes and 3 acrylates, this hetero-dimerization reaction is tolerant of a number of common organic functional groups (e.g., aromatic substituents, halides, isolated mono- and di-substituted double bonds, esters, silyl ethers, and silyl enol ethers). The novel results including ligand, counterion, and solvent effects uncovered during the course of these investigations show a unique role of a possible cationic Co(I) intermediate in these reactions. The rational evolution of a mechanism-based strategy that led to the eventual successful outcome and the attendant support studies may have further implications for the expanding use of low-valent group 9 metal complexes in organic synthesis.

A new route for the preparation of enriched iso-polylactide from rac-lactide via a Lewis acid catalyzed ring-opening of an epoxide.

Tetraphenylporphyrin aluminum(iii) salts, TPPAlX, where X = Me, OEt, OiPr, OCHMeCH2Cl, and Cl, and bis(triphenylphosphine)imminium chloride, PPN+Cl- (1 : 1) react with rac-lactide, rac-LA, in neat propylene oxide, PO, to yield chains of enriched isotactic polylactide, PLA, with end groups of PO-Cl and with time these yield cyclic polymers (PO)n(PLA) where n = 2 or 3 and even higher. There is no reaction between TPPAlOR (R = Et or iPr), PPN+Cl-, and rac-LA in neat THF at 25 °C even though TPPAlOR (R = Et or iPr) and PPN+Cl- in neat PO yields polypropylene oxide with a terminal OR group, H(PO)nOR. Taken together, Al(iii) acts as a Lewis acid in the ring-opening of PO, in which PPN+Cl- is present and the incipient ClCH2CHMeO- initiates the ROP of LA to yield anion chains of [(PLA)-OCHMeCH2Cl]-, and then the ring-opening of PO yields cycles, (PO)n(PLA), with the liberation of Cl-. The polymer was isolated by the addition of MeOH/HCl and end group analysis by mass spectrometry.

Bismuth-lithium bonding in the ion pairs: LiBiL2, where L = a porphyrin or a salen ligand.

From the reaction between BiCl3 (1 equiv.) and LiN(SiMe3)2 (4 equiv.) and LH2 (2 equiv.), where L = a tetraphenylporphyrin, TPP, an octaethylporphyrin, OEP and phsalen in THF the title compounds have been obtained LiBiTPP2, LiBiOEP2, and LiBi(phsalen)2 and LiBi(phsalen)2·THF. Crystals grown from CH2Cl2-hexanes are colored; (green), (red-purple) and and (red-orange). The molecular structures of compound , and were determined by single-crystal X-ray crystallography and are shown to have short LiBi bonds of distance 2.8 Å involving the LiL(-)BiL(+). Compound shows a slipped structure involving Li to two oxygens and a LiBi distance of 3.1 Å. Compounds and undergo a rapid reversible exchange in toluene-d8 at 90 °C. The MALDI-MS yields weak molecular ions due to LiBiL2(+/-) with more intense ions due to BiL(+) and LiL(-) in the positive and negative modes. The short Li(+) to Bi(3+) distances are comparable to those seen in LiBi compounds, such a LiBiR2, and are comparable to those seen by Pyykkö (P. Pyykkö, J. Phys. Chem. A, 2015, 119, 2326; P. Pyykkö and M. Atsumi, Chem. - Eur. J., 2009, 15, 186; P. Pyykkö and M. Atsumi, Chem. - Eur. J., 2009, 15, 12770) for Li-Bi bonds. These can be seen to be involving Bi6s6p hybrid lone-pairs to Li(+) atoms. The lithium bis(bistrimethylsilyl)amide (2 equiv.) and phsalenH2 in THF gave a compound having Li2L·2THF, . Crystallographically compound contains two Li(+) atoms, one coordinated to five atoms LiO2N2·THF. and the other being coordinated to three atoms, LiO2·THF. By (7)Li and (1)H NMR both lithium atoms share an equivalent environment.

Coupling of propylene oxide and lactide at a porphyrin chromium(III) center.

5,10,15,20-Tetraphenylporphyrin chromium chloride (TPPCrCl) with added [Ph3P═N═PPh3](+)Cl(-) (PPN(+)Cl(-)) selectively polymerizes lactide (L and rac) dissolved in neat propylene oxide (PO) to yield polylactide (PLA) terminated by the -OCHMeCH2Cl group. At 0 °C and below, rac-LA yields polymers highly enriched in isotactic tetrads (iii). At 25 °C, some stereoselectivity is lost as transesterification becomes significant, and at 60 °C and above, enchainment of PO leads to the formation of 3,6-dimethyl-1,4-dioxan-2-one by a backbiting mechanism. At 0 °C, after the enchainment of L-(S,S)-LA in neat (R)-(+)-PO, the formation of (3S,6R)-3,6-dimethyl-1,4-dioxan-2-one occurs, while at higher temperatures the ratio of (3S,6R)-3,6-dimethyl-1,4-dioxan-2-one to (3R,6R)-3,6-dimethyl-1,4-dioxan-2-one falls to 3:2.

Ethyl 2-hydroxy-2-methylpropanoate derivatives of magnesium and zinc. The effect of chelation on the homo- and copolymerization of lactide and ε-caprolactone.

The preparation, single crystal and molecular structures of the compounds (BDI)Mg(OCMe2COOEt)(L) are described where BDI = 2-[(2,6-diisopropylphenyl)amino]-4-[(2,6-diisopropylphenyl)imino]pent-2-ene and L = pyridine (py) and 4-dimethylaminopyridine (DMAP). In the solid-state the molecules have five coordinate Mg(2+) ions that may be considered as derived from a trigonal bipyramid where the chelating BDI and OCMe2COOEt ligands span equatorial-axial sites. IR spectroscopic data obtained in CH2Cl2 indicate that the chelating nature of the OCMe2COOEt ligand is maintained in solution and variable temperature NMR studies reveal that the ligands py or DMAP undergo a rapid reversible dissociation in toluene-d8 and especially in THF-d8. This leads to a facile exchange involving coordinated and free L upon the addition of L and evidence is presented to support the view that this occurs exclusively by a dissociative interchange mechanism. Zinc forms a related complex (BDI)Zn(OCMe2COOEt) lacking additional coordination of L but maintaining the chelating nature of the OCMe2COOEt ligand and undergoing reversible coordination of L in the presence of added ligands L. The magnesium and zinc complexes will initiate and sustain the living polymerization of lactide (L- or rac-LA) but not the ring-opening polymerization of ε-caprolactone.

On the molecular structure and bonding in a lithium bismuth porphyrin complex: LiBi(TPP)(2).

A new BiLi porphyrin sandwich compound, LiBi(TPP)2 has been synthesized and characterized (TPP=tetraphenylporphyrin). The unique molecular structure of LiBi(TPP)2 is such that the Bi sits between the porphyrins and is directed towards the Li. This complex was shown to remain intact in solution by temperature-dependent 2D NMR spectroscopy. In order to investigate the potential interaction between these two metals, DFT calculations were used and showed a Bi 6s orbital polarized towards Li which could be indicative of a BiLi dative bond. This bond is remarkably short, 2.87 Å, and is among the shortest BiLi distances seen in a small molecule.

Single-site bismuth alkoxide catalysts for the ring-opening polymerization of lactide.

Salen bismuth alkoxides, where the salen ligand contains 2,4-di-tert-butylphenoxy groups and one of ethylene, cyclohexane or ortho-phenyl as a backbone have been prepared from reactions involving Bi[N(SiMe3)2]3 and the free salen ligand followed by alcoholysis (ButOH, PriOH and 2,6-But2C6H3OH). The molecular structures of the salen ligand with the cyclohexyl back-bone have been determined for the complexes salenBiCl and salenBiOC6H3-2,6-But2. The chloro compound is a dimer with chloride bridges while the phenoxide is monomeric with an unusually distorted five-coordinate geometry. The phenoxide and tert-butoxide complexes have been employed in the ring-opening polymerization of lactides (L- and rac-) to give polylactides, PLAs. With rac-LA heterotactic PLA is formed preferentially, Pr = ~0.9, in dichloromethane or toluene at room temperature. The reaction is first order in [Bi] and is notably faster than most aluminum and zinc initiators as well as tin(II) octanoate. These results are discussed in terms of a recent report on the polymerization of LA by Peptobismol® and bismuth subsalicylate.

Use of over the counter oral relief aids or dietary supplements for the ring-opening polymerization of lactide.

Over the counter oral relief aids or dietary supplements, namely calcium carbonate, calcium hydroxyapatite, magnesium oxide, zinc oxide, Tums® and Pepto-Bismol® have been investigated in the melt ring-opening polymerization of lactide and a direct comparison with that involving tin(II) octanoate has been made. Of these, Pepto-Bismol® is shown to be the most active and comparable to tin octanoate. The active ingredient in Pepto-Bismol® is bismuth subsalicylate and when this is employed as the initiator similar results are obtained in melt polymerizations which suggests it could be employed as an alternative to the commercially used tin(II) octanoate.

Pd-MCM-48: a novel recyclable heterogeneous catalyst for chemo- and regioselective hydrogenation of olefins and coupling reactions.

A novel, heterogeneous Pd-MCM-48 catalyst has been developed by encapsulating palladium nanoparticles into the cubic phase of mesoporous MCM-48 matrix at room temperature. The catalyst demonstrated excellent chemo- and regioselectivity for the hydrogenation of olefins at room temperature within 30-80 min. The turnover frequency for the hydrogenation is very high (4400 h(-1)). Interestingly, selectivity of the catalyst was significantly influenced by the mode of addition of palladium precursor. Moreover, the catalyst was also very effective for the coupling reactions with the formation of carbon-carbon and carbon-nitrogen bonds under ligand-free and aerobic conditions.