Poly(lactides) (PLAs), or poly(lactic acid)s, are among the first commercial biodegradable polymers that have the potential to become commodity plastics.
meso
-Lactide, a by-product of
l
-lactide ...production, will become more easily available. Highly stereoregular poly(
meso
-lactides) should be crystalline and thus interesting as polymeric material. In this short review, initiators capable of inducing the stereoselective ring-opening polymerization of
meso
-lactide to ideally give syndiotactic or heterotactic PLAs will be discussed. Mechanistic discussions with regard to understanding the reactivity differences between the various lactide monomers are included.
This review presents initiators capable of catalyzing the stereoselective ring-opening polymerization of
meso
-lactide resulting in syndiotactic or heterotactic polylactide.
The molecular strontium hydride (Me3TACD)3Sr3(μ3-H)2SiPh3 (2) was isolated as the dark red benzene solvate 2·C 6 H 6 in 69% yield from the reaction of Sr(SiPh3)2(thf)3 (1′) with (Me3TACD)H ...(1,4,7-trimethyl-1,4,7,10-tetraazacyclododecane). This reaction can be considered as a redox process, with the Brønsted acidic amine proton in (Me3TACD)H transformed into the hydride by the anion SiPh3−. Trace amounts of water resulted in the formation of (Me3TACD)3Sr3(μ3-H)(μ3-OH)SiPh3 (2*), which cocrystallized with 2. Single-crystal X-ray diffraction of 2 revealed a substitutional disorder of a bridging hydride with a hydroxide ligand. Hydride complex 2 was also obtained by hydrogenolysis of (Me3TACD)Sr(SiPh3) (3), although pure 3 proved difficult to isolate. In the presence of a 2-fold excess of (Me3TACD)H, the reaction with disilyl 1′ gave (Me3TACD)SiPh3 (4). Complex 2 underwent facile H/D exchange with D2 (1 bar), with the anion SiPh3− decomposing concurrently. In the reaction of 2 with 1,1-diphenylethylene (DPE), the anion SiPh3− was added to the CC bond in DPE to give (Me3TACD)3Sr3H2Ph2CCH2SiPh3 (5), whereas the cationic cluster (Me3TACD)3Sr3H2+ remained unchanged. 9-Fluorenone underwent one-electron reduction with 2 to give the paramagnetic ketyl complex {(Me3TACD)H}Sr(OC13H8 •)2(thf)2 (6). These strontium compounds are structurally similar to the lighter calcium congeners, but more reactive, in particular with regard to fast H/D exchange and SiPh3− anion decomposition. DFT studies on the cationic hydride clusters suggest a more pronounced covalent character for strontium compared to calcium. Disilyl 1, strontium diketyl 6, and the calcium congener of 6, {(Me3TACD)H}Ca(OC13H8 ·)2 (10), were also characterized by X-ray diffraction.
Tactical response: High heterotactic stereocontrol was observed from the ring‐opening polymerization of rac‐lactide on using a series of bis(phenolato)scandium complexes (see scheme). This ...stereocontrol appears to involve a dynamic monomer‐recognition process as a result of the interconversion of the configuration from Λ into Δ.
The fifteen‐membered NNNNN macrocycle Me5PACP (Me5PACP=1,4,7,10,13‐pentamethyl‐1,4,7,10,13‐pentaazacyclopentadecane) stabilized the CaH+ fragment as a dimer with a distorted pentagonal bipyramidal ...coordination geometry at calcium. The hydride complex was prepared by protonolysis of calcium dibenzyl with the conjugate acid of Me5PACP followed by hydrogenolysis or treating with nOctSiH3 of the intermediate calcium benzyl cation. The calcium hydride catalyzed the hydrogenation and hydrosilylation of unactivated olefins faster than the analogous calcium complex stabilized by the twelve‐membered NNNN macrocycle Me4TACD (Me4TACD=1,4,7,10‐tetramethyl‐1,4,7,10‐tetraazacyclododecane). Kinetic investigations indicate that higher catalytic efficiency for the Me5PACP stabilized calcium hydride is due to easier dissociation of the dimer in solution when compared to the Me4TACD analogue.
Macrocyclic ligands recognize ion size, but are rarely used to influence metal‐centered catalytic activity. Calcium hydride cation CaH+ supported by the fifteen‐membered NNNNN macrocycle Me5PACP (Me5PACP=1,4,7,10,13‐pentamethyl‐1,4,7,10,13‐pentaazacyclopentadecane) catalyzes hydrogenation and hydrosilylation of unactivated olefins significantly faster than the analogous calcium catalyst containing the twelve‐membered NNNN macrocycle (see figure).
Organometallic allyl compounds are important as allylation reagents in organic synthesis, as polymerization catalysts, and as volatile metal precursors in material science. Whereas the allyl ...chemistry of synthetically relevant transition metals such as palladium and of the lanthanoids is well‐established, that of main group metals has been lagging behind. Recent progress on allyl complexes of Groups 1, 2, and 12–16 now provides a more complete picture. This is based on a fundamental understanding of metal–allyl bonding interactions in solution and in the solid state. Furthermore, reactivity trends have been rationalized and new types of allyl‐specific reactivity patterns have been uncovered. Key features include 1) the exploitation of the different types of metal–allyl bonding (highly ionic to predominantly covalent), 2) the use of synergistic effects in heterobimetallic compounds, and 3) the adjustment of Lewis acidity by variation of the charge of allyl compounds.
No longer an aside: The allyl chemistry of main group metals was traditionally less investigated, but there is now a fundamental understanding of metal–allyl interactions. Furthermore, reactivity trends and new allyl‐specific reaction patterns have been identified. In this Review, the coordination modes and reactivity of allyl compounds of main group metals as well as trends in the periodic system of the elements (PSE) are discussed.
Redox‐unstable cuprous hydridotriphenylborate was isolated as an N‐heterocyclic carbene adduct (IPr)Cu(HBPh3) (IPr=1,3‐bis(2,6‐diisopropylphenyl)imidazol‐2‐ylidene) with good thermal stability. ...Although this compound displays a contact ion‐pair structure, CuIH‐like catalytic activity was envisaged in carbonyl hydrosilylation. Sufficient moisture stability allowed the catalysis in aqueous/organic media. Mechanistic study further showed that a phenyl group on the borate anion is ed by (IPr)Cu+ to give the cationic organocopper complex (IPr)2Cu2(μ‐Ph)BPh4.
Monomeric Cu(HBPh3) supported by an N‐heterocyclic carbene is thermally stable and catalyzes the carbonyl hydrosilylation in water as a co‐solvent.
This article summarizes recent work related to the chemistry of cationic rare-earth organometallic complexes, that is, molecular compounds of the rare-earth metals that contain at least one ...metal-carbon bond and that carry a positive ionic charge. The synthesis, structure, and reactivity of each will be presented.
•Metallocenes helped understanding heterogeneous olefin polymerization catalysts.•Bulky cyclopentadienyl ligands were crucial for designing stereospecific catalysts.•Polymerization catalysts for ...larger monomers require half-sandwich complexes.•Chelating cyclopentadienyl complexes can polymerize bulky monomers.
This personal account summarizes the development of molecularly defined organometallic polymerization catalyst precursors based on cyclopentadienyl metal complexes. Departure from the bent metallocene structure provided opportunities to considerably expand the coordination sphere of early transition metal centers relevant to olefin polymerization. Thus, half-sandwich metal complexes such as those featuring linked amido-cyclopentadienyl ligands (“constrained geometry catalysts“) allowed copolymerization of ethylene with larger comonomers, while mono(cyclopentadienyl) complexes of titanium and rare earth metals offered new opportunities to develop catalysts for the stereoselective polymerization of monomers such as styrene. The quest for molecular understanding of heterogeneous Ziegler-type catalysts has inspired the organometallic chemistry of the early transition metals, initially limited to that of zirconocenes. The importance of modifying cyclopentadienyl ligands for the rational design of homogeneous polymerization catalysts is highlighted.
Display omitted
The combination of the presence of ST-segment depression in lead aVR and the absence of ST-segment elevation in lead V1 identified TC with 91% sensitivity, 96% specificity, and 95% predictive ...accuracy, which was superior to any other electrocardiographic findings (Fig. 1B). To clarify electrocardiographic characteristics of TC, we studied only patients who were admitted within 6 h of symptom onset. ...most previous studies assessing electrocardiographic findings of TC have paid little attention to limb leads.
PDF
