Single-Site Calcium Initiators for the Controlled Ring-Opening Polymerization of Lactides and Lactones

Summary Single-site calcium initiators containing chelating tmhd (H-tmhd = 2,2,6,6-tetramethylheptane-33-dione) ligands [(THF)Ca(tmhd)_]2[-N(SiMe₃)₂](-tmhd) (2) and [(THF)Ca(tmhd)]₂[-OCH(Me)Ph](-tmhd) (3) have been synthesized and applied for the ring-opening polymerization of L-lactide and -caprolactone. Both 2 and 3 were highly reactive and promoted a fast polymerization of L-lactide and -caprolactone to high monomer conversions under mild conditions (THF as a solvent, room temperature). More importantly, results showed that the ring-opening polymerizations of lactides and lactones initiated by either 3 or 2 in the presence of equivalent 2-propanol are living, to provide polymers and block copolymers of controlled molecular weights and tailored end-groups. The polymerizations were first-order in monomer up to high conversions, in which the in situ initiating system 2/2-propanol revealed no induction period and much faster polymerization kinetics as compared to 3.     

The role of the corpus callosum in dichotic listening: A combined morphological and diffusion tensor imaging study.

The objective of the present study was to examine the role of the corpus callosum (CC) in dichotic listening. For this purpose, 40 right-handed healthy male participants were assessed with morphological and diffusion tensor imaging, which yielded macrostructural (midsagittal area) and microstructural (mean diffusion, fractional anisotropy) measures of the total CC and predefined subregions. Applying the standard consonant-vowel Bergen Dichotic Listening Test (including 3 different attentional instructions), the authors found dichotic listening performance to be substantially related to the integrity of the CC at both macro- and microstructural levels. Results indicate a dual role of the CC; it is relevant not only for the stimulus-driven (bottom-up) transfer of left-ear input to the left hemisphere but also for its attentional (top-down) modulation. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Dppm stabilized nickelacyclic carboxylates as building blocks of oligonuclear nickel complexes

The reaction of 1,1-bis(diphenylphosphino)methane (dppm) and [py₂Ni(C₂H₄COO)] with trace amounts of water leads to the formation of the novel pentanuclear organonickel compound [(μ²-dppm)₂{Ni(C₂H₄COO)}₄ * Ni(OH)₂] (1) bearing nickel centres in square planar as well as in octahedral coordination sphere. In presence of NiBr₂, the related trinuclear complex [(μ²-dppm){Ni(C₂H₄COO)}₂ * NiOH(H₂O)(py)₂]⁺ Br^? (2) was isolated and characterized by elemental analysis, IR measurements and X-ray single crystal structural analysis. These compounds are rare examples of organonickel compounds with μ³-hydroxide ligands and to best of our knowledge the first nickelacyclic carboxylates stabilized by such simple inorganic compounds as ligands.     

Transesophageal echocardiography in the detection of cardiovascular sources of peripheral vascular embolism

The purpose of this study was to determine the impact of transesophageal echocardiography (TEE) on the management of patients with peripheral vascular emboli. We prospectively evaluated the role of TEE in 15 patients with documented peripheral emboli and no evidence of occlusive peripheral vascular disease. The patients were divided in two groups for analysis: group 1 (n = 8) had no clinical evidence of heart disease and group 2 (n = 7) had clinically significant heart disease. TEE provided information regarding the source of embolism in four (50%) patients in group 1, and these findings significantly affected the management of all. Three patients underwent thoracic surgery to remove the source of embolism (aortic valve mass in one and a thrombus in the descending thoracic aorta in two); the other patients was treated with thrombolytic agents. TEE findings had high diagnostic value in all patients in group 2, but the results had a possible effect on clinical management in only two of these patients. TEE provides diagnostic information in most patients with peripheral vascular emboli and this information has a significant influence on management, particularly in those without clinically evident heart disease. TEE should be performed in all patients with documented peripheral embolism.     

Calcium-mediated hydrophosphination of diphenylethyne and diphenylbutadiyne as well as crystal structure of 1,4-diphenyl-1,4-bis(diphenylphosphanyl)buta-1,3-diene

Hydrophosphination reactions of phenyl substituted alkynes are catalyzed effectively by [(thf)₄Ca(PPh₂)₂] (1). The reactions of diphenylethyne and diphenylbutadiyne with diphenylphosphane in THF in the presence of catalytic amounts of 1 (approx. 5 mol%) yield quantitatively the cis-addition products trans-1,2-diphenyl-1-diphenylphosphanylethene (2) and 1,4-diphenyl-1,4-bis(diphenylphosphanyl)buta-1,3-diene (3), respectively. The phenyl groups in 3 are oriented nearly perpendicular to the butadiene backbone and therefore, the π-systems of the phenyl groups and the butadiene unit show no interaction with each other.     

Synthesis and molecular structure of the sodium arsanediyl zincate [(thf)Na]2[(EtZn)6(AsSiiPr3)4]

The reaction of NaAs(H)SiiPr₃ with ZnEt₂ in the presence of additional triisopropylsilylarsane yields the heterobimetallic complex [(thf)Na]₂ [(EtZn)₆(AsSiiPr₃)₄] (1) in the shape of red prisms. The Na–As and Zn–As bond lengths show values of 301/309 pm and between 247 and 260 pm, respectively.     

Synthesis and crystal structure of dinuclear [(thf)3YCl2{μ-P(H)C6H2-2,4,6-Me3}]2 with seven-coordinate yttrium atoms

The metathesis reaction of KP(H)Mes with YCl₃ in tetrahydrofuran yields dinuclear [(thf)₃YCl₂{μ-P(H)Mes}]₂ (1) with a yield of 26% (Mes = mesityl, C₆H₂-2,4,6-Me₃). The seven-coordinate yttrium atoms exhibit pentagonal bipyramidal coordination spheres with bridging mesitylphosphanide substituents. The Y₂PC fragments are nearly planar with an average Y–P bond length of 296.0 pm. The P-bound hydrogen atoms are perpendicular to these planes. The small ¹J(P,H) coupling constant of 125 Hz is in agreement with the picture that the H atom binds mainly to the p-orbital of the phosphorus atom.     

Synthesis and characterization of rare examples of stable potassium and arylcalcium triethylboranate complexes

Stabilization of potassium triethylboranate can be achieved in solution as well as in the solid state with tridentate 1,3,5-trimethyl-1,3,5-triazinane (tmta) as dimeric [(tmta)K(μ-H)BEt₃]₂ (1). The metathesis reaction of the post-Grignard reagent [2,6-(tol)₂C₆H₃-Ca(thf)₃I] (2) with potassium triethylboranate yields an unusual hydrogen bridged organocalcium contact ion pair as hydrocarbon soluble [(thf)(dme)Ca(C₆H₃-2,6-tol₂)HBEt₃] (3) which shows no tendency to dismutate to the homoleptic derivatives.     

Reinvestigation of the synthesis of phenylcalcium iodide and the first structural characterization of a heavy Grignard reagent as [((thf)2CaPhI)3 · (thf)CaO] with a central Ca4 tetrahedron

Phenylcalcium iodide is accessible by direct synthesis from activated calcium and iodobenzene in THF at low temperatures. The use of chloro- or bromobenzene as well as of another solvent such as diethylether lowers the yield significantly. Phenylcalcium iodide cleaves THF already at −20 °C under formation of benzene. From this reaction solution single crystals of the title compound precipitate. The central structural fragment consists of a distorted calcium tetrahedron, centered by an oxygen atom and with Ca atoms being bridged by phenyl groups (average Ca–C 257 pm) and iodide ligands.     

Calcium methoxide initiated ring-opening polymerization of ε-caprolactone and L-lactide

Summary A commercial calcium dimethoxide and an in-situ generated calcium methoxide prepared from bis(tetrahydrofuran)calcium bis[bis(trimethylsilyl)amide] and methanol, were investigated as initiators for the ring-opening polymerization of ε-caprolactone and L-lactide. Commercial calcium dimethoxide initiated rapid ε-caprolactone polymerization at 120°C in bulk to give quantitatively a polymer with a polydispersity index around 1.3. Significant racemization was observed for L-lactide polymerization. The In-situ formed calcium methoxide promoted the solution polymerization of both ε-caprolactone and L-lactide to high conversion at room temperature over a short time period, yielding the corresponding polyesters with narrow molecular weight distribution. NMR spectra showed that the poly(L-lactide) isolated had a purely isotactic microstructure. The initiator efficiency could be tuned by varying the molar ratio of methanol and bis(tetrahydrofuran)calcium bis[bis(trimethylsilyl)amide]. Received: 11 August 2000/Revised version: 21 December 2000/Accepted: 3 January 2001