Bis(guanidinate) alkoxide complexes of lanthanides: synthesis, structures and use in immortal and stereoselective ring-opening polymerization of cyclic esters

A series of new bis(guanidinate) alkoxide Group 3 metal complexes [Ln((Me3Si)2NC(NiPr)2)2(OR)] (R=OtBu, Ln=Y, Nd, Sm, Lu; R=OiPr, Ln=Y, Nd, Lu) has been synthesized. X-ray structural determinations revealed that bis(guanidinate) tert-butoxides are monomeric complexes. The isopropoxide complex [Y((Me3Si)2NC(NiPr)2)2(OiPr)] undergoes slow decomposition in solution, to afford the unusual dimeric amido complex [(Y((Me3Si)2NC(NiPr)2)2(mu-N(iPr)C triple chemical bond N))2]. Complexes [Ln((Me3Si)2NC(NiPr)2)2(OR)] (R=OtBu, Ln=Y, Nd, Sm, Lu; R=OiPr, Ln=Y, Nd, Lu) are active catalysts/initiators for the ROP of rac-lactide and rac-beta-butyrolactone under mild conditions. Most of those polymerizations proceed with a significant degree of control. Bis(guanidinate) alkoxides appear to be well suited for achieving immortal polymerization of lactide, through the introduction of large amounts of isopropanol as a chain-transfer agent. The synthesized complexes are able to promote the stereoselective ROP of rac-beta-butyrolactone to afford syndiotactic poly(hydrobutyrate) through a chain-end control mechanism, while they are surprisingly non-stereoselective for the ROP of lactide under strictly similar conditions.     

Scandium alkyl and amide complexes containing a cyclen-derived (NNNN) macrocyclic ligand: synthesis, structure and ring-opening polymerization activity toward lactide monomers

Cyclic polyamine 1,4,7-trimethyl-1,4,7,10-tetraazacyclododecane, (Me(3)TACD)H (= Me(3)[12]aneN(4)), reacted with [K{N(SiHMe(2))(2)}] in benzene-d(6) to give [K{(Me(3)TACD)SiMe(2)N(SiHMe(2))}] (1) under hydrogen evolution. Single-crystal X-ray diffraction of 1 shows a dinuclear structure in the solid state, featuring a bridging μ-amido and a weak β-agostic Si-H bond. 1,7-Dimethyl-1,4,7,10-tetraazacyclododecane (Me(2)TACD)H(2) (= Me(2)[12]aneN(4)) and (Me(3)TACD)H were reacted with [Sc{N(SiHMe(2))(2)}(3)(thf)] in benzene-d(6) to give [{(Me(2)TACD)SiMe(2)N(SiHMe(2))}Sc{N(SiHMe(2))(2)}] (2) and [(Me(3)TACD)Sc{N(SiHMe(2))(2)}(2)SiMe(2)] (3), respectively. Both compounds are monomeric in solution and X-ray diffraction studies showed the scandium metal centers to be six-coordinate. The scandium alkyl complex [Sc(Me(3)TACD)(CH(2)SiMe(3))(2)] (4) was obtained by reacting (Me(3)TACD)H with [Sc(CH(2)SiMe(3))(3)(thf)] in benzene-d(6). The scandium amide complexes 2 and 3 catalyzed the ring-opening polymerization (ROP) of meso-lactide to give syndiotactic polylactides.     

微波辅助壳聚糖接枝聚乳酸共聚物的合成及表征

在微波辅助下,以辛酸亚锡为催化剂、壳聚糖(CS)为大分子引发剂引发消旋丙交酯（D,L-LA）本体开环聚合制备了壳聚糖接枝聚乳酸共聚物。通过正交实验研究了微波功率、催化剂用量、反应温度和反应时间对聚合反应的影响,确定了最佳合成条件。并通过红外光谱、元素分析、核磁共振氢谱、X-射线衍射和热分析对接枝共聚物的结构与性能进行了表征。结果表明,在微波条件下,能快速、有效地合成预定结构的壳聚糖接枝聚乳酸共聚物;聚乳酸支链的引入,有效削弱了壳聚糖分子间和分子内较强的氢键作用,与相应的壳聚糖比较,共聚物的结晶性能下降,热分解温度降低;原料配比对共聚物的结构与性能有显著影响,随nD,L-LA/nCS糖环数值增大,共聚物中平均乳酰单元数逐渐增大,共聚物的结晶性能、起始分解温度逐渐下降。     

Bimetallic aluminum complexes supported by bis(salicylaldimine) ligand: Synthesis,characterization and ring-opening polymerization of lactide

Two types of bifunctional bis(salicylaldimine) ligands (syn-L and anti-L) were designed and synthesized to support bimetallic aluminum complexes.Owing to the rigid anthracene skeleton,syn-L and anti-L successfully locked two Al centers in close proximity (syn-Al2) and far apart (anti-Al2),respectively.The distance between two Al centers in syn-Al2 was defined by X-ray diffraction as 6.665 (A),which is far shorter than that in anti-Al2.In the presence of stoichiometrical BnOH,syn-Al2 and anti-Al2 were both efficient for ring-opening polymerization (ROP) of rac-LA with the former being more active.In the presence of excess BnOH,syn-Al2 showed an efficient and immortal feature,consistent with high conversions,matched Mns,narrow molecular weight distributions and end group fidelity,while anti-Al2 had a much lower activity or even became entirely inactive due to rapid decomposition,indicated by in situ 1H-NMR experiments of A1 complexes with BnOH.     

Zinc and magnesium complexes incorporated by bis(amine) benzotriazole phenoxide ligand: synthesis, characterization, photoluminescent properties and catalysis for ring-opening polymerization of lactide

A new bis(amine) benzotriazole phenoxide ligand, (C8NN)BTP-H (1) was prepared from the Mannich condensation of 2-(2H-benzotriazol-2-yl)-4-(2,4,4-trimethylpentan-2-yl)phenol with the mixtures of excess paraformaldehyde and N,N,N-trimethylethylenediamine under reflux conditions. Zinc and magnesium complexes bearing the N,N,O-tridentate (C8NN)BTP ligand were synthesized and fully characterized. The reaction of (C8NN)BTP-H with ZnEt(2) (1.2 molar equiv.) in toluene gives the monomeric tetra-coordinated zinc complex [((C8NN)BTP)ZnEt] (2). The homoleptic and monomeric complexes [((C8NN)BTP)(2)M] (M = Zn (3) and M = Mg (4)) result from treatment of ZnEt(2) or Mg(n)Bu(2) with (C8NN)BTP-H (two equiv.), in which the metal center is hexa-coordinated by two tridentate (C8NN)BTP ligands. Luminescent properties and catalysis for lactide (LA) polymerizations of complexes 2-4 are studied. Complexes 2-4 produce bright green fluorescence with emission maxima ranging from 510 to 520 nm in the solid state. Experimental results indicate that complex 2 catalyzes the ring-opening polymerization of LA in the presence of 9-anthracenemethanol (9-AnOH) with efficient catalytic activities in a controlled fashion, yielding poly(rac-lactide) with a slight isotactic predominance (P(m) = 0.59).     

Bis(phosphinic)diamido yttrium amide, alkoxide, and aryloxide complexes: an evaluation of lactide ring-opening polymerization initiator efficiency

The synthesis and characterization of a series of bis(phosphinic)diamido yttrium alkoxide, amide, and aryloxide initiators are reported. The new complexes are characterized using multinuclear nuclear magnetic resonance (NMR) spectroscopy, elemental analysis, and, in some cases, X-ray crystallography. The alkoxide complexes are all dimeric in both the solid state and in solution, as are the amide complexes substituted with iso-propyl or phenyl groups on the phosphorus atoms. On the other hand, increasing the steric hindrance of the phosphorus substituents (tert-butyl), enables isolation of mononuclear yttrium amide complexes with either 2,2-dimethylpropylene or ethylene diamido ligand backbones. The complex of 2,6-di-tert-butyl-4-methylphenoxide is also mononuclear. All the new complexes are efficient initiators for rac-lactide ring-opening polymerization. The polymerization kinetics are compared and pseudo first order rate constants, k(obs), determined. The polymerization control is also discussed, by monitoring the number-averaged molecular weight, M(n), and polydispersity index, PDI, obtained using gel permeation chromatography (GPC). The alkoxide complexes are the most efficient initiators, showing very high rates and good polymerization control, behavior consistent with rapid rates of initiation. The phenoxide and amide complexes are less efficient as manifest by nonlinear regions in the kinetic plots, lower values for k(obs), and reduced polymerization control. One of the mononuclear yttrium amide complexes shows heteroselectivity in the polymerization of rac-lactide; however, this effect is reduced on changing the initiating group to phenoxide or on changing the ancillary ligand diamido backbone group.     

Homoleptic lanthanide complexes of chelating bis(phosphanyl)amides: synthesis,structure, and ring-opening polymerization of lactones

Treatment of the bis(phosphanyl)amide (Ph2P)2NH with KH in boiling THF followed by crystallization from THF/n-pentane leads to [K(thf)n][N(PPh2)2] (n = 1.25, 1.5). Reaction of [K(thf)n][N(PPh2)2] with anhydrous yttrium or lanthanide trichlorides in a 3:1 molar ratio afforded homoleptic bis(phosphanyl)amide complexes [Ln[N(PPh2)2]3] (Ln = Y, Er) as large crystals in good yields. [Ln[N(PPh2)2]3] can also be obtained by reaction of the homoleptic bis(trimethylsilyl)amides of Group 3 metals and lanthanides [Ln[N(SiMe3)2]3] (Ln = Y, La, Nd) with three equivalents of (Ph2P)2NH in boiling toluene. The single-crystal X-ray structures of these complexes always show eta 2 coordination of the ligand. Dynamic behavior of the ligand is observed in solution and is caused by rapid exchange of the two different phosphorus atoms. [Ln[N(PPh2)2]3] was used as catalyst for the polymerization of epsilon-caprolactone. Significant differences in terms of correlation of theoretical and experimental molecular weights as well as polydispersities were observed depending on the nature of Ln. On the basis of the crystal structure of the heteroleptic complex [Lu[N(PPh2)2]3(thf)], we suggest that in the initiation step of epsilon-caprolactone polymerization the lactone adds to the lanthanide atom to form a sevenfold coordination sphere around the central atom.     

Ytterbium amides of linked bis(amidinate): synthesis, molecular structures, and reactivity for the polymerization of L-lactide

The steric effect of an amide group on the synthesis, molecular structures and reactivity of ytterbium amides supported by linked bis(amidinate) L (L = [Me3SiNC(Ph)N(CH2)3NC(Ph)NSiMe3]) is reported. Reaction of LYbCl(THF)2 with equimolar NaNHAr' and NaNHAr (Ar' = 2,6-Me2C6H3; Ar = 2,6-iPr2C6H3), respectively, gave the corresponding monometallic amide complexes LYb(NHAr')(DME) 1 and LYb(NHAr)(DME) 2, in which the linked bis(amidinate) is coordinated to the metal center as a chelating ligand. The similar reaction with NaN(SiMe3)2 afforded a bimetallic amide complex (TMS)2NYb(L)2YbN(TMS)2 3 formed through the rearrangement reaction of L induced by the bulky N(SiMe3)2 group. In complex 3 the two linked bis(amidinate)s act as bridging ancillary ligands to link two YbN(TMS)2 species in one molecule. The definite molecular structures of 1-3 were provided by single-crystal X-ray analysis. Complexes 1-3 are efficient initiators for the polymerization of L-lactide, and their catalytic performance is highly dependent on the amido groups and molecular structures. The polymerizations initiated by complexes 1 and 2 proceeded in a living fashion as evidenced by the narrow polydispersities of the resulting polymers, together with the linear natures of the number average molecular weight versus conversion plots, while the polymerization system with complex 3 provided polymers with rather broad molecular weight distributions.     

Synthesis of highly isotactic poly(N,N‐diethylacrylamide) by anionic polymerization with grignard reagents and diethylzinc

N,N‐Dimethylacrylamide (DMA) and N,N‐diethylacrylamide (DEA) were polymerized with various Grignard reagents in tetrahydrofuran at −78 °C in the presence of diethylzinc (Et₂Zn). Highly isotactic poly(DEA) was produced in quantitative yield with tert‐butylmagnesium bromide and Et₂Zn, whereas atactic poly(DEA) was generated in the absence of Et₂Zn. No stereospecific polymerization of DMA proceeded with Grignard reagent in the presence of Et₂Zn. The highly isotactic poly(DEA) obtained was soluble in water and showed the characteristic coil–globule transition phenomenon. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 4677–4685, 2000     

A highly efficient stereoselective synthesis of (Z)- and (E)-allyl iodides from Baylis-Hillman adducts

A highly efficient synthesis of (Z)- and (E)-allyl iodides has been accomplished by treatment of Baylis-Hillman adducts with iodine and triphenylphosphine in methylene chloride at room temperature. The method is associated with mild reaction conditions, high yields and excellent stereoselectivity.     

Rheology of a polymer-based hybrid suspension composed of concentrated poly[(D,L-lactide)-co-glycolide] solution and inorganic salt particles

Suspensions composed of a solution of a biodegradable polyester in a volatile organic solvent, and solid inorganic particles of size in the order of hundreds of microns have been very important in the fabrication of porous scaffolds in the field of tissue engineering. This article reports the basic rheological investigations of this complex fluid type. A Couette geometry covered by silicon oil was found to be an appropriate geometry to retain stability of the rheological measurements. Suspension viscosity increased with the particle volume fraction, and the extent of the increase was much larger than that predicted by the Einstein suspension equation. Both start-up dynamics at the inception of steady shear and relaxation after an abrupt change of oscillatory shear frequency in the suspension showed significantly different behaviors from those in the associated polymer solution. Particle reorganization upon change of rheological state was anticipated.     

Aluminum and zinc complexes based on an amino-bis(pyrazolyl) ligand: synthesis, structures, and use in MMA and lactide polymerization

The coordination chemistry of bis[2-(3,5-dimethyl-1-pyrazolyl)ethyl]amine (1, LH) with aluminum- and zinc-alkyls has been studied. Reaction of 1 with AlR3 affords the adducts [LH] x AlR3 (R = Me, 2; Et, 3), which undergo alkane elimination upon heating to yield the amido complexes [L]AlR2 (R = Me, 4; Et, 5). Reaction of LiO(iPrO)C=CMe2 with 2 proceeds via N-H deprotonation to give Li[L]AlMe3 (6), while the former enolate adds to 4 to generate [Me2C=C(OiPr)OLi] x [L]AlMe2 (7). Similarly, the 1:1 reaction of ZnEt2 with 1 gives [LH] x ZnEt2 (9), which is transformed into [L]ZnEt (10) upon heating. When an excess of ZnEt2 was used in the latter reaction, the bimetallic complex [L]ZnEt x ZnEt2 (11) was isolated beside 10. Performing the same reaction in the presence of O2 traces yielded selectively the dinuclear ethyl-ethoxide complex [L]Zn2Et2(mu-OEt) (12), which was alternatively prepared from the reaction of 10 and ZnEt(OEt). Zinc chloride complexes [LH] x ZnRCl (R = Et, 13; p-CH3C6H4CH2, 14) and [L]ZnCl (15) were prepared in high yields following similar strategies. Ethyl abstraction from 10 with B(C6F5)3 yields [L]Zn+EtB(C6F5)3- (16). All complexes have been characterized by multinuclear nuclear magnetic resonance (NMR), elemental analysis, and single-crystal X-ray diffraction studies for four-coordinate Al complexes 2, 4, and 6 and Zn complexes 9-12 and 14. Aluminate species 6 and 7 initiate the polymerization of methyl methacrylate, and the monomer conversions are improved in the presence of neutral complexes 2 or 4, respectively; however, these methyl methacrylate (MMA) polymerizations are uncontrolled. Polymerization of rac-lactide takes place at 20 degrees C in the presence of zinc ethoxide complex 12 to yield atactic polymers with controlled molecular masses and relatively narrow polydispersities.     

Ligand self-recognition in the stereoselective assembly of [2 + 2] metallomacrocycles from racemic chiral bisbipyridyl molecular clefts and zinc(II)

The synthesis of racemic and optically pure ligand L, in which two 6,6'-disubstituted bipyridines are connected by methyleneoxy linkers to the molecular cleft dibenzobicyclo[b,f][3.3.1]nona-5a,6a-diene-6,12-dione, is reported. In the presence of 2 equivalents of zinc(ii) trifluoromethansulfonate (+/-)- undergoes slow reversible coordination over 24 h to form a pair of enantiomeric [2 + 2] metallomacrocycles, [Zn2(+)L2](OTf)(4) and [Zn2(-)L2](OTf)(4) respectively, that contain either two (+)-L ligands or two (-)-L ligands. This assignment was confirmed by independent studies with either (+)-L or (-)-L which formed the same complexes but at a significantly faster rate (3 h), and circular dichroism spectra of [Zn2(+)L2](OTf)(4) and [Zn2(-)L2](OTf)(4) which gave signals of the same intensity with the opposite sign. Treatment of (+/-)-L or optically pure L with copper(I) showed rapid formation of a mixture of oligomers as well as the [2 + 2] metallomacrocycle. The complex Zn2L2(OTf)(4) exhibits slow exchange between two species on the NMR time scale at room temperature. The results are consistent with the formation of a library of metal complexes in which the zinc(ii) binds initially to the most accessible bipyridyl binding sites in (+/-)-. Equilibration over several hours results in self-recognition of enantiomeric ligands to form a pair of enantiomeric metallomacrocycles, which have been tentatively assigned as having the helical configuration. Slow exchange is attributed to the preference for both metal centres to adopt 6-coordinate geometries involving the linker oxygens, but are limited to exchanging 5-coordinate complexes due to the shape of the cleft and the short linker.     

Rare-earth metal complexes supported by 1,omega-dithiaalkanediyl-bridged bis(phenolato) ligands: synthesis, structure, and heteroselective ring-opening polymerization of rac-lactide

Monomeric yttrium and lutetium bis(phenolato) complexes [Ln(OSSO){N(SiHMe 2) 2}(THF)] (Ln = Y, Lu) were prepared from the reaction of silylamido complexes [Ln{N(SiHMe 2) 2} 3(THF) 2] with 1 equiv of tetradentate 1,omega-dithiaalkanediyl-bridged bis(phenol) (OSSO)H 2 1- 9 in moderate to high yields. In contrast to the rigid configuration of scandium analogues, the yttrium complexes 2b and 3b and the lutetium complex 3c that contain a C 2 bridge between the two sulfur donors of the ligand are symmetric in solution. The monomeric nature of these complexes was indicated by an X-ray diffraction study of the yttrium complex 6b. The yttrium center in 6b is coordinated to the tetradentate [OSSO]-type ligand, one silylamido group and one THF ligand with the two oxygen donors of the [OSSO]-type ligand located trans. Corresponding bis(phenolato) silylamido complexes of larger rare-earth metals could not be obtained from similar reactions: Reaction of [La{N(SiHMe 2) 2} 3(THF) 2] with 1,2-xylylene-linked bis(phenol) gave a dinuclear lanthanum complex 6d of the formula [La 2(OSSO) 3] with two inequivalent eight-coordinate metal centers. The yttrium and lutetium complexes efficiently initiated the ring-opening polymerization (ROP) of lactides in THF. The heteroselectivity during the ROP of rac-lactide was enhanced when the steric demand of the bis(phenolato) ligand was increased, either by extending the bridge length or by introducing bulky ortho-substituents in the phenoxy units. A C 3 bridge within the ligand backbone is essential to allow configurational interconversion of the active site between Lambda and Delta configuration during polymerization, allowing accommodation of both enantiomers of the monomer in an alternating fashion.     

A facile and highly stereoselective synthesis of (2E)-, (2E,4E)-unsaturated amides and related natural products

A facile synthesis of (2E)-, (2E, 4E)-unsaturated amides was achieved via arsonium bromides with high stereoselectivity. Its application to the synthesis of related natural products 4 and 5 is also reported.     

Shell-cross-linked vesicles synthesized from block copolymers of Poly(D,L-lactide) and Poly(N-isopropyl acrylamide) as thermoresponsive nanocontainers

A polylactide (D,L-PLA) macroRAFT agent was prepared by utilizing a hydroxyl-functional trithiocarbonate as a coinitiator for the ring-opening polymerization. The length of the resultant polymer was controlled by the concentration of the coinitiator leading to the formation of two PLA polymers with M(n) = 12500 g mol(-)(1) (PDI = 1.46) and M(n) = 20500 g mol(-)(1) (PDI = 1.38) each with omega-trithiocarbonate functionality. Chain extension of PLA via the RAFT (free radical) polymerization of N-isopropyl acrylamide (NIPAAm) resulted in the formation of amphiphilic block copolymers with the PNIPAAm block increasing in size with conversion. TEM measurements of the aggregates obtained by self-organization of the block copolymers in aqueous solutions indicated the formation of vesicles. The sizes of these aggregates were influenced by the ratio of both blocks and the molecular weight of each block. The lower critical solution temperature (LCST) of the block copolymer was largely unaffected by the size of each block. UV turbidity measurements indicated a higher LCST for the block copolymers than for the corresponding PNIPAAm homopolymers. Stabilization of the vesicles was attained by a cross-linking chain extension of the PNIPAAm block using hexamethylene diacrylate. As the trithiocarbonate group was located between the PLA and PNIPAAm blocks, the chain extension resulted in a cross-linked layer between the core and corona of the vesicles.     

A highly stereoselective synthesis of 3,4-dihydro-1(2H)-isoquinolinones and 8-oxoberbines from homophthalic anhydrides and azomethines

The interaction between 1,3-isochromanediones(homophthalic anhydrides) 1 and acyclic azomethines of types 2 and 3 and the cyclic 6,7-dimethoxy-3,4-dihydroisoquinoline 4 is investigated. The former lead in high yields to the (±)-trans-3-aryl-4-carboxy-3,4-dihydro-1(2H)-isoquinolinones 5, while the latter gives the (±)-cis-13-carboxy-8-oxoberbines 7. The relative configurations of compounds 5–8 as well as the preferred conformations of some compounds of type 5 and of all of type 6 are determined by chemical correlation and NMR.