Synthesis and solid-state structures of pyrazolylmethane complexes of the rare earths

In this paper, we report the first examples of trispyrazolylmethane complexes of rare earths. Reaction of LnCl3 with Tpm* (tris(3,5-dimethylpyrazolyl)methane) in THF or acetonitrile gives good yields of the [Ln(Tpm*)Cl3] (Ln = Y, Ce, Nd, Sm, Gd, Yb). Tpm* adducts of the lanthanide triflates [Ln(Tpm*)(OTf)3(THF)] (Ln = Y, Ho, Dy) may also be prepared. The X-ray crystal structures of [Y(Tpm*)Cl3], [Sm(Tpm*)Cl3(THF)], and [Ln(Tpm*)(OTf)3(THF)] (Ln = Y, Ho) are reported. The halide/triflate complexes may be used to prepare the aryloxide complexes [Ln(Tpm*)(OArMe2)3] (Ln = Y, Nd, Sm, Yb; ArMe2 = C6H3-2,6-(CH3)2), which are fluxional in solution as a result of interactions between the Tpm* and the aryloxide groups. The structures of the Nd and Sm complexes have been determined. Finally, the reaction of [Nd(BH4)3(THF)3] with Tpm* in THF results in the displacement of two THF molecules to give [Nd(Tpm*)(BH4)3(THF)]. Infrared spectra are consistent with tridentate borohydride coordination. The X-ray structures of these compounds indicate that the Tpm* ligand is less strongly bound than its anionic trispyrazolylborate analogues.     

Stereoblock copolymers and tacticity control in controlled/living radical polymerization

Three controlled/living radical polymerization processes, atom transfer radical polymerization (ATRP), reversible addition-fragmentation transfer (RAFT) polymerization, and nitroxide-mediated polymerization (NMP), were investigated for the polymerization of N,N-dimethylacrylamide in the presence of Lewis acids known to enhance isotacticity, such as yttrium trifluoromethanesulfonate (Y(OTf)(3)) and ytterbium trifluoromethanesulfonate (Yb(OTf)(3)). Poly(N,N-dimethylacrylamide) with controlled molecular weight, low polydispersity (M(w)/M(n) < 1.2), and a high proportion of meso dyads ( approximately 85%) was prepared by ATRP (with initiating system methyl 2-chloropropionate/CuCl/Me(6)TREN) and RAFT (with cumyl dithiobenzoate transfer agent) in the presence of Y(OTf)(3). The combination of NMP (using N-tert-butyl-1-diethylphosphono-2,2-dimethylpropyl nitroxide, SG1) and a Lewis acid complexation technique led to less precise control over chain architecture and microstructure ( approximately 65% meso dyads), as compared to RAFT/Y(OTf)(3) or ATRP/Y(OTf)(3). The latter two systems were used for the first one-pot synthesis of stereoblock copolymers by radical polymerization. Well-defined stereoblock copolymers, atactic-b-isotactic poly(N,N-dimethylacrylamides), were obtained by adding Y(OTf)(3) at a given time to either RAFT or ATRP polymerizations, initially started without the presence of the Lewis acid.     

C-H activation on a diphosphine and hydrido-bridged diiridium complex: generation and detection of an active IrII-IrII species [(Cp*Ir)2(micro-dmpm)(micro-H)]+

Reaction of [Cp*Ir(micro-H)](2) (5) (Cp* = eta(5)-C(5)Me(5)) with bis(dimethylphosphino)methane (dmpm) gives a new neutral diiridium complex [(Cp*Ir)(2)(micro-dmpm)(micro-H)(2)] (3). Treatment of 3 with methyl triflate at -30 degrees C results in the formation of [(Cp*Ir)(H)(micro-dmpm)(micro-H)(Me)(IrCp*)][OTf] (6). Warming a solution of above 0 degrees C brings about predominant generation of 32e(-) Ir(II)-Ir(II) species [(Cp*Ir)(micro-dmpm)(micro-H)(IrCp*)][OTf] (7). Further heating of the solution of 7 up to 30 degrees C for 14 h leads to quantitative formation of a new complex [(Cp*Ir)(H)(micro-Me(2)PCH(2)PMeCH(2))(micro-H)(IrCp*)][OTf] (8), which is formed by intramolecular oxidative addition of the methyl C-H bond of the dmpm ligand. Intermolecular C-H bond activation reactions with 7 are also examined. Reactions of 7 with aromatic molecules (benzene, toluene, furan, and pyridine) at room temperature result in the smooth sp(2) C-H activation to give [(Cp*Ir)(H)(micro-dmpm)(micro-H)(Ar)(IrCp*)][OTf] (Ar = Ph (9); Ar = m-Tol (10a) or p-Tol (10b); Ar = 2-Fur (11)) and [(Cp*Ir)(H)(micro-dmpm)(micro-C(5)H(4)N)(H)(IrCp*)][OTf] (12), respectively. Complex also reacts with cyclopentene at 0 degrees C to give [(Cp*Ir)(H)(micro-dmpm)(micro-H)(1-cyclopentenyl)(IrCp*)][OTf] (13). Structures of 3, 8 and 12 have been confirmed by X-ray analysis.     

Ruthenium-mediated cycloaromatization of acyclic enediynes and dienynes at ambient temperature

The ruthenium(II) cation, [Cp*Ru(NCMe)3]OTf (4), triggers the Bergman cycloaromatization of acyclic endiynes at room temperature in THF solvent. Treatment of 1,2-di(1-alkynynyl)cyclopentenes (13-Me, alkynyl = propynyl; 13-Prn, alkynyl = pentynyl; 13-Bui, alkynyl = 4-methyl-pent-1-ynyl) with 4 in THF solvent at room temperature gives rise to the ruthenium arene complexes: [Cp*Ru{(3a,4,5,6,7,7a-eta)-2,3-dihydro-5,6-dialkyl-1H-indene}]OTf (15-Me, alkyl = methyl, 64% yield; 15-Prn, alkyl = n-propyl, 73% yield; 15-Bui, alkyl = 4-methyl-1-pentynyl, 88% yield). In a similar fashion, the room-temperature reaction of 4 with 1-ethynyl-2-(1-propynyl)cyclopentene (11) and [2-(1-propynyl)-1-cyclopenten-1-yl]trimethylsilane (14) leads to the formation of [Cp*Ru{(3a,4,5,6,7,7a-eta)-2,3-dihydro-5-methyl-1H-indene}]OTf (12, 92% yield) and [Cp*Ru{(3a,4,5,6,7,7a-eta)-2,3-dihydro-6-methyl-1H-inden-5-yl)trimethylsilane}]OTf (16, 77% yield), respectively. The bis(TMS)-substituted enediyne (1-cyclopentene-1,2-diyldi-2,1-ethynediyl)bis(trimethylsilane) (9-TMS) and 4 underwent reaction at 100 degrees C to give [Cp*Ru{(3a,4,5,6,7,7a-eta)-2,3-dihydro-1H-inden-5-yl)trimethylsilane}]OTf (10, 69% yield). Deuterium-labeling studies rule out a mechanism that involves a ruthenium-vinylidene intermediate, and provide support for the involvement of a p-benzyne intermediate. In a similar fashion, complex 4 is shown to trigger the cycloaromatization of the conjugated dienyne, 1-ethenyl-2-(1-pentynyl)cyclopentene (19), at room temperature in chloroform-d1 solvent to give [Cp*Ru{(3a,4,5,6,7,7a-eta)-2,3-dihydro-5-(1-propyl)-1H-indene}]OTf (20, 96% yield), with no deuterium enrichment. In the absence of ruthenium the thermal cyclization reactions of unsubstituted acyclic enediynes (Bergman cycloaromatization) and acyclic conjugated dienynes (Hopf cyclization) typically require elevated temperatures (150-250 degrees C). Complexes 10 and 15-Prn were characterized structurally by X-ray crystallography.     

The chemoselective reactions of tyrosine-containing G-protein-coupled receptor peptides with [Cp*Rh(H2O)3](OTf)2, including 2D NMR structures and the biological consequences

The bioconjugation of organometallic complexes with peptides has proven to be a novel approach for drug discovery. We report the facile and chemoselective reaction of tyrosine-containing G-protein-coupled receptor (GPCR) peptides with [Cp*Rh(H(2)O)(3)](OTf)(2), in water, at room temperature, and at pH 5-6. We have focused on three important GPCR peptides; namely, [Tyr(1)]-leu-enkephalin, [Tyr(4)]-neurotensin(8-13), and [Tyr(3)]-octreotide, each of which has a different position for the tyrosine residue, together with competing functionalities. Importantly, all other functional groups present, i.e., amino, carboxyl, disulfide, phenyl, and indole, were not prominent sites of reactivity by the Cp*Rh tris aqua complex. Furthermore, the influence of the Cp*Rh moiety on the structure of [Tyr(3)]-octreotide was characterized by 2D NMR, resulting in the first representative structure of an organometallic-peptide complex. The biological consequences of these Cp*Rh-peptide complexes, with respect to GPCR binding and growth inhibition of MCF7 and HT29 cancer cells, will be presented for [(η(6)-Cp*Rh-Tyr(1))-leu-enkephalin](OTf)(2) and [(η(6)-Cp*Rh-Tyr(3))-octreotide](OTf)(2).     

Evidence for Cu-O2 intermediates in superoxide oxidations by biomimetic copper(II) complexes

The mechanism by which [Cu(II)(L)](OTf)2 and [Cu(II)N3(L)](OTf) (L = TEPA: tris(2-pyridylethyl)amine or TMPA: tris(2-pyridylmethyl)amine; OTf = trifluoromethanesulfonate) react with superoxide (O2*-) to form [Cu(I)(L)](OTf) and O2 is described. Evidence for a CuO2 intermediate is presented based on stopped-flow experiments and competitive oxygen (18O) kinetic isotope effects on the bimolecular reactions of (16,16)O2*- and (18,16)O2*- ((16,16)k/(18,16)k). The (16,16)k/(18,16)k fall within a narrow range from 0.9836 +/- 0.0043 to 0.9886 +/- 0.0078 for reactions of copper(II) complexes with different coordination geometries and redox potentials that span a 0.67 V range. The results are inconsistent with a mechanism that involves either rate-determining O2*- binding or one-step electron transfer. Rather a mechanism involving formation of a CuO2 intermediate prior to the loss of O2 in the rate-determining step is proposed. Calculations of similar inverse isotope effects, using stretching frequencies of CuO2 adducts generated from copper(I) complexes and O2, suggest that the intermediate has a superoxo structure. The use of 18O isotope effects to relate activated oxygen intermediates in enzymes to those derived from inorganic compounds is discussed.     

Multiple C-H bond activation in group 3 chemistry: synthesis and structural characterization of an yttrium-aluminum-methine cluster

Complete donor-induced alkylaluminate cleavage of halfmetallocene complex Cp*Y(AlMe4)2, that is, treatment of Cp*Y(AlMe4)2 with 2 equiv of diethyl ether, produces [Cp*Y(mu2-Me)2]3 in high yield (95%). In contrast, the equimolar reaction of Cp*Y(AlMe4)2 with diethyl ether reproducibly formed complex [Cp*4Y4(mu2-CH3)2{(CH3)Al(mu2-CH3)2}4(mu4-CH)2] in low yield (10-30%) via a multiple C-H bond activation. The synthesis of the heterooctametallic yttrium-aluminum-methine cluster was also accomplished in moderate yield (47%) by the equimolar reaction of discrete Cp*Y(AlMe4)2 and [Cp*Y(mu2-Me)2]3 in the absence of any donor solvent and "free" AlMe3. This gives strong evidence that preformed heterometal-bridged Y-CH3-Al moieties are prone to multiple hydrogen abstraction in the presence of a highly basic reagent such as [Cp*Y(mu2-Me)2]3. The monocylopentadienyl complexes [Cp*Y(mu2-Me)2]3 and [Cp*4Y4(mu2-CH3)2{(CH3)Al(mu2-CH3)2}4(mu4-CH)2] were structurally characterized.     

A highly diastereoselective decarboxylative mannich reaction of β-keto acids with optically active N-sulfinyl α-imino esters

A range of protected γ-oxo-α-amino esters have been prepared in a highly regio- and stereoselective manner through the decarboxylative Mannich reaction of β-keto acids with optically active N-tert-butanesulfinyl α-imino esters in the presence of 3 mol % La(OTf)(3) or 5 mol % Y(OTf)(3) at 20 °C. Preliminary mechanistic studies indicate that the reaction proceeds through imine addition followed by decarboxylation.     

Single-molecule magnets constructed from cyanometalates: {[Tp*Fe(III)(CN)3M(II)(DMF)4]2[OTf]2} x 2DMF (M(II) = Co, Ni)

Treatment of several divalent transition-metal trifluoromethanesulfonates [M(II)(OTf)2; M(II) = Mn, Co, Ni] with [NEt4][Tp*Fe(III)(CN)3] [Tp* = hydridotris(3,5-dimethylpyrazol-1-yl)borate] in DMF affords three isostructural rectangular clusters of {[Tp*Fe(III)(CN)3M(II)(DMF)4]2[OTf]2} x 2DMF (M(II) = Mn, 3; Co, 4; Ni, 5) stoichiometry. Magnetic studies of 3-5 indicate that the Tp*Fe(CN)3(-) centers are highly anisotropic and exhibit antiferromagnetic (3 and 4) and ferromagnetic (5) exchange to afford S = 4, 2, and 3 spin ground states, respectively. ac susceptibility measurements suggest that 4 and 5 exhibit incipient single-molecule magnetic behavior below 2 K.     

Stereoregulation of thermoresponsive polymer brushes by surface-initiated living radical polymerization and the effect of tacticity on surface wettability

In this study, stereocontrolled poly(N-isopropylacrylamide) (PIPAAm) brushes were grafted from surfaces by atom transfer radical polymerization (ATRP) in the presence of a Lewis acid, and the effect of PIPAAm brush tacticity on the thermoresponsive wettabiliy was investigated. PIPAAm grafted by ATRP in the presence of Y(OTf)(3) showed high isotacticity, while the control brush polymerized in the absence of Y(OTf)(3) was clearly atactic. The isotacticity and molecular weight of PIPAAm brushes were controlled by polymerization conditions. The wettability of isotactic PIPAAm-grafted surfaces decreased slightly below 10 °C, although the phase transition temperature of atactic surface was 30 °C, and the bulk isotactic polymer was water-insoluble between 5 and 45 °C.     

光学活性N-邻噁唑啉苯基甲基丙烯酰胺聚合物的合成及其手性光学性质

在三氟甲磺酸稀土盐(Ln(OTf)3,Ln=La,Nd,Sm,Y)的存在下,光学纯N-邻唑啉苯基甲基丙烯酰胺((S)-MeOPMAM)经自由基聚合反应得到相应的光学活性聚合物.考察了稀土盐种类、用量及溶剂性质等因素对聚合反应立体化学的影响.研究发现,以Y(OTf)3为调节剂、正丁醇为溶剂的体系能在一定程度上提高聚合反应的全同立体定向性.聚合物的手性光学性质明显依赖于立构规整度,随全同含量增大,聚合物的比旋光度和π-π*电子跃迁区域的Cotton效应强度呈下降趋势.利用1H-NMR技术研究了上述聚合物与1,1′-联-2-萘酚(BINOL)的对映选择性相互作用,结果表明,全同三元组含量较高的聚合物不仅使酚羟基质子峰向低场位移,而且导致信号分裂.     

Stereocontrol in radical polymerization of acrylic monomers

A clear effect of Lewis acids, such. as scandium trifluoromethanesulfonate [Sc(OTf)₃], on stereocontrol during the radical polymerization of a designed monomer, benzyl α-(methoxymethyl)acrylate was found. This Lewis acid also influenced the stereochemistry in the radical polymerization of methyl methacrylate giving a less syndiotactic and more isotactic polymer, although many Lewis acids were not effective. A catalytic amount of Lewis acids, such as Y(OTf)₃ and Yb(OTf)₃, also significantly enhanced isotactic-specificity during the radical polymerization of acrylamide and its derivatives, N-isopropylacrylamide (NIPAM) and N,N-dimethylacrylamide. Obvious solvent and temperature effects on tacticity were observed in these polymerizations, and poly(NIPAM) with >80% triad isotactic content has been obtained in the presence of Lewis acids.     

Stereocontrol using Lewis acids in radical polymerization

The radical polymerization of various (meth)acrylamides in the presence of Lewis acids such as Yb(OTf)₃ and Y(OTf)₃ was carried out. The polymerization with Lewis acids led to highly isotactic polymers, while the polymers synthesized without Lewis acids were atactic or syndiotactic. The dependence of the polymer properties on the tacticity was also demonstrated.     

Hypervalent, low-coordinate phosphorus(III) centers in complexes of the phosphadiazonium cation with chelate ligands

Trifluoromethylsulfonyloxy-(2,4,6-tri-tert-butylphenylimino)phosphine, Mes*NPOTf (Mes = 2,4,6-tri-tert-butylphenyl, OTf = trifluoromethanesulfonate, triflate) reacts quantitatively with the multifunctional ligands 2,2'-bipyridine (2,2'-BIPY), N,N,N',N'-tetramethylethylenediamine (TMEDA), 1,2-bis(diethylphosphino)ethane (DEPE), 1,2-bis(diphenylphosphino)ethane (DIPHOS), and N,N,N',N' ',N' '-pentamethyldiethylenetriamine (PMDETA) to give the Lewis acid-base complexes [Mes*NP(2,2'-BIPY)][OTf], [Mes*NP(TMEDA)][OTf], [Mes*NP(DIPHOS)][OTf], [Mes*NP(DEPE)][OTf], and [Mes*NP(PMDETA)][OTf], respectively. Single-crystal X-ray diffraction studies indicate that the closest contact of the ligand donor atoms occurs at phosphorus in all cases, affecting significant displacement of the OTf anion. The resulting cations [Mes*NP(L)]+ are best described as complexes of a neutral chelating ligand on a phosphadiazonium Lewis acceptor, and highlight the potential for electron-rich centers to behave as Lewis acids despite the presence of a lone pair of electrons at the acceptor site. More importantly, the new complexes represent rare examples of systems containing hypervalent, low-coordinate phosphorus(III) centers.     

Stereocontrol Study in Radical Polymerization of 2-Acrylamido-2-methylpropane Sulfonic Acid in the Presence of Lewis Acids

Tacticity of poly-2-acrylamido-2-methylpropane sulfonic acid samples that were prepared via free radical polymerization in four different solvents, ethanol, dimethyl sulphoxide (DMSO), N, N-dimethylformamide (DMF) and water, were studied by ¹³C nuclear magnetic resonance (¹³C-NMR) techniques. In order to change isotacticity, two rare metal triflates, yttrium trifluoromethane sulfonate (Y(OTf)₃) and ytterbium trifluoromethane sulfonate (Yb(OTf)₃), were used. The stereoregularity of the obtained polymers in DMSO, water and DMF in the presence of the mentioned triflates did not show a noticeable alteration. But isotacticity of the samples prepared in ethanol was increased by about 17% in the presence 75 wt% Y(OTf)₃. In the absence of metal triflates, solvents did not have any significant effect on stereocontrol. In addition, the influence of temperature on isotacticity of the prepared samples was studied. By decreasing the reaction temperature, the isotacticity increased slightly.     

Lewis acid catalyzed benzylic C-H bond functionalization of azaarenes: addition to enones

A Lewis acid catalyzed benzylic C-H bond functionalization of alkyl-substituted azaarenes is described. Sc(OTf)(3) and Y(OTf)(3) promoted the direct addition of alkyl-substituted azaarenes and benzoxazole to enones and an α,β-unsaturated N-acylpyrrole. Products were obtained in 60-96% yield.     

New cationic and zwitterionic Cp*M(kappa2-P,S) complexes (M = Rh, Ir): divergent reactivity pathways arising from alternative modes of ancillary ligand participation in substrate activation

Treatment of 0.5 equiv of [Cp*IrCl(2)](2) with 1/3-P(i)Pr(2)-2-S(t)Bu-indene afforded Cp*Ir(Cl)(kappa(2)-3-P(i)Pr(2)-2-S-indene) (1) in 95% yield (Cp* = eta(5)-C(5)Me(5)). Addition of AgOTf or LiB(C(6)F(5))(4) x 2.5 OEt(2) to 1 gave [Cp*Ir(kappa(2)-3-P(i)Pr(2)-2-S-indene)](+)X(-) ([2](+)X(-); X = OTf, 78%; X = B(C(6)F(5))(4), 82%), which represent the first examples of isolable coordinatively unsaturated [Cp'Ir(kappa(2)-P,S)](+)X(-) complexes. Exposure of [2](+)OTf(-) to CO afforded [2 x CO](+)OTf(-) in 91% yield, while treatment of [2](+)B(C(6)F(5))(4)(-) with PMe(3) generated [2 x PMe(3)](+)B(C(6)F(5))(4)(-) in 94% yield. Treatment of 1 with K(2)CO(3) in CH(3)CN allowed for the isolation of the unusual adduct 3 x CH(3)CN (41% isolated yield), in which the CH(3)CN bridges the Lewis acidic Cp*Ir and Lewis basic indenide fragments of the targeted coordinatively unsaturated zwitterion Cp*Ir(kappa(2)-3-P(i)Pr(2)-2-S-indenide) (3). In contrast to the formation of [2 x CO](+)OTf(-), exposure of 3 x CH(3)CN to CO did not afford 3 x CO; instead, a clean 1:1 mixture of (kappa(2)-3-P(i)Pr(2)-2-S-indene)Ir(CO)(2) (4) and 1,2,3,4-tetramethylfulvene was generated. Treatment of [2](+)OTf(-) with Ph(2)SiH(2) resulted in the net loss of Ph(2)Si(OTf)H to give Cp*Ir(H)(kappa(2)-3-P(i)Pr(2)-2-S-indene) (5) in 44% yield. In contrast, treatment of [2](+)B(C(6)F(5))(4)(-) with Ph(2)SiH(2) or PhSiH(3) proceeded via H-Si addition across Ir-S to give the corresponding [Cp*Ir(H)(kappa(2)-3-P(i)Pr(2)-2-S(SiHPhX)-indene)](+)B(C(6)F(5))(4)(-) complexes 6a (X = Ph, 68%) or 6b (X = H, 77%), which feature a newly established S-Si linkage. Compound 6a was observed to effect net C-O bond cleavage in diethyl ether with net loss of Ph(2)Si(OEt)H, affording [Cp*Ir(H)(kappa(2)-3-P(i)Pr(2)-2-SEt-indene)](+)B(C(6)F(5))(4)(-) (7) in 77% yield. Furthermore, 6a proved capable of transferring Ph(2)SiH(2) to acetophenone, with concomitant regeneration of [2](+)B(C(6)F(5))(4)(-); however, [2](+)X(-) did not prove to be effective ketone hydrosilylation catalysts. Treatment of 1/3-P(i)Pr(2)-2-S(t)Bu-indene with 0.5 equiv of [Cp*RhCl(2)](2) gave Cp*Rh(Cl)(kappa(2)-3-P(i)Pr(2)-2-S-indene) (8) in 94% yield. Combination of 8 and LiB(C(6)F(5))(4) x 2.5 Et(2)O produced the coordinatively unsaturated cation [Cp*Rh(kappa(2)-3-P(i)Pr(2)-2-S-indene)](+)B(C(6)F(5))(4)(-) ([9](+)B(C(6)F(5))(4)(-)), which was transformed into [Cp*Rh(H)(kappa(2)-3-P(i)Pr(2)-2-S(SiHPh(2))-indene)](+)B(C(6)F(5))(4)(-) (10) via net H-Si addition of Ph(2)SiH(2) to Rh-S. Unlike [2](+)X(-), complex [9](+)B(C(6)F(5))(4)(-) was shown to be an effective catalyst for ketone hydrosilylation. Treatment of 3 x CH(3)CN with Ph(2)SiH(2) resulted in the loss of CH(3)CN, along with the formation of Cp*Ir(H)(kappa(2)-3-P(i)Pr(2)-2-S-(1-diphenylsilylindene)) (11) (64% isolated yield) as a mixture of diastereomers. The formation of 11 corresponds to heterolytic H-Si bond activation, involving net addition of H(-) and Ph(2)HSi(+) fragments to Ir and indenide in the unobserved zwitterion 3. Crystallographic data are provided for 1, [2 x CO](+)OTf(-), 3 x CH(3)CN, 7, and 11. Collectively, these results demonstrate the versatility of donor-functionalized indene ancillary ligands in allowing for the selection of divergent metal-ligand cooperativity pathways (simply by ancillary ligand deprotonation) in the activation of small molecule substrates.     

Iron‐catalyzed radical polymerization of acrylamides in the presence of Lewis acid for simultaneous control of molecular weight and tacticity

This work is directed to the stereospecific living radical polymerization of acrylamides such as N,N‐dimethylacrylamide and N‐isopropylacrylamide with an iron complex and a Lewis acid. DMAM was polymerized with [FeCp(CO)₂]₂ in conjunction with an alkyl iodide [(CH₃)₂C(CO₂Et)I] as an initiator in the presence of Y(OTf)₃ in toluene/methanol (1/1) at 60 °C to be converted almost quantitatively to the polymers with controlled molecular weights and high isotacticity (m > 80%), wherein the Fe‐complex generates radical species from a covalent C? I bond of the dormant species and the Lewis acid controls the stereochemistry of the polymerization via coordination with the amide groups of the polymer terminal and the monomer. A series of Lewis acids were also used for the iron(I)‐catalyzed DMAM polymerization, and Yb(OTf)₃ and Yb(NTf₂)₃ proved effective in giving isotactic polymers without deteriorating the molecular weight control similar to Y(OTf)₃. Furthermore, a slight enhancement of isospecificity was observed for the iron‐catalyzed system in comparison with the α,α‐Azobisisobutyronitrile‐initiated, when coupled with Y(OTf)₃. Stereoblock polymerization of DMAM via a one‐pot reaction was also achieved by just adding the Y(OTf)₃ methanol solution in the course of the polymerization to give atactic‐b‐isotactic poly(DMAM). A similar but slightly lower control in the molecular weight and tacticity was achieved in the polymerization of NIPAM with [FeCp(CO)₂]₂/Y(OTf)₃. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 2086–2098, 2006     

Stereocontrol during photo-initiated controlled/living radical polymerization of acrylamide in the presence of Lewis acids

Poly(acrylamide) (PAM) with controlled molecular weight and tacticity was prepared by UV-irradiation-initiated controlled/living radical polymerization in the presence of dibenzyl trithiocarbonate (DBTTC) and Y(OTf)₃. The rapid and facile photo-initiated controlled/living polymerization at ambient temperature led to controlled molecular weight and narrow polydispersity (M_w/M_n = 1.12-1.24) of PAM. The coordination of Y(OTf)₃ with the last two amide groups in the growing chain radical effectively enhanced isotacticity of PAM. The isotactic sequence of dyads (m), triads (mm) and pentads (mmmm) in PAM were 70.32%, 50.95%, and 29.97%, respectively, which were determined by the resonance of methine (CH) groups in PAM under ¹³C NMR experiment. Factors affecting stereocontrol during the polymerization were studied, including the type of Lewis acids, concentration of Y(OTf)₃, and monomer conversion. It is intriguing that the meso tacticity increased gradually with chain propagation and quite higher isotacticity (m = 93.01%, mm = 86.57%) was obtained in the later polymerization stage (conversion 65-85%).     

Ligand-modification effects on the reactivity, solubility, and stability of organometallic tantalum complexes in water

Tantalum complexes [TaCp*Me{κ(4)-C,N,O,O-(OCH(2))(OCHC(CH(2)NMe(2))=CH)py}] (4) and [TaCp*Me{κ(4)-C,N,O,O-(OCH(2))(OCHC(CH(2)NH(2))=CH)py}] (5), which contain modified alkoxide pincer ligands, were synthesized from the reactions of [TaCp*Me{κ(3)-N,O,O-(OCH(2))(OCH)py}] (Cp* = η(5)-C(5)Me(5)) with HC≡CCH(2)NMe(2) and HC≡CCH(2)NH(2), respectively. The reactions of [TaCp*Me{κ(4)-C,N,O,O-(OCH(2))(OCHC(Ph)=CH)py}] (2) and [TaCp*Me{κ(4)-C,N,O,O-(OCH(2))(OCHC(SiMe(3))=CH)py}] (3) with triflic acid (1:2 molar ratio) rendered the corresponding bis-triflate derivatives [TaCp*(OTf)(2){κ(3)-N,O,O-(OCH(2))(OCHC(Ph)=CH(2))py}] (6) and [TaCp*(OTf)(2){κ(3)-N,O,O-(OCH(2))(OCHC(SiMe(3))=CH(2))py}] (7), respectively. Complex 4 reacted with triflic acid in a 1:2 molar ratio to selectively yield the water-soluble cationic complex [TaCp*(OTf){κ(4)-C,N,O,O-(OCH(2))(OCHC(CH(2)NHMe(2))=CH)py}]OTf (8). Compound 8 reacted with water to afford the hydrolyzed complex [TaCp*(OH)(H(2)O){κ(3)-N,O,O-(OCH(2))(OCHC(CH(2)NHMe(2))=CH(2))py}](OTf)(2) (9). Protonation of compound 8 with triflic acid gave the new tantalum compound [TaCp*(OTf){κ(4)-C,N,O,O-(OCH(2))(HOCHC(CH(2)NHMe(2))=CH)py}](OTf)(2) (10), which afforded the corresponding protonolysis derivative [TaCp*(OTf)(2){κ(3)-N,O,O-(OCH(2))(HOCHC(CH(2)NHMe(2))=CH(2))py}](OTf) (11) in solution. Complex 8 reacted with CNtBu and potassium 2-isocyanoacetate to give the corresponding iminoacyl derivatives 12 and 13, respectively. The molecular structures of complexes 5, 7, and 10 were established by single-crystal X-ray diffraction studies.