Antonio Otero

Ring‐Opening Polymerization of Cyclic Esters by an Enantiopure Heteroscorpionate Rare Earth Initiator

With a sting in its tail: An enantiopure neodymium complex (see scheme) acts as an efficient single-site initiator for the controlled ring-opening polymerization of rac-lactide, forming isotactic polyester. The heteroscorpionate complex was characterized spectroscopically and by X-ray diffraction.

Hybrid scorpionate/cyclopentadienyl magnesium and zinc complexes: synthesis, coordination chemistry, and ring-opening polymerization studies on cyclic esters.

The reaction of the hybrid scorpionate/cyclopentadienyl lithium salt [Li(bpzcp)(THF)] [bpzcp = 2,2-bis(3,5-dimethylpyrazol-1-yl)-1,1-diphenylethylcyclopentadienyl] with 1 equiv of RMgCl proceeds cleanly to give very high yields of the corresponding monoalkyl kappa(2)-NN-eta(5)-C(5)H(4) magnesium complexes [Mg(R)(kappa(2)-eta(5)-bpzcp)] (R = Me 1, Et 2, (n)Bu 3, (t)Bu 4, CH(2)SiMe(3) 5, CH(2)Ph 6). Hydrolysis of the hybrid lithium salt [Li(bpzcp)(THF)] with NH(4)Cl/H(2)O in ether cleanly affords the two previously described regioisomers: (bpzcpH) 1-[2,2-bis(3,5-dimethylpyrazol-1-yl)-1,1-diphenylethyl]-1,3-cyclopentadiene (a) and 2-[2,2-bis(3,5-dimethylpyrazol-1-yl)-1,1-diphenylethyl]-1,3-cyclopentadiene (b). Subsequent reaction of the bpzcpH hybrid ligand with ZnR(2) quantitatively yields the monoalkyl kappa(2)-NN-eta(1)(pi)-C(5)H(4) zinc complexes [Zn(R){kappa(2)-eta(1)(pi)-bpzcp}] (R = Me 7, Et 8, (t)Bu 9, CH(2)SiMe(3) 10). Additionally, magnesium alkyls 1, 2, 4, and 5 can act as excellent cyclopentadienyl and alkyl transfers to the zinc metal center and yield zinc alkyls 7-10 in good yields. The single-crystal X-ray structures of the derivatives 4, 5, 7, and 10 confirm a 4-coordinative structure with the metal center in a distorted tetrahedral geometry. Interestingly, whereas alkyl magnesium derivatives 4 and 5 present a eta(5) coordination mode for the cyclopentadienyl fragment, zinc derivatives 7 and 10 feature a peripheral eta(1)(pi) arrangement in the solid state. Furthermore, the reaction of the hybrid lithium salt [Li(bpzcp)(THF)] with 1 equiv of ZnCl(2) in tetrahydrofuran (THF) affords very high yields of the chloride complex [ZnCl{kappa(2)-eta(1)(pi)-bpzcp}] (11). Compound 11 was used as a convenient starting material for the synthesis of the aromatic amide zinc compound [Zn(NH-4-MeC(6)H(4)){kappa(2)-eta(1)(pi)-bpzcp}] (12), by reaction with the corresponding aromatic primary amide lithium salt. Alternatively, aliphatic amide and alkoxide derivatives were only accessible by protonolysis of the bis(amide) complexes [M{N(SiMe(3))(2)}(2)] (M = Mg, Zn) and the mixed ligand complex [EtZnOAr)] with the hybrid ligand bpzcpH to afford [Zn(R){kappa(2)-eta(1)(pi)-bpzcp}] (R = N(SiMe(3))(2) 13, R = 2,4,6-Me(3)C(6)H(2)O 14) and [Mg{N(SiMe(3))(2)}(kappa(2)-eta(5)-bpzcp)] (15). Finally, alkyl and alkoxide-containing complexes 1-10 and 14 can act as highly effective single-component living initiators for the ring-opening polymerization of epsilon-caprolactone and lactides over a wide range of temperatures. Epsilon-caprolactone is polymerized within minutes to give high molecular weight polymers with medium-broad polydispersities (M(n) > 10(5), M(w)/M(n) = 1.45). Lactide afforded poly(lactide) materials with medium molecular weights and polydispersities as narrow as M(w)/M(n) = 1.02. Additionally, polymerization of L-lactide occurred without racemization in the propagation process and offered highly crystalline, isotactic poly(L-lactides) with very high melting temperatures (T(m) = 165 degrees C). Microstructural analysis of poly(rac-lactide) by (1)H NMR spectroscopy revealed that propagations occur without appreciable levels of stereoselectivity. Polymer end group analysis showed that the polymerization process is initiated by alkyl transfer to the monomer.

Synthesis of Cyclic Carbonates Catalysed by Aluminium Heteroscorpionate Complexes.

New aluminium scorpionate based complexes have been prepared and used for the synthesis of cyclic carbonates from epoxides and carbon dioxide. Bimetallic aluminium(heteroscorpionate) complexes 9-14 were synthesised in very high yields. The single-crystal X-ray structures of 12 and 13 confirm an asymmetric κ(2)-NO-μ-O arrangement in a dinuclear molecular disposition. These bimetallic aluminium complexes were investigated as catalysts for the synthesis of cyclic carbonates from epoxides and carbon dioxide in the presence of ammonium salts. Under the optimal reaction conditions, complex 9 in combination with tetrabutylammonium bromide acts as a very efficient catalyst system for the conversion of both monosubstituted and internal epoxides into the corresponding cyclic carbonates showing broad substrate scope. Complex 9 and tetrabutylammonium bromide is the second most efficient aluminium-based catalyst system for the reaction of internal epoxides with carbon dioxide. A kinetic study has been carried out and showed that the reactions were first order in complex 9 and tetrabutylammonium bromide concentrations. Based on the kinetic study, a catalytic cycle is proposed.

Synthesis of cyclic carbonates using monometallic, and helical bimetallic, aluminium complexes

The aluminium complexes of a series of bis(pyrazol-1-yl)methane derived ligands were investigated as catalysts for the synthesis of cyclic carbonates from carbon dioxide and epoxides. A bimetallic, helical, heteroscorpionate complex displayed significantly higher catalytic activity than the corresponding monometallic complexes. The optimal reaction conditions were found to involve the use of 5 mol% of the bimetallic complex and 5 mol% of tetrabutylammonium bromide under solvent free conditions in the presence of 10 bar of wet carbon dioxide. Under these conditions, eleven monosubstituted epoxides were converted into the corresponding cyclic carbonates in 51–91% yield. A kinetic study showed that the reactions were first order in epoxide, catalyst and tetrabutylammonium bromide and allowed a catalytic cycle to be proposed.

Synthesis of cyclic carbonates catalysed by aluminium heteroscorpionate complexes

Parallel catalyst screening was used to develop new aluminium scorpionate based catalysts for the synthesis of cyclic carbonates from epoxides and carbon dioxide. Nineteen complexes were included in the catalyst screening, which resulted in the development of trimetallic complex 27 which had the same catalytic activity at one bar carbon dioxide pressure that the initial lead complex (8) had at ten bar carbon dioxide pressure. The combination of complex 27 and tetrabutylammonium bromide could be used to form cyclic carbonates from a range of terminal epoxides and kinetic studies showed that the reactions were first order in epoxide, complex 27 and tetrabutylammonium bromide concentrations. Based on this data a catalytic cycle has been proposed which accounts for all of features of the catalyst system.

Heteroscorpionate magnesium alkyls bearing unprecedented apical σ-C(sp3)-mg bonds: heteroselective ring-opening polymerization of rac-lactide.

The previously described reaction of the low sterically hindered heteroscorpionate lithium acetamidinates [Li(κ(3)-pbpamd)(THF)] and [Li(κ(3)-tbpamd)(THF)] with a series of commercially available Grignard reagents RMgCl in an equimolecular ratio yielded the magnesium monoalkyls [Mg(R)(κ(3)-NNN)] (NNN = pbpamd, R = CH2SiMe3, Et (1), Bn (2); NNN = tbpamd, R = CH2SiMe3, Et (3), Bn (4)). However, subsequent reaction of these monoalkyls [Mg(R)(κ(3)-NNN)] with two additional equivalents of the same RMgCl in tetrahydrofuran gave rise to dinuclear dialkyls of the type [RMg(κ(3)-N,N,N;κ(2)-C,N)MgR(thf)] (κ(3)-N,N,N;κ(2)-C,N = pbpamd(-), R = CH2SiMe3 (5), Et (6); κ(3)-N,N,N;κ(2)-C,N = tbpamd(-), R = CH2SiMe3 (7), Et (8)). Furthermore, when the reaction was carried out in a mixture of tetrahydrofuran/dioxane with the same stoichiometry, a new family of tetranuclear tetraalkyl magnesium complexes [{RMg(κ(3)-N,N,N;κ(2)-C,N)MgR}2{μ-O,O-(C4H8)}] (κ(3)-N,N,N;κ(2)-C,N = pbpamd(-), R = CH2SiMe3 (9), Et (10), Bn (11); κ(3)-N,N,N;κ(2)-C,N = tbpamd(-), R = CH2SiMe3 (12), Et (13), Bn (14)) was obtained. In both families, an apical methine C-H activation process on the heteroscorpionate takes place. The single-crystal X-ray structures of 4, 8, 9, and 12 confirm the nuclearity of each family, with 4-coordinative arrangements for all magnesium atoms. More importantly, the presence in the di- and tetranuclear complexes of unprecedented apical carbanions with a direct σ-C(sp(3))-Mg covalent bond, and as a result, the existence of stereogenic magnesium centers, have been unambiguously confirmed. Interestingly, the dinuclear dialkyls 5 and 7, as well as the tetranuclear tetraalkyls 9, 10, and 12, can act as highly efficient single-component living initiators for the ring-opening polymerization of ε-caprolactone and lactides. Lactide (LA) polymerizations afforded polylactide (PLA) materials with medium molecular weights in only a few minutes even at 20 °C for L-LA and in a few hours at 50 °C for rac-LA propagations. More importantly, microstructural analysis of the poly(rac-lactide) materials revealed that the tetranuclear tetra-alkyl 12 exerts enhanced levels of heteroselectivity on the PLAs under mild conditions, with Ps values up to 0.78.

Oxo- and imido-alkoxide vanadium complexes as precatalysts for the guanylation of aromatic amines

Vanadium oxo-alkoxide complexes [V(O)(Cl)(3-x)(OR)x] (x = 1, R = Me (1), Et (2), n-Pr (3); x = 2, R = Me (4), Et (5), n-Pr (6)) were obtained by a clean reaction between [V(O)Cl3] and different excesses of ROSiMe3 silicon ethers. Imido complexes [V(NAr)(Cl)(3-x)(OR)x]2 (Ar = 2,6-i-Pr2C6H3; x = 1, R = Me (7), Et (8), n-Pr (9); x = 2, R = Me (10), Et (11), n-Pr (12)) were prepared in excellent yields by reaction of the oxo-alkoxide precursors and ArNCO. X-Ray crystal structure determinations for 7, 8 and 12 revealed dimeric structures with alkoxide bridges. Complexes 7-12 are precursor of highly active guanylation catalysts. The study of the catalytic reaction allow us to propose the non-participation of the imido moiety. This was confirmed when complexes 1-6 gave better catalysts under the same conditions. The simple and commercial [V(O)Cl3] (16) can be proposed as the most effective catalyst in this series of vanadium complexes.

Synthesis, Characterization and Dynamic Behavior of Mono- and Dinuclear Palladium(II) Carbene Complexes Derived From 1,1′-Methylenebis(4-alkyl-1,2,4-triazolium) Diiodides

Mononuclear palladium carbene complexes 2a–c and 3 derived from 1,1′-methylenebis(4-alkyl-4,5-dihydro-1H-1,2,4-triazole-5-ylidene) have been obtained by the reaction of 1,1′-methylenebis(4-alkyl-1,2,4-triazolium) diiodides 1a–d with palladium acetate under mild conditions. The mononuclear complexes 2a–c have been transformed into their corresponding trans-binuclear complexes 4a–c. All compounds were characterized by spectroscopic techniques and the dynamic behavior of 2a–cand 4a–c has been studied. The X-ray structures of 2a and 4c are reported.

On the search for NNO-donor enantiopure scorpionate ligands and their coordination to group 4 metals.

The preparation of new chiral bis(pyrazol-1-yl)methane-based NNO-donor scorpionate ligands in the form of the lithium derivatives [Li(bpzb)(THF)] [1; bpzb = 1,1-bis(3,5-dimethylpyrazol-1-yl)-3,3-dimethyl-2-butoxide] and [Li(bpzte)(THF)] [2; bpzte = 2,2-bis(3,5-dimethylpyrazol-1-yl)-1-p-tolylethoxide] or the alcohol ligands (bpzbH) (3) and (bpzteH) (4) has been carried out by 1,2-addition reactions with trimethylacetaldehyde or p-tolualdehyde. The separation of a racemic mixture of the alcohol ligand 3 has been achieved and gave an enantiopure NNO alcohol-scorpionate ligand in three synthetic steps: (i) 1,2-addition of the appropriate lithium derivative to trimethylacetaldehyde, (ii) esterification and separation of diastereoisomers 5, (iii) saponification. Subsequently, the enantiopure scorpionate ligand (R,R)-bpzmmH {6; R,R-bpzmmH = (1R)-1-[(1R)-6,6-dimethylbicyclo[3.1.1]2-hepten-2-yl]-2,2-bis(3,5-dimethylpyrazol-1-yl)ethanol} was obtained with an excellent diastereomeric excess (>99% de) in a one-pot process utilizing the aldehyde (1R)-(-)-myrtenal as a chiral substrate to control the stereochemistry of the newly created asymmetric center. These new chiral heteroscorpionate ligands reacted with [MX(4)] (M = Ti, Zr; X = NMe(2), O(i)Pr, OEt, O(t)Bu) in a 1:1 molar ratio in toluene to give, after the appropriate workup, the complexes [MX(3)(kappa(3)-NNO)] (7-18). The reaction of Me(3)SiCl with [Ti(NMe(2))(3)(bpzb)] (7) or [Ti(NMe(2))(3)(R,R-bpzmm)] (11) in different molar ratios gave the halide-amide-containing complexes [TiCl(NMe(2))(2)(kappa(3)-NNO)] (19 and 20) and [TiCl(2)(NMe(2))(kappa(3)-NNO)] (21 and 22) and the halide complex [TiCl(3)(kappa(3)-NNO)] (23 and 24). The latter complexes can also be obtained by reaction of the lithium compound 1 with TiCl(4)(THF)(2) and deprotonation of the alcohol group of 6 with NaH, followed by reaction with TiCl(4)(THF)(2) in a 1:1 molar ratio, respectively. Isolation of only one of the three possible diastereoisomers of the complexes 19 and 22 revealed that chiral induction from the ligand to the titanium center took place. The structures of these complexes were elucidated by (1)H and (13)C{(1)H} NMR spectroscopy, and the X-ray crystal structures of 3-7, 12, and 24 were also established. Finally, we evaluated the influence that the chiral center of the new heteroscorpionate complexes has on the enantioselectivity of the asymmetric epoxidation of allylic alcohols.

Synthesis and fluxional behaviour of allylpalladium complexes with poly (pyrazol-1-yl)methane ligands

Abstract The reaction of the solvento-complex [(ν 3 -2-McC 3 H 4 )Pd(S) 2 ]X (S = Me 2 CO) with the stoichiometric amounts of the following poly(pyrazol-1-yl)methanes: bis(pyrazol-1-yl)methane (bpzm), bis(3,5-dimethylpyrazol-1-yl)methane(3,5-Me2bpzm), tris(pyrazol-lyl)methane (tpzm) and tris(3,5dimethylpyrazol-1-yl) methane (3,5-Me 2 -tpzm) leads to the cationic complexes [(ν 3 -C 4 H 7 )Pd(bpzm)]BF 4 , (1), [(ν 3 -C4H,)Pd(bpzm)]PF6, ( 2 ), [(ν 3 -C 4 H 7 )Pd(tpzm)]B F4 , (3), [(ν 3 -C 4 H 7 )Pd(3,5-Me 2 bpzm)]PF 6 , (4), and [(ν 3 _ C 4 H 7 )Pd(3,5Me 2 tpzm)]PF 6 , ( 5 ). Resonances in the 1H NMR spectra have been assigned by considering the NOE effects between the methylene or methyne protons and the H( 5 ) or Me(5) groups. NOE effects have also been observed between the H(3) or Me(3) and the H(syn) allylic protons. The 13C NMR resonances have been assigned using 1 H- 13 C HETCOR experiments. The fluxional behaviour of 4 and 5 has been studied by 1 H NMR spectroscopy. Two conformers of 4 are discernible at low temperature, and they interchange when the temperature is increased. The AB systems corresponding to methylene groups of both conformers coalesce to a single A, system. A mechanism is proposed on the basis of this observation and the activation free energies at the coalescence temperature calculated. The 1 H NMR spectrum of 5 shows the equivalence of the coordinated and uncoordinated pyrazole rings. The energy barrier of this phenomenon is too low to be determined by NMR spectroscopy. A tumbling motion, as proposed for similar observations, seems a likely pathway of exchange.