Neutral, cationic and anionic organonickel and -palladium complexes supported by iminophosphine/phosphinoenaminato ligands.

We report a series of organometallic nickel and palladium complexes containing iminophosphine ligands R2PCH2C(Ph) = N-Dipp (Dipp = 2,6-diisopropylphenyl; R = iPr, La; R = Ph, Lb; and R = o-C6H4OMe, Lc), synthesized by ligand exchange or oxidative addition reactions, and we investigate the capacity of such ligands to undergo reversible deprotonation to the corresponding phosphinoenaminato species. In the attempted ligand exchange reaction of the nickel bis(trimethylsilyl)methyl precursor [Ni(CH2SiMe3)2Py2] with Lb, the iminophosphine acts as a weak acid rather than a neutral ligand, cleaving one of the Ni-C bonds, to afford the phosphinoenaminato complex [Ni(CH2SiMe3)(L'b)(Py)] (L'b = conjugate base of Lb). We disclose a general method for the syntheses of complexes [Ni(CH2SiMe3)(L)(Py)]+ (L = La, Lb or Lc), and demonstrate that iminophosphine deprotonation is a general feature and occurs reversibly in the coordination sphere of the metal. By studying proton exchange reactions of the cation [Ni(CH2SiMe3)(Lb)(Py)]+ with bases of different strength we show that the conjugate phosphinoenaminato ligand in [Ni(CH2SiMe3)(L'b)(Py)] is a base with strength comparable to DBU in THF. The acyl group in the functionalized aryl complex [Ni(p-C6H4COCH3)(Br)(La)] does not interfere in the iminophosphine deprotonation with NaH. The latter reaction affords the unusual anionic hydroxide species [Ni(p-C6H4COCH3)(OH)(L'a)]-Na+, which was isolated and fully characterized.

Aluminium(iii) dialkyl 2,6-bisimino-4R-dihydropyridinates(-1): selective synthesis, structure and controlled dimerization.

A family of stable and otherwise selectively unachievable 2,6-bisimino-4-R-1,4-dihydropyridinate aluminium (III) dialkyl complexes [AlR'2(4-R-iPrBIPH)] (R = Bn, Allyl; R' = Me, Et, iBu) have been synthesized, taking advantage of a method for the preparation of the corresponding 4-R-1,4-dihydropiridine precursors developed in our group. All the dihydropyrdinate(-1) dialkyl aluminium complexes have been fully characterized by 1H- 13C-NMR, elemental analysis and in the case 2'a, also by X-ray diffraction studies. Upon heating in toluene solution at 110 °C, the dimethyl derivatives 2a and 2'a dimerize selectively through a double cycloaddition. This reaction leads to the formation of two new C-C bonds that involve the both meta positions of the two 4-R-1,4-dihydropyridinate fragments, resulting the binuclear aluminium species [Me2Al(4-R-iPrHBIP)]2 (R = Bn (3a); allyl (3'a)). Experimental kinetics showed that the dimerization of 2'a obeys second order rate with negative activation entropy, which is consistent with a bimolecular rate-determining step. Controlled methanolysis of both 3a and 3'a release the metal-free dimeric bases, (4-Bn-iPrHBIPH)2 and (4-allyl-iPrHBIPH)2, providing a convenient route to these potentially useful ditopic ligands. When the R' groups are bulkier than Me (2b, 2'b and 2'c), the dimerization is hindered or fully disabled, favoring the formation of paramagnetic NMR-silent species, which have been identified on the basis of a controlled methanolysis of the final organometallic products. Thus, when a toluene solution of [AlEt2(4-Bn-iPrBIPH)] (2b) was heated at 110 °C, followed by the addition of methanol in excess, it yields a mixture of the dimer (4-Bn-iPrHBIPH)2 and the aromatized base 4-Bn-iPrBIP, in ca. 1 : 2 ratio, indicating that the dimerization of 2b competes with its spontaneous dehydrogenation, yielding a paramagnetic complex containing a AlEt2 unit and a non-innocent (4-Bn-iPrBIP)˙- radical-anion ligand. Similar NMR monitoring experiments on the thermal behavior of [AlEt2(4-allyl-iPrBIPH)] (2'b) and [AliBu2(4-allyl-iPrBIPH)] (2'c) showed that these complexes do not dimerize, but afford exclusively NMR silent products. When such thermally treated samples were subjected to methanolysis, they resulted in mixtures of the alkylated 4-allyl-iPrBIP and non-alkylated iPrBIP ligand, suggesting that dehydrogenation and deallylation reactions take place competitively.

Monomeric alkoxide and alkylcarbonate complexes of nickel and palladium stabilized with the iPrPCP pincer ligand: a model for the catalytic carboxylation of alcohols to alkyl carbonates.

Monomeric alkoxo complexes of the type [(iPrPCP)M-OR] (M = Ni or Pd; R = Me, Et, CH2CH2OH; iPrPCP = 2,6-bis(diisopropylphosphino)phenyl) react rapidly with CO2 to afford the corresponding alkylcarbonates [(iPrPCP)M-OCOOR]. We have investigated the reactions of these compounds as models for key steps of catalytic synthesis of organic carbonates from alcohols and CO2. The MOCO-OR linkage is kinetically labile, and readily exchanges the OR group with water or other alcohols (R'OH), to afford equilibrium mixtures containing ROH and [(iPrPCP)M-OCOOH] (bicarbonate) or [(iPrPCP)M-OCOOR'], respectively. However, [(iPrPCP)M-OCOOR] complexes are thermally stable and remain indefinitely stable in solution when these are kept in sealed vessels. The constants for the exchange equilibria have been interpreted, showing that CO2 insertion into M-O bonds is thermodynamically more favorable for M-OR than for M-OH. Alkylcarbonate complexes [(iPrPCP)M-OCOOR] fail to undergo nucleophilic attack by ROH to yield organic carbonates ROCOOR, either intermolecularly (using neat ROH solvent) or in intramolecular fashion (e.g., [(iPrPCP)M-OCOOCH2CH2OH]). In contrast, [(iPrPCP)M-OCOOMe] complexes react with a variety of electrophilic methylating reagents (MeX) to afford dimethylcarbonate and [(iPrPCP)M-X]. The reaction rates increase in the order X = OTs < IMe ≪ OTf and Ni < Pd. These findings suggest that a suitable catalyst design should combine basic and electrophilic alcohol activation sites in order to perform alkyl carbonate syntheses via direct alcohol carboxylation.

Interaction of an imidazolium-2-amidinate (NHC-CDI) zwitterion with zinc dichloride in dichloromethane: role as ligands and C-Cl activation promoters.

Adducts of imidazolium carbenes and carbodiimides (NHC-CDI) are emerging as a new class of thermally stable and modular zwitterions with many potential applications. Our study of the interaction of a representative NHC-CDI zwitterion with ZnCl2 in dichloromethane led to the serendipitous discovery of a highly selective, double activation of dichloromethane C-Cl bonds.

Potenciación de la efectividad en tiros libres de basquetbolistas juveniles: vinculación biomecánica / Strengthening the effectiveness in free throws of young basketball players: Biomechanical linking

Introducción: El baloncesto es un deporte de conjunto en el que cada acción está encaminada a anotar puntos, una de esas acciones son los tiros libres que no posee un modelo oficial estándar para sus lanzamientos, pero es posible determinar patrones en cada tiro según el estilo del jugador. Objetivo: Medir la influencia del entrenamiento con referencias biomecánicas en la efectividad del tiro libre a partir del establecimiento de un patrón ideal de los tiros efectivos del basquetbolista. Métodos: Se empleó un test inicial de efectividad de tiros libres con carga, del cual a su vez se obtuvo las variables biomecánicas de análisis, posterior a éste se dio conciencia de los resultados con el modelo de lanzamiento más efectivo a cada jugador, modelo empleado para el entrenamiento durante cinco sesiones, finalmente se tomó el mismo test de efectividad de tiro libre con carga en un segundo momento. El estudio se realizó a 12 basquetbolistas del equipo juvenil femenino del Club Roosters de Guayaquil, Ecuador. Resultados: Al finalizar la intervención se encontró que al finalizar el micro ciclo de 5 sesiones de entrenamiento el equipo presentó un ligero incremento en la efectividad en un 59 por ciento en el test inicial a 67por ciento en el test final. La comparación de la efectividad antes y después de implementada la propuesta fue significativa (p=0,021), existiendo correlaciones lineales positivas y negativas entre las variables rendimiento final y las variables biomecánicas estudiadas. Conclusiones: Se comprueba que el modelo propuesto resulta beneficioso a corto plazo pese a no representar grandes cambios en las correlaciones lineales positivas y negativas. En el caso de la efectividad de los tiros libres se comprobó mejoras significativas(AU)

Introduction: Basketball is a sport in which each action is aimed at scoring points, one of those forms are free throws that do not have a standard official model for their throws, but it is possible to determine patterns in each shot according to player with greater or lesser effectiveness. Objective: To measure the training influence with biomechanical references on the free throw effectiveness from establishment of an ideal pattern of the effective shots. Methods: An initial test of free throws effectiveness with load was used, which in turn was obtained the analysis biomechanical variables, after this one was aware of results with the most effective launch model for each player, same as it was used for five sessions training, finally the same free throw effectiveness test was taken with load. The study was carried out on 12 basketball players from girls' youth team of Guayaquil Roosters Club, Ecuador. Results: At the intervention end it was found of micro cycle of 5 training sessions in general the team presented a slight increase in effectiveness from 59 percent in the initial test to 67 percent in the final test. The effectiveness before and after comparison of proposal implementation was significant (p = 0.021), there being positive and negative linear correlations between final performance and biomechanical variables studied. Conclusions: It is proven that the proposed model is beneficial in short term, despite not representing large changes in positive and negative linear correlations. In the case of free throws effectiveness, significant improvements were found(AU)

Educación emocional. Abordaje del proceso de la muerte en la escuela / Emotional education. Approach to the process of death at schooll

El tema de la muerte es raramente abordado en las escuelas, existiendo una negación a la misma, propia de valores y arraigos presentes en la sociedad occidental actual. La educación socio-emocional en las escuelas, es un área emergente con escasos programas rigurosos y bien diseñados en relación a la muerte y el duelo; educar hacia el proceso de pérdida y lo que esta genera en las personas, debería incluirse como contenido curricular. Realizar un análisis sistemático sobre las diferentes propuestas metodológicas orientadas a la educación para la muerte(AU)

The issue of death is rarely addressed in schools, there is a denial of it, typical of values and roots present in today's Western society. Social-emotional education in schools is an emerging area with few rigorous and well-designed programs in relation to death and grief; educate towards the process of loss and what it generates in people, should be included as curricular content. Carry out a systematic analysis on the different methodological proposals aimed at education for death(AU)

Nickel Pincer Complexes with Frequent Aliphatic Alkoxo Ligands [(iPrPCP)Ni-OR] (R = Et, nBu, iPr, 2-hydroxyethyl). An Assessment of the Hydrolytic Stability of Nickel and Palladium Alkoxides.

A series of nickel pincer complexes with terminal alkoxo ligands [(iPrPCP)Ni-OR] (R = Et, nBu, iPr, CH2CH2OH; iPrPCP is the 2,6-bis(diisopropylphosphinomethyl)phenyl pincer ligand) was synthesized and fully characterized. Together with the previously reported methoxo analogues of Ni and Pd, these complexes constitute a unique series of isostructural late transition-metal alkoxides. Spectroscopic and X-ray diffraction data provide direct indications of the strong polarization of their covalent Ni-OR bonds. One of the most salient features of this class of compounds is their facile hydrolysis with traces of moisture, leading to equilibrium mixtures with the corresponding hydroxides [(iPrPCP)M-OH] (M = Ni or Pd) and alcohols, ROH. To compare the hydrolytic stability of nickel and palladium alkoxides, we performed NMR titrations of both hydroxides with several alcohols and determined the corresponding equilibrium constants. In general, these constants are ca. 1 order of magnitude smaller for M = Ni than Pd, indicating that Ni alkoxide complexes are more readily hydrolyzed than their Pd counterparts. For alkoxide complexes containing heteroatom-free R groups, the tendency to hydrolyze decreases as the parent alcohol ROH becomes more acidic, that is, R = Me > Et > iPr. This intuitive trend is broken for 2-methoxyethanol, the most acidic alcohol investigated. The hydroxo/2-methoxyethanol exchange equilibrium constants are comparable to those of ethanol (M = Ni) or methanol (M = Pd), showing that the corresponding 2-methoxyethoxide complexes are more prone to hydrolysis than anticipated. These experimental observations were rationalized in the light of density functional theory calculations.

Obesidad, hábitos alimenticios y actividad física en alumnos de educación secundaria / Obesity, eating habits and physical activity in secondary school students

Introducción: La obesidad, los hábitos alimenticios y la actividad física son variables interrelacionadas a tener en cuenta para el análisis y corrección de problemas de salud. Objetivo: Analizar la prevalencia de la obesidad y el sobrepeso en jóvenes estudiantes de educación secundaria de la Unidad Educativa FICOA de Guayaquil (Ecuador), así como sus hábitos alimenticios y de actividad física. Métodos: Se estudia una muestra representativa de 82 alumnos de la escuela mencionada (40 mujeres y 42 hombres), sometiéndolos al cálculo del Índice de Masa Corporal (IMC), aplicándoles un cuestionario con preguntas relacionadas con la dieta y la práctica de actividad física. Resultados: La prevalencia de la obesidad y el sobrepeso fue de un 17,1 por ciento. Los jóvenes obesos o con sobrepeso tienen peores hábitos alimenticios que la media y su consumo de dulces, bollería y embutidos está por encima de lo deseable. Se han constatado conductas alimenticias de riesgo, con un 22 por ciento de jóvenes delgados que no desayuna y un 14 por ciento de ellos que sigue dietas hipocalóricas. El consumo de alcohol declarado fue mínimo, y la práctica de actividad física fue menor de la recomendable en los jóvenes obesos. Conclusiones: La prevalencia de la obesidad de los adolescentes analizados se sitúa en niveles ligeramente inferiores a los de países desarrollados. Aun así, consumen demasiados dulces, bollería, embutidos y carnes, en detrimento de verduras y pescado, siendo necesario cambiar estos hábitos y vigilar, al mismo tiempo, las conductas de restricción calórica peligrosas. Los poderes públicos deben insistir en las políticas de fomento de la práctica deportiva(AU)

Introduction: Obesity, eating habits and physical activity are interrelated variables to be taken into account for the analysis and correction of health problems. Objective: To analyze the obesity prevalence and overweight in young high school students of Unidad Educativa FICOA de Guayaquil (Ecuador), as well as their eating habits and physical activity. Methods: A representative sample of 82 students from mentioned school (40 women and 42 men) was studied, being submitted to calculation of Body Mass Index (BMI), applying a questionnaire with questions related to diet and physical activity practice. Results: The obesity prevalence and overweight was 17.1 percent. Young obese or overweight have worse eating habits than average and their consumption of sweets, pastries and sausages is above desirable. Dietary risk behaviors have been observed, with 22 percent of thin young people who do not eat breakfast and 14 percent of them who follow low-calorie diets. The alcohol consumption declared was minimal, and the physical activity practice was lower than recommended in obese youngsters. Conclusions: The obesity prevalence among adolescents analyzed is slightly lower than in developed countries. Even so, they consume too much sweets, pastries, sausages and meats, to detriment of vegetables and fish, being necessary to change these habits, at the same time, the dangerous behaviors of caloric restriction. The public authorities must insist on policies to promote sports practice(AU)

Copper(I) Complexes of Zwitterionic Imidazolium-2-Amidinates, a Promising Class of Electroneutral, Amidinate-Type Ligands.

The first complexes containing imidazolium-2-amidinates as ligands (betaine-type adducts of imidazolium-based carbenes and carbodiimides, NHC-CDI) are reported. Interaction of the sterically hindered betaines ICyCDI(DiPP) and IMeCDI(DiPP) [both bearing 2,6-diisopropylphenyl (DiPP) substituents on the terminal N atoms] with Cu(I) acetate affords mononuclear, electroneutral complexes 1a and 1b, which contain NHC-CDI and acetate ligands terminally bound to linear Cu(I) centers. In contrast, the less encumbered ligand ICyCDI(p-Tol), with p-tolyl substituents on the nitrogen donor atoms, affords a dicationic trigonal paddlewheel complex, [Cu2(µ-ICyCDI(p-Tol))3](2+)[OAc(-)]2 (2-OAc). The nuclear magnetic resonance (NMR) resonances of this compound are broad and indicate that in solution the acetate anion and the betaine ligands compete for binding the Cu atom. Replacing the external acetate with the less coordinating tetraphenylborate anion provides the corresponding derivative 2-BPh4 that, in contrast with 2-OAc, gives rise to sharp and well-defined NMR spectra. The short Cu-Cu distance in the binuclear dication [Cu2(µ-ICyCDI(p-Tol))3](2+) observed in the X-ray structures of 2-BPh4 and 2-OAc, ca. 2.42 Å, points to a relatively strong "cuprophilic" interaction. Attempts to force the bridging coordination mode of IMeCDI(DiPP) displacing the acetate anion with BPh4(-) led to the isolation of the cationic mononuclear derivative [Cu(IMeCDI(DiPP))2](+)[BPh4](-) (3b) that contains two terminally bound betaine ligands. Compound 3b readily decomposes upon being heated, cleanly affording the bis-carbene complex [Cu(IMe)2](+)[BPh4(-)] (4) and releasing the corresponding carbodiimide (C(═N-DiPP)2).

ß-Hydrogen Elimination Reactions of Nickel and Palladium Methoxides Stabilised by PCP Pincer Ligands.

Nickel and palladium methoxides [((iPr)PCP)M-OMe], which contain the (iPr)PCP pincer ligand, decompose upon heating to give products of different kinds. The palladium derivative cleanly gives the dimeric Pd(0) complex [Pd(µ-(iPr)PCHP)]2 ((iPr)PCHP = 2,6-bis(diisopropylphosphinomethyl)phenyl) and formaldehyde. In contrast, decomposition of [((iPr)PCP)Ni-OMe] affords polynuclear carbonyl phosphine complexes. Both decomposition processes are initiated by ß-hydrogen elimination (BHE), but the resulting [((iPr)PCP)M-H] hydrides undergo divergent reaction sequences that ultimately lead to the irreversible breakdown of the pincer units. Whereas the Pd hydride spontaneously experiences reductive C-H coupling, the decay of its Ni analogue is brought about by its reaction with formaldehyde released in the BHE step. Kinetic measurements showed that the BHE reaction is reversible and less favourable for Ni than for Pd for both kinetic and thermodynamic reasons. DFT calculations confirmed the main conclusions of the kinetic studies and provided further insight into the mechanisms of the decomposition reactions.

Dibenzyl and diallyl 2,6-bisiminopyridinezinc(II) complexes: selective alkyl migration to the pyridine ring leads to remarkably stable dihydropyridinates.

Diorganozinc compounds (ZnR2) with R = CH2Ph or CH2CH=CH2 react with 2,6-bisiminopyridines ((iPr)BIP) to afford thermally stable dihydropyridinate(-1) complexes, and do not react if R = CH2SiMe3 or CH2CMe2Ph. NMR studies reveal that dibenzylzinc binds (iPr)BIP at -80 °C, yielding the unstable complex [Zn(CH2Ph)2((iPr)BIP)]. Above -20 °C, this undergoes selective alkyl migration to the remote 4 position of the central pyridine ring.

Well-defined alkylpalladium complexes with pyridine-carboxylate ligands as catalysts for the aerobic oxidation of alcohols.

Neophylpalladium complexes of the type [Pd(CH(2)CMe(2)Ph)(N-O)(L)], where N-O is picolinate or a related bidentate, monoanionic ligand (6-methylpyridine-2-carboxylate, quinoline-2-carboxylate, 2-pyridylacetate or pyridine-2-sulfonate) and L is pyridine or a pyridine derivative, efficiently catalyze the oxidation of a range of aliphatic, benzylic and allylic alcohols with oxygen, without requiring any additives. A versatile method is described which allows the synthesis of the above-mentioned complexes with a minimum synthetic effort from readily available materials. Comparison of the rates of oxidation of 1-phenylethanol with different catalysts reveals the influence of the structure of the bidentate N-O chelate and the monodentate ligand L on the catalytic performance of these complexes.

Migratory insertion reactions of nickel and palladium σ-alkyl complexes with a phosphinito-imine ligand.

Ligand exchange reactions have been used for the synthesis of metallacyclic complexes of Ni and Pd of the type [M[upper bond 1 start](CH(2)CMe(2)-o-C(6)[upper bond 1 end]H(4))(P-N)], where P-N is the phosphinito-imine ligand P(iPr)(2)OC(Me)[double bond, length as m-dash]N(2,6-C(6)H(3)(iPr)(2). The protic acid [H(OEt(2))(BAr'(4))] (Ar' = 3,5-C(6)H(3)(CF(3))(2)) selectively cleaves one of the two σ metal-carbon bonds, affording cationic monoalkyl complexes. Nickel monoalkyls stabilized with Et(2)O or MeCN ligands are thermally unstable and spontaneously undergo a decomposition process that ultimately leads to the breakdown of the phosphinito-imine ligand. In contrast, cationic alkylpalladium derivatives are thermally very stable, allowing the isolation of a formally unsaturated monoalkyl complex stabilized by an intramolecular π-arene interaction. Although monoalkynickel and -palladium phosphinito-imine derivatives are inactive as ethylene polymerization or copolymerization catalysts, they readily experience migratory insertion reactions. A palladium chelate arising from the successive insertion of CO and ethylene has been isolated and characterized.

Selective reduction of a Pd pincer PCP complex to well-defined Pd(0) species.

Well-defined dimeric or polymeric Pd(0) complexes [Pd(µ-(iPr)PCHP)](n) (n = 2 or ∞) containing the bridging ligand α,α'-bis(diisopropylphosphino)-m-xylene ((iPr)PCHP) are produced under mild conditions when the cyclometallated PCP pincer complex ((iPr)PCP)Pd-OH reacts with methanol or isopropanol.

Neutral and cationic alkylmanganese(II) complexes containing 2,6-bisiminopyridine ligands.

Manganese alkyl complexes stabilised by 2,6-bis(N,N'-2,6-diisopropyl-phenyl)acetaldiminopyridine ((iPr)BIP) have been selectively prepared by reacting suitable alkylmanganese(II) precursors, such as homoleptic dialkyls [(MnR(2))(n)] or the corresponding THF adducts [{MnR(2)(thf)}(2)] with the mentioned ligand. For R=CH(2)CMe(2)Ph or CH(2)Ph, formally Mn(I) derivatives are produced, in which one of the two R groups migrates to the 4-position of the central pyridine ring in the (iPr)BIP ligand. In contrast, a true dialkyl complex [MnR(2)((iPr)BIP)] can be isolated for R=CH(2)SiMe(3). In solution, this compound slowly evolves to the corresponding Mn(I) monoalkyl derivative. A detailed study of this reaction provides insights on its mechanism, showing that it proceeds through successive alkyl migrations, followed by spontaneous dehydrogenation. Protonation of [Mn(CH(2)SiMe(3))(2)((iPr)BIP)] with the pyridinium salt [H(Py)(2)][BAr'(4)] (Ar'=3,5-C(6)H(3)(CF(3))(2)) leads to the cationic species [Mn(CH(2)SiMe(3))(Py)((iPr)BIP)](+). Alternatively, the same complex can be produced by reaction of the pyridine complex [{Mn(CH(2)SiMe(3))(2)(Py)}(2)] with the protonated ligand salt [H(iPr)BIP](+)[BAr'(4)](-). This last reaction allows the synthesis of analogous cationic alkylmanganese(II) derivatives, when precursors of type [MnR(2)((iPr)BIP)] are not available. Treatment of these neutral and cationic (iPr)BIP alkylmanganese derivatives with a range of typical co-catalysts (modified methylaluminoxane (MMAO), B(C(6)F(5))(3), trimethyl or triisobutylaluminum) does not lead to active ethylene polymerisation catalysts.

Palladium(II) carboxylates and palladium(I) carbonyl carboxylate complexes as catalysts for olefin cyclopropanation with ethyl diazoacetate.

Palladium(I) carbonyl carboxylate complexes [Pd(mu-CO)(mu-RCO2)](n) (R = Me, n = 4; R = CMe(3), n = 6) and the corresponding palladium(II) carboxylates (acetate and pivalate) catalyze the cyclopropanation of olefins with ethyl diazoacetate. The performance of these catalysts is similar in terms of selectivity and cyclopropane yields, regardless of the oxidation state of the metal center. However the rates of the cyclopropanation reactions are significantly higher for the acetate based catalysts than for the pivalate derivatives, which suggests that the main catalytic species are carboxylate containing palladium complexes. Kinetic measurements show that reaction rates are independent of the olefin concentration when these are 1-hexene or styrene, but norbornene exerts an inhibitory effect. In spite of this, competition experiments indicate that the cyclopropanation of styrene is 2.2 times as favorable as that of 1-hexene for any of the four catalysts. These observations indicate that while the rate-determining formation of the intermediate palladium carbenoid species is controlled by the catalyst structure, this is followed by a rapid and less specific cyclopropanation step that is not affected by the nature of the carboxylate groups present in the catalyst. An independent test using a 1:1 benzene/cyclohexane mixture of solvents showed that the transfer of ethoxycarbonylcarbene (:C(CO2Et)H) to these molecules is unselective (relative rate of benzene/cyclohexane functionalization approximately 1.8, independent of the catalyst). This result can be interpreted as an indication of the involvement of free ethoxycarbonylcarbene in the carbene transfer step.

Studies on the atropisomerism of Fe(II) 2,6-bis(N-arylimino)pyridine complexes.

NMR spectra of free 2,6-bis(N-arylimino)pyridine (PDI) ligands displaying different substituents at the ortho and ortho' positions of the two N-aryl rings indicate that they can exist in syn (meso) and anti (chiral) configurations. These interconvert in solution at room temperature, via rotation of the aryl group. The corresponding paramagnetic FeX(2)(PDI) complexes exhibit the same kind of isomerism, a property that is thought to be important for their activity as alpha-olefin polymerization catalysts. For the first time, this has been detected by (1)H NMR and studied in solution. Although the conformational stability of the diastereoisomeric complexes varies widely (depending on the size of the substituents at the imine and the aromatic rings), a moderate degree of steric hindrance suffices to allow their chemical separation. A simple procedure is developed for the preparation of these complexes in diastereoisomerically pure form. In addition, introduction of a prochiral substituent in the pyridine ring enables positive assignment of the stereoisomers. Isomerization rate measurements of the Fe(II) complexes in solution suggest that isomerization very likely involves the dissociation of the corresponding Fe-N(imino) bond prior to the rotation of N-aryl groups. DFT calculations provide additional support to the conformational assignment as well as the dissociative isomerization mechanism.

Separation of a diiminopyridine iron(II) complex into rac- and meso- diastereoisomers provides evidence for a dual stereoregulation mechanism in propene polymerization.

Separation of a diiminopyridine iron(II) complex into its rac- and meso- diastereoisomers provides for first time the opportunity of observing the enantiomorphic site control competing with the chain-end control mechanism in a non-metallocene catalyst system.

Conjugate additions of cyclic oxygen-bound nickel enolates to alpha,beta-unsaturated ketones.

The reaction of nickel enolates displaying a metallacyclic structure with the alpha,beta-unsaturated ketones methyl vinyl ketone (MVK) or methyl propenyl ketone (MPK) takes place in two stages, affording initially bicyclic adducts, which subsequently isomerize to the corresponding open-chain products. The former are generated with high stereoselectivity and can be considered as the products of the [2+4] cycloaddition of the enolate to the enone. The ring opening process involves a prototropic rearrangement that can be catalyzed by water. In the case of the reaction of the parent nickel enolate complex 1 (which displays an unsubstituted Ni-O=C(R)CH2 arrangement) with MVK, a double-addition process has been observed, consisting of two successive cycloaddition/isomerization reactions. The carbonylation of the different cyclic and noncyclic products affords the corresponding lactones that retain the stereochemistry of the organometallic precursors. This methodology allowed trapping the primary product of the reaction of 1 with MPK as the corresponding organic lactone, demonstrating that the cycloaddition process takes place with exo selectivity. DFT modeling of the latter reaction provides further support for a quasi-concerted cycloaddition mechanism, displaying a nonsymmetric transition state in which the C-C and the C-O bond are formed in an asynchronous manner.