AM1 calculations of parts of the enthalpy surfaces for additions of active methylene nitriles to α β-unsaturated nitriles

The nucleophilic additions of active methylene nitriles (MNs), R₁R₂CH⁻, where R₁=CN and R₂=CN, and CSNH₂, to acetaldehyde and to the resultant α, β-unsaturated nitriles have been studied theoretically by the AM1 semiempirical MO method. The additions of MNs anions to acetaldehyde are found to be endothermic with late productlike transition states (TSs) on the reaction coordinate. Their additions to α,β-unsaturated nitriles may conceivably proceed via two pathways: addition to the C=C double bond and addition to the C≡N triple bond. It has been found that the nucleophilic attack at the &alpha,β-unsaturated linkage is exothermic, while that at the nitrile group is endothermic and has a relatively high enthalpy barrier. Both additions have late productlike transition states. The reactivity of the nucleophilic attack has been discussed in the light of the frontier molecular orbital theory and in terms of the HOMO–LUMO two-electron interaction. The calculations have been compared with experimental results. © 1997 John Wiley & Sons, Inc.     

Reaktionen von Nickel(0)-Komplexen mit p-Chinonen. Bildung phosphinsubstituierter Radikalanionen

(dipy)Ni(COD) react with duroquinone (Dch) or anthraquinone (Ach) to yield the complexes (dipy)Ni(η⁴ -Dch) or (dipy)Ni(η⁴ -Ach). Chloranil (CA), however, reacts as an oxidant and depending on the temperature (dipy)Ni^II(CA^2-) or following an oxidative addition (dipy)Ni^II(Cl)(CAH^-)(THF) are formed.By substitution of (Cy₃P)₂Ni(C₂H₄) the complexes (Cy₃P)Ni(η⁴-Dch) or (Cy₃P)₂Ni(η⁴ -Ach) are obtained, whereas a 1,1-coupling of quinone and the coordinated phosphine proceeds during the reaction between p-benzoquinone of chloranil and (Cy₃P)₂Ni(C₂H₄). By ESR studies it was demonstrated that with Ni(Cy₃P?Ch)₂ or Ni(Cy₃P?CA)₂, resp., complexes are obtained, in which radical anions, which are derived from the product of this 1,1-coupling, are coordinated to low-spin nickel (II). There is a significant difference between (Cy₃P)₂Ni(C₂H₄) and the analogous platinum or palladium complexes, which are substituted by p-benzoquinone while an oxidative addition proceeds with chloranil.     

Synthesis and Structure of Low‐valent η4‐Cinnamaldehyde Cobalt Complexes

Three η⁴‐(C=C–C=O) coordination cobalt(I) complexes 1 – 3 were synthesized by the reactions of cinnamaldehyde, p‐fluorocinnamaldehyde, and p‐chlorocinnamaldehyde with CoMe(PMe₃)₄. Complex 4 as η²‐(C=C) coordination was prepared by the reaction of chalcone with Co(PMe₃)₄. The structures of complexes 1 – 4 were confirmed by single‐crystal X‐ray diffraction. Although the reactions didn't undergo C–H bond activation and decarbonylation, the formation of complexes 1 – 4 deepens our understanding of the reactions between α,β‐unsaturated aldehyde or ketone with low‐valent central cobalt atom.     

Synthese und Struktur von η1‐Phosphaallyl‐, η1‐Arsaallylund η1‐Stibaallyleisen‐Komplexen [(η5‐C5Me5)(CO)2Fe–E(SiMe3)C(OSiMe3)=CPh2] (E = P,As, Sb)

Syntheses and Structures of η¹‐Phosphaallyl, η¹‐Arsaallyl, and η¹‐Stibaallyl Iron Complexes [(η⁵‐C₅Me₅)(CO)₂Fe–E(SiMe₃)C(OSiMe₃)=CPh₂] (E = P, As, Sb) The reaction of equimolar amounts of [(η⁵‐C₅Me₅)(CO)₂Fe–E(SiMe₃)₂] ( 1 a : E = P; 1 b : As; 1 c : Sb) and diphenylketene afforded the η¹‐phosphaallyl‐, η¹‐arsaallyl‐, and η¹‐stibaallyl complexes [(η⁵‐C₅Me₅)(CO)₂Fe–E(SiMe₃)C(OSiMe₃)=CPh₂] ( 2 a : E = P; 2 b : As; 2 c : Sb). The molecular structures of 2 b and 2 c were elucidated by single crystal X‐ray analyses.     

Thermische Lactonisierung von Estern γ-Brom-α, β-ungesättigter Carbonsäuren zu Δα-Butenoliden. Direkte γ-Bromierung von α, β-ungesättigten Säuren

By heating with iron powder at 120–150° some γ-bromo-α, β-unsaturated carboxylic methyl esters, and, less smothly, the corresponding acids, were lactonized to Δ^7alpha;-butenolides with elimination of methyl bromide. The following conversions have thus been made: methyl γ-bromocrotonate ( 1c ) and the corresponding acid ( 1d ) to Δ^α-butenolide ( 8a ), methyl γ-bromotiglate ( 3c ) and the corresponding acid ( 3d ) to α-methyl-Δ^α-butenolide ( 8b ), a mixture of methyl trans- and cis-γ-bromosenecioate ( 7c and 7e ) and a mixture of the corresponding acids ( 7d and 7f ) to β-methyl-Δ^α-butenolide ( 8c ). The procedure did not work with methyl trans-γ-bromo-Δ^α-pentenoate ( 5c ) nor with its acid ( 5d ). Most of the γ-bromo-α, β-unsaturated carboxylic esters ( 1c, 7c, 7e and 5c ) are available by direct N-bromosuccinimide bromination of the α, β-unsaturated esters 1a, 7a and 5a ; methyl γ-bromotiglate ( 3c ) is obtained from both methyl tiglate ( 3a ) and methyl angelate ( 4a ), but has to be separated from a structural isomer. The γ-bromo-α, β-unsaturated esters are shown by NMR. to have the indicated configurations which are independent of the configuration of the α, β-unsaturated esters used; the bromination always leads to the more stable configuration, usually the one with the bromine-carrying carbon anti to the carboxylic ester group; an exception is methyl γ-bromo-senecioate, for which the two isomers (cis, 7e , and trans, 7d ) have about the same stability. The N-bromosuccinimide bromination of the α,β-unsaturated carboxylic acids 1b , 3b , 4b , 5b and 7b is shown to give results entirely analogous to those with the corresponding esters. In this way γ-bromocrotonic acid ( 1 d ), γ-bromotiglic acid ( 3 d ), trans- and cis-γ-bromosenecioic acid ( 7d and 7f ) as well as trans-γ-bromo-Δ^α-pentenoic acid ( 5d ) have been prepared. Iron powder seems to catalyze the lactonization by facilitating both the elimination of methyl bromide (or, less smoothly, hydrogen bromide) and the rotation about the double bond. α-Methyl-Δ^α-butenolide ( 8b ) was converted to 1-benzyl-( 9a ), 1-cyclohexyl-( 9b ), and 1-(4′-picoly1)-3-methyl-Δ^α-pyrrolin-2-one ( 9 c ) by heating at 180° with benzylamine, cyclohexylamine, and 4-picolylamine. The butenolide 8b showed cytostatic and even cytocidal activity; in preliminary tests, no carcinogenicity was observed. Both 8b and 9c exhibited little toxicity.     

η3-Allylnickel alkoxides and their use as homogeneous and heterogeneous catalysts for the dimerization of olefins

η³-Allylnickel alkoxides {η³-C₃H₅NiOR}₂ (R = Me, Et, i-Pr, Ph, SiPh₃) may be activated by gaseous boron trifluoride (BF₃) to give active catalysts for the dimerization of propene in homogeneous phase. In CH₂Cl₂ at ?20 °C catalytic turnover numbers of 5000 mol propene(mol Ni)^?1h^?1 were measured. The nature of the OR group influences both the catalytic activity and the oligomerization product distribution. The ratio of methylpentenes to dimethylbutenes in the dimer fraction may be controlled by the presence of additional phosphine ligands at the nickel atom. The nickel alkoxide precursor was heterogenized on alumina to give {Al₂O₃}–O–Ni–(η³-C₃H₅). Subsequent activation using gaseous BF₃ generates a powerful heterogeneous olefin dimerization catalyst which converts 50 × 10³ mol propene (mol Ni)^?1 at ?10° to ?5°C in a batchwise process and 143 × 10³ mol propene (mol Ni)^?1 continuously to give 75% dimers and 25% higher oligomers. The solvent-free treatment of oxide supports, e.g. alumina or silica, with gaseous BF₃ produces strong ‘solid acids’. The activated hydroxyl groups on the support surface serve as effective anchor sites for organometallic complexes to form heterogenous catalysts. By reaction of Ni(cod)₂ with {Al₂O₃}O(BF₃)H or {SiO₂}O(BF₃)H, η¹, η²-cyclo-octenylnickel–O fragments may be fixed to the surface. In the absence of halogenated solvents, the resulting catalysts, e.g. {SiO₂}O–(BF₃)–Ni–(η¹, η²-C₈H₁₃), dimerize propene continuously at +5°C at the rate of 800 × 10³ mol liquid propene (mol Ni)^?1.     

1H NMR structural increments for alkyl substituted η3-allyl transition metal complexes

The ¹H NMR spectra of various alkyl substituted η³-allyl transition metal complexes (M?Ni, Ru) have been analysed. The chemical shifts of the η³-allyl protons can be calculated using additive increments; the values of the syn and anti vicinal proton-proton coupling constants approach each other on alkyl substitution of the η³-allyl group.     

Darstellung neuer,unsymmetrisch substituierter N-(2-Aminophenyl)azomethine von β-Ketoaldehyden durch Template-Kondensation mit Nickel(II) als potentielle Vorstufen unsymmetrisch substituierter makrocyclischer Komplexe

Synthesis of New Unsymmetrically Substituted N-(2-Aminophenyl)-azomethines of β-Ketoaldehydes via Template Condensation with Nickel(II) as Potential Precursors of Unsymmetrically Substituted Macrocyclic Complexes The synthesis of new unsymmetrically substituted nickel(II) complexes of type 4 is described. These complexes arises via template condensation of the unsymmetrically substituted tridentate amino-azomethines 3 , whereas only the amino group of the less reactive and the carbonyl group of the more reactive aminoazomethine react with each other. Diamagnetic square-planar nickel(II) complexes with a [N₃O] donor set and an uncoordinated amino group are formed. The complexes are precursors of carbonyl-substituted macrocyclic chelates. Further condensations of the amino group with e. g. dialdehydes or diketons are possible. All the new complexes are characterized by visible and ¹H and ¹³C n.m.r. spectra as well. The influence of several peripheral substituents on the distribution of the electron density of the ligand is investigated. Strong O…H bonds forms a 6-membered ring between the uncoordinated amino group, the coordinated carbonyl group and the substituents at the R³ position.     

Reaction of perfluoroalkanesulfonyl halide: 3. The reactions of perfluoroalkanesulfonyl bromide with α, β-unsaturated esters and chemical conversions of the products

The reactions of perfluoroalkanesulfonyl bromide with α, β-unsaturated esters were studied in detail. The reaction products were further converted to a series of perfluoroalkyl-substituted α, β-unsaturated acids or esters, α-amino acids and γ-lactones. A peculiar peak (M+15) was found to appear in the mass spectra of some perfluoroalkyl-substituted methyl esters. It was interpreted to be the result of a CH₃ group transfer to the molecular ion. Magnetic nonequivalence was observed in the ¹⁹F NMR spectra of CF₂ group linked to CH₂ in compounds 2f, g and 3f, g which showed a typical AB pattern, and was attributed to the effect of steric hindrance.     

Übergangsmetallphosphidokomplexe. XI. Diphosphenkomplexe des Typs (R3P)2Ni[η2-(PR′)2] und phosphidoverbrückte Nickel(I)-Komplexe des Typs [R3PNiP(SiMe3)2]2 (Ni?Ni)

Transition Metal Phosphido Complexes. XI. Diphosphene Complexes of the Type (R₃P)₂Ni[η²-(PR′)₂] and Phosphido-Bridged Nickel(I) Complexes of the Type [R₃PNiP(SiMe₃)₂]₂(Ni? Ni) From reactions of complexes of the type (R₃P)₂NiCl₂ 1 (R = Me a , Et b , nBu c , iBu d , Ph e , iPr f , Cy g ) with LiP(SiMe₃)₂ in a 1:2 molar ratio the diphosphene complexes (R₃P)₂Ni[η²-(PSiMe₃)₂] 4a–c and the phosphido-bridged nickel(I) complexes [R₃PNiP(SiMe₃)₂]₂ (Ni? Ni) 7a–g are available. Using low temperature n.m.r. measurements the monosubstitution products (R₃P)₂NiClP(SiMe₃)₂ 2a–c and the nickel(0) diphosphane complexes R₃PNi[η¹-P₂(SiMe₃)₄] 6a–g can be detected as intermediates. In reactions in a 1:1 molar ratio the formation of the diphosphorus complexes [(R₃P)₂Ni]₂P₂ 9b, 9c is n.m.r. spectroscopically detectable. 1b reacts with LiP(SiMe₃)CMe₃ to give first the nickel(0) diphosphane complex Et₃PNi[η¹-P(SiMe₃)CMe₃? P(SiMe₃)CMe₃] 10 , which can be isolated at low temperatures, finally yielding (Et₃P)₂Ni[η²-(PCMe₃)₂] 11 and [Et₃PNiP(SiMe₃)CMe₃]₂ (Ni? Ni) 12. 11 as well as (Et₃P)₂Ni[η²-(PPh)₂] 14 can also be obtained reacting 1b with R′P(SiMe₃)₂ (R′ = CMe₃, Ph). The best yields of diphosphene complexes result from [2+1] cyclocondensation reactions of 1a–c with P₂(SiMe₃)₄ to give 4a–c and of 1b with [P(SiMe₃)CMe₃]₂ to give 11 . N.m.r. and mass spectral data are reported.     

Thermal decomposition of 3,3,5-trisubstituted-4,4-dimethyl-4,5-dihydro-5-hydroperoxy-3H-pyrazoles: Route to β,γ-unsaturated ketones

The thermal decomposition of a series of cyclic α-azo hydroperoxides (3,3,5-R₁, R₂, R₃-4,4-dimethyl-4,5-dihydro-5-hydroperoxy-3H-pyrazoles; 2a R₁ = R₂ = R₃ = Ph; 2b R₁ = R₃ = Ph, R₂ = Me; 2c R₁ = R₃ = p-Anisyl, R₂ = Me; 2d R₁ = R₂ = Me, R₃ = Ph; 2e R₁ = R₃ = Me, R₂ = Ph), synthesized by oxidation of the corresponding 3,4-dihydro-2H-pyrazoles, proceeded smoothly with evolution of nitrogen. The relative stability series was found to be 2a > 2c ≈? 2b > 2d > 2e . For 2a , the products were 1,4,4-triphenyl-2,2-dimethyl-1-propanone and 1,1-dimethyl-2,2-diphenylethylene. For 2b-e , β,γ-unsaturated ketones [R₁-C(= CH₂)-CMe₂-C(= O)R₃, 5a-d] were obtained as the major products in ～60% yield from the thermolyses. The products are consistent with a free-radical mechanism involving initial homolysis of the O-O bond followed by loss of nitrogen to yield a free-radical beta to the carbonyl group. For 2a, β -scission and hydrogen-atom abstraction of the hydroperoxy proton by the β-keto radical (induced decomposition) are the major pathways leading to products. For 2b-c , abstraction of a γ-hydrogen atom of the β-keto radicals by hydroxy radical accounts for the formation of the β,γ-unsaturated compounds as the major product.     

Komplexkatalyse. XXXII. Synthese und Charakterisierung von η3-Allyl-, η3-Crotyl-und η1,η2-Cyclooct-4(Z)-en-1-yl-nickel(II)-bis(brenzcate-chinato)-borat und ihre Eignung als Katalysatoren für die stereospezifische Butadienpolymerisation

Complex Catalysis. XXXII. Synthesis and Characterization of η³-Allyl-, η³-Crotyl-, and η¹,η²-Cyclooct-4(Z)-en-1-yl-nickel(II)-bis(brenzcatechinato)borate and their Suitability as Catalysts for the Stereospecific Butadiene Polymerization By reaction of [(η³-C₃H₅)₂Ni], [(η³-C₄H₇)₂Ni], and [Ni(cycloocta-1,5-diene)₂] with one equivalent bis(brenzcatechinato)boric acid HB(O₂C₆H₄)₂ in ether the complexes given in the title could be synthesized in good yields. The allyl complex [η³-C₃H₅NiB(O₂C₆H₄)₂] reacts with cycloocta-1,5-diene (COD) to give a cationic complex [η³-C₃H₅Ni(COD)]B(O₂C₆H₄)₂ and catalyses the 1,4-trans-polymerization of butadiene with an activity of ca. 150 ml C₄H₆/mol Ni · h and a selectivity of 78% under standard conditions at room temperature.     

Variable Bonding Modes of Pyrimidine‐2‐thione in PdII/PtII Complexes [M(η2‐N,S‐pymS)(η1‐S‐pymS)(PPh3)] and [M(η1‐S‐pymS)2(L‐L)] (L‐L = dppm,dppe)

Reactions of pyrimidine‐2‐thione (HpymS) with Pd^II/Pt^IV salts in the presence of triphenyl phosphine and bis(diphenylphosphino)alkanes, Ph₂P‐(CH₂)_m‐PPh₂ (m = 1, 2) have yielded two types of complexes, viz. a) [M(η²‐N, S‐ pymS)(η¹‐S‐ pymS)(PPh₃)] (M = Pd, 1 ; Pt, 2 ), and (b) [M(η¹‐S‐pymS)₂(L‐L)] {L‐L, M = dppm (m = 1) Pd, 3 ; Pt, 4 ; dppe (m = 2), Pd, 5 ; Pt, 6 }. Complexes have been characterized by elemental analysis (C, H, N), NMR spectroscopy (¹H, ¹³C, ³¹P), and single crystal X‐ray crystallography ( 1 , 2 , 4 , and 5 ). Complexes 1 and 2 have terminal η¹‐S and chelating η²‐N, S‐modes of pymS⁻, while other Pd/Pt complexes have only terminal η¹‐S modes. The solution state ³¹P NMR spectral data reveal dynamic equilibrium for the complexes 3 , 5 and 6 , whereas the complexes 1 , 2 and 4 are static in solution state.     

Metalated 1, 3‐Azaphospholes: η1‐(1H‐1, 3‐Benzazaphosphole‐P)M(CO)5 and μ2‐[(1, 3‐Benzazaphospholide‐P)(cyclopentadienide)nickel] Complexes

1H‐1, 3‐Benzazaphospholes react with M(CO)₅(THF) (M = Cr, Mo, W) to give thermally and relatively air stable η¹‐(1H‐1, 3‐Benzazaphosphole‐P)M(CO)₅ complexes. The ¹H‐ and ¹³C‐NMR‐data are in accordance with the preservation of the phosphaaromatic π‐system of the ligand. The strong upfield ³¹P coordination shift, particularly of the Mo and W complexes, forms a contrast to the downfield‐shifts of phosphine‐M(CO)₅ complexes and classifies benzazaphospholes as weak donor but efficient acceptor ligands. Nickelocene reacts as organometallic species with metalation of the NH‐function. The resulting ambident 1, 3‐benzazaphospholide anions prefer a μ₂‐coordination of the η⁵‐CpNi‐fragment at phosphorus to coordination at nitrogen or a η³‐heteroallyl‐η⁵‐CpNi‐semisandwich structure. This is shown by characteristic NMR data and the crystal structure analysis of a η⁵‐CpNi‐benzazaphospholide. The latter is a P‐bridging dimer with a planar Ni₂P₂ ring and trans‐configuration of the two planar heterocyclic phosphido ligands arranged perpendicular to the four‐membered ring.     

Synthesis of highly branched polyethylene using para‐phenyl‐substituted α‐diimine nickel catalysts

A series of para‐phenyl‐substituted α‐diimine nickel complexes, [(2,6‐R₂‐4‐PhC₆H₂N═C(Me))₂]NiBr₂ (R = ⁱPr ( 1 ); R = Et ( 2 ); R = Me ( 3 ); R = H ( 4 )), were synthesized and characterized. These complexes with systematically varied ligand sterics were used as precatalysts for ethylene polymerization in combination with methylaluminoxane. The results indicated the possibility of catalytic activity, molecular weight and polymer microstructure control through catalyst structures and polymerization temperature. Interestingly, it is possible to tune the catalytic activities ((0.30–2.56) × 10⁶ g (mol Ni·h)^?1), polymer molecular weights (M_n = (2.1–28.6) × 10⁴ g mol^?1) and branching densities (71–143/1000 C) over a very wide range. The polyethylene branching densities decreased with increasing bulkiness of ligand and decreasing polymerization temperature. Specifically, methyl‐substituted complex 3 showed high activities and produced highly branched amorphous polyethylene (up to 143 branches per 1000 C).     

[(8a,9,9a‐η)‐9‐(η5‐Cyclopentadienyl)‐9‐nickelafluorenyl](η5‐pentamethylcyclopentadienyl)nickel(II)

The title compound, [Ni₂(C₅H₅)(C₁₀H₁₅)(C₁₂H₈)] or [Ni(C₁₀H₁₅){Ni(C₅H₅)(C₁₂H₈)}], is a rare example (and the first obtained from nickelafluorenyllithium) of an analogue of nickelocene in which the central Ni atom is coordinated to one pentamethylcyclopentadienyl ring and one nickelafluorenyl ring. Both rings lie almost parallel to one another: the dihedral angle between the planes which include these rings is 4.4 (1)°. Slip parameter analysis indicates that the bonding mode of the central Ni atom to the nickelacyclic ring is between η³ and η⁵. Two‐dimensional layers of molecules are formed by C—H...π interactions.     

Studies in negative ion mass spectrometry. XIII—Electron attachment to a series of Nickel(II) β-diketonate complexes

Results of electron attachment reactions and negative ion mass spectra are presented for a group of selected nickel(II) β-diketonate complexes of formula Ni[R¹COCHCOR²]₂, where R¹ is a perfluoroalkyl group and R² either an alkyl or aryl group. Molecular negative ions together with ligand ions are the major contributors to the total ion currents for each compound, and the degree of fragmentation has been shown to be dependent on the substituents R¹ and R². Fragmentation schemes have been elucidated for all the major ion decomposition pathways, and all significant ions have been identified in the negative ion mass spectra of each compound. Bis(1,1,1-trifluoro-5,5-dimethyl-2,4-hexanedionato) nickel(II), with R¹?CF₃ and R²?tert-butyl is the complex which shows considerable potential for analytical quantitation in the negative ion mode, because of the stability of its negative molecular ion, the high negative ion yield given after electron attachment, as well as the volatility of the compound.     

Übergangsmetallphosphidokomplexe. XV. (DRPE)Ni-Komplexe mit PH-haltigen η2-koordinierten Diphosphenliganden und die Diphosphorkomplexe [(DRPE)Ni]2P2

Transition Metal Phosphido Complexes. XV. (DRPE)Ni-Complexes with PH-Containing, η²-Coordinated Diphosphene Ligands and the Diphosphorus Complexes [(DRPE)Ni]₂P₂ The complexes (DRPE)NiCl₂ 1 (DRPE = R₂PCH₂CH₂PR₂; R = Et: DEPE a ; R = Cy: DCPE b ; R = Ph: DPPE c ) react with the silylphosphines (Me₃Si)₃P, (Me₃Si)₂PH, Me₃SiPH₂ and [(Me₃Si)₂P]₂ to form the diphosphorus complexes [(DRPE)Ni]₂P₂ 3a–c and the nickel(0) complexes (DRPE)₂Ni 4a–c . In the reaction of 1b with Me₃SiPH₂ the P₂H₂ complex (DCPE)Ni[η²-(PH)₂] 5b can be isolated at low temperature as an intermediate. Cleaving the Si? P bonds in (DRPE)Ni[η²-(PSiMe₃)₂] 2a, 2b with CH₃OH gives also the P₂ complexes 3a, 3b . Intermediates containing HP=PSiMe₃ and P₂H₂ as ligands can be detected nmr spectroscopically. Reacting 1a–c with (Me₃Si)₂PP(SiMe₃)CMe₃ the complexes (DRPE)Ni(η²-Me₃SiP?PCMe₃) 7a–c containing asymmetric diphosphene ligands can be obtained. 7a reacts with CH₃OH yielding the P₂ complex 3a directly, while 7b with CH₃OH first gives (DCPE)Ni(η²-HP?PCMe₃) 8b . In solution 8b can be transformed into 3b upon heating to 80°C. N.m.r. and mass spectral data are reported.     

1,3-Dipolar cycloaddition of nitrones with α,β-unsaturated sulfones

C,N-Diphenyl nitrones react with substituted α,β-unsaturated phenylsulfones yielding isoxazolidinic cycloadducts whose structure and stereochemistry were assigned on the basis of ¹H and ¹³C nmr data. The ycloaddition regioselectivity is discussed in accordance with frontier orbital considerations.     

Polymerization of ethylene with nickel α-diimine catalysts

Several nickel α-diimine compounds of the general formula (ArNC(R) C(R)NAr)NiX₂ (Ar = 2,6-alkyl substituted Ph, R = H or CH₃, X = Br or CH³) were tested in ethylene polymerization after activation with different co-catalysts, such as methylaluminoxane, Al(C₂H₅)₂Cl or other aluminium alkyls, and ionizing reagents like B(C₆F₅)₃, [CPh₃][B(C₆F₅)₄] or HBF₄. The performances of the different catalytic systems were compared with reference to polymer productivity and structure. The degree of branching of the obtained polyethylenes was shown to depend not only on the ligand environment at the Ni centre but also on the type of co-catalyst.