Olefin Rotation Research Articles

Conformational Isomerization as a Process to Determine Selectivity over Reaction Pathways: Effect of Alkene Rotation in Chain Walking via Cis Alkene Intermediates.

In organic reactions, bond-forming and bond-cleaving processes are generally considered to be more important than other processes such as conformational isomerization. We report herein an example where a conformational isomerization process, propeller-like alkene rotation, is considered to determine the selectivity over the reaction pathways. The transition state with the highest energy barrier in some alkylpalladium isomerization (chain walking) events was theoretically indicated to correspond to alkene rotation, while transition states for bond-cleaving β-hydride elimination and bond-forming migratory insertion were not even observed. It was also suggested both theoretically and experimentally that the palladium chain walking over internal carbons in alkyl chains proceeds via cis alkene intermediates rather than thermodynamically more stable trans alkene intermediates, due to their relative difficulty to undergo alkene rotation.

Structure and Isotope Effects of the β‐H Agostic (α‐Diimine)Nickel Cation as a Polymerization Intermediate

AbstractSingle‐crystal X‐ray characterization of cationic (α‐diimine)Ni‐ethyl and isopropyl β‐agostic complexes, which are key intermediates in olefin polymerization and oligomerization, are presented. The sharp Ni‐Cα‐Cβ angles (75.0(3)° and 74.57(18)°) and short Cα−Cβ distances (1.468(7) and 1.487(5) Å) provide unambiguous evidence for a β‐agostic interaction. An inverse equilibrium isotope effect (EIE) for ligand coordination upon cleavage of the agostic bond highlights the weaker bond strength of Ni−H relative to the C−H bond. An Eyring plot for β‐hydride elimination–olefin rotation–reinsertion is constructed from variable‐temperature NMR spectra with 13C‐labeled agostic complexes. The enthalpy of activation (ΔH≠) for β‐H elimination is 13.2 kcal mol−1. These results offer important mechanistic insight into two critical steps in polymerization: ligand association upon cleavage of the β‐agostic bonds and chain‐migration via β‐H elimination.

Structure and Isotope Effects of the β-H Agostic (α-Diimine)Nickel Cation as a Polymerization Intermediate.

Single-crystal X-ray characterization of cationic (α-diimine)Ni-ethyl and isopropyl β-agostic complexes, which are key intermediates in olefin polymerization and oligomerization, are presented. The sharp Ni-Cα -Cβ angles (75.0(3)° and 74.57(18)°) and short Cα -Cβ distances (1.468(7) and 1.487(5) Å) provide unambiguous evidence for a β-agostic interaction. An inverse equilibrium isotope effect (EIE) for ligand coordination upon cleavage of the agostic bond highlights the weaker bond strength of Ni-H relative to the C-H bond. An Eyring plot for β-hydride elimination-olefin rotation-reinsertion is constructed from variable-temperature NMR spectra with 13 C-labeled agostic complexes. The enthalpy of activation (ΔH≠ ) for β-H elimination is 13.2 kcal mol-1 . These results offer important mechanistic insight into two critical steps in polymerization: ligand association upon cleavage of the β-agostic bonds and chain-migration via β-H elimination.

Quantum mechanical study on the different fluxional mechanisms in a [Pd(η2-olefin) complex

In this work, using quantum mechanics, different fluxional mechanisms in a Pd(η2-olefin) complex including sulfur inversion, olefin pseudo rotation, Pd–olefin bond dissociation, propeller-like olefin rotation and a bimolecular process have been investigated theoretically in a solvent environment using the polarisable continuum model (PCM). The activation energy, activation enthalpy and activation Gibbs free energy of the different pathways have been calculated and compared with each other and with experimental results. These activation energies for olefin pseudo rotation, Pd–olefin bond dissociation and propeller-like olefin rotation are greater in comparison with those for the sulfur inversion and bimolecular process, which demonstrates that these latter mechanisms are the dominating paths and make the main contributions in the reaction. The computed activation enthalpies of the sulfur inversion and bimolecular process pathways are in good agreement with the experimental values. All of the calculations have been performed using a hybrid density functional method (B3LYP) in the solution phase.

Noninnocent Behavior of Ancillary Ligands: Apparent Trans Coupling of a Saturated N-Heterocyclic Carbene Unit with an Ethyl Ligand Mediated by Nickel

Oxidative addition of the tridentate N-heterocyclic carbene (NHC) diphosphine ligand precursor ([PCP]H)PF(6) (1) {[PCP] = o-(i)Pr(2)PC(6)H(4)(NC(3)H(4)N)o-C(6)H(4)P(i)Pr(2)} to Ni(COD)(2) results in the formation of the nickel(II) hydride complex ([PCP]NiH)PF(6) (2). This hydride undergoes a rapid reaction with ethylene to generate a nickel(0) complex in which an ethyl group has been transferred to the carbene carbon of the original NHC-diphosphine ligand. If the first intermediate is the anticipated square-planar nickel(II) ethyl species, then the formation of the product would require a process that involves a trans C-C coupling of the NHC carbon and a presumed Ni-ethyl intermediate. Deuterium-labeling studies provide evidence for migratory insertion of the added ethylene into the Ni-H bond rather than into the Ni-carbene linkage; this is based on the observed deuterium scrambling, which requires reversible beta-elimination, alkene rotation, and hydride readdition. However, density functional theory studies suggest that a key intermediate is an agostic ethyl species that has the Ni-C bond cis to the NHC unit. A possible transition state containing two cis-disposed carbon moieties was also identified. Such a process represents a new pathway for catalyst deactivation involving NHC-based metal complexes.

Qualitative and quantitative discrimination of fake and true alkene rotation processes in pd(η 2-olefin) complexes. A new bimolecular mechanism

The fluxional behaviour of [Pd(η 2-fn)(N-SMe)] ( 2) (fn = fumaronitrile, N-SMe = 2-methylthiomethylpyridine) and of [Pd(η 2-tmetc)( N- N′-4-anisyl)] ( 3) (tmetc = tetramethylethylenetetracarboxylate, N- N′-4-anisyl = 2-(4-methoxyphenyliminomethane)pyridine) were monitored by 1H NMR spectroscopy and quantitatively determined by line-shape analyses (for 2) and selective inversion recovery experiments (for 3). The coalescence of the AB multiplet of fn hydrogens of 2 is concentration dependent and presents a strongly negative Δ S ≠, suggesting the intermediacy of a dimeric complex and ruling out the hypothesis of olefin rotation. The accurate evaluation of all spectral features also allows determination of the approaching mode of the monomeric units. The inversion transfer between the tmetc methyls of 3 reveals a true propeller-like olefin rotation. The presence of a nucleophilic electron pair at sulfur in 2 triggers the formation of the dimeric intermediate.

Rhodium Complexes with Pincer Diphosphite Ligands. Unusual Olefin in-Plane Coordination in Square-Planar Compounds

A family of rhodium complexes bearing pincer diphosphite (PCP) and neutral L (L = PPh3, CO, CNXy, C2H4) ligands has been prepared and characterized. Reactions between Rh(Cl)(PPh3)3 and the diphosphites 2 lead to the chloro hydrides Rh(H)(Cl)(PCP)(PPh3) (4), which can be deprotonated to yield the complexes Rh(PCP)(PPh3) (5). In the latter, the PPh3 ligand is labile and can be exchanged by CO, CNXy, and C2H4 to give derivatives 6−8. Most noteworthy is the fact that ethylene complexes 8a,b show a remarkable in-plane (ip) conformation in the solid state. Solution NMR studies for these complexes show fast olefin rotation in all the temperature ranges, while solution and solid-state 13C{1H} NMR experiments give practically superimposable spectra, also indicating the prevalence of the ip conformer in solution. Complementary detailed DFT calculations performed with models of compounds 8 indicate that the olefin conformational preference is due to a dual combination of steric effects arising from the reduction of the P−Rh−P angle from 180°, caused by pincer ligand chelation and by the nature of the phosphite substituents, which in the case of t-Bu groups perfectly outline a cavity for an ip coordination of the olefin.

Syn−Anti Isomerization by Intramolecular Alkene Rotation: Study of the Dynamics of the Bicyclo[2.2.1]hept-2-ene Ligands in a Diplatinum(I) Complex and Its Lewis Acid Adducts

Intramolecular exchange by alkene rotation observed for the first time in a dinuclear d9-Pt(I) olefin complex was evidenced for [Pt2(µ-dppm)(µ-PPh2)(η2-bicyclo[2.2.1]hept-2-ene)2](O3SCF3) (1) (dppm = bis(diphenylphosphino)methane). The dynamic behavior leads to the occurrence of two diastereomers in solution with relative syn and anti orientation of the two in-plane coordinated bicyclo[2.2.1]hept-2-ene ligands. The low activation energy ΔG⧧rot of 43 kJ/mol found for this exchange process was attributed (a) to a favorable geometry due to the ample P−Pt−P angle of the phosphorus ligands in mutual trans position promoting flexibility and reducing steric hindrance in the intermediate or transition state with upright alkene orientation and (b) to the positive charge of the complex cation, which reduces the ability of platinum for metal-to-ligand π-back-donation compared to neutral or anionic complexes. Intramolecular alkene rotation was also observed for the Lewis acid adducts at the Pt−Pt bond of 1 with Ag+, ...

A possible 2,1 → 3,1 isomerization mechanism in zirconocene-catalyzed propene polymerization: An application of the density functional theory and combined ONIOM approach

The unimolecular isomerization mechanism from a secondary 2,1 → 3,1 propene unit promoted by the prototype zirconocene system H 2SiCp 2Zr +(P) (P = polymeryl chain) has been investigated by using density functional theory calculations (B3LYP) for both gas and solvent phases. The typical route occurring through β-hydrogen elimination to the metal, olefin rotation around the metal center and olefin reinsertion into the metal hydrogen bond has been calculated by using QM/MM calculations with the ONIOM model in the presence of the counterion CH 3 B(C 6 F 5 ) 3 - and compared to the alternative intramolecular reversible formation of a zirconocene allyl dihydrogen complex. Our calculations show that the alternative route remains energetically less accessible, at least for the prototype metallocene system used here.

Synthesis of Cp‘Ru(CO)(L)(η2-olefin)+ Complexes and Kinetic Studies of Olefin Substitution

The ruthenium(II)−olefin complexes [Cp‘Ru(CO)(L)(η2-olefin)]+ (Cp‘ = Cp, Cp*; L = CO, PPh3; olefin = methyl oleate, cis-3-hexene, trans-3-hexene, 1,4-pentadiene) have been synthesized and characterized by IR and 1H and 13C NMR spectroscopy. An X-ray structure of [CpRu(CO)(PPh3)(η2-cis-3-hexene)]+ shows that the olefinic bond is nearly parallel to the plane of the Cp ligand. The olefins in [CpRu(CO)2(η2-trans-3-hexene)]+ and [Cp‘Ru(CO)2(η2-trans-3-hexene)]+ rotate rapidly about the Ru−olefin bond, even at −30 °C, as established by the presence of a single methyl signal for the olefin in the 1H NMR spectrum, whereas olefin rotation in [CpRu(CO)(PPh3)(η2-trans-3-hexene)]+ is slow on the 1H NMR time scale at −25 °C. The [CpRu(CO)(PPh3)]+ unit exhibits a unique diastereoselectivity by binding to only one face of trans-3-hexene, due to steric repulsion between the ethyl groups of the trans-3-hexene and the bulky Cp and PPh3 ligands. Kinetic studies of the substitution of the olefin in [Cp'Ru(CO)2(η2-olefin)]+ by PPh3 show that the lability of the large methyl oleate is similar to that of the smaller cis-3-hexene. Replacement of Cp by Cp* and of cis-3-hexene by trans-3-hexene increases substantially the rate of olefin substitution, due to an increase in steric repulsion.

Stereoselectivity in a Chiral Ruthenium Ethylene Hydride Complex: Evidence of an Agostic Intermediate

Treatment of Ru(η6:η1-Me2NC6H4C6H4PCy2)Cl2 with MeLi resulted in the formation of Ru(η6:η1-Me2NC6H4C6H4PCy2)Me2, which reacted with Ph3CPF6 to form the cationic olefin hydride complex [Ru(η6:η1-Me2NC6H4C6H4PCy2)(H2CCH2)(H)]PF6. The new complexes were characterized by spectroscopy and by X-ray crystallographic analysis. The diastereomeric olefin hydride complexes were observed to exhibit two fluxional processes: a facile olefin rotation and another process that results in the exchange of the hydride and olefin protons. As the rate of diastereomer interconversion is much slower than that of olefin insertion, our studies suggest that the latter exchange takes place through an agostic species which undergoes dynamic methyl rotation.

Novel Hydridoirida-β-diketones Containing Small Molecules, CO, or Ethylene: Their Behavior in Coordinating Solvents Such as Dimethylsulfoxide or Acetonitrile

New hydridoirida-beta-diketones [IrH[(PPh2(o-C6H4CO))2H](CO)]ClO4 2 and [IrH[(PPh2(o-C6H4CO))2H](olefin)]BF4 (olefin = C2H4, 5; 1-hexene, 10) have been prepared. These complexes may afford new diacylhydridoiridium(III) derivatives. In chloroform solution, complex 2 is in equilibrium with the deprotonated diacylhydride trans-[IrH(PPh2(o-C6H4CO))2(CO)] complex 3. In DMSO, deprotonation of 2 occurs to yield the kinetically favored product 3, which isomerizes to the thermodynamically favored complex cis-[IrH(PPh2(o-C6H4CO))2(CO)] 4. Reprotonation of 4 with HBF4 in chlorinated solvents gives the cation in 2. In coordinating solvents such as dimethyl sulfoxide or acetonitrile, complex 5 undergoes displacement of ethylene to afford [IrH{(PPh2(o-C6H4CO))2H](L)]BF4 (L = DMSO, 7; CH3CN, 9). Complexes 5 and 7 undergo deprotonation by NEt3 to give the corresponding diacylhydrides. The ethylene complex gives only trans-[IrH(PPh2(o-C6H4CO))2(C2H4)] 6, while the dimethyl sulfoxide derivative affords a mixture of trans- and cis-[IrH(PPh2(o-C6H4CO))2(DMSO)] 8. Complex 10 shows inhibited alkene rotation around the Ir-olefin axis. All of the complexes were fully characterized spectroscopically. Single-crystal X-ray diffraction analysis was performed on complexes 3, 4, and 9. The 13C NMR and X-ray data point to a carbenoid character in the carbon atoms bonded to iridium in the irida--diketone fragment, so that it can be considered as an acyl(hydroxycarbene) moiety.

Chain epimerization during propylene polymerization with metallocene catalysts: mechanistic studies using a doubly labeled propylene.

The mechanisms of chain epimerization during propylene polymerization with methylaluminoxane-activated rac-(EBTHI)ZrCl(2) and rac-(EBI)ZrCl(2) catalysts (EBTHI = ethylenebis(eta(5)-tetrahydroindenyl); EBI = ethylenebis(eta(5)-indenyl)) have been studied using specifically isotopically labeled propylene: CH(2)=CD(13)CH(3). These isospecific catalysts provide predominantly the expected [mmmm] pentads with [minus signCH(2)CD(13)CH(3)(-)] repeating units ((13)C NMR). Under relatively low propylene concentrations at 50 and 75 degreesC, where stereoerrors attributable to chain epimerization are prevalent, (13)C NMR spectra reveal (13)C-labeled methylene groups along the polymer main chain, together with [CD(13)CH(3)] units in [mmmr], [mmrr], and [mrrm] pentads and [CH(13)CH(3)] units in [mmmmmm] and [mmmmmr] heptads, as well as [mrrm] pentads. The isotopomeric regiomisplacements and stereoerrors are consistent with a mechanism involving beta-D elimination, olefin rotation and enantiofacial interconversions, and insertion to a tertiary alkyl intermediate [Zr-C(CH(2)D)((13)CH(3))P] (P = polymer chain), followed by the reverse steps to yield two stereoisomers of [Zr-CHDCH((13)CH(3))P] and [Zr-(13)CH(2)CH(CH(2)D)P], as well as unrearranged [Zr-CH(2)CD((13)CH(3))P]. The absence of observable [-CH(2)CH(13)CH(2)D-] in the [mrrm] pentad region of the (13)C NMR spectra provides evidence that an allyl/dihydrogen complex does not mediate chain epimerization.

Selective dynamics of [Rh(1,5-COD)(bidentate)]BF 4 complexes via NMR exchange spectroscopy

NMR exchange measurements on [Rh(1,5-COD)(bidentate)]BF 4 complexes (bidentate=chiral bis-phosphine, a P,N-phosphino-oxazoline, a P,S-phosphito-thioether and a bis-pyrazolylborate) show selective 1,5-COD dynamics which can be superficially attributed to olefin rotation. It is suggested that the mechanism actually involves: (i) ML 1 bond breaking (L 1=N or S-donor); (ii) isomerization of the T-shaped species; (iii) rotation around the remaining ML 2 bond and (iv) recomplexation. The solid state structures of the two compounds [M(1,5-COD)( 10)]BF 4, M=Rh, Ir and 10=( S,R)-2-[4-(isopropyl)oxazol-2-yl]-2′-diphenylphosphino-1,1′-binaphthyl, were determined by X-ray diffraction methods.

Synthesis and Fluxional Behavior, Including a Comparative Analysis of the Pd−N Bond Rupture, of New Chiral Complexes of Palladium(0) and -(II) with a Rigid (Aminoferrocenyl)phosphine Ligand. Crystal Structure of the Two Rotamers of a Palladium(0) Maleic Anhydride Complex

The (aminoferrocenyl)phosphine ligand 1-diphenylphosphino-2,1‘-(1-dimethylaminopropanediyl)ferrocene, 1, was used to synthesize new palladium(0) and -(II) complexes. The reaction of Pd2(dba)3·CHCl3 with 1 in the presence of the electron-withdrawing olefins dimethylfumarate (DMFU) and maleic anhydride (MA) gave the new complexes Pd(1)(DMFU) (2) and Pd(1)(MA) (3). The allyl complex [Pd(η3-2-Me-C3H4)(1)]Tf (4) was obtained from the reaction of 1 with [Pd(η3-2-Me-C3H4)(Cl)]2 in the presence of AgTf. In solution all these compounds exist as mixtures of two diastereomers, with either the alkene or the allyl group differently oriented with respect to the aminophosphine ligand. The orientation of these ligands in the major isomers has been determined by means of NOEs. Alkene rotation takes place in complexes 2 and 3 with free energies of activation ΔG340⧧ = 73.1 kJ mol-1 (2) and ΔG368⧧ = 79.9 kJ mol-1 (3), respectively. These barriers are compared with those of some analogous ferrocenyl aminophosphine ligands, PPFA (2-(1-dimethylaminoethyl)-1-diphenylphosphinoferrocene) and PTFA (1-diphenylphosphino-2,3-endo-(α-dimethylamino)tetramethyleneferrocene) complexes. For 3, the alkene rotation leads to isomer interconversion, while the observed isomerization of 2 must proceed via an olefin face exchange. Some experiments in relation to the nature of this process are discussed. Starting from PdRR‘L‘ precursors and 1 or PPFA, other Pd(II) derivatives of formulas PdRR‘(1), R = Cl, R‘ = Me, L‘ = cod, 5; R = R‘ = Me, L‘ = tmeda, 6; R = R‘ = C6F5, L‘ = cod, 7, or PdClMe(PPFA), 8, were prepared. For 5 and 8, only the isomers with the methyl group trans to nitrogen were obtained. The Pd−N bond rupture in the new complexes of 1 and in similar derivatives of PPFA and PTFA has been analyzed by variable-temperature 1H NMR studies, and in some cases a line shape analysis has been carried out. The influence of the ferrocenyl aminophosphine and ancillary ligands as well as of the oxidation state of the palladium center on this process is discussed. The molecular structures of both rotamers of 3, present in the same crystal, were determined by X-ray structure analysis.

Preparation and reactions of palladium(0)–olefin complexes with iminophosphine ligands

The complexes [Pd( η 2- ol){ o-(Ph 2P)–C 6H 4–CHNR}] [ ol, dimethyl fumarate ( dmf), 1,4-naphtoquinone ( nq), fumaronitrile ( fn); R=C 6H 4OMe-4, CMe 3, Me, bornyl] can be prepared in good yields from the reaction of the allyl derivatives [Pd( η 3-C 3H 5) { o-(Ph 2P)–C 6H 4–CHNR}]BF 4 with an excess of NHEt 2 in the presence of the activated olefin ol. The complex [Pd( η 2- ma){ o-(Ph 2P)–C 6H 4-CHNC 6H 4OMe-4}] ( ma, maleic anhydride) is more conveniently obtained via olefin substitution from [Pd( η 2- dmf){ o-(Ph 2P)–C 6H 4–CHNC 6H 4OMe-4}]. The α-diimine ligand of [Pd( η 2- fn)(py-2-CHNC 6H 4OMe-4)] is quantitatively displaced by the appropriate iminophosphine to give [Pd( η 2- fn){ o-(Ph 2P)–C 6H 4–CHNC 6H 4OMe-4}]. The new zerovalent complexes with P–N ligands are characterized by multinuclear NMR spectroscopy. In solution, olefin rotation or olefin exchange are generally slow. The compound [Pd( η 2- fn){ o-(Ph 2P)–C 6H 4–CHNC 6H 4OMe-4}] reacts with a second molecule of iminophosphine yielding [Pd( η 2- fn){ o-(Ph 2P)–C 6H 4–CHNC 6H 4OMe-4} 2] in which the iminophosphines act essentially as P-monodentate ligands. [Pd( η 2- dmf) { o-(Ph 2P)–C 6H 4–CHNC 6H 4OMe-4}] undergoes fast oxidative addition of allyl chloride to [Pd( η 3-C 3H 5){ o-(Ph 2P)–C 6H 4–CHNC 6H 4OMe-4}] +.

Iron dinitrosyl complexes of TCNE: a synthetic, X-ray crystallographic, high field NMR and electrochemical study

Treatment of Fe(NO) 2[PR 3](CO) with tetracyanoethylene (TCNE) in diethyl ether leads to the formation of Fe(NO) 2[PR 3]( η 2-TCNE), where PR 3=P(OCH 3) 3, 1, P( n-Bu) 3, 2, PMe 2Ph, 3, and PEt 2Ph, 4. An X-ray crystallographic study of 1 shows the iron to be situated in a nearly tetrahedral environment with a π-bonded tetracyanoethylene and two linearly bound nitrosyl groups. From the ambient-temperature NMR spectral data, it is evident that there exist two non-equivalent cyanocarbon environments, indicating that the rotation about the Fe-TCNE π-bond is slowed at room temperature; variable-temperature NMR studies on Fe(NO) 2[P(OMe) 3]( η 2-TCNE), 1, yielded an activation energy barrier of approximately 18.1±0.5 kcal mol −1 for this rotational process. Electrochemical studies revealed that the neutral Fe(NO) 2[PR 3]( η 2-TCNE) complexes undergo irreversible reductions at positively shifted potentials, relative to the related Fe(NO) 2[PR 3](CO) complexes. Moreover, a trend toward cathodic shift of the reduction potentials with increasing phosphine p K a has been observed. The high energy barrier for alkene rotation and the shift towards positive reduction potentials are rationalized in terms of a strong π-interaction between the iron center and TCNE.

Zirconium−Alkyl Isomerizations in Zirconocene-Catalyzed Olefin Polymerization: A Density Functional Study

The migration of a Zr center between adjacent carbon atoms of a Zr-bound alkyl group is investigated by a density functional study of alternative reaction paths available to a (C5H5)2Zr−alkyl cation. This migration is found to occur by the classical reaction route, i.e. by β-H transfer, olefin rotation, and reinsertion into the Zr−H bond without loss of the olefin ligand, rather than by a concerted Zr/H exchange. The activation barrier is determined to decrease from 75 kJ/mol for the degenerate isomerization of a zirconocene ethyl cation to 49 kJ/mol for the isomerization of a primary to a secondary zirconocene propyl cation and 41 kJ/mol for its back-reaction. It is further reduced to 40 kJ/mol for the isomerization of a primary to a tertiary zirconocene isobutyl cation and 31 kJ/mol for its reverse, which model the isomerization process competing with stereoregular chain growth in zirconocene-based catalysts. The facility with which the zirconocene isobutyl cation transfers its β-H atom is connected wit...

Synthesis and Chemistry of Titanacyclopentane and Titanacyclopropane Rings Supported by Aryloxide Ligation

Treatment of the titanacyclopentadiene compound [Ti(OC6H3Ph2-2,6)2(C4Et4)] (3) (OC6H3Ph2-2,6 = 2,6-diphenylphenoxide) with olefins leads to the formation of a variety of stable titanacyclopentane derivatives along with one equivalent of substituted 1,3-cyclohexadiene. The structural and spectroscopic properties of the ethylene product [Ti(OC6H3Ph2-2,6)2(CH2)4] (4) show a ground state titanacyclopentane structure, but facile fragmentation on the NMR time scale to form a bis(ethylene) complex has been detected. The substituted products [Ti(OC6H3Ph2-2,6)2(C4H6R2)] (R = Me, 5; Et, 6; Ph, 7) formed from α-olefins RCHCH2 exist as a mixture of regio- and stereoisomers in hydrocarbon solution. Analysis of a crystal obtained from solutions of 7 showed a trans-2,5-diphenyl-titanacyclopentane ring to be present in the solid state. Alternative routes to the titanacyclopentane compounds involve treatment of the dichlorides [Ti(OC6H3Ph2-2,6)2Cl2] (1) or [Ti(OC6HPh4-2,3,5,6)2Cl2] (2) with either sodium amalgam (2 Na per Ti) or 2 equiv of BunLi in the presence of the substrate olefin. Using these conditions the titanabicyclic compounds [(ArO)2Ti{CH2CH(C4H8)CHCH2}] (ArO = OC6H3Ph2-2,6, 10; OC6HPh4-2,3,5,6, 11) can be obtained by intramolecular coupling of 1,7-octadiene. The spectroscopic properties of 10 and 11 as well as a single-crystal X-ray diffraction analysis of 11 show an exclusive trans stereochemistry is present. Thermolysis of 10 or 11 in the presence of excess 1,7-octadiene leads to the catalytic formation of 2-(methylmethylene)cyclohexane (80%) along with E,Z isomers of 2,6-octadiene (20%). A kinetic study shows the reaction to be zero order in diene with activation parameters, ΔH⧧ = +18.7(5) kcal mol-1 and ΔS⧧ = −26(5) eu. The diphenyltitanacyclopentane 7 will catalyze the dimerization of styrene to trans-1,3-diphenylbut-1-ene followed by isomerization to 1,3-diphenylbut-2-ene. This result shows that although a 2,5-diphenyl regiochemistry was observed in the solid state, styrene dimerization occurs via the 2,4-diphenyltitanacyclopentane intermediate. The facile fragmentation of these titanacyclopentane compounds accounts for the products observed in a number of reactions. Addition of phosphine donor ligands (L) leads to a series of titanacyclopropane compounds [Ti(OC6H3Ph2-2,6)2(η2-CHRCH2)(L)] (R = H, 14; Me, 15; Et, 16; Ph, 17) along with 1 equiv of olefin. The solid-state structure of the ethylene complex 14 shows the C2H4 unit lies approximately coplanar with the Ti−PMe3 bond. This structure is not only maintained in solution but slow olefin rotation is observed on the NMR time scale. In the case of the α-olefin products, two isomers are detected by 1H, 13C, and 31P NMR spectroscopy. Addition of Ph2CO or PhCHNR (R = Ph, CH2Ph) to the titanacyclopentane and titanacyclopropane compounds leads to different products depending upon the reagent and reaction conditions. These can be classified as 2-oxa(aza)titanacycloheptanes, 2-oxa(aza)titanacyclopentanes, 2,5-dioxa(diaza)titanacyclopentanes, and examples of 2-oxatitanacyclopropane (η2-ketone) and 2,7-dioxatitanacycloheptane compounds. The 2-azatitanacyclopentane compounds [Ti(OC6H3Ph2-2,6)2{(PhCH2)NCH(Ph)CH2CH2}] (30) and trans-[Ti(OC6H3Ph2-2,6)2{(Ph)NCH(Ph)CH2CH(Ph)}] (31) react with alkynes to produce the corresponding 2-azatitanacyclopent-4-ene which hydrolyze to produce a stoichiometric equivalent of allylamine. Reaction of 30 with benzonitrile produces the 2,5-diazatitanacyclopent-2-ene [Ti(OC6H3Ph2-2,6)2(NCPhCHPhNR)] (35) along with ethylene.

Density Functional Study of the Mechanism of the Palladium(II)-Catalyzed Ethylene Polymerization Reaction

The hybrid density functional B3LYP method has been used to study the mechanism of ethylene polymerization catalyzed by the Pd(II) diimine complex. The chain initiation starts with coordination of ethylene to the active catalyst [L2PdCH3]+, followed by ethylene migratory insertion into the Pd−alkyl bond to form a γ-agostic intermediate, which rearranges to a more stable β-agostic form. The chain propagation starting with the β-agostic complex 7 can proceed via pathway A through coordination, migratory insertion, and rearrangement, leading to linear polyethylene, or pathway B, through β-hydride elimination, olefin rotation, and reinsertion to give a branched alkyl intermediate, which can then coordinate ethylene and produce branched polyethylene. The rate-determining step is olefin insertion, and the olefin alkyl complexes are the resting states in the catalytic cycle. Associative displacement, where the coordinated polymer is associatively replaced from the same diimine−Pd−hydrido−olefin complex 7 by ethy...