Highly Selective Olefin Trimerization Catalysis by a Borane-Activated Titanium Trimethyl Complex

Reaction of a trimethyl titanium complex, (FI)­TiMe₃ (FI = phenoxy-imine), with 1 equiv of B­(C₆F₅)₃ gives [(FI)­TiMe₂]­[MeB­(C₆F₅)₃], an effective precatalyst for the selective trimerization of ethylene. Mechanistic studies indicate that catalyst initiation involves generation of an active Ti^II species by olefin insertion into a Ti–Me bond, followed by β-H elimination and reductive elimination of methane, and that initiation is slow relative to trimerization. (FI)­TiMe₃/B­(C₆F₅)₃ also leads to a competent catalyst for the oligomerization of α-olefins, displaying high selectivity for trimers (>95%), approximately 85% of which are one regioisomer. This catalyst system thus shows promise for selectively converting light α-olefins into transportation fuels and lubricants

Alkyne Hydroamination and Trimerization with Titanium Bis(phenolate)pyridine Complexes: Evidence for Low-Valent Titanium Intermediates and Synthesis of an Ethylene Adduct of Titanium(II)

A class of titanium precatalysts of the type (ONO)­TiX₂ (ONO = pyridine-2,6-bis­(4,6-di-<i>tert</i>-butylphenolate); X = Bn, NMe₂) has been synthesized and crystallographically characterized. The (ONO)­TiX₂ (X = Bn, NMe₂, X₂ = NPh) complexes are highly active precatalysts for the hydroamination of internal alkynes with primary arylamines and some alkylamines. A class of titanium imido/ligand adducts, (ONO)­Ti­(L)­(NR) (L = HNMe₂, py; R = Ph, ^<i>t</i>Bu), have also been synthesized and characterized and provide structural analogues to intermediates on the purported catalytic cycle. Furthermore, these complexes exhibit unusual redox behavior. (ONO)­TiBn₂ (<b>1</b>) promotes the cyclotrimerization of electron-rich alkynes, likely via a catalytically active Ti^II species that is generated in situ from <b>1</b>. Depending on reaction conditions, these Ti^II species are proposed to be generated through Ti benzylidene or imido intermediates. A formally Ti^II complex, (ONO)­Ti^II(η²-C₂H₄)­(HNMe₂) (<b>7</b>), has been prepared and structurally characterized

Highly Selective Olefin Trimerization Catalysis by a Borane-Activated Titanium Trimethyl Complex

Reaction of a trimethyl titanium complex, (FI)­TiMe₃ (FI = phenoxy-imine), with 1 equiv of B­(C₆F₅)₃ gives [(FI)­TiMe₂]­[MeB­(C₆F₅)₃], an effective precatalyst for the selective trimerization of ethylene. Mechanistic studies indicate that catalyst initiation involves generation of an active Ti^II species by olefin insertion into a Ti–Me bond, followed by β-H elimination and reductive elimination of methane, and that initiation is slow relative to trimerization. (FI)­TiMe₃/B­(C₆F₅)₃ also leads to a competent catalyst for the oligomerization of α-olefins, displaying high selectivity for trimers (>95%), approximately 85% of which are one regioisomer. This catalyst system thus shows promise for selectively converting light α-olefins into transportation fuels and lubricants

Guanidine-Functionalized Rhenium Cyclopentadienyl Carbonyl Complexes: Synthesis and Cooperative Activation of H–H and O–H Bonds

Catalytic reactions utilizing carbon monoxide as a substrate are numerous, and they typically involve selective functionalization of a metal-bound CO. We have developed group 7 carbonyl complexes where secondary coordination sphere, Lewis acidic functionalities can assist in the activation of substrate molecules, mainly in the context of syngas conversion. This work describes a new class of cyclopentadienyl (Cp) rhenium carbonyl compounds of the type [Re­(η⁵-C₅H₄DMEG)­(CO)_3<i>–n</i>(NO)_<i>n</i>]^<i>n</i> (DMEG = dimethylethyleneguanidine, <i>n</i> = 0, 1), where a tethered guanidine base is appended to the Cp ring to participate in cooperative substrate activation with the electrophilic carbonyl. A reliable synthetic route for these complexes is presented, with crystallographic characterization of the free-base and protonated forms for both the carbonyl and mixed carbonyl-nitrosyl complexes. The latter are employed as platforms to study heterolytic H–H and O–H bond cleavage reactions that result in nucleophilic CO functionalization. The corresponding formyl complex is prepared by hydride transfer, and by measuring its hydricity (Δ<i><i>G</i>°</i>_H–) and p<i>K</i>_a of the protonated base, the free energy of H₂ cleavage is found to be +3.3(6) kcal/mol. The activation of methanol to form methoxycarbonyl complexes is found to be more favorable, with Δ<i><i>G</i>°</i> ≈ 0 for the intramolecular addition of methanol to the guanidine-appended carbonyl complex. A detailed thermodynamic study is described for both the intramolecular methanol activation reaction and related intermolecular reactions with external bases. The results highlight some tangible thermodynamic benefits of tethering the base in the secondary coordination sphere

Mechanistic Studies of Ethylene and α-Olefin Co-Oligomerization Catalyzed by Chromium–PNP Complexes

To explore the possibility of producing a narrow distribution of mid- to long-chain hydrocarbons from ethylene as a chemical feedstock, co-oligomerization of ethylene and linear α-olefins (LAOs) was investigated, using a previously reported chromium complex, [CrCl₃(PNP^OMe)] (<b>1</b>, where PNP^OMe = <i>N</i>,<i>N</i>-bis­(bis­(<i>o</i>-methoxyphenyl)­phosphino)­methylamine). Activation of <b>1</b> by treatment with modified methylaluminoxane (MMAO) in the presence of ethylene and 1-hexene afforded mostly C₆ and C₁₀ alkene products. The identities of the C₁₀ isomers, assigned by detailed gas chromatographic and mass spectrometric analyses, strongly support a mechanism that involves five- and seven-membered metallacyclic intermediates comprised of ethylene and LAO units. Using 1-heptene as a mechanistic probe, it was established that 1-hexene formation from ethylene is competitive with formation of ethylene/LAO cotrimers and that cotrimers derived from one ethylene and two LAO molecules are also generated. Complex <b>1/</b>MMAO is also capable of converting 1-hexene to C₁₂ dimers and C₁₈ trimers, albeit with poor efficiency. The mechanistic implications of these studies are discussed and compared to previous reports of olefin cotrimerization

Spectral Studies of a Cr(PNP)–MAO System for Selective Ethylene Trimerization Catalysis: Searching for the Active Species

Variable temperature spectroscopic, kinetic, and chemical studies were performed on a soluble Cr^IIICl₃(PNP) (PNP = bis­(diarylphosphino)­alkylamine) ethylene trimerization precatalyst to map out its methylaluminoxane (MAO) activation sequence. These studies indicate that treatment of Cr^IIICl₃(PNP) with MAO leads first to replacement of chlorides with alkyl groups, followed by alkyl abstraction, and then reduction to lower–valent species. Reactivity studies demonstrate that the majority of the chromium species detected are not catalytically active

Synthesis of a Bis(thiophenolate)pyridine Ligand and Its Titanium, Zirconium, and Tantalum Complexes

A precursor to a new tridentate LX₂ type ligand, bis­(thiophenol)­pyridine ((SNS)­H₂ = (2-C₆H₄SH)₂-2,6-C₅H₃N), was prepared. Bis­(thiophenolate)­pyridine complexes of Ti, Zr, and Ta having dialkylamido coligands were synthesized and structurally characterized. The zirconium complex (SNS)­Zr­(NMe₂)₂ (<b>4</b>) displays <i>C</i>₂ symmetry in the solid state, unlike a related bis­(phenolate)­pyridine compound, <i>C</i>_<i>s</i>-symmetric (ONO)­Ti­(NMe₂)₂. This change is likely the result of strain about the sulfur atom in the six-membered chelate with longer metal–sulfur and carbon–sulfur bonds. Solid-state structures of tantalum complexes (SNS)­Ta­(NMe₂)₃ (<b>5</b>) and (SNS)­TaCl­(NEt₂)₂ (<b>6</b>) also display pronounced <i>C</i>₂ twisting of the SNS ligand. 1D and 2D NMR experiments show that <b>5</b> is fluxional, with rotation about the Ta–N­(amide) bonds occurring on the NMR time scale that interchange the equatorial amide methyl groups (Δ<i><i>G</i></i>^⧧₃₉₃ = 25.0(3) kcal/mol). The fluxional behavior of <b>6</b> in solution was also studied by variable-temperature ¹H NMR. Observation of separate signals for the diastereotopic protons of the methylene unit of the diethylamide indicates that the complex remains locked on the NMR time scale in one diastereomeric conformation at temperatures below −50 °C, fast rotation about the equatorial amide Ta–N bonds occurs at higher temperature (Δ<i><i>G</i></i>^⧧₃₉₃ = 13.4(3) kcal/mol), and exchange of diastereomeric methylene protons occurs via inversion at Ta that interconverts antipodes (Δ<i><i>G</i></i>^⧧₃₉₃ ≈ 14(1) kcal/mol)

Diverse C–C Bond-Forming Reactions of Bis(carbene)platinum(II) Complexes

The platinum(0) complex Pt­(PPh₃)₄ catalyzes coupling of the carbene ligands of (CO)₅Cr­{C­(OMe)­(<i>p-</i>MeOC₆H₄)} (<b>1</b>). The stable bis­(carbene)­platinum­(II) complexes Cl₂Pt­{C­(OMe)­(Me)}₂ (<b>3</b>), Br₂Pt­{C­(OMe)­(Me)}₂ (<b>4</b>), and Cl₂Pt­{C­(O^<i>i</i>Pr)­(Me)}₂ (<b>5</b>) can be induced to undergo C–C coupling reactions by several means. Reduction of <b>3</b>–<b>5</b> to platinum(0) with cobaltocene results in formation of internal olefins, (<i>E/Z</i>)-2,3-dimethoxybut-2-ene (<b>6</b>) or (<i>E/Z</i>)-2,3-diisopropoxybut-2-ene (<b>7</b>). Reaction of <b>3</b>–<b>5</b> with PPh₃ yields terminal olefins, 2,3-dimethoxybut-1-ene (<b>13</b>) or 2,3-diisopropoxybut-1-ene (<b>15</b>), along with Cl₂Pt­(PPh₃)₂ (<b>12</b>) or Br₂Pt­(PPh₃)₂ (<b>14</b>). In contrast, addition of pyridine to <b>3</b>–<b>5</b> does not effect C–C coupling; instead, the acyl complexes <i>cis</i>-Cl­(py)­Pt­(COMe)­{C­(OMe)­(Me)} (<b>8</b>), <i>cis</i>-Br­(py)­Pt­(COMe)­{C­(OMe)­(Me)} (<b>9</b>), and <i>cis</i>-Cl­(py)­Pt­(COMe)­{C­(O^<i>i</i>Pr)­(Me)} (<b>10</b>) are obtained, with concomitant formation of alkyl halide. Possible mechanistic pathways for C–C bond formation are discussed, as well as explanations for the different reactivities observed for pyridine and PPh₃

Alkyne Hydroamination and Trimerization with Titanium Bis(phenolate)pyridine Complexes: Evidence for Low-Valent Titanium Intermediates and Synthesis of an Ethylene Adduct of Titanium(II)

A class of titanium precatalysts of the type (ONO)­TiX₂ (ONO = pyridine-2,6-bis­(4,6-di-<i>tert</i>-butylphenolate); X = Bn, NMe₂) has been synthesized and crystallographically characterized. The (ONO)­TiX₂ (X = Bn, NMe₂, X₂ = NPh) complexes are highly active precatalysts for the hydroamination of internal alkynes with primary arylamines and some alkylamines. A class of titanium imido/ligand adducts, (ONO)­Ti­(L)­(NR) (L = HNMe₂, py; R = Ph, ^<i>t</i>Bu), have also been synthesized and characterized and provide structural analogues to intermediates on the purported catalytic cycle. Furthermore, these complexes exhibit unusual redox behavior. (ONO)­TiBn₂ (<b>1</b>) promotes the cyclotrimerization of electron-rich alkynes, likely via a catalytically active Ti^II species that is generated in situ from <b>1</b>. Depending on reaction conditions, these Ti^II species are proposed to be generated through Ti benzylidene or imido intermediates. A formally Ti^II complex, (ONO)­Ti^II(η²-C₂H₄)­(HNMe₂) (<b>7</b>), has been prepared and structurally characterized

Scope and Mechanism of Homogeneous Tantalum/Iridium Tandem Catalytic Alkane/Alkene Upgrading using Sacrificial Hydrogen Acceptors

An in-depth investigation of a dual homogeneous catalyst system for the coupling of alkanes and alkenes based on an early-/late-transition-metal pairing is reported. The system is composed of Cp*TaCl₂(alkene) for alkene dimerization and pincer-iridium hydrides for alkane/alkene transfer hydrogenation. Because there is no kinetically relevant interaction between the two catalysts, the tandem mechanism can be entirely described using the two independent catalytic cycles. The alkene dimerization mechanism is characterized by an entropically disfavored pre-equilibrium between Cp*TaCl₂(1-hexene) + 1-hexene and Cp*TaCl₂(metallacyclopentane) (Δ<i>H</i>° = −22(2) kcal/mol; Δ<i>S</i>° = −16(2) eu); thus, the overall rate of alkene dimerization is positive order in 1-hexene (exhibiting saturation kinetics), and increases only modestly with temperature. In contrast, the rate of 1-hexene/<i>n</i>-heptane transfer hydrogenation catalyzed by <i>t</i>-Bu­[PCP]­IrH₄ is inverse order in 1-hexene and increases substantially with temperature. Styrene has been investigated as an alternate sacrificial hydrogen acceptor. Styrene dimerization catalyzed by Cp*TaCl₂(alkene) is considerably slower than 1-hexene dimerization. The conversion of styrene/heptane mixtures by the Ta/Ir tandem system leads to three product types: styrene dimers, coupling of styrene and heptane, and heptene dimers (from heptane). Through careful control of reaction conditions, the production of heptene dimers can be favored, with up to 58% overall yield of heptane-derived products and cooperative TONs of up to 12 and 10 for Ta and Ir catalysts, respectively. There is only slight inhibition of Ir-catalyzed styrene/<i>n</i>-heptane transfer hydrogenation under the tandem catalysis conditions