Synthesis, Characterization, and Reduction Chemistry of Mixed-Cyclopentadienyl/Arylsulfide Titanium Dichlorides

A series of titanium derivatives [CpTi(SAr)Cl2] containing both cyclopentadienyl and various substituted arylsulfide ligands (SAr = SC6H4Me-4 (1), SC6H2Me3-2,4,6 (2), SC6H2Pri3-2,4,6 (3), SC6H2Ph3-2,4,6 (4)) has been synthesized from the reaction of [CpTiCl3] with 1 equiv of the lithium salt of the corresponding arylsulfides in benzene. X-ray diffraction studies show that each metal center possesses a pseudo-tetrahedral geometry. The compounds undergo one-electron reduction to produce sulfur-bridged dimers of the type [CpTiCl(μ-SAr)]2 (SAr = SC6H2Me3-2,4,6 (5), SC6H2Pri3-2,4,6 (6)). The solid-state structures of these dimers show that the cyclopentadienyl rings are arranged in a transoid fashion about the [Ti(μ-SAr)2Ti] core. The Ti−Ti distances, 3.242(1) and 3.225(1) Å, respectively, are similar to those found in previously reported Cp/aryloxide dimers with terminal aryloxides and bridging chlorides

Ancillary Ligand Control of the Regiochemistry of Coupling of 3,3-Dimethyl-1-butyne at Titanium Metal Centers

The ambient-temperature sodium amalgam (2 Na per Ti) reduction of hydrocarbon solutions of [Cp(ArO)TiCl2] (ArO = 2,3,5,6-tetraphenylphenoxide) (1) in the presence of 3,3-dimethyl-1-butyne yields the 2,5-di(tert-butyl)titanacyclopentadiene compound [Cp(ArO)Ti(C4H2But2 -2,5)] (2). An X-ray diffraction study of 2 confirms the regiochemistry and shows carbon−carbon distances of 1.343(3) and 1.492(3) Å for the double and single bonds, respectively. In contrast the analogous reaction of either [Cp2TiCl2] or [(ArO)2TiCl2] in the presence of 3,3-dimethyl-1-butyne yields the corresponding 2,4-di(tert-butyl)titanacyclopentadiene compounds. When 2 is heated at 100 °C for a few days in C6D6 solution, isomerization to the more stable 2,4-di(tert-butyl) regioisomer 3 is observed by NMR. An attempt is made to rationalize the regiochemistry of the kinetically formed titanacyclopentadiene in terms of steric factors within the intermediate bis(alkyne) complex

Synthesis of Group 1 Metal 2,6-Diphenylphenoxide Complexes [M(OC₆H₃Ph₂-2,6)] (M = Li, Na, K, Rb, Cs) and Structures of the Solvent-Free Complexes [Rb(OC₆H₃Ph₂-2,6)]<i>_x</i> and [Cs(OC₆H₃Ph₂-2,6)]<i>_x</i>: One-Dimensional Extended Arrays of Metal Aryloxides

Reaction of 2,6-diphenylphenol (HOC6H3Ph2-2,6) with nBuLi, NaH, KH, or Rb or Cs metal in benzene gives the solvent-free complexes [M(OAr)]x in excellent yield. The complex [Rb(OC6H3Ph2-2,6)]x exhibits a ladderlike structure in the solid state with triply bridging oxygen atoms and Rb−O distances of 2.743(3), 2.930(2), and 2.973(2) Å. The Rb cations interact with the π-electron cloud of the arene moieties, giving rise to a high Rb coordination number. The cesium-containing congener forms a layered, columnlike structure consisting of [Cs2(μ2-OAr)2] units, with nearly identical Cs−O distances of 2.945(2) and 2.947(2) Å. The individual layers are held together solely by Cs−arene π-interactions

Regio- and Stereoselective Synthesis of the 1,3-Cyclohexadiene Nucleus by [2 + 2 + 2] Cycloaddition Reactions Catalyzed by Titanium Aryloxide Compounds

A variety of titanium aryloxide reagents catalyze the cross coupling of two alkyne units with 1 equiv of olefin to produce the 1,3-cyclohexadiene nucleus. Catalysts include isolated titanacyclopentadiene or titanacyclopentane complexes. The reaction proceeds via attack of the olefin upon a titanacyclopentadiene compound initially formed by coupling of two alkyne units. The reaction is limited to bulky alkyne substrates that undergo slow catalytic cyclotrimerization via competing attack of a third alkyne upon the titanacyclopentadiene ring. The organic products isolated are typically the result of an isomerization within the initially produced 1,3-cyclohexadiene nucleus. Mechanistic studies show that these isomerization processes occur via sequential, metal-mediated 1,5-hydrogen shifts upon a single face of the six-membered ring, exclusively leading to a cis-stereochemistry within the final products. In the reactions of the diynes RC⋮C(CH2)4C⋮CR (R = Et, SiMe3), coupling with ethylene and α-olefins produces a variety of substituted hexalins. A combination of NMR spectroscopy, photochemistry, and molecular mechanics calculations has been applied to determine the stereochemistry and ground state conformations adopted by the product 1,3-cyclohexadienes and hexalins. The primary and secondary photoproducts obtained from some of these 1,3-cyclohexadiene compounds have been characterized

Reactivity of a Tungsten(II) Aryloxide with Isocyanides and Isocyanates

The 16-electron W(II) aryloxide [W(OC6HPhMe2-η6-C6H4)(OC6HPh2-2,6-Me2-3,5)(η1-dppm)] (2); OC6HPh2-2,6-Me2-3,5 = 2,6-diphenyl-3,5-dimethylphenoxide) reacts with 2,6-dimethylphenyl isocyanide (xyNC) to produce the structurally distorted compounds cis-[W(OC6HPh2-2,6-Me2-3,5)2(xyNC)4] (3). The addition of PhNCO to 2 produces a product (5) in which both cleavage of an isocyanate CN double bond and insertion of an isocyanate into a WOAr bond has occurred

Reactivity of Tantalum Hydride Aryloxide Complexes toward Organic Isocyanide Reagents

The tantalum monohydride compound [Ta(OC6H3Pri2-2,6)2Cl2(H)(PMe2Ph)2] (1) reacts with organic isocyanides RNC to produce the phosphine adduct η2-iminoformyl complexes [Ta(OC6H3Pri2-2,6)2Cl2{HC(PMe2Ph)NR}] (R = 2,6-diisopropylphenyl (2a), 2,6-dimethylphenyl (2b), tert-butyl (2c)). Compounds 2b,c were only detected spectroscopically as intermediates, while 2a was isolated and reacted with PMe3 to produce the complex [Ta(OC6H3Pri2-2,6)2Cl2{HC(PMe3)NC6H3Pri2-2,6}] (3), which was structurally characterized. The structure of 3 is best described as trigonal bipyramidal about tantalum with a chloride atom and the phosphine adduct of an η2-iminoformyl group occupying axial sites. Important structural parameters for 3 are Ta−N = 1.956(4), Ta−C = 2.210(5), and N−C = 1.421(7) Å. Compounds 2a and 3 do not react with a further 1 equiv of 2,6-diisopropylphenyl isocyanide. In contrast 2b reacts with 2,6-dimethylphenyl isocyanide to produce the ylide derivative [Ta(OC6H3Pri2-2,6)3Cl2{RNCHC(PMe2Ph)NR}] (4) (R = C6H3Me2-2,6), while 2c reacts with an additional 2 equiv of ButNC to produce the compound [Ta(OC6H3Pri2-2,6)3Cl2{ButNCHC(CNBut)NBut}] (5). Spectroscopic data on 4 and 5 and a single-crystal X-ray diffraction study of 5 show both molecules contain 2,5-diazacyclopent-2-ene rings with either ylide (4) or keteneimine (5) groups attached to the C-4 position. Addition of ButNC to 2a produces the complex [Ta(OC6H3Pri2-2,6)2Cl2{RNCHC(CNBut)NBut}] (6) (R = C6H3Pri2-2,6). The tantalum dihydride compound [Ta(OC6H3But2-2,6)2Cl2(H)(PMePh2)] (7) reacts with RNC to produce the monocyclometalated derivatives [Ta(OC6H3But-CMe2CH2)(OC6H3But2-2,6)Cl2{N(R)CH3}] (8) (a, R = C6H3Me2-2,6; b, R = But), in which a total of three hydride groups have been transferred to the isocyanide substrate to produce the dialkylamido ligand

Synthesis of the Tantalum Hydride Complex (<i>R,R</i>)-[Ta(O₂C₂₀H₁₀{SiMe₃}₂-3,3‘)₂(H)] and Reactivity with Aldehydes, Ketones, Acetylenes, and Related Substrates: A Reagent for the Asymmetric Hydrogenation of Prochiral Carbonyl Species^†

The tantalum phenyl complex (R,R)-[Ta(O2C20H10{SiMe3}2-3,3‘)2(Ph)], (R,R)-1, has been prepared from [H2NEt2][Ta(O2C20H10{SiMe3}2-3,3‘)2Cl2] and has been employed for the synthesis of the hydride species (R,R)-[Ta(O2C20H10{SiMe3}2-3,3‘)2(H)], (R,R)-2, by reaction with diisobutylaluminum hydride (DIBAL-H). The reaction proceeds cleanly and in high yield. Compound (R,R)-2 undergoes clean reactions with acetophenone and benzaldehyde-d to give alkoxide complexes that are chiral at the α-carbon. The progress of these reactions was monitored by 13C NMR spectroscopy. An X-ray crystal structure of one of the products (R,R,R)/(R,R,S)-[Ta(O2C20H10{SiMe3}2-3,3‘)2(OCH{CH3}{Ph})(OC{CH3}{Ph})] was obtained, and the catalytic conversion of benzaldehyde-d to benzyl alcohol-d was carried out using (R,R)-2. The hydride complex also reacts with acetylenes to give vinyl metal complexes, with 2,6-dimethylphenyl isocyanide to give an iminoformyl complex, and with allene to yield a labile η3-π-allyl species

Reactivity of a Tungsten(II) Aryloxide with Isocyanides and Isocyanates

The 16-electron W(II) aryloxide [W(OC6HPhMe2-η6-C6H4)(OC6HPh2-2,6-Me2-3,5)(η1-dppm)] (2); OC6HPh2-2,6-Me2-3,5 = 2,6-diphenyl-3,5-dimethylphenoxide) reacts with 2,6-dimethylphenyl isocyanide (xyNC) to produce the structurally distorted compounds cis-[W(OC6HPh2-2,6-Me2-3,5)2(xyNC)4] (3). The addition of PhNCO to 2 produces a product (5) in which both cleavage of an isocyanate CN double bond and insertion of an isocyanate into a WOAr bond has occurred

Reactivity of Group 4 Metal Alkyl and Metallacyclic Compounds Supported by Aryloxide Ligands Toward Organic Isocyanides

The reactivity of the dimethyl compound [Ti(OC6H3Ph2-2,6)2Me2] (1) and titanabicyclic compounds [(ArO)2Ti(CH2CH(C4H8)CHCH2)] (6, 7) (formed by trans-coupling of 1,7-octadiene) toward organic isocyanides has been investigated. Dimethyl 1 reacts with 1 equiv of 2,6-dimethylphenylisocyanide (xyNC) to produce a cyclometalated product [Ti(OC6H3Ph2-2,6)(OC6H3Ph-η1-C6H4){N(xy)CHMe2}] (2), which was structurally characterized. Compound 2 is believed to arise via CH bond activation within an intermediate η2-imine (azatitanacyclopropane) formed by transfer of both methyl groups to the isocyanide. With an excess of xyNC, 1 produces an intermediate bis(η2-iminoacyl) 4, which slowly converts to the enediamide [Ti(OC6H3Ph2-2,6)2{N(xy)CMeCMeN(xy)}] (5). The solid-state structure of 5 shows the presence of a folded diazatitanacyclopent-3-ene ring formed by intramolecular coupling of the two iminoacyl groups. Titanabicyclic compounds 6 and 7 react with ButNC and xyNC respectively to generate products 8 and 9, which both contain amide ligands derived by CH bond activation and an iminoacyl group formed by insertion into the new Ti−C bond. The solid-state structure of [Ti(OC6H3Ph2-2,6){OC6H3Ph-(η2-(C6H4)CNBut)}{ButNCH(C8H14)}] (8) shows a trans-fusion between the five- and six-membered carbon rings as found in the initial titanabicycle 6. The treatment of 1 with xyNC in the presence of excess 3-hexyne or styrene produces azatitanacyclopent-2-ene 10 and azatitanacyclopentane 11 derivatives, respectively. These arise via coupling of the alkyne or olefin with an intermediate η2-imine. The solid-state structure of [Ti(OC6H3Ph2-2,6)2{N(xy)C(Me2)C(Et)C(Et)}] (10) confirms its formulation

Synthesis, Characterization, and Reduction Chemistry of Mixed-Cyclopentadienyl/Arylsulfide Titanium Dichlorides

A series of titanium derivatives [CpTi(SAr)Cl2] containing both cyclopentadienyl and various substituted arylsulfide ligands (SAr = SC6H4Me-4 (1), SC6H2Me3-2,4,6 (2), SC6H2Pri3-2,4,6 (3), SC6H2Ph3-2,4,6 (4)) has been synthesized from the reaction of [CpTiCl3] with 1 equiv of the lithium salt of the corresponding arylsulfides in benzene. X-ray diffraction studies show that each metal center possesses a pseudo-tetrahedral geometry. The compounds undergo one-electron reduction to produce sulfur-bridged dimers of the type [CpTiCl(μ-SAr)]2 (SAr = SC6H2Me3-2,4,6 (5), SC6H2Pri3-2,4,6 (6)). The solid-state structures of these dimers show that the cyclopentadienyl rings are arranged in a transoid fashion about the [Ti(μ-SAr)2Ti] core. The Ti−Ti distances, 3.242(1) and 3.225(1) Å, respectively, are similar to those found in previously reported Cp/aryloxide dimers with terminal aryloxides and bridging chlorides