Reactions of Secondary Allylamines with Bis(η5:η1-pentafulvene)titanium Complexes: Selective Formation of Monoazabutadiene Titanium Complexes by N–H and C–H Bond Activation

Titanium-Catalyzed Polymerization of a Lewis Base-Stabilized Phosphinoborane

The reaction of the Lewis base-stabilized phosphinoborane monomer tBuHPBH2 NMe3 (2 a) with catalytic amounts of bis(η5 :η1 -adamantylidenepentafulvene)titanium (1) provides a convenient new route to the polyphosphinoborane [tBuPH-BH2 ]n (3 a). This method offers access to high molar mass materials under mild conditions and with short reaction times (20 °C, 1 h in toluene). It represents an unprecedented example of a transition metal-mediated polymerization of a Lewis base-stabilized Group 13/15 compound. Preliminary studies of the substrate scope and a potential mechanism are reported.

Cp2Ti(II) Mediated Rearrangement of Cyclopropyl Imines

Ti-catalyzed oxidative alkyne carboamination with alkenes and azo compounds can yield either α,β-unsaturated imines or cyclopropyl imines through a common azatitanacyclohexene intermediate. Herein, we report the synthesis of a model azatitanacyclohexene complex (3) through the ring-opening of a cyclopropyl imine with Cp2Ti(BTMSA) (BTMSA = bis(trimethylsilyl)acetylene). 3 readily undergoes thermal or reductant-catalyzed ring contraction to an azatitanacyclopentene (4), analogous to the proposed mechanism for forming α,β-unsaturated imines in the catalytic reaction. A cyclopropyl imine or an α,β-unsaturated imine could be liberated via the oxidation of 3 or 4 with azobenzene, respectively, further implicating the role of these metallacycles in the Ti-catalyzed carboamination reaction.

Selective propargylic C(sp3)-H activation of methyl-substituted alkynes versus [2 + 2] cycloaddition at a titanium imido template

The reaction of the titanium imido complex 1b with 2-butyne leads to the formation of the titanium azadiene complex 2a at ambient temperature instead of yielding the archetypical [2 + 2] cycloaddition product (titanaazacyclobutene) which is usually obtained by combining titanium imido complexes and internal alkynes. The formation of 2a is presumably caused by an initial propargylic C(sp³)–H activation step and quantum chemical calculations suggest that the outcome of this unexpected reactivity is thermodynamically favored. The previously reported titanaazacyclobutene I (which is obtained by reacting 1b with 1-phenyl-1-propyne) undergoes a rearrangement reaction at elevated temperature to give the corresponding five-membered titanium azadiene complex 2b.

An unexpected reactivity between a titanium imido complex and internal alkynes was unveiled yielding titanaazacyclobutenes instead of the expected [2 + 2] cycloaddition products.

Advantages of Group 4 Metallocene Bis(trimethylsilyl)acetylene Complexes as Metallocene Sources Towards Other Synthetically used Systems

Active species for synthetic and catalytic applications are formed from well defined complexes or mixtures of compounds. For group 4 metallocenes, three pathways for the formation of the reactive complex fragment [Cp'2M] are known: (i) reductive mixtures and well defined complexes which are able to form the metallocene fragments either by (ii) addition or (iii) substitution reactions. In this account for each of theses systems (i)-(iii) a prominent example will be discussed in detail, (i) the Negishi reagent Cp2ZrCl2/n-BuLi, (ii) bis(η5 : η1-pentafulvene) complexes and (iii) metallocene bis(trimethylsilyl)acetylene complexes, to show the advantages and the disadvantages for each of these methods for synthetic applications. This account summarizes some main advantages of group 4 metallocene bis(trimethylsilyl)acetylene complexes as metallocene generating agents over other synthetically used systems. For each of the special purposes, all described systems have advantages as well as disadvantages. The aim of this overview is to help synthetic chemists in selecting the most effective system on the basis of [Cp'2M] (M=Ti, Zr) for synthetic or catalytic puposes.

Synthesis, characterization, and reactivity of dinuclear organo-rare-earth-metal alkyl complexes supported by 2-amidate-functionalized indolyl ligands: substituent effects on coordination and reactivity

Two series of new dinuclear organo-rare-earth-metal alkyl complexes supported by 2-amidate-functionalized indolyl ligands with different haptic modes were synthesized and characterized. The treatment of [RE(CH2SiMe3)3(THF)2] with 1 equiv. of 2-(2,6-iPr2C6H3NHC[double bond, length as m-dash]O)C8H5NH (H2L1) and 2-(2-tBuC6H4NHC[double bond, length as m-dash]O)C8H5NH (H2L2) in toluene yielded the dinuclear organo-rare-earth-metal alkyl complexes {[η1:(μ2-η1:η1)-L1]RE(CH2SiMe3)(THF)2}2 [RE = Gd (1a), Dy (1b), Y (1c), Er (1d), and Yb (1e)] and {[η1:(μ2-η1:η1):η1-L2]RE(CH2SiMe3)(THF)2}2 [RE = Gd (2a), Dy (2b), Y (2c), Er (2d), and Yb (2e)] in good yields. When [RE(CH2SiMe3)3(THF)2] were treated with 2 equiv. of H2L1 or H2L2 in THF, the dinuclear organo-rare-earth-metal complexes {(η1:η1-HL)[η1:(μ2-η1:η1):η1-L]RE(THF)}2 (1ca: RE = Y, L = L1; 2ea: RE = Yb, L = L2) were obtained. The complexes could react with small organic molecules such as N,N'-diisopropylcarbodiimide (DIC), phenyl isocyanate, N-methylallylamine, phenylacetylene, pyridine, N-phenylimidazole, or 4-dimethylaminopyridine (DMAP) to yield a series of new complexes with different reactivity patterns along with the reported rare-earth-metal alkyl complexes. In the presence of cocatalysts, these dinuclear organo-rare-earth-metal alkyl complexes could initiate isoprene polymerization with high activity (100% conversion of 2000 equiv. of isoprene in 12 h), yielding polymers with high regioselectivity (1,4 polymers up to 96.1%).

Strong Preference of the Redox-Neutral Mechanism over the Redox Mechanism for the TiIV Catalysis Involved in the Carboamination of Alkyne with Alkene and Diazene

Titanium catalysis generally prefers redox-neutral mechanisms. Yet it has been reported that titanium could promote bond formations in a way similar to reductive elimination. Accordingly, redox catalytic cycles involving Ti^IV /Ti^II cycling have been considered. By studying, as an example, the carboamination of alkynes with alkenes and azobenzene catalyzed by the [Ti^IV ]=NPh imido complex, we performed DFT computations to gain an understanding of how the "abnormal" catalysis takes place, thereby allowing us to clarify whether the catalysis really follows Ti^IV /Ti^II redox mechanisms. The reaction first forms an azatitanacyclohexene by alkyne addition to the [Ti^IV ]=NPh bond, followed by alkene insertion. The azatitanacyclohexene can either undergo C^α -C^γ coupling, to afford bicyclo[3.1.0]imine, or β-H elimination, to yield a [Ti^IV ]-H hydride, which then undergoes C^α =C^β or C^γ =C^δ insertion to give an α,β- or β,γ-unsaturated imine, respectively. Both the geometric and electronic structures indicate that the catalytic cycles proceed through redox-neutral mechanisms. The alternative redox mechanisms (e.g., by N-H or C-H reductive elimination) are substantially less favorable. We concluded that electronically, the Ti^IV catalysis intrinsically favors the redox-neutral mechanism, because a redox pathway would involve Ti^II structures either in the triplet ground state or in the high-lying open-shell singlet state, but the involvement of triplet Ti^II structures is spin-forbidden and that of singlet Ti^II structures is energetically disadvantageous.

Crystal structure of the η4-ketimine titanium complex (di-phenyl-amido-κN){3-methyl-6-[(4-methyl-phen-yl)(phenyl-aza-nid-yl)methyl-idene]cyclo-hexa-2,4-dien-1-yl-κ2N,C1}(η5-penta-methyl-cyclo-penta-dien-yl)titanium(IV)

The mol-ecular structure of the title titanium(IV) half-sandwich complex, [Ti(η5-C10H15)(η4-C21H19N)(C12H10N)], shows a three-legged piano-stool geometry at the central TiIV atom, comprising of one penta-methyl-cyclo-penta-dienyl ligand, one bidentate ketimine ligand in an η4-coordination mode and one monodentate di-phenyl-amide ligand. Except for van der Waals forces, there are no significant inter-molecular inter-actions in the crystal.