Synthesis and Reactions of a Cyclopentadienyl-Amidinate Titanium tert-Butoxyimido Compound

Eco-friendly methoximation of aromatic aldehydes and ketones using MnCl2.4H2O as an easily accessible and efficient catalyst

Methoximes are important as a class of intermediates and products, among fine chemicals and specialties. The development of a new, facile and efficient method for their synthesis is reported. The methoximes were properly accessed from the corresponding aromatic aldehydes and ketones in good to excellent yields, under mild conditions, employing the inexpensive and environmentally friendly MnCl2.4H2O as a catalyst (at low loading and without the addition of ligand), in EtOH at 50°C. The scope of the process was systematically assessed.

Formation of amidino-borate derivatives by a multi-component reaction

Cyclohexene reacts with the (Fmes)BH2·SMe2 borane reagent and three molar equivalents of the isonitrile CN-Xyl to give the five membered 1,3-BN heterocyclic product 7 that contains a zwitterionic borata-amidinium moiety and a cyclohexenyl substituent. The analogous five-component coupling between cyclopentene, (Fmes)BH2·SMe2 and CN-Xyl in a 1 : 1 : 3 molar ratio gives the related cyclic amidino-borate derivative 10. The reaction of the (Fmes)BH2 derived frustrated Lewis pair 12, in situ generated or employed as the isolated dimer, reacts with 3 CN-Xyl equivs. at elevated temperature (60 °C) to yield the analogous coupling product 13.

Multicomponent Pyrazole Synthesis from Alkynes, Nitriles, and Titanium Imido Complexes via Oxidatively Induced N-N Bond Coupling

Pyrazoles are an important class of heterocycles found in a wide range of bioactive compounds and pharmaceuticals. Pyrazole synthesis often requires hydrazine or related reagents where an intact N-N bond is conservatively installed into a pyrazole precursor fragment. Herein, we report the multicomponent oxidative coupling of alkynes, nitriles, and Ti imido complexes for the synthesis of multisubstituted pyrazoles. This modular method avoids potentially hazardous reagents like hydrazine, instead forming the N-N bond in the final step via oxidation-induced coupling on Ti. The mechanism of this transformation has been studied in-depth through stoichiometric reactions of the key diazatitanacyclohexadiene intermediate, which can be accessed via multicomponent coupling of Ti imidos with nitriles and alkynes, ring opening of 2-imino-2H-azirines, or direct metalation of 4-azadiene-1-amine derivatives. The critical transformation in this reaction is the 2-electron oxidation-induced N-N coupling on Ti. This is a rare example of formal N-N coupling on a metal center, which likely occurs through an electrocyclic mechanism analogous to a Nazarov cyclization. Conveniently, these 2-electron-oxidized diazatitanacyclohexadiene intermediates can be accessed via disproportionation of the 1-electron-oxidized species, which allows utilization of weak oxidants such as TEMPO.

Metal-Involving Synthesis and Reactions of Oximes

This review classifies and summarizes the past 10-15 years of advancements in the field of metal-involving (i.e., metal-mediated and metal-catalyzed) reactions of oximes. These reactions are diverse in nature and have been employed for syntheses of oxime-based metal complexes and cage-compounds, oxime functionalizations, and the preparation of new classes of organic species, in particular, a wide variety of heterocyclic systems spanning small 3-membered ring systems to macroheterocycles. This consideration gives a general outlook of reaction routes, mechanisms, and driving forces and underlines the potential of metal-involving conversions of oxime species for application in various fields of chemistry and draws attention to the emerging putative targets.

New Titanium Borylimido Compounds: Synthesis, Structure, and Bonding

We report a combined experimental and computational study of the synthesis and electronic structure of titanium borylimido compounds. Three new synthetic routes to this hitherto almost unknown class of Group 4 imide are presented. The double-deprotonation reaction of the borylamine H₂NB(NAr'CH)₂ (Ar' = 2,6-C₆H₃ⁱPr₂) with Ti(NMe₂)₂Cl₂ gave Ti{NB(NAr'CH)₂}Cl₂(NHMe₂)₂, which was easily converted to Ti{NB(NAr'CH)₂}Cl₂(py)₃. This compound is an entry point to other borylimides, for example, reacting with Li₂N₂^pyrN^Me to form Ti(N₂^pyrN^Me){NB(NAr'CH)₂}(py)₂ and with 2 equiv of NaCp to give Cp₂Ti{NB(NAr'CH)₂}(py) (23). Borylamine-tert-butylimide exchange between H₂NB(NAr'CH)₂ and Cp*Ti(N^tBu)Cl(py) under forcing conditions afforded Cp*Ti{NB(NAr'CH)₂}Cl(py), which could be further substituted with guanidinate or pyrrolide-amine ligands to give Cp*Ti(hpp){NB(NAr'CH)₂} (16) and Cp*Ti(N^pyrN^Me₂){NB(NAr'CH)₂} (17). The Ti-N_im distances in compounds with the NB(NAr'CH)₂ ligand were comparable to those of the corresponding arylimides. Dialkyl- or diaryl-substituted borylamines do not undergo the analogous double-deprotonation or imide-amine exchange reactions. Reaction of (Cp″₂Ti)₂(μ₂:η¹,η¹-N₂) with N₃BMes₂ gave the base-free, diarylborylimide Cp″₂Ti(NBMes₂) (26) by an oxidative route; this compound has a relatively long Ti-N_im bond and large Cp″-Ti-Cp″ angle. Reaction of 16 with H₂N^tBu formed equilibrium mixtures with H₂NB(NAr'CH)₂ and Cp*Ti(hpp)(N^tBu) (Δ_rG = -1.0 kcal mol^-1). In contrast, the dialkylborylimide Cp*Ti{MeC(NⁱPr)₂}(NBC₈H₁₄) (2) reacted quantitatively with H₂N^tBu to give the corresponding tert-butylimide and borylamine. The electronic structures and imide-amine exchange reactions of half-sandwich and sandwich titanium borylimides have been evaluated using density functional theory (DFT), supported by quantum theory of atoms in molecules (QTAIM) and natural bond orbital (NBO) analysis, and placed more generally in context with the well-established alkyl- and arylimides and hydrazides. The calculations find that Ti-N_im bonds for borylimides are stronger and more covalent than in their organoimido or hydrazido analogues, and are strongest for alkyl- and arylborylimides. Borylamine-tert-butylimide exchange reactions fail for H₂NBR₂ (R = hydrocarbyl) but not for H₂NB(NAr'CH)₂ because the increased strength of the new Ti-N_im bond for the former is outweighed by the increased net H-N bond strengths in the borylamine. Variation of the Ti-N_im bond length over short distances is dominated by π-interactions with any appropriate orbital on the N_im atom organic substituent. However, over the full range of imides and hydrazides studied, overall bond energies do not correlate with bond length but with the Ti-N_im σ-bond character and the orthogonal π-interaction.

Generation of TiII Alkyne Trimerization Catalysts in the Absence of Strong Metal Reductants

Low-valent Ti^II species have typically been synthesized by the reaction of Ti^IV halides with strong metal reductants. Herein we report that Ti^II species can be generated simply by reacting Ti^IV imido complexes with 2 equiv of alkyne, yielding a metallacycle that can reductively eliminate pyrrole while liberating Ti^II. In order to probe the generality of this process, Ti^II-catalyzed alkyne trimerization reactions were carried out with a diverse range of Ti^IV precatalysts.

Theoretical insights into C–C bond formation through isonitrile insertion into a Cp*Ti complex

Reaction of Cp*(Cl)Ti(2,3-dimethylbutadiene) with isonitriles is studied using DFT, detailed elementary reaction steps and N-substitution effects of isonitrile are examined.

Metallo-Wittig chemistry of an alkylidene to form a terminal titanium oxo complex

The synthesis and characterization of a terminal titanium oxo complex generated by alkylidene benzophenone cross-metathesis is reported.

ε-Caprolactone polymerization using titanium complexes immobilized onto silica based materials functionalized with ionic liquids: insights into steric, electronic and support effects

Heterogeneous catalysts comprised of titanium immobilized on silica imidazolium-based systems have been synthesized for the ring opening polymerization of ε-caprolactone.