Titanium redox catalysis: insights and applications of an earth-abundant base metal

A combined computational and experimental study on the mechanism of Ti-catalyzed formal [2 + 2 + 1] pyrrole synthesis from alkynes and aryl diazenes is reported. This reaction proceeds through a formally Ti/Ti redox catalytic cycle as determined by natural bond orbital (NBO) and intrinsic bond orbital (IBO) analysis. Kinetic analysis of the reaction of internal alkynes with azobenzene reveals a complex equilibrium involving Ti═NPh monomer/dimer equilibrium and Ti═NPh + alkyne [2 + 2] cycloaddition equilibrium along with azobenzene and pyridine inhibition equilibria prior to rate-determining second alkyne insertion. Computations support this kinetic analysis, provide insights into the structure of the active species in catalysis and the roles of solvent, and provide a new mechanism for regeneration of the Ti imido catalyst via disproportionation. Reductive elimination from a 6-membered azatitanacyclohexadiene species to generate pyrrole-bound Ti is surprisingly facile and occurs through a unique electrocyclic reductive elimination pathway similar to a Nazarov cyclization. The resulting Ti species are stabilized through backbonding into the π* of the pyrrole framework, although solvent effects also significantly stabilize free Ti species that are required for pyrrole loss and catalytic turnover. Further computational and kinetic analysis reveals that in complex reactions with unysmmetric alkynes the resulting pyrrole regioselectivity is driven primarily by steric effects for terminal alkynes and inductive effects for internal alkynes.

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“…In fact, Ti-catalyzed reactions involving a formal oxidation state change at the metal center (i.e., Ti II /Ti IV ) are quite rare. 5…”

Section: Introductionmentioning

confidence: 99%

Mechanism of Ti-Catalyzed Oxidative Nitrene Transfer in [2 + 2 + 1] Pyrrole Synthesis from Alkynes and Azobenzene

et al. 2018

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“…[9] However, our initial report was limited to the homocoupling of alkynes, and unsymmetric alkynes yielded poor, substrate-controlled regioselectivity. Regioselectivity in other intermolecular [2+2+1] reactions such as the Pauson-Khand reaction is often difficult to achieve in the absence of clear stereoelectronic differentiation of alkyne substituents, and remains a siginficant synthetic challenge.…”

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confidence: 99%

“…[6a, 8] Recently,w ed emonstrated an atom-economical method to synthesize pentasubstituted pyrroles from simple alkynes and azobenzene through at itanium-catalyzed formal [2+ +2+ +1] reaction (Figure 1, top). [9] However,o ur initial report was limited to the homocoupling of alkynes,a nd unsymmetrical alkynes yielded poor, substrate-controlled regioselectivity.R egioselectivity in other intermolecular [2+ +2+ +1] reactions,s uch as the Pauson-Khand reaction, is often difficult to achieve in the absence of clear stereoelectronic differentiation of alkyne substituents,a nd remains asignificant synthetic challenge. [10] In an effort to design more useful and practical pyrrole syntheses,w eh ave explored several routes for the chemo-and regioselective heterocoupling of alkynes in the [2+ +2+ +1] reaction to yield highly substituted pyrroles.H erein, we report that silyl-protected alkynes serve as excellent heterocoupling partners,y ielding pentasubstituted N-aryl pyrroles with high chemo-and regioselectivity (Figure 1,bottom).…”

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confidence: 99%

Trimethylsilyl‐Protected Alkynes as Selective Cross‐Coupling Partners in Titanium‐Catalyzed [2+2+1] Pyrrole Synthesis

Chiu

Tonks

2018

Angew Chem Int Ed

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Trimethylsilyl (TMS)-protected alkynes serve as selective alkyne cross coupling partners in Ti-catalyzed [2+2+1] pyrrole synthesis. Reactions of TMS-protected alkynes with internal alkynes and azobenzene catalyzed by Ti imido catalysts yield pentasubstituted 2-TMS-pyrroles with greater than 90% selectivity over the other 9 possible pyrrole products. The steric and electronic effects of the TMS group have both been identified to play key roles in this highly selective pyrrole synthesis. This strategy provides a convenient methodology to synthesize multisubstituted pyrroles as well as an entrypoint into further pyrrole diversification through facile modification of the resulting 2-silylpyrrole products, as demonstrated through a short formal synthesis of the marine natural product Lamellarin R.

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“…One promising material is titanium oxide (TiO x ), which has tunable oxidation states with the titanium ion charged from 2+ to 4+. [ 14 ] Moreover, TiO x is earth‐abundant [ 15 ] and has been widely used in important applications, such as photovoltaics, [ 16 ] photocatalysis, [ 17 ] and ultraviolet absorption. [ 18 ] Particularly, in the rocksalt titanium monoxide (Ti 2+ O), the d xy , d xz , and d yx orbits overlap and form the partially filled t 2g orbit, leading to a unique metallic behavior, [ 19,20 ] different from the other semiconducting TiO x families.…”

Section: Introductionmentioning

confidence: 99%

Bipolar Conduction and Giant Positive Magnetoresistance in Doped Metallic Titanium Oxide Heterostructures

Huang

Wang

Jin

et al. 2021

Adv Materials Inter

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Empowering conventional materials with unexpected magnetoelectric properties is appealing to the multi‐functionalization of existing devices and the exploration of future electronics. Recently, owing to its unique effect in modulating a matter's properties, ultra‐small dopants, for example, H, D, and Li, attract enormous attention in creating emergent functionalities, such as superconductivity, and metal–insulator transition. Here, an observation of bipolar conduction accompanied by a giant positive magnetoresistance in D‐doped metallic Ti oxide (TiOxDy) films is reported. To overcome the challenges in intercalating the D into a crystalline oxide, a series of TiOxDy is formed by sequentially doping Ti with D and surface/interface oxidation. Intriguingly, while the electron mobility of the TiOxDy increases by an order of magnitude larger after doping, the emergent holes also exhibit high mobility. Moreover, the bipolar conduction induces a giant magnetoresistance up to 900% at 6 T, which is ≈6 times higher than its conventional phase. This study paves a way to empower conventional materials in existing electronics and induce novel electronic phases.

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Titanium redox catalysis: insights and applications of an earth-abundant base metal

Abstract: π-acid ancillary ligands, reactants, or products can stabilize reactive low valent Ti intermediates through backbonding, and present opportunities for the development of vast new classes of Ti-catalyzed redox reactions with practical applications.

Cited by 51 publications

References 73 publications

Mechanism of Ti-Catalyzed Oxidative Nitrene Transfer in [2 + 2 + 1] Pyrrole Synthesis from Alkynes and Azobenzene

Mechanism of Ti-Catalyzed Oxidative Nitrene Transfer in [2 + 2 + 1] Pyrrole Synthesis from Alkynes and Azobenzene

Trimethylsilyl‐Protected Alkynes as Selective Cross‐Coupling Partners in Titanium‐Catalyzed [2+2+1] Pyrrole Synthesis

Bipolar Conduction and Giant Positive Magnetoresistance in Doped Metallic Titanium Oxide Heterostructures

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