Dimethyltitanocene Cp2TiMe2: A Useful Reagent for C—C and C—N Bond Formation

Siebeneicher, Hölger; Doye, Sven

doi:10.1002/(sici)1521-3897(200001)342:1<102::aid-prac102>3.0.co;2-n

Cited by 33 publications

(2 citation statements)

References 15 publications

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“…This electrocyclic reductive elimination is unique and provides an important piece of data on C–X (X ≠ H) reductive eliminations on Ti: direct M–C σ reductive coupling on Ti is virtually unknown; it appears as though in almost all cases Ti organometallics would energetically prefer H abstraction, , radical decomposition, substitution, or electrocyclization to classical two-electron reductive elimination pathways seen with late transition metals. In fact, in our previous report on Ti-catalyzed alkyne/alkene/nitrene coupling, direct C–N bond forming was never observed .…”

Section: Resultsmentioning

confidence: 96%

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

et al. 2018

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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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Section: Resultsmentioning

confidence: 96%

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

et al. 2018

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“…Ti organometallics are more likely to undergo H abstraction or radical reaction than σ-bond reductive elimination. Thus, it is surprising that IM6 undergoes facile C–N reductive elimination (Figure , TS4 ).…”

Section: Catalytic [2 + 2 + 1] Synthesis Of Pyrrolesmentioning

confidence: 99%

Ti-Catalyzed and -Mediated Oxidative Amination Reactions

Tonks

2021

Acc. Chem. Res.

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Metrics & MoreArticle Recommendations CONSPECTUS: Titanium is an attractive metal for catalytic reaction development: it is earth-abundant, inexpensive, and generally nontoxic. Howeverlike most early transition metalscatalytic redox reactions with Ti are difficult because of the stability of the high-valent Ti IV state. Understanding the fundamental mechanisms behind Ti redox processes is key for making progress toward potential catalytic applications. This Account details recent progress in Ti-catalyzed (and -mediated) oxidative amination reactions that proceed through formally Ti II /Ti IV catalytic cycles. This class of reactions is built on our initial discovery of Ti-catalyzed [2 + 2 + 1] pyrrole synthesis from alkynes and azobenzene, where detailed mechanistic studies have revealed important factors that allow for catalytic turnover despite the inherent difficulty of Ti redox. Two important conclusions from mechanistic studies are that (1) low-valent Ti intermediates in catalysis can be stabilized through coordination of π-acceptor substrates or products, where they can act as "redox-noninnocent" ligands through metal-toligand π back-donation, and (2) reductive elimination processes with Ti proceed through π-type electrocyclic (or pericyclic) reaction mechanisms rather than direct σ-bond coupling.The key reactive species in Ti-catalyzed oxidative amination reactions are Ti imidos (TiNR), which can be generated from either aryl diazenes (RNNR) or organic azides (RN 3 ). These Ti imidos can then undergo [2 + 2] cycloadditions with alkynes, resulting in intermediates that can be coupled to an array of other unsaturated functional groups, including alkynes, alkenes, nitriles, and nitrosos. This basic reactivity pattern has been extended into a broad range of catalytic and stoichiometric oxidative multicomponent coupling reactions of alkynes and other reactive small molecules, leading to multicomponent syntheses of various heterocycles and aminated building blocks. For example, catalytic oxidative coupling of Ti imidos with two different alkynes leads to pyrroles, while stoichiometric oxidative coupling with alkynes and nitriles leads to pyrazoles. These heterocycle syntheses often yield substitution patterns that are complementary to those of classical condensation routes and provide access to new electron-rich, highly substituted heteroaromatic scaffolds. Furthermore, catalytic oxidative alkyne carboamination reactions can be accomplished via reaction of Ti imidos with alkynes and alkenes, yielding α,β-unsaturated imine or cyclopropylimine building blocks. New catalytic and stoichiometric oxidative amination methods such as alkyne α-diimination, isocyanide imination, and ring-opening oxidative amination of strained alkenes are continuously emerging as a result of better mechanistic understanding of Ti redox catalysis.Ultimately, these Ti-catalyzed and -mediated oxidative amination methods demonstrate the importance of examining oftencontinued...

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Microwave-Assisted Catalytic Intermolecular Hydroamination of Alkynes

Bytschkov

Doye

2001

Eur. J. Org. Chem.

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Irradiation of reaction mixtures containing an alkyne, an amine, and a catalytic amount of Cp 2 TiMe 2 in toluene with microwaves at a frequency of 2.45 GHz and a power output of 180−300 W results in fast reactions to give the corresponding hydroamination products. The initially formed imines can easily be reduced to secondary amines by use of H 2 /Pd, LiAlH 4 , or NaCNBH 3 /p-TsOH. The microwave-assisted hydroamination reactions go to completion within one tenth (or less) of the time required for reactions run conventionally in [a]

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Dimethyltitanocene Cp2TiMe2: A Useful Reagent for C—C and C—N Bond Formation

Cited by 33 publications

References 15 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

Ti-Catalyzed and -Mediated Oxidative Amination Reactions

Microwave-Assisted Catalytic Intermolecular Hydroamination of Alkynes

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