Ti-Catalyzed and -Mediated Oxidative Amination Reactions
1Department of Chemistry, University of Minnesota-Twin Cities, 207 Pleasant Street SE, Minneapolis, Minnesota 55455, United States.
Accounts of Chemical Research
|August 23, 2021
Summary
Titanium catalysis enables new oxidative amination reactions by stabilizing low-valent titanium intermediates. This research details titanium imido chemistry for synthesizing diverse heterocycles and aminated compounds.
Area of Science:
- Organometallic Chemistry
- Catalysis
- Synthetic Organic Chemistry
Background:
- Titanium (Ti) is earth-abundant and non-toxic, making it attractive for catalysis.
- Catalytic redox reactions with Ti are challenging due to the stable Ti(IV) state.
- Understanding Ti redox mechanisms is crucial for developing catalytic applications.
Purpose of the Study:
- To detail recent advancements in Ti-catalyzed oxidative amination reactions.
- To explore reactions proceeding through Ti(II)/Ti(IV) catalytic cycles.
- To elucidate mechanistic factors enabling Ti redox catalysis despite inherent difficulties.
Main Methods:
- Mechanistic studies of Ti-catalyzed [2+2+1] pyrrole synthesis from alkynes and azobenzene.
- Generation and reaction of key titanium imido (Ti≡NR) intermediates.
- Exploration of cycloaddition reactions with alkynes and other unsaturated functional groups.
Main Results:
- Low-valent Ti intermediates are stabilized by π-acceptor ligands via metal-to-ligand back-donation.
- Reductive elimination in Ti catalysis proceeds via π-type electrocyclic mechanisms.
- Ti imidos react with alkynes, alkenes, nitriles, and nitrosos to form diverse heterocycles and aminated building blocks.
Conclusions:
- Ti-catalyzed oxidative amination reactions provide complementary synthetic routes to classical methods.
- These methods enable access to novel, highly substituted heteroaromatic scaffolds.
- Early transition metals like Ti possess significant, often overlooked, potential in modern catalysis.
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