Preparation of Novel Carbenes and their Catalytic Application to the Cross-Benzoin, Aza-Benzoin and Stetter Reactions
Date
2020-02-11
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ORCID
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Thesis
Degree Level
Doctoral
Abstract
N-Heterocyclic carbenes (NHCs) derived from bicyclic triazolium salts possessing a fused 6 membered ring were found to induce chemoselectivity in the cross-benzoin between aromatic and aliphatic aldehydes. Use of these catalysts gave rise to predominantly one cross-benzoin product which could be isolated in good to excellent yield (68-99%). Consistently high chemoselectivity was realized across a wide range of substrates. Examples were shown using benzaldehyde and several different unbranched aliphatic aldehydes. The reaction was also effective with ortho, meta or para substituted aromatic aldehydes (possessing either electron donating or withdrawing substituents) and was tolerant to α and β branched aliphatic aldehydes as well.
A library of bis(amino)cyclopropenium salts were prepared, and the corresponding bis(amino)cyclopropenylidenes (BACs) generated in situ were screened in the Stetter reaction. Among those tested, bis(diethylamino)cyclopropenylidene (EtBAC) was found to be the most efficient BAC catalyst for the Stetter reaction. EtBAC was also found to be superior to common NHCs in reactions with β-alkyl ketone acceptors or electron rich aromatic aldehydes. A chiral bis-1,4-dimethylpyrrolidine derived BAC catalyst demonstrated superior enantioselectivity (73% ee) compared to triazolium systems. Reaction monitoring and crossover experiments revealed no formation of benzoin product with EtBAC, unlike other carbenes.
EtBAC was found to be an excellent catalyst for aza-benzoin reactions with N-phosphinoyl imines and their derivatives. Anhydrous conditions were necessary, but the reaction was otherwise robust and afforded good yields (69-96%) with a number of aromatic and heteroaromatic aldehydes. Substitution on the N-phosphinoyl imine was also tolerated. An excess of aldehyde improved reactivity in some instances, and no homo-benzoin side-products were observed in aza-benzoin reactions with EtBAC.
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Keywords
Stetter, benzoin, aza-benzoin, NHC, carbene bis(amino)cyclopropenium, bis(amino)cyclopropenylidene, umpolung
Citation
Degree
Doctor of Philosophy (Ph.D.)
Department
Chemistry
Program
Chemistry