While asymmetric transition-metal catalysis has become a powerful method for constructing chiral products, a challenge in this field is the identification of the correct ligand for high selectivity. We report here a simple approach to chiral catalyst formation: coupling of an available pool of Brønsted acids, namely, amino acid derivatives, with tunable ligands on copper catalysts. This system can be used to generate many different chiral environments simply by changing the amino acid or ligand employed and provides a scaffold for rapid screening and identification of the correct combination for high enantioselectivity. The latter is illustrated in the copper-catalyzed alkynylation of imines in up to 99% ee.
Having the munchies: A mild palladium‐catalyzed method to activate the carbon–oxygen bond of α‐amidoethers has been developed and applied to carbonylation chemistry (see scheme). A münchnone intermediate is generated in situ and undergoes a 1,3‐dipolar cycloaddition with alkynes to give diversely substituted pyrroles.
A one-step method to assemble pyrroles from alpha,beta-unsaturated imines and acid chlorides has been developed. This reaction is mediated by triphenylphosphine, which eliminates phosphine oxide to allow cyclization. This reaction has been employed to access a diverse range of pyrroles via modulation of the two building blocks and applied as well to the synthesis of lukianol A.
A copper-catalyzed Petasis-type reaction of imines, acid chlorides, and organoboranes to form α-substituted amides is described. This reaction does not require the use of activated imines or the transfer of special units from the organoboranes and represent a useful generalization of the Petasis reaction.
Palladium-catalyzed Saegusa-Ito oxidation of trimethylsilyl enol ethers is possible using Oxone as a stoichiometric oxidant and sodium hydrogen phosphate as a buffer. Cyclic and acyclic enones as well as α,β-unsaturated aldehydes are obtained in good to excellent yields.
A Lewis acid mediated method to induce the carbon-oxygen bond of amide-substituted ethers to undergo addition to palladium is described. The product of this reaction has been crystallographically characterized. This reaction suggests the potential use of such ethers as an alternative to organic halides in palladium catalyzed carbon-carbon bond formation. As an illustration of this potential, this reaction has been used to design a mild, catalytic route to alpha-amino acid derivatives from alpha-phenoxyamides and carbon monoxide.
Eine milde Palladiumkatalyse aktiviert die Kohlenstoff‐Sauerstoff‐Bindung von α‐Amidoethern in Carbonylierungen (siehe Schema). Ein in situ erzeugtes Münchnon geht eine 1,3‐dipolare Cycloaddition mit Alkinen ein, die zu substituierten Pyrrolen als Produkten führt.
The palladium-catalyzed carbonylative coupling of imines, acid chlorides, and dipolarophiles can provide efficient routes to prepare nitrogen-containing heterocycles. One challenge in developing this reaction, and in the creation of more active catalyst systems, is the lack of data on how this complex transformation proceeds. To address this, we report here the results of our mechanistic studies on this system, and in particular the formation of mesoionic münchnones. This includes the synthesis of key catalytic intermediates, model reactions, and kinetic studies that support the role of these compounds in catalysis. Together, these studies provide a clear picture of the impact of catalyst structure, ligands, and palladium nanoparticles on facilitating the carbonylation of in situ generated iminium salts, and suggest an avenue for the creation of more active catalyst systems.
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