Metal-Organic Frameworks Invert Molecular Reactivity: Lewis Acidic Phosphonium Zwitterions Catalyse The Aldol-Tishchenko Reaction

G. Bauer, D. Ongari, X. Xu, D. Tiana, B. Smit, and M. Ranocchiari, Metal-Organic Frameworks Invert Molecular Reactivity: Lewis Acidic Phosphonium Zwitterions Catalyse The Aldol-Tishchenko Reaction J Am Chem Soc 139 (50), 18166–18169 (2017) http://dx.doi.org/10.1021/jacs.7b10928

Abstract: The influence of metal-organic frameworks (MOFs) as additives is herein described for the reaction of n-alkyl aldehydes in the presence of methylvinylketone and tri-phenylphosphine. In the absence of a MOF, the expected Morita-Baylis-Hillman product – a β-hydroxy enone - is observed. In the presence of MOFs with UMCM-1 and MOF-5 topologies, the reaction is selective to Aldol-Tishchenko products - the 1 and 3 n-alkylesters of 2-alkyl-1,3-diols - that is unprecedented in organocatalysis. The (3-oxo-2-butenyl)triphenylphosphonium zwitterion, a commonly known nucleophile, is identified as the catalytic active species. This zwitterion favours nucleophilic character in solution, whereas once confined within the framework, it becomes an electrophile yielding Aldol-Tishchenko selectivity. Computational investigations reveal a structural change in the phosphonium moiety induced by the steric confinement of the framework that makes it accessible and an electrophile.


© Berend Smit 2019