Highly efficient Lewis acid catalytic activity of the tritylium ion at the node of a tensile organic framework.
Chem Sci 2021;
12:9786-9793. [PMID:
34349952 PMCID:
PMC8293798 DOI:
10.1039/d1sc02594e]
[Citation(s) in RCA: 3] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/11/2021] [Accepted: 06/10/2021] [Indexed: 12/18/2022] Open
Abstract
Tritylium salts have been used as Lewis acid catalysts in organic synthesis for a long time. In this work, we found that the Lewis acid catalytic activity of tritylium ions at the node of a tensile framework is significantly improved compared to that of the free tritylium salts. The tritylium-based framework, PAF-201 (PAF, porous aromatic framework), was prepared by acidification of a semi-rigid triphenylcarbinol-based parent framework, PAF-200. When PAF-200 was alternately exposed to HCl and NH3 gas, a fast allochroic cycle was observed due to repeated formation of tritylium species. Interestingly, the pseudo-first-order reaction rate of a Povarov model reaction catalyzed by PAF-201 as a Lewis acid was ∼3.7 times and ∼4.7 times as those of tritylium tetrafluoroborate and tri(4-biphenyl)carbonium tetrafluoroborate, respectively. Theoretical calculations revealed that the tritylium ion at the node of PAF-201 has a quasi-planar structure. The transformation of triphenylcarbinol in PAF-200 to tritylium in PAF-201 can make the framework taut, and the rebounding force toward the tetrahedral structure is stored. This is favorable for tritylium to activate the imine substrate along with a deformation of the quasi-plane to tetrahedron. PAF-201 could be easily recycled at least three times without evident loss of catalytic activity. This work presents the catalytic activity of the tritylium ion under stress.
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