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. 2021 Apr 16;23(8):2853-2857.
doi: 10.1021/acs.orglett.1c00248. Epub 2021 Mar 26.

A Phosphine-Mediated Dearomative Skeletal Rearrangement of Dianiline Squaraine Dyes

Emily P Bacher  1 , Kevin J Koh  1 , Antonio J Lepore  1 , Allen G Oliver  1 , Olaf Wiest  1 , Brandon L Ashfeld  1
Affiliations

A Phosphine-Mediated Dearomative Skeletal Rearrangement of Dianiline Squaraine Dyes

Emily P Bacher et al. Org Lett. .

Abstract

A phosphorus(III)-mediated dearomatization of ortho-substituted dianiline squaraine dyes results in an unusual skeletal rearrangement to provide exotic, highly conjugated benzofuranone and oxindole scaffolds bearing a C3 side chain comprised of a linear conflagration of an enol, a phosphorus ylide, and 2,4-disubstituted aniline. Employing experimental and computational analysis, a mechanistic evaluation revealed a striking dependence on the acidity of the aniline ortho substituent. Notably, the rearrangement adducts underwent rapid and complete reversion to the parent squaraine in the presence of a Brønsted acid.

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Conflict of interest statement

The authors declare no competing financial interest.

Figures

Figure 1.
Figure 1.
Dearomative functionalizations. (a) Visible light-mediated, dearomative induced fragmentation of 2-naphthols. (b) Disruption of the squaraine aromaticity via the addition of PIII. (c) Dearomatization-induced skeletal rearrangement of dianiline squaraines.
Figure 2.
Figure 2.
Electrostatic potential surface of 6a.
Figure 3.
Figure 3.
Potential free energy diagram for the formation of 6a/e from 4a/e (B3LYP+D3/6-311+G**+CPCM (CHCl3)).
Figure 4.
Figure 4.
Structure of 10a (C–H bonds hidden for the sake of clarity).
Scheme 1.
Scheme 1.
Dearomative Rearrangement of the Squaraine Core
Scheme 2.
Scheme 2.
Acid-Promoted Reversion to the Parent Squaraine
Scheme 3.
Scheme 3.
Mechanistic Considerations
See this image and copyright information in PMC

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