A Phosphine-Mediated Dearomative Skeletal Rearrangement of Dianiline Squaraine Dyes

Article abstract of DOI:10.1021/acs.orglett.1c00248

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 Br?nsted acid.

Full text of DOI:10.1021/acs.orglett.1c00248

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Letter
A Phosphine-Mediated Dearomative Skeletal Rearrangement of
Dianiline Squaraine Dyes
Emily P. Bacher, Kevin J. Koh, Antonio J. Lepore, Allen G. Oliver, Olaf Wiest,* and Brandon L. Ashfeld*
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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.
hemical dearomatizations have evolved into a powerful
Cstrategy for the construction of more complex, saturated
synthetic building blocks from readily available starting
materials.1 While most arene dearomatizations enable the
installation of high-value functionalities that retain the core
carbon framework, the initiation of a ring fragmentation
cascade to assemble architecturally distinct scaffolds is
substantially less common. A rare example of this was recently
reported by Glorius and co-workers, who exploited visible light
photocatalysis employing an IrIII photosensitizer to initiate a
dearomative [2+2] cycloaddition of 2-naphthol derivatives to
unveil a second photosensitive functionality.2 The subsequent
energy transfer process resulted in a core skeletal reorganiza-
tion of the starting naphthol framework to provide a
cyclobutylarene (Figure 1a). Recently, we reported a reversible
addition of phosphorus(III) reagents to dearomatize dianiline
squaraine dyes 1 as chemodosimeters for transition metal
complex detection,3a in which the resulting thermo- and
chemoreversible dynamic equilibrium resulted in a metal-
dependent spectroscopic response (Figure 1b).3 Over the
course of this study, we speculated that upon dearomatization,
the increased level of polarization renders the indicated
squaraine C−C bond labile, and thus prone to spontaneous
and reversible heterolytic cleavage. The installation of a
nucleophilic functionality proximal to the squaraine core
would promote a skeletal reorganization leading to a new
aromatic framework. Herein, we report a dearomative ring
fragmentation of ortho-heteroatom-substituted dianiline squar-
aine dyes 3/4 to the exotic enol ylide scaffolds 5/6 under
exceptionally mild reaction conditions (Figure 1c). Exper-
imental and computational evaluation of the reaction
mechanism revealed that retroversion of this unusual skeletal
rearrangement readily occurs in the presence of a Brønsted
acid, highlighting a peculiar pKa dependence on incorporation
of phosphine and providing a basis for chemodosimeter
development.
Despite the potential for architectural diversification around
a central cyclobutanedione core, the utility of squaraine dyes as
synthetic building blocks constitutes an underdeveloped area
in organic chemistry. Characterized by their absorption and
emission of light in the near-infrared region (NIR), these
highly conjugated compounds are widely utilized in physical
and analytical chemistry,4 and their fluorescence and
absorption properties have led to impressive advances in
non-invasive imaging of biological structures and as sensitizers
in photodynamic therapy.5,6
The electrophilic character of the cyclobutenedione nucleus
is frequently an impediment to the translational implementa-
tion of these dyes, which has led to macromolecular protection
strategies to impede the addition of exogenous nucleophiles.7
Received: January 21, 2021
Published: March 26, 2021
https://doi.org/10.1021/acs.orglett.1c00248
© 2021 American Chemical Society
Org. Lett. 2021, 23, 2853−2857
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P−C bond length of 1.846 Å, and an apparent electrostatic
attraction between the phosphonium cation and benzofuran
carbonyl. It is noteworthy that the presumptive zwitterionic
phosphonium adduct 7a was not observed as a co-product in
this reaction.
To evaluate the architectural limitations of this dearomative
rearrangement, we assessed the generality of this process across
an array of o-hydroxy and o-amino dianiline squaraine dyes
(Table 1). Treatment of 2-hydroxy squaraines 3b and 3c with
a
Table 1. Assembly of Benzofuranones 5 and Oxindoles 6
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.
In contrast, we sought to exploit this electrophilicity to initiate
a dearomative, skeletal reorganization to provide functionalized
benzofurans (X = O) and oxindoles (X = NR′). In contrast to
our previous work, the addition of PnBu3 to o-hydroxy
squaraine 3a resulted in complete bleaching within 5 min
and the formation of benzofuranone 5a in 94% yield (Scheme
1).8 The polycyclic framework of 5a was confirmed by X-ray
Scheme 1. Dearomative Rearrangement of the Squaraine
Core
a
PnBu3 (0.11 mmol) was added to a suspension of 3/4 (0.1 mmol) in
CHCl3 (0.1 M) at 25 °C. See the Supporting Information for detailed
experimental procedures.
PnBu3 at room temperature resulted in rapid conversion to the
corresponding benzofuranones 5b and 5c bearing N,N-
dibenzyl and N,N-diphenyl substitution at the arene 4-position
in >99% and 93% yields, respectively. However, the 4-
morpholine and 4-pyrrolidine squaraines gave benzofuranones
5d and 5e in diminished yields, presumably due to the
insolubility of the parent squaraines. Addition of excess PnBu3
(>3 equiv) failed to ameliorate the issue. Exposure of the o-
amino-substituted squaraines 4 underwent ring fragmentation
to provide the corresponding oxindoles 6. The 2-N-acyl and 2-
N-sulfonyl derivatives gave oxindoles 6a−d in high yields
(from 72% to >99%). In contrast, squaraines 6e−h bearing o-
N-alkyl, -benzyl, -allyl, and -propargyl substitutions failed to
provide the expected oxindoles. It is noteworthy that the
squaraine dye was retained unaltered, indicating that the
equilibrium favors the intact squaraine 4. Variations of the N-
crystallography, which revealed a number of intriguing
architectural features specific to this class of compounds.
The resulting adduct is characterized by a high degree of
functionalization, as illustrated by the presence of a 1,3-
dicarbonyl motif residing as its enol, a conjugated phosphorus
ylide, and a benzofuranone core with specific C6-dialkylamino
substitution. The ylide motif exhibits less double bond
character than a standard phosphorus ylide as indicated by a
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alkyl groups at the para position did not impact the efficacy of
oxindole formation as illustrated by N,N-diallylamine and N,N-
dipropargylamine derivatives yielding oxindoles 6i and 6j in
78% and 94% yields, respectively. Likewise, the N-allyl-N-
methyl and N-methyl-N-propargyl squaraines cleanly provided
the corresponding oxindoles in quantitative yields.
Scheme 2. Acid-Promoted Reversion to the Parent
Squaraine
Employing unsymmetrical dianiline squaraines 8 sets the
stage for competitive ring fragmentation to incorporate either
X or Y into the heterocyclic core (Table 2). To examine the
a
Table 2. Unsymmetrical Dianiline Squaraine Dyes
regenerate the parent dye under mild and specific conditions
has significant implications to sensing applications.
To gain insight into this unusual oxindole ylide reactivity
profile, the mechanism for its formation, and the lack of
reactivity observed with 4e−h, we performed a series of
computational studies at the B3LYP+D3/6-311+G**+CPCM-
(CHCl3) level of theory using Gaussian16.9 The electrostatic
potential across the framework of oxindole 6a reveals extensive
delocalization of charge density across the extended π-system
(Figure 2). As a result, the ylide anion is highly stabilized with
a
PnBu3 (0.11 mmol) was added to a suspension of 3/4 (0.1 mmol) in
CHCl3 (0.1 M) at 25 °C. See the Supporting Information for detailed
experimental procedures.
Figure 2. Electrostatic potential surface of 6a.
relative propensity of N- and O-substitution to undergo
addition to the cyclobutenedione, we assembled unsym-
metrical squaraine dyes 8a−f and subjected each to Bu3P in
the pKa of its conjugate acid estimated to be 9.17.10 This
provides a rationale for the observed lack of ylide reactivity
with a variety of electrophiles. It is also consistent with the X-
ray crystal structure analysis of 5a and 6a showing a fully
conjugated, extended π-system that includes the anionic
carbon of the phosphorus ylide.
n
CHCl3 at 25 °C. Treatment of those squaraine dyes bearing a
single ortho-substituted aryl group (8a−d) led to the formation
of the anticipated benzofurans 9a and 9b and oxindoles 9c and
9d in excellent yields (79−94%). Treatment of 2-hydroxy-2′-
acylamino squaraine dye 8e led to the formation of benzofuran
9e via selective o-hydroxy-induced ring fragmentation in 94%
yield. Rearrangement to N-acyl oxindole 9f in the presence of
an o-NMeAc proceeded in quantitative yield wherein the
presence of a labile N−H likely determines the product
distribution. The ability to employ unsymmetrical dianiline
squaraine dyes provides an opportunity for orthogonal
functionalization.
To elucidate the mechanism for the conversion of 3/4 to 5/
6, we began with the assumption that formation of the
phosphine adduct 7/10 precedes ring fragmentation given the
rapid addition of PL3 to squaraines we observed in the
development of squaraine-based chemodosimeters (Scheme
3a).3a As it relates to the formation of oxindole 6a, three
possible pathways can be envisioned to form 11a, which would
then undergo a final proton transfer to form 6a: (i) ring closure
to form tricyclic species 13, which would then undergo a ring
opening event, (ii) a concerted pathway that proceeds via TS1,
or (iii) an initial ring opening to form α-ketoketene 12a
followed by nucleophilic attack of the amide nitrogen on the
ketene sp carbon (Scheme 3b). Any attempts to optimize the
geometry of 11a resulted in spontaneous formation of 6a.
Similarly, the structures of 13a or TS1 could not be located
and consistently resulted in the formation of 6a, suggesting
that the nucleophilic addition pathway is not feasible and the
reaction proceeds via the initial ring fragmentation to provide
intermediate ketene 12a.
Attempts to synthetically modify the products led to a
notable resiliency given the multitude of functionality present.
Exposure of oxindole ylide 6a to an array of carbon
electrophiles (e.g., benzaldehyde, methyl iodide, allyl bromide,
etc.) did not provide the anticipated C- or O-alkylation
products at the ylide carbon, 3-oxindole position, enol or
oxindole carbonyl. Intrigued by the unexpected robustness of
these typically reactive functional groups, we probed the
behavior of these Brønsted basic motifs in the presence of acid
by exposure of oxindole 6a to p-tolylsulfonic acid. Surprisingly,
we discovered that rather than products resulting from direct
protonation, within minutes reversion back to squaraine dye 4a
occurred in quantitative yield (Scheme 2). In combination
with the inherent stability of adducts 5 and 6, the ability to
The Gibbs free energy diagram shows the computed
pathways for the formation of 6a (R = Ac; black), which
proceeds in quantitative yield, and 6e (R = Me; blue) where
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Scheme 3. Mechanistic Considerations
Figure 4. Structure of 10a (C−H bonds hidden for the sake of
clarity).
relative to MeN−H. This effect is stronger after the addition of
the phosphine, which leads to increased localization of the
negative charge on the carbonyls, but disappears once the
skeletal rearrangement to 6a/e occurs.11
As shown in Figure S1, scans of the N−C bond lengths in
12a show a barrierless transformation of 12a to 11a in
agreement with the results discussed above. Similarly, a scan of
the N−C bond in 11a leads to a barrierless N-deprotonation
en route to 6a. These mechanistic steps are consistent with a
classic general acid/base-catalyzed ring closure of 12a to 6a
where the protonation/deprotonation step is coupled to N−C
bond formation.12 It is well understood that such reactions
cannot be accurately predicted by implicit solvent models or
even a small number of solvent molecules.13 These results are
consistent with the experimentally observed reversibility of the
reaction upon treatment with a strong acid (Scheme 2). On
the basis of the results shown in Figure 3, this step is
thermoneutral but requires protonation of the weakly basic
oxindole nitrogen, which is unlikely to occur spontaneously.
In conclusion, this study presents a versatile route by which
ortho-substituted dianiline squaraine dyes serve as synthetic
building block precursors to highly functionalized oxindoles
and benzofurans through the addition of phosphines.
Disruption of the aromatic core squaraine framework leads
to a rapid and reversible dearomative ring fragmentation to
produce a conjugated system bearing resilient enol and
phosphonium ylide motifs in excellent yield. The surface
electrostatic potential of this exotic scaffold revealed an
extended conjugation that portends an unusual stability to a
diverse array of stimuli. An exploration of the mechanism of
this transformation revealed a dependency on the relative
acidity of the proton residing on the ortho substituent of the
starting dianiline squaraine dye whereby dearomative ring
fragmentation leads to a proposed 1,2-ketoketene intermediate.
Current studies are focused on the exploitation of this
discovery in the context of target-directed alkaloid synthesis
and a new class of chemodosimeters with wide-ranging
potential for analyte detection.
product formation is not observed (Figure 3). The formation
of 10a from 4a is exergonic by 3.9 kcal/mol. The ring
Figure 3. Potential free energy diagram for the formation of 6a/e
from 4a/e (B3LYP+D3/6-311+G**+CPCM (CHCl3)).
fragmentation via TS2 to form 12a occurs with an activation
energy of 22.6 kcal/mol, which is consistent with a reaction
occurring at room temperature on the hour time scale.
Nucleophilic attack on the ketene followed by a proton transfer
yields final product 6a in a reaction pathway that is overall
thermoneutral.
A key difference in the reaction of 4e is that the formation of
the initial phosphonium adduct 10e is endergonic by 3.4 kcal/
mol, 7.3 kcal/mol less favorable than the formation of 10a.
Similar free energy differences between the two pathways are
also obtained by TS2 and 12, but not in 6. Consequently, the
reaction of 4e is not feasible at room temperature, in
agreement with our experimental results. The similarity of
the free energy difference along the pathway of the N-acyl and
N-methyl derivative suggests the same origin for the energy
difference in 10a/e, TS2, and 11a/e. The calculated structure
of 10a highlights key differences in H-bonds between the
carbonyl and the NH moieties in 10a and 10e (Figure 4). The
H-bonds are significantly elongated in 10e compared to 10a,
which corresponds to the greater H-bonding ability of AcN−H
ASSOCIATED CONTENT
* Supporting Information
■
sı
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.1c00248.
Experimental procedures, spectroscopic data, and
crystallographic data (PDF)
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tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
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AUTHOR INFORMATION
Corresponding Authors
■
Brandon L. Ashfeld − Department of Chemistry and
Biochemistry, University of Notre Dame, Notre Dame,
Indiana 46556, United States; orcid.org/0000-0002-
4552-2705; Email: bashfeld@nd.edu
Olaf Wiest − Department of Chemistry and Biochemistry,
University of Notre Dame, Notre Dame, Indiana 46556,
United States; orcid.org/0000-0001-9316-7720;
Email: owiest@nd.edu
(6) (a) Prabhakar, C.; Bhanuprakash, K.; Rao, V. J.;
Balamuralikrishna, M.; Rao, D. N. J. Phys. Chem. C 2010, 114,
6077. (b) Wang, S.; Hall, L.; Diev, V. V.; Haiges, R.; Wei, G.; Xiao, X.;
Djurovich, P. I.; Forrest, S. R.; Thompson, M. E. Chem. Mater. 2011,
23, 4789. (c) Arunkumar, E.; Ajayaghosh, A.; Daub, J. J. Am. Chem.
Soc. 2005, 127, 3156. (d) Gassensmith, J. J.; Matthys, S.; Lee, J.-J.;
Wojcik, A.; Kamat, P. V.; Smith, B. D. Chem. - Eur. J. 2010, 16, 2916.
(7) (a) Arunkumar, E.; Forbes, C. C.; Noll, B. C.; Smith, B. D. J. Am.
Chem. Soc. 2005, 127, 3288. (b) Arunkumar, E.; Fu, N.; Smith, B. D.
Chem. - Eur. J. 2006, 12, 4684.
Authors
(8) Yang, M.; Jiang, Y. Phys. Chem. Chem. Phys. 2001, 3, 4213.
(9) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Petersson,
G. A.; Nakatsuji, H.; Li, X.; Caricato, M.; Marenich, A. V.; Bloino, J.;
Janesko, B. G.; Gomperts, R.; Mennucci, B.; Hratchian, H. P.; Ortiz, J.
V.; Izmaylov, A. F.; Sonnenberg, J. L.; Williams-Young, D.; Ding, F.;
Lipparini, F.; Egidi, F.; Goings, J.; Peng, B.; Petrone, A.; Henderson,
T.; Ranasinghe, D.; Zakrzewski, V. G.; Gao, J.; Rega, N.; Zheng, G.;
Liang, W.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.;
Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.;
Throssell, K.; Montgomery, J. A., Jr.; Peralta, J. E.; Ogliaro, F.;
Bearpark, M. J.; Heyd, J. J.; Brothers, E. N.; Kudin, K. N.; Staroverov,
V. N.; Keith, T. A.; Kobayashi, R.; Normand, J.; Raghavachari, K.;
Rendell, A. P.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.;
Millam, J. M.; Klene, M.; Adamo, C.; Cammi, R.; Ochterski, J. W.;
Martin, R. L.; Morokuma, K.; Farkas, O.; Foresman, J. B.; Fox, D. J.
Gaussian 16, rev. C.01; Gaussian, Inc.: Wallingford, CT, 2016.
(10) (a) Bochevarov, A. D.; Harder, E.; Hughes, T. F.; Greenwood,
J. R.; Braden, D. A.; Philipp, D. M.; Rinaldo, D.; Halls, M. D.; Zhang,
J.; Friesner, R. A. Int. J. Quantum Chem. 2013, 113, 2110. (b) Jaguar
Emily P. Bacher − Department of Chemistry and
Biochemistry, University of Notre Dame, Notre Dame,
Indiana 46556, United States
Kevin J. Koh − Department of Chemistry and Biochemistry,
University of Notre Dame, Notre Dame, Indiana 46556,
United States; orcid.org/0000-0002-0412-4516
Antonio J. Lepore − Department of Chemistry and
Biochemistry, University of Notre Dame, Notre Dame,
Indiana 46556, United States
Allen G. Oliver − Department of Chemistry and Biochemistry,
University of Notre Dame, Notre Dame, Indiana 46556,
United States
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.1c00248
Notes
The authors declare no competing financial interest.
̈
pKa, release 2019-4; Schrodinger, LLC: New York, 2019.
(11) See the Supporting Information for further details.
(12) (a) Jencks, W. P. Chem. Rev. 1972, 72, 705. (b) Dougherty, D.
A.; Anslyn, E. V. Modern Physical Organic Chemistry; University
Science: Sausalito, CA, 2006.
ACKNOWLEDGMENTS
■
This work was supported by the National Science Foundation
(CHE-1665440, 1956170, and 1855908). E.P.B. was sup-
ported by the CBBI Program and National Institutes of Health
Training Grant T32GM075762.
(13) Plata, R. E.; Singleton, D. A. J. Am. Chem. Soc. 2015, 137, 3811.
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