Synthesis and photochromism of some mono and bis (thienyl) substituted oxathiine 2,2-dioxides

Article abstract of DOI:10.1039/c9ob02128k

1,2-Oxathiine 2,2-dioxides have been obtained from their respective 3,4-dihydro-4-dimethylamino precursors, for the first time, by a mild Cope elimination of the 4-dimethylamino function. The application of the 1,2-oxathiine 2,2-dioxide scaffold in materi

Full text of DOI:10.1039/c9ob02128k

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Synthesis and photochromism of some mono and
bis (thienyl) substituted oxathiine 2,2-dioxides†
Cite this: Org. Biomol. Chem., 2019,
17, 9578
Stuart Aiken,
Dimitrios Zonidis
Christopher D. Gabbutt,
*
B. Mark Heron, * Craig R. Rice
and
Received 1st October 2019,
Accepted 9th October 2019
DOI: 10.1039/c9ob02128k
rsc.li/obc
1,2-Oxathiine 2,2-dioxides have been obtained from their respect- This work constitutes part of our ongoing programme of hetero-
ive 3,4-dihydro-4-dimethylamino precursors, for the first time, by cyclic synthesis concerning strategies to 1,2-oxathiine 2,2-dioxides
a mild Cope elimination of the 4-dimethylamino function. The in which we are exploring the versatility of sulfene additions to
application of the 1,2-oxathiine 2,2-dioxide scaffold in materials enaminoketones to afford relatively inaccessible substitution pat-
chemistry is exemplified by the efficient P-type photochromism of terns on the 4-dimethylamino-3,4-dihydro-1,2-oxathiine 2,2-
the 5,6-bis(2,5-dimethyl-3-thienyl) substituted oxathiine 2,2- dioxide core and subsequent mechanistic investigations concern-
dioxides.
ing the elimination of the 4-dimethylamino function to access
diversely substituted unsaturated 1,2-oxathiine 2,2-dioxides.29
The addition of sulfenes to enaminoketones to afford
4-amino-3,4-dihydro-1,2-oxathiine 2,2-dioxides has been
explored by Schenone et al.30 Interestingly, the formation of
the unsaturated 1,2-oxathiine 2,2-dioxides was a relatively scar-
cely observed feature in these initial studies.31 Indeed when
chlorosulfene was added to an enaminoketone a subsequent
facile base-promoted elimination of HCl was observed and the
unsaturated 4-amino-1,2-oxathiine 2,2-dioxide resulted32 and
attempts to effect dehydrogenation of 4-amino-3,4-dihydro-1,2-
oxathiine 2,2-dioxides using excess DDQ met with variable
results.33 We elected to utilise the foregoing sulfene addition
chemistry30,34 and explore the subsequent elimination step
required to obtain the unsaturated 1,2-oxathiine 2,2-dioxides.
Fundamental to the present study was access to a series of
methylene ketones and whilst deoxybenzoin 11 is widely com-
mercially available the isomeric thienylketones 9 and 10 and
the 1,2-bis(2,5-dimethylthiophen-3-yl)ethan-1-one 6 required
preparation (Scheme 2). Examination of the literature revealed
that the 2,5-dimethylthienyl-3-acetic acid 4 has been prepared
from 2,5-dimethylthiophene in three steps, acylation,
P-type photochromic dithienylethenes such as 1-open, which
readily undergoes reversible conversion to the coloured isomer
1-closed, (Scheme 1) are fundamental switching units which
have been used to modulate a variety of physical and optical
properties.1 Structural variation of the essential 1,2-bis(2,5-di-
methylthiophen-3-yl)ethene core has been frequently explored
by modification of the 2,5-dimethylthiophene moiety1,2 and to a
somewhat lesser extent by variation of the perfluorocycle, par-
ticularly replacement of the latter with a 5-membered hetero-
cyclic unit to afford 2, wherein the heterocycle has been selected
from imidazole,3,4 imidazolium,5 pyrrole,6 thienophosphole,7
phosphindolothiophene,8
thiophene,9–11
thiopyranothio-
phene,12 silole,13,14 1,3-dithiole15 and thiazole16 and less com-
monly with a six-membered unit leading to 2 where the hetero-
cyclic unit includes quinoxaline,17 triazoloquinoline,18 pyrida-
zine,19 thiazine,20 1,2-oxazine21 and 1,2,4-triazine.22
We have previously studied the synthesis and performance
of
various
T-type
photochromic
systems,
e.g.
naphthopyrans23–25 and naphthoxazines,26,27 and we have
recently explored negatively photochromic systems.28 In this
study we describe the preliminary examples of an efficient syn-
thetic route to the relatively little studied 1,2-oxathiine 2,2-
dioxide unit and in doing so define a route to new photochromic
dithienylethenes with a central 1,2-oxathiine 2,2-dioxide core.
Department of Chemical Sciences, School of Applied Sciences, University of
Huddersfield, Queensgate, Huddersfield, HD1 3DH, UK. E-mail: m.heron@hud.ac.uk,
dimitrios.zonidis@hud.ac.uk
†Electronic supplementary information (ESI) available: Experimental details and
characterization data. CCDC 1905437. For ESI and crystallographic data in CIF
or other electronic format see DOI: 10.1039/c9ob02128k
Scheme 1 Representative photochromic response of a dithienylethene
system.
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Scheme 2 Synthesis of thienyl ketones 6, 9 and 10. Reagents and conditions: (i) Ethyl oxalyl chloride, AlCl3, anyhd., MeNO2, 5 °C–RT; (ii)
NH2NH2·H2O, HOCH2CH2OH, then KOH, 70 °C – reflux; (iii) SOCl2, cat. DMF, CH2Cl2; (iv) 2,5-dimethylthiophene, AlCl3, anyhd., MeNO2, 5 °C–RT; (v)
PhH, AlCl3, anyhd., MeNO2, 5 °C–RT; (vi) MeNHOMe·HCl, pyridine, CH2Cl2, 0–5 °C–RT; (vii) PhLi in Bu2O, anhyd., THF, N2, −78 °C–RT; (viii)
PhCH2COCl, AlCl3, anyhd., CH2Cl2, MeNO2, 5 °C–RT.
Willgerodt–Kindler reaction and hydrolysis, in moderate dimethylformamide dimethylacetal (DMFDMA) (Scheme 3).
overall yield.35 Given the unappealing nature of this sequence Phenylsulfene, generated in situ by the action of Et3N on phe-
we elected to examine an alternative protocol. Friedel–Crafts nylmethanesulfonyl chloride, added cleanly to the foregoing
acylation of 2,5-dimethylthiophene with ethyl oxalyl chloride enaminoketones to afford the 3,4-dihydro-1,2-oxathiine 2,2-
gave the glyoxalate 3 which underwent a smooth Wolff– dioxides 13a-open–d-open (53–77%) after either chromato-
Kishner reduction with concomitant hydrolysis to afford 4 in graphy or recrystallization. Oxathiine 2,2-dioxide 13e-open was
64% yield (two-steps). The acid chloride 5 was prepared and obtained in a similar manner in 55% yield from the addition
1
used directly in a Friedel–Crafts acylation with 2,5-dimethyl- of sulfene to 12a. The H NMR spectrum of 13e-open revealed
thiophene to afford 6 (47%) which displayed a characteristic an AA′B spin pattern for the C-3 and C-4 hydrogens with
singlet in its 1H NMR spectrum at δ 3.92 assigned to the J3,4(trans) = 9.1 Hz, J3′,4(cis) = 7.8 Hz and 2J3,3′ = 13.8 Hz.
methylene unit. Interestingly, applying this Friedel–Crafts
Attempts to effect the acid elimination of dimethylamine
strategy to benzene failed to afford 9 and instead 5 underwent from 13d-open using increasing amounts of 4-TsOH (0.05–5
a ‘homo Friedel–Crafts’ reaction followed by cyclisation to eq.) at either RT or reflux in PhMe were unsuccessful and at
afford the novel thieno[3,4-c]pyranone 7 (δCH 7.41; δmethylene elevated temperature some yellowing of the reaction mixture
3.67; δCvO 166.44); the geometry of which was established as was observed together the formation of minor amounts of
the Z-isomer by a NOESY experiment. Evidently the thiophene polar ‘degradation’ material as indicated by TLC. The magni-
moiety of 5 is more electron rich than benzene and is thus the tude of the coupling constants between 3-H and 4-H (3J3,4
=
favoured substrate in the foregoing acylation reaction. 7.8–8.1 Hz) of the 3-phenyl substituted series 13a-open–13d-
Undeterred by this setback, the Weinreb amide 8 (δOMe 3.60; open suggest that these protons occupy an anti-peri-planar
δNMe 3.18; δmethylene 3.59) was obtained (65%) from 5 by stan- arrangement.
dard methodology.36 The addition of PhLi to 8 proceeded
A crystal of 13a-open was obtained from Et2O and hexane
without complication to afford target ketone 9 (δmethylene 4.12) (stored at −20 °C for 24 h) and an X-ray crystal structure
in 66% yield (Scheme 2). (Fig. 1) confirmed the arrangement of 3-H and 4-H which have
Ketone 10 was readily prepared by the Friedel–Crafts reac- a torsion angle of ca. 39.5° with the torsion angle between the
tion of phenylacetyl chloride with 2,5-dimethylthiophene in 3-Ph and 4-NMe2 moieties as 78.9°. The SO2 unit of the
77% yield. Here the characteristic methylene singlet appeared oxathiine 2,2-dioxide ring protruded out of the main oxathiine
at δ 4.11 in the 1H NMR spectrum.
ring plane (O1–C3–C4–C5–C6) with C3–S2–O1 angle of ca.
Ketones 6, 9–11 were transformed into their respective 110°. The thiophene rings adopt an anti-parallel conformation
enaminoketones 12a–d (60–86% yield) upon reaction with N,N- which favours the reversible photocyclistion process.1,2,37
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Scheme 3 Synthesis of thienyl substituted 1,2-oxathiine 2,2-dioxides. Reagents and conditions: (i) DMFDMA, reflux; (ii) either phenylmethanesulfo-
nyl chloride (for 13a–d) or methanesulfonyl chloride (for 13e), Et3N, anhyd. THF, 0 °C–RT; (iii) m-CPBA, CH2Cl2, 0 °C–RT.
Given the syn-relationship between 3-H and the 4-NMe2
moiety a Cope elimination protocol was adopted to affect the
syn-elimination of the dimethylamine unit.39 Pleasingly, treat-
ing a DCM solution of 13d-open with an excess of m-CPBA
(0–5 °C → RT, 4 h) afforded 14d-open (δ4-H 7.01) as pale-yellow
crystals in 62% yield. Repeating this procedure enabled the
isolation of 14a-open (84%, δ4-H 6.89), 14b-open (78%, δ4-H
6.89), 14c-open (71%, δ4-H 7.04) and 14e-open (78%, δ3-H 6.61,
δ4-H 6.90 (J3,4 = 10.2 Hz)) without the detection of any S oxi-
dised products (Scheme 3). This facile elimination protocol
provides an efficient strategy to form unsaturated 1,2-oxathiine
2,2-dioxides from the 4-dialkylamino substituted 3,4-dihydro-
1,2-oxathiine 2,2-dioxides which are easily obtained from
sulfene additions to enaminoketones.
With the series of oxathiine 2,2-dioxides 13-open and 14-
open to hand their photochromic response was examined.
Irradiating hexane solutions of 13a-open, b-open, c-open and
e-open revealed very weak to moderate yellow – orange colour
development (Table 1) due to photoinduced ring closure
(Scheme 4) with λmax in the range 413 to 441 nm after pro-
probability level and disordered Et2O solvent molecule omitted for clarity).38 longed irradiation to a steady state (ca. 145 min, λirr
Fig. 1 Crystal structure of 13a-open (thermal ellipsoids shown at 50%
=
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Table 1 Photochromic response of series 13-open and series 14-open
b
Absorbance at λmax
λmaxa (nm)
Hexane
A0
APSS
εm at PSSc (mol−1 dm3 cm−1
)
% closed formd
13a-open/13a-closed
14a-open/14a-closed
13b-open/13b-closed
14b-open/14b-closed
13c-open/13c-closed
14c-open/14c-closed
13e-open/13e-closed
14e-open/14e-closed
414
503
413
481
474
513
441
494
0.01
0.03
0.01
0.01
0
0
0.01
0.01
0.55
0.82
0.23
0.07
0.03
0.01
0.38
0.94
1300
1708
489
125
61
25
1524
1541
2
38
5
0.5
2
2
8
9
a Wavelength of maximum absorption of the closed species. b Absorbance A0 before UV irradiation and absorbance APSS at photostationary state
(PSS) for hexane solution of ca. 0.5 mmol dm−3 c Molar extinction coefficient of closed form at photostationary state as calculated using the
.
Beer–Lambert Law. d % closed form determined by comparison of the relative integrals of the signals for the thiophene ring methyl group
protons in the original open forms and the closed forms at PSS.
Fig. 2 Absorption spectra (in hexane) of 3,4-dihydro-1,2-oxathiine 2,2-
dioxides 13a-open, b-open, c-open, e-open before irradiation and at
photostationary state.
Scheme 4 Structures of 13a-open/14a-open before and after
irradiation.
260–380 nm, 150 W) (Fig. 2). 1H NMR spectra for 13a-open
before and after irradiation are provided in the ESI.† The
remaining 5,6-diphenyl analogue 13d-open showed no photo-
chromic response.
The photochromic behaviour of the unsaturated series 14a-
open–e-open was next examined in hexane solution.
Irradiation of a hexane solution of 14a-open resulted in the
generation of an intense red hue (λmax = 503 nm, PSS 45 min)
(Fig. 3 and insert 1, Table 1) which is assigned to the ring-
closed isomer 14a-closed (Scheme 4). Visible light bleaching
(455–650 nm) of the foregoing red solution of 14a-closed was
efficiently accomplished after 25 min. The colouration and
bleaching of 14a-open (PhMe solution) was repeated 10 times
to illustrate the reversibility of the system (insert 2 on Fig. 3).
Repeating the irradiation experiment of 14a-open in CDCl3
and recording the 1H NMR spectra over time revealed the pres-
Fig. 3 Absorption spectra of 14a-open (initial, after UV activation and
after visible light bleaching); inset shows recyclability with UV activation
and visible light bleaching cycles.
ence of new signals attributed to 14a-closed at δ 2.05 (Th-Me), 4-H in 14a-open (δ = 6.89) and 14a-closed (δ = 6.83) of the CDCl3
2.11 (Th-Me), 2.12 (Th-Me), 2.22 (Th-Me), 5.96 (Th-H) 6.07 solution revealed a ratio of ca. 5 : 3 (14a-open/14a-closed) at the
(Th-H) and 6.83 (4-H) (Fig. 4). Comparison of the integrals for photostationary state (Fig. 4, Table 1).
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Fig. 6 Photograph of 14a-open powdered sample on white paper
background (LHS pre-irradiated, RHS post-irradiation).
Fig. 4 1H NMR spectra (δ 5.5–7.7, CDCl3) showing evolution of signals
for photochemical ring-closure of 14a-open.
resulted in the largest (89 nm) shift. The photochromic
response of the series 13-open and 14-open is summarised in
Table 1.
The solid-state photochromism of 14a-open was also briefly
examined; with a powdered sample irradiated for 30 s with a
TLC inspection lamp (Spectroline E Series 365 nm, 8 Watt).
The change in appearance of the sample is clearly visible from
the photograph presented in Fig. 6 with the unirradiated
sample appearing pale yellow and a post-irradiated sample
developing a red/brown hue.
Irradiation of 14e-open, similarly substituted with 2,5-di-
methylthiophen-3-yl units, offered inferior performance to
14a-open with λmax at 494 nm (50 min irradiation, ratio 14e-
open : 14e-closed ca. 10 : 1 based on the relative integrals of the
doublets for 3-H at δ 6.61 (open) and δ 6.38 (closed) (see ESI†
1
for H NMR spectra for 14e-open before and after irradiation).
In the 1H NMR spectrum of 14e-closed signals were
observed at δ 2.18, 2.31, 2.39 and 2.64 for the methyl groups,
at ca. δ 5.95 for the thiophene ring protons and at δ 6.82 (d, J =
10.3 Hz) for 4-H. Unfortunately, 14b-open and 14c-open only
showed an exceptionally weak red hue upon irradiation to
generate their ring-closed forms (Fig. 5) and the diphenyl
analogue 14d showed no photochromism, emphasising the
requirement for at least one 2,5-dimethylthiophene unit on
the central ethene bond.
In summary, 1,2-oxathiine 2,2-dioxides with combinations
of aryl and heteroaryl substituents have been efficiently
obtained for the first time by a Cope elimination protocol from
their respective 4-dimethylamino-3,4-dihydro precursors which
were derived from sulfene additions to enaminoketones. The
3,4-dihydro-1,2-oxathiine 2,2-dioxide series 13 exhibited very
weak photo-colouration (low percentage ring closed form and
hence weak molar extinction coefficients), perhaps due to
limited activation as a consequence of a relatively low degree
of unsaturation resulting in a low absorption in the activating
UV region. Of the series 13 the 5,6-bis(2,5-dimethylthien-3-yl)
analogues 13a-open and 13e-open exhibited the best photo-
chromism with UV irradiation generating their closed ring
isomers which exhibited a weak yellow – orange hue. The
series of unsaturated oxathiines 14 typically exhibited better
photochromism then the dihydro precursors 13. The oxathiine
2,2-dioxides 14a-open and 14e-open offered the best photo-
chromism with λmax of their closed ring forms bathochromi-
cally shifted relative to their dihydro-precursors, leading to
moderately intense red-brown hues as a consequence of the
extended lateral conjugation. The presence of a phenyl substi-
tuent on the oxathiine ring lead to a further small bathochro-
mic shift in λmax viz. 14a-open (503 nm) and 14e-open
(494 nm). The photochromic response of 14a-open and
14e-open of this preliminary series of novel dithienylethenes
Interestingly, the introduction of the C-3–C-4 double bond
induced a bathochromic shift in λmax of 14a-closed (503 nm)
and 14e-closed (494 nm) relative to their dihydro precursors
[13a-closed (414 nm), 13e-closed (441 nm)] presumably as a
result of the extended lateral conjugation. It should be noted
that the conjugation with the C-3 phenyl group (14a-open)
containing
a
1,2-oxathiine 2,2-dioxide core offered
comparable performance to other heterocyclic bridged
dithienylethenes.17–22 However, it is clear from this study that
for good photochromic performance the presence of two sub-
stituted thiophene units at the termini of the central ethene
bond combined with further unsaturation in the central
oxathiine moiety is essential.1,2 Our exploration of the appli-
Fig. 5 Absorption spectra (in hexane) of oxathiine 2,2-dioxides 14a-
open, b-open, c-open, e-open before irradiation and at photostationary
state.
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cation of the oxathiine moiety as the central core in photochro- 18 M. M. Krayushkin, B. V. Lichitskii, D. V. Pashchenko,
mic systems continues and is presently focussed on both
further extending the lateral conjugation using substituted
I. A. Antonov, B. V. Nabatov and A. A. Dudinov, Russ. J. Org.
Chem., 2007, 43, 1357–1363.
(hetero)aromatic systems and enhancing the photocolouration 19 M. M. Krayushkin, D. V. Pashchenko, B. V. Lichitsky,
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B. V. Nabatov, A. M. Komogortsev, L. G. Vorontsova and
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Conflicts of interest
20 L. I. Belen’kii, A. V. Kolotaev, V. Z. Shirinyan,
M. M. Krayushkin, Yu. P. Strokach, T. M. Valova,
Z. O. Golotyuk and V. A. Barachevskii, Chem. Heterocycl.
Compd., 2005, 41, 86–92.
There are no conflicts to declare.
21 K. P. Schultz, D. W. Spivey, E. K. Loya, J. E. Kellon,
L. M. Taylor and M. R. McConville, Tetrahedron Lett., 2016,
57, 1296–1299.
22 S. N. Ivanov, B. V. Lichitskii, A. A. Dudinov,
A. Yu. Martynkin and M. M. Krayushkin, Chem. Heterocycl.
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Acknowledgements
DZ thanks the University of Huddersfield for funding for his
PhD studies.
23 J. D. Hepworth and B. M. Heron, in Functional Dyes, ed.
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Org. Biomol. Chem., 2019, 17, 9578–9584 | 9583

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Communication
Organic & Biomolecular Chemistry
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38 Crystal for 13a. Crystal data for C26H32NO3.50S3, M = 510.70,
triclinic, a = 9.869 (5), b = 11.897 (5), c = 13.181 (5) Å, α =
113.322 (17), β = 109.14 (2), γ = 90.79 (2)°, V = 1324.1 (10) Å3,
0.0778 (all data). The final wR(F2) = 0.1481 (all data). The
goodness of fit on F2 was 1.027. Peak and hole =
0.823/−1.120. CCDC 1905437† contains the supplementary
crystallographic data for this paper. The structure con-
tained a disordered diethyl ether solvent molecule which
was modelled in two positions using the PART instruction
in the refinement. The anisotropic displacement
parameters were restrained using the DELU and SIMU
instructions.
ˉ
T = 150 K, space group P1, Z = 2, 19 176 reflections
measured, 8011 independent reflections (Rint = 0.0392). 39 P. C. Astles, S. V. Mortlock and E. J. Thomas, in
The final R1 values were 0.0519 (I > 2σ(I)). The final wR(F2)
values were 0.1309 (I > 2σ(I)). The final R1 values were
Comprehensive Organic Synthesis, Elsevier, Amsterdam,
1991, vol. 6, ch. 5.3, pp. 1011–1039.
9584 | Org. Biomol. Chem., 2019, 17, 9578–9584
This journal is © The Royal Society of Chemistry 2019