5-(Diarylimino)- And 5-(sulfoximido)dibenzothiophenium triflates: syntheses and applications as electrophilic aminating reagents

Article abstract of DOI:10.1039/d1ob00285f

The one-pot synthesis of well-defined 5-(diarylimino) and 5-(sulfoximido)dibenzothiophenium triflates, respectively from diarylimines or sulfoximines, is reported and the structures of a series of these compounds are elucidated by X-ray crystallography. In analogy to their hypervalent I(iii) analogues, the iminoyl and sulfoximidoyl groups of these compounds can be selectively transferred to organic substrates. Specifically, the uncatalyzed imination of thiols or sulfinates proceeds with good yields, while under the mild reaction conditions offered by visible light photoredox catalysis, the radical amination of hydrazones or the sulfoximidation of benzylic, allylic and propargylic C-H bonds takes place satisfactorily.

Full text of DOI:10.1039/d1ob00285f

Organic &
Biomolecular Chemistry
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5-(Diarylimino)- and 5-(sulfoximido)
dibenzothiophenium triflates: syntheses and
applications as electrophilic aminating reagents†
Cite this: Org. Biomol. Chem., 2021,
19, 2941
Zhen Li,‡ Gonela Vijaykumar, ‡ Xiangdong Li,
Manuel Alcarazo
Christopher Golz and
*
The one-pot synthesis of well-defined 5-(diarylimino) and 5-(sulfoximido)dibenzothiophenium triflates,
respectively from diarylimines or sulfoximines, is reported and the structures of a series of these com-
pounds are elucidated by X-ray crystallography. In analogy to their hypervalent I(III) analogues, the iminoyl
and sulfoximidoyl groups of these compounds can be selectively transferred to organic substrates.
Specifically, the uncatalyzed imination of thiols or sulfinates proceeds with good yields, while under the
mild reaction conditions offered by visible light photoredox catalysis, the radical amination of hydrazones
or the sulfoximidation of benzylic, allylic and propargylic C–H bonds takes place satisfactorily.
Received 15th February 2021,
Accepted 8th March 2021
DOI: 10.1039/d1ob00285f
rsc.li/obc
functional groups such as azido,6 phthalimido,7 diarylimino,8
Introduction
or sulfoximido are available,9 only a few S-analogues of these
The reactivity of S-(aryl/alkyl) sulfonium salts shows many compounds have been reported to date and the studies regard-
similarities with that of hypervalent I(III) reagents of an ana-
logue substitution pattern.1 Both types of compounds do react
with low valent metals such as Ni(0) or Pd(0) via oxidative
addition at the C–S or C–I bond, respectively, making these
compounds suitable partners for cross coupling reactions.2 In
addition, the highly electrophilic nature of the central hetero-
atom in these species facilitates one-electron reduction of both
sulfonium salts and hypervalent I(III) reagents, which ultimately
leads to the homolytic fragmentation of one of the C–E (E =
heteroatom) bonds and generates a carbon-centred radical.3
This fragmentation pattern paves the way to the use of both
compounds as radical precursors; however, the synthetic utility
of this methodology can only be fully exploited if the chemo-
selective cleavage of only one of the C–E bonds is achieved.4
Finally, the uncatalyzed reaction of both species with suitable
nucleophiles is also possible. This last process can be seen as
an attack of the nucleophile on the central heteroatom followed
by reductive elimination of a Nu–R moiety (Scheme 1a).5
Much more obscure is the chemistry of sulfonium salts
bearing nitrogen-based substituents. Specifically, while many
hypervalent iodine reagents containing transferable nitrogen
Institut für Organische und Biomolekulare Chemie Georg-August-Universität
Göttingen Tammannstr 2, 37073 Göttingen, Germany.
E-mail: manuel.alcarazo@chemie.uni-goettingen.de
†Electronic supplementary information (ESI) available. CCDC 2062172–2062181;
2063032–2063039. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/d1ob00285f
Scheme 1 Reactivity overview and previous and current work on the
use of hypervalent iodine and sulfonium salts as electrophilic sulfoxi-
mide/imine transfer reagents.
‡These authors contributed equally.
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ing their reactivity are even more scarce.10 We speculated place at −50 °C in the presence of pyridine as a base, affording
however that they should also share similarities with their 1a as a beige crystalline solid in 83% yield. Scaling the reaction
iodine counterparts and hypothesized that imino- and sulfoxi- up to multigram quantities is possible without a drop in the
mido-substituted sulfonium salts might potentially function isolated yield (see the ESI for details†). This simple synthetic
as electrophilic aminating reagents or efficient sources of protocol seems to be general and it has been successfully
imine/sulfoximide radicals (Scheme 1b).
extended to the preparation of sulfonium salts 1b and 1c, con-
With this idea in mind, we report herein the multigram- taining two different sulfoximidoyl moieties (Scheme 2a).
scale syntheses of a series of sulfonium salts containing sul- Following a very similar methodology, but just replacing the
foximido 1 and diarylimino 3 functional groups and prelimi- sulfoximide with benzophenone imines 5, one obtains diaryli-
narily evaluate their use in the electrophilic amination of mino-substituted sulfonium salts 3a–c. Compounds 3d–f,
thiols and organic sulfonites. The generation of sulfoximidoyl which employ 2,8-difluorodibenzothiophene, phenoxathiin
and iminyl radicals from the same reagents under photoche- and thianthrene units as sulfonium platforms, are also pre-
mical conditions and their subsequent reaction with hydra- pared from the respective sulfoxides through the same route.
zones and benzylic C–H bonds to deliver N-(amino)amidines
and N-benzylic sulfoximides, respectively, are also studied.
X-ray diffraction analysis of single crystals of 1a–c and 3a–f
confirmed the expected connectivity of the newly prepared
compounds (see Fig. 1 for the structures of 1b, 1c, 3a and 3e
and the ESI for the others†). In 1c, the sulfur atom from the
dibenzothiophenium unit (S1) remains in the plane defined
by that heterocycle and adopts a trigonal-pyramidal bond geo-
metry, the sum of the bond angles around S1 being 304.6°.
The S1–N1 bond distance of 1.652(1) Å falls in between the
ranges for S–N single bonds and double bonds, while the
N1–S2 length for the sulfoximide moiety (1.597(1) Å) gets
closer to the typical SvN(sp2) double bond distance of neutral
sulfoximides (1.517 Å).13 Although the general bonding situ-
ation in 3a is relatively similar to that in 1c, two differences are
of note. An S1–N1 bond distance of 1.705(1) Å was determined,
which more clearly indicates that a single bond connects both
atoms. Moreover, in 3a, the sum of the bond angles around S1
decreases to 297.9°. Both facts can be attributed to the lack of
significant π-interaction between S1 and N1 in this molecule;
this is also confirmed by the N1–C13 bond distance, 1.304(2)
Results and discussion
Synthesis and structure of S-imino- and S-sulfoximido-
substituted sulfonium salts
Our initial efforts focused on the synthesis of the parent sulfo-
nium salts 1, which have a sulfoximidoyl moiety attached to
the dibenzothiophene platform (Scheme 2). While the prepa-
ration of structurally analogous sulfonium salts has been
described by the reaction of N-halosulfilimines with dialkyl/
arylsulfides,10e,11 we decided to explore alternative methods,
which employ less elaborated starting materials and minimize
the number of synthetic steps. Hence, the direct reaction of
diphenylsulfoximide with Tf2O-activated dibenzothiophene
S-oxide was evaluated.12 Gratifyingly, the desired reaction took
Fig. 1 Molecular structures of compounds 1b (top left), 1c (top right),
3a (bottom left) and 3e (bottom right) in the solid state. Anisotropic dis-
placement shown at the 50% probability level; triflate anions and hydro-
gen atoms are omitted for clarity.14
Scheme 2 Synthesis of target sulfonium salts and scope of the
transformation.
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Å, which is just slightly longer than the typical one for CvN
Inspection of the frontier orbitals reveals that in both 1c
and 3a the very low-lying LUMO+1 largely corresponds to a
double bonds in imines (1.297 Å).13
In an attempt to better understand the bonding situation mixture between the π*-system of the dibenzothiophene plat-
just described, the geometry of each compound was optimized form and the σ*(S1–N1) bond (see the ESI†). This distribution
at the PBE/def2TZVP/univ-JFIT level of theory and an IBO of orbital coefficients suggests that any donation of electron
(Intrinsic Bond Orbital) analysis was carried out for 1c and 3a density to this orbital weakens the S1–N1 bond. Hence, one
(see Fig. 2 for the most important IBOs and the ESI for full might expect that the amination of appropriately selected
details†). In line with our interpretation of the crystallographic nucleophiles might occur via the formation of a sulfurane
data, a weak π-interaction (2e; 3c) is detected between the intermediate and subsequent reductive elimination from the
central nitrogen and the two flanking sulfur atoms in 1c, S-center. In addition, single electron transfer to 1 or 3 should
which may account for the reduced bond length in between deliver dibenzothiophene and N-center radicals, as in the case
these atoms. Conversely, in 3a the bond situation is better rep- of the analogous I(III)-compounds.8,9
resented by a clear CvN π-bond. Non-shared electron pairs are
found at S1 and N1 in both compounds.
Reactivity studies
Along these lines, natural population analysis (B3LYP/6- Once compounds 1a–c and 3a–f were completely characterized,
31G*) in 1c and 3a indicates that the sulfur atoms belonging their potential as electrophilic amination reagents was prelimi-
to the dibenzothiophene unit (S1) bear an entire positive narily examined. Our study started with an attempt to prepare
charge in both compounds (+1.050e, 1c, and +1.021e, 3a, sulfenylimines 6 by direct reaction of 3a–f with thiols under
respectively), while the nitrogen atoms significantly differen- mildly basic conditions. While several methods are already
tiate from each other. N1 bears a negative charge in 1c available for the synthesis of these products,15 to the best of
(−1.060e) while this atom is only partially charged in 3a our knowledge, the disconnection approach we report here is
(−0.613e). Wiberg bond indices for the S1–N1 interaction are new. Moreover, most protocols start from disulfides, and the
1.023 and 0.966 for 1c and 3a, respectively, 1.043 for N1–S2 in only one described employing thiol substrates requires metal
1c and 1.672 for the N1vC13 bond in 3a. This analysis, catalysis.16 This could be attributed to the facile oxidation of
together with the X-ray structures, points to a slightly stronger thiols, resulting in undesired products.
S1–N1 interaction in 1c than in 3a.
4-(t-Butyl)benzenethiol 7a was initially used as a model sub-
strate (Scheme 3). Treatment of that thiol with salts 3a,d–f
smoothly afforded the desired sulfenylimine 6a, with reagent
3a being the one that delivered the desired product with better
yields. Employing 3a, the reaction indistinctly works with elec-
tron-rich or electron-poor thiols, albeit all yields are moderate.
On the other hand, the functional group tolerance of the trans-
formation is quite broad; ether, ester, amide and nitro groups
as well as a range of heterocycles did not disturb. As a side
reaction, the formation of 5–10% of the corresponding di-
sulfide was systematically observed in all experiments. When
TEMPO (1.0 equiv.) was added to the imination reaction, the
formation of 6a was completely suppressed. It seems therefore
reasonable to believe that this reaction takes place via one-elec-
tron oxidation of the thiolate by 3a followed by coupling of the
iminyl and thiyl radicals.
The electrophilic amination could also be extended to sulfi-
nates 8 to obtain sulfonamides 9. In this case, bisimine 10 is
often observed as a homocoupled side product. Moreover, the
addition of TEMPO (1.0 equiv.) to the reaction mixture signifi-
cantly reduced the isolated yield of 9b (23%). For these
reasons we also believe that the aryl sulfinate salts, which have
low oxidation potentials (E1/2 = −0.37 V vs. SCE for sodium
phenylsulfinate), undergo SET oxidation in the presence of 3a
(E1/2 = −0.46 V vs. SCE) to furnish sulfonyl and iminyl radicals,
which recombine to form 9 and small amounts of 10
(Scheme 4).17
After this initial contact with the reactivity of salts with the
general formula of 3, and given the number of precedents in
which sulfonium salts are reduced via single electron transfer
under photochemical conditions to produce radicals of
Fig. 2 Selected IBO plots for complexes 1c (a) and 3a (b); assigned
partial charges for the given IBOs to the individual atoms. Threshold
value for printing: 80.
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Scheme 5 (a) Stern–Volmer plot of photocatalyst luminescence
quenching by 3a and (b) intramolecular radical trap experiment employ-
ing substrate 3c.
an o-biphenyl substituent, in the presence of [Ru(bpy)3]Cl2
(1 mol%) quantitatively afforded the product of radical trap-
ping, phenanthridine 11 (Scheme 5).18
Having established the conditions to form iminyl radicals
from 3a, we set about determining the potential of this photo-
catalytic system on the C(sp2)–H amination of aldehyde-
derived hydrazones 12 into hydrazonamides 13, which would
represent the most step-economical method to carry out this
transformation.19 We were pleased to see that the desired
transformation actually proceeded, leading to the formation of
C(sp2)–H aminated products; the yields however are moderate
and they drop dramatically if the hydrazones used as sub-
strates are not derived from electron-rich aromatic aldehydes
(Scheme 6).
Scheme 3 Substrate scope of the electrophilic amination of thiols. The
reaction was conducted at room temperature. Unless specified, all reac-
tions were performed using reagent 3a (1.5 equiv.). All reactions were
quenched after 1 h and yields are of the isolated products.
Based on these results and literature precedents, we
proposed the reaction pathway shown in Scheme 6b.
Initial irradiation of [Ru(bpy)3]2+ generates photoexcited
([Ru(bpy)3]2+)*, which reduces reagent 3a via single-electron
transfer and generates the transient iminyl radical (Int-I) via
homolytic S–N bond cleavage. Addition of the N-centered
radical to the azomethine carbon of 12 delivers an aminyl
radical intermediate (Int-II), which is stabilized by the adjacent
N ðIaIItÞo=ðmIIÞ. At this stage, Int-II can be oxidized by [Ru(bpy)3]3+
(E1=2
= 1.29 V vs. SCE) to produce a diazenium cation with
regeneration of the photocatalyst, followed by deprotonation to
deliver 13.20 Alternatively, deprotonation of Int-II may occur
first leading to the formation of the corresponding radical
anion, which would undergo oxidation to 13.21
Finally, we also checked the reactivity of salt 1a in the sul-
foximidation of benzylic C–H bonds. A closely related trans-
formation to this one has been already described by Bolm and
co-workers, also under photochemical conditions, but employ-
Scheme 4 Substrate scope of the electrophilic amination of sodium
sulfinates with 3a. All reactions were conducted at room temperature
and quenched after 1 h. Yields are of isolated products.
different nature,2 it was hypothesized that under these soft ing hypervalent I(III) reagent 2.9c These authors report the for-
activation conditions, 3a also might be able to generate iminyl mation of the desired sulfoximides in moderate to good yields.
radicals. An experiment often used to determine the feasibility Mechanistic studies suggest the formation of a sulfoximidoyl
of that process consists of the evaluation of fluorescence inten- radical as a key intermediate, which is able to abstract a
sity quenching of the photoexcited state of a photocatalyst benzylic hydrogen atom from the substrate delivering the N–H
(Stern–Volmer) in the presence of the desired substrate. In sulfoximine and a benzyl radical. Oxidation of this radical by
fact, this analysis revealed an effective oxidative quenching of the oxidized photocatalyst delivers a benzylic carbocation,
ðIIÞ*=ðIIIÞ
the photoexcited Ru catalyst (E
¼ ꢀ0:81 V vs. SCE) in which reacts with the sulfoximine to afford the
1=2
the presence of 3a (Ered = −0.46 V vs. SCE). The formation of N-functionalized products. Hence, if sulfoximidoyl radicals
the envisaged iminyl radical was subsequently evidenced by a could be generated from sulfonium salts 1a–c, then, similar
radical scavenger experiment. Hence, irradiation of 3c, bearing reactivity could be expected.
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determined for the photoexcited state of [Ru(bpy)3][PF6]
ðIIÞ*=ðIIIÞ
(E
¼ ꢀ0:81 V vs. SCE) indicate the feasibility of the
1=2
necessary SET event. Moreover, a Stern–Volmer experiment
confirms that 1c effectively quenches the excited state of
[Ru(bpy)3][PF6] (Fig. 3a). A final confirmation of the generation
of sulfoximidoyl radicals by one-electron reduction of 1a was
provided by the reaction of that compound with Cp2Co (Ered
=
Scheme 6 (a) Synthesis of hydrazonamides 13 and (b) plausible reac-
tion mechanism.
We initially evaluated this possibility by cyclic voltammetry.
These experiments showed an irreversible reduction with
Ered = −0.462 V and −0.606 V vs. SCE for 1a and 1c, respect-
ively; values that are considerably less negative than that
Fig. 3 Mechanistic studies: (a) Stern–Volmer plot of photocatalyst
luminescence quenching by 1a and (b) simulated (blue) and experi-
mental (black) EPR spectra of 14.
Scheme 7 Reaction scope for the sulfoximidation of benzylic C–H
bonds.
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−0.93 vs. SCE) in the presence of N-(t-butyl)phenylnitrone Synthesis of 3a
(PBN) as a radical trap. The EPR spectrum of the reaction
Triflic acid anhydride (0.62 g, 2.2 mmol, 1.1 equiv.) was slowly
added at −50 °C to a suspension of dibenzothiophene-S-oxide
(0.40 g, 2.0 mmol, 1.0 equiv.) in dry dichloromethane (16 mL).
After stirring the reaction mixture for 30 min at that tempera-
ture, pyridine (0.17 g, 2.2 mmol, 1.1 equiv.) and benzophenone
imine (0.39 g, 2.2 mmol, 1.1 equiv.) were added as a solution
in dichloromethane (3 mL). After this, the reaction mixture
was slowly warmed to −15 °C and stirred for another 6 h at the
same temperature. Then the resulting mixture was washed
with aq. K2CO3 and extracted with dichloromethane. The
solvent was evaporated in vacuo and the residue was washed
with dry Et2O (2 × 10 mL) and dried in vacuo to obtain the
mixture shows a well-resolved splitting as a result of hyperfine
coupling with a N- and a H-atom (aN = 14.61; aH = 2.22 G), a
pattern that fits with that expected for radical 14.
Once the generation of the desired sulfoximidoyl radical
was confirmed, the amination of benzylic C–H bonds was eval-
uated. In fact, we could perform this reaction with a scope very
similar to that reported by Bolm,9c which includes not only
benzylic positions 16a–aa, but also allylic 16ab–ag and pro-
pargylic ones 16ah–aj (Scheme 7).22 A light on/off experiment
confirmed the necessity of continuous irradiation for the reac-
tion to proceed, and the quantum yield of the reaction to form
16a (Φ = 0.044) indicates that a radical chain process cannot
be a predominant pathway.
1
desired product as a pale-yellow solid (0.84 g, 82%). H NMR
(400 MHz, CDCl3) δ = 8.03 (d, J = 7.8 Hz, 2H), 7.88 (d, J =
7.3 Hz, 2H), 7.84–7.75 (m, 5H), 7.68 (d, J = 8.0 Hz, 2H),
7.57–7.53 (m, 5H), 7.35 (t, J = 7.8 Hz, 2H) ppm; 13C NMR
(101 MHz, CDCl3) δ = 190.7, 139.5, 135.9, 135.5, 135.3, 134.6,
132.9, 132.5, 131.6, 131.3, 130.4, 129.7, 129.0, 128.9, 124.1,
121.0 (q, J = 322.2 Hz) ppm; 19F NMR (377 MHz, CDCl3) δ =
−78.11 ppm; IR (ATR): 3061, 1710, 1657, 1582, 1526, 1482,
1446, 1259, 1223, 1155, 1029, 999, 955, 761, 707, 637, 572,
Conclusions
In summary, we report herein the straightforward syntheses of
sulfonium salts containing imino and sulfoximido substitu-
ents and demonstrated that under photochemical conditions,
these compounds are effective sources of iminyl and sulfoximi-
doyl radicals, respectively, which can be subsequently
employed in the imination of hydrazones or the sulfoximida-
tion of benzylic C–H positions. The straightforward synthesis,
easy handling and safe profile of compounds 1 and 3 confirm
these sulfonium salts as convenient surrogates of iodonium
salts of analogue structure and reactivity.
517 cm−1; HRMS calculated m/z for C25H18NS+ [M-OTf]+
364.1154, found 364.1156; Mp: 122–123 °C.
:
Author contributions
Zhen Li synthesized compounds 1a–c and studied their reactiv-
ity, Gonela Vijaykumar synthesized 3a–f and studied their reac-
tivity, Xiangdong Li performed the mechanistic studies,
Christopher Golz performed the calculations and determined
the X-ray structures, and Manuel Alcarazo supervised the
project and wrote the manuscript.
Experimental section
Synthesis of 1a
Triflic anhydride (1.4 g, 5.0 mmol, 1.0 equiv.) was dropwise
added to a solution of dibenzothiophene-S-oxide (1.00 g,
5.0 mmol) in dry DCM (40 mL) at −50 °C. After stirring the
resulting mixture for 1 hour, pyridine (403 μL, 5.0 mmol) and
diphenylsulfoximide (1.09 g, 5.0 mmol) dissolved in DCM
(10 mL) were sequentially added dropwise, and the mixture
was further stirred at −50 °C for 6 additional hours. Then, the
cooling system was removed, and the obtained solution was
allowed to reach room temperature. After this, an aqueous
K2CO3 solution was added, upon which the phases separated
and the organic one was dried with anhydrous MgSO4 and
evaporated in vacuo. The thus obtained residue was washed
with dry Et2O (3×) and finally dried under vacuum to deliver 1a
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
Support from the European Research Council (ERC CoG
771295) and the DFG (INST 186/1237-1 and INST 186/1324-1)
is gratefully acknowledged. We also thank Mr M. Simon for
assistance with preparative HPLC and Dr S. I. Kozhushkov for
proof checking this manuscript.
1
as a beige solid (2.28 g, 83%). H NMR (400 MHz, CD3CN) δ =
8.15–8.06 (m, 2H), 8.04–7.95 (m, 6H), 7.89–7.77 (m, 4H),
7.74–7.55 (m, 6H). 13C NMR (101 MHz, CD3CN) δ = 138.6,
136.6, 135.8, 135.2, 133.6, 131.2, 130.6, 128.7, 128.0, 124.0,
121.2 (q, J = 321.0 Hz); IR (neat): 3566, 3088, 1709, 1577, 1448,
1362, 1258, 1224, 1152, 1088, 1029, 1005, 969, 758, 738, 707,
685, 637, 613, 589, 573, 541, 518, 422 cm−1. HRMS calculated
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2948 | Org. Biomol. Chem., 2021, 19, 2941–2948
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