α-Diazo Sulfonium Triflates: Synthesis, Structure, and Application to the Synthesis of 1-(Dialkylamino)-1,2,3-triazoles

Article abstract of DOI:10.1002/anie.202014775

The one-pot synthesis of a series of sulfonium salts containing transferable diazomethyl groups is described, and the structure of these compounds is elucidated by X-ray crystallography. Under photochemical conditions, reaction of these salts with N,N-dialkyl hydrazones affords 1-(dialkylamino)-1,2,3-triazoles via diazomethyl radical addition to the azomethine carbon followed by intramolecular ring closure. The straightforward transformation of the structures thus obtained into mesoionic carbene–metal complexes is also reported and the donor properties of these new ligands characterized.

Full text of DOI:10.1002/anie.202014775

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Accepted Article
Title: α-Diazo Sulfonium Triflates: Synthesis, Structure and Application
to the Synthesis of 1-(Dialkylamino)-1,2,3-triazoles
Authors: Manuel Alcarazo, Xiangdong Li, and Christopher Golz
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α-Diazo Sulfonium Triflates: Synthesis, Structure and Application
to the Synthesis of 1-(Dialkylamino)-1,2,3-triazoles
Xiangdong Li,[a] Christopher Golz,[a] and Manuel Alcarazo*[a]
[a]
Dr. X. Li, Dr. C. Golz, Prof. Dr. M. Alcarazo
Institut für Organische und Biomolekulare Chemie
Georg-August-Universität Göttingen
Tammannstr. 2, 37077-Göttingen, Germany
E-mail: manuel.alcarazo@chemie.uni-goettingen.de
Supporting information for this article is given via a link at the end of the document.
Abstract: The one pot synthesis of a series of sulfonium salts
containing transferable diazomethyl groups is described, and the
structure of these compounds elucidated by X-ray crystallography.
Under photochemical conditions, reaction of these salts with N,N-
dialkyl hydrazones affords 1-(dialkylamino)-1,2,3-triazoles via
diazomethyl radical addition to the azomethine carbon followed by
intramolecular ring closure. The straightforward transformation of the
structures thus obtained into mesoionic carbene-metal complexes is
also reported and the donor properties of these new ligands
characterized.
exhibit.[ 14 ] Herein, the one pot synthesis of a series of such
reagents from sulfoxides and diazo compounds is described, and
their ability to transfer the diazo moiety to hydrazones under
photochemical conditions is preliminary evaluated (Figure 1b).
Sulfonium ions are defined as positively charged organosulfur
species of general formula [R3S]+, in which a central sulfur(IV)
atom is attached to three organic rests while keeping a non-
shared electron pair.[1] The reactivity of these salts is dominated
by the positive charge that they bear, mainly located at sulfur.[2]
Thus, on treatment with bases, α-CH deprotonation occurs on
alkyl sulfonium salts leading to the formation of sulfur ylides; a
class of compounds of demonstrated utility for the synthesis of
three-membered rings such as cyclopropanes, epoxides and
aziridines.[3] Also as consequence of their positive charge, the R2S
fragment in aryl sulfonium salts behaves as an excellent leaving
group making possible the direct attack of nucleophiles to form
Nu-R coupling products,[ 4 ] or the oxidative addition of low
oxidation-state metals into the Ar-S bond. This reactivity makes
sulfonium salts suitable electrophilic partners for typical Pd- or Ni-
catalyzed coupling reactions.[5] One electron reduction is also a
relatively facile process in sulfonium salts; it leads to a fast
mesolytic scission of one of the C-S bonds, generating thioethers
and organic radicals;[6] being the latter able to further engage in
synthetically useful transformations such as cross coupling,
oxygenation, amination or fluorination reactions among others
(Figure 1a).[7]
The reactivity pattern just described shows clear overlaps
between the chemistry of sulfonium salts and that of hypervalent
I(III) reagents.[8] The selection of one or the other for a determined
transformation often depends on factors such as the ease of
synthesis of the necessary reagent, their relative stability, and the
balancing between the levels of reactivity and selectivity that they
provide for the specific process under study.[9,10] Considering
these precedents and being aware that α-diazo iodonium salts
such as A,[11] and B have been used as diazo transfer reagents
under thermal[12] and photochemical conditions,[13] we questioned
ourselves whether sulfonium equivalents of these reagents could
be efficiently prepared, and if so, what kind of reactivity would they
Figure 1. Reactivity outline of sulfonium salts.
Our initial efforts were focused on the synthesis of the parent
sulfonium salt 1a, derived from the dibenzothiophene platform
and containing an ethyldiazoacetate substituent. Sulfonium salts
of related structure have been previously reported; however, they
were prepared without exception by reaction of dialkyl sulfides
with iodonium salt A.[12c,d] Hence, alternative routes which avoid
the use of that I(III)-reagent were explored. Considering the
natural nucleophilicity of diazo compounds at the carbon adjacent
to the dinitrogen moiety, we hypothesized that ethyldiazoacetate
might directly react with sulfoxide 2 after its activation with triflic
acid anhydride. Gratifyingly, the desired reaction took place in
acceptable yield (61%), and compound 1a was isolated as a pale
yellow crystalline solid (Scheme 1a). Moreover, the reaction could
be scaled up to 30 mmol without drop in the isolated yield (see
the Supporting Information). This simple protocol also allowed the
preparation of 1b starting from trifluoromethyl diazomethane; or
1c, 1d and 1e, which are based on the thianthrene, phenoxathiine
and tetrahydrothiophene platforms, respectively (Scheme 1b).
Monocrystals suitable for X-ray diffraction analysis were grown by
slow diffusion of Et2O into saturated CH2Cl2 solutions of 1a-d. The
thus obtained molecular diagrams confirmed the expected
1
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connectivity (Figure 2 and the Supporting Information). In 1a the
sulfur atom remains at the plane defined by the dibenzothiophene
of 384 J•g-1 (Figure 3a). Similar values were obtained for 1c (396
J•g-1) and 1d (348 J•g-1), all significantly lower than those reported
for I(III) analogues (549 J•g-1).[13] On the basis of these results,
the use of the sulfonium salts herein described should be safer;
we carefully recommend however not to expose multigram
quantities 1a-e to temperatures above 80 ºC.
skeleton and adopts
a
trigonal-pyramidal coordination
environment, being the sum of the bond angles around S1
303.8(1)°. The S1-C1 bond distance of 1.7357(12) Å falls within
the range for S-C(sp2) single bonds, while the C1-N1 and N1-N2
lengths (1.3367(15)Å and 1.1111(2)Å) are identical, within the
experimental error, to those found in non-charged diazo
At this stage we evaluated the possibility to generate diazomethyl
radicals through single electron reduction of 1a-e; specifically, via
photoredox catalysis. An experiment often used to determine the
feasibility of the desired activation consists in the evaluation of
fluorescence intensity quenching of the photoexcited state of a
photocatalyst in the presence of the reaction substrate (Stern
Volmer relation). In fact, this analysis revealed an effective
oxidative quenching of the photoexited [Ru(bipy)3][PF6]2 catalyst
(E1/2(II)*/(III)= -0.81V vs SCE) in the presence of 1a (Ered = -0.50V vs
SCE). The formation of the envisaged diazomethyl radical was
subsequently evidenced by testing 1a in the benchmark C-H
diazomethylation of arenes recently reported by Suero.[13]
Gratifying, irradiation of 1a in the presence of [Ru(bpy)3][PF6]2 (1
mol %) and 1,3,5-trimethylbenzene (2 equiv.) afforded the
diazomethylated arene product 3a in 73% isolated yield (Figure
3). Under identical conditions salts 1c and 1d also promoted the
formation of 3a, albeit with lower isolated yields; when employing
reagent 1e not even traces of 3a were detected.
15
]
compounds.[
Very similar parameters are found in the
structures of 1b-d; the only difference of note is the increase in
the sum of bond angles around S1 in 1c (306.4(1) Å) and 1d
(311.5(1) Å), which is a consequence of the geometry imposed by
the heterocyclic skeleton. It is also worth noting that all
compounds 1a-d are found to form dimeric aggregates in the solid
state where two triflate anions bridge simultaneously the sulfur
atoms of two sulfonium moieties. In all cases the S…O distances
are definitely shorter than the sum of the van der Waals radii of
both elements (3.32 Å), revealing a remarkable electrophilicity at
S1.
Scheme 1. Synthesis of α-diazo sulfonium triflates 1a-e.
Figure 3. a) DSC analysis for 1a in air; b) Quenching of the fluorescence of
[Ru(bipy)3][PF6]2 following the addition of 1a; c) trapping the postulated
diazomethyl radical intermediate with 1,3,5-trimethylbenzene to afford product
3a and formation of diethyl acetylenedicarboxylate via radical dimerization and
dinitrogen elimination.
Figure 2. Molecular structures of compounds 1a-d in the solid state.
Anisotropic displacements shown at 50% probability level. Hydrogen atoms and
solvent molecules removed for clarity. Selected bond lengths [Å]: 1a: S1-C1,
1.7357(12); C1-N1, 1.3367(15); S1-O3, 2.893(1); 1b: S1-C1, 1.7309(11); C1-
N1, 1.3252(13); S1-O1, 2.970(1); 1c: S1-C1, 1.7386(11); C1-N1, 1.3138(14);
S1-O3, 2.938(1); 1d: S1-C1, 1.7386(2); C1-N1, 1.339(3); S1-O4, 3.073(2).[16]
Having confirmed that possibility, we set about determining the
potential of the photocatalytic system just developed on the
C(sp2)-H diazomethylation of aldehyde-derived hydrazones 4. It
was reasoned at that stage that in case of a successful transfer
Differential scanning calorimetry (DSC) analysis for 1a shows
decomposition starting at 130 ºC that leads to an energy release
2
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of the diazo group, the initially obtained α-diazohydrazones 5’
We were pleased to observe that under slightly modified reaction
conditions to those employed for the obtention of 3a, the triazole
products 5a-v were formed in good to excellent yields from the
corresponding N,N-dibenzyl hydrazones, being those slightly
better for hydrazone substrates derived from electron poor
aldehydes. The cyclic nature of the products isolated was
unambiguously assigned from the single-crystal structures of 5i
and 5s (Scheme 2 and the Supporting Information).
should tautomerize to the corresponding 1,2,3-triazoles
5
providing not only a synthetically useful entry to N-dialkylamino
triazoles of potential pharmacological interest, [17] but also easy to
handle precursors for α-imino metal carbenoids in case a ring to
chain equilibrium between 5’ and 5 could be established.[18]
Interestingly, in these structures the dialkylamino nitrogen is
pyramidal. As illustrative example, the sum of angles around N(4)
is 329.4° in 5i; therefore, at least in the solid state the exo-amino
group is not conjugated with the heterocyclic ring. Morpholine
derived hydrazones also get involved in this transformation; yields
however are slightly lower, 5w-5an. Scheme 2 also depicts
compound 5ao derived from 1,1-dimethylamino hydrazine, which
has been included in our product scope table for completeness.
Note that the most commonly used route to prepare 1,2,3-
triazoles precursors is a [2+3] dipolar cycloaddition of alkynes and
alkyl/aryl azides.[ 19 ] This synthetic approach however, is not
suitable for the preparation of the corresponding 1-(dialkylamino)
derivatives due to the unavailability of di(alkylamino)azides, which
are known to be explosive compounds.[20,21]
A plausible reaction pathway for the formation of 5 from 1a/b and
4 is depicted in Scheme 3. Initially, irradiation of [Ru(bpy)3]2+
generates photo exited ([Ru(bpy)3]2+)*, which reduces reagent
1a/b via single-electron transfer and generates the transient
diazomethyl radical (Int-I) via homolytic S-C bond cleavage.
Addition of the latter intermediate to the azomethine carbon of 4
delivers an aminyl radical intermediate (Int-II), which is
(iii)/(ii)
subsequently oxidized by [Ru(bpy)3]3+ (E1/2
= 1.29V vs
SCE).[22] Deprotonation would re-establish the C-N double bond
of the hydrazone moiety leading to 5’, which ultimately cyclizes to
the observed 1,2,3-triazole 5. Alternatively, deprotonation of Int-II
may occur first leading to the corresponding radical anion, which
would undergo oxidation to 5’.[ 23 ] A light on/off experiment
confirmed the necessity of continuous irradiation for the reaction
to proceed; moreover, measurement of the quantum yield of the
reaction to form 5a (Φ = 0.18) suggests that a radical chain
process cannot be a predominant path.[24]
Scheme 3. Plausible reaction mechanism for the formation of 5a-ao.
Finally, the plausible utility of the triazole products obtained as
precursors for mesoionic 1-(dialkylamino)-1,2,3-triazol-4-ylidene
ligands was evaluated.[25] Hence, 5a,d,w,x were submitted to
reaction with one equivalent of methyl triflate, affording triazolium
salts 6a,d,w,x. Methylation selectively occurs at position 3- of the
heterocycle (Scheme 4). Subsequent saponification of these
species in the presence of LiOH took place with concomitant lost
of CO2 delivering azolium salts 7a,d,w,x. The connectivity of 7w
and 7x was additionally confirmed by X-ray analysis. This opened
Scheme 2. Synthesis of 1,2,3-triazoles 5a-ao and molecular structure of
compounds 5i and 5y in the solid state. Anisotropic displacements shown at
50% probability level.[16]
3
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the stage to the reaction with [RhCl(COD)]2 in the presence of
KHMDS to afford the desired carbene-metal complexes 8a,d,w,x
as air stable yellow solids. Compounds 8w and 8x were also
crystallized and their structures in the solid state are depicted in
Scheme 4 and the Supporting Information.
Rhodium dicarbonyl complexes 9a and 9x were synthesized by
reaction of precursors 8a and 8x with CO, respectively, and their
CO stretching vibrations (υav = 2029 cm-1, 9a and υav = 2032 cm-1,
9x) compared with that of 1-alkyl analogues (υav = 2027 cm-1).[26]
This analysis suggests that the introduction of the dialkylamino
group at N(1) slightly reduces the donor ability exhibited by the
original 1,2,3-triazol-4-ylidenes probably due to the inductive
effect of a dialkylamino moiety, which is not conjugated with the
ring.[27] Both carbene ligands remain anyway stronger donors than
conventional NHCs (υav = 2039-2041 cm-1).[28]
Scheme 4. Synthesis of Rh complexes 8a,d,w,x via decarboxylative metalation,
evaluation of the donor properties of the new ligands and molecular structure of
8x. Anisotropic displacements shown at 50% probability level and H atoms
removed for clarity.[16] Reaction conditions: a) MeOTf (1.2 equiv.), DCM, 8h, 0°C
→ r.t.; b) (LiOH 3.0 equiv.), THF/H2O/MeOH (1:1:1), 1h, 60°C, 7a, 89%; 7d,
89%;7w, 71%;7x, 61%, (two steps yield); c) KHMDS (1.2 equiv.), [RhCl(COD)]2
(0.5 equiv.), THF, 0°C → r.t., 8a, 60%, 8d, 60%; 8w, 70%; 8x, 81%; d) CO,
DCM, 20 min., r.t., 9a, 75%, 9x, 72%.
In conclusion, diazomethyl radicals can be efficiently generated
under photochemical conditions from α-diazosulfonium triflates,
whose synthesis is described along this paper. Addition of these
species to hydrazones followed by intramolecular cyclization
affords 1-(dialkylamino)-1,2,3-triazoles in moderate to excellent
yields. Finally, these heterocyclic compounds were further
elaborated into mesoionic carbene ligands having an
unprecedented dialkylamino rest at position 1. The development
of additional synthetic applications of sulfonium salts is an active
area of investigation in our laboratory and further progress on the
topic will be reported in due course.
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. Open access funding enabled and
organized by Projekt DEAL.
Keywords: diazo compounds • sulfonium salts • photoredox
catalysis • mesoionic carbenes • 1,2,3-triazoles
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5
This article is protected by copyright. All rights reserved.

10.1002/anie.202014775
Angewandte Chemie International Edition
COMMUNICATION
A radically new approach to the synthesis of 1-(dialkylamino)-1,2,3-triazoles via addition of diazomethyl radicals to hydrazones is
described. This reactivity confirms the utility of the α-diazosulfonium salts herein introduced as synthetic equivalents of the
diazomethyl cation.
6
This article is protected by copyright. All rights reserved.