Copper-mediated one-pot transformation of 2-N-sulfonylaminoaryl diselenides into benzo[b][1,4]selenazines

Article abstract of DOI:10.1002/adsc.201100587

A simple procedure for the one-pot transformation of 2-N-tosylamino diselenides into benzo[b]ACHTUNGTRENUNG[1,4]selenazines has been established. The data collected indicate that the six-membered het- ACHTUNGTRENUNGerocyclic ring is the result of a [4+2]

Full text of DOI:10.1002/adsc.201100587

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DOI: 10.1002/adsc.201100587
Copper-Mediated One-Pot Transformation of 2-N-Sulfonyl-
aminoaryl Diselenides into Benzo[b][1,4]selenazines
N
Caterina Viglianisi,a Lavinia Simone,a and Stefano Menichettia,*
a
Dipartimento di Chimica “U. Schiff”, Polo Scientifico e Tecnologico Universitꢀ di Firenze, Via della Lastruccia, 3-13,
50019 Sesto Fiorentino, Italy
Fax. (+39)-055-457-3531
E-mail: stefano.menichetti@unifi.it
Received: July 26, 2011; Revised: October 10, 2011; Published online: January 12, 2012
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.201100587.
Abstract: A simple procedure for the one-pot trans-
formation of 2-N-tosylamino diselenides into
In our continuous investigation on the chemistry[1g,3]
and applications[4] of thioquinones, we demonstrated
that benzo[b]ACHTGUNTERNNU[G 1,4]thiazines of type 1 can be prepared
by exploiting an inverse electron demand hetero
Diels–Alder reaction of electron-rich alkenes with o-
iminothioquinones 2 acting as electron-poor hetero-
dienes as depicted in Scheme 2.[5]
benzo[b]
data collected indicate that the six-membered het-
ACHTUNGTRENNUNGerocyclic ring is the result of a [4+2]cycloaddition
[1,4]selenazines has been established. The
reaction of a transient electron-poor o-iminoseleno-
quinone, acting as diene, with an electron-rich
alkene, performing as dienophile. The key step of
the synthesis is a 1,4-elimination at selenium of a
selenolate ion, leading to the generation of the het-
erodiene, that requires catalytic amounts of cop-
per(II) trifluoromethanesulfonate, for the activation
of the selenium-selenium bond.
Keywords: copper; cycloaddition; Lewis acids; sele-
nium; synthetic methods
Hetero Diels–Alder reactions are a powerful tool for
the stereocontrolled construction of six-membered
heterocycles. In this context, the preparation of
Scheme 2. Hetero Diels–Alder approach to benzo[b]-
AHCTUNGTREG[NNNU 1,4]thiazines 1 via 1,4-elimination at sulfur mediated by
benzo-fused derivatives is of particular interest since, generation of o-iminothioquinones 2.
synthetically, it is a challenge requiring that an aro-
matic moiety is transformed, transiently, in a dienic o-
quinoid species[1] and, practically, it offers a straight-
Such transient dienes are obtained, under very mild
forward opportunity for preparing a plethora of bio- reaction condition, by base-promoted 1,4-elimination
logically relevant compounds (Scheme 1).[1,2]
at sulfur in N-tosyl-2-thiophthalimides 3, in turn, pre-
pared by SEAr of properly substituted N-tosylanilines
with the phthalimidesulfenyl chloride (PhtNSCl,
Pht=phthaloyl). The key steps of this procedure are:
(i) the direct introduction, via SEAr, of the sulfenic
sulfur armed with a suitable leaving group (i.e., the
phthalimide residue), and (ii) the deprotonation at ni-
trogen that promotes the 1,4-elimination at sulfur
with generation of the o-iminothioquinone[5]
(Scheme 2).
Scheme 1. Hetero Diels–Alder approach to benzo-fused six-
membered heterocycles.
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One of the drawbacks of the above procedure is
the impossibility to apply it for the synthesis of the
corresponding selenium derivatives. In fact, no exam-
ples of electrophilic selenyl reagents suitable for the
introduction of a selenium atom armed with an effi-
cient leaving group are reported in the literature,[6]
thus preventing the possibility to obtain an o-iminose-
lenoquinone species via a 1,4-elimination at selenium,
as depicted in Scheme 2.
Nevertheless, selenium is a highly important ele-
ment in organic chemistry and several reviews de-
scribe its versatility in synthesis.[7] Moreover, it is an
essential dietary component for many living systems,
including humans. Recently, selenium-containing com-
pounds are attracting increased attention for their
ability to act as glutathione peroxidase (GPX)
mimics, catalytic antioxidants and cancer-chemopre-
ventive compounds.[8] Thus, we decided to study the
possibility of generating o-iminoselenoquinones from
easily available starting materials that are accessible
circumventing the introduction of the selenium
moiety via a SEAr of a functionalized selenyl halide.
We reasoned that, possibly, an o-N-tosyl diselenide
could serve as precursor of a selenoquinone. In fact,
considering that in 1,4-elimination at chalcogens the
leaving group ability can be approximated to the con-
jugated acid pKa values, the elimination of a phenyl-
selenolate ion could be even easier than that of the
phthalimide anion.[9] Thus, we imagined that when re-
acting o-N-tosyl diselenide 4 with Et3N the deproto-
nation at the ArNHTs residue, could be followed by a
1,4-elimination at selenium (of an arylselenolate ion)
with generation of the o-iminoselenoquinones 5. If
this latter species was an electron-poor diene, carrying
out the reaction in the presence of an electron-rich
alkene, at the end of the process we could isolate the
benzoselenazine 7 as depicted in Scheme 3.
Scheme 3. Preparation of benzo[b]ACTHUNTGRNEUNG[1,4]selenazines from o-
N-tosyl diselenides via hetero Diels–Alder reaction of o-imi-
noselenoquinones.
Table 1. Effect of copper-based Lewis acids on the formation
of benzo[b]ACTHNUTRGNEU[GN 1,4]selenazine 7a.
Entry
LA
Mol%
Yield [%][a]
1
2
3
4
5
6
–
–
–
–
30
36
63
75
CuO[b]
20
20
20
20
20
N-Tosylated diselenide 4 was prepared from the
corresponding 2-amino diselenide, in turn, obtained
by reacting o-iodoaniline with powdered Se(0) in the
presence of CuO nanoparticles as recently report-
ed.[10] However, reacting 4 with 1 equivalent of Et3N
and 2 equivalents of styrene 6a, as suitable trapping
electron-rich dienophile, in CHCl3 at 608C, i.e., under
the reaction conditions that we found to be effective
for the generation of o-imonothioquinones,[5] no reac-
Cu
CuI
Cu
Cu
A
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
[a]
Isolated yield after column chromatography.
CuO nanoparticles.
[b]
tion occurred and the starting diselenide 4 was recov- we could verify that copper-based Lewis acids are
ered almost quantitatively after an acid work-up. able to promote the reaction,[12] and the best results
Thus, although the pKa values indicated that the sele- were obtained using 20 mol% of copper(II) triflate,
nolate ion should be a better leaving group than the Cu
phthalimide anion, the Se Se bond in 4 was not in 75% yield after column chromatographic purifica-
broken under the reaction conditions tested. A possi- tion.
(OTf)2, that allows us to isolate the selenazine 7a
À
ble solution, also strengthened by an inspection of the
This new approach for the generation of selenoqui-
literature[11] reporting the activation of chalcogen- nones from diselenides foresees that one half of 4 is
chalcogen bonds, was to use a Lewis acid, and in par- transformed into the heterodiene, while the second
ticular a copper salts, to promote the reaction. half behaves as a leaving group. Actually, as a crucial
Indeed, after a fairly short investigation (see Table 1), point for the success of this procedure, the yields of
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Copper-Mediated One-Pot Transformation of 2-N-Sulfonylaminoaryl Diselenides into Benzo[b]ACTHNUTRGNEUNG[1,4]selenazines
Table 2. Benzo[b]
ACHTUNGTRENNUNG
Entry Dienophile
t[a] Yield[b]
1
72 75
48 58
72 55
6
7
8
48 75
2
48 79
3
4
72
-
48 60
48 59
9
60 43
48 60
5
10
[a]
Reaction times (hours) for the complete consumption of diselenide 4 at 608C in CHCl3.
Isolated yields [%] after column chromatography.
[b]
selenazine 7a reported in Table 1 have been calculat- acceptable to good yields of selenazines 7 isolated as
ed considering the consumption of both aminoselenyl single regioisomers with retention of the alkene ge-
units of diselenide 4. This means that the selenolate ometry. In particular, the reaction proceeded reasona-
ion, that has acted as leaving group, is not wasted bly well with EDG-substituted styrenes, vinyl ethers,
during the process but, in turn, converted in the final N-vinylamides (entries 1–7, 10) and with cyclopenta-
derivative 7a. The simplest rational for this hypothesis diene (entry 9), while, surprisingly, it failed when
is that the selenolate ion is oxidized by molecular using phenyl vinyl sulfide 6h as potential dienophile
oxygen[11] to the starting diselenide 4 until its com- (entry 8).[13] As expected, we verified that electron-
plete consumption. Indeed, when carrying out the re- poor or electron-neutral alkenes, like methyl vinyl
action of 4 with 6a in a deoxygenated solution and ketone or cyclohexene respectively, do not afford any
under a positive nitrogen atmosphere, after 72 h at selenazine derivative. Actually, the mechanism depict-
1
608C, the H NMR spectra of the crude reaction mix- ed in Scheme 3, involving a hetero Diels–Alder cyclo-
ture showed a large amount of unreacted diselenide 4 addition as key step, is not the only one able to ra-
with a 78:22 ratio between diselenide 4 and selenazine tionalize the formation of the selanazines 7. For ex-
7a. Compound 7a was fully characterized and the ample, the possibility that the attack of the nucleo-
structure solidly confirmed also taking in consider- philic alkene to the LA-activated diselenide afforded
ation the spectroscopic data of the corresponding an ionic intermediate, a classic episeleniranium ion[7]
thiazine.[5] The scope of the procedure was studied by 8a possibly in equilibrium with an open carbocation
reacting diselenide 4 with alkenes 6b–l under the 8b (vide infra), which evolves into selenazines 7 via
above optimized conditions. The results obtained are an intramolecular ring closing, seems similarly feasi-
reported in Table 2.
ble (Scheme 4). Conceivably, both mechanisms fore-
The reaction was quite general and using the elec- see the liberation of a selenolate ion, since its oxida-
tron-rich alkenes[4,5] that were effective in the case of tion to diselenide 4 is mandatory to justify the isola-
o-thioquinones and o-iminothioquinones, it affords tion of selenazines 7 with yields higher than 50%.
Adv. Synth. Catal. 2012, 354, 77 – 82
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Caterina Viglianisi et al.
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must be seriously considered, above all when 8b is a
highly stabilized a-hetero-substituted carbocation
(Table 2, entries 4, 5, 7 and 10). Moreover, it must be
taken into consideration that the internal NTs nucleo-
phile, linked to the selenium in the intermediate 8a,
appears sterically prevented to bring an anti-attack to
one of the carbons of the episeleniranium ion.[17] In
other words, the stereoelectronic characteristics of the
ionic intermediate strongly support the presence of
the open species 8b that seems mandatory in order to
achieve an intramolecular cyclization to selenazines 7
as depicted in Scheme 4. Thus, the overall retention
of the alkene geometry makes such a mechanism
poorly convincing.[18]
Scheme 4. Lewis acids-mediated access to [1,4]selenazines
via intramolecular cyclization.
In conclusion, 2-N-sulfonylamino diselenides can be
transformed in transient o-iminoselenoquinones via
À
Looking for indications on the more realistic mecha- copper(II)-catalyzed activation of the Se Se bond
nism, the possibility of using cyclopentadiene 6i as di- and base-mediated 1,4-elimination at the selenium of
enophile (entry 9) is, in our opinion, worthy of men- a selenolate anion. The selenolate ion, exploited as a
tion. In fact, despite the low yield of derivative 7i, the leaving group, is oxidized by molecular oxygen to the
aptitude of 1,3-dienes to behave as electron-rich dien- starting diselenide which is completely transformed
ophiles is typical of those reactions involving quinone into the final selenazine The hitherto unknown 2-imi-
methides and the corresponding heterofunctionalized noselenoquinone can be efficiently trapped as elec-
systems as dienes.[1]
tron-poor diene in an inverse electron demand hetero
In addition, the possibility of using styrene 6c as a Diels–Alder reaction with several electron-rich al-
suitable reagent is another proof for the Diels–Alder kenes to give, regio- and stereoselectively, benzo[b]-
mechanism, since the free amino moiety would proba-
ACHTUNGTREN[NUGN 1,4]selenazines, an interesting yet very poorly studied
bly strongly interact with any selenium electrophile class of selenium-containing heterocycles. The scope
preventing the formation of selenazine 7c. Clearly, and application of this new procedure as well as the
this is on the way to be conclusive in favour of a [4+ validation of the proposed mechanism[18] are under in-
2]-guided mechanism, and, on the other hand, we vestigation.
were unable to point out any retro Diels–Alder pro-
cess from selenazines 7.[14] However, the stereochemi-
cal features of this reaction are, in our opinion, a Experimental Section
solid indication of the actual mechanism in action. In
Spectroscopic data for diselenide 4 and selenazines 7a–g, 7i
and 7l are reported in the Supporting Information. The fol-
lowing protocol, describing the synthesis of derivative 7a,
can be considered as the general procedure used for the
preparation of all selenazines 7.
any case selenazines 7 were isolated as single re-
gioisomers and, for entries 2, 4, 5, 9 and 10, with over-
all retention of the alkene geometry. The complete re-
gioselectivity is typical but not exclusive of a cycload-
dition path, thus, definitely, a single regioisomer is ex-
pected if the reaction occurs as depicted in
Scheme 3,[4,5] but, reasonably, a complete regiocontrol
could also be observed for the attack of the nucleo-
philic NTs group to the ionic intermediate 8 described
in Scheme 4. The retention of the alkene geometry is
much more supportive for a cycloaddition process.
3,4-Dihydro-3-(4-methoxyphenyl)-4-tosyl-2H-
benzo[b]ACTHNUGRTENUNG[1,4]selenazine (7a)
In a reaction vial, to a solution of diselenide 4 (50 mg,
0.08 mmol) in dry CHCl3, Cu(OTf)2 (5 mg, 0.015 mmol), sty-
rene 6a (21 mg, 0.16 mmol) and Et3N (8 mg, 0.08 mmol)
AHCTUNGTRENNUNG
Retention of the alkene geometry has been reported, were added in sequence and the mixture was heated at
608C until complete disappearance of diselenide 4 as moni-
tored by TLC (72 h). Then, the crude reaction mixture was
diluted with dichloromethane (20 mL), washed with saturat-
ed NH4Cl (2ꢂ20 mL), dried over anhydrous Na2SO4 and
evaporated to dryness. Silica gel column chromatographic
purification (eluent dichloromethane:petrol=3:1) allowed
the isolation of derivative 7a as a white solid; yield: 55 mg
(75%); mp 50–518C. H NMR (CDCl3, 400 MHz): d=7.65
(dd, J=8.0, 1.2 Hz, 1H) 7.47–7.43 (m, 2H), 7.34–7.30 (m,
2H), 7.21 (dd, J=7.6, 1.6 Hz, 1H), 7.20–7.15 (m, 3H), 7.04
at very low temperature, for both E- and Z-alkenes in
asymmetric cyclization reactions with diselenides[15]
due to an anti-attack of an internal nucleophile to an
episelenACHTUNGTRENNUNGiranium ion intermediate. At the same time,
an equilibrium between an episeleniranium ion and
an open carbocation has already been described in
the selenylation of enolethers, under reaction condi-
tions milder than those required for this transforma-
tion,[16] indicating that, in our system, the possibility
1
of an equilibrium between 8a and the open cation 8b (td, J=7.6, 1,6 Hz, 1H), 6.81–6.77 (m, 2H), 5.74 (at, J=6.6,
80
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Copper-Mediated One-Pot Transformation of 2-N-Sulfonylaminoaryl Diselenides into Benzo[b]ACTHNUTRGNEUNG[1,4]selenazines
1H), 3.75 (s, 3H), 3.36 (dd, J=12.2, 6.6 Hz, 1H), 3.04 (dd,
J=12.2, 6.6 Hz, 1H), 2.40 (s, 3H); 13C NMR (CDCl3,
100 MHz): d=158.9, 143.5, 136.7, 135.5, 132.1, 131.3, 130.4,
129.3 (2C), 128.8, 127.8 (2C), 127.5 (2C), 126.8, 126.6, 113.9
(2C), 59.7, 55.2, 27.0, 21.6; MS: m/z (rel. abs. %)=459 (M·+,
28), 304 (78), 224 (55), 223 (61), 134 (76), 91 (100); anal.
calcd. for C22H21NO3SSe: C 57.64, H 4.62, N 3.06; found: C
58.01, H 4.95, N 3.02.
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from 1 to 2 orders of magnitude more acidic than
PhtNH.
Acknowledgements
The authors are indebted to the reviewers for valuable sug-
gestions and to Prof. Andrea Temperini, University of Peru-
gia, for helpful discussions. Fondazione CARIPLO is ac-
knowledged for a grant to C.V.
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[12] Several non copper LAs, such as AgACHTNUGTRNENUG(OTf), LaACHTUNGTRENNUNG(OTf)3,
CoCl2, CeCl3, were unable to promote the reaction.
[13] It is difficult to explain the lack of reactivity of elec-
tron-rich alkene 6h. We may consider that sulfide 6h
could compete with the diselenide 4 in coordinating the
LA. This, at the same time, decreases the reactivity of
the vinyl group as dienophile and prevents the activa-
À
tion of the Se Se bond. As a matter of fact, when re-
acting 4 with 2 equiv. of 6a and 2 equiv. of 6h, we ob-
served the exclusive formation of selenazine 7a which
was isolated in 51% yield. Thus, sulfide 6h definitely
does not participate as dienophile to the cycloaddition,
while it demonstrated a limited, if any, ability to inhibit
the reaction of a different dienophile.
[5] M. Campo, G. Lamanna, S. Menichetti, Synlett 2007,
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[6] Actually, to the best of our knowledge, there is only
one example of a similar reagent (namely PhSO2SeCl)
that had found a very limited number of applications:
a) M. R. Brice, A. Chesney, J. Chem. Soc. Chem.
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[14] Selenazines 7 are thermally stable under the reaction
conditions required for their formation. In fact, no
trace of selenazine 7f was detected after heating selen-
AHCTUNGTRENNUNG
[7] a) Organoselenium Chemistry: Modern Developments
ACHTUNGTRENNUNG
in Organic Synthesis, (Ed.: T. Wirth), Top. Curr. Chem.
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Caterina Viglianisi et al.
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tion pattern in the mass spectra of many of selenazines
prepared (see Supporting Information for details).
[15] G. Fragale, T. Wirth, Eur. J. Org. Chem. 1998, 1361–
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[16] W. R. Roush, D. P. Sebesta, R. A. James, Tetrahedron
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[17] Recently, Wirth and co-workers reported the formation
of 2,3-dihydro-1,4-benzoselenothiines via an intramo-
lecular attack to an open cationic intermediate very
similar to that postulated in Scheme 4: D. M. Freuden-
dahl, M. Iwaoka, T. Wirth, Eur. J. Org. Chem. 2010,
3934–3944.
[18] Although we are confident that the mechanism opera-
tive in our system foresees a hetero Diels–Alder reac-
tion as key step, we cannot rule out the possibility that
the open cation 8b behaves as a very tight ion pairs al-
lowing the formation of the new carbon-nitrogen bond
with retention of configuration. A set of experiments is
under consideration to definitely clarify this point.
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ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2012, 354, 77 – 82