[1 + 2] Cycloadditions of sulfur monoxide (SO) to alkenes and alkynes and [1 + 4] cycloadditions to dienes (polyenes). Generation and reactions of singlet SO?

Article abstract of DOI:10.1021/ja072044e

The Diels-Alder adduct, produced from 3,4-di-tert-butylthiophene 1-oxide and dimethyl acetylenedicarboxylate, spontaneously extrudes sulfur monoxide (SO) at room temperature and serves as a new method for generation of SO under mild conditions. Thus, the SO generated underwent [1 + 2] cycloadditions to a series of alkenes and alkynes to produce thiirane oxides and thiirene 1-oxides, respectively, providing new syntheses of the sulfur-containing three-membered heterocycles. The SO also reacted with a variety of cyclic and acyclic dienes to give the corresponding 2,5-dihydrothiophene 1-oxides. Copyright

Full text of DOI:10.1021/ja072044e

Published on Web 05/17/2007
[
1 + 2] Cycloadditions of Sulfur Monoxide (SO) to Alkenes and Alkynes and
[
1 + 4] Cycloadditions to Dienes (Polyenes). Generation and Reactions of
Singlet SO?
Juzo Nakayama,* Yumi Tajima, Piao Xue-hua, and Yoshiaki Sugihara
Department of Chemistry, Graduate School of Science and Engineering, Saitama UniVersity, Urawa-ku, Saitama,
Saitama 338-8570, Japan
Received March 23, 2007; E-mail: nakaj@post.saitama-u.ac.jp
Recently, we reported that the Diels-Alder reaction of 3,4-di-
tert-butylthiophene 1-oxide (1) with dimethyl acetylenedicarboxy-
late (DMAD) at room temperature affords dimethyl 4,5-di-tert-
butylphthalate (3) in high yield probably through extrusion of sulfur
stereoselective formation of 5a-5e, where the SdO group is anti
to the resulting double bond. It was also reported that the SO,
generated from thiirane oxide, did not add to cycloheptatriene to
give 5d, but gave 7,7′-dicycloheptatriene probably through hydrogen
1
2b
monoxide (SO) from the initial adduct 2. The SO, generated here,
abstraction by SO. Any sign of such reaction was not observed
would be singlet if the SO extrusion from 2 takes places and is
concerted. We have thus examined a range of chemical trapping
experiments of the SO to characterize its reactivities and to compare
them with those of SO generated from other organic sources.2
in the present case. In addition, throughout this study, [2 + 4]
adducts of the SO with dienes never formed, in contrast with
1
8
9
10
reactions of O
2 2
, diatomic sulfur (S ), and sulfur dioxide which
,3
afford [2 + 4] cycloadducts.
Thus, the reaction of 1 with an equimolar amount of DMAD
was examined at room temperature in the presence of excess diene.4
2
1
-Methyl-1,3-butadiene, 2,3-dimethyl-1,3-butadiene, 2,3-diphenyl-
,3-butadiene, myrcene, 1,2-dimethylenecyclopentane, and 1,2-
Thiirane oxide 6a was reported to generate SO with formation
2j,k
of 2,2′-biadamantylidene (7).
We have now found that the
dimethylenecyclohexane successfully trapped the SO to give the
reverse reaction does take place, that is, the SO, generated from 2,
added to 7 affording 6a in 17% yield. Angle-strained alkenes,
norbornene, norbornadiene, and benzonorbornadiene also reacted
2
f
2a
2a
2k
[
(
1 + 4] cycloadducts 4a (49%), 4b (74%), 4c (28%), 4d
27%), 4e (76%), and 4f (44%), respectively; the phthalate 3 was
isolated in good yield in all cases. Reportedly, SO, generated by
thermolysis of thiirane oxide in refluxing toluene, added to each
of the three 2,4-hexadiene isomers to provide [1 + 4] adducts, in
which the stereochemistry of the dienes was not retained. The
authors thus concluded that triplet SO was generated and the
addition took place through a radical intermediate. By contrast, the
SO generated from 2 failed to add to these dienes, and thus we
could not determine the stereochemistry of the addition.
11
11
with the SO to produce 6b (2%), 6c (19%), and 6d (4%),
respectively, thus providing a new one-pot conversion of alkenes
to thiirane oxides. Dicyclopentadiene gave 6e (5%) and 6e′ (3%).
The SO addition to cis- and trans-cyclooctene is stereospecific and
gave c-6f (2%) and t-6f (17%) as the sole adduct, respectively,
2f,g
5
indicating that the addition may occur in
a concerted
12
manner. However, more general information on the stereochem-
istry was not obtained because the SO did not add to cis- and trans-
cyclodecenes and cis- and trans-3-hexenes. Incidentally, we should
note the fact that thiirane oxides correspond to perepoxides 8, which
were often postulated as the intermediates that lead to the formation
1
of dioxetanes and other products for the reactions of O
2
with
alkenes.13
Synthetically most important is the addition of the SO to alkynes,
1
4
The SO was also trapped by cyclic dienes (polyenes) such as
,3-cyclohexadiene, 1,3-cycloheptadiene, 1,3-cyclooctadiene, cy-
which provided a simple synthesis of thiirene 1-oxides. Thus, the
SO added to 2-phenylacetylene, 3,3-dimethyl-1-butyne, 2-butyne,
di(1-adamantyl)acetylene, and diphenylacetylene to give thiirene
1
cloheptatriene, and cyclooctatetraene to give 5a (68%), 5b (27%),
2f
2i
15
14e-g
14a,b
5
c
(2%), 5d (26%), and 5e (39%), respectively, although
1-oxides 9a (20%), 9b (10%), 9c (17%), 9d
(7%), and 9e
cyclopentadiene failed to give the corresponding adduct. Reportedly,
the SO, generated from thiirane oxide in refluxing toluene, gave a
mixtureof5e(23.6%)and5e′(4.5%)onreactionwithcyclooctatetraene,2
whereas in our case, 5e is the sole adduct. 5e isomerized to 5e′,
when heated in refluxing toluene, to produce a 9:1 equilibrium
mixture of 5e and 5e′.6 The other adducts 5a-5d did not undergo
such isomerization under the same conditions. Thus, the present
(5%), respectively. 9a and 9b are the first examples of monosub-
stituted thiirene 1-oxides. Even structurally simple thiirene 1-oxides
such as 9a-c are much more stable than expected and were
satisfactorily purified by silica gel column chromatography, re-
i
1
crystallization, or distillation. In the H NMR spectrum of 9a and
,7
9b, the vinyl proton appeared at δ 8.69 and 8.20, respectively,
values comparable to those of cyclopropenone (δ 9.0) and methyl-
cyclopropenone (8.6).16
[1 + 4] addition is kinetically controlled and results in the
7250
9
J. AM. CHEM. SOC. 2007, 129, 7250-7251
10.1021/ja072044e CCC: $37.00 © 2007 American Chemical Society

C O M M U N I C A T I O N S
Supporting Information Available: General procedure of the
trapping experiments and spectroscopic data for new compounds and
a few selected compounds. This material is available free of charge
via the Internet at http://pubs.acs.org.
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(
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2
d
(
5) The formation of alkene from thiirane 1-oxide is not stereospecific.
(
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8
(
(
(
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3
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18
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2 4
9
to norbornene and norbornadiene was reported. The reaction is not
stereoselective; for example, the transfer to norbornene gave a mixture
1
2
1 is thermally unstable and converted to 12 by extrusion of SO .
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1
(
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2
, see: (a) Stratakis, M.; Organopoulos, M.
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5
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3, 424-430.
(
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1
292. However, 9c was not isolated, and its structural evidence only comes
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(
(
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7
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Evidently the reactivities of the present SO differ from those of
(
18) Nakayama, J.; Aoki, S.; Takayama, J.; Sakamoto, A.; Sugihara, Y.; Ishii,
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SO that was generated from thiirane oxides and other sources and
considered to be triplet. Its capability of adding to alkenes and
alkynes resembles singlet carbenes. Therefore, seemingly, the singlet
SO was generated from 2 and directly involved in the trapping
reactions without decay to the more stable triplet.1
(
19) The recent calculations showed that the triplet is more stable than the
singlet by ca. 20 kcal/mol: Ishikawa, Y.; Gong, Y.; Weiner, B. R. Phys.
Chem. Chem. Phys. 2000, 2, 869-876.
(
20) A bimolecular mechanism between 2 and a trapping agent would be least
possible sterically. For example, the double bond of 7 is highly protected
sterically, and also the SdO group of 2 is sterically protected by tert-
butyl groups. Therefore, these two functional groups cannot come close
enough to undergo the SdO transfer. This would be true for the reaction
between 2 and di(1-adamantyl)acetylene.
9,20
Acknowledgment. This work was supported by a Grant-in-Aid
#16350019) for Scientific Research the Ministry of Education,
(
Culture, Sports, Science, and Technology, Japan.
JA072044E
J. AM. CHEM. SOC.
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