9
154
J . Org. Chem. 1998, 63, 9154-9155
Th er m a l a n d P h otoch em ica l 1,3-Dip ola r
Cycloa d d ition of a Su lfin e (F lu or en eth ion e
S-Oxid e) to th e Str a in ed Tr ip le Bon d of
Cyclooctyn e
Ta ble 1. P r od u ct Stu d ies of th e Th er m a l a n d
P h otoch em ica l Su lfu r Tr a n sfer fr om Su lfin e 1 to
Cyclooctyn e in Ch lor ofor m
Waldemar Adam,* Bettina Fr o¨ hling, and
Stephan Weink o¨ tz
Institut f u¨ r Organische Chemie der Universit a¨ t,
Am Hubland, 97074 W u¨ rzburg, Germany
Received August 18, 1998
Thiocarbonyl S-oxides (sulfines) are known to react as 1,3
dipoles in [3 + 2] cycloadditions with thioketones. The latter1
have been designated as “super dipolarophiles” by Huisgen,
and they are the only effective ones for the sluggish sulfine
1
,3-dipoles. However, recent work from our group showed
convna,b (mmol)
productsa,b (mmol)
2a
entry modec (mmol) (h)
t
that fluorenethione S-oxide (1) cycloadds as 1,3-dipole to
trans-cyclooctene, the first example of a thermal 1,3-dipolar
2
1
2
4
5
7
cycloaddition between a sulfine and a CC double bond. The
resulting sultene was isolated and displayed a remarkable
propensity for sulfur transfer toward cis/trans-cyclooctene
and norbornene under acid catalysis. In view of this exalted
1
2
∆T
∆T
∆Te
∆T
hν
2.10 24 0.545
88 0.783
4.71 41 0.927
0.990 0.271
0.512 traces
0.767 traces
1.42
1.71
0.341
0.355
0.749 traces
d
d
(0.249) (0.556) traces
3
4
5
6
3.65 20 0.771
3.51 20 0.674
1.22
1.43
1.52
2.08
0.289
0.279
0.319
0.392
0.652 traces
0.645 traces
0.363 0.543
0.452 0.528
1
,3-dipolarophilic reactivity of such strained molecules, we
f
have investigated the thermal and photochemical reaction
of sulfine 1 with cyclooctyne (2) to assess whether the
reactive triple bond may serve this purpose. Indeed, we
report herein that the initial step in both the thermal and
photochemical reaction is a 1,3-dipolar cycloaddition of
sulfine 1 to cyclooctyne, followed by efficient sulfur transfer
2.01
3.55
3
3
1.00
1.00
hν
a
Relative to 1.00 mmol of sulfine 1. b Determined from the H
1
NMR spectra of the crude reaction mixture by comparison of
characteristic signals with the sum of aromatic signals as internal
c
3
standard (error ( 5% of the stated values). ∆T: 40 °C, exclusion
to cyclooctyne to afford dithiin 4 (Scheme 1).
d
of light. hν: 350 nm (Rayonet), 5 °C. Yield of isolated product
The thermolysis (40 °C, 20-88 h) of a CDCl3 solution of
sulfine 1 in the presence of excess cyclooctyne (2) led to
dithiin 4 and enone 5 as the desulfurized product (Table 1).
The extent of conversion of sulfine 1 depends on the
concentration of 2 (Table 1, entries 1,2), which indicates
that cyclooctyne (2) is involved in the rate-determining step.
after silica gel chromatography. e K2CO3 (0.04 mmol). NEt3 (0.02
f
mmol).
the other hand, 1,3-dipolar cycloaddition between the sulfine
with cyclooctyne generates the 1,2-oxathiole 3, which after
sulfur transfer leads to the enone 5 (Scheme 1). The sulfur
that is not transferred to afford the dithiin 4 is extruded as
elemental sulfur, an undesired side reaction that is well-
1
2
Unlike trans-cyclooctene, the analogous unsaturated sul-
tene expected as 1,3 cycloadduct between sulfine 1 and
cyclooctyne could not be detected even in the presence of
bases (entries 3 and 4). Nevertheless, the formation of the
enone 5 unequivocally speaks for 1,3-dipolar cycloaddition
of the sulfine to cyclooctyne.
4
known for photochemical sulfur-transfer reactions. Since
there are only traces of enone 5 formed within 3 h in the
thermal process, the photochemical 1,3 cycloaddition must
operate.
The 1,2-oxathiole 3 proposed for both the thermal and
photochemical reactions of sulfine 1 with cyclooctyne (2),
analogous to the sultene derived from the cycloaddition of
sulfine 1 with trans-cyclooctene, transfers its sulfur atom
directly to cyclooctyne to form thiirene 10, which then
dimerizes to dithiin 4. The formation of a thiirene has been
The results of the photolysis (5 °C, 350 nm, 3 h) of
fluorenethione S-oxide in the presence of cyclooctyne were
rather surprising. While sulfine 1 was converted solely to
fluorenone (7) when irradiated in the presence of trans-
4
cyclooctene, for cyclooctyne we observed both enone 5 and
fluorenone in almost equal amounts (Table 1, entries 5 and
6
). The yield of the photochemical sulfur-transfer product,
the dithiin 4, was the same as for the thermal reaction.
These results imply that two different pathways operate in
the photochemical sulfur transfer by sulfine 1: On one hand,
we propose photochemical cyclization to the oxathiirane 6,2
followed by sulfur transfer and formation of fluorenone; on
,5
*
To whom correspondence should be attached. Fax: Int + 49-931-
884756. E-mail: adam@chemie.uni-wuerzburg.de.
1) Fisera, L.; Huisgen, R.; Kalwinsch, I.; Langhals, E.; Li, X.; Mloston,
G.; Polborn, K.; Rapp, J .; Sicking, W.; Sustmann, R. Pure Appl. Chem. 1996,
proposed in the literature when elemental sulfur is allowed
to react with triple bonds. Therefore, an alternative reaction
8
6
(
pathway could be the extrusion of elemental sulfur from the
1,2-oxathiole 3 and subsequent formation of thiirene 10 and
dimerization of the latter to the dithiin 4. Indeed, a small
amount of elemental sulfur is observed in these reactions;
however, a control experiment showed that cyclooctyne and
elemental sulfur afford, besides the dithiin 4, also equal
6
8, 789-798.
(
(
(
2) Adam, W.; Weink o¨ tz, S. J . Am. Chem. Soc. 1998, 130, 4861-4862.
3) B u¨ hl, H.; Timm, U.; Meier, H. Chem. Ber. 1979, 112, 3728-3736.
4) Adam, W.; Deeg, O.; Weink o¨ tz, S. J . Org. Chem. 1997, 62, 7084-
7
085.
(5) (a) Carlsen, L.; Harrit, N.; Holm, A. J . Chem. Soc., Perkin Trans. 1
1
976, 1404-1407. (b) Senning, A.; Hansen, H. C.; Abdel-Megeed, M. F.;
Mazurkiewicz, W.; J ensen, B. Tetrahedron 1986, 42, 739-746. (c) Williams,
C. R.; Harpp, D. N. Tetrahedron Lett. 1991, 32, 7633-7636. (d) Still, I. W.;
Kutney, G. W.; McLean, D. J . Org. Chem. 1982, 47, 555-560.
(6) Verkoczy, B.; Sherwood, A. G.; Safarik, I.; Strausz, O. P. Can. J . Chem.
1983, 61, 2268-2279.
1
0.1021/jo9816764 CCC: $15.00 © 1998 American Chemical Society
Published on Web 11/20/1998