992
Chemistry Letters Vol.35, No.9 (2006)
Isolation and Characterization of an Allenylthioketene and 4-Methylenecyclobutenethiones
Generated by Thermal Reaction of Alkynyl Propargyl Sulfides
Shigenobu Aoyagi,ꢀ Katsuaki Sugimura, Nobuya Kanno, Dai Watanabe,
Kazuaki Shimada, and Yuji Takikawa
Department of Chemical Engineering, Faculty of Engineering,
Iwate University, Morioka 020-8551
(Received May 17, 2006; CL-060581; E-mail: aoyagi@iwate-u.ac.jp)
S
R1
R2
R1
R2
Alkynyl propargyl sulfides underwent [3,3] sigmatropic re-
S
•
•
R1
R2
S
[3,3]
arrangement followed by ring closure to afford isolable 4-meth-
ylenecyclobutenethiones, and an intermediary allenylthioketene
2 was isolated by heating t-butyldimethylsilylethynyl propargyl
sulfide.
(1)
1
2
3
Alkynyl propargyl sulfides 1a–1f were readily prepared as
a slightly air-sensitive pale yellow oil in an analogous manner
reported for alkynyl propargyl selenides.5c After heating a
solution of sulfides 1 in the absence of a trapping agent up to
complete consumption of 1, the resultant reaction mixture was
subjected to column chromatography on silica gel for purifica-
tion. Sulfides 1a–1d bearing a bulky substituent, t-Bu, Me3Si,
or t-BuMe2Si, at R1 were efficiently converted into cyclobutene-
thiones 3 as air-sensitive reddish oils (Table 1). The required re-
action temperature and time were affected by the bulkiness of R1
and R2 of 1 due to the formation of almost planar transition state
in [3,3] sigmatropic rearrangement of 1. Thermal reaction of 1c
in refluxing MeCN for 14 h afforded thione 3c in 54% yield
along with polymeric products, presumably formed by further
reaction of 3c in the polar solvent. In contrast with the analogous
reactions in the selenium series, 1,3-dithietanes, dimers of 3,
were not found at all in the crude product. The structures of
thiones 3 were fully characterized by means of MS, IR,
1H NMR, 13C NMR spectra, and elemental analysis. While 3a–
3d were relatively stable in hexane solution to be stored for a
week, exposure of neat thiones 3a–3d to air caused gradual de-
composition to give uncharacterizable insoluble solid. Because
of lability of 3e relative to 3a–3d, thermal reaction of 1e was car-
ried out in an NMR tube. The 13C NMR signals of 3a–3e in the
range of ꢁ ¼ 230{236 ppm are assigned to thiocarbonyl carbons
and characteristic electronic absorptions of 3a–3d in Et2O rang-
ing from 528 to 560 nm were assigned to n–ꢂꢀ transitions of
thiocarbonyl groups (Table 1). Thiones 3a, 3c, and 3d showed
red shift in electronic absorption in comparison with 3b, presum-
ably due to the higher ꢂ-conjugation involving a phenyl group.
For a few decades, generation and trapping of chalcogeno-
ketenes1–5 have been widely investigated and their high reactiv-
ity to amines and imines has been providing efficient approaches
to the corresponding chalcogenoamides and ꢀ-chalcogenolac-
tams, respectively.2 Furthermore, isolations of thermodynami-
cally and/or kinetically stabilized chalcogenoketenes were per-
formed to reveal their unique structures and reactivities.3 Among
the reported methods for generation of chalcogenoketenes, [3,3]
sigmatropic rearrangement of alkynyl propargyl chalcogenides
and alkynyl allyl chalcogenides was recognized to be the most
convenient way in the light of relatively mild reaction condition
and easy access to their precursors.4,5 We reported an efficient
generation and trapping of allenylselenoketenes by [3,3] sigma-
tropic rearrangement of alkynyl propargyl selenides,5c and
Koketsu et al. also observed allenylselenoketene generated
through a similar route.5f Accordingly, these results prompted
us to isolation of intermediary allenylchalcogenoketenes and cy-
clobutenechalcogenones to confirm the formation pathway in-
volving [3,3] sigmatropic rearrangement and the subsequent ring
closure. Considering the relative stability, less dimerizing na-
tures, and the synthetic applicability of cyclobutenethiones,6
thermal reaction of alkynyl propargyl sulfides seems to be a
promising way to isolate 4-methylenecyclobutenethiones 3 as
a monomeric form. Herein, we wish to report the first isolation
of an allenylthioketene 2 and 4-methylenecyclobutenethiones 3
by thermal reaction of alkynyl propargyl sulfides 1, as well as
the reactivity of 2 and 3.
Table 1. Thermal reaction of 1 and the selected spectral data of 2 and 3
Yield/%a
Substrate
R1
R2
Solvent
Temp/ꢁC
Time
13C NMRb
UV–visc
2
3
1a
1b
1c
1d
1ed
1fg
t-Bu
TMS
TMS
TBS
Ph
Me
Ph
Ph
H
Toluene
Hexane
Benzene
Benzene
CDCl3
Reflux
Reflux
Reflux
Reflux
50
72 h
10 h
14 h
48 h
50 min
10 min
0
0
0
0
0
57 (3a)
64 (3b)
85 (3c)
52 (3d)
68 (3e)e
0
230.2
234.8
234.3
235.2
231.3
242.3
c
560 (" ¼ 126)
528 (" ¼ 13)
555 (" ¼ 59)
558 (" ¼ 52)
540f
9-Triptycyl
TBS
H
THF
50
51 (2f)
532 (" ¼ 24)
aIsolated yield. The signal assigned to thiocarbonyl carbons. ꢁ value in ppm measured in CDCl3. Selected ꢃmax value in nm
measured in Et2O. dThermal reaction of 1e was carried out in an NMR tube. eEstimated by integration of 1H NMR spectrum using
dichloroethane as an internal standard. fElectronic absorption of crude 3e. g1f was prepared in THF and the resulting solution was
subjected to thermal reaction without isolation of 1f.
b
Copyright Ó 2006 The Chemical Society of Japan
Aoyagi, Shigenobu
