Angewandte
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optimized by performing the reaction of the diyne 1a with
1 mol% [CpRu(MeCN)3]PF6 (Ru1; Cp = h5-C5H5) at 808C
using DMF while open to air. It was revealed that an electron-
withdrawing group on the N-benzyl moiety of 3 has a favor-
able impact on the sulfur-atom transfer. The p-chlorinated
analogue 3b gave the best yield (71%), albeit with 21% of 1a
remaining. The m-chloro and p-fluoro analogues 3c and 3d,
respectively, also produced 2a, albeit in slightly lower yields,
and 3e, bearing an electron-donating methoxy group on the
N-benzyl moiety, was inefficient. Notably, the reaction was
sluggish in anhydrous DMF and the yield of 2a decreased to
35%, and unreacted 1a and 3b were recovered in 63 and 64%
yield, respectively. This result implies that water plays an
important role. Moreover, [Cp*Ru(MeCN)3]PF6, bearing the
bulky and electron-donating Cp* (h5-C5Me5) ligand, proved
to be a less efficient catalyst.
obtained in 62% yield from the stoichiometric reaction of
a preformed rhodacyclopentadiene with elemental sulfur.[5a]
The present protocol successfully improved the yield by using
catalytic amounts of the less expensive ruthenium complex.
The fused thiophene structure was unambiguously confirmed
by single-crystal X-ray analysis of 2k.
Next, the influence of the terminal aryl groups of diyne
substrates was investigated (Figure 3). The reactions of the
By using 3b as the optimal sulfur donor, the general
applicability of this method was investigated in terms of the
diyne substrates 1 (Figure 2). The reaction of 1a was repeated
Figure 3. Influence of terminal aryl groups.
diynes 1o–r, possessing electron-rich aryl terminals, pro-
ceeded with a 2 mol% catalyst loading, except for 1p, which
bears sterically demanding o-substituted aryl terminals
(10 mol%). The corresponding bicyclic thiophenes 2o–r
were obtained in high yields. In addition, the diynes 1s–u,
bearing p-fluoro, p-iodo, and p-boryl groups on the aryl
terminals, respectively, afforded the corresponding products
in greater than 90% yields, although 5 mol% catalyst
loadings were required for 1s and 1u. In contrast, the
reaction of the diyne 1v, possessing p-formylphenyl terminal
groups, hardly produced the desired thiophene under the
standard reaction conditions with a 2 mol% catalyst load-
ing.[10] This protocol enables the synthesis of unsymmetrical
push-pull thiophenes 2w, 2x, and 2y in high yields.[11]
Moreover, the tristhiophenes 2za–zc were synthesized in
67–92% yields. It is particularly significant that the reactive
Figure 2. Scope with respect to the diyne substrates. TBS=tert-butyldi-
methylsilyl, Ts=4-toluenesulfonyl.
with an increased catalyst loading (2 mol%) to observe
complete conversion in 5 hours, thus affording 2a in 90%
yield upon isolation. Similarly, other diynes (1b–j) were
subjected to the reaction with a 2–5 mol% catalyst loading,
thus affording the corresponding bicyclic thiophenes in 71–
90% yield. Notably, various functional groups, including
ester, ketone, amide, nitrile, sulfide, sulfone, and silyl ether,
were well tolerated under the reaction conditions.
The spirocyclic fluorene derivative 2k and acenaphtho-
[1,2-c]thiophene derivative 2l were also synthesized in high
yields (Figure 2). Moreover, this method proved to be
applicable to six-membered ring formations, as the 4H-
thieno[3,4-c]chromene derivative 2m and naphtho[2,3-c]thio-
phene-4,9-dione derivative 2n were obtained in 87 and 75%
yield, respectively. The latter compound was previously
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C I, C Br, and C B bonds, which can be utilized for further
transformations, were preserved in the final products.
This method was also applicable to diyne substrates
bearing alkyl or silyl terminal groups (Scheme 2). The diynes
4a and 4b, each bearing at least one alkyl terminal group
afforded the corresponding bicyclic thiophenes 5a and 5b,
albeit in moderate yields. In our previous study, silyldiynes
could be converted into 2-silylfurans using nitrones as oxygen
donors.[9b] In striking contrast, TMS groups were not tolerated
under the present reaction conditions: the reaction of diyne
6a, bearing a TMS terminal group, afforded the desilylated
bicyclic thiophene 7 in 56% yield. Thus, the diyne 6b, bearing
a bulkier TBS terminal group was examined, and afforded the
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Angew. Chem. Int. Ed. 2016, 55, 1 – 5
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