3,4-Didehydrothiophene: Generation, trapping reactions, and ab initio study

Full text of DOI:10.1021/ja953747b

J. Am. Chem. Soc. 1996, 118, 2511-2512
2511
of trifluoromethanesulfonic acid gave 4 in 53% yield.15 Ad-
dition of a CH2Cl2 solution of 4 to a CH2Cl2 solution of KF in
the presence of 18-crown-6 as anticipated generated cumulene
2, whose presence was convincingly endorsed by its trapping
reactions with several dienes as shown in Scheme 1. Such a
trapping exercise was first explored by generating 2 at room
temperature in the presence of anthracene. In this manner a
chromatographically inseparable mixture of the known 9,10-
adduct 5a16 and the 1,4-adduct 5b was obtained in a total yield
of only 10%. The structures and ratio (2.7:1) of 5a and 5b
were confirmed by 1H- and 13C-NMR spectral analyses.15
Careful partial recrystallization of a mixture of 5a and 5b from
MeOH nonetheless afforded a pure sample of 5a, mp 267-
268 °C (lit.16 mp 268 °C).15 Lower reaction temperatures did
not seem to cause any significant effect on the yield and ratio
of the products. Compelling proof for the remarkable reactivity
of 2 was procured by its reaction with benzene, which yielded
the adduct 6 in a meager 7% yield.15 Moreover, the reaction
of 2 with 2,3-dimethyl-1,3-butadiene unexpectedly gave in 27%
total yield a chromatographically separable mixture (silica gel,
n-pentane) of a [2 + 2] adduct 7a, as well as an ene reaction
product 7b, in the ratio of 1:1.15 The isolation of 7b is consistent
with the observation that ene reaction generally competes with
cycloaddition in a trapping process comprising a distorted
π-system and an alkene having allylic hydrogen atoms.17 In
other Diels-Alder reactions, furan, 2-methylfuran, and 2,5-
dimethylfuran were all found to react readily with 2 to furnish
adducts 8a,18 8b, and 8c, in 31%, 17%, and 13% yields,
respectively.15 It appeared that 2 did not dimerize to provide
2,5-dithiabisnorbiphenylene,13,19 even for the condition in which
no trapping reagent was involved.
3,4-Didehydrothiophene: Generation, Trapping
Reactions, and Ab Initio Study1
Xin-Shan Ye, Wai-Kee Li,2a and Henry N. C. Wong*,2b
Department of Chemistry
The Chinese UniVersity of Hong Kong
Shatin, New Territories, Hong Kong
ReceiVed NoVember 6, 1995
Despite the considerable efforts devoted in the last decade
to the synthesis of strained cyclic cumulenes,3 in the literature
only the isolable 1,2,3-cyclononatriene4 and the fugitive 1,2,3-
cycloheptatriene5 and 1,2,3-cyclohexatriene6 have been regis-
tered. 1,2,3-Cyclooctatriene and 1,2,3-cyclopentatriene still
remain unknown, but their structural features and energetics have
been studied by computation.4a On the other hand, five-
membered hetarynes have also aroused widespread synthetic
endeavor7 and theoretical curiosity8 because of their inherent
strain. Although both 2,3-didehydrothiophene (1)9 and 3,4-
didehydrothiophene (2)10 had been suggested as reactive
intermediates by Wittig in the early 1960s, the validity of such
claims was later questioned.7bc,11 Subsequently, hetaryne 1 was
generated from thiophene-2,3-dicarboxylic anhydride and ac-
cordingly trapped.12 Evidence for the existence of 2 has
surprisingly never been obtained despite many experimental
attempts.12c,13 Inspired by the recent generation of benzyne from
(phenyl)[o-(trimethylsilyl)phenyl]iodonium triflate,14 we report
herein unequivocal verification of 2 as an intermediate. It is
believed that the five-membered 2 is the smallest cyclic
cumulene ever characterized.
Cumulene 2 was also studied with the complete-active-space
SCF (CASSCF) method.20 This method was chosen instead of
the more conventional single-configuration models because it
was found that single-configuration descriptions such as RHF
would not be adequate for benzyne and related compounds.8b
The basis sets adopted in these calculations were 3-21G(d),
6-31G(d), and 6-311G(d,p). The standard ab initio molecular
orbital calculations, including geometry optimization and fre-
quency computations, were carried out using the Gaussian 94
programs.21
The precursor of 2, namely, phenyl[4-(trimethylsilyl)thien-
3-yl]iodonium triflate (4), was prepared from 3,4-bis(trimeth-
ylsilyl)thiophene (3)1 according to the literature procedure.14
Thus, treatment of 3 with iodobenzene diacetate in the presence
(1) 3,4-Disubstituted Thiophenes. 2. Part 1: Ye, X.-S.; Wong, H. N. C.
J. Chem. Soc., Chem. Commun., in press.
(15) See supporting information for the spectrometric data of the
compounds prepared.
(16) De Wit, J.; Wynberg, H. Tetrahedron 1973, 29, 1379-1391.
(17) Wittig, G.; Du¨rr, H. Justus Liebigs Ann. Chem. 1964, 672, 55-62.
Crews, P.; Beard, J. J. Org. Chem. 1973, 38, 522-528.
(2) (a) To whom correspondence concerning the ab initio study should
be addressed. (b) To whom other correspondence should be addressed.
(3) Johnson, R. P. Chem. ReV. 1989, 89, 1111-1124.
(4) (a) Angus, R. O., Jr.; Johnson, R. P. J. Org. Chem. 1984, 49, 2880-
2883. (b) Angus, R. O., Jr.; Janakiraman, M. N.; Jacobson, R. A.; Johnson,
R.P. Organometallics 1987, 6, 1909-1912.
(18) Typical experimental procedures for the preparation of 8a from 3:
To a stirred suspension of PhI(OAc)2 (3.3 g, 10.2 mmol) in dry CH2Cl2
(50 mL) was added trifluoromethanesulfonic acid (1.8 mL, 20.3 mmol)
slowly at 0 °C with a syringe. The mixture was stirred for 1 h at room
temperature during which time the mixture became a clear yellowish
solution. The solution was then cooled to 0 °C and 3,4-bis(trimethylsilyl)-
thiophene (3) (2.5 g, 11.0 mmol) in CH2Cl2 (25 mL) was added dropwise
with a syringe. After addition, the reaction mixture was stirred at room
temperature for 20 min. After evaporation of the solvent, Et2O was added
to crystallize the residue. The solid formed was filtered, washed with Et2O,
and dried in Vacuo to yield 4 (2.7 g, 53%): mp 169-172 °C; 1H NMR
(250.13 MHz, CDCl3) δ 0.28 (s, 9H, Me3Si), 7.43 (t, J ) 6.5, 6.5 Hz, 2H,
ArH), 7.52 (dd, J ) 6.7, 1.1 Hz, 1H, ArH), 7.55 (d, J ) 2.9 Hz, 1H, H-5),
7.80 (d, J ) 6.4 Hz, 2H, ArH), 8.51 (d, J ) 2.9 Hz, 1H, H-2); 13C NMR
(62.89 MHz, CDCl3) δ -0.41, 100.82, 115.19, 131.99, 132.27, 132.98,
137.09, 140.86, 144.14. Anal. Calcd for C14H16O3S2SiF3I: C, 33.08; H,
3.17. Found: C, 33.03; H, 2.90. To a mixture of 4 (254 mg, 0.5 mmol),
18-crown-6 (79 mg, 0.3 mmol), and furan (1 mL) in dry CH2Cl2 (5 mL)
under N2 was added KF (87 mg, 1.5 mmol). The reaction mixture was
stirred for 30 min at room temperature. The resulting precipitate was
removed by filtration. The filtrate was evaporated under reduced pressure,
and the residue was chromatographed on a silica gel column (hexanes-
EtOAc, 6:1) to give 8a (23 mg, 31%) as colorless crystals: mp 70-72 °C;
1H NMR (250.13 MHz, CDCl3) δ 5.55 (d, J ) 0.9 Hz, 2H), 6.73 (s, 2H),
6.92 (t, J ) 0.9, 0.9 Hz, 2H); 13C NMR (62.89 MHz, CDCl3) δ 79.77,
112.08, 141.34, 151.38; MS m/e 150 (M+, 32). Anal. Calcd for C8H6OS:
C, 63.98; H, 4.03. Found: C, 64.15; H, 4.14.
(5) Zoch, H.-G.; Szeimies, G.; Ro¨mer, R.; Schmitt, R. Angew. Chem.,
Int. Ed. Engl. 1981, 20, 877-878. Zoch, H.-G.; Szeimies, G.; Ro¨mer, R.;
Germain, G.; Declercq, J.-P. Chem. Ber. 1983, 116, 2285-2310.
(6) Shakespeare, W. C.; Johnson, R. P. J. Am. Chem. Soc. 1990, 112,
8578-8579.
(7) (a) Kauffmann, T.; Wirthwein, R. Angew. Chem., Int. Ed. Engl. 1971,
10, 20-33. (b) Reinecke, M. G. Tetrahedron 1982, 38, 427-498. (c)
Reinecke, M. G. In ReactiVe Intermediates; Abramovitch, R. A., Ed.; Plenum
Press: New York, 1982; Vol. 2, pp 367-526.
(8) (a) Yonezawa, T.; Konishi, H.; Kato, H. Bull. Chem. Soc. Jpn. 1969,
42, 933-942. (b) Radom, L.; Nobes, R. H.; Underwood, D. J.; Li, W.-K.
Pure Appl. Chem. 1986, 58, 75-88.
(9) Wittig, G.; Wahl, V. Angew. Chem. 1961, 73, 492.
(10) Wittig, G. Angew. Chem., Int. Ed. Engl. 1962, 1, 415-419.
(11) Wittig, G. Pure Appl. Chem. 1963, 7, 173-191. Hoffmann, R. W.
Dehydrobenzene and Cycloalkynes; Academic Press: New York, 1967; p
293.
(12) (a) Reinecke, M. G.; Newsom, J. G. J. Am. Chem. Soc. 1976, 98,
3021-3022. (b) Reinecke, M. G.; Chen, L.-J.; Almqvist, K. A. J. Chem.
Soc., Chem. Commun. 1980, 585-586. (c) Reinecke, M. G.; Newsom, J.
G.; Chen, L.-J. J. Am. Chem. Soc. 1981, 103, 2760-2769. (d) Reinecke,
M. G.; Newsom, J. G.; Almqvist, K. A. Tetrahedron 1981, 37, 4151-
4157. (e) Teles, J. H.; Hess, B. A., Jr.; Schaad, L. J. Chem. Ber. 1992, 125,
423-431.
(13) David, M. P.; McOmie, J. F. W. Tetrahedron Lett. 1973, 1361-
1362. Ayres, B. E.; Longworth, S. W.; McOmie, J. F. W. Tetrahedron 1975,
31, 1755-1760.
(14) Kitamura, T.; Yamane, M. J. Chem. Soc., Chem. Commun. 1995,
983-984.
(19) Wynberg, H. Acc. Chem. Res. 1971, 4, 65-73.
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0002-7863/96/1518-2511$12.00/0 © 1996 American Chemical Society

2512 J. Am. Chem. Soc., Vol. 118, No. 10, 1996
Communications to the Editor
Scheme 1a
a
(a) PhI(OAc)2, TfOH, CH2Cl2. (b) KF, 18-crown-6, CH2Cl2, room temperature. (c) Anthracene. (d) C6H6. (e) CH2dCMeCMedCH2. (f) Furan,
2-methylfuran, or 2,5-dimethylfuran.
3-21G(d) basis set, the CASSCF(2,2) optimized structure shows
an elongation of C3-C4 of 0.065 Å relative to the RHF value.
As a comparison, it is noted that the corresponding elongation
for benzyne is 0.036 Å.8b At the CASSCF(2,2)/6-311G(d,p)
level, the occupancies of the S and A orbitals are 1.64 and 0.36,
respectively; there is very little variation in these occupancies
when the basis set is changed to either 3-21G(d) or 6-31G(d).
These occupancies indicate that the in-plane π bond of 2 has a
fairly large degree of biradical character, as expected for such
a highly bent bond.
The vibrational frequencies of 2 have also been calculated.
At the CASSCF(2,2)/6-311G(d,p) level, they are (in cm-1) 3405
Figure 1.
(A1), 3400 (B2), 1635 (B2), 1616 (A1), 1421 (A1), 1279 (B2),
1199 (A1), 884 (B1), 866 (B2), 832 (A1), 815 (A2), 739 (B2),
660 (A1), 620 (A2), and 466 (B1). The two highest-energy
frequencies correspond to C-H stretching, the next five are
mostly bond stretching around the ring, while the remaining
eight are the bending modes. Considering that the scaling factor
Figure 2.
for HF/6-31G(d) frequencies is 0.8929,22 the CASSCF(2,2)/6-
311G(d,p) frequencies should be at most 10% too high. Finally,
it is noted that, at all four levels of calculations reported here,
all of the vibrational frequencies of 2 are real, indicating the
structure to be an energy minimum.
In conclusion, evidence for 3,4-didehydrothiophene (2) as an
intermediate has been obtained by the formation of its [2 + 2]-
and [4 + 2]-cycloaddition adducts with dienes.
Optimized CASSCF structures for 2, along with that calcu-
lated at the RHF/3-21G(d) level,8b are summarized in Figure 1.
It is seen that the CASSCF results are in close agreement with
each other, while the RHF/3-21G(d) method yields an exceed-
ingly short C3-C4 bond.
In our CASSCF calculations, we considered two configura-
tions corresponding to double occupation of one or the other
of the molecular orbitals which are predominantly the symmetric
Acknowledgment. The financial support by a Research Grants
Council (Hong Kong) Earmarked Grant for Research (CU93/7.01) is
(S) and antisymmetric (A) combinations of the in-plane sp2-
gratefully acknowledged. X.-S.Y. is on study leave from Department
type orbitals at C3 and C4 (Figure 2). In other words, they are
of Fundamental Courses, The Agricultural University of Central China,
CASSCF(2,2) calculations. If 2 were a pure biradical, CASSCF-
(2,2) treatments would lead to a substantial lengthening of the
C3-C4 bond (compared to the RHF results) and almost equal
populations for the S2 and A2 configurations. In fact, with the
Wuhan, China.
Supporting Information Available: Listing of physical data, 1H-
and 13C-NMR spectra, and the microanalysis and/or high-resolution
mass spectrometric results for compounds prepared (13 pages). This
material is contained in many libraries on microfiche, immediately
follows this article in the microfilm version of the journal, can be
ordered from the ACS, and can be downloaded from the Internet; see
any current masthead page for ordering information and Internet access
instructions.
(21) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Gill, P. M. W.;
Johnson, B. J.; Robb, M. A.; Cheeseman, J. R.; Keith, T. A.; Petersson, G.
A.; Montgomery, J. A.; Raghavachari, K.; Al-Laham, M. A.; Zakrzewski,
V. G.; Ortiz, J. V.; Foresman, J. B.; Cioslowski, J.; Stefanov, B. B.;
Nanayakkara, A.; Challacombe, M.; Peng, C. Y.; Ayala, P. Y.; Chen, W.;
Wong, M. W.; Andres, J. L.; Replogle, E. S.; Gomperts, R.; Martin, R. L.;
Fox, D. J.; Binkley, J. S.; Defrees, D. J.; Baker, J.; Stewart, J. J. P.; Head-
Gordon, M.; Gonzalez, C.; Pople, J. A. Gaussian 94, Revision A1; Gaussian
Inc.: Pittsburgh, PA, 1995.
JA953747B
(22) See, for example: Pople, J. A.; Head-Gordon, M.; Fox, D. J.;
Raghavachari, K.; Curtiss, L. A. J. Chem. Phys. 1989, 90, 5622-5629.