4720 J. Am. Chem. Soc., Vol. 118, No. 19, 1996
Communications to the Editor
Table 1. 2,6-Dithiabicyclo[3.1.0]hex-3-enes 4a-d from Irradiation of 1,2-Dithiins 1a-d
compound
yieldd
1H NMR chemical shift (J)a,b
13C NMR chemical shift
4a, [4a′]c,d
91
2.04 (s), 5.88 (11.1, 2.1), 5.98 (17.6, 2.3), 6.08 (17.6, 11.1),
6.66/5.03 (2.6), [7.01/4.86 (2.8)e]
4.48 (2.4, 5.1), 5.17 (1.8, 5.1), 6.38 (2.1, 5.7), 6.51 (2, 5.6)
4.67 (2.7), 6.77 (2.7), 7.40-7.51 (m, 8H), 7.60 (1.5, 7.8, 2H)
3.83 (12.1), 4.09 (12.1), 4.20 (14.5, 1.6), 4.35 (17.4),
4.37 (s), 6.19 (s)
3.94 (CH3), 53.88 (CH),f 58.65 (C),f 69.81, 71.34, 72.90,
76.47, 80.58, 95.21 115.83, 125.77, 129.41, 133.76g
51.16, 52.63, 125.09, 129.41
58.21, 118.64, 126.14, 128-129, 132.37, 136.30, 146.90
55.20, 60.18, 69.22, 73.16, 119.63, 150.64
4b
4c
88
86
96
4dc
a Unless otherwise indicated solvent is CDCl3 and each signal corresponds to 1H. b Coupling constants in Hz. c In CD3C(O)CD3. d Minor component
in brackets; major isomer 88.8%, minor isomer 2.6%; only thiirane ring protons seen for minor isomer. e Ring protons. f Thiirane ring. g Major
component signals only. h From NMR analysis; also formed 8.6% 2a from 1a, 12% 2b from 1b, 14% 2c from 1c, 4% 2d from 1d.
Scheme 2
at 500-600 nm (CHdS nπ*).15,17b Compound 9 displays
characteristic thioketene IR absorption at 1760 cm-1 15,17
.
To confirm the structural assignments, ab initio calculations
(HF, 6-31G*) were carried out of the vibrational spectra of 3b
and 9.15a While cZt-3b was twisted ca. 40° out of plane, tZt-
3b (not shown) was found to be planar; no energy minimum
was found corresponding to cZc-3b in either planar or twisted
geometries.6a,15a The frequencies and intensities calculated for
the cZt-3b geometry fit the experiments nicely.15a An energy
minimum was found for 9 in a slightly twisted s-cis geometry.
The calculated vibrational spectrum for this conformer of 9
agrees well with experiment.15a
We propose that photoproducts 4a-d, identified spectro-
scopically as 2,6-dithiabicyclo[3.1.0]hex-3-enes,12 are formed
by a π4a
+ π2a rearrangement of initally formed (Z)-2-
Time-resolved irradiations of 1b in solution were also carried
out.18 Laser flash photolysis of 1b (Freon-113, 308 nm, 25 °C)
produced a transient absorption with λmax ) 329 nm. The trans-
ient was fairly long lived, decaying by apparent first-order
kinetics with a lifetime of 62 µs. The similarity of this absorp-
tion to that obtained in the low-temperature matrix isolation
experiments suggests that the transient corresponds to cZt-3b.
Flash vacuum pyrolysis of 1b at 500 °C also affords thiophene
7. This result could be explained either in terms of formation
of 4b followed by surface-catalyzed rearrangement or by con-
version of cZt-3b to 9 which can give 4b by an intramolecular
thiol addition.19 Microwave spectroscopy reveals that 1b is
nonplanar with a 53.9° CSSC dihedral angle (Scheme 2).20 Elec-
trocyclic ring opening followed by limited rotation about the
C1-C2 bond could lead directly to cZt-3b. Rotation in the
opposite direction would afford cZc-3b, disfavored relative to
cZt-3b.2b,6a Compound cZt-3b can then undergo either a 1,5-
sigmatropic shift13a,21 giving 9 or a 4a + 2a rearrangement
butenedithials (3), a process analogous to that reported by Padwa
for photolysis of 4-phenylisothiochromene (see below for
evidence for cZt-3b).13 At least in the cases of 4a and 4c, the
process can be partially reversed under thermal conditions,
regenerating dithiins 1a and 1c. Rearrangement of photoprod-
ucts 4b and 4c to mercaptothiophenes 7 and 8 presumably
involves acid-catalyzed ring opening giving the most stable
carbocation, 5 and 6, respectively. In common with photo-
chemical and thermal loss of sulfur from related thiiranes,10b,12c,13a
further irradiation of 4b yields 2b.
To obtain additional information on intermediates in 1,2-
dithiin photochemistry, Ar matrix isolation spectroscopy at 25
K14,15a was performed with 1b. Irradiation of the matrix with
visible light (436 nm, 2 h) caused the complete disappearance
of 1b and the simultaneous production of s-trans-Z-s-cis
2-butenedithial (cZt-3b).15 Irradiation of cZt-3b at shorter
wavelengths (313 nm, 8 h) gave (Z)-4-mercapto-1,3-butadi-
enethial (9) (Scheme 2).15 Irradiation of 9 at 254 nm caused
the reappearance of the bands of 3b which grew until a
photostationary state containing both species was reached (5
h). In this fashion, 3b and 9 could be interconverted over
several cycles by changes in irradiation wavelength.16 The IR
spectrum of 3b exhibited strong bands in the 1100-1200 cm-1
region (CdS)15,17b and a weaker band at 1558 cm-1 (conjugated
CdC). In addition to an intense UV band at 319 nm, an
extremely weak absorption was observed in the visible region
π
π
giving 4b. Formation of 9 is apparently favored at 25 K in the
Ar matrix.
Acknowledgment. We gratefully acknowledge support from the
National Science Foundation (E.B., R.S.S.), the Natural Sciences and
Engineering Research Council, Canada (J.E.P., G.H.N.T.), and the
Petroleum Research Fund, administered by the ACS (R.S.S.). We thank
the NSF for partial funding for purchase of the Convex C210 computer
used in the work at Nevada.
Supporting Information Available: Text describing matrix isola-
tion conditions, UV-vis spectra of Ar matrix dithiin before and after
irradiation, matrix IR spectra of 1b, 3b, and 9, ab initio calculation
methods, calculated geometries, and IR spectra (tabular and graphical)
for cZt-3 and 9, and NMR spectra of 4b,c (12 pages). Ordering
information is given on any current masthead page.
(12) (a) The NMR assignments for 4 are supported by comparison with
NMR data for 2-cyano-5-thiabicyclo[2.1.0]pent-2-ene12b and 2,3-
dihydrothiophene.12c (b) Barltrop, J. A.; Day, A. C.; Irving, E. J. Chem.
Soc. Chem. Commun. 1979, 966-967. (c) Sauer, N. N.; Angelici, R. J.;
Huang, Y. C. J.; Trahanovsky, W. S. J. Org. Chem. 1986, 51, 113-114.
(13) (a) Padwa, A.; Au, A.; Lee, G. A.; Owens, W. J. Org. Chem. 1975,
40, 1142-1149. (b) George, M. W.; Mitra, A.; Sukumaran, K. B. Angew.
Chem., Int. Ed. Engl. 1980, 19, 973-983.
JA960589V
(14) (a) Kesselmayer, M. A.; Sheridan, R. S. J. Am. Chem. Soc. 1986,
108, 99-107. (b) Hayes, J. C.; Sheridan, R. S. J. Am. Chem. Soc. 1990,
112, 5879-5881.
(17) (a) Elam, E. U.; Rash, F. H.; Dougherty, J. T.; Goodlett, V. W.;
Brannock, K. C. J. Org. Chem. 1968, 33, 2738-2741. (b) Duus, F. In
ComprehensiVe Organic Chemistry; Barton, D., Ollis, W. D., Eds.;
Pergamon: New York, 1979; Vol. 3, p 373. (c) Schaumann, E.; Ehlers, J.;
Mrrotzek, H. Liebigs Ann. Chem. 1979, 1734-1745.
(18) Laser flash photolysis experiments were carried out at Ohio State
University in the laboratories of Professor M. S. Platz.
(19) However, cZt-3b f 9 is predicted by ab initio calculations (HF,
6-31G*) to be thermodynamically unfavorable by ca. 10 kcal/mol.
(20) Gillies, J. Z.; Gillies, C. W.; Cotter, E. A.; Block, E.; DeOrazio, R.
Submitted for publication in J. Mol. Spectrosc.
(21) Matrix isolation photochemical conversion of thioacrolein to
methylthioketene via 1,3-H shift: Korolev, V. A.; Baskir, E. G. IzV. Akad.
Nauk., Ser. Khim. 1995, 464-470.
(15) (a) See the supporting information for details. (b) cZt-3b: IR 3024
(w), 2990 (w), 2953 (vw), 2948 (vw), 1558 (w), 1425 (s), 1422 (s), 1366
(m), 1357 (w), 1163 (s), 1151 (vs), 1076 (m), 1074 (m), 1033 (m), 998
(m), 871 (w), 786 (w), 691 (w) cm-1; UV λmax ca. 218, 319 (both strong),
500-600 (very weak) nm. 9: IR 1773 (s), 1763 (s), 1753 (s), 1742 (s),
3082 (vw), 3039 (vw), 3015 (vw), 1903 (w), 1593 (m), 1526 (m), 1387
(w), 1351 (m), 1348 (m), 1221 (w), 957 (vw), 934 (vw), 880 (vw) cm-1
;
UV λmax ca. 215, 274, 327 (sh) nm, tailing out to ca. 700 nm.
(16) Broad-band irradiation (>200 nm, 10 h) of the matrix converted
3b and 9 to CS2 together with unknown material. No 2b or 7 was observed
in the IR by comparison to authentic material. No discrete products could
be identified upon warming a 2-methylpentane matrix containing 9.