Aryl-aryl bond formation by flash vacuum pyrolysis of benzannulated thiopyrans

Article abstract of DOI:10.1021/jo800379x

(Chemical Equation Presented) In contrast to fully unsaturated 7-membered ring sulfur heterocycles (thiepines), some of which extrude sulfur and give the ring-contracted hydrocarbon even at room temperature in solution, benzannulated thiopyrans (6-membere

Full text of DOI:10.1021/jo800379x

1
Aryl-Aryl Bond Formation by Flash Vacuum
strained, geodesic polyarenes, including the first chemical
6
synthesis of C60 in isolable quantities.
Pyrolysis of Benzannulated Thiopyrans
In an effort to expand the scope of this methodology, we
have begun looking for alternatives to aryl halides as sources
of aryl radicals in the gas phase under FVP conditions. Aromatic
sulfides first attracted our attention in this connection, because
†
‡
Aaron W. Amick, Atsushi Wakamiya, and
†
,
Lawrence T. Scott *
Merkert Chemistry Center, Boston College, Chestnut Hill,
Massachusetts 02467-3860 and Department of Chemistry,
Graduate School of Science, Nagoya UniVersity, Furo,
Chikusa, Nagoya 464-8602, Japan
the bond dissociation energy of the C-S bond in Ph-S-Ph
7
(
78.3 kcal/mol) is reported to lie close to that of the C-Br
8
bond in Ph-Br (82.7 kcal/mol) and well below that of the
8
C-Cl bond in Ph-Cl (97.8 kcal/mol). The prevalence of
Ar-S-Ar′ structural subunits in the complex architectures of
lawrence.scott@bc.edu
9
many coals underscores the importance of learning about the
fundamental behavior of Ar-S bonds at high temperatures.
At the outset of this investigation, we were aware that
substituted thiepines suffer thermal ring contraction and des-
ReceiVed February 15, 2008
10
ulfurization under relatively mild conditions (e.g., Scheme 1);
however, very little seems to be known about the thermal
chemistry of fully unsaturated 6- and 5-membered ring sulfur
1
1
heterocycles (thiopyrans and thiophenes, respectively). We
report here our findings on the high temperature chemistry of
representatives from both families.
In contrast to fully unsaturated 7-membered ring sulfur
heterocycles (thiepines), some of which extrude sulfur and
give the ring-contracted hydrocarbon even at room temper-
ature in solution, benzannulated thiopyrans (6-membered
sulfur heterocycles) require flash vacuum pyrolysis (FVP)
conditions in the gas phase at temperatures in the range of
Our examination of benzannulated thiopyrans began with the
12
13
FVP of benzo[kl]thioxanthene (1). At 1000 °C (0.25 mmHg),
the mass recovery is essentially quantitative, and sulfur extrusion
occurs to an extent of about 50%, giving the ring-contracted
(6) (a) Scott, L. T.; Boorum, M. M.; McMahon, B. J.; Hagen, S.; Mack, J.;
1000-1200 °C to promote the corresponding reaction. Thus,
Blank, J.; Wegner, H.; de Meijere, A. Science 2002, 295, 1500–1503. (b) Scott,
L. T. Angew. Chem., Int. Ed. 2004, 43, 4994–5007.
FVP of benzo[kl]thioxanthene (1) gives fluoranthene, and
naphtho[2,1,8,7-klmn]thioxanthene (6) gives benzo[ghi]-
fluoranthene (7). FVP of thioxanthone (9) gives fluorenone
(7) A value of 78.3 kcal/mol for the C-S bond dissociation energy of Ph-
S-Ph is reported by: (a) Luo, Y.-R. Handbook of Bond Dissociation Energies
in Organic Compounds; CRC Press: Boca Raton, FL, 2002. Using the Benson
additivity tables, a value of 76 kcal/mol has been estimated for the BDE of
Ph-S-Ph by: (b) Bausch, M. J.; Guadalupe-Fasano, C.; Gostowski, R. Energy
Fuels 1991, 5, 419–423. (c) Estimates of the BDE of Ph-S-Ph based on the
(
(
10), together with lesser amounts of dibenzo[b,d]thiophene
11), from competing decarbonylation.
7
d
7e
experimental heats of formation of•S-Ph and•Ph, together with calculated
heats of formation of Ph-S-Ph, fall in the range of 77.7 to 81.6 kcal/mol (PM3
and AM1 levels of theory, respectively). (d) Colussi, A. J.; Benson, S. W. Int.
J. Chem. Kinet. 1977, 9, 295–306. (e) Davico, G. E.; Bierbaum, V. M.; DePuy,
C. H.; Ellison, G. B.; Squires, R. R. J. Am. Chem. Soc. 1995, 117, 2590–2599.
Flash vacuum pyrolysis (FVP) of aryl halides is known to
generate aryl radicals (•Ar) by homolytic cleavage of carbon-
(8) Berkowitz, J.; Ellison, G. B.; Gutman, D. J. Phys. Chem. 1994, 98, 2744–
halogen bonds at temperatures above 900 °C for Ar-Br and
2765.
(9) (a) Winans, R. E.; Kim, Y.; Hunt, J. E.; McBeth, R. L. Coal Sci. Technol.
995, 24, 87–90. (b) Winans, R. E.; McBeth, R. L.; Young, J. E. Coal Sci.
1
above 1000 °C for Ar-Cl. Depending on the structure of the
1
arene, aryl radicals formed in this manner have been shown to
Technol. 1990, 16, 53–65. (c) Van Aelst, J.; Yperman, J.; Franco, D. V.; Van
Poucke, L. C.; Buchanan, A. C., III.; Britt, P. F. Energy Fuels 2000, 14, 1002–
2
3
4
5
rearrange by 1,2-, 1,3(peri)-, 1,4-, and 1,5-shifts of hydrogens
1
008. (d) Mullens, S.; Yperman, J.; Reggers, G.; Carleer, R.; Buchanan, A. C.;
and/or to cyclize by intramolecular addition to the π-system of
Britt, P. F.; Rutkowski, P.; Gryglewicz, G. J. Anal. Appl. Pyrolysis 2003, 70,
469–491. (e) Vorres, K. S. Kirk-Othmer Encyclopedia of Chemical Technology,
1
another aromatic ring in the molecule. Our laboratory has
5
th ed.; John Wiley & Sons: New York, 2004; Vol. 6, pp 703-771.
10) (a) Murata, I.; Tatsuoka, T.; Sugihara, Y. Angew. Chem., Int. Ed. Engl.
974, 13, 142. (b) Nakasuji, K.; Kawamura, K.; Ishihara, T.; Murata, I. Angew.
employed such chemistry extensively for the synthesis of highly
(
1
†
Boston College.
Nagoya University.
Chem. 1976, 88, 650–1. (c) Murata, I.; Nakasuji, K. Top. Curr. Chem. 1981,
97, 33–70. (d) Murata, I. Phosphorus, Sulfur Silicon Relat. Elem. 1989, 43, 243–
59. (e) Yamamoto, K.; Yamazaki, S. ComprehensiVe Heterocyclic Chemistry
II; Elsevier: New York, 1996; Vol. 9, pp 67-111, 1039-1146.
‡
(
1) For many examples, see: Tsefrikas, V. M.; Scott, L. T Chem. ReV. 2006,
1
06, 4868–4884.
(
(
2) Brooks, M. A.; Scott, L. T. J. Am. Chem. Soc. 1999, 121, 5444–5449.
(11) (a) Brown, R. F. C. Pyrolytic Methods in Organic Chemistry: Application
of Flow and Flash Vacuum Pyrolytic Techniques; Academic Press: New York,
1980. (b) McNab, H. Contemp. Org. Synth. 1996, 3, 373–396. (c) McNab, H.
Aldrichimica Acta 2004, 37, 19–26.
3) (a) Hill, T. J.; Hughes, R. K.; Scott, L. T. Abstracts of Papers, 234th
ACS National Meeting; Boston, MA, August 19-23, 2007; abstr. no. ORGN-
7
48. (b) Hill, T. J. Ph.D. Dissertation, Boston College, 2007.
(4) Peng, L.; Scott, L. T. J. Am. Chem. Soc. 2005, 127, 16518–16521.
5) (a) Tsefrikas, V. M.; Scott, L. T. EleVenth International Symposium on
(12) Details of the technique and the apparatus used are described in ref 1
and in: Necula, A.; Scott, L. T. J. Anal. Appl. Pyrolysis 2000, 54, 65–87.
(13) Benzo[kl]thioxanthene (1) was prepared according to the procedure of:
De Luca, G.; Pizzabiocca, A.; Renzi, G Tetrahedron Lett. 1983, 24, 821–824.
(
NoVel Aromatic Compounds; St. John’s, Newfoundland, August 14-18, 2005;
abstr. no. 289. (b) Tsefrikas, V. M. Ph.D. Dissertation,Boston College, 2007.
1
0.1021/jo800379x CCC: $40.75  2008 American Chemical Society
J. Org. Chem. 2008, 73, 5119–5122 5119
Published on Web 05/30/2008

SCHEME 1
SCHEME 2
SCHEME 3
SCHEME 4
SCHEME 5
product, fluoranthene, and recovered starting material in an
1
4
approximate ratio of 1:1 (Scheme 2). Clearly, the energetic
requirements for this process greatly exceed those for the
corresponding loss of sulfur in the thiepine case (Scheme 1).
For reasons discussed below, the FVP of 1 was not examined
at higher temperatures.
Two additional compounds were detected as very minor
1
products in this pyrolysis. Several peaks in the H NMR
spectrum of the crude pyrolysate could be identified as arising
1
5
from small amounts of acephenanthrylene (2), an isomer of
the first C-S bond in the Ar-S-Ar substrate, and that may
account for the unexpected robustness of this heterocycle,
relative to typical Ar-Br.
fluoranthene, and even smaller peaks revealed trace amounts
1
6
of benzo[b]naphtho[1,2-d]thiophene (3), )an isomer of 1.
A second benzannulated thiopyran, naphtho[2,1,8,7-klmn]thio-
1
9
xanthene (6), was also subjected to FVP. Again, sulfur
extrusion was found to promote aryl-aryl bond formation,
giving the corresponding ring-contracted hydrocarbon, ben-
zo[ghi]fluoranthene (7). As in the pyrolysis of thiopyran 1,
however, the primary product begins to suffer skeletal isomer-
ization even before the temperature is high enough to effect
We have previously demonstrated that fluoranthene inter-
5
0% conversion of the starting material. The 5/6-ring swap
converts with acephenanthrylene (2) at 1100 °C in the gas phase
20
rearrangement in this case produces cyclopenta[cd]pyrene (8),
1
5
by a 5/6-ring swap rearrangement, the mechanism for which
which becomes the major product at temperatures in the
we believe involves a hydrogen shift and benzene ring contrac-
1
150-1200 °C range (Scheme 5 and Figure 1). The isomer-
1
7
tion (Scheme 3.
ization of 7 to 8 under FVP conditions at 1100 °C is already
well documented.
In view of this known isomerization pathway, it would be
reasonable to assume that 2 is formed in the FVP of 1 as a
secondary rearrangement product from the primary product,
fluoranthene. We cannot rule out the possibility, however, that
a competing pathway from 1 might also lead to “benzyne” 4,
which would be expected to ring contract to carbene 5 and then
cyclize to 2 by insertion into an ortho C-H bond on the phenyl
20
Heterocycle 6 must be heated to a significantly higher
temperature than heterocycle 1 to induce loss of the sulfur atom.
The greater rigidity of the intermediate formed by the first C-S
bond scission in 6, relative to that in 1, may contribute to this
difference in reactivity and is in accord with our tentative
conclusion (Vide supra) that rupture of the second C-S bond
is rate limiting.
1
8
substituent (Scheme 4).
The formation of benzo[b]naphtho[1,2-d]thiophene (3), albeit
only in trace amounts, suggests that the initial step in the
fragmentation of 1 may involve stepwise homolytic cleavage
of just one C-S bond. Those molecules in which sulfur
dissociates first from the naphthalene ring will generate a freely
rotating intermediate that can cyclize to 3 before it loses sulfur.
Rupture of the second C-S bond to lose sulfur from the •S-Ar
radical thus appears to be more difficult than dissociation of
1
(
14) Product distributions were determined by integration of H NMR spectra
of the pyrolysis mixtures prior to any separation of the components.
(
15) Scott, L. T.; Roelofs, N. H. J. Am. Chem. Soc. 1987, 109, 5461–5465.
(
16) Benzo[b]naphtho[1,2-d]thiophene (3) was prepared according to the
procedure of: Tominaga, Y.; Lee, M. L.; Castle, R. N. J. Heterocycl. Chem.
981, 18, 967–972.
17) Scott, L. T.; Hashemi, M. M.; Schultz, T. H.; Wallace, M. B. J. Am.
Chem. Soc. 1991, 113, 9692–9693.
18) Brown, R. F. C.; Eastwood, F. W. Synlett 1993, 9–19, and references
cited therein.
1
(
FIGURE 1. Product composition from flash vacuum pyrolysis of
naphtho[2,1,8,7-klmn]thioxanthene (6).
(
5
120 J. Org. Chem. Vol. 73, No. 13, 2008

SCHEME 6
SCHEME 8
SCHEME 7
2
3
atom to produce benzo[b]naphtho[1,2-d]furan (14) in 64%
isolated yield. The remainder of the recovered material was
unchanged 13. Scheme 8 outlines a plausible mechanism for
this transformation. An analogous mechanism has previously
been proposed to explain the formation of dibenzofuran from
2
4
FVP of the parent dibenzothiophene dioxide.
In conclusion, flash vacuum pyrolysis of benzannulated
thiopyrans has been found to cause the anticipated extrusion of
the sulfur atom and aryl-aryl bond formation to yield the ring
contracted hydrocarbon product. Unfortunately, the temperatures
required to achieve this chemistry are high enough to promote
The process of sulfur extrusion from a thiopyran under FVP
conditions to form a ring-contracted hydrocarbon resembles the
21
high temperature decarbonylation of aromatic ketones. To gain
an appreciation for the relative difficulties of these two high
energy transformations, we subjected thioxanthone (9) to FVP
at 1100 °C (0.25 mmHg). Analysis of the product mixture by
5
/6-ring swap rearrangements of the primary products, thereby
rendering the transformation ill-suited for applications in organic
synthesis. The thermal extrusion of sulfur from benzannulated
thiopyrans is found to be only slightly less demanding energeti-
cally than thermal decarbonylation of aromatic ketones, and a
new demonstration of the thermal stability of annulated thiophenes
is reported.
1
H NMR spectroscopy revealed fluorenone (10, from loss of
sulfur), dibenzo[b,d]thiophene (11, from loss of CO), and
recovered starting material in a ratio of 3:1:2 (Scheme 6). Thus,
sulfur extrusion is about 3 times more facile than decarbony-
lation in this case. The recovery of thioxanthone (9) in significant
amounts reinforces the emerging picture that sulfur extrusion
and decarbonylation are both more demanding energetically than
the thermal homolysis of aryl bromides and aryl chlorides.
Thermal extrusion of sulfur from a thiophene should be
even more costly energetically than sulfur extrusion from a
thiopyran, because thiophenes enjoy an aromatic stabilization
Experimental Section
Materials. Benzo[kl]thioxanthene (1) was synthesized in three
steps from 1,8-diaminonaphthalene according to the procedure of
1
3
De Luca et al. Naphtho[2,1,8,7-klmn]thioxanthene (6) was
synthesized in four steps from thioxanthone (9) according to the
2
2
that the 6-membered ring heterocycles do not. In keeping
19
procedure of Donovan and Scott. Thioxanthone (9) was obtained
with this prediction, we found that benzo[b]naphtho[1,2-
from Aldrich Chemical Co. Benzo[b]naphtho[1,2-d]thiophene (3)
was synthesized in three steps from benzo[b]thiophene according
1
6
d]thiophene (3) survives FVP at 1100 °C (0.25 mmHg)
unchanged. As a point of comparison, the corresponding
ketone, 7H-benzo[c]fluoren-7-one (12) has previously been
observed to decarbonylate under these conditions, yielding
1
6
to the procedure of Tominaga et al. Benzo[b]naphtho[1,2-
2
5
d]thiophene dioxide (13) was synthesized in quantitative yield
from 3 by oxidation with meta-chloroperoxybenzoic acid according
2
6
2
1g
to the procedure of Nelsen et al. All of the pyrolysis products
obtained in this work are known compounds; their NMR spectra
are readily available in standard databases and/or the chemical
literature.
fluoranthene (Scheme 7).
Oxidation of 3 with MCPBA and FVP of the corresponding
thiophene dioxide (13) also failed to produce fluoranthene.
Instead of losing SO2, the thiophene dioxide retains one oxygen
Flash Vacuum Pyrolysis Details. All pyrolyses were run the
1
2
same way in the same apparatus, as described here for FVP
(
19) Naphtho[2,1,8,7-klmn]thioxanthene (6) was prepared according to the
procedure of: Donovan, P. M.; Scott, L. T. J. Am. Chem. Soc. 2004, 126, 3108–
112.
20) (a) Necula, A. Ph.D. Dissertation, Boston College, 1996. (b) Sarobe,
of naphtho[2,1,8,7-klmn]thioxanthene (6). A 50 mg sample of
3
(
(
23) Marrocchi, A.; Minuti, L.; Taticchi, A.; Scheeren, H. W. Tetrahedron
001, 57, 4959–4965.
24) Fields, E. K.; Meyerson, S. J. Am. Chem. Soc. 1966, 88, 2836–2837.
M.; Zwikker, J. W.; Snoeijer, J. D.; Wiersum, U. E.; Jenneskens, L. W. J. Chem.
Soc., Chem. Commun. 1994, 89–90, Erratum: J. Chem. Soc., Chem. Commun.
2
(
1
994, 1404.
21) See, for example: (a) Schaden, G. Z. Naturforsch., B: Anorg. Chem.,
Org. Chem. 1980, 35B, 1328. (b) Schaden, G. J. Anal. Appl. Pyrolysis 1982, 4,
3–101. (c) Schaden, G. J. Org. Chem. 1983, 48, 5385–5386. (d) Schaden, G.
(
TABLE 1. FVP of Naphtho[2,1,8,7-klmn]thioxanthene (6)
8
temp. (°C)
starting material
mass balance
6
7
8
J. Anal. Appl. Pyrolysis 1985, 8, 135–151. (e) Hagen, S.; Nuechter, U.; Nuechter,
M.; Zimmermann, G. Polycyclic Aromat. Compd. 1995, 4, 209–217. (f) Preda,
D. V.; Scott, L. T. Org. Lett. 2000, 2, 1489–1492. (g) Preda, D. V.; Scott, L. T.
Polycyclic Aromat. Compd. 2000, 19, 119–131.
1050
1085
50 mg
50 mg
50 mg
40 mg
36 mg
85.2
81.0
35.9
3.2
13.4
19.0
45.9
37.8
1.4
0.0
18.2
59.0
36.8 mg
30.6 mg
20.3 mg
1
1
125
200
(
22) Minkin, V. I.; Glukhovtsev, M. N.; Simkin, B. Y. Aromaticity and
Antiaromaticity; Wiley: New York, 1994.
J. Org. Chem. Vol. 73, No. 13, 2008 5121

thiopyran 6 was sublimed at a pressure of 0.2-0.6 mmHg into
a slow stream of nitrogen gas that carried the sample through a
hot quartz tube and into a trap cooled with liquid nitrogen. The
crude product was removed from the trap, weighed, and analyzed
gratefully acknowledged. We thank Prof. K. Komatsu (Kyoto
University) for his support and encouragement of this work and
Restek Corp. for capillary GC columns and supplies. A.W.
thanks the JSPS for a fellowship to study abroad.
1
directly by H NMR spectroscopy. The results of several runs
over a range of temperatures are summarized in Table 1.
Elemental sulfur was detected in the crude pyrolysate, but the
amount was not quantified.
JO800379X
(
25) Klemm, L. H.; Pou, S.; Detlefsen, W. D.; Higgins, C.; Lawrence, R. F.
J. Heterocycl. Chem. 1984, 21, 1293–1296.
26) Nelsen, S. F.; Luo, Y.; Weaver, M. N.; Lockard, J. V.; Zink, J. I. J.
Org. Chem. 2006, 71, 4286–4295.
Acknowledgment. Financial support of this work from the
Department of Energy and the National Science Foundation is
(
5
122 J. Org. Chem. Vol. 73, No. 13, 2008