Phenyl migrations in dehydroaromatic compounds. A new mechanistic link between alternant and nonalternant hydrocarbons at high temperatures

Article abstract of DOI:10.1021/ol005849z

(Matrix presented) Flash vacuum pyrolysis of benzo[b]biphenylene, an alternant polycyclic aromatic hydrocarbon (PAH), gives fluoranthene, a nonalternant PAH, as the major product at 1100 °C in the gas phase. The most reasonable mechanism to explain this isomerization involves equilibrating diradicals of 2-phenylnaphthalene that rearrange by the net migration of a phenyl group to give equilibrating diradicals of 1-phenylnaphthalene, one isomer of which then cyclizes to fluoranthene.

Full text of DOI:10.1021/ol005849z

ORGANIC
LETTERS
2000
Vol. 2, No. 10
1489-1492
Phenyl Migrations in Dehydroaromatic
Compounds. A New Mechanistic Link
between Alternant and Nonalternant
Hydrocarbons at High Temperatures
Dorin V. Preda and Lawrence T. Scott*
Boston College, Merkert Chemistry Center, Chestnut Hill, Massachusetts, 02467-3860
lawrence.scott@bc.edu
Received March 23, 2000
ABSTRACT
Flash vacuum pyrolysis of benzo[b]biphenylene, an alternant polycyclic aromatic hydrocarbon (PAH), gives fluoranthene, a nonalternant PAH,
as the major product at 1100 °C in the gas phase. The most reasonable mechanism to explain this isomerization involves equilibrating
diradicals of 2-phenylnaphthalene that rearrange by the net migration of a phenyl group to give equilibrating diradicals of 1-phenylnaphthalene,
one isomer of which then cyclizes to fluoranthene.
In the course of developing a new and general method for
the synthesis of indeno-fused polycyclic aromatic hydrocar-
bons (PAHs),1 we encountered unexpected byproducts from
the gas-phase pyrolysis of 2-bromobenzoyl PAHs that
appeared to have arisen from phenyl group migrations around
the perimeter of a polycyclic ring system. Mechanistically,
the migration process could best be explained by a series of
elementary steps consisting of reversible 1,5- and 1,6-
hydrogen atom transfers (labeled as ∼H in the schemes that
follow) and reversible intramolecular radical couplings to
form transient biphenylene-type compounds, the whole
sequence ending only when one of the intermediate aryl
diradicals could finally close to form a relatively stable five-
or six-membered ring. We now report more direct evidence
for this type of “phenyl walk”.
vacuum pyrolysis (FVP) of benzo[b]biphenylene (1).2 At
temperatures above 900 °C in the gas phase, two principal
products are formed: indeno[2,1-a]indene (2) and fluoran-
thene (3) (Scheme 1).3,4 The ratio of products 2 to 3 is 7:3
Scheme 1
In an effort to uncover the simplest possible (nondegen-
erate) thermal migration of a phenyl group around the
perimeter of a PAH diradical, we chose to examine the flash
at 900 °C and 3:7 at 1100 °C.5,6 Thus, isomerization to 2 is
clearly favored at lower temperatures, but the pathway
leading to 3 becomes dominant at higher temperatures.
(1) (a) Preda, D. V.; Scott, L. T. Polycyclic Aromat. Compd., in press.
(b) Scott, L. T.; Preda, D. V. Abstracts of Papers, 218th National Meeting
of the American Chemical Society, New Orleans, LA, Aug, 1999; American
Chemical Society: Washington, D C, 1999; ORGN-500. (c) Preda, D. V.;
Scott, L. T. SeVenteenth International Symposium on Polycyclic Aromatic
Compounds, Bordeaux, France, Oct, 1999, abstract 138.
(2) (a) Jensen, F. R.; Coleman, W. E. Tetrahedron Lett. 1959, No. 20,
7-11. (b) Baker, W.; Barton, J. W.; McOmie, J. F. W.; Searle, R. J. G. J.
Chem. Soc. 1962, 2633-6.
10.1021/ol005849z CCC: $19.00 © 2000 American Chemical Society
Published on Web 04/25/2000

The rearrangement of 1 to 2 was not unexpected, as it
closely resembles the known isomerization of biphenylene
(4) to benzo[a]pentalene (5).7 That skeletal reorganization
can be viewed as a simple example of the well-known
benzene ring contraction rearrangement,6 driven by relief of
strain in the four-membered ring (Scheme 2).
whereas the isomerizations of 6 to 7, 7 to 8, and 11 to 12 all
involve 1,5-hydrogen atom transfers.
There is good experimental precedent for the isomerization
of 1,2-aryl diradicals (ortho-benzynes) to 1,5-aryl diradicals
by intramolecular hydrogen atom transfers of the sort
proposed in Scheme 3,6c,10,11 and 1,3-aryl diradicals (meta-
benzynes) should behave similarly. Moreover, computational
studies have shown that reversible intramolecular transfers
of hydrogen atoms via five- and six-membered ring transition
states are expected to be facile at high temperatures in aryl
radicals.12
Scheme 2
The isomerization of 2-phenylnaphthalene-1,3-diyl (7) to
1-phenylnaphthalene-2,8-diyl (11) constitutes the net migra-
tion of a phenyl group in a dehydroaromatic compound that
we set out to expose.
To be absolutely complete, of course, this mechanistic
picture should include a minor variant of Scheme 3 in which
the benzene ring, rather than the naphthalene ring, serves as
the temporary diradical moiety to progress from intermediate
6 to 8 and from 10 to 12. This completely parallel series of
steps (not shown) likewise serves to move the benzene ring
in 6 from the â-position on the naphthalene ring system to
the R-position (also by way of 8, 9, and 10), leading
ultimately to fluoranthene (3).
The best evidence to date indicates that the benzene ring
contraction rearrangement is not a concerted dyotropic
process but proceeds in a stepwise manner through a carbene
intermediate.8 In the case of benzo[b]biphenylene (1), the
ring contraction occurs exclusively in the naphthalene portion
of the molecule, as expected on energetic grounds, to give
indeno[2,1-a]indene (2) rather than naphtho[2,3-a]pentalene.
The formation of fluoranthene (3) from benzo[b]biphen-
ylene (1) at high temperatures is difficult to explain without
invoking a phenyl migration from the â-position of the
naphthalene ring system to the R-position. This process is
most likely initiated in the present case by homolytic
cleavage of a strained C-C bond in 1 at high temperatures
to generate a reactive diradical of 2-phenylnaphthalene (6).9
From there, we propose a mechanistic pathway involving
two successive 1,5-hydrogen atom transfers (discussed further
below) to give an isomeric diradical (8) that is capable of
cyclizing to benzo[a]biphenylene (9) (Scheme 3). Further
isomerization to fluoranthene (3) could then proceed by a
repeat of the same sequence of steps, viz., homolytic cleavage
of a strained C-C bond in 9, two successive intramolecular
hydrogen atom transfers, and a final ring closure. A 1,6-
hydrogen atom transfer is proposed to produce 11 from 10,
This mechanistic picture suggests that independent genera-
tion of diradical 6 from another precursor might yield a
proportion of fluoranthene (3), relative to indeno[2,1-a]indene
(2), higher than that obtained from the FVP of 1. In accord
with this prediction, we have found that FVP of benzo[b]-
fluorenone (13)13,14 affords fluoranthene (3) as a major
product at both 900 and 1100 °C (Scheme 4). Thus, a
significant fraction of the initial diradical (6) formed by
decarbonylation of 13 goes on to fluoranthene directly
(Scheme 4); the remainder cyclizes to benzo[b]biphenylene
(1), which then rearranges according to the partitioning
reported above (Scheme 1).
Not surprisingly, in light of the results presented above,
FVP of 5,12-naphthacene quinone (14)15,16 affords fluoran-
thene (3) as the dominant product at both temperatures (900
and 1100 °C) and gives only small amounts of indeno[2,1-
Scheme 3
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Org. Lett., Vol. 2, No. 10, 2000

Scheme 4
Scheme 6
a]indene (2) (Scheme 5). A portion of the diradical formed
by the initial decarbonylation in this case (15) presumably
Scheme 5
cyclizes to benzo[b]fluorenone (13), which then yields
products 2 and 3 in the ratios reported in Scheme 4. The
shift in the product distribution toward fluoranthene (3),
however, suggests that a significant fraction of 15 must be
cyclizing to benzo[c]fluorenone (16) (Scheme 6). Decarbon-
ylation of 16 would generate diradical 10, an intermediate
in Scheme 3 that is well on its way to fluoranthene (3), which
could isomerize to 2 only by going all the way back to benzo-
[b]biphenylene (1).
In conclusion, we have studied new transformations that
occur on the C16H10 energy surface at high temperatures. Our
(3) Flash vacuum pyrolyses (FVPs) were performed on 100 mg samples
of compound 1 with a steady flow of nitrogen carrier gas as previously
described.4 At 900 °C, the mass recovery was 75% (75 mg), whereas at
1100 °C, the mass recovery was 70% (70 mg). Approximately 30-35% of
unchanged 1 was recovered from the 900 °C pyrolysis, but none survives
in the 1100 °C pyrolysis.
1
(5) The product ratio (2:3) was determined by H NMR spectroscopic
analysis of the crude product mixture. At 1100 °C, fluoranthene equilibrates
with acephenanthrylene by the well-known benzene ring contraction
rearrangement (“5/6 ring-switch”).6 The numbers reported for fluoranthene
(3) at 1100 °C therefore represent the sum of the concentrations of
fluoranthene and acephenanthrylene.
(4) Scott, L. T.; Bratcher, M. S.; Hagen, S. J. Am. Chem. Soc. 1996,
118, 8743-4.
Org. Lett., Vol. 2, No. 10, 2000
1491

results provide one of the simplest examples imaginable of
a phenyl group migration around the perimeter of a dehydro-
PAH (in this case, dehydronaphthalene). Such a process may
well play a role in the formation of large PAHs, fullerenes,
soot, and other carbonaceous material during the combustion
or pyrolysis of low molecular weight hydrocarbons and fossil
fuels.17 More specifically, it could play a major role in the
formation of nonalternant PAHs at high temperatures from
oxygenated precursors, such as ketones and/or quinones. The
thermal generation of nonalternant hydrocarbons from al-
ternant hydrocarbons is not unprecedented,7,18 but we know
of no prior examples that involve phenyl migrations of the
sort described here.19
(6) (a) Scott, L. T.; Roelofs, N. H. J. Am. Chem. Soc. 1987, 109, 5461-
5. (b) Scott, L. T.; Roelofs, N. H. Tetrahedron Lett. 1988, 29, 6857-60.
(c) Anderson, M. R.; Brown, R. F. C.; Coulston, K. J.; Eastwood, F. W.;
Ward, A. Aust. J. Chem. 1990, 43, 1137-50. (d) Brown, R. F. C.; Eastwood,
F. W.; Wong, N. R. Tetrahedron Lett. 1993, 34, 3607-8.
(7) (a) Wiersum, U. E.; Jenneskens, L. W. Tetrahedron Lett. 1993, 34,
6615-18. (b) Brown, R. F. C.; Choi, N.; Coulston, K. J.; Eastwood, F. W.;
Wiersum, U. E.; Jenneskens, L. W. Tetrahedron Lett. 1994, 35, 4405-8.
(8) Scott, L. T.; Hashemi, M. M.; Schultz, T. H.; Wallace, M. B. J. Am.
Chem. Soc. 1991, 113, 9692-3.
(9) For examples of other high-temperature reactions that appear to
involve homolytic cleavage of strained sp2-sp2 carbon-carbon single bonds,
see: (a) Sarobe, M.; Kwint, H. C.; Fleer, T.; Jenneskens, L. W.; Wesseling,
J. Tetrahedron Lett. 1998, 39, 9823-6. (b) Sarobe, M.; Kwint, H. C.; Fleer,
T.; Hevenith, R. W. A.; Jenneskens, L. W.; Vlietstra, E. J.; Van Lenthe, J.
H.; Wesseling, J. Eur. J. Org. Chem. 1999, 1191-1200.
(10) Brown, R. F. C.; Choi, N.; Eastwood, F. W. Aust. J. Chem. 1995,
48, 185-98.
Acknowledgment. We thank the Department of Energy
for financial support of this work.
OL005849Z
(17) See, for example: (a) Howard, J. B.; Lafleur, A. L.; Makarovsky,
Y.; Mitra, S.; Pope, C. J.; Yadav, T. K. Carbon 1992, 30, 1183-201. (b)
Ahrens, J.; Bachmann, M.; Baum, T.; Griesheimer, J.; Kovacs, R.;
Weilmuenster, P.; Homann, K. H. Int. J. Mass Spectrom. Ion Processes
1994, 138, 133-48. (c) Homann, K.-H. Angew. Chem., Int. Ed. Engl. 1998,
37, 2435-51. (d) Richter, H.; Grieco, W. J.; Howard, J. B. Chem. Phys.
Processes Combust. 1997, 135-138. (e) Richter, H.; Grieco, W. J.; Howard,
J. B. Combust. Flame 1999, 119, 1-22. (f) Marr, L. C.; Kirchstetter, T.
W.; Harley, R. A.; Miguel, A. H.; Hering, S. V.; Hammond, S. K. EnViron.
Sci. Technol. 1999, 33, 3091-9.
(11) Bratcher, M. S. Ph.D. Dissertation, Boston College, Chestnut Hill,
MA, 1996.
(12) Cioslowski, J.; Liu, G.; Moncrieff, D. J. Org. Chem. 1996, 61,
4111-4.
(13) Latif, A.; Soliman, G. J. Chem. Soc. 1944, 56-8.
(14) FVPs were performed on 300 mg samples of compound 13 with a
steady flow of nitrogen carrier gas as previously described.4 At 900 °C,
the mass recovery was 45% (120 mg), whereas at 1100 °C, the mass
recovery was 38% (100 mg).5
(18) (a) Scott, L. T. Pure Appl. Chem. 1996, 68, 291-300. (b) Neilen,
R. H. G.; Wiersum, U. E. Chem. Commun. 1996, 149-50. (c) Sarobe, M.;
Jenneskens, L. W.; Wiersum, U. E. Tetrahedron Lett. 1996, 37, 1121-2.
(d) Matzger, A. J.; Vollhardt, K. P. C. Chem. Commun. 1997, 1415-6. (e)
Necula, A.; Scott, L. T. J. Anal. Appl. Pyrolysis 2000, 54, 65-87.
(19) For acid-catalyzed phenyl group migrations around the perimeter
of aromatic hydrocarbons, see: Necula, A.; Racoveanu-Schiketanz, A.;
Gheorghiu, M. D.; Scott, L. T. J. Org. Chem. 1995, 60, 3448-51 and
references therein.
(15) Purchased from Aldrich Chemical Co.
(16) FVPs were performed on 1000 mg samples of compound 14 with
a steady flow of nitrogen carrier gas as previously described.4 At 900 °C,
the mass recovery was 47% (370 mg), whereas at 1100 °C, the mass
recovery was 43% (340 mg).5
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