Microwave flash pyrolysis

Article abstract of DOI:10.1021/jo900245v

(Chemical Equation Presented) In a microwave reactor, graphite heats rapidly to high surface temperatures; applications of graphite thermal "sensitization" have been described previously. We report here that microwave thermal sensitization with graphite, carbon nanotubes, or silicon carbide can be used to carry out reactions more typically accomplished by flash vacuum pyrolysis (FVP) and which usually require temperatures much higher than the nominal limit of a microwave reactor. The graphite-sensitized microwave reaction of azulene in the solid phase at temperatures of 100 to 300°C affords rapid rearrangement to naphthalene, a reaction typically observed by FVP at 700-900°C. Multiwall carbon nanotubes give similar results when used as a thermal sensitizer. Other graphite-sensitized reactions that we have observed include the following: conversion of 2-ethynylbiphenyl to phenanthrene, fragmentation of phthalic anhydride to benzyne, cleavage of iodobenzene to phenyl radical, aryl-aryl bond cleavage, and a variety of cycloaromatizations. An advantage is seen for less volatile substrates. Rearrangement of azulene and generation of benzyne from phthalic anhydride have also been observed on powdered silicon carbide. Because of the high temperature, rapid heating, and frequent ejection of material from the irradiation zone, we refer to this general method as microwave flash pyrolysis (MFP).

Full text of DOI:10.1021/jo900245v

Microwave Flash Pyrolysis
Hee Yeon Cho, Aida Ajaz, Dibya Himali, Prashant A. Waske, and Richard P. Johnson*
Department of Chemistry, UniVersity of New Hampshire, Durham, New Hampshire 03824
rpj@cisunix.unh.edu
ReceiVed February 7, 2009
In a microwave reactor, graphite heats rapidly to high surface temperatures; applications of graphite
thermal “sensitization” have been described previously. We report here that microwave thermal sensitization
with graphite, carbon nanotubes, or silicon carbide can be used to carry out reactions more typically
accomplished by flash vacuum pyrolysis (FVP) and which usually require temperatures much higher
than the nominal limit of a microwave reactor. The graphite-sensitized microwave reaction of azulene in
the solid phase at temperatures of 100 to 300 °C affords rapid rearrangement to naphthalene, a reaction
typically observed by FVP at 700-900 °C. Multiwall carbon nanotubes give similar results when used
as a thermal sensitizer. Other graphite-sensitized reactions that we have observed include the following:
conversion of 2-ethynylbiphenyl to phenanthrene, fragmentation of phthalic anhydride to benzyne, cleavage
of iodobenzene to phenyl radical, aryl-aryl bond cleavage, and a variety of cycloaromatizations. An
advantage is seen for less volatile substrates. Rearrangement of azulene and generation of benzyne from
phthalic anhydride have also been observed on powdered silicon carbide. Because of the high temperature,
rapid heating, and frequent ejection of material from the irradiation zone, we refer to this general method
as microwave flash pyrolysis (MFP).
8
Introduction
Microwave-assisted organic synthesis (MAOS) has advanced
have been reviewed by Laporterie, who developed this method,
9
and more recently by Besson. At present, few reported graph-
ite sensitized reactions venture toward the more extreme regime
Some notable examples (Scheme
) include the ene cyclization of citronellal (1), “microwave
assisted pyrolysis” of urea to give cyanuric acid (2), and
ketodecarboxylation of adipic acid (3). There is also a growing
body of literature on the generation of biofuels by sewage
pyrolysis by using graphite as a microwave absorber. Silicon
1
-3
at a rapid pace during the past decade.
The scope of
10-13
of flash vacuum pyrolysis.
microwave reactions generally is limited by absorption char-
acteristics of solvent, safety concerns, or the temperature range
of commercial microwave reactors. Allotropes of carbon offer
the potential for carrying out higher temperature processes in
1
4
1
1
5
16
4
the solid phase. In a microwave oven, graphite and carbon
1
7
5
-7
nanotubes
absorb radiation with high efficiency and heat
rapidly to high surface temperatures. Applications of graphite
as a thermal susceptor or “sensitizer” in microwave chemistry
(
8) Laporterie, A.; Marquie, J.; Dubac, J. MicrowaVes in Organic Synthesis;
Wiley-VCH: Weinheim, Germany, 2002; p 219.
(
9) Besson, T.; Thiery, V.; Dubac, J. MicrowaVes in Organic Synthesis, 2nd
(
(
(
(
1) Kappe, C. O. Chem. Soc. ReV. 2008, 37, 1127.
ed.; Wiley-VCH: Weinheim, Germany, 2006; Vol. 1, p 416.
(10) Tsefrikas, V. M.; Scott, L. T. Chem. ReV. 2006, 106, 4868.
(11) McNab, H. Aldrichim. Acta 2004, 37, 19.
(12) Brown, R. F. C. Eur. J. Org. Chem. 1999, 3211.
(13) Brown, R. F. C. Organic Chemistry; Vol. 41: Pyrolytic Methods in
Organic Chemistry: Application of Flow and Flash Vacuum Pyrolytic Techniques;
Wiley: New York, 1980.
(14) Garrigues, B.; Laurent, R.; Laporte, C.; Laporterie, A.; Dubac, J. Liebigs
Ann. Chem. 1996, 743.
(15) Chemat, F.; Poux, M. Tetrahedron Lett. 2001, 42, 3693.
(16) Marquie, J.; Laporterie, A.; Dubac, J.; Roques, N. Synlett 2001, 493.
2) Kappe, C. O. Angew. Chem., Int. Ed. Engl. 2004, 43, 6250.
3) Polshettiwar, V.; Varma, R. S. Pure Appl. Chem. 2008, 80, 777.
4) Walkiewicz, J. W.; Kazonich, G.; McGill, S. L. Miner. Metall. Process.
1
988, 5, 39.
5) Imholt, T. J.; Dyke, C. A.; Hasslacher, B.; Perez, J. M.; Price, D. W.;
Roberts, J. A.; Scott, J. B.; Wadhawan, A.; Ye, Z.; Tour, J. M. Chem. Mater.
003, 15, 3969.
6) Nadagouda, M. N.; Varma, R. S. Prepr. Symp.-Am. Chem. Soc., DiV.
Fuel Chem. 2007, 52, 734.
7) Paton, K. R.; Windle, A. H. Carbon 2008, 46, 1935.
(
2
(
(
1
0.1021/jo900245v CCC: $40.75  2009 American Chemical Society
J. Org. Chem. 2009, 74, 4137–4142 4137
Published on Web 05/08/2009

Cho et al.
SCHEME 1. Microwave Reactions on Graphite
nitrogen atmosphere with use of 100-300 W of power in quartz
tubes for periods of 1-5 min. In general, higher power and
temperatures differentiates our approach from earlier graphite-
8,9
sensitized experiments. A loose plug of glass wool above the
graphite minimized ejection of material from the reaction zone.
8
As noted earlier by Laporterie, quartz glassware is desirable
because of the potentially high surface temperatures. Sparks
were seen occasionally during irradiation and a nitrogen
atmosphere minimizes explosion hazards. Products were col-
lected by rinsing the cooled reactor and graphite with several
portions of solvent. The CEM microwave reactor uses an
infrared sensor to limit temperatures to a default value of 300
°
C; we did not modify this upper limit. Measurements of the
graphite temperature with a thermocouple immediately after
microwave irradiation showed temperatures only slightly above
this limit. High initial power is essential; lower power micro-
wave irradiation tends to cause more sublimation than chemical
reaction.
In a second “closed vessel” method, pyrolyses were carried
out in 10 mL capped Pyrex tubes lined with a smaller quartz
tube (12 × 70 mm; glassblower fabrication) and purged with
nitrogen. The quartz insert minimizes thermal risk to the Pyrex
tube and is commonly reusable. Reaction tubes were cooled
prior to opening and products were isolated by rinsing the tube
carbide absorbs efficiently and has been used as a “passive
1
8,19
heating element” in microwave reactions.
In principle,
powdered SiC might function like graphite as a sensitizer for
high-temperature chemistry.
The potential for higher energy microwave chemistry is
suggested by a variety of literature. Microwave irradiation has
2
0
been used to exfoliate graphite and graphite intercalation
21
5
6
compounds. Tour and more recently Varma have shown that
in a microwave field, carbon nanotubes can undergo structural
changes, glow, or even ignite in air. Clearly, the limits and
preparative applications of chemistry under these extreme
conditions bear further exploration. The question posed in the
present work was whether many higher temperature reactions
that have typically been accomplished by flash vacuum pyrolysis
and graphite with CDCl . Samples were filtered and analyzed
3
directly by NMR, TLC, or GC. Control experiments with pure
graphite heated by microwave to 300 °C showed minimal
pressure buildup from outgassing.
Under these high-power conditions, reaction tubes usually
fill with aerosols, while solid product is deposited above the
heated zone. Tubes may look charred but mass balances are
good in most cases. MFP reactions are not stirred but most
reactants are rapidly redistributed by vaporization. With their
10-13
(FVP)
might be carried out in a microwave reactor through
thermal sensitization with graphite or other substances. FVP
reactions often generate radicals, arynes, and other reactive
intermediates; thus, the chemical inertness of graphite is a further
advantage. As our work progressed, silicon carbide emerged
3
low density (<0.6 g/cm ), graphite and nanotubes appear to
undergo substantial agitation during reaction, while SiC (density
3
3.2 g/cm ) does not.
1
8,19
as another good candidate for a thermal sensitizer.
We report here that with high power, solid-phase graphite or
carbon nanotube sensitization extends the range of microwave
reactions to include broad classes of high-energy processes.
Powdered silicon carbide shows similar behavior but does not
seem to reach as extreme temperatures. Because of the high
temperature, rapid reaction, and frequent ejection of material
from the heated reaction zone, we refer to this general method
as microwave flash pyrolysis (MFP).
Results and Discussion
Rearrangement of Azulene. We initially investigated the
thermal isomerization of azulene (4) to naphthalene (5), a reac-
2
2
tion first reported in 1947 by Heilbronner and later studied in
2
3
24
25
great detail by Scott, Alder, and Wentrup. Vapor-phase
rearrangement of 4 typically is carried out in the range of
7
00-900 °C. Barriers to unimolecular rearrangement of azulene
24,26
have been estimated to be >80 kcal/mol.
Lower energy
Methodology Development. Two general microwave meth-
ods were employed in this work. Additional details are provided
in the Supporting Information. A CEM Discover reactor was
used for all reactions. Substrates were mixed with graphite,
carbon nanotubes, or silicon carbide by light grinding prior to
reaction. Although some earlier studies employed precoating
2
4
radical initiated processes are also likely.
SCHEME 2. Azulene Thermal Rearrangement on Graphite
8,9
of graphite by solvent evaporation, we prefer to avoid residual
solvent impurities and find that reactants are rapidly redistributed
during pyrolysis.
Our first method was “open vessel”, i.e., ambient pressure,
with mixtures of substrate and graphite irradiated under a
Microwave reaction of 4 on graphite was carried out both at
atmospheric pressure and in sealed tubes, with similar results.
Irradiation was conducted for one to three 1 min periods initially
at 300 °C/150 W, interspersed with 1 min cooling periods.
Following these reaction conditions, we observe 60-77%
conversion of 4 to naphthalene (Scheme 2) as the sole product
seen by NMR or GC. Limitations on conversion to product seem
primarily due to sublimation of reactant out of the graphite hot
zone.
(
17) Dominguez, A.; Menendez, J. A.; Inguanzo, M.; Pis, J. J. Bioresour.
Technol. 2006, 97, 1185.
(
(
18) Kremsner, J. M.; Kappe, C. O. J. Org. Chem. 2006, 71, 4651.
19) Razzaq, T.; Kremsner, J. M.; Kappe, C. O. J. Org. Chem. 2008, 73,
6
321.
(
(
20) Tryba, B.; Morawski, A. W.; Inagaki, M. Carbon 2005, 43, 2417.
21) Falcao, E. H. L.; Blair, R. G.; Mack, J. J.; Viculis, L. M.; Kwon, C.-
W.; Bendikov, M.; Kaner, R. B.; Dunn, B. S.; Wudl, F. Carbon 2007, 45, 1367.
4
138 J. Org. Chem. Vol. 74, No. 11, 2009

MicrowaVe Flash Pyrolysis
SCHEME 3. Thermal Rearrangement of 2-Ethynylbiphenyl on Graphite
SCHEME 4. Microwave Generation of Benzyne
Not surprisingly, a nearly identical reaction was observed
when the graphite was replaced with inexpensive Baeyer
multiwalled nanotubes (BayTubes). Amorphous carbon was less
effective, giving only ca. 20% conversion under similar condi-
tions. We conducted a series of sealed tube experiments on
graphite in which the limiting temperature was stepped from
of 94:6. Traces (<1%) of benzazulene 11 were seen by NMR.
Anthracene was an unexpected product but is easily explained
as a secondary product from 11. Whittaker and Alder reported
2
9
in 1975 that pyrolysis of 11 gives 9 and 10 in a 2.5:1 ratio.
Thus the initial reaction products apparently are 9 and 11, with
10 a secondary product of 11. This implies an initial product
ratio (9: 11) of ca. 85:15.
1
2
00 to 300 °C while maintaining the maximum power level at
00 W. At 100 °C, we still observed 34% conversion of 4 to 5
Generation of Benzyne from Phthalic Anhydride. In 1965,
Fields and Meyerson reported evidence that benzyne is generated
after only 1 min of irradiation. Control experiments showed that
heating pure azulene under the same reaction conditions (150
W/300 °C) resulted only in sublimation of reactant to the upper
part of the tube.
3
0a
by FVP of phthalic anhydride. Other benzynes have been
generated from anhydrides and similar precursors under similar
3
0,31
conditions.
We can find only one published example of
1
8,19
We briefly explored finely powdered silicon carbide
as a
benzyne generation in a microwave reactor; this was carried
3
2
thermal sensitizer for 4. Heating a mixture of azulene and silicon
carbide for 1 min at 300 °C resulted in 16% conversion to
naphthalene. The ramp time to maximum temperature was
longer than with graphite, however, and most of the azulene
sublimed out of the reaction zone.
out by base-promoted elimination.
MFP reactions of phthalic anhydride (12) were carried out
at atmospheric pressure or in sealed tubes, resulting in high
conversions of starting material but only a modest mass balance.
Sealed-tube reactions of 12 (with careful monitoring of pres-
sure!) gave the best results. The product mixture (Scheme 4)
consisted of unreacted 12 (34%), benzene (38%), biphenyl (18,
Rearrangements of 2-Ethynylbiphenyl. FVP reaction of 6
(700 °C/0.01 Torr) has been reported by Brown and co-workers
to give a mixture of phenanthrene (9) and benzazulene (11) in
1
7%), naphthalene (21, 9%), biphenylene (15, 0.8%), and
2
7
a ratio of 72:28. These products (Scheme 3) may result from
triphenylene (16, 0.5%). Addition of 1 equiv of anthracene to
the mixture in open vessel experiments produced a low yield
1
2
Brown rearrangement (1,2-H shift to vinylidene 7) although
we have recently shown by computations that there is a
2
8
competitive route to 9 passing through cyclic allene 8.
(
25) Gugel, H.; Zeller, K. P.; Wentrup, C. Chem. Ber. 1983, 116, 2775.
(26) Stirling, A.; Iannuzzi, M.; Laio, A.; Parrinello, M. ChemPhysChem 2004,
, 1558.
27) Brown, R. F. C.; Eastwood, F. W.; Harrington, K. J.; McMullen, G. L.
The reaction was carried out with graphite at 300 °C/150W
maximum power in a closed vessel and the product mixture
was analyzed directly by NMR. When pyrolyzed by MFP for
only 1 min, ca. 80% of 6 was converted to products. The major
products were phenanthrene (9) and anthracene (10) in a ratio
5
(
Aust. J. Chem. 1974, 27, 2393.
(28) Mackie, I. D.; Johnson, R. P. J. Org. Chem. 2009, 74, 499.
(
(
29) Alder, R. W.; Whittaker, G. J. Chem. Soc. Perkin Trans. 2 1975, 714.
30) (a) Fields, E. K.; Meyerson, S. J. Chem. Soc., Chem. Commun. 1965,
4
74. (b) Brown, R. F. C.; Solly, R. K. Aust. J. Chem. 1966, 19, 1045.
(31) Wenk, H. H.; Winkler, M.; Sander, W. Angew. Chem., Int. Ed. Engl.
(22) Heilbronner, E.; Plattner, P. A.; Wieland, K. Experientia 1947, 3, 70.
(23) Scott, L. T.; Kirms, M. A. J. Am. Chem. Soc. 1981, 103, 5875.
(24) Alder, R. W.; East, S. P.; Harvey, J. N.; Oakley, M. T. J. Am. Chem.
2003, 42, 502.
(32) Shi, L.; Wang, M.; Fan, C.-A.; Zhang, F.-M.; Tu, Y.-Q. Org. Lett. 2003,
5, 3515.
Soc. 2003, 125, 5375.
J. Org. Chem. Vol. 74, No. 11, 2009 4139

Cho et al.
SCHEME 5. Aryl Iodide Cleavage on Graphite
SCHEME 6. Microwave Cycloaromatizations on Graphite
of trypticene (19), as seen by NMR. These observations support
the intermediacy of o-benzyne (13), presumably formed through
Cycloaromatizations. Gas phase pyrolysis of aromatic com-
pounds often yields highly complex mixtures in which rings
37
loss of CO and CO
2
from 12. Hydrogen abstraction leads to
are broken, formed, rearranged, or joined together. Polycyclic
phenyl radical (14) and then 18 through dimerization. Although
aromatic ring synthesis has been widely studied because of its
3
3
38,39
metal-catalyzed trimerization of benzyne is well-known,
relationship to soot formation in combustion chemistry.
This
30
thermal trimerization is rare. This suggests a high concentra-
tion of reactive intermediates, consistent with the rapid reaction
we observe. Naphthalene is likely to be a secondary product
arising from cycloreversion of benzobarrelene (20), itself formed
from cycloaddition of 13 and 17. This sequence was noted
is also a critical area of materials science where many high-
1
0,40,41
temperature cycloaromatizations have been described.
We have carried out a series of model cycloaromatization
reactions (Scheme 6). Stilbenes are known to cyclize under FVP
42,43
conditions, although cis-trans isomerization is clearly faster.
3
4
earlier from pyrolysis of benzyne precursors in benzene. We
MFP reaction of cis-stilbene (23) on graphite afforded primarily
cis-trans isomerization to 24, with only a low yield of 9. Similar
results occurred with stereoisomer 24. Better success was
observed with pyrolysis of o-terphenyl (25) on graphite, which
gave 7-15% efficiency in cyclization to 16. Sealed tube
pyrolyses of 25 showed that biphenyl and benzene are the major
reaction products. As expected, 1,1′-binaphthyl (26) cyclized
in both directions to give 27 and 28; unexpectedly, the major
reaction product was naphthalene. Sublimation from the hot zone
is a major limiting factor in these reactions.
35
found that graphite-sensitized pyrolysis of benzobarrelene (20)
under similar conditions yielded naphthalene.
Again, we briefly explored replacing graphite with powdered
silicon carbide. With use of comparable reaction parameters,
the SiC + 12 mixture heated more slowly but the same
collection of products appeared in similar ratios, including
15-18 and 21. The similarity of product distributions observed
with graphite and SiC suggests that the source of hydrogen
atoms in these experiments must be other molecules of substrate.
This would also explain the poor mass balances.
Perhaps most astonishing is the observation that MFP of both
Aryl Iodide Cleavage to Aryl Radicals. Aryl iodides undergo
2
5 and 26 on graphite gives major products arising from
3
6
facile thermal homolysis thus a direct MFP route to phenyl
radical is the pyrolysis of iodobenzene (22). In a sealed tube,
this reaction proceeded smoothly (82% conversion), affording
cleavage of aryl-aryl bonds. Indeed, naphthalene is the major
product from MFP of 26. Aryl-aryl bonds have dissociation
44
energies of >115 kcal/mol, thus direct dissociation seems
(Scheme 5) benzene and biphenyl in a ratio of 84:16. Molecular
unlikely. Lower activation barriers have been reported for bond
cleavage in biphenyl, with radical addition to the ipso carbon
iodine is also generated in these reactions as judged by the deep
purple coloration of the reaction mixture. As noted above,
hydrogen atoms are likely to derive from other molecules of
substrate.
(
37) See for example: Winans, R. E.; Tomczyk, N. A.; Hunt, J. E.; Solum,
M. S.; Pugmire, R. J.; Jiang, Y. J.; Fletcher, T. H. Energy Fuels 2007, 21, 2584.
38) Pope, C. J.; Miller, J. A. Proc. Combust. Inst. 2000, 28, 1519.
(
(
(
33) Pena, D.; Perez, D.; Guitian, E. Chem. Rec. 2007, 7, 326.
(39) Richter, H.; Howard, J. B. Prog. Energy Combust. Sci. 2000, 26, 565.
(40) Rajadurai, M. S.; Shifrina, Z. B.; Kuchkina, N. V.; Rusanov, A. L.;
Mullen, K. Russ. Chem. ReV. 2007, 76, 767.
34) (a) Miller, R. G.; Stiles, M. J. Am. Chem. Soc. 1963, 85, 1798. (b)
Friedman, L.; Lindow, D. F. J. Am. Chem. Soc. 1968, 90, 2329.
(
35) Johnson, R. P.; Exarchou, A.; Jefford, C. W.; Hahn, R. C. J. Org. Chem.
977, 42, 3758.
36) Kominar, R. J.; Krech, M. J.; Price, S. J. W. Can. J. Chem. 1976, 54,
981.
(41) Wu, J.; Pisula, W.; Mullen, K. Chem. ReV. 2007, 107, 718.
(42) Petrocelli, F. P.; Klein, M. T. Macromolecules 1984, 17, 161.
(43) Plater, M. J. Tetrahedron Lett. 1994, 35, 6147.
1
(
2
(44) Zavitsas, A. A. J. Phys. Chem. A 2003, 107, 897.
4
140 J. Org. Chem. Vol. 74, No. 11, 2009

MicrowaVe Flash Pyrolysis
SCHEME 7. Elbs Cyclizations and Related Reactions
Conclusions
Graphite-sensitized microwave reactions were first developed
8
by Laporterie and co-workers. Many types of graphite-
8
,9,14-16
sensitized chemistry have been reported earlier,
but the
upper limits of this method remained to be explored. We have
found that with higher initial power, graphite sensitization
extends easily to reactions that have previously been ac-
complished at high temperature (>500 °C) by flash vacuum
1
0-13
pyrolysis
even though the bulk graphite temperature
remains much lower. Formation of surface “hot spots” seems
the most likely explanation for our results, but much further
study is required to understand processes in these extreme
regions of microwave chemistry.
Microwave flash pyrolysis experiments use no solvents or
expensive reagents, can be run in minutes, apply readily to
nonvolatile substratessan advantage over FVP methods in some
casessand appear to generate high concentrations of reactive
intermediates. This technique is rapid and simple but requires
quartz glassware and careful attention to safety concerns. In
general, results are similar to other methods for thermal reactions
but novel chemistry has been observed in some cases.
4
5
suggested as a mechanism. Thus it is possible that radical
processes are occurring in these MFP reactions.
We explored a variety of cycloaromatization reactions
related to the Elbs cyclization, a venerable route to aromatic
4
6-48
compounds.
-methylbenzophenone (29) on graphite (Scheme 7) afforded
primarily fluorene (30) with smaller amounts of 10 and 9;
Atmospheric pressure MFP reaction of
2
Graphite clearly is not unique as a thermal sensitizer but may
provide the best combination of properties and economy. Solid
phase reaction on carbon nanotubes yielded similar chemistry
but nanotubes are less inert and more expensive than graphite.
Powdered silicon carbide offers another alternative. We were
able to effect rearrangement of azulene and generation of
benzyne from phthalic anhydride on silicon carbide and suggest
that this inert solid deserves further exploration, not just as a
4
9
these products are consistent with earlier FVP results.
-Methybenzhydrol (31) cyclized more smoothly to 10,
presumably through initial loss of water, but also yielded
2, from homolytic cleavage. Flash vacuum pyrolysis of 31
has not been reported previously and was explored for
comparison; we find efficient conversion to anthracene and
suggest this may be a useful route to some types of aromatic
compounds.
Mechanism of Thermally Sensitized Pyrolysis. How can
azulene rearrange at a bulk temperature of 100 °C in a 1 min
reaction? There has been much discussion in the literature of
thermal vs. nonthermal microwave effects.
isomerization to naphthalene (5) may be an ideal reaction for
2
3
18,19
“passive heating element”
sensitizer.
but as a high-temperature thermal
We have developed a simple method for small-scale closed-
vessel microwave pyrolysis, but caution that such reactions
should be carried out under inert atmosphere and with careful
monitoring of pressure. In our closed-vessel pyrolytic method,
all reaction products are captured in the reaction tube. This
clearly has preparative applications where volatile products are
created. Additionally, this method is ideal for reactions requiring
headspace analysis and may be useful in analytical method
development where decomposition of samples is desirable.
Solid phase thermal sensitization with graphite, carbon nano-
tubes, silicon carbidesand almost certainly other substances yet
to be exploredsgreatly extends the range of chemistry that can
be accomplished in a common microwave reactor. We are
continuing to explore novel applications of this simple technique.
50-53
The azulene (4)
studying unusual effects in microwave chemistry. As suggested
1
5
previously for urea pyrolysis on graphite, we believe our
results with azulene provide compelling evidence for the
existence of localized heating (“hot spots”) created by a
combination of microwave absorption by the sensitizer and
polarity of the substrate. Reaction may be localized just inside
the tube because deep penetration of strongly absorbing solid
materials is unlikely. Hot spots have been observed to form
54-56
and grow in microwave irradiation of other solid materials.
Cleavage of phthalic anhydride to benzyne and the apparent
secondary reaction of benzazulene 11 provide additional ex-
amples of polar substances reacting rapidly while nonpolar
reactants such as o-terphenyl (24) undergo less efficient reaction.
Experimental Section
Caution! Graphite, carbon nanotubes and silicon carbide heat
rapidly in a microwave field, generating high surface temperatures
and occasional sparks. To minimize explosion hazards, thermally
sensitized experiments should be conducted in quartz glassware
under nitrogen. An external explosion shield is recommended for
open vessel experiments. For closed-vessel experiments, the pres-
sure should be monitored closely; tubes may hold residual pressure
after cooling.
(
45) Cook, B. R.; Wilkinson, B. B.; Culross, C. C.; Holmes, S. M.; Martinez,
L. E. Energy Fuels 1997, 11, 61.
(
(
(
46) Bergmann, E. D.; Blum, J. J. Org. Chem. 1961, 26, 3214.
47) Fieser, L. F. 1942, pp 129.
48) Lupes, M. E.; Lupes, S. A.; Sarbu, C. G. Mem. Sect. Stiint.-Acad. Repub.
Soc. Rom. 1984, 5, 147.
(
49) Gu, T. Y.; Weber, W. P. J. Org. Chem. 1980, 45, 2541.
50) de la Hoz, A.; Diaz-Ortiz, A.; Moreno, A. Chem. Soc. ReV. 2005, 34,
(
64.
Product analyses were carried out by a combination of 400 MHz
H NMR, TLC, or GCMS. Representative NMR spectra are
1
1
(
51) Perreux, L.; Loupy, A. MicrowaVes in Organic Synthesis, 2nd ed.; Wiley-
VCH: Weinheim, Germany2006; Vol. 1, p 134.
reproduced in the Supporting Information. Unless otherwise stated,
quoted percentages represent product ratios not absolute yields.
Pyrolysis of Azulene on Graphite: Closed Vessel. Azulene (20
mg, 0.16 mmol) and graphite (50 mg; Aldrich synthetic graphite,
(
52) Conner, W. C.; Tompsett, G. A. J. Phys. Chem. B 2008, 112, 2110.
53) Herrero, M. A.; Kremsner, J. M.; Kappe, C. O. J. Org. Chem. 2008,
(
7
3, 36.
(
(
(
54) Haas, P. A. Am. Ceram. Soc. Bull. 1979, 58, 873.
55) Kriegsmann, G. A. Mater. Res. Soc. Symp. Proc. 1994, 347, 473.
56) Kriegsmann, G. A. Mater. Res. Soc. Symp. Proc. 1996, 430, 181.
2
0 µm) were mixed with a mortar and pestle and transferred to a
quartz tube (12 × 70 mm). A small plug of glass wool was placed
J. Org. Chem. Vol. 74, No. 11, 2009 4141

Cho et al.
above the graphite. The quartz tube was inserted into a Pyrex
microwave reaction tube, purged with nitrogen, and sealed. The
reaction was run “closed vessel” in a CEM Discover reactor at 300
for 1 min. Products were isolated by washing the tube with small
volumes of CDCl and filtering through neutral alumina directly
3
into an NMR tube. NMR analysis showed the mixture to be
composed of starting material (53%), triphenylene (7%), biphenyl
(11%), and benzene (29%).
°
C/150 W for 1 min hold time. Product was isolated by washing
the cooled tube with small volumes of CDCl and filtering through
3
1
neutral alumina. Analysis by 400 MHz H NMR showed the mixture
to be 35% azulene and 65% naphthalene.
Pyrolysis of 1,1′-Binaphthyl on Graphite: Closed Vessel. 1,1′-
Binaphthyl (26, 25 mg, 0.100 mmol) and graphite (100 mg) were
mixed with a mortar and pestle and pyrolyzed as above at 300 °C/
100 W for 1 min. Product analysis by NMR showed the following:
1,1′-binaphthyl (26, 77%), perylene (27, 5%), naphthalene (27.5%),
Pyrolysis of Azulene on Graphite: Open Vessel. To a quartz test
tube (25 × 200 mm) were added 1.00 g of graphite and 40 mg
(0.31 mmol) of powdered azulene. The tube was mixed well, purged
5
7
with nitrogen, sealed with a rubber septum, and vented to a nitrogen
line. The mixture was heated by microwave for 1 min (hold time)
at 150 W/ 300 °C, then cooled for 2 min. The same heating process
was repeated twice more. After cooling, the tube was rinsed several
times with hexane. The solution was filtered through neutral
alumina, eluting with hexane. The eluate (250 mL) was concentrated
to yield 32.5 mg (81% mass recovery) of blue solid. NMR and GC
analysis showed the product to be 23% of azulene and 77% of
naphthalene.
and benzo[j]fluoranthene (28, 0.6%).
Pyrolysis of 2-Methylbenzophenone on Graphite: Open Ves-
sel. To a quartz test tube (25 × 200 mm) were added 1.0 g of
graphite and 300 mg (1.53 mmol) of 2-methylbenzophenone (29).
The tube was stirred well and an additional 1.0 g of graphite was
added on top. The tube was purged with nitrogen, sealed with a
rubber septum, vented by needle to a nitrogen line, and heated in
the microwave for 30 s at 300 W/300 °C, then cooled for 2 min.
This cycle was repeated. After cooling, the product was isolated to
yield 170 mg (57% mass recovery) of yellow oil. No starting
material remained. NMR analysis showed fluorene (86%), an-
thracene (12%), and phenanthrene (2%).
Flash Vacuum Pyrolysis of 2-Methylbenzhydrol. 2-Methylben-
zhydrol (31, 100 mg, 0.507 mmol) was subjected to flash vacuum
pyrolysis by flow through a quartz reaction tube maintained at 900
°C and 0.02 Torr. The product (59 mg, 66%) was obtained within
5 min. NMR analysis showed no starting material. GC and NMR
analysis showed anthracene (91.6%) along with phenanthrene
(3.1%), fluorene (30, 2.2%), and 2-benzyltoluene (32, 3.1%).
Pyrolysis at 700 °C and 0.02 Torr yielded similar products but only
ca. 30% conversion.
Pyrolysis of Azulene on Silicon Carbide: Closed Vessel. Azulene
20 mg, 0.16 mmol) and silicon carbide (500 mg; Alfa Aesar, 2
(
µm powder, alpha-phase) were ground lightly together and trans-
ferred to a quartz tube (12 × 70 mm). Reaction was run “closed
vessel” as above at 300 °C/150 W for a 2 min hold time. Blue
solid sublimed into the upper part of the tube. NMR analysis of
the product showed 84% azulene and 16% naphthalene.
Pyrolysis of 2-Ethynylbiphenyl on Graphite: Closed Vessel.
2
7
2
-Ethynylbiphenyl (6, 23 mg, 0.13 mmol) and graphite (68 mg)
were mixed and pyrolyzed as above. NMR analysis showed ca.
0% conversion to products, with formation of phenanthrene (9)
9
and anthracene (10) (ratio 17:1), as well as traces (<1%) of
benzazulene 11 and several unidentified products.
Pyrolysis of Phthalic Anhydride on Graphite: Closed Vessel.
Phthalic anhydride (12, 27 mg, 0.18 mmol) and graphite (100 mg)
were mixed with a mortar and pestle and pyrolyzed as above at
Acknowledgment. Funding for portions of this work was
provided by a grant from DARPA, administered by Los Alamos
National Laboratory, as well as Summer Undergraduate Re-
search Fellowships from the University of New Hampshire and
Merck Research Laboratory, Boston. We are grateful to Dr.
Robert Lockerman of the CEM Corporation for technical
assistance and to Professor Larry Scott, Boston College, for
advice and encouragement.
2
00 °C/100 W for 1 min. Product analysis by NMR showed the
following composition: phthalic anhydride (34%), benzene (38%),
biphenyl (17%), naphthalene (9%), biphenylene (0.8%), triph-
enylene, (0.5%).
Pyrolysis of Phthalic Anhydride on Silicon Carbide: Closed
Vessel. Phthalic anhydride (12, 27 mg, 0.18 mmol) and silicon
carbide (100 mg) were mixed with a mortar and pestle and
pyrolyzed as above at 200 °C/100 W for 1 min. Products were
Supporting Information Available: General procedures,
additional experimental descriptions, and NMR spectra of
product mixtures for microwave flash pyrolysis reactions. This
material is available free of charge via the Internet at
http://pubs.acs.org.
isolated by washing the quartz tube with small volumes of CDCl
3
and filtering through neutral alumina directly into an NMR tube.
Analysis by NMR showed the following composition: phthalic
anhydride (26%), benzene (45%), biphenyl (21%), naphthalene
(
7%), biphenylene (1.2%), and triphenylene, (0.5%).
Pyrolysis of o-Terphenyl on Graphite: Closed Vessel. o-Ter-
JO900245V
phenyl (25, 23 mg, 0.010 mmol) and graphite (80 mg) were mixed
with a mortar and pestle and pyrolyzed as above at 300 °C/150 W
(57) Rice, J. E.; Cai, Z. W. J. Org. Chem. 1993, 58, 1415.
4
142 J. Org. Chem. Vol. 74, No. 11, 2009