Formation of dibenzofurans by flash vacuum pyrolysis of aryl 2-(allyloxy)benzoates and related reactions

Article abstract of DOI:10.1039/c002480e

Flash vacuum pyrolysis (FVP) of aryl 2-(allyloxy)benzoates 5 and of the corresponding aryl 2-(allylthio)benzoates 6 at 650°C, gives dibenzofurans 19 and dibenzothiophenes 20, respectively. The mechanism involves generation of phenoxyl (or thiophenoxyl) radicals by homolysis of the O-allyl (or S-allyl) bond, followed by ipso attack at the ester group, loss of CO₂ and cyclisation of the resulting aryl radical. Synthetically, the procedure works well for p-substituted substrates, which lead to 2-substituted dibenzofurans 19b-f (73-90%) and dibenzothiophenes 20b-c (90-94%). Little selectivity is shown in the cyclisation of m-substituted substrates and competing interactions of the radical with the substituent - and ipso-attack - complicate the pyrolyses of o-substituted substrates. FVP of related radical precursors including 2-(allyloxy)phenyl benzoates 43 gave no dibenzofurans, whereas 2-(allyloxy-5-methyl)azobenzene 44 gave a much reduced yield. No carbazoles were obtained by FVP of 4-methylphenyl 2-(allylamino)benzoate 42.

Full text of DOI:10.1039/c002480e

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www.rsc.org/obc | Organic & Biomolecular Chemistry
Formation of dibenzofurans by flash vacuum pyrolysis of aryl
2-(allyloxy)benzoates and related reactions†
Michael Black,a J. I. G. Cadogan*b and Hamish McNab*a
Received 10th February 2010, Accepted 6th April 2010
First published as an Advance Article on the web 11th May 2010
DOI: 10.1039/c002480e
Flash vacuum pyrolysis (FVP) of aryl 2-(allyloxy)benzoates 5 and of the corresponding aryl
2-(allylthio)benzoates 6 at 650 ◦C, gives dibenzofurans 19 and dibenzothiophenes 20, respectively. The
mechanism involves generation of phenoxyl (or thiophenoxyl) radicals by homolysis of the O-allyl (or
S-allyl) bond, followed by ipso attack at the ester group, loss of CO2 and cyclisation of the resulting aryl
radical. Synthetically, the procedure works well for p-substituted substrates, which lead to 2-substituted
dibenzofurans 19b–f (73–90%) and dibenzothiophenes 20b–c (90–94%). Little selectivity is shown in the
cyclisation of m-substituted substrates and competing interactions of the radical with the
substituent—and ipso-attack—complicate the pyrolyses of o-substituted substrates. FVP of related
radical precursors including 2-(allyloxy)phenyl benzoates 43 gave no dibenzofurans, whereas
2-(allyloxy-5-methyl)azobenzene 44 gave a much reduced yield. No carbazoles were obtained by FVP of
4-methylphenyl 2-(allylamino)benzoate 42.
Introduction
The gas-phase behaviour of phenoxyl and thiophenoxyl radicals
under flash vacuum pyrolysis (FVP) conditions often differ signifi-
cantly. Whereas the former are prone to hydrogen atom abstraction
processes,1–5 the latter species are much more likely to undergo
cyclisation. The formation of benzofurans6 and benzothiophenes7
from 2-allyloxy- and 2-allythio-cinnamate esters, respectively,
via the corresponding phenoxyl and thiophenoxyl species, is a
notable exception. Here, we present full details of results published
in a preliminary communication8 where we showed that FVP
of aryl esters of 2-allyloxy- and 2-allylthio-benzoic acids gives
dibenzofurans and dibenzothiophenes, respectively, in good yields.
We discuss the scope, limitations and mechanism of the process
and a brief investigation of related radical precursors. This work
complements our earlier study of the formation of dibenzofurans
and dibenzothiophenes by direct cyclisation of aryl radicals.9
Standard routes to dibenzofurans10 and dibenzothiophenes11
include solution-phase free-radical cyclisations, but surprisingly
few general methods are available, despite the environmental
importance of many derivatives. Therefore, the development of
concise and cost-effective synthetic methods from readily available
starting materials was an important aim of this work.
Scheme 1 Reagents and conditions: (i) ArOH, POCl3, 135 ◦C; (ii) allyl
bromide, DMF, K2CO3, 20 ◦C.
thiosalicylic acid 2) was first esterified using a modification of
the Organic Syntheses procedure12 to give the aryl esters 3 and
4, respectively, which were treated with allyl bromide in DMF in
the presence of potassium carbonate to provide the allyloxy and
allylthio compounds 5 and 6, respectively. Yields were variable
(see Experimental section and ESI†) but no attempt was made
to optimise the formation of these precursors. Substituents were
chosen to provide a range of 2-, 3- and 4-substituted aryl esters
and, in one case (7), a substrate with a further substituent on the
salicylate ring was synthesised (see below).
The general method proved to be ineffective if the phenol
contains an electron withdrawing group in the para-position.
Instead, a Smiles rearrangement takes place under the basic
conditions used for the allylation, to provide allyl esters (e.g. 8).
In their 13C NMR spectra, allyl esters 8 show a signal at around
Results and discussion
Most of the radical precursors, aryl 2-(allyloxy)benzoate esters 5
and the corresponding 2-(allylthio)benzoate esters 6, were made
by the sequence shown in Scheme 1, in which salicylic acid 1 (or
aSchool of Chemistry, The University of Edinburgh, West Mains Road,
Edinburgh, UK EH9 3JJ. E-mail: H.McNab@ed.ac.uk; Fax: +44 131 650
4743; Tel: +44 131 650 4718
bDepartment of Chemistry, University of Wales Swansea, Swansea, UK SA2
3PP
† Electronic supplementary information (ESI) available: Experimental. See
DOI: 10.1039/c002480e
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dC 65 ppm due to the O-CH2, whereas the corresponding signal of
the aryl ester system 5 appears around dC 70 ppm.
Compounds 5e and 5f were made by an alternative route
(Scheme 2) in which the order of allylation and esterification was
reversed. Because the phenol was introduced at the final stage of
the sequence, this alternative route was advantageous for more
complex phenols, though an additional step was required.
Scheme 3 Reagents and conditions: (i) FVP, 650 ◦C; (ii) FVP, 850 ◦C.
are therefore compatible with substituents (e.g. methoxy-groups)
which decompose at the higher temperatures.9
Our previous work has also demonstrated that aryl radicals
of the type 17 can equilibrate by hydrogen atom abstraction
from the neighbouring ring,9 one of a class of hydrogen atom
rearrangements which take place at high temperatures.14 Thus,
FVP of the allyl ester 23 at 850–1000 ◦C leads to the two isomeric
methyl benzofurans 19gb and 19ga in a 3 : 1 ratio via equilibration
and cyclisation of the two aryl radicals 24 and 25 (Scheme 4).9
FVP of 7 at 650 ◦C also gave 19gb and 19ga in a 3 : 1 ratio, which
suggests that the radicals are fully equilibrated even at 650 ◦C. This
rearrangement can lead to regioselectivity problems for certain
targets.
Scheme 2 Reagents and conditions: (i) allyl bromide, DMF, K2CO3, 20 ◦C;
(ii) NaOH, H2O–MeOH, reflux; (iii) SOCl2, then ArOH, DMAP, Et3N,
20 ◦C.
However, the alternative route (Scheme 2) proved unsuitable for
S-allyl compounds because a rearrangement of the allyl group to a
propenyl group (to give 12) takes place under the basic conditions
used for the ester hydrolysis. The unwanted rearrangement was
avoided by using S-isopropyl derivatives 6¢a and 6¢k as alternative
radical generators.
As expected from our earlier study of related alkyl benzoate
systems,13 the EI mass spectra of the precursors 5 and 6 display
a-cleavage of the ester function to produce a peak at (M-OAr)+,
followed by loss of CO. Analogous processes are not found in the
thermal behaviour of the substrates.
FVP of 5a and of 6a at 650 ◦C provides dibenzofuran (62%)
and dibenzothiophene (88%) after purification. As usual for gas-
phase pyrolyses, the Claisen rearrangement does not compete
with radical formation.5 The mechanism involves ipso-attack of
the initial radicals 13 and 14, respectively, to provide spirodienyl
radicals 15 and 16, which collapse with loss of CO2 to generate
the aryl radicals 17 and 18, which cyclise (Scheme 3). The
mechanism bears some resemblance to a radical variant of the
Smiles rearrangement. We have previously shown that the key aryl
radicals 17 and 18 can also be generated by FVP of allyl esters
21 and 22, but the present method has the major advantage that
the furnace temperature required for the reaction is ca. 200 ◦C
lower than that needed for the previous method. Conditions
Scheme 4 Reagents and conditions: (i) FVP, 650 ◦C; (ii) FVP 850–1000 ◦C.
By employing substituents in the aryl ester ring, the method is
synthetically useful for 2-substituted dibenzofurans 19b–f and 2-
substituted dibenzothiophenes 20b–c. The reaction proceeds well,
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irrespective of the nature of the substituent, which includes an
alkyl group (19b, 70%; 20b, 90%) a halogen (19c, 75%; 20c, 92%),
an electron donating group (19d, 91%) and electron withdrawing
groups (19e, 90%; 19f, 73%). The only significant by-product is
a trace of the phenol (see below), which can be easily removed
on a preparative scale by base extraction. The method therefore
provides a three-step route from a p-substituted phenol to a 2-
substituted dibenzothiophene or dibenzofuran.
of salicylaldehyde 29 (12%) confirms that the small amounts of
phenols detected in these pyrolysates are formed by competitive
radical cleavage of the aryl ester moiety followed by hydrogen atom
abstraction (cf. ref. 1–5).
FVP of the 3-methyl precursors 5g and 6g showed little
regioselectivity in the pyrolysis step, giving mixtures of 1- and 3-
methyl isomers 19ga, 19gb and 20ga, 20gb, respectively, identified
by the characteristic chemical shifts of the methyl groups.9 No
other 3-substituted substrates were investigated.
The scope of the reaction was further tested by extension to
some more unusual ring systems. The heterocyclic and naphthyl
precursors 30, 33, 34 and 37 were synthesised in the standard way
and pyrolysed at 650 ◦C.
Scheme 6 Reagents and conditions: (i) FVP 650 ◦C.
FVP of the 3-pyridyl compound 30 gave a 55% yield of a
3 : 1 mixture of benzofuro[3,2-b]pyridine 31 and benzofuro[2,3-
c]pyridine 32, identified by their 13C NMR spectra17 and by
isolation of the picrate salt of the minor isomer 32 (Scheme 6).18
Formation of the major isomer therefore involves attack at C-2
of the pyridine ring, the preferred site for intramolecular radical
attack of the pyridine system in solution-phase cyclisations.19 In
contrast, FVP of the quinolinyl system 33 gave 8-hydroxyquinoline
(34%), by radical cleavage of the ester, rather than significant
amounts of cyclised products.
The 2-methyl precursors 5h and 6h gave the expected cyclised
products 19h (31%) and 20h (39%), respectively, together with
small amounts of o-cresol (ca.10%), and of the parent compounds
19a and 20a. The latter are formed by ipso-attack of the aryl
radical at the site of the methyl group followed by ejection of the
substituent; related sequences have been previously observed.15
The 1H NMR spectrum of the pyrolysate from 5h suggested
the presence of 1-hydroxyfluorene 27 (2%), formed by a known4
sequence of hydrogen-transfer and rearrangement processes via
the benzyl radical 26 (Scheme 5). In agreement with this mecha-
nism, a trace of thioxanthene 28 was identified in the pyrolysate
from 6h; 2-(thiophenoxy)benzyl radicals are known to cyclise to
thioxanthene.2
Scheme 7 Reagents and conditions: (i) FVP 650 ◦C.
FVP of the a-naphthyl system 34 gave two isomeric cyclised
products in 43% overall yield in ca. 1 : 1 ratio which were
not separated (Scheme 7). One component was identified as
benzo[k,l]xanthene 36 by comparison of the 13C NMR spectrum of
the mixture with reported data for 36.20 The remaining signals were
tentatively assigned to benzo[b]naphtho[2,1-d]furan (a-brazan)
35 (see Experimental section). Two products (in 4 : 1 ratio) were
also obtained by FVP of the b-naphthyl system 37 (91% overall
yield). The minor isomer was isolated by recrystallisation and
identified as benzo[b]naphtho[2,3-d]furan, (b-brazan) 39 (19%),
so the major isomer is benzo[b]naphtho[1,2-d]furan (g-brazan) 38
(Scheme 8). Intermolecular radical attack at the a- and b-positions
of naphthalene in solution gives comparable regioselectivity to the
present example.21
Scheme 5 Reagents and conditions: (i) FVP 650 ◦C.
Although the 4-chloro compounds 19i and 20i were both
significant components of the pyrolysates from the 2-chloro
substrates 5i and 6i, ipso-attack and ejection of the substituent
also occurred, leading to the major product in the latter case.
This problem could be overcome to some extent by FVP of
the 2,6-dichloro compounds 5k and 6¢k. FVP of the 2-methoxy-
compound 5j gave 4-methoxydibenzofuran 19j (ca. 30%) as the
major component. It is known that methoxy groups ortho to
radical centres can rearrange to aldehydes,16 so the detection
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Scheme 9 Reagents and conditions: (i) FVP 650 ◦C.
Conclusions
In conclusion, the work reported in this paper defines the scope of a
three step synthesis of dibenzofurans and dibenzothiophenes.8 The
key step involves radical generation, cyclisation, CO2 extrusion,
recyclisation and hydrogen atom expulsion, accomplished by FVP
(650 ◦C) of aryl allyloxy- (or allythio)-benzoates 5 and 6. If the aryl
group contains a para-substituent, good yields of 2-substituted
cyclised products are obtained, independent of the nature of the
substituent. If the aryl group has an ortho- or meta- substituent,
mixtures of products are usually obtained which, in principle,
could be separated. FVP of the ester substrates 5 and 6 is a
better route to dibenzofurans and dibenzothiophenes than that
of corresponding azo-compounds (e.g. 42) or of the ‘reversed’
esters 41, and requires a much lower furnace temperature than
that of the allyl esters 21 and 22. The method was extended to
some heterocyclic and polycyclic substrates.
Scheme 8 Reagents and conditions: (i) FVP 650 ◦C.
It was easy to show that the synthetic strategy shown in Scheme 3
is not applicable to the formation of carbazoles (see Experimental
section). As previously found in the FVP reactions of anthranilic
esters,9,22 elimination of the phenol and formation of an unstable
ketenimine by a cyclic transition state is probably the major
pathway from 40.
Experimental
1H and 13C NMR spectra were recorded at 200 and 50 MHz,
respectively, for solutions in [2H]chloroform unless otherwise
stated. Coupling constants are quoted in Hz. All mass spectra
were recorded under electron impact conditions.
The FVP reactions of two related sets of precursors were studied
in which the two aryl groups are again linked by a potential thermal
leaving group. In the first of these, 41, the leaving group is again
CO2 but arranged in the opposite fashion to that in 5. The synthesis
of these ‘reversed esters’ was achieved by standard esterification
of 2-allyloxyphenol, itself obtained (20%) by reaction of catechol
with a slight excess of allyl bromide (ESI†). FVP of 41a or 41b
gave only traces of dibenzofurans and other monomeric products;
considerable amounts of polymeric material were formed, which
suggests that other reaction pathways are open to the phenoxyl
radical, rather than ipso-attack. The nature of these pathways has
not been investigated.
The second precursor related to 5 was the azo-compound
42, designed so that any ipso-attack of the phenoxyl could be
followed by extrusion of dinitrogen (ESI†). Compound 42 was
synthesised by azo-coupling of benzenediazonium chloride with
p-cresol followed by allylation. Although 2-methyldibenzofuran
19b was indeed formed (28%) by FVP of 42 at 650 ◦C or higher,
the yield was surprisingly much lower than when the ester 5b was
used as the precursor, despite the obvious efficiency of the leaving
group. Clearly the juxtaposition of functionality in radicals 13 and
14 is ideally suited to the sequence of events shown in Scheme 3,
leading to dibenzofurans and dibenzothiophenes.
Aryl benzoates 3 and 412
Salicyclic acid or thiosalicyclic acid (0.127 mol) was melted◦with
the appropriately substituted phenol (0.127 mol) at 135 C.12
Phosphoryl chloride (7 g, 0.046 mol) was then added gradually and
the temperature was moderated until the evolution of hydrogen
chloride had ceased. The reaction mixture was cooled in ice, water
(250 cm3) was added and the product was obtained by trituration
of the organic precipitate. The reaction mixture was then washed
with aqueous sodium carbonate (4 M, 50 cm3) to remove any
unreacted acid.
If necessary, non-crystalline products were extracted with
dichloromethane (3 ¥ 50 cm3), the extracts washed with water (2 ¥
50 cm3), dried (MgSO4), the solvent removed and the products
purified by distillation. Typical products are reported here, the
remainder are described in the ESI†:
4-Methylphenyl (2-hydrox◦y)benzoate 3b. (46%), mp 38–40 ◦C
(from methanol) (lit.,23 38.5 C); dH 10.56 (1H, br s), 8.09 (1H, m),
7.27–6.97 (7H, m) and 2.39 (3H, s); m/z 228 (M+, 27%), 121 (100),
107 (24) and 93 (20).
Finally, the direct cyclisation of the 2-phenylphenoxyl radical
44, generated by FVP of 2-allyloxybiphenyl 43, gave dibenzofuran
19a (65%) (ESI†). A considerable amount of hydrogen capture
product, 2-hydroxybiphenyl 45 (17%) was also formed (Scheme 9),
despite the proximity of the 2-phenyl group for cyclisation.
Phenyl (2-mercapto)benzoate 4a. (60%), mp 82–85 ◦C (from
ethanol) (lit.,24 91 ◦C) dH 8.27 (1H, d), 7.49–7.20 (8H, m) and 4.47
(1H, s); m/z 230 (M+, 89%), 137 (100) and 109 (82).
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Aryl 2-(allyloxy)benzoates 5 and aryl 2-(allylthio)benzoates 6
30 min. Excess thionyl chloride was removed in vacuo (using a
water pump). The acid chloride thus formed was dissolved in
dichloromethane (25 cm3) and 4-dimethylaminopyridine (DMAP)
(0.02 g, 0.0002 mol) was added, followed by the addition of the
appropriate phenol (0.02 mol). Triethylamine (2.02 g, 0.02 mol) in
dichloromethane (25 cm3) was added dropwise and the solution
was stirred for 3 h. The dense white precipitate of triethylamine hy-
drochloride was removed by filtration and the filtrate was washed
with water (3 ¥ 15 cm3), dried (MgSO4) and the solvent removed
in vacuo.. Typical products are reported here, the remainder are
described in the ESI†:
The following S- and O-allyl compounds were prepared by
treatment of the appropriate aryl 2-mercaptobenzoate or aryl 2-
hydroxybenzoate with allyl bromide in DMF in the presence of
potassium carbonate as previously described.25 Typical products
are reported here, the remainder are described in the ESI†:
Phenyl 2-(allyloxy)benzoate 5a. (83%), bp 137–140 ◦C (0.4
Torr) (Found: C, 75.3; H, 5.45. C16H14O3 requires C, 75.6; H,
5.5%); dH 8.09 (1H, m), 7.68–6.95 (8H, m), 6.06 (1H, m), 5.66–5.23
(2H, m) and 4.64 (2H, m); dC 163.95 (quat), 158.22 (quat), 150.74
(quat), 133.58, 132.19, 131.58, 128.90, 125.16, 121.36, 120.01,
119.42 (quat), 116.83, 113.35 and 68.94; m/z 254 (M+, 4%), 161
(100), 133 (13) and 92 (7).
4-Nitrophenyl 2-(allyloxy)benzoate 5e. (70%) mp 62–64 ◦C
◦
(from ethanol) (lit.,28 62–63 C) (Found: C, 64.0; H, 4.4; N, 4.7.
C16H13NO5 requires C, 64.2; H, 4.35; N, 4.7%); dH 8.11 (1H, m),
7.55 (1H, m), 7.17–7.05 (2H, m), 6.99–6.93 (3H, m), 6.08 (1H,
m), 5.52–5.39 (2H, m) and 4.80 (2H, m); dC 167.20 (quat), 162.38
(quat), 157.44 (quat), 140.90 (quat), 135.49, 133.47, 130.59, 125.96,
122.37, 120.52, 117.14 (quat), 115.64, 113.17 and 70.86; m/z 299
(M+, 0.3%), 161 (100), 133 (12) and 41 (20).
The following compounds were prepared using 2-isopropyl-
thiobenzoic acid13 in place of 2-allyloxybenzoic acid, by the same
method:
Phenyl 2-(allylthio)benzoate 6a. (89%), mp 60–62 ◦C (from
ethanol) (Found: C, 69.4; H, 5.2. C16H14O2S·0.3H2O requires C,
69.5; H, 5.3%); dH 8.19 (1H, m), 7.54–7.19 (7H, m), 6.00 (1H,
m), 5.39–5.16 (2H, m) and 3.65 (2H, m); dH 164.54 (quat), 150.69
(quat), 141.74 (quat), 132.46, 131.41, 129.16, 126.52, 125.59 (quat),
124.05, 121.53, 118.31, 77.52, 76.88 and 35.34; m/z 270 (M+,
1.5%), 177 (100), 149 (18) and 108 (26).
Base-promoted reaction of 4-nitrophenyl (2-hydroxy)benzoate and
allyl bromide
Phenyl 2-(isopropylthio)benzoate 6¢a. (75%) mp 57–59 ◦C
(from ethanol) (Found: C, 68.9; H, 5.4. C16H16O2S·0.4H2O requires
C, 68.8; H, 6.0%); dH 8.14 (1H, m), 7.58–7.19 (8H, m), 3.55 (1H, m)
and 1.38 (6H, m); dC 164.89 (quat), 150.73 (quat), 141.23 (quat),
132.31, 131.33, 129.23, 127.89, 125.98 (quat), 125.67, 124.25,
121.65, 35.56 and 22.52; m/z 272 (M+, 13%), 179 (100), 137 (64)
and 109 (22).
Under the basic conditions employed in the general alkylation
procedure this benzoate can undergo Smiles’ rearrangement26 and
it is the rearranged product [2-(4-nitrophenoxy)benzoic acid] that
is subsequently alkylated. Thus, allyl 2-(4-nitrophenoxy)benzoate
8 was obtained as a brown solid (72%), mp 63–65 ◦C (from ethanol)
(Found: C, 64.0; H, 4.35; N, 4.7. C16H13NO5 requires C, 64.2; H,
4.35; N, 4.7%); dH 8.16 (2H, m), 8.02 (2H, m), 7.60 (1H, m), 7.35
(1H, m), 7.13 (1H, m), 6.91 (2H, m), 5.78 (1H, m), 5.37–5.20
(2H, m) and 4.62 (2H, m); dC 164.14 (quat), 163.53 (quat), 153.37
(quat), 142.32 (quat), 134.22, 132.34, 131.40, 125.74 (2C), 124.01
(quat), 123.00, 118.47, 116.07 and 65.73; m/z 299 (M+, 48%), 242
(46), 196 (100) and 168 (27).
2,6-Dichlorophenyl 2-(isopropylthio)benzoate 6¢k. (73%), mp
◦
65–67 C (from ethanol) (Found: C, 56.7; H, 4.2. C16H14C12O2S
requires C, 56.3; H, 4.1%); dH 8.33 (1H, m), 7.53–7.11 (6H,
m), 3.57 (1H, m) and 1.38 (6H, m); dC 162.27 (quat), 143.99
(quat), 142.80 (quat), 132.98, 132.09, 129.08 (quat), 128.45, 127.40,
127.00, 126.03 (quat), 124.05, 35.26 and 22.46; m/z 340 (M+, 5%),
179 (100), 161 (45), 137 (80) and 136 (22).
2-(2-Propenylthio)benzoic acid 12
FVP experiments
Thiosalicylic acid (6.17 g, 0.04 mol), allyl bromide (14.52 g,
0.12 mol) and anhydrous potassium carbonate (16.59 g, 0.12 mol)
were reacted as described in ref. 27 After work-up, allyl 2-
(allylthio◦)benzoate was obtained as a clear oil. (8.57 g, 87%), bp
155–160 C (0.4 Torr) dH 7.95 (1H, m), 7.40–7.28 (2H, m), 7.12
(1H, m), 6.09–5.83 (2H, m), 5.43–5.12 (4H, m), 4.80 (2H, m) and
3.57 (2H, m). Crude allyl 2-(allylthio)benzoate (5.5 g, 0.024 mol)
was then treated with aqueous sodium hydroxide (5 M, 30 cm3) in
methanol (100 cm3) by the method previously described.13 Work-
up afforded 2-(2-propenylthio)benzoic acid 12 (3.87 g, 85%), mp
143–145 ◦C (from ethyl acetate) (lit.,27 144–146 ◦C); dH 9.60 (1H,
br s), 8.09 (1H, m), 7.50–7.10 (3H, m), 6.30–6.12 (2H, m) and 1.90
(3H, d); m/z 194 (M+, 100%) and 153 (32).
The precursors were distilled under vacuum through a silica
pyrolysis tube (35 ¥ 2.5 cm), which was heated by a laboratory
tube furnace. Products were collected in a U-tube trap cooled
by liquid nitrogen and situated at the exit point of the furnace.
Upon completion of the pyrolysis, the trap was allowed to warm
up to room temperature under nitrogen and the product was
removed from the trap. Pyrolysis parameters are quoted as follows:
furnace temperature (Tf) inlet temperature (Ti), pressure (range if
appropriate) (P) and reaction time (t).
When the FVP reaction was carried out on >100 mg scale, the
entire pyrolysate was dissolved in dichloromethane (ca. 30 cm3)
and the solution washed with aqueous sodium hydroxide (2 M,
15 cm3) to remove phenols. The neutral fraction was washed
with water (2 ¥ 15 cm3), dried (MgSO4) and the solvent re-
moved in vacuo to give the non-acidic components. Typical
products are reported here, the remainder are described in
the ESI†:
Aryl 2-(allyloxy)benzoates 5 and aryl 2-(isopropylthio)benzoates 6¢
2-Allyloxybenzoic acid13 11 (3.56 g, 0.02 mol) and thionyl
chloride (2.86 g, 0.024 mol) were heated under reflux for
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FVP of phenyl 2-(allyloxy)benzoate 5a
127.02, 125.74, 124.26, 123.33, 123.03, 121.81, 112.58 and 111.75
(consistent with a spectrum of g-brazan 38 prepared by a different
method31); dC (minor isomer) 128.20 (2C), 127.35, 125.72, 124.15,
122.82, 121.13, 118.97, 111.41 and 106.77, consistent with the
spectrum of isolated b-brazan 39 reported below.
FVP of 5a [1.106 g (5 mmol), Ti 160 ◦C, Tf 650 ◦C, P 0.005
Torr, t 40 min] provided phenol (0.03 g, 6%) and dibenzofuran 19a
◦
(0.52 g, 62%) mp 79–81 C (from ethanol) (lit.,29 83–84 ◦C); dC
156.02 (quat), 126.97, 124.06 (quat), 122.52, 120.49 and 111.50.
On a preparative scale [0.448 g (1.5 mmol), Ti 130 ◦C, Tf 650 ◦C,
P 0.001 Torr, t 60 min], work-up afforded both isomers (0.30 g,
91%) and the minor isomer was isolated by recrystallisation to
constant melting poi◦nt and thus identified as b-brazan 39 (0.061 g,
19%), ◦mp 201–202 C (from ethanol–acetic acid) (lit.,32 209.2–
209.8 C); dC 157.55 (quat), 154.76 (quat), 132.96 (quat), 130.08
(quat), 128.22 (2C), 127.65, 125.66, 125.33 (quat), 124.17, 123.81
(quat), 122.63, 121.15, 119.00, 111.44 and 106.80, consistent with
published data.33
FVP of phenyl 2-(allylthio)benzoate 6a
FVP of 6a [1.037 g (4 mmol), Ti 150 ◦C, Tf 650 ◦C, P 0.001 Torr, t
75 min] gave dibenzothiophene 20a as a white solid (0.62 g, 88◦%),
mp 96–99 ◦C (from ethanol), mixed mp 96–98 ◦C (lit.,29 98–100 C)
dC 139.21 (quat), 135.32 (quat), 126.45, 124.11, 122.55 and 121.34,
compatible with published data.9
FVP of 3-pyridyl 2-(allyloxy)benzoate 30
Acknowledgements
FVP of 30 [0.831 g (4 mmol), Ti 150 ◦C, Tf 650 ◦C, P 0.001
Torr, t 60 min] gave, after work-up a 3 : 1 mixture of isomers
(0.37 g, 55%), which can be distinguished by 13C NMR (DEPT)
spectroscopy: benzofuro[3,2-b]pyridine 31 (major); dC 144.99,
129.01, 123.39, 121.06, 120.95, 118.39 and 111.95 (consistent with
reported data17); benzofuro[2,3-c]pyridine 32 (minor); dC 142.77,
134.20, 129.74, 123.22, 121.88, 114.97 and 112.25 (consistent with
reported data17).
The picrate mixture was made as follows:30 picric acid (wet with
ethanol) (0.6 g, 2.6 mmol) was dissolved in acetone (0.6 cm3)
and added to the isomeric mixture (0.21 g, 1.2 mmol). The
addition of ether (6 cm3) completed the crystallisation of the
products (0.74 g, 72%). The picrate of the minor isomer 32 was
partially purified by recrystallisation (0.26 g, 25%), mp 204–207 ◦C
(from nitromethane) (lit.,18 240–241 ◦C). In solution one drop of
trifluoroacetic acid was added to maintain the protonation dH (d6-
DMSO) 9.63 (1H, s), 8.92–8.81 (2H, m), 8.58 (2H, s), 8.54 (1H,
m), 8.04 (1H, m), 7.95 (1H, m) and 7.65 (1H, m); the singlet at dH
9.63 defines the regiochemistry of this product.
We are grateful to British Petroleum for a research studentship
(to M. B.).
Notes and references
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FVP of 1-naphthyl 2-(allyloxy)benzoate 34
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