4
896
J . Org. Chem. 1997, 62, 4896-4897
Communications
An In ter estin g Ben zyn e-Med ia ted
Sch em e 1
An n u la tion Lea d in g to Ben zo[a ]p yr en e
Agust ´ı n Cobas, Enrique Guiti a´ n,* and L. Castedo
Departamento de Qu ´ı mica Org a´ nica, Universidad de
Santiago and Unidad Asociada al CSIC,
Sch em e 2
1
5706 Santiago de Compostela, Spain
Received April 7, 1997
The Diels-Alder reactionsone of the most useful
synthetic transformations in organic synthesisshas been
the subject of a huge amount of theoretical and experi-
mental work in the last 10 years. In spite of this, the
closely related cycloaddition reaction between a conju-
gated enyne and a dienophile, known as the dehydro
Diels-Alder reaction (DDAR), remains relatively un-
known.1 Scheme 1 shows a general representation of this
reaction (solid lines), assuming a concerted mechanism
through an allene intermediate (an acid-catalyzed step-
wise mechanism has been proposed for some of these
reactions).
Sch em e 3a
After more than 10 years of working on the synthesis
of natural products by means of Diels-Alder reactions
2
between styrene derivatives and arynes, we focused our
attention on the corresponding DDAR between aryl-
alkynes and arynes (Scheme 1).3 In view of the recent
advances made in the synthesis of arylacetylenes, and
the new procedures available for the generation of
benzyne, we envisaged that the DDAR might be useful
for the synthesis of policyclic aromatic hydrocarbons. To
test this hypothesis, we examined the reaction between
the readily available arylalkyne, 1,8-diethynylnaphtha-
lene (1), and benzyne, which we expected would afford
naphtho[1,2-c]chrysene (2) via a double DDAR (Scheme
a
Key: (a) (PH3P)2PdCl2, CuI, 1.1 equiv of HCCTMS, Et2NH,
5
(
8%; (b) (Ph3P)2PdCl2, CuI, 2.0 equiv of HCC-TMS, Et2NH, 42%;
c) (Ph3P)2PdCl2, CuI, 2.8 equiv of HCC-TMS, Et2NH, 30%; (d)
+
-
n-Bu4N F , THF, 95%.
lyl)acetylene. Finally, 5 was desilylated by treatment
with n-Bu
4
NF/THF to yield 1 in 95% yield.5
For the DDAR of 1 with benzyne, the latter was
generated by two alternative procedures: firstly (method
A), by the thermal decomposition of benzenediazonium
-carboxylate in a refluxing 1,2-dichloroethane solution
of diyne 1; later (method B), by room-temperature
treatment of 2-(trimethylsilyl)phenyl triflate with fluoride
ion in a THF solution of 1. In both cases, benzopyrene
6) was obtained as the only isolable product, in 23% yield
2
2
). We also considered the possibility that a formal [2 +
+ 2] cycloaddition between the alkyne units and
2
6
benzyne might occur since intramolecular interactions
between the triple bonds of compound 1 have been
reported.4
7
(
Diyne 1 was made from 1,8-naphthalene diiodide (3)
as shown in Scheme 3. Transformation of 3 into 5 was
accomplished either in two steps through iodoalkyne 4
by method A and in 30% yield by method B.
Scheme 4 shows a possible mechanism for this unex-
pected transformation. The first step is a DDAR afford-
ing strained cyclic allene 7.8 Strained 1,2-cyclohexa-
dienes have previously been proposed as intermediates
,9
or in a one-pot reaction, in both cases using Pd(PPh
Cl in the presence of Et NH and Cu to catalyze the
coupling reaction between the iodides and (trimethylsi-
3 2
) -
2
2
(5) For the preparation of 1 by Pd-catalyzed reactions of 1,8-
diiodonaphthalene and acetylene, see: Badrieh, Y.; Blum, J .; Schu-
mann, H. J . Mol. Catal. 1994, 90, 231. Compound 1 has also been
prepared by dehydrohalogenation of 1,8-bis(1′,2′-dibromoethyl)naph-
thalene; see: Mitchell, R. H.; Sondheimer, F. Tetrahedron 1968, 24,
1397.
(6) A description of the experimental procedure can be found in ref
2.
(7) Himeshima, Y.; Sonoda, T.; Kobayashi, H. Chem. Lett. 1983,
1211.
(8) For a comprehensive review of strained cyclic cumulenes see:
J ohnson, R. P. Chem. Rev. 1989, 89, 1111.
(9) The actual structure of compounds formulated as strained allene
intermediates is not obvious. The spatial structure and electronic
distribution of these strained intermediates has been studied. See: (a)
Angus, R. O., J r.; Schmidt, M. W.; J ohnson, R. P. J . Am. Chem. Soc.
1985, 107, 532. (b) J anoschek, R. Angew. Chem., Int. Ed. Engl. 1992,
31, 476.
(
1) (a) Kloetgel, M. C.; Holmes, H. L. Org. React. 1948, 4, 60. (b)
Onischenko, A. S. Diene Synthesis; Daniel Dave & Co.: New York,
964. (c) Butz, L. W.; Gaddis, A. M.; Butz, E. J . W.; Davis, R. E. J .
1
Org. Chem. 1969, 34, 1857. (d) Whitlock, H. W., J r.; Wu, E. M.;
Withlock, B. J . J . Org. Chem. 1969, 34, 1857. (e) Miller, B.; Ionescu,
D. Tetrahedron Lett. 1994, 35, 6615. (f) Danheiser, R. L.; Gould, A. E.;
Fern a´ ndez de la Pradilla, R.; Helgason, A. L. J . Org. Chem. 1994, 59,
5
514. (g) Burrell, R. C. Daoust, K. J .; Bradley, A. Z.; DiRico, K. J .;
J ohnson, R.P. J . Am. Chem. Soc. 1996, 118, 4218.
2) See, for example: Atanes, N.; Castedo, L.; Guiti a´ n, E.; Sa a´ , C.;
Sa a´ , J . M.; Suau, R. J . Org. Chem. 1991, 56, 2984.
3) For previous examples of this reaction see: (a) Dyke, S. F.;
(
(
Marshall, A. R.; Watson, J . P. Tetrahedron 1966, 22, 2515. (b) Stiles,
M.; Burckhardt, U.; Haag, A. J . Org. Chem. 1962, 27, 4715.
(4) Staab, H. A.; Nissen, A; Ipaktschi, J . Angew. Chem., Int. Ed.
Engl. 1968, 7, 226.
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