3432
J . Org. Chem. 1997, 62, 3432-3433
Ta ble 1. Diels-Ald er Cycloa d d ition of
Cyclop en ta d ien on es 1a -c w ith Alk yn es 5a -i
Syn th esis a n d X-r a y Ch a r a cter iza tion of a n
Octa a lk yn yld iben zoocta d eh yd r o[12]-a n n u len e
J ohn D. Tovar, Norbert J ux, Thibaut J arrosson,
Saeed I. Khan, and Yves Rubin*
Department of Chemistry and Biochemistry, University of
California, Los Angeles, California 90095-1569
Received February 27, 1997
We have recently reported a straightforward synthesis
of a variety of tetraalkynylcyclopentadienones (1).1 In
view of the inverse-demand Diels-Alder reactivity of
cyclopentadienones,2 we were interested in the use of
these systems for the preparation of 1,2,3,4/5,6-differen-
tially protected hexaalkynylbenzenes (HEBs, 2), which
are otherwise not accessible by known methods.3,4 Dif-
ferentially protected HEBs are precursors of well-defined
molecular fragments (e.g., 3b, 4) representing novel
planar carbon networks in which the graphitic texture
is extended by acetylene or butadiyne units (graphynes).5
Some of these networks are predicted to be semiconduc-
tors having large third-order nonlinear susceptibilities.6,7
Their synthesis is out of the reach of current method-
ology;8 however, their properties can be investigated on
smallerseven though sizablesfragments such as com-
pounds 3b and 4.4
dienone
yield
(%)
1
R
alkyne
R1
R2
product
1a
TIPS
5a
5b
5c
5d
5e
5f
5g
5h
5i
Et
Ph
Ph
Et
Ph
6a
6b
6c
6d
6e
6fc
6g
6h
6i
84a
30a
CtCPh
CO2Me
CH2OH
58a
CO2Me
H
CtC-TMS CtC-TMS
92a
74a
55d (12)a
46b
1b
t-Bu
Br
CO2Et
CH(OEt)2
CtC-TMS
CHO
TMS
61a
20a
5f
5f
CtC-TMS CtC-TMS
CtC-TMS CtC-TMS
6jc
6k
41d (9)a
a,e
1c
CtC-TIPS
a
b
Toluene, reflux, 1.3-2 equiv of alkyne. Xylenes, reflux. c R1
d
) TMS, R2 ) CtCCtC-TMS. Neat TMS(CtC)3TMS (20-40
equiv), 100 °C. e Complex mixture.
The synthesis and characterization of the cyclic dimer
3a , obtained in pure form next to trimer 3b and tetramer
3c by Hay coupling of the 1,2-diacetylene 2a , is described
in this report. The Diels-Alder reactivity of the dienones
1a -c was first investigated on several alkynes (5a -i,
Table 1). Heating the dienones in the presence of a slight
excess of alkyne afforded tetraalkynyl- and pentaalkynyl-
benzenes in good yields, except for alkynes with strong
steric requirements (6b,f,i,j).
Since each alkyne moiety of 1a -c can act as its own
dienophile to give dimers or higher oligomers, it is
remarkable that the adducts 6a -j are formed with such
fidelity. Indeed, in the lower yielding reactions of dienone
1b, a single purple dimer (7a or 7b) was formed com-
F igu r e 1. HOMO and LUMO frontier orbitals for 1b (PM3).
petitively as a byproduct.9 As judged by the larger size
of the HOMO coefficients (PM3) and reduced steric
hindrance at carbons a ,b, compared to carbons c,d
(Figure 1), it is most likely that the dimerization product
is regioisomer 7a , although a definitive structural proof
will require X-ray characterization. Dimer formation was
largely inhibited by running reactions in the melt of
excess dienophile (6f,j).
Interestingly, bis(trimethylsilyl)hexatriyne (5f)10 did
not add to dienones 1a ,b across its central, least sterically
hindered CtC bond11 to give the desired C2v-symmetric
HEBs, but rather at one of the two peripheral triple
bonds to give 6f and 6j. This can be understood as a
result of the large steric repulsion created between the
TMS and t-Bu/TIPS groups at the transition state (T.S.,
Figure 2);12 an unsymmetrical approach is more favor-
able. Since the more extended diynyl system of the green
(1) J ux, N.; Holczer, K.; Rubin, Y. Angew. Chem., Int. Ed. Engl. 1996,
35, 1986-1990.
(2) Ogliaruso, M. A.; Romanelli, M. G.; Becker, E. I. Chem. Rev.
1965, 65, 261-367.
(3) (a) Diercks, R.; Armstrong, J . C.; Boese, R.; Vollhardt, K. P. C.
Angew. Chem., Int. Ed. Engl. 1986, 25, 268-269. (b) Boese, R.; Green,
J . R.; Mittendorf, J .; Mohler, D. L.; Vollhardt, K. P. C. Angew. Chem.,
Int. Ed. Engl. 1992, 31, 1643-1645. (c) Praefcke, K; Kohne B; Singer,
D. Angew. Chem., Int. Ed. Engl. 1990, 29, 177-179. (d) Kondo, K.;
Yasuda, S.; Sakaguchi, T.; Miya, M. J . Chem. Soc., Chem. Commun.
1995, 55-56.
(4) Anthony, J . E.; Khan, S. I.; Rubin, Y. Tetrahedron Lett. 1997, in
press.
(9) See the accompanying paper: Tobe, Y.; Kubota, K.; Naemura,
K. J . Org. Chem. 1997, 62, 3430-3431.
(5) (a) Diederich, F.; Rubin, Y. Angew. Chem., Int. Ed. Engl. 1992,
31, 1101-1123. (b) Diederich, F. Nature 1994, 369, 199-207.
(6) Baughman, R. H.; Eckhardt, H.; Kertesz, M. J . Chem. Phys.
1987, 57, 6687-6699.
(7) Nalwa, H. S. Adv. Mater. 1993, 5, 341-358.
(8) See, for example: Feldman, K. S.; Mareska, D. A.; Weinreb, C.
K.; Chasmawala, M. J . Chem. Soc., Chem. Commun. 1996, 865-866.
(10) Rubin, Y.; Lin, S. S.; Knobler, C. B.; Anthony, J .; Boldi, A. M.;
Diederich, F. J . Am. Chem. Soc. 1991, 113, 6943-6949.
(11) Rubin, Y.; Knobler, C. B.; Diederich, F. J . Am. Chem. Soc. 1990,
112, 4966-4968.
(12) The PM3 calculated T.S. (Spartan 4.0) gave one imaginary
frequency at 591.2 cm-1 by vibrational analysis.
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