A R T I C L E S
Teply´ et al.
Scheme 1. Calculated Gibbs Energy Content of
cis,cis-Dienetriyne 1 and [5]Helicene 2
Scheme 2. Preparation of cis,cis-Dienetriynes 1 and 9a
feasibility of a synthetic strategy,7 we proposed the model
isomeric cis,cis-dienetriyne 1 as a potential precursor of [5]-
helicene 2.
Computational Study. For the reaction 1 f 2 in tetrahy-
drofuran, the value of ∆G0298(THF) ) -136.6 kcal‚mol-1 has
been calculated (for details, see the Experimental Section). It
can be decomposed into three significant contributions: ∆E )
-148.0 kcal‚mol-1 (difference between potential energies of
reactant and product), ∆G0298(gas phase) - ∆E ) 14.3
kcal‚mol-1 (difference in the gas-phase corrections to Gibbs
energy), and ∆Gsolv ) -2.9 kcal‚mol-1 (difference in the
solvation energies of reactant and product in tetrahydrofuran).
It can be seen that dominant energy contribution comes from
the decay of three triple bonds and the formation of three
aromatic rings.8 The calculations support the idea of “energy-
rich” intermediate 1 and demonstrate that its conversion into
[5]helicene 2 is thermodynamically highly favored.9
a Conditions: (a) TIPS-CtCH (1.0 equiv), Pd(PPh3)4 (5%), CuI (10%),
iPr2NH, 0 °C to room temperature, 2 h, 99%. (b) nBu-CtCH (2.1 equiv),
i
Pd(PPh
3)4 (5%), CuI (10%), Pr2NH, 0 °C to room temperature, 12 h, 99%.
t
(c) BuLi (2.0 equiv), ether, -90 °C, 10 min, then I2 (1.7 equiv), -90 °C,
10 min, 98%. (d) BuLi (2.0 equiv), ether, -90 °C, 10 min, then I2 (1.7
t
equiv), -90 °C, 10 min, 97%. (e) HCtCH (gaseous), Pd(PPh3)4 (5%), CuI
(10%), piperidine, room temperature, 3 h, 79%. (f) HCtCH (gaseous),
Pd(PPh3)4 (5%), CuI (10%), piperidine, room temperature, 1 h, 78%. (g)
nBu4NF (2.4 equiv), THF, room temperature, 5 min, 71%. (h) TMS-CtCH
(1.1 equiv), Pd(PPh3)4 (5%), CuI (10%), iPr2NH, room temperature, 50 min,
99%. (i) CH3ONa (2.4 equiv), methanol, room temperature, 2 h, 98%.
(6) For the syntheses of helicenes and heterohelicenes based on Diels-Alder
approach, see: (a) Phillips, K. E. S.; Katz, T. J.; Jockusch, S.; Lovinger,
A. J.; Turro, N. J. J. Am. Chem. Soc. 2001, 123, 11899. (b) Weix, D. J.;
Dreher, S. D.; Katz, T. J. J. Am. Chem. Soc. 2000, 122, 10027. (c) Paruch,
K.; Vyklicky´, L.; Katz, T. J.; Incarvito, C. D.; Rheingold, A. L. J. Org.
Chem. 2000, 65, 8774. (d) Paruch, K.; Katz, T. J.; Incarvito, C.; Lam, K.
C.; Rhatigan, B.; Rheingold, A. L. J. Org. Chem. 2000, 65, 7602. (e)
Thongpanchang, T.; Paruch, K.; Katz, T. J.; Rheingold, A. L.; Lam, K. C.;
Liablesands, L. J. Org. Chem. 2000, 65, 1850. (f) Dreher, S. D.; Katz, T.
J.; Lam, K. C.; Rheingold, A. L. J. Org. Chem. 2000, 65, 815. (g) Dreher,
S. D.; Paruch, K.; Katz, T. J. J. Org. Chem. 2000, 65, 806. (h) Fox, J. M.;
Katz, T. J.; Van Elshocht, S.; Verbiest, T.; Kauranen, M.; Persoons, A.;
Thongpanchang, T.; Krauss, T.; Brus, L. J. Am. Chem. Soc. 1999, 121,
3453. (i) Nuckolls, C.; Katz, T. J.; Katz, G.; Collings, P. J.; Castellanos,
L. J. Am. Chem. Soc. 1999, 121, 79. (j) Dreher, S. D.; Weix, D. J.; Katz,
T. J. J. Org. Chem. 1999, 64, 3671. (k) Fox, J. M.; Katz, T. J. J. Org.
Chem. 1999, 64, 302. (l) Nuckolls, C.; Katz, T. J. J. Am. Chem. Soc. 1998,
120, 9541. (m) Fox, J. M.; Goldberg, N. R.; Katz, T. J. J. Org. Chem.
1998, 63, 7456. (n) Katz, T. J.; Liu, L. B.; Willmore, N. D.; Fox, J. M.;
Rheingold, A. L.; Shi, S. H.; Nuckolls, C.; Rickman, B. H. J. Am. Chem.
Soc. 1997, 119, 10054. (o) Dai, Y. J.; Katz, T. J. J. Org. Chem. 1997, 62,
1274. (p) Nuckolls, C.; Katz, T. J.; Castellanos, L. J. Am. Chem. Soc. 1996,
118, 3767. (q) Dai, Y. J.; Katz, T. J.; Nichols, D. A. Angew. Chem., Int.
Ed. Engl. 1996, 35, 2109. (r) Shi, S.; Katz, T. J.; Yang, B. V.; Liu, L. J.
Org. Chem. 1995, 60, 1285. (s) Willmore, N. D.; Hoic, D. A.; Katz, T. J.
J. Org. Chem. 1994, 59, 1889. (t) Willmore, N. D.; Liu, L. B.; Katz, T. J.
Angew. Chem., Int. Ed. Engl. 1992, 31, 1093. (u) Liu, L.; Katz, T. J.
Tetrahedron Lett. 1990, 31, 3983.
Despite the encouraging fact that thermodynamic factors stood
on our side, the basic questions about synthetic availability/
stability of energy-rich, fully unsaturated cis,cis-dienetriynes and,
most importantly, about kinetics of the key isomerization to
helicenes under transition metal catalysis primarily still has to
be answered.
Synthesis of cis,cis-Dienetriynes. The synthesis of cis,cis-
dienetriynes 1 and 9 began from known cis-dibromide 3
(Scheme 2).10 Sonogashira coupling with TIPS-CtCH or
nBu-CtCH proceeded smoothly under Pd0/CuI catalysis with
excellent regioselectivity at the vinyl part of 3 to deliver cis-
enyne 4 and 6, respectively. To ensure further clean Sonogashira
coupling with acetylene or TMS-CtCH without participation
of the ortho enyne moiety, we converted bromide 4 and 6 to
t
iodide 5 and 7, respectively, by routine lithiation with BuLi
and subsequent quenching aryllithiums with iodine. Then, on
reaction with gaseous acetylene under Pd0/CuI catalysis, we
obtained the symmetrical cis,cis-dienetriynes 8 and 9. Smooth
desilylation of 8 with nBu4NF ensued to provide cis,cis-
dienetriyne 1. In addition, iodide 5 was treated with TMS-Ct
CH under Pd0/CuI catalysis to afford cis-endiyne 10. The
(7) Synthesis of parent pentahelicene 2 by photocyclization of stilbene-type
precursors is difficult without using a bromine auxiliaries trick, see: (a)
Liu, L.; Katz, T. J. Tetrahedron Lett. 1991, 32, 6831. For other preparations
of pentahelicene 2, see refs 5j-l,w and: (b) Bestmann, H. J.; Both, W.
Chem. Ber. 1974, 107, 2923. (c) Bestmann, H. J.; Both, W. Angew. Chem.
1972, 84, 293. (d) Mukharji, P. C.; Ray, S. B.; Ghosh, A. Indian J. Chem.
1971, 408. (e) Bestmann, H. J.; Armsen, R.; Wagner, H. Chem. Ber. 1969,
102, 2259. (f) Altman, Y.; Ginsburg, D. J. Chem. Soc. 1959, 466.
(8) The studied reaction is anisodesmic. The local electronic environments of
most of the atoms change significantly in the course of the reaction.
Therefore we carried out model calculations for the simple reaction (3C2H2
f C6H6) to ascertain the accuracy of the calculated data. It was found that
the ∆E value calculated at B3LYP/6-311++G(d,p) level (∆E ) -151.7
kcal‚mol-1) was in a very good agreement with reference CCSD(T)/6-
311++G(d,p) values (∆E ) -148.6 kcal‚mol-1). The B3LYP method
performed even better than MP2/6-311++G(d,p), which yielded ∆E )
-153.6 kcal‚mol-1. In this respect, we consider the adopted computational
scheme as reasonably accurate and estimate the error in the calculated value
(9) The difference between the potential energy ∆E ) -148.0 kcal‚mol-1 for
the simultaneous formation of three aromatic rings in 2 and the enthalpy
value ∆H ) -143.1 kcal‚mol-1 (calculated from tabulated data, see ref
24) for the formation of benzene from three acetylenes is not so significant
as expected. This incongruity may be attributed to the presence of a torsional
strain in pentahelicene 2. Its value calculated at the B3LYP/6-31+G(d)
level as the difference in thermodynamic stability of 2 and picene is 11.9
kcal‚mol-1, in agreement with the previous findings of Schulman for [n]-
helicenes and [n]phenacenes (ref 25).
(10) cis-Bromide 3 was readily available from commercial trans-2-bromocin-
namic acid in two steps, see: (a) Yasuike, S.; Shiratori, S.-I.; Kurita, J.;
Tsuchiya, T. Chem. Pharm. Bull. 1999, 47, 1108. (b) Tietze, L. F.; Noebel,
T.; Spescha, M. J. Am. Chem. Soc. 1998, 120, 8971.
of ∆G0298(THF) for reaction 1 f 2 to be 5-10 kcal‚mol-1
.
9
9176 J. AM. CHEM. SOC. VOL. 124, NO. 31, 2002