nisms, based on symmetrical and unsymmetrical coupling
between two phenalenone radical anions (2•-):
undecacyclic LPAHs which potentially could have been
formed in such a reductive “dimerization” of 4.11 Neither
the expected McMurry reaction product 6-(6H-benzo[de]-
pyrenylidene)-6H-benzo[de]pyrene (11) nor its electro-
cyclization-aromatization C38H16 product, naphth[2′,1′,8′,7′:
4,10,5]anthra[1,9,8-abcd]coronene (12),26 were found in the
reaction mixture.
The absence of any of the LPAHs related topologically
to (E)-7, (Z)-7, and 11 among the products of “dimerization”
of 3 and of 4 suggests that the reductive coupling reactions
of phenalenone-type PAKs leading to LPAHs follow the
unsymmetrical route (vide supra) of “dimerization”: a
coupling between the carbonyl carbon and peri-aromatic
carbon, â-elimination of TiOH, 1,3,5-hexatriene-1,3-cyclo-
hexadiene electrocyclic reaction, and aromatization by a
second â-elimination of TiOH (Scheme 1). The alternative
1. A symmetrical 2•- + 2•- carbonyl-carbonyl coupling
at C1 and C1′ (C1 of another molecule of 2•-), leading to
(Z)-5, followed by a successive 1,3,5-hexatriene f 1,3-
cyclohexadiene electrocyclic reaction of (Z)-5 and aroma-
tization. The formation of (E)-5 may lead to 1 only indirectly,
via a thermal (E)-5 h (Z)-5 isomerization.
2. An unsymmetrical 2•- + 2•- carbonyl-peri-aromatic
carbonyl coupling at C1 and C9′, followed by successive
â-elimination of hydroxytitanium species, 1,3,5-hexatriene
f 1,3-cyclohexadiene electrocyclic reaction of (Z)-5, and
aromatization by a second â-elimination of hydroxytitanium
species. This mechanism implies spin densities also at the
peri-positions in 2•-.
The titanium-induced reductive “dimerizations” of 3 and
of 4 may, in principle, distinguish between these two
mechanisms.
Treatment of 3 with low-valent titanium species generated
from TiCl4 and LiAlH4 in THF for 126 h gave the
overcrowded chiral LPAH tetrabenzo[a,cd,j,lm]perylene
(6)15-17 in 19% yield. Another product of the reaction was
7H-benz[de]anthracene18 (21% yield). The reaction was
highly regioselective: LPAH 6 was the only isomer among
the 12 C34H18 nonacyclic LPAHs that could have potentially
been formed in such a reductive “dimerization” of 3.11
Furthermore, neither the expected conventional McMurry
reaction products, (E)- and (Z)-7-(7H-benz[de]anthracen-
ylidene)-7H-benz[de]anthracene ((E)-7 and (Z)-7),19 nor their
electrocyclization-aromatization C34H16 products, dibenzo-
[fg,ij]phenanthro[2,1,10,9,8,7-pqrstuV]pentaphene (8) and
perylo[3,2,1,12-pqrab]perylene (9),20 were found in the
reaction mixture. An analogous low-valent titanium-induced
reductive “dimerization” of 421 afforded the overcrowded
chiral LPAH dibenzo[jk,uV]dinaphtho[2,1,8,7-defg;2′,1′,8′,7′-
opqr]pentacene (10)22-24 in 1.3% yield. The major product
of the reaction was 6H-benzo[cd]pyrene25 (37% yield).
LPAH 10 was the only isomer among the eight C38H18
Scheme 1
(14) Ueda, T.; Iwashima, S.; Aoki, J.; Takekawa, M. Magn. Reson. Chem.
1995, 33, 95.
(15) A total of 1.61 g of 3 afforded hydrocarbon 6 as yellow needles,
285 mg, 19% (38%, based on consumed 3): mp 335-340 °C (lit.16 mp
333-334 °C); 1H NMR δ ) 9.17 (d, J ) 9.2 Hz, H8, H17), 9.02 (d, J ) 6.9
Hz, H3, H12), 9.01 (d, J ) 7.7 Hz, H7, H16), 8.95 (d, J ) 7.9 Hz, H4, H13),
8.28 (d, J ) 7.8 Hz, H1, H10), 8.15 (d, J ) 10.0 Hz, H9, H18), 8.13 (t, J )
7.7 Hz, H2, H11), 7.82 (q, J ) 7.1 Hz, J ) 8.0 Hz, H5, H14), 7.77 (q, J )
7.0 Hz, J ) 8.0 Hz, H6, H15) (cf. ref. 14); 13C NMR δ ) 131.48 (C3b,
C
C
12b), 131.14 (C18a, C9a), 130.95 (C7, C16), 130.21 (C7a, C16a), 129.89 (C3a,
12a), 127.39 (C5, C14), 127.07 (C8, C17), 126.65 (C9, C18), 126.61 (C6,
C15), 126.53 (C1, C10), 126.46 (C2, C11), 125.23 (C7b, C16b), 125.17 (C7c,
16c), 123.60 (C9c, C18c), 123.86 (C9b, C18b), 123.86 (C4, C13), 120.74 (C3,
C
C12) (cf. ref. 14).
(16) Zinke, A.; Ott, R.; Weinhardt E. Monatsh. Chem. 1950, 81, 878.
(17) Kohno, Y.; Konno, M.; Saito, Y.; Inokuchi, H. Acta Crystallogr.
1975, B31, 2076.
(18) Bally, O.; Scholl, R. Ber. Dtsch. Chem. Ges. 1911, 44, 1656.
(19) The alkenes (E)-7 and (Z)-7 have previously been reported, but their
structures have not been characterized.16
(20) Clar, E.; Fell, G. S.; Ironside, C. T.; Balsillie, A. Tetrahedron 1960,
10, 26.
(21) (a) Bradley, W.; Sutcliffe, F. K. J. Chem. Soc. 1951, 2118. (b)
Fujisawa, S.; Oonishi, I.; Aoki, J.; Iwashima, S. Bull. Chem. Soc. Jpn. 1976,
49, 3454.
(22) Clar, E.; Kelly, W.; Robertson, J. M.; Rossmann, M. G. J. Chem.
Soc. 1956, 3878.
symmetrical route would be less favored, due to significant
overcrowding (fjord regions) existing in the intermediate
alkenes (E)-7, (Z)-7, and 11. It is noted that the regioselec-
tivity revealed in the syntheses of 6 indicates not only a
Org. Lett., Vol. 1, No. 9, 1999
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