Formation of benzo[ghi]fluoranthenes by palladium catalyzed intramolecular coupling

Article abstract of DOI:10.1016/S0040-4039(99)01940-1

A novel synthesis of benzo[ghi]fluoranthenes by palladium mediated coupling is reported. The high yields, mild reaction conditions and functional group tolerance are an advantage of this method.

Full text of DOI:10.1016/S0040-4039(99)01940-1

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 41 (2000) 285–288
Formation of benzo[ghi]fluoranthenes by palladium catalyzed
intramolecular coupling
Le Wang and Philip B. Shevlin∗
Department of Chemistry, Auburn University, Auburn, AL 36849-5312, USA
Received 5 August 1999; revised 2 October 1999; accepted 4 October 1999
Abstract
A novel synthesis of benzo[ghi]fluoranthenes by palladium mediated coupling is reported. The high yields, mild
reaction conditions and functional group tolerance are an advantage of this method. © 2000 Elsevier Science Ltd.
All rights reserved.
Stimulated by interest in fullerenes, the synthesis of polycyclic aromatic hydrocarbons (PAHs) has
become an active field. Benzo[ghi]fluoranthene, 1, a sub-unit of C60 has been synthesized in low yield by
dehydrogenation of benzo[c]phenanthrene,1 and by flash vacuum pyrolysis of appropriate precursors.2,3
The major drawback of FVP is low to modest yield, the small scale involved, and the minimum functional
group tolerance. Since intramolecular palladium mediated coupling of bromides or iodides with aromatic
rings has been reported,4 palladium chemistry with its mild reaction conditions and wide functional
group compatibility is an attractive route to PAHs. Rice and Cai have reported the synthesis of several
fluoranthenes and benzofluoranthenes by a palladium catalyzed intramolecular arene–triflate coupling.5
We now report an efficient synthesis of benzo[ghi]fluoranthenes by palladium mediated intramolecular
coupling employing bromides, triflates, and chlorides as leaving groups (Scheme 1).
trans-Stilbenes, 4, were prepared by the Heck reaction from 2 and 3 in yields of 66 to 88% using
[1,10-bis(diphenylphosphino)ferrocene]palladium(II) chloride:CH2Cl2 (1:1) [Pd(dppf)Cl2] as catalyst.
Although the bromide 2g with an o-methoxy was much less reactive with this catalyst, the use of
trans-di-µ-acetatobis[2-di-o-tolyphosphino)benzyl]dipalladium(II) [palladacyles]6 with higher boiling
tributylamine in DMA gave 4g in 71% yield. (Table 1)
Table 2 illustrates the photolysis of 4 to 5 in the presence of propylene oxide and iodine in Pyrex.7 Al-
though most functional groups were tolerated in the photolysis reaction, when both of OCOPh and OMe
were present in 4g, the photolysis gave complex products possibly from the debenzoylation–oxidation
during the photolysis.8 However, using benzoxy rather than benzoate as the protecting group gave 5g in
89% yield.
∗
Corresponding author.
0040-4039/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(99)01940-1

286
Scheme 1.
Table 1
Formation of stilbenes by the Heck reaction
Table 2
Formation of benzo[c]fluoranthenes by photolysis
Triflates, 7, were obtained in excellent yield either by standard demethylation of OMe (5a and 5b) or
debenzylation of OBn (5f and 5g) followed by treatment with triflic anhydride.
Table
3 shows that the treatment of triflate or bromo benzo[g]phenanthrenes with
bis(triphenylphosphine)palladium(II) chloride [Pd(PPh3)2Cl2] in DMF or DMA gives 1 in excellent
yield.9 Contrary to previous reports in which one or more equiv. of PPh3 was needed to achieve
satisfactory yields in the presence of electron-donating substituents,5,10 the formation of 1g proceeded
well without additional PPh3.
Table 4 demonstrates the formation of benzo[ghi]fluoranthenes from chloro benzo[g]phenanthrenes.
When an electron-donating methyl group was para to chlorine, neither Pd(PPh3)2Cl2 nor palladacyles

287
Table 3
Palladium catalyzed intramolecular couplings from triflates and the bromide
Table 4
Palladium catalyzed intramolecular couplings from chlorides
gave ring closed product, although it has been reported that palladacyles were effective for cataly-
zing aryl chloride Heck reactions.6 However, when bis(tricyclohexylphosphine)palladium(II) chloride
[Pd(PCy3)2Cl2], a catalyst known to be effective for some palladium-catalyzed carbonylation11 and
Suzuki reactions involving activated aryl chlorides,12 was used, about 20% of desired product was formed
when sodium acetate was used as a base. Various organic and inorganic bases were then investigated, and
we were pleased to find that the strong organic base, DBU, brought about the conversion of 5d to 1d in ca.
90% yield. The use of tris(dibenzylideneacetone)dipalladium(0) [Pd2(dba)3] and P(t-Bu)3 as the catalyst
in DMA reduced the conversion to 50%.13 The introduction of the electron-withdrawing carbomethoxy
group para to chlorine greatly enhanced the reactivity of the chloride. Again, Pd(PCy3)2Cl2, which gave
the desired product in good yield in 12 h, proved to be superior to either Pd(PPh3)Cl2 or palladacyles that
gave a three component mixture after 20 h. The effectiveness of Pd(PCy3)2Cl2 over Pd(PPh3)2Cl3 may
result from the higher electron density and greater steric bulk of the former.14 The formation of 8e in the
reaction of 5e suggests that the palladation of the aromatic ring with either Pd(PPh3)2Cl2 or palladacyles
is slow allowing dechlorination to occur.
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