In the synthetic methods used for both the coronene derivatives and the nitrogen containing heterosuperbenzenes discussed in §1.1 the key step is the oxidative carbon- carbon bond formation between two aromatic rings (Scheme 1.5).
[ox.]
-H2
Scheme 1-5: The general form of cyclodehydrogenation reactions.
A number of possible metal based oxidants have been reported. Initial work on dehydrogenation reactions carried out by Copland and McNeil used a palladium- platinum-charcoal catalyst in a hydrogen atmosphere to oxidise a naturally occurring naphthalene derivative at high temperature (Scheme 1-6).32
(i)
(i) Platinum/Palladium/Charcoal, H2 gas, 3 hours, 490°C.
In the 1960’s Kovacic examined the oxidation of benzene to para-polyphenyl at room temperature using a mixture of aluminium chloride and copper chloride.33 Soon afterwards it was also shown that the same oxidation could be achieved using iron trichloride.34 The oxidation mechanism (Scheme 1-7) resembles that of a catalysed olefin polymerisation,35 for which an acid is required.
H+ H H H H H H Metal -2H H H H H Metal -2H H H n -H+ n Scheme 1-7: Acid and metal catalysed polymerisation of benzene.
In this polymerisation the initiation step is the protonation of a phenyl ring to give the reactive cation, this then undergoes propagation with another benzene ring to form a biphenyl cation. The non-cationic ring is then oxidised by the metal to regain its aromaticity with the loss of hydrogen bound to the metal, the cation on the other ring then undergoes reactions so that the polymerisation is further propagated. This cationic mechanism is only possible in the presence of a metal catalyst. Lewis acid catalysts such as iron trichloride or aluminium trichloride are used as they form weak Bronsted acids in dichloromethane solution and can act as the oxidant in the reaction. The reaction is quenched by the removal of the cationic species by the addition of methanol or a weak base.
In the work of King et al. the oxidation of smaller arene systems was studied. Here by examining the effect of changes in the substituent groups, the directed oxidation of
smaller oligophenyl arenes was achieved.36 For small unsubstituted oligophenylenes such as ortho-terphenyl I.18 (Scheme 1-8) oxidation results in the formation of triphenylene
I.19 in a 20% yield. An insoluble powder was also formed in a 75% yield which was found to be a mixture of products (I.20 and I.21) on which annulation has occurred; a small amount (< 5%) of larger fused polyphenylenes was also observed.
FeCl3
+
+
I.18 I.19 I.20 I.21
Scheme 1-8: Dehydrogenation products observed for the oxidation of ortho-terphenyl.
The low concentrations of the larger fused polyphenylenes was due to the insolubility of the species I.20 and I.21, indeed the limiting factor in these oxidative coupling reactions is the solubility of the first products formed. In reactions where the fused products are insoluble further reactivity is not observed. These conditions have been used in the work of Mullen to prepare coronenes from their polyaromatic phenylene precursors, in these systems the oxidation leads to the formation of coronene systems which cannot undergo further oxidations due to their insolubility. Both aluminium and iron oxidation catalysts have been used in the synthesis of such systems in a condensation called the Scholl reaction (Scheme 1-9).37-38 R R R R R R FeCl3 or AlCl3/CuCl2 R R R R R R R= H, tBu, n-alkyl
The mechanism for the Scholl reaction of hexaphenylbenzene has been modelled computationally by King et al.39-40 In these studies two possible mechanisms for the reaction were proposed, the first goes via aromatic cation intermediates similar to those seen for the polymerisation of benzene and the other goes via radical cations. The aromatic cation mechanism (Scheme 1-10) is the more likely with each bond formation lowering the energy required for the next in a concerted cascade effect.
BH+ H H -B H H H H +B -BH + -H+ [ox.]
Scheme 1-10: The cationic Scholl mechanism for the oxidative dehydrogenation of hexaphenylbenzene. In the case of the N-HSB systems, nitrogen atoms in the periphery increase the possibility of the formation of stable, partially cyclised systems such as N-1/2HSB I.5. There are three factors contributing to this: (i) protonation of the nitrogen atoms makes the pyrimidine rings more electrophilic, such that cationic bond formation involving pyrimidines are easier than those involving only phenyl groups. (ii) The nitrogen- protonated PAH core is a poor nucleophile making it difficult for it to attack ortho phenyl groups. (iii) The basicity of the pyrimidine nitrogens makes the protonation of the phenyl groups more difficult, again hindering the formation of bonds not involving the pyrimidine rings.
The conditions employed for cyclodehydrogenation to give HBC compounds are similar to those used for Friedel-Crafts reactions. In recognition of this Rathore and Burns have shown that cyclodehydrogenation can be combined with Friedel-Crafts alkylation to give substituted HBC systems in high yields (Scheme 1-11).41
(i)
(i) tBuCl, FeCl3, MeNO2, 22°C
Scheme 1-11: Synthesis of a substituted HBC reported by Rathore and Burns.41