Synthesis of PAHs

The synthesis of PAHs is generally thought not to occur within the clouds themselves, but in the envelopes of old carbon-rich stars. These stars possess large envelopes in which material expands outward and cools as it expands. In the inner... 

Scheme 4. The two-step procedure for the synthesis of PAHs (HBC is given as an example). Hexaphenylbenzenes are afforded in the first step either by cobalt-catalyzed eye-lotrimerization of suitable diphenylethynylenes 22 or by...

Scheme 11. Hexa(dodecyloxy)hexaphenylbenzene 41 is used as a precursor in a template-mediated synthesis of PAHs.

The topic Polycyclic Aromatic Hydrocarbons has been reviewed by different authors in the last decades [26]. The most complete summary of classical synthetic routes to PAHs may still be found in Eric Clar s Polycyclic Hydrocarbons [26a]. Most recently, Ronald G. Harvey reviewed the syntheses and characteristics of PAHs up to seven rings in a very comprehensive fashion [26 f] in his book Polycyclic Aromatic Compounds . Recent advances in the synthesis of PAHs are published in all major journals of organic chemistry, in Polycyclic Aromatic Hydrocarbons [27], and presented during the regular ISNA [28] and ISPAC [29] conferences. 

This contribution will focus on new developments in the key step for the synthesis of PAHs the C-C bond formation as one of the most fundamental and challenging processes in organic and organometallic chemistry. In addition, these new synthetic approaches often give access to substituted aromatic structures. The authors attempt to cover the literature until the end of 1996. Important contributions in the synthesis of PAHs during the work on this review in the beginning of 1997 will also be considered whenever possible. The functionalization of aromatic compounds [10] and the chemistry of fullerenes [18] are beyond the limits of this contribution. As mentioned above, this review will concentrate on modern routes to preferably large condensed PAHs. Classical approaches towards aromatic hydrocarbons like the Pschorr, Elbs, Hayworth,... 

Although the [2 + 2] photocycloaddition is preferably prevented in the synthesis of PAHs, it has been proven to be a very feasible approach in the selective synthesis of syn-[2.2]cyclophanes [86,87]. Topological reaction control in solution has been achieved for a multistep intramolecular [2 + 2] cycloaddition reaction in the photochemical formation of [n]-ladderanes from pseudo-gem-bis(polyene) substituted [2.2]paracyclophanes [88]. The probably most well-known example of an intramolecular photocycloaddition process is one of the... 

Additional Reductive Coupling Reactions for the Synthesis of PAHs... 

The palladium-catalyzed arylation of alkenes and arenes offers one of the conceptually most intelligent solutions for the synthesis of PAHs from the appropriate aryl halides or triflates. The reaction is usually carried out in a polar solvent (AT,AT-dimethylformamide (DMF) or AT,AT-dimethylacetamide (DMA)) at relatively high temperatures (100-170°C) with a base to trap the acid formed, and a phase transfer catalyst. Most often Pd(II) acetate or a Pd(II) complex are used, that are converted to a catalytically active Pd(0) species in the course of the reaction. The mechanisms of these Heck reactions have been discussed widely throughout the pertinent literature [78,79]. 

Scheme 13.10 Synthesis of PAHs with zigzag peripheries.

The addition of organometallic compounds to benzoquinone derivatives was an early approach for the synthesis of PAHs. The diol obtained was subsequently aromatized and cyclodehydrogenated to give the PAH [19-22]. An example of such an approach is depicted in Scheme 3. Clar et al. synthesized a cata-hexabenzocoronene 9 by this method. 

The Diels-Alder reaction represents one of the most efficient methods for assembling six-membered carbon cycles. As this structural element is the characteristic of graphene-type structures, Diels-Alder reactions have been applied successfully for the synthesis of PAHs such as acene and coronene. 

Arynes are also highly suitable dienophiles for the synthesis of PAHs via Diels-Alder reactions [27, 28]. For example, Kitamura et al. reported a new hypervalent iodide precursor 27 for the in situ generation of naphthyne, which can be trapped in a Diels-Alder reaction to form PAHs, such as 28 and 29 (Scheme 8) [29]. 

Scheme 8 Synthesis of PAHs 28 and 29 using an aryne precursor [29]...

The synthesis of PAHs was also achieved by an intramolecular Diels-Alder reaction of an ene-diene stUbene derivative 32 (Scheme 10) followed by cyclodehydrogenation [31, 32],... 

Scheme 10 Synthesis of PAH 34 via intramolecular Diels-Alder reaction [31, 32]...

In all cases presented above, annulation reactions were limited to intramolecular arrangements. There are also transformations involving two independent reaction partners. In 2005 a Cr-mediated synthesis of PAHs from halobiaryls was published [94]. The 2,2 -dihalobiaryl starting material 150 is first treated with BuLi before addition of the Cr reagent. Final treatment with an acetylenic reaction partner delivers the phenanthrene core 151 (Scheme 38). Recently, Wu and coworkers presented a Pd-catalyzed version starting from 2,2 -diiodobiphenyls to access substituted phenanthrenes [95]. 

SCHEME 16.40 Synthesis of PAH through cycloaddition/dehydrogenation method. 

Scheme 3.4. Examples for the synthesis of PAHs via intramolecular cydodehydration steps.

In the 1960s and 1970s, the synthesis of PAHs by photocyclization was frequently investigated. For example, benzoperylene (119) can be obtained from several precursors under light irradiation with/without iodine (Scheme 8.60). 

In the enantioselective synthesis of (+)-pah.sadin B 46, (- -)-12-hydroxy-palisadin B 47, (+)-palisadin A 48, and (—)-aplysistain 49, the key bromide intermediate 45 was formed with this method. Treatment of epoxide 44 with LiBr and pyridinium p-toluenesulfonate (PPTS) in A-methyl-2-pyrrolidone (NMP) led to the corresponding bromohydrin 45 (Scheme 42.15). 

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