Arynes trimers

The generation of the aryne (31) from the sodium derivative 237 or the halocarboxylate 238 has also been attempted without apparent success. However, treatment of the 3-chloro compound 233 with NaOH at 200°C leads to the aryne adduct 232 with tetracyclone (151), to oxindole 239 in up to 31% yield by cine-substitution, and to an aryne trimer (240) in up to 16% yield. As already mentioned, the fact that 233 alone reacts with tetracyclone (151) to give the adduct 232 suggests that an addition-elimination rather than an aryne process is involved in this reaction. The cine-substitution reaction of... 

As in the case of the corresponding anhydride (23) (Section III.1.C), didehydromaleimide (30) probably does not qualify as a true aryne, but is expected to be sufficiently similar in properties to be covered in this review (Section I.2.C). This species (30) was first suggested as an intermediate along with the 1,2-dithiete (274) in the decomposition of the dithiin (275) in pyridine at 120°C to explain the formation of the aryne trimer (276). When this reaction was carried out in the presence of cyclopentadiene (146b), however, the failure to obtain the appropriate aryne adduct (277) was rationalized as being due to its presumed instability. The only products obtained from this reaction, as well as from butadiene (146c) and anthracene (147), were Diels-Alder adducts such as 278 and 279 of the dithiin (275) and the dithiete (274), respectively. 

Finally, thermolysis of the A/ -phenyldiiodomaleimide (280c) did not give any aryne trimer (282), but only a polymer 283 whose empirical formula was that of the aryne (284). By analogy with the related diiodomaleic anhydride (210) (Section IILl.c) there is, therefore, no reason to postulate the formation of the aryne (284) in this reaction. In fact, as was concluded for the oxaarynes (Section III. 1), there is no evidence at present for the formation of any azaaryne intermediate. 

The synthesis of HBT (73), which contains three [5]helicene units, illustrates the power of the cyclotrimerization of polycyclic arynes for the synthesis of he-licenes. More examples are shown in Table 2. Again, Pd2(dba)3 is the catalyst of choice for trimerization of the asymmetric arynes 77-79, which are generated from the corresponding o-(trimethylsilyl)aryl triflates 74-76. In the reactions of 1,2-didehydronaphthalene (77) and 1,2-didehydrophenanthrene (78), mixtures of regioisomers are obtained, whereas 84 is the only isomer isolated from the cyclotrimerization of 79. Compounds 80 and 82 contain a [5]helicene unit, while compound 84 is the first example of a double helicene formed by a pen-tahelicene and a heptahelicene with two rings in common. 

The palladium-catalyzed cyclotrimerization of arynes can also be used to construct flat hydrocarbons with extended conjugation, such as hexabenzotri-naphthylene (supertriphenylene, 87). In this case the required aryne 86 was generated in the presence of palladium by treatment of triflate 85 with tetra-butylammonium fluoride, and afforded trimer 87 in 20% isolated yield from 85 (Scheme 18) [51]. As in other cases (see below), the use of a soluble fluoride source was necessary because the extended planar reaction product is extremely... 

Pyrolysis of silver phthalates (122) also leads to products which have suggested the intermediacy of either arynes or the related diradicals (le). Mercuric phthalates (123) lose COj in a stepwise manner to give the trimeric phenylmercury carboxylates (124) n = 3) and the o-phenylenemercury trimers (125) n = 3) before going on to aryne-type products. Although it has been claimed that the monomeric form of 124 decomposes to benzyne (1), both the trimer and hexamer of 125 apparently do not. 

In 1998, Perez, Guitian, et al. disclosed the Pd-catalyzed trimerization of arynes [89], demonstrating that these intermediates are able to undergo transition metal-catalyzed transformations and so, greatly expanding the potential of synthetic applications of arynes. The success of these processes depends on both the metal catalyst and the method of the aryne generation. 

The first metal-catalyzed reaction of arynes was described by Guitian in the late 1990s consisting in the cyclotrimerization of benzyne to yield triphenylene 100 (Scheme 12.51) [89]. These and other authors have extended this process to the trimerization of a variety of arynes, leading to polycyclic aromatic hydrocarbons (PAHs). 

In palladium-catalyzed [2 - - 2 - - 2] cycloaddition, arynes can be used as alkyne components. In 1994, Pena, Romero, and co-workers reported palladium-catalyzed homo-[2- -2- -2] cycloaddition of arynes (Scheme 6.9) [12]. The reactions of 2-silylaryl trifluoromethanesulfonates 26 with CsF generated the corresponding arynes 27, which were trimerized in the presence of a catalytic amount of Pd(PPh3)4 or Pd2(dba)3 to afford substituted triphenylenes 28. In the reactions of unsymmetric arynes, moderate to high regioselectivities were observed. A mechanism via pallada-cycle intermediates, generated through the oxidative cyclization of two molecules of arynes, was proposed. 

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