Aryne-Diene adducts

The failure to obtain such a furan adduct from an aryne precursor which yields other aryne-diene adducts (148) does not necessarily indicate the absence of arynes, however. One example of precisely this phenomenon was traced to the unique ability of furan to preferentially consume the aryne precursor 411 in a nonaryne pathway leading to 2-phenylfuran (172). Other examples are due to the instability of the benzyne-furan adduct 148a to strong bases. ... 

This new benzyne precursor combines practical facility in its preparation with relative stability and safety in handling in addition, its conversion into benzyne does not require a strong base or high temperature. Another useful feature of this methodology is that, as with substituted diphenyliodonium 2-carboxylates, only one aryne can be formed from substituted precursors. Indeed, two methyl analogues of 2-trimethylsilyl-diphenyliodonium triflate, in the presence of furan, afforded quantitatively the corresponding adducts high yields of benzyne-diene adducts were also obtained with other 1,3-dienes. 

Photolysis or thermolysis of diarylzirconocenes or arylalkylzirconocenes lead to reactive aryne-zirconocene complexes which can be trapped with several types of electron-donating ligands including dienes, phosphines, and carbon monoxide [13-15]. After being postulated by VoPpin et al. [14] and Erker [13], benzyne-zirconocene 1 (Eq. 1) was fully characterized by Buchwald et al. [16] by X-ray crystallographic studies of its trimethylphosphine complex adduct ... 

The first steroidal examples of Diels-Alder reaction in which the dienophile is an aryne have been reported in fulL Thus, cholesta-2,4-diene affords two adducts with benzyne, in low yield, as the result of both a- and -face attack... 

Thiophenes undergo Diels-Alder reactions, but their diene reactivity is lower than that of furans. The [4+2] cycloaddition occurs, therefore, only with very reactive dienophiles (arynes and alkynes with acceptor substituents) or under high pressure (example 3). 1,2-Disubstituted benzene derivatives 2 are formed with alkynes, as the primary Diels-Alder adduct 1 eliminates sulfur ... 

Styrenes may give Diels-Alder adducts with arynes, the exo-cyclic double bond being part of the diene moiety Yields are generally rather low. The competition between [4+2] cycloaddition and Michael addition of benzyne (generated from chlorobenzene and... 

Di- and tri-aryne equivalents other than explored. Bistriazole 466 was synthesized as a with lead tetraacetate (LTA) in the presence of various dienes give bis-adducts in good yield (mainly 70-90%, 17 examples). The main advantages over tetrahalobenzenes 440 is that dienes with functionality that might not survive BuLi can be tolerated, as shown in examples 467-469 (the latter a bis-l,3-dipolar cycloaddition). The regioselectivity in these cycloadditions is also striking. The major disadvantage is that precursor 466 requires a multistep synthesis. 

Bis-arynic reagents in which the two arynes are in separate, insulated arene rings have also been developed. For example, bis-anthranilic acid 478 is a synthetic equivalent for diaryne 479 aprotic diazotization of 478 in the presence of various dienes (i.e., tetraphenylcyclone, 2,5-dimethylfuran, anthracene) gave the expected bis-adducts in 40-... 

The benzyne created from 2-(trimethylsilyl)phenyl triflate was used in a highly diastereoselective aryne Diels-Alder reaction with a diene bearing Oppolzer s sultam. This approach to cis-functionalized 1,4-dihydronaphthalenes was reportedly the first aryne Diels-Alder reaction to provide enantioenriched cyclo-adducts. An unusual route to /3-aminoketones involves the treatment of 2-(trimethylsilyl)phenyl triflate with TBAF and an asymmetric vinyldihydropyridone. The resultant aryne Diels-Alder cycloadduct undergoes aromatization/eUmination to create the )V-acyl-/3-aminoketone (eq 13). This method was featured in a multistep synthesis of an unnatural a-amino acid. 

The retro-Diels-Alder reaction has found considerable use in the preparation of reactive molecules and intermediates, particularly under flash vacuum thermolysis conditions.Application of this methodology to the adducts (85) of arynes and various dienes (Section II.2.B.b) does not generally regenerate the aryne and the diene but either gives the isobenzo derivative 86 and an alkyne by an alternate retro-Diels-Alder reaction, or... 

The well-advised caution with which the above traps should be utilized has led to other traps being suspected of leading to false aryne adducts as well. Most prominent in this category has been the first such diene trap to be discovered, furan (146a). Gilman considered and presented cogent arguments... 

A closely related reaction is observed with chloromaleic anhydride (218), which gives the apparent aryne adduct 216 with tetracyclone (151) but stops at the Diels-Alder adduct 219 with other dienes such as cyclopentadiene (146b), butadiene (146e), and anthracene (147). Not only does this result suggest the absence of an elimination-addition mechanism via the aryne (23) in this system. 

What remains to be considered is whether the adducts 390 might arise by an addition-elimination process via 397 rather than an elimination-addition mechanism. The variety of dienes (146) used and their apparent lack of ambiguity as aryne probes (Sections II.2.B.C and II.2.B.d) is strong evidence (Section III.l.C) in favor of the latter mechanism unless it is argued that the anhydride 229 is an unusually powerful dienophile such as the halobenzo-quinones (173) (Section II.2.B.d). In effect 229 would have to behave like a... 

CO, then the alleged aryne adduct (390) could be formed via an adduct such as 401 and hence without the intervention of the aryne (4). Although such dienophilic reactivity has not been detected for benzocyclopropenones (33), even in the presence of several traditional dienes (146), it has been observed for cyclopropenone itself (but not for diphenylcyclopropenone) and for benzocyclopropene (402). The second step of the proposed sequence 400 to... 

Although research on the FVT of the thiophene anhydride 229 has been primarily directed at obtaining evidence for the intermediacy of the aryne 4, it has also yielded some information on the chemistry of this species. The most studied reaction of thiophyne (4) thus far is with dienes 146 to give thianaphthenes 293, presumably via the Diels-Alder adducts 390. The ubiquity of this reaction, as discussed above, appears to make it virtually diagnostic for the presence of the aryne (4). Thus the formation of thianaphthene (293) when benzene is the diene has been interpreted to involve a retro-Diels-Alder loss of acetylene from the initially formed thiophyne-ben-zene adduct (390c). Conversely the absence of thianaphthene (293) in the photolysis of the mercury compound 286 in benzene (Section III.3.A.a) strongly suggests that the aryne 4 is not produced in this reaction. 

The claim for this aryne is based on the isolation of the appropriate adducts 631 or 632 when a crown-ether complex of the diazonium carboxylate potassium salt 633 is decomposed in the presence of tetracyclone (151) or the diphenyltetrazine (169). As pointed out in Section II.2.B.C, however, both of these diene traps may be ambiguous as aryne probes because of their tendency to react with the aryne precursors by an addition-elimination process via 634 and 635 rather than by an elimination-addition mechanism involving the aryne 617. The failure of several other dienes with unambiguous records as aryne traps (Section II.2.B.b.d), such as anthracene (147), to give aryne adducts with the precursor 633 tends to support the former mechanism. On the other hand. 

Diphenylisobenzofuran proves to be a very reactive 1,3-diene in (4 + 2)-cycloadditions. It is used to trap unstable alkenes, alkynes, or arynes (e.g., dehydrobenzene, cf p. 67) with formation of DiELs-ArDER-adducts, for example, 6 ... 

Aryne Annulation. Treatment of o-(trimethylsilyl)aryl triflates with TBAT results in the formation of ben-zyne intermediates. Fluoride-induced 1,2-eliminations of o-(trimethylsilyl)aryl triflates, which are readily prepared from phenols, have been shown to provide the respective aryne products, which, at room temperature, precipitously undergo cycloaddition in the presence of Diels-Alder adducts, including, but not limited to, tethered acyclic dienes, tethered enynes (eq 15), azomethine imines (eq 16),ene carbamates, and acetamidoacry-lates (eq 17). ... 

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