Allenes, cyclization arenes

Toste observed the aromatization of the new cycle in a different type of reaction, in which the allene had been formed in situ by acyl migration of a propargyl carboxylate. In a similar vein, cyclizations of diynes bearing propargyl carboxylates have been described to synthesize dienes and pyrones (equation 90). In the last case, a new cascade allows the intermolecular addition of electron-rich arenes. ... 

A further variation of these functionalizations of cyanoarenes is the NOCAS process [14, 15]. As shown in Scheme 14.2, path g, this involves the addition of a nucleophile (which is often the solvent) to the donor radical cation. The thus-formed neutral radical adds to the acceptor radical anion, while rearomatization by the loss of an anion leads again to an overall ipso-substitution. AUenes could be used as the donors in these reactions, as shown recently by Arnold [50]. Accordingly, the irradiation of TCB in the presence of tetramethylaUene (15) in a 3 1 MeCN/MeOH mixture afforded 1 1 1 arene-allene-methanol adduct 16 in 48% yield (Scheme 14.9, central part). Interestingly, the addition of methanol took place exclusively at the central allene carbon, while aromatic substitution occurred through the terminal carbons. co-Alkenols, in which an O-nucleophile and an easily oxidized moiety are both present, could also be used. In the latter case, the initial ET was followed by a cyclization, yielding aryl-substituted tetra-hydrofurans or tetrahydropyrans as the final products via a tandem Ar—C, C—O bond formation [51]. 

Allenes as an interesting class of unsaturated compounds have been explored in cyclizations with 2-iodoanilines and 2-iodophenols as well. In 1991, Larock and co-workers reported the cyclization of allenes with double functionalized arenes. Various five- and six-membered heterocycles were formed in high yields (Scheme 2.85). But the main problem is the long reaction time one or two days are usually needed. 

A highly electrophilic phosphite gold(I) catalyst was applied to the intramolecular allene hydroarylation reaction to give a vinyl-substituted carbocycle [17], This cyclization proceeds effectively in cases where the aryl groups are electron-rich arene nucleophiles. A dimethoxybenzene bearing an allene group furnished a vinyl-substituted tetralin derivative in 85% yield (Scheme 18.18). From the mechanistic... 

---