Iron benzannulations

Cyclotrimerization of triynes has been reported using cobalt or iron complexes of NHCs without CO and Cp ligands.31 Benzannulated compounds have been isolated with up to quantitative yields. 

Iron(lll)-catalyzed benzannulation of 2-phenacylbenzaldehyde has been reported (Scheme 16.44) [49]. This reaction provides high substrate scope and high yields of the benzannulated naphthalene derivatives. The mechanism of the reaction is shown in Scheme 16.45. It involves enohzation of 2-phenacylbenzaldehyde to give an ortho quinodimethide derivative that undergoes Diels-Alder reaction and elimination of a carboxyhc acid from the adduct to yield the naphthalene derivative. 

Alkynes are coupled with 2-biaryl, 2-heteroarylphenyl, or 2-alkenylphenyl Grignard reagents to give phenanthrenes (Scheme 4-282), naphtho[6]heteroarenes, or naphthalenes, respectively. This [4+2] benzannulation is catalyzed by tris(acetyl-acetonato)iron in the presence of 4,4 -di-terr-butyl-2,2 bipyridyl (dtby) as a ligand and 1,2-dichloro-2-methylpropane as an oxidant. ... 

Scheme 4-282. [4+2] Benzannulation between alkynes and 2-biaryl Grignard reagents catalyzed by tris(acetylacetonato)iron.

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