Lewis Acid-Mediated Cyclization of Allenyl Aryl Ketones

Abstract

The cyclization of a series of nonheterocyclic allenyl aryl ketones was examined using boron trifluoride etherate and indium triflate to mediate the reaction. Yields with BF3 were low in most instances due mainly to competitive destruction of the substrates. With In­(OTf)3, there was less decomposition, and the yields of the cyclized product were much higher, but only for substrates with electron-donating substituents. Cyclization did not occur without those substituents. A computational study using the ωB97X-D/6-311+G­(2d,p)//ωB97X-D/6-31+G­(d,p) method confirmed better stability of the σ-complexed substrate by indium­(III) and that meta-substituents on the phenyl ring of the substrate significantly influenced the activation barrier of the cyclization, whereas the effect of para-substituents was almost negligible. The computational results supported the idea that the cyclization is a 4π-electrocyclization and not a 5-endo-dig ring closure as had been proposed in the literature.

Highlighting the Rich Chemistry of the Allenone Moiety

“…It has been found that electron‐donating substituents notably increased the yield of the transformation. Also, DFT calculations have pointed to a more plausible 4π‐electrocyclization mechanism versus a 5‐ endo ‐ dig ring closing reaction (Scheme 43a) [138] . In a similar report, cationic iron catalysts have been employed to perform the cyclization of related AAKs 228 b (Scheme 45b) [139] .…”

“…In recent years, different reports have described the synthesis of those systems within more complex structures. [128][129][130][131][132][133][134][135][136][137][138][139][140][141] Also, tandem processes have been described involving Lewis acid-promoted Nazarov cyclizations towards the synthesis of cyclopentenones exhibiting diverse functionalization. Thus, AVK molecules have been recurrently employed in the so-called "interrupted Nazarov cyclization", where the Nazarov carbocation intermediate 215 is trapped with different nucleophiles at either position C2' or C4 generating highly substituted cyclopentenones.…”

“…Also, DFT calculations have pointed to a more plausible 4π-electrocyclization mechanism versus a 5-endo-dig ring closing reaction (Scheme 43a). [138] In a similar report, cationic iron catalysts have been employed to perform the cyclization of related AAKs 228 b (Scheme 45b). [139] The methodology is compatible with a large range of substituents on the allenic moiety in compounds 228 b, generating highly substituted indanones 229 b (Scheme 43b).…”

“…In this regard, allenyl vinyl ketones (AVKs) are perfect candidates to undergo Nazarov‐type cyclization and yield the cyclopentenone skeleton. In recent years, different reports have described the synthesis of those systems within more complex structures [128–141] . Also, tandem processes have been described involving Lewis acid‐promoted Nazarov cyclizations towards the synthesis of cyclopentenones exhibiting diverse functionalization.…”

Transition‐Metal‐Free Synthesis of 1‐Indanone Skeleton: A Brief Update

“…OEt 2 ) or indium triflate to afford indanone derivatives 46 along with the formation of indenone as minor product 47 (Scheme 15). [42] Yields with BF 3 were low in most of the cases due to mainly competitive destruction of the substrates. With indium triflate, there was less decomposition of product but only for the substrates containing electron donating groups.…”