Coupling of β-Cyanocarbene-Chromium Complexes with 2-Alkynylbenzoyl Derivatives: A [5+5]-Cycloaddition Approach to Phenanthridines

Abstract

The coupling of β-cyanocarbene complexes and 2-alkynylbenzoyl derivatives has been examined. The reaction afforded phenanthridine derivatives in a complex tandem process involving carbene-alkyne coupling, isobenzofuran formation, intramolecular Diels-Alder reaction using a nitrile dienophile, and deoxygenation. The chemistry could not be reproduced in non-chromium-based systems.

carbene complexes - chromium - cycloadditions - alkynes - nitriles - Diels-Alder reactions

References

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This compound was prepared from o-tolunitrile in a three-step sequence involving: (1) deprotonation with LDA followed by allylation with allyl bromide, (2) oxidation of the alkene to the aldehyde using OsO₄/NaIO₄, and (3) reaction of the aldehyde with 2-(dimethoxymethyl)phen-yllithium.

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This is a variant of a literature procedure in reference 9b. This procedure is very reliable if one is careful to cut the potassium into small pieces and if the tetramethyl-ammonium acylate salt is prepared prior to the alkylation step. Otherwise we noted wide variances in the yields between different experimental runs.

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Prepared from reaction of the acid chloride (ref. 21) with aqueous dimethylamine.

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