Influencing the Balance between ipso and ω Nucleophile Addition to (1‐Arylcyclohexadienyl)iron Intermediates in Alkaloid Synthesis

Abstract

AbstractAryllithium reagents react with tricarbonyl(η5‐1,4‐dimethoxycyclohexadienyl)iron hexafluorophosphate to give η4 products, which are converted into a series of (1‐arylcyclohexadienyl)iron(1+) complexes with H, CHO, CH2OAc, and CH(OAc)2 on the arene, ortho to the point of attachment to the dienyl ligand. The structures of these complexes have been compared by X‐ray crystallography. Correlations between spectroscopic and structural data indicate that the crystallographically defined conformations are also significant in the interpretation of the pathways of nucleophile addition in solution. Reactions with nucleophiles are influenced by the conformational properties of the 1‐aryl substituent, and examples of ω addition and a new intramolecular ipso addition relative to the arene have been characterised. Intermolecular ipso addition has been used to synthesise the alkaloid mesembrine in which 3,4‐dimethoxy electron‐donating substituents flatten the arylcyclohexadienyl portion and facilitate the ipso pathway. Electron‐withdrawing formyl substituents cause the arene to adopt a more perpendicular conformation, and a through‐space interaction between CHO and the exo face of the cationic dienyliron complex activates the aldehyde for reaction with (PhC≡C)2CuLi, generating an alkoxide that forms a spirocyclic ether by intramolecular reaction with the aryl‐substituted terminus of the dienyl ligand. Full spectroscopic assignments of the (1‐arylcyclohexadienyl)iron(1+) complexes are reported and correlated with structural data from crystallography to show that the complexes fall into two groups in terms of the relationship between the dihedral angle of the arene and the dienyl systems and the weighted means for ν(CO) in their infrared spectra. The relationship between this structural data, the distribution of positive charge on the dienyliron complex, and the regioselectivity of nucleophile addition explains how the correct choice of substitution patterns influences the selectivity between ipso and ω reactivity in 1,1‐iterative strategies that use (1‐arylcyclohexadienyl)iron(1+) complexes as key building blocks in synthetic routes to the galanthamine and crinine subclasses of the Amaryllidaceae alkaloids.