Stereochemical Requirements of Oxidative Cyclizations in Extended Iterative Organoiron-Mediated Routes to Alkaloids

Oxidative cyclization by reaction of benzylic and phenolic
OH groups
on tricarbonyl­(η4-cyclohexa-1,3-diene)­iron­(0) complexes
has been achieved with the hypervalent iodine oxidant PIFA which was
shown to be compatible with the tricarbonyliron complex. The reaction
proceeds with substrates with the nucleophilic substituent on the
opposite face of the ligand to the iron. IBX gives efficient oxidation
of the benzyl alcohol to the aldehyde in the presence of the Fe­(CO)3 group. Reduction of 1-arylcyclohexadienyliron­(1+) complexes
with sodium borohydride to access the endo series
also gave a novel rearranged 2-aryl reduction product with a 5-endo OMe group. The cis relative stereochemistry
of the oxidative cyclization product, the exo delivery
of hydride to the 1-arylcyclohexadienyliron­(1+) complex, and the 2-aryl-5-endo-methoxy relative stereochemistry of the rearranged
product were proved by X-ray crystallography.