Stereoselective Formation of η6‑Arene Ruthenium(II) Complexes via Metal-Triggered Bergman and Hopf Cycloaromatizations

A stereoselective metal-mediated cycloaromatization of chiral conjugated dienynes and enediynes is described. For dienyne cycloaromatization, placement of the carbon stereocenter in the allylic position gives the highest diastereomeric ratios (dr). The observed stereoselectivity depends on the steric bulk of the alkyne substituent, as replacing a propargylic methyl for trimethylsilyl increases the dr from 56:44 to 80:20. For both enediyne and dienyne substrates, [(η5-C5Me5)­Ru­(NCMe)3]­PF6 exhibits greater diastereoselectivity than does [(η5-C5H5)­Ru­(NCMe)3]­PF6. For the same chiral enediyne substrate, [(η5-C5Me5)­Ru­(NCMe)3]­PF6 generates a 4:1 ratio of diastereomeric arene products, whereas both [(η5-C5Me4CF3)­Ru­(NCMe)3]­PF6 and [(η5-C5H5)­Ru­(NCMe)3]­PF6 generate a 1:1 product mixture, indicative of a significant electronic influence of the ancillary ligand on diastereoselectivity. X-ray structure determination of several isolated ruthenium arene diastereomers confirms the assigned relative stereochemistry for the major and minor stereoisomeric metal arene products. Arene-binding experiments demonstrate that the observed stereoselectivity does not involve complexation of free arene by ruthenium.