Organometallic Helices: The Mechanism of Formation of “Metallospiralenes”

A complete investigation of the mechanism of formation of chelated syn-facial heterobimetallic benzyl complexes, also referred to as Cr/Mn-spiralenes, is presented. Spectroscopic
evidence for the intermediacy of a metal carbene species was established by 13C NMR
spectroscopy. The nature of the substitution pattern at the carbene carbon atom is shown
to play a determining role in the stereoselectivity of spiralene formation. The sequential
addition of alkyllithium reagents and MeOTf to a series of cyclomanganated 2-phenylpyridine
derivatives afforded mixtures of two isomeric spiralenes, of which a pair of examples has
been characterized by X-ray diffraction. The more structurally flexible hetero-bimetallic
isomers display an ability to interconvert with the probable mediation of biradical species
formed upon homolytic disruption of a benzylic C−Mn bond. Investigation of 2-oxazolinyl
derivatives unveiled the origin of the diastereoselective formation of spiralenes. Sequential
addition of PhLi and MeOTf to a cyclomanganated 2-[(η6-4-tolyl)Cr(CO)3]-2-oxazoline
derivative afforded a mixture of two products, both resulting from the insertion of a Ph(MeO)C:  moiety into the CAr−Mn bond. X-ray diffraction analyses reveal that the two
compounds are pseudoconformers which differ from the position of the Mn(CO)3 moiety:  in
one compound, the Mn(CO)3 group is located close to the Cr(CO)3 group, while in the other
compound, it is part of a nearly planar six-membered manganacyclic ring coplanar with the
π-coordinated arene. These results indicate that in “spiralenes” the helical and spiral-type
arrangements are stereoselectively produced if at least one substituent of the transient
carbene complex is an aromatic ring capable of electronic interaction with the Mn(I) center
during the cis-migration process.