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 C-13 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-[(eta (6)-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 C-Ar-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 pi -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.