Over the last years radicals have been used increasingly in multistep syntheses due to the mild reaction conditions, high functional group tolerance, and broad accessibility of interesting structures, often obtained in sequential transformations.[1] In this context we wish to report our first results on a novel radical tandem reaction [1b] featuring an unprecedented formal homolytic substitution reaction at a Ti-O bond for the formation of tetrahydrofurans. The planned sequence is shown in Scheme1. The initial step is based on the titanocene-mediated opening of 1 described by Nugent and RajanBabu,[2] which we have developed into a catalytic reaction,[3] to give the radicals cis-and trans-2. The second and conceptually novel step of our tandem reaction constitutes the attack of the tertiary radical 2 on the Ti-O bond. Mechanistically, this can be viewed as a homolytic substitution of the [Cp2TiCl] radical.[4] Our work therefore introduces metal–oxygen bonds as a very useful class of radical traps. As a consequence of this homolytic substitution the redox catalyst [Cp2TiCl] is regenerated. To the best of our knowledge, this concept of a catalytic redox isomerization is unknown in the literature.

Because the second cyclization is counterintuitive, a supposedly strong Ti-O bond is cleaved and a CO bond formed, we analyzed the transformation of model system 4 by density functional theory (DFT) calculations as shown in Scheme 2.[5] As expected [1c, 6] both the ring opening of 4