Reactions of Rhenacyclobutadiene Complexes with Allenes

The reactivity of low-valent metallacyclobutadienes toward allenes was studied. Rhenacyclobutadienes of the type Re­{−C­(Ar)C­(CO2Et)­C­(OR)}­(CO)4 are unreactive toward allenes such as PhCHCCH2, MeOCHCCH2, and pyrrolyl-CHCCH2 but reacted with the electron-rich aminoallenes R2NCHCCH2. The products of the reactions are dependent on or can be controlled by the steric properties of the OR groups of the metallacyclobutadienes as well as the substituents of the aminoallenes. The complex Re­{−C­(Ph)C­(CO2Et)­C­(OEt)}­(CO)4 reacted with Me2NCHCCH2 to give a mixture of methylenecyclopentene and 1,3-dimethylenecyclopentane, derived from mono-insertion of allene into the Re–C­(OEt) bond and bis-insertion of the allene into the Re–C­(Ph) bond, respectively. The complexes Re­{−C­(Ph)C­(CO2Et)­C­(OR)}­(CO)4 (R = iPr, tBu) containing a bulky alkoxy group reacted with Me2NCHCCH2 to selectively give 1,3-dimethylenecyclopentane derivatives. The complex Re­{−C­(Ar)C­(CO2Et)­C­(OR)}­(CO)4 (Ar = Ph, naphthyl) reacted with the sterically more demanding aminoallene Et2NCHCCH2 to selectively give methylenecyclopentene derivatives. The origin of the product selectivity has been investigated computationally.