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.