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.