Reactions of Rhenacyclobutadiene Complexes with Allenes

The reactivity of low-valent metallacyclobutadienes toward allenes was studied. Rhenacyclobutadienes of the type Re{−C(Ar)═C(CO 2 Et)C(OR)═}(CO) 4 are unreactive toward allenes such as PhCH═C═CH 2, MeOCH═C═CH 2, and pyrrolyl-CH═C═CH 2 but reacted with the electron-rich aminoallenes R 2 NCH═C═CH 2 . 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(CO 2 Et)C(OEt)═}(CO) 4 reacted with Me 2 NCH═C═CH 2 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(CO 2 Et)C(OR)═}(CO) 4 (R = i Pr, t Bu) containing a bulky alkoxy group reacted with Me 2 NCH═C═CH 2 to selectively give 1,3-dimethylenecyclopentane derivatives. The complex Re{−C(Ar)═C(CO 2 Et)C(OR)═}(CO) 4 (Ar = Ph, naphthyl) reacted with the sterically more demanding aminoallene Et 2 NCH═C═CH 2 to selectively give methylenecyclopentene derivatives. The origin of the product selectivity has been investigated computationally.