Reactions of Osmium Hydrido Alkenylcarbyne with Allenoates: Insertion and [3 + 2] Annulation

Treatment
of the osmium hydrido alkenylcarbyne complex [OsH­{CC­(PPh3)CHPh}­(PPh3)2Cl2]­BF4 (1) with allenoates (CH2CCHCOOR,
R = Me, Et) in the presence of excess AgBF4 leads to the
insertion products 2/3. The reactions of
complex 1 with substituted allenoates, i.e., diethyl
2-vinylidenesuccinate (CH2CC­(CH2COOEt)­(COOEt)) and ethyl penta-2,3-dienoate (CH­(CH3)CCH­(COOEt)), result in the formation of [3 + 2]
annulation products 4 and 6, respectively.
Mechanisms of the reactions have been proposed with the isolation
of the key intermediates. The results reveal that the divergent pathways
for these reactions are mainly of steric origin.