Ruthenium Hydroxycarbenes as Key Intermediates in Cycloisomerization and Decarbonylative Cyclization of Terminal Alkynals

The complex [Ru­(η5-C5H5)­(CO)­(κ1-OCMe2)­(PiPr3)] BF4 (1) reacts with 3,3-bis­(methoxycarbonyl)-5-hexyn-1-al to give the α,β-unsaturated cyclopentenylhydroxycarbene derivative [Ru­(η5-C5H5)­{C­(OH)CCHCH2C­(CO2CH3)2 CH2}­(CO)­(PiPr3)]­BF4 (2), which undergoes deprotonation with Al2O3 to afford Ru­(η5-C5H5)­{C­(O)CCHCH2C­(CO2CH3)2 CH2}­(CO) (PiPr3) (3). In the presence of P2O5, the reaction of 1 with the alkynal leads to the alkenylvinylidene [Ru­(η5-C5H5)­{CCCHCH­C(CO2CH3)2CH2}­(CO)­(PiPr3)]­BF4 (4), which yields the β,γ-unsaturated cyclopentenylhydroxycarbene [Ru­(η5-C5H5)­{C­(OH)CHCHCHC­(CO2CH3)2 CH2}­(CO)­(PiPr3)]­BF4 (5) by means of a 1,2-addition of water. Complex 5 slowly isomerizes into 2. The deprotonation of 5 with Al2O3 gives Ru­(η5-C5H5)­{C­(O)CHCHCHC­(CO2CH3)2 CH2}­(CO)­(PiPr3) (6). Solvate Ru complex 1 and Ru hydroxycarbene 2 catalyze the cyclization of 3,3-bis­(methoxycarbonyl)-5-hexyn-1-al to give mixtures of the cycloisomerized aldehyde 1,1-bis­(methoxycarbonyl)-3-formylcyclopent-3-ene (7) and cycloalkene 1,1-bis­(methoxycarbonyl)­cyclopent-3-ene (8).