Ruthenium hydroxycarbenes as key intermediates in cycloisomerization and decarbonylative cyclization of terminal alkynals

Abstract

The complex [Ru(η5-C5H5)(CO) (κ1-OCMe2)(PiPr3)] BF 4 (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=CHCH 2C(CO2CH3)2CH2}(CO) (PiPr3)]BF4 (2), which undergoes deprotonation with Al2O3 to afford Ru(η5-C 5H5){C(O)C=CHCH2C(CO2CH 3)2CH2}(CO) (PiPr3) (3). In the presence of P2O5, the reaction of 1 with the alkynal leads to the alkenylvinylidene [Ru(η5-C5H 5){=C=C-CH=CH-C(CO2CH3)2-CH 2}(CO)(PiPr3)]BF4 (4), which yields the β,γ-unsaturated cyclopentenylhydroxycarbene [Ru(η 5-C5H5){C(OH)CHCH=CHC(CO2CH 3)2CH2}(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(CO 2CH3)2CH2}(CO)(PiPr 3) (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). © 2014 American Chemical Society.