A comprehensive study of [2 + 2] cycloadditions and ene reactions of alkynyl chromium and tungsten carbene complexes with enol ethers and ketene acetals and of the stereochemistry of the electrocyclic ring opening of cyclobutenyl carbene complexes

Abstract

The reactions of several alkynyl carbene complexes [(CO)₅MC(OMe)CCR¹, M = Cr, W, R¹ = Me, Me₃Si, Ph, i-Pr, t-Bu) with a variety of acyclic enol ethers and ketene acetals [CH₂C(OR²)R³, R² = Et, Me, i-Bu, SiMe₂t-Bu, R³ = H, Me, EtO, p-MeC₆H₄] are examined. These reactions occur to give [2 + 2] cycloaddition products in all cases except with R¹ = Me₃Si where ene products predominate. The cyclobutenyl carbene complexes produced in the [2 + 2] cycloadditions undergo rapid electrocyclic ring-opening at room temperature when R³ = H to give butadienyl carbene complexes as the isolated products. The reactions of the alkynyl carbene complexes with cyclic enol ethers derived from cyclohexanone and cyclopentanone are more prone to give ene products than the acyclic enol ethers. Greater proportions of ene products are seen for six- rather than five-membered ring enol ethers and for silyl rather than alkyl enol ethers and for silyl rather than carbon substituents at R¹. Only small differences are seen between chromium and tungsten complexes. The [2 + 2] cycloadditions with the E- and Z-isomers of ethyl prop-1-enyl ether are stereospecific with complexes in which R¹ = Me but not with those with R¹ = SiMe₃. The cyclobutenyl carbene complexes from the latter reactions with tungsten derivatives were found to undergo stereoselective electrocyclic ring-opening at 70 °C to give only Z,E-butadienyl carbene complexes which result from the conrotatory ring-opening in which the ethoxy group rotates in an outward direction. An E,E-isomer was also isolated from the thermolysis mixture; however, it was shown not to be a primary product but rather the result of an isomerization of the Z,E-butadienyl carbene complex under the reaction conditions. The stereoselectivity of the electrocyclic ring-opening of these cyclobutenyl carbene complexes was shown to be the same as that found for their corresponding cyclobutenyl esters. In one case, an interesting cine-rearrangement of a cyclobutenyl carbene complex was observed. The metal can be oxidatively removed from the cyclobutenyl carbene complexes to give the corresponding cyclobut-1-enyl esters in good yield. Thus, alkynyl carbene complexes can serve as synthons for alkynyl esters in [2 + 2] cycloadditions with enol ethers and have the attractive feature of greatly increased reaction rates. Additional synthetic interest can be associated with processes in which the [2 + 2] cycloaddition of the alkynyl carbene complex is coupled in tandem with other reactions of the carbene complex functionality in the cycloadducts. This is illustrated with Diels–Alder reactions of the butadienyl carbene complexes and cyclohexadienone annulations of a cyclobutenyl carbene complex.