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

The reactions of several alkynyl carbene complexes [(CO)(5)M=C(OMe)C=CR1, M = Cr, W, R-1 = Me, Me3Si. Ph, i-Pr, t-Bu) with a variety of acyclic enol e thers and ketene acetals [CH2=C(OR2)R-3, R-2 = Et, Me. i-Bu, SiMe(2)t-Bu, R -3 = H, Me, EtO, p-MeC6H4] are examined. These reactions occur to give [2 2] cycloaddition products in all cases except with R-1 = Me3Si where ene p roducts predominate. The cyclobutenyl carbene complexes produced in the [2 + 2] cycloadditions undergo rapid electrocyclic ring-opening at room temper ature when R-3 = H to give butadienyl carbene complexes as the isolated pro ducts. The reactions of the alkynyl carbene complexes with cyclic enol ethe rs 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 R-1. Only small differences are seen between chromium and tungsten comp lexes. The [2 + 2] cycloadditions with the E- and Z-isomers of ethyl prop-1 -enyl ether are stereospecific with complexes in which R-1 = Me but not wit h those with R-1 = SiMe3. The cyclobutenyl carbene complexes from the latte r reactions with tungsten derivatives were found to undergo stereoselective electrocyclic ring-opening at 70 degrees C to give only Z,E-butadienyl car bene complexes which result from the conrotatory ring-opening in which the ethoxy group rotates in an outward direction. An E,E-isomer was also isolat ed 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 car bene complex under the reaction conditions. The stereoselectivity of the el ectrocyclic ring-opening of these cyclobutenyl carbene complexes was shown to be the same as that found for their corresponding cyclobutenyl esters. I n one case, an interesting cine-rearrangement of a cyclobutenyl carbene com plex was observed. The metal can be oxidatively removed from the cyclobuten yl carbene complexes to give the corresponding cyclobut-1-enyl esters in go od 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 ca rbene complex functionality in the cycloadducts. This is illustrated with D iels-Alder reactions of the butadienyl carbene complexes and cyclohexadieno ne annulations of a cyclobutenyl carbene complex.