STEREOELECTRONIC EFFECTS ON PRODUCT FORMATION FROM THE E-ISOMER AND Z-ISOMER OF ETA(1),ETA(3)-VINYL CARBENE COMPLEXED INTERMEDIATES IN THE REACTIONS OF FISCHER CARBENE COMPLEXES WITH ALKYNES

The reactions of 2-dihydrapyranyl(methoxy)methylene pentacarbonyl chro mium 19 with the series of five acetylenic ketones CH3(CH2)(2)C=C(O)(p -X-C6H4) 26 (X = CF3, Cl, H, OCH3, NMe2) gave a mixture of phenol and lactone products. The most electron-rich alkyne (X = NMe2) gave a grea ter proportion of the phenol product 27 (81:19),ana the most electron- poor alkyne (R = CF3) gave the lactone 28 as the major product (64:36) . Since the lactone product must come from a Z-isomer of a vinyl carbe ne complexed intermediate and the phenol product must come an E-isomer , this reveals that the stereochemistry of the reactive intermediates involved can be influenced by electronic effects of the substituents o n the alkyne. A similar set of experiments with 3-hexyn-2-one 20 and t he series of five phenoxy carbene complexes (CO)(5)Cr=C(C5H6O)O(p-X-C6 H4) 29 (X = CF3, Cl, H, OCH3, NMe2) produced the opposite result, wher e predominantly the phenol product was produced from the most electron -pear carbene complex (X = CF3) and for the most electron-rich carbene complex (X =NMe2) the lactone product was predominant. These results can be explained by a trans effect; in the eta(1),eta(3)-vinyl carbene complexed intermediate where the most electron-rich substituent, of t he beta-carbon of this intermediate prefers to be trans to the most el ectron-poor substituent of the alpha-carbon. These data are consistent with observation that the complex 19 will react with methyl-2-hexynoa te 23 to give only phenol products and with the observation that the d iaryl carbene complex (CO)(5)Cr=C(p-Me-C6H4)p-CF3C6H4 7 will react wit h diphenylacetylene to give the phenol product which results from cycl ization onto the more electron-poor aryl substituent by a factor of 3: 1. These data are also consistent with and offer an explanation for th e fact that under normal conditions the reactions of Fischer carbene c omplexes with unactivated alkynes give high chemoselectivities for phe nol products. Finally the stereoselectivity of lactone formation was e xamined in the reaction of 1-(2,6-dimethyl-1-cyclohexenyl)(methoxy)met hyl pentacarbonyl chromium 32 with 4-phenyl-3-butyn-2-one 33 to give o nly the lactone product 34 as a single diastereomer. The stereochemist ry of 34 was determined to have a trans relationship of the two methyl s on the six-membered ring by X-ray diffraction. It is interesting tha t the same stereochemistry has been previously observed for the reacti on of this complex with 1-hexyne, where the cyclohexadienone product i s produced form an E-isomer of the vinyl carbene complexed intermediat e rather than from a Z-isomer.