REGIOCHEMISTRY OF ACETYLATION OF FERROCENYLARYLETHYLENES

We describe the synthesis and Friedel-Crafts acetylation of a series of ferrocenylarylethylenes, C5H5FeC5H4CH = CH(C6H4-p-X), where X = NO2 (3a), Br (3b), and NMe2 (3c). Compounds 3a-c provide a direct comparison of the reactivity of ferrocene, olefin, and aryl functionalities. The regiochemistry of substitution of these compounds depends on the nature of the aryl substituent. Acetylation occurs predominantly at the olefin and the unsubstituted cyclopentadienyl ring, substitution does not occur at the aryl ring or at the substituted cyclopentadienyl ring. Reaction at the olefin is accompanied by olefin isomerization. With the strongly activating dimethylamino substituent (3c), substitution at the unsubstituted cyclopentadienyl ring (5c) is slightly favored over substitution at the olefin (4c). The regiochemistry of olefin substitution suggests that a ferrocenyl substituent is better able to stabilize an adjacent positive charge than a p-(dimethylamino)aryl substituent. With the bromine substituent (3b), substitution at the olefin (4b) is slightly favored over substitution at the unsubstituted cyclopentadienyl ring (5b). The nitro group is sufficiently deactivating that 3a fails to react under our conditions.