CATIONIC AND NEUTRAL [4]-CUMULENES C=C=C=C=C WITH 5 CUMULATED CARBONSAND 3 TO 4 FERROCENYL TERMINI

Starting from 1,1-diferrocenyl-1-methoxypropyne, the first air-stable C-5-cumulenium salt (Fc)(2)C=C=C=C-C(Fc)+BF4- (Fc = ferrocenyl) can be synthesized by a metalation, iodination, cross-coupling, and dehydrat ion sequence. Attempted nucleophilic attack of ferrocenyl carbanion af fords only radical decomposition products and none of the desired tetr aferrocenyl-C-5-cumulene (Fc)(2)C=C=C=C=C(Fc)(2). In contrast, reactio n with phenyllithium yields the nucleophilic addition product (Fc)(2)C =C=C(R)CrCFe (R = C6H5) with 100% regioselectivity. Starting from dife rrocenyl ketone, the first air-stable protonated C-5-cumulenium salt ( Fc)(2)CCH=C=C=C(Fc)(2)+BF4- is prepared by stepwise substitution of pr opyne with diferrocenyl(methoxy)methyl groups and subsequent 2-fold de hydration. Addition of methoxide affords (Fc)(2)C=C=C(R)CH=C(Fc)(2) (R = OCH3); attempted conversion to tetraferrocenyl-C-5-cumulene (Fc)2C= C=C=C=C(Fc)2 by thermal elimination of methanol in vacuo fails but yie lds an unusual cyclobutene dimer with eight ferrocenyl substituents. F inally, deprotonation of (Fc)(2)CCH=C=C=C(Fc)(2)+BF4- with ''super bas e'' n-BuLi/t-BuOK gives access to tetraferrocenyl-C-5-cumulene (Fc)(2) C=C=C=C=C(Fc)(2), one of the very few C-5-cumulenes known. IR, Raman, UV-vis, MS, NMR, Mossbauer spectroscopy, cyclic voltammetry, and X-ray crystallography are used for a detailed characterization of these met allocenylcumulene compounds.