B. Bildstein et al., CATIONIC AND NEUTRAL [4]-CUMULENES C=C=C=C=C WITH 5 CUMULATED CARBONSAND 3 TO 4 FERROCENYL TERMINI, Organometallics, 17(12), 1998, pp. 2414-2424
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.