Cationic and Neutral [4]-Cumulenes CCCCC with Five Cumulated Carbons and Three to Four Ferrocenyl Termini

Abstract

Starting from 1,1-diferrocenyl-1-methoxypropyne, the first air-stable C5-cumulenium salt (Fc)2CCCCC(Fc)+BF4 - (Fc = ferrocenyl) can be synthesized by a metalation, iodination, cross-coupling, and dehydration sequence. Attempted nucleophilic attack of ferrocenyl carbanion affords only radical decomposition products and none of the desired tetraferrocenyl-C5-cumulene (Fc)2CCCCC(Fc)2. In contrast, reaction with phenyllithium yields the nucleophilic addition product (Fc)2CCC(R)C⋮CFc (R = C6H5) with 100% regioselectivity. Starting from diferrocenyl ketone, the first air-stable protonated C5-cumulenium salt (Fc)2CCHCCC(Fc)2 +BF4 - is prepared by stepwise substitution of propyne with diferrocenyl(methoxy)methyl groups and subsequent 2-fold dehydration. Addition of methoxide affords (Fc)2CCC(R)CΗC(Fc)2 (R = OCH3); attempted conversion to tetraferrocenyl-C5-cumulene (Fc)2CCCCC(Fc)2 by thermal elimination of methanol in vacuo fails but yields an unusual cyclobutene dimer with eight ferrocenyl substituents. Finally, deprotonation of (Fc)2CCHCCC(Fc)2 +BF4 - with “super base” n-BuLi/t-BuOK gives access to tetraferrocenyl-C5-cumulene (Fc)2CCCCC(Fc)2, one of the very few C5-cumulenes known. IR, Raman, UV−vis, MS, NMR, Mössbauer spectroscopy, cyclic voltammetry, and X-ray crystallography are used for a detailed characterization of these metallocenylcumulene compounds.

Synthesis and properties of long [n]cumulenes (n≥ 5)

“…Bildstein and coworkers have reported the formation of [4]Fc based on an adaptation of the diol approach described above for [3]cumulenes (Scheme 4). 106,109 A homopropargylic ether is formed, lithiated, and then added to diferrocenylketone to complete the carbon skeleton. Treatment with acid (HBF 4 ) results in the stabilized (and isolable) [3]cumulene intermediate, which can then be converted to [4]Fc through base induced elimination.…”

α,ω-Diferrocenyl Cumulene Molecular Wires. Synthesis, Spectroscopy, Structure, and Electrochemistry

“…Indeed, "superbases" or "Schlosser bases" (equimolar mixtures of potassium alkoxides with butyllithium) 27 deprotonate a purple solution of 17a,b,c under color change to give a red solution of cumulene 18, similar to that in the case of tetraferrocenyl[4]cumulene. 24 However, compound 18, containing two ferrocenyl and two phenyl termini, is sterically less protected than its tetraferrocenyl analogue, and on attempted chromatographic workup hydrolysis proved unavoidable, affording pent-4-en-2-yn-1-ol 19 (Figure 7) as the sole isolable product. Interestingly, treatment with tetrafluoroboric acid regenerates the starting material 17a,b,c as outlined in Scheme 4.…”

Structures of a Tetradentate Ferrocenyl Ligand and Its Oxorhenium(V) Complex in Solution and in the Solid State

“…These rings are coplanar in molecules that contain a sequence of Fc-Fc-Fc units 31 or are separated by an even number of cummulene carbons. 32 The eclipsed conformation is favored in unsubstituted and monosubstituted ferrocene compounds, but in disubstituted complexes the staggered arrangement of the Cp rings is often encountered. A comparison of the structural data for ferrocene and some of its derivatives with [Pd(dppf-P,P′)(anthracenyl)Br] 33 outlines these orientations.…”