Hexasubstituted hybrid tetradentate (N,N′,P,P′)-ferrocenes bearing phosphino and aminomethyl groups, plus hindering tert-butyl moieties, were synthesized by using two different strategies: a "diverging" synthesis involving successive functionalization of preformed di-tert-butylated ferrocene and a "converging" assembly of the species from appropriately substituted cyclopentadienyl rings. While the new cyclopentadienyl salts formed are of interest, their assembly with iron dichloride used as a "converging" way to produce tetradentate ferrocene ligands presented several drawbacks. Conversely, the synthesis of new tert-butylated (aminomethyl)ferrocene derivatives was found convenient to further form (N,N′)-aminomethyl,(P,P′)-tert-butylated-ferrocenyl diphosphines by N-directed ortho-metalation. The novel N 2didentate and N 2 P 2 -tetradentate tert-butylated ferrocene compounds were all synthesized in good to high yields (48−96%) and tolerated aryl, alkyl, and heteroaryl phosphino groups as substituents on nitrogen and phosphorus atoms. They were characterized by X-ray diffraction and multinuclear NMR ( 1 H, 13 C, 31 P, 15 N). We observed the conformation control provided to rac-(N,N′)-diaminomethyl-(P,P′)-tert-butylated-ferrocenyldiphosphines with in particular the systematic near-eclipsed conformation of aminomethyl groups. This conformation is at the origin of the unexpected formation at RT of a zwitterionic cyclopalladate from an (aminomethyl)ferrocene derivative, arising from intramolecular Cp-proton transfer to the proximate free amino group by simple C−H activation reaction in the presence of palladium dichloride.
A variety
of aminomethyl-substituted ferrocenes and the parent compounds (iminomethyl)ferrocenes,
azaferrocenophanes, and diferrocenylamines can be selectively synthesized
from reductive amination of 1,1′-diformylferrocene or formylferrocene.
The optimized one- or two-step reactions have delivered 13 new compounds,
isolated in 65–97% yields, which include tertiary (ferrocenylmethyl)amines
and azaferrocenophanes by using NaBH(OAc)3 as a mild reducing
agent and (iminomethyl)ferrocenes and secondary (ferrocenylmethyl)amines
by using LiAlH4. X-ray structures of representative members
of these ferrocene derivative families have evidenced the preferred
conformation adopted by ferrocene backbones, in which surprisingly
the steric hindrance is apparently not systematically minimized. 15N NMR measurements on aminomethyl-substituted ferrocenes
and derivatives are provided for the first time, establishing benchmark
values ranging from −330 to −305 ppm (nitromethane δ
0 ppm). The cyclic voltammetry of these species evidences two clearly
distinct oxidation potentials related to the iron(II) center and the
amino function. These aminomethyl-substituted ferrocenes are potentially
valuable for further ortho-directed functionalization
of ferrocene.
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