Intramolecular Conversions of (Aminoferrocenylpenta-1,4-dienyl)-ferrocenylcarbenes: Synthesis of Diferrocenylmono-, bi-, tricycles and Amino(diferrocenyl)hexa-1,3,5-trienes

Synthesis of 3,4-diferrocenyltoluene (7), 1-morpholino- and 1-piperidino-2,3-diferrocenylbicyclo[3.1.0]hex-2-enes 8a, 8b, 1-morpholino- and 1-piperidino-7-ferrocenyl-3,4-ferrocenobicyclo[3.2.1]oct-6-enes 9a, 9b, 2- and 3-amino(diferrocenyl)-hexa-1,3,5-trienes 10a,b, 11a,b by reactions of amino(diferrocenyl)cyclopropenylium tetrafluoro-borates with 1-methylprop-2-enylmagnesium chloride at 80 °C is described. The structures of the compounds obtained were determined by IR, 1H- and 13C-NMR spectroscopy and mass spectrometry. X-ray diffraction data for 1-piperidino-7-ferrocenyl-3,4-ferroceno-bicyclo[3.2.1]oct-6-ene (9b), 2-morpholino- and 2-piperidino-1,3-diferrocenyl-4-methyl-hexa-1,3,5-trienes 10a and 10b is presented. The electrochemical behaviour of compounds 7, 8a, 10a and 10b was investigated by means of cyclic voltammetry and square wave voltammetry. For 7 and 8a two electrochemical processes (I-II), attributed to the oxidation of the ferrocene moieties were found. On the other hand for compounds 10a and 10b a single electron transfer for both ferrocene groups and the electrochemical generation of the monocation and dication species were detected.

The interest in these compounds, especially with heteroaryl substituents in their molecules, may stem from the peculiarities of their chemical behavior due to the mutual effects of the ferrocene system and the heterocyclic fragment [8]. These effects may result in the emergence of diverse valuable properties, such as biological activity, dyeing ability, possible use as propellant additives or light-sensitive materials, redox switching receptors in supramolecular chemistry, etc., which have previously been observed for a number of heteroarylferrocenes [8].
Both reaction mixtures were separated by column chromatography on alumina, and the structures of the isolated products were established based on the data from UV, IR, and 1 H-and 13 C-NMR spectroscopy, mass spectrometry, and elemental analysis. The physicochemical characteristics of compounds 6 and 7 corroborate completely their structures.
We attempted to assign endo forms of compounds 8a and 8b based on previously reported criteria [10][11][12]. Thus, compounds 8a and 8b were characterized as endo isomers based on the fact that the signals from three of the protons of the C 5 H 4 groups are observed at higher field than the singlets from the protons of the C 5 H 5 groups [10]; the 1 H-NMR spectra of compounds 8a and 8b each contain characteristic doublets for the protons of the methyl group (δ 1.04 and 0.97 ppm), two doublets of doublets for the methylene fragment, two multiplets for the methine fragments (δ 0.94 and 1.77; 0.89 and 1.76 ppm) [11], signals for the protons of the morpholine or piperidine substituents, two singlets for unsubstituted cyclopentadienyl rings and the appropriate number of multiplet signals for two substituted C 5 H 4 moieties of the ferrocenyl substituents. The 13 C-NMR spectra of compounds 8a and 8b each contained two signals for C ipso Fc belonging to two ferrocene fragments, as well as the necessary number of signals for cyclopropyl-fragments δ 13.12, 20.54 ppm (2CH), 20.63 (CH 3 ) from 8a, and δ 12.92, 20.26 (2CH), 21.34 ppm (CH 3 ) from 8b [12]. The position of the heterocyclic substituents at the C(1) carbon atom of the bicycles 8a,b was established based on the data from 1 H-NMR spectra with a one-dimensional NOE experiment, which showed the interaction of the protons of the two CH groups with the 2-CH 2 -protons of the heterocycles, that completely corroborated their structures. Additional proof of the structures of compounds 9a and 9b was obtained from X-ray diffraction analysis of a single crystal of compound 9b, which was grown by crystallization from CH 2 Cl 2 . A general view of 9b is shown in Figure 1a, the character of packing of molecules in a crystal is shown in Figure 1b, and the main geometrical parameters are given in Table 1. Data from the X-ray analysis proved the structure of 9b as 7-ferrocenyl-3,4-ferroceno-6-methyl-1-piperidinobicyclo-[3.2.1]oct-6-ene.
As inferred from the 1 H-NMR spectra of trienes 10a and 10b, they are formed as single isomers. The spectra contained each signals for the olefinic protons of the -CH= fragments and the protons of the terminal =CH 2 group, singlets for the CH 3 -group and two C 5 H 5 fragments, as well as multiplets for the protons of the heterocyclic substituents. The data from 13 C-NMR spectroscopy of these compounds are in full accord with the proposed structures; however, the positions of the ferrocenyl and heterocyclic substituents remained uncertain. To address this issue, we performed X-ray diffraction analysis of their single crystals grown by crystallization from dichloromethane. The general views of the molecules 10a and 10b are shown in Figures 2a,b and Figures 3a,b, respectively, and the main geometrical parameters are given in Table 1. Data from the X-ray analysis demonstrated that 10a is (E,E)-1,3-diferrocenyl-4-methyl-2-morpholinohexa-1,3,5-triene, and 10b is (E,E)-1,3-diferrocenyl-4-methyl-2-piperidinohexa-1,3,5-triene. Compounds 11a and 11b were isolated as single geometrical isomers of (Z,E)-1,2-diferrocenyl-3heteryl-4-methylhexa-1,3,5-trienes as inferred from the following data: the 1 H and 13 C-NMR spectra of each of them contain signals for two ferrocenyl, one methyl, and one heterocyclic substituents, two olefinic -CH= and one terminal CH 2 = group. Presumably, the Z-arrangement of two ferrocenyl groups at the C(2) = C(1) double bond was assigned based on comparison with cis-positions of two Fc-substituents in bicyclic products 8a,b. The E-configuration of the C(3) = C(4) double bond was assigned based on comparison with E-configuration of the C(3) = C(4) in trienes 10a,b.
The most feasible mechanism of the formation of compounds 6, 7, 8a,b, and 11a,b involves the initial nucleophilic attack of the 1-methylprop-2-enyl anion on the heteroaryl-substituted carbon atom of the three-carbon ring of the cations 1a and 1b to afford  The primary nucleophilic attack on the C(2) atom of cyclopropenylium cations 1a,b affords unstable tetrasubstituted cyclopropenes 16a,b. Small-ring opening in these transient species proceeds via 1,3-diferrocenyl-2-heterylallylcarbenes 17a,b and 18a,b, their intramolecular transformations result in tricyclic 9a,b and linear 10a,b final products. Compounds 9a,b formed, like bicyclic products 8a,b, from carbenes with configuration 18a,b, while configuration 17a,b of these carbenes gives rise to trienes 10a,b.

Electrochemistry
In the framework of this work, we studied electrochemical behavior of compounds 7, 8a, 10a and 10b by means of cyclic voltammetry and square wave voltammetry. Figure 4 shows a typical voltammogram of compound 7 obtained in a platinum electrode. Two oxidation signals (I a and II a ) with its complementary reduction signals (I c and II c ) were detected. The anodic and cathodic peak potential values were independent of scan rate, which indicates the reversibility of the two processes.
The logarithmic values of peak current and scan rate were linearly dependent, with a slope near 0.5; this behaviour is characteristic for a diffusion-controlled process. This evidence indicates that processes I and II are attributed to two consecutive electron transfers of ferrocene moieties.
In order to obtain the formal electrode potential for both processes, square wave voltammetry experiments were carried out ( Figure 5). The obtained values were E 0' (I) = 0.004 V/Fc-Fc + and E 0' (II) = 0.232 V/Fc-Fc + with its corresponding difference ΔE 0' (II-I) = 0.228 V and comproportionation constant K com of 7151.3 [13,14]. The electrochemical response of 8a was similar than those described above for 7. This fact suggests that in the K com values electron withdrawing an electron donating effects must be consider. Also, the difference in the conjugation manner in compounds 7 and 8a could be the reason for the difference in ΔE 0' (II-I) values. Figure 6 presents cyclic voltammoamperometric response of compound 10a. When the potential scan was initiated in the positive direction, three oxidation signals (I a , II' a , and II'' a ) were observed, and when the potential scan was reversed, two reduction signals (I c and II' c ) were also observed. We propose that the electrochemical process I with its corresponding oxidation and reduction signals, I a and I c, are attributed to electron transfer ferronece/ferricium + for both moieties. This is in agreement with the fact that a single electrochemical process is observed when there is no electronic communication between two redox centers, due to the long distance within a molecule, as occurs in compound 10a ( Figure 6). On the other hand, based on studies reported previously for molecules with conjugated double bonds, the processes II' a and II'' a can be related to electrochemical generation of the monocation and dication species [15][16][17][18][19][20][21]. Controlled potential coulommetry experiments acquired at potential step corresponding to anodic peak potential values for processes I, and II, led us to calculate the value of two electrons transferred for each process. The formal electrode potential values were evaluated also using square wave voltammetry experiments (Figure 7). The values for E 0' (I), E 0' (II), and E 0' (II * ) were −0.260, 0.152, and 0.256 V/Fc-Fc + , respectively. The electrochemical response of compound 10b is similar that obtained for 10a. However, different values of formal electrode potentials were observed. Table 2 shows a summary of electrochemical behaviour of compounds 7, 8a, 10a and 10b. Table 2. Formal electrode potential E 0 (I), E 0 (II) and Δ E 0 (II-I), and constant K com for compounds 7, 8a, 10a, and 10b.

General
All the solvents were dried according to standard procedures [22] and were freshly distilled before use. Column chromatography and TLC were carried out on alumina (Brockmann activity III). The 1 H and 13 C-NMR spectra of the compounds 6, 7 8a,b, 11a,b were recorded on a Unity Inova Varian spectrometer (300 and 75 MHz) for solutions in CDCl 3 , with Me 4 Si as the internal standard. The IR spectra were measured with an FTIR spectrophotometer (Spectrum RXI Perkin-Elmer instruments) using KBr pellets. UV spectra were recorded on a Specord UV-VIS spectrophotometer.The mass spectra were obtained on a Varian MAT CH-6 instrument (EI MS, 70 eV). A LECO CHNS-900 Elementar Analysensysteme was used for elemental analyses.
Electrochemical measurements were carried out in acetonitrile containing 0.1 M tetra-n-butylammonium tetrafluoroborate (TBABF 4 ) with sample concentration c.a. 1 mM. An Epsilon-BAS potentiostat/galvanostat was used for all experiments. A typical three-electrode array was employed; using a platinum disk as working electrode and a platinum wire as counter-electrode. A silver wire immersed in acetonitrile solution with 0.1 M tetra-n-butylammonium chloride (TBACl) was used as a pseudo reference electrode. Before each measurement all solutions were bubbled with nitrogen. Cyclic voltammetry experiments were acquired from the open circuit potential (E ocp ) to positive direction, at different scan rates (from 0.1 to 0.5 V s −1 ). Square wave voltammetry experiments using amplitude of 50 mV with a frequency of 10 Hz were also performed. All potentials were reported versus the Fc/Fc + couple, according to the IUPAC convention [23].

Conclusions
The reactions of 1-amino-2,3-diferrocenylcyclopropenylium cations 1a,b with strong C-nucleophiles are nonregioselective (unlike the reactions with β-dicarbonyl compounds). The carbanions attack the C(1) and C(2) atoms of the cyclopropenylium cations with equal probability to afford two types of tetrasubstituted cyclopropenes, viz., 3-amino-1,2-diferrocenyl-and 1-amino-2,3diferrocenylcyclopropenes. Intramolecular transformations of the latter formed due to opening of the three-membered ring giving rise to two types of diferrocenylvinylcarbene species 13a,b, 14a,b, 15 and  17a,b, 18a,b differing in location of ferrocenyl, methyl, allyl and heteroaryl substituents in the carbon chain. Their intramolecular conversions to cyclic, bicyclic, tricyclic and linear final products certainly form the basis for further investigations in the fields of theoretical and synthetic organic chemistry, the search for new methods for an access to practically valuable materials.
The electrochemical behaviour of compounds 7, 8a, 10a and 10b was investigated by means of cyclic voltammetry and square wave voltammetry. For 7 and 8a two electrochemical processes (I-II), attributed to the oxidation of the ferrocene moieties were found. On the other hand for compounds 10a and 10b a single electron transfer for both ferrocene groups and the electrochemical generation of the monocation and dication species were detected.