Stereo chain template effect on the s ynthesis of conjugated 1,1´-di-[ p -(X-phenyl)ethenyl]ferrocenes

The synthesis of the 1-and 1,1'-phenylethenylferrocenes of controlled stereochemistry exhibiting π -extended conjugation was satisfactorily carried out by reaction of 1,1'-ferrocene dicarboxaldehyde with p -(X-benzyl)(triphenyl)phosphorane (X= NO 2 , NMe 2 ). Moreover, the Wittig reaction of ( E )- or ( Z )-1'- p -(X-phenyl)ethenyl-1-ferrocenecarboxaldehyde shows a template effect produced by the phenylethenyl chain, which was analysed. Cyclic voltammetry studies of 1-and 1,1'- p -(X-phenyl)ethenylferrocenes exhibit shifts in oxidation potentials which are consistent with the more effective conjugation of the E than Z isomers, and the electron acceptor character of NO 2 vs. NMe 2 or CHO vs. H.


Introduction
The use of organic molecular materials for conductor and nonlinear optics is an area of considerable activity, due to their inherent synthetic flexibility, which permits the design of different molecular properties. 1 Ferrocene has attracted the interest of many scientists and research groups because of its possible applications in materials science 2 and asymmetric synthesis. 3Compounds containing ferrocene have been of use in electrochemistry and catalyst chemistry, 4 as molecular sensors, 5 ferromagnets, 6 and in nonlinear optics. 7It is now well established that molecular structures with differences in the dipole moments of their ground and excited-states have high second-order nonlinearities. 8Molecules with π donor-acceptor interactions, such as conjugated vinylferrocene, are promising candidates to show these properties.Thus, the vinylferrocene moiety has been used as a π-electron donor in several compounds, 9 with high second harmonic generation (SHG) efficiencies. 10he synthesis of conjugated ethenyl or ethynylferrocenes has been recently reported. 11,12ow, we report the synthesis of (E,E)-1,1'-di(p-X-phenyl)ethenylferrocenes with π-extended conjugation.The different electronic demands of the phenyl substituents can produce a syn or anti configuration of the chains, with a low syn/anti rotational barrier (about 0.8 Kcal.mol -1 ).5k, 13 Structures showing eclipsed syn conformation in 1,1'-diethynylferrocene or (E,E)-1,1'-di(p-Xphenyl)ethenylferrocenes have been reported. 14
Method B is a good synthetic alternative to prepare 1,1'-di(p-X-phenyl)ethenylferrocenes with different X groups, such as p-NO 2 and p-NMe 2 , with opposite electronic effects, that would permit the formation of a charge-transfer complex between the phenyl rings forced into a syn intramolecular conformation or an anti conformation by the same intermolecular association.Hence, the syn/anti ferrocene conformational equilibrium would be achieved by self-chargetransfer complex stabilisation.11d The synthesis of 1,1'-di(p-nitrophenyl)ethenylferrocene (3) was carried out using the Wittig reaction between 1,1'-ferrocenedicarboxaldehyde (FDC) and p-nitrobenzyl(triphenyl)phosphorane (NTP) in toluene (in 1,1.5 excess, method A). (E,Z)-3 was identified as the only disubstituted stereoisomer, as a red solid (46%).14a Moreover, from the reaction mixture were also isolated (E)-and (Z)-1'-p-(nitrophenyl)ethenyl-1-ferrocenecarboxaldehyde (E-1, 24 and Z-1, 18%) as a red solid, mp 142-143 ºC and a red oil respectively, Scheme 1.
Hence, the isolation and unambiguous 1 H NMR identification of the 1,1'-di(p-Xphenyl)ethenylferrocene derivatives obtained from E-1 or Z-1 with the same p-X group (X= NO 2 or NMe 2 ) and the phosphorane, showed a unique (E,Z) or (E,E)-isomer.The (Z,Z) isomer was never detected.Thus, the stereochemistry of the products obtained from (E)-1 or (Z)-1 and E-2 or Z-2 isomer, seems to play a template role on the reaction of the phosphorane with the formyl group.In this way, method B was used to explore the influence of the (E) or (Z) stereochemistry of the p-X-phenylethenyl chain on the stereochemistry of the 1,1'-disubstituted reaction products.Preparation of (E/Z)-1, 14a,14b and (E/Z)-2 pure isomers was carried out by reaction of DFC and the corresponding phosphorane, followed by acid hydrolysis.
Thus, following the Method B, compound 3 was obtained by the Wittig reaction between E-1 or Z-1 and NTP.
A mixture of (E)-1 and (Z)-1 isomers was obtained by reaction between DFC and NTP in toluene, after acid hydrolysis, in a E/Z 3,2 molar ratio, and in good yield (76%).The E-1 and Z-1 isomers were isolated pure by crystallization from hexane, Scheme 1.
Compound (E,Z)-3 was obtained as the unique isomer by reaction between (Z)-1 and NTP in dry toluene, as a red solid in good yield (62%).Wittig reaction in dry tetrahydrofuran instead toluene also gave the (E,Z) isomer as the main product (65%) but the (Z,Z) isomer was also isolated in very low yield (EZ/ZZ, 15,1, by NMR).The synthesis and structure of the (E,Z)-3 was recently reported, 14b Table 1.
Compound (E,Z)-3 was also obtained as the unique isomer, by reaction between (E)-1 and NTP in dry toluene, in 74% yield.Hence, both monosubstituted Z-1 or E-1 isomers always yield the disubstituted (E,Z)-3 isomer, although the reaction can be slightly influenced by the solvent.14a In the same way, 1,1'-di(p-N,N-dimethylaminophenyl)ethenylferrocene (4) was obtained by method B, from (E)-2 or (Z)-2 isomers and the appropriate phosphorane.The mixture of (E/Z)-2 was obtained by reaction between DFC and DTP in toluene, after acid hydrolysis, as a mixture of the (E/Z)-2 isomers in a 3,1 molar ratio (by 1 H NMR), with an excellent yield (89%).The E-2 isomer was isolated pure by crystallization from hexane, while Z-2 remained in solution, Scheme 1.Thus, (E,E)-4 was obtained by Wittig reaction between (Z)-2 and DTP in dry toluene, as a red solid, which was unambiguously identified as the only stereoisomer, in 75% yield.The same isomer (E,E)-4 was also obtained by Wittig reaction between (E)-2 and DTP in dry toluene, as the unique isomer, in 78% yield.
Thus, the Wittig reaction between E-2 or Z-2 isomers and DTP always yields the (E,E)-4 isomer.Hence, during the Wittig reaction there must be an isomerization of Z to E in order to give (E,E)-4 as the only product.Compound 5, having two different 1,1' chains, was obtained by reaction between Z-1 and DTP in dry toluene, as a red oil, which was identified as the unique isomer (Z,E)-5 in 69% yield.Hence, the stereochemistry of the (Z)-p-(nitrophenyl)ethenyl moiety was maintained throughout the reaction in the (Z,E)-5 product.However, (E,E)-5 isomer was obtained by reaction between the (E)-1 isomer and DTP in dry toluene, as a red solid, as the unique isomer, in 71% yield.The same (E,E)-5 isomer was also obtained by Wittig reaction between Z-2 and NTP in toluene, as a red solid, in 76% yield and the same (E,E)-5 isomer was also obtained by Wittig reaction between E-2 and NTP in toluene, in 84% yield, Table 1.
Hence, whatever the stereochemistry of the starting material, (E)-2 or (Z)-2, the (E,E)-5 product was obtained as the only isomer.Thus, during the Wittig reaction the starting Z isomer completely transforms to the E geometry.
In summary, the reaction of FDC and the corresponding phosphorane takes place through the 1'-p-X-phenylethenylferrocenecarboxaldehyde.The first 1-p-X-phenylethenyl chain induces a template effect on the stereochemistry of the second p-X-phenylethenyl chain during the Wittig reaction, Table 1.Moreover, the NO 2 and NMe 2 functional groups, with opposite electronic character, show different behaviour.The electron releasing character of the p-N,Ndimethylaminophenyl substituent produces a decrease in the double bond energy allowing the Z → E isomerization, while the resonance of the withdrawing electronic effect of the p-nitrophenyl group, does not influence the double bond isomerization.On the other hand, (Z,E) and (Z,Z) isomers can be entirely transformed into the (E,E) stereoisomer by prolonged sunlight exposure in solution of ethanol.11d,12a Cyclic Voltammetry Electrochemical data for the ferrocenes 1-5 are summarised in Table 2.The peak to peak separations (∆E p = E pc -E pa ) are, however, significantly greater than the ideal value of 56.5 mV at 25 ºC for a fully reversible one-electron process, probably due to a combination of uncompensated solution resistance and slightly slow electron-transfer kinetics.2d However, this difference is similar to that measured for ferrocene under comparable experimental conditions.
Processes attributable to the [m] + /[m] couple, as reversible as the ferrocenium/ferrocene couple under the same conditions, were observed for all the ferrocenyl compounds studied.The voltammogram of compound (E,E)-5 is presented in Figure 1.When compared with the previously reported 4-nitrophenylethenyl or 4-(N,N-dimethylamino)phenylethenylferrocenes, the disubstituted ferrocenes show a shift to a more anodic half-wave potential, suggesting that the extra electron-accepting substituent has an important electronic effect on the ferrocenyl fragment. 16owever, for the ferrocene complexes with analogous stereochemical features, the nitro compounds have a highest oxidation potential than the homologous (N,N-dimethylamino)phenyl derivatives, thus confirming the better electron-accepting nature of the nitro group.
The ferrocene complexes with highest degree of (E) stereochemistry (i.e.E,E > E,Z) exhibit a lower half-wave potential than those with a higher degree of (Z) stereochemistry.This is in good agreement with the greater stabilisation of the positive charge corresponding to the more effective conjugation of the (E) isomers, and indicates a higher degree of charge transfer from the metal centre to the polyene backbone. 17The aldehydes 1 and 2 show a higher half-wave potential than ferrocenyl complexes with the 4-nitro-or 4-(N,N-dimethylamino)phenylethenyl, probably due to the electron-acceptor character of the aldehyde group.Changing the aldehyde group for the corresponding conjugated chain, such as in 3-5, causes a shift to more cathodic potentials, probably because of the enhanced stabilization of the positive charge of the resultant ferrocenium species at the two conjugated ancillary chains.In order to quantify these electronic effects, E ½ can be correlated linearly with the summation of the values of the Hammett constant (σ p ) for disubstituted ferrocene derivatives, in agreement with the additive nature of the effect of the substituents, Table 2.It is evident that the increase in the electron-withdrawing ability of the substituent, as measured by the Hammett constant, is associated with an increase in the half-wave oxidation potential of the complex, as expected.

Experimental Section
General Procedures.Melting points were determined in a Reichert hot-stage microscope and are uncorrected.UV-visible spectra were recorded using a Hewlett Packard 8453 spectrophotometer.Infrared spectra were recorded using a Bruker Vector 22 spectrophotometer.NMR spectra were recorded at 300 MHz using a Bruker spectrometer; chemical shifts are given in ppm, using TMS as internal reference ( 1 H/ 1 H was used for structure assignation).Elemental analyses were performed with a LECO CHN-600.Cyclic voltammetry was carried out at room temperature with a conventional three-electrode cell in which the working and auxiliary electrodes were platinum and the reference electrode was Ag/AgCl containing 3M KCl.The working electrode consisted of a Pt-disk ultramicroelectrode (with diameter of 2 mm), sealed in glass.The supporting electrolyte tetra-n-butylammonium perchlorate (TBAP) was used as received.The solvent used in all experiments was dichloromethane of HPLC grade.The measurements were made at a scan rate of 50 mV/s.E 1/2 values were determined as (E pa + E pc )/2, where E pa and E pc are the anodic and cathodic peak potentials, respectively.All reported potentials are not corrected for the junction potential.

1'-p-(nitrophenyl)ethenyl-1-p-(N,N-dimethylaminophenyl)ethenylferrocene 5 from (Z)-2.
To a suspension of p-(nitrobenzyl)(triphenyl)phosphonium bromide (122 mg, 0.26 mmol) in dry toluene (20 ml), under argon atmosphere at room temperature, was added potassium tert-butoxide (44 mg, 0.39 mmol).The solution was stirred for 30 min and then a solution of (Z)-2 (60 mg, 0.17 mmol) in dry toluene (15 ml) was slowly added.The mixture was stirred at room temperature overnight and after evaporation of solvent, the residual solid was extracted with dichloromethane and a little water.The organic layer was dried on magnesium sulfate and after filtration and evaporation of solvent, the residual solid was purified by silica gel column chromatography using hexane/ethyl acetate 3,1 as the eluent, to give (E,E)-5, (61 mg, 76%) as a red solid, mp > 250 ºC.