Organometallics 1995,14, 4334-4342
4334
Tetraferrocenylethylene,a Chiral, Organometallic Propeller: Synthesis, Structure, and Electrochemistry Benno Bildstein,*lt Peter Denifl,? Klaus Wurst,? Max Andre,$ Martin Baumgarten,s Jan Friedrich,$ and Ernst Ellmerer-Miillerl' Institut f i r Allgemeine, Anorganische, und Theoretische Chemie, Universitat Innsbruck, Innrain 52a, A-6020 Innsbruck, Austria, Biochemie GmbH, A-6250 Kundl, Austria, Max-Planck-Institut f i r Polymerforschung, 0-55021 Mainz, Germany, and Institut f i r Organische Chemie, Universitat Innsbruck, Innrain 52a, A-6020 Innsbruck, Austria Received April 24, 1995@ Tetraferrocenylethylene is synthesized from diferrocenyl ketone by three different reductive carbon-carbon bond-forming methodologies: (a) an ultrasound-promoted McMurry reaction with low-valent titanium, (b) a modified Clemmensen reduction with zinc and trimethylchlorosilane, and (c) an aluminum-assisted oxygen-tellurium exchange in diferrocenyl ketone and subsequent thermolysis. Mechanistically, the first two methods involve carbenoid intermediates, whereas the third method consists of a twofold extrusion process from a preformed cyclic dimer of diferrocenyl telluroketone. Tetraferrocenylethylene shows spectral properties which are in accord with a sterically highly congested conformation. Noteworthy features include the very low C-C stretching vibration of 1474 cm-' in the Raman spectrum, indicative of an elongated and weak C-C double bond, and the magnetic inequivalence of the lH and 13CNMR signals of the hydrogens and carbons of the substituted cyclopentadienyl rings, indicative of a frozen molecular propeller conformation. An X-ray single-crystal structure analysis shows tetraferrocenylethylene to be a chiral, strongly twisted, and sterically congested olefin. The bond length of 138.1 pm of the central double bond and the angles of twisting and torsion are close in value to those of the most distorted olefins known. The helical chirality stems from the uniform twisting of the four alternatingly arranged ferrocenyl substituents. Electrochemically, tetraferrocenylethylene can be oxidized to the tetracation in accord with the number of ferrocenyl units. The donor ability of tetraferrocenylethylene compared to ferrocene itself is strongly enhanced with A,?P1/2 = -220 mV.
Introduction The normal properties of the olefinic double bond in ethylene can be substantially altered by fourfold attachment of sterically demanding, electron-donating or electron-acceptinggroups. Perturbation of the preferred molecular geometry by bulky substituents can lead to twisting of the double bond or to pyramidalization of the olefinic carbon atoms and to elongation of the C-C double bond. Electron-donatingsubstituents reduce the oxidation potential, whereas electron-accepting substituents increase the electron affinity of the olefinic double bond. In comparison to ethylene these effects result in molecular distortion1 of these tetrasubstituted olefins and in unusual redox behavior and reactivitya2
' Institut fur Allgemeine, Anorganische, und Theoretische Chemie, Universitat Innsbruck. Biochemie GmbH. 8 Max-Planck Institut fur Polymerforschung. I' Institut fur Organische Chemie, Universitat Inssbruck. Abstract published in Advance ACS Abstracts, July 15, 1995. (1)(a)Luef, W., Keese, R. Strained Olefins: Structure and Reactivity of Nonplanar Carbon-Carbon Double Bonds. Top. Stereochem. l S S l p 0 , 231. (b) Bock, H.; Ruppert, K.; Nather, C.; Havlas, Z.; Herrmann, H.F.; h a d , C.; Gobel, I.; John, A.; Meuret, J.; Nick, S.; Rauschenbach, A.; Seitz, W.; Vaupel, T.; Solouki, B. Angew. Chem. 1992, 104, 564; Angew. Chem., Int. Ed. Engl. lSS2,31, 550. (c) Borden, W. T. Chem. Rev. 1989,89, 1095. ( 2 )(a) Wiberg, N. Angew. Chem. 1968,80,809;Angew. Chem., Int. Ed. Engl. 1968, 7 , 766. tb) Hoffmann, R. W. Angew. Chem. 1968,80, 823;Angew. Chem., Int. Ed. Engl. 1968, 7 , 7 5 4 .(c) Hocker, J.; Merten, R. Angew. Chem. 1972,84, 1022; Angew. Chem., Int. Ed. Engl. 1972, 1 1 , 964. (d) Lappert, M. F. J. Orgunomet. Chem. 1988, 358, 185. (e) Fatiadi, A. J. Synthesis 1986, 249. (0 Fatiadi, A. J. Synthesis 1987, 85. (g) Fatiadi, A. J . Synthesis 1987, 959.
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In this context, tetraferrocenylethyleneis an interesting target compound because of the electron-donating properties and steric requirements of the ferrocenyl moiety. Structurally, the fourfold substitution of the carbon-carbon double bond should result in a highly strained molecule with an exceptionally elongated olefinic bond. Stereochemically,the twisting of the cyclopentadienyl rings in relation to the C=C plane should cause atropisomerism. Electronically, the powerful electron-donating capacity of the ferrocenyl moieties3 will substantially ease oxidation. Here we report the synthesis, characterization (NMR, IR, W-vis, Raman, MS), structure (X-ray),attempted separation of enantiomers by HPLC, and electrochemistry (CV,PES) of tetraferr~cenylethylene.~
Results and Discussion
Preparation of Tetraferrocenylethylene (1). Compound 1 can be prepared by three different synthetic routes, as outlined in Scheme 1. Method a. The ultrasound-promotedMcMuny reac( 3 )(a) Watts, W. E. J. Organomet. Chem. Libr. 1979, 7 , 399. (b) Watts, W. E. In Comprehensive Organometallic Chemistry; Wilkinson, G., Stone, F. G. A., Abel, E. W., Eds.; Pergamon: Oxford, U.K., 1982; Vol. 8,Chapter 59, p 1051. ( 4 )Part of this work has been presented as a poster at the XVth International Conference on Organometallic Chemistry, Warsaw, Poland, Aug 9-14, 1992 and at the Xth FECHEM Conference on Organometallic Chemistry, Agia Pelagia, Crete, Greece, Sept 5-10, 1993.
0276-733319512314-4334$09.00/0 0 1995 American Chemical Society
Organometallics,Vol. 14,No.9, 1995 4335
Tetraferrocenylethylene
Scheme 2. Twofold Te-ExtrusionMechanism for Method ca
Scheme 1"
2
Fc Fc
:C=Te
I '
+
Fc
Fc, Fc"'"Fc
Fc 2
3
a Legend: (a)TiC1~~3THF/Li/DME/ultrasoun& (b)Zn/Me&CY THF; (c) Med-Te-AlMeddioxane; Fc = ferrocenyl.
tion5 of diferrocenyl ketone gives a high yield of 1 (80%) in a mixture with tetraferrocenylethane6 (2) and traces of hexaferrocenylcyclopropane (3). Unsuccessful attempts to synthesize compound 1 from diferrocenyl ketone or from diferrocenyl thioketone by a McMurry reaction have been r e p ~ r t e d . The ~ failure to obtain olefinic products has been attributed to steric hindrance, but the combination of the proper choice of reducing agent (TiCly3THFLi versus the more common hydridecontaining TiCldLiAlH4 or versus TiClJZn) and improved reaction conditions (ultrasonic enhancements of the heterogeneous McMurry reaction) results in complete consumption of the starting material diferrocenyl ketone with a good yield of a product mixture of 1, 2, and 3. Although the exact nature of the low-valent Ti reagent and the mechanism of the McMurry reaction is still under discus~ion,~ the small traces of hexaferrocenylcyclopropane (31, a formal trimer of diferrocenylmethylidene, indicates the involvement of carbenoid intermediates, which either couple to olefin 1 or oligomerize to cyclopropane 3. Ethane 2 is formed most likely by hydrogen abstraction from the solvent. Method b. Reduction of diferrocenyl ketone by Z d Me3SiCl similarly affords a mixture of mainly 1 with 2 as byproduct. Mechanistically, this reaction can be viewed as a modificationlo of the more familiar Clemmensen reduction, in which the proton has been replaced by a silicon electrophile, whose high oxophilicity removes the carbonyl oxygen as hexamethyldisiloxane and generates an organozinc carbenoid, which forms products 1 and 2 in a manner analogous to that in method a. In this case, no trimerization product 3 can be detected. Method c. Thermolysis of diferrocenyl telluroketone, prepared from diferrocenyl ketone by reaction with bis(dimethylaluminum) telluride,ll yields almost quanti( 5 )(a) McMurry, J . E. Chem. Rev. 1989,89, 1513. (b) Lenoir, D. Synthesis 1989,883. (c) Betschart, D.; Seebach, D. Chimia 1989,43, 39.( d ) McMurry, J. E. Acc. Chem. Res. 1983,16,405. (6)Paulus, H.; Schlogl, K.; Weissensteiner, W. Monatsh. Chem. 1982,113,767. (7) (a) Lenoir, D.,Burghard, H. J . Chem. Res. Synop. 1980,396.(b) Sato, M.; Asai, M. J . Organomet. Chem. 1992,430,105. (8) Navak. S. K.: Banerii, A. J . 0r.g. Chem. 1991,56,1940. (9) Fuistner, A. Angew."Chem. 1953,105,171;Angew. Chem., Int. Ed. Engl. 1993,32,164. (10)(a) Motherwell, W. B.; Nutley, C. J. Contemp. Org. Synth. 1994, 1,219.(b) Motherwell, W. B. Aldrzchim. Acta 1992.25,71.(c)Banerjee, A.K.; de Carrasco, M. C. S.; Frydych-Houge, C. S. V.; Motherwell, W. B. J . Chem. SOC.,Chem. Commun. 1986,1803. (11)Denifl, P.;Bildstein, B. J . Organomet. Chem. 1993,453,53.
1
> W C
Fc Fc, ,Fc F=C,Fc Fc
-
r
[2 + 21
,Fc
- Te
Fc, ,Te Fc
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