[12078-17-0]  · C7H4FeO3  · Tricarbonyl(cyclobutadiene)iron  · (MW 191.96)

(stable, storeable source of cyclobutadiene)

Physical Data: pale yellow solid, mp 26 °C; bp 68-70 °C/3.0 mmHg; X-ray diffraction.2

Solubility: sol polar solvents; sparingly sol in pentane.

Form Supplied in: not commercially available.

Analysis of Reagent Purity: NMR, elemental analysis.

Preparative Methods: may be prepared in good overall yields by the reaction of 3,4-dihalocyclobutene or 1,2,3,4-tetrahalocyclobutane with Nonacarbonyldiiron,3 or by the photolysis of photo-a-pyrone with Pentacarbonyliron (eq 1).4 The reaction of cis-3,4-carbonyldioxycyclobutene with Fe2(CO)9 or Disodium Tetracarbonylferrate(-II) generates the title reagent (1), and while this reaction is stereospecific it is not known if it occurs with inversion or retention of configuration.5 Thermal reaction of Fe(CO)5 with Ni, Pd, or Pt cyclobutadiene complexes results in p-ligand exchange to give the iron cyclobutadiene complexes in high yield.6 A variety of simple substituted (cyclobutadiene)Fe(CO)3 complexes may be prepared by these methods. More recently, methodology for the regiospecific preparation of 1,2- or 1,3-differentially disubstituted (cyclobutadiene)Fe(CO)3 complexes from squaric esters has been reported.7 1,2-Disubstituted (cyclobutadiene)Fe(CO)3 complexes with two different substituents are chiral, and the two enantiomers may be separated by standard resolution techniques.8 The optically active cyclobutadiene complexes are stable to racemization at 120 °C for 48 h.

Purification: recrystallization from pentane.

Handling, Storage, and Precautions: may be stored under an inert atmosphere for extended periods.

Reactions of (Cyclobutadiene)Fe(CO)3.

The organic chemistry of (1) is much like that of Ferrocene. Acylation, formylation, chloromethylation, aminomethylation, mercuration, sulfonation, and deuteration of (1) can be effected by electrophilic substitution (eq 2).9 Friedel-Crafts alkylation of (1) to give simple monoalkyl derivatives is not possible. Acylation of monoalkyl-substituted (cyclobutadiene)Fe(CO)3 complexes generates 1,3-disubstituted (major) and 1,2-disubstituted (minor) derivatives.

In comparison to ferrocene, direct lithiation of (1) by treatment with methyl- or butyllithium is not possible.1 However, treatment of the chloromercury derivative (2) with Methyllithium or Phenyllithium at -78 °C effects transmetalation (eq 3). (Lithiocyclobutadiene)Fe(CO)3 can serve as a nucleophile toward triphenylmethyl chloride, Chlorotrimethylsilane, and ketones.10

Functionalized sidechains attached to (cyclobutadiene)Fe(CO)3 undergo a variety of standard organic transformations without disturbing the organometallic portion of the molecule.8,9a,11 Solvolysis of (chloromethylcyclobutadiene)Fe(CO)3 (3) in 5% aqueous acetone proceeds at a rate 108 times faster than that of Benzyl Chloride (eq 4).10,12 The rapid rate of this SN1 solvolysis is attributed to the stability of the a-carbocation. Chemistry of this type has been used to generate the (cyclobutadiene)Fe(CO)3 analog of phenylalanine (4) (eq 5).13

Liberation of the Ligand.

The cyclobutadiene ligand may be liberated by oxidation of the complex with Cerium(IV) Ammonium Nitrate or Lead(IV) Acetate. In the absence of other reactants the cyclobutadiene ligand undergoes dimerization to afford a mixture of syn- and anti-tricyclo[,5]octa-3,7-diene (eq 6).14

In the presence of alkenes, alkynes, or dienes, the liberated cyclobutadiene can act as either a diene or dienophile in Diels-Alder cycloadditions.15-22 The cyclobutadiene thus generated appears to be a singlet diene. Cycloaddition occurs in a stereospecific fashion with respect to the geometry of the alkene component and with endo selectivity. Compound (1) has been used as a source of cyclobutadiene for the synthesis of a wide variety of theoretically interesting molecules, including cubane,16 homocubanone,17 caged keto sulfides,18 tricyclo[,5]heptanes,19 tricyclo[,5]hept-3-ene,20 bicyclo[4.2.0]octa-2,4,6-trienes,21 Dewar benzenes,22 and Dewar furan.23 Oxidation of optically active disubstituted (cyclobutadiene)Fe(CO)3 complexes in the presence of a symmetrical alkene generates racemic products (eq 7).8 Thus the chemical oxidation appears to generate free cyclobutadiene. Additional evidence for the presence of the free ligand was provided by the three-phase test. Transfer of the ligand from a polymer-bound (cyclobutadiene)iron complex by oxidation in the presence of a separately polymer-bound dienophile can only be accounted for by the generation of the free ligand, since there is negligable contact between the functionalized sites of the two different polymeric supports.24 In contrast, oxidation of the optically enriched complex (5) containing a dangling dienophile produced the achiral phthalan and the optically active tricyclic ketone (6) (eq 8).25 The product (6) is proposed to arise via initial coordination of the dienophile to the oxidized complex, followed by alkene insertion, carbonyl migration, and reductive elimination.

Photolysis of substituted (cyclobutadiene)Fe(CO)3 complexes in the presence of alkynes affords substituted benzenes as a mixture of regioisomers (eq 9).26 The distribution of products is not statistical, which suggests that the free cyclobutadiene ligand is not involved. It is proposed that the reaction occurs via initial loss of carbon monoxide and coordination of the alkyne. Insertion of the alkyne generates a bridged species which reductively eliminates to a Dewar benzene. Secondary photolysis of the Dewar benzene gives the aromatic product.

The photochemical reduction of (benzoylcyclobutadiene)Fe(CO)3 in acetic acid gives 2-cyclobutenyl phenyl ketone as the product (eq 10).27

1. Efraty, A. CRV 1977, 77, 691.
2. For a compilation of cyclobutadieneiron X-ray diffraction structures, see: Herndon, W. C. JOM 1982, 232, 163.
3. (a) Emerson, G. F.; Watts, L.; Pettit, R. JACS 1965, 87, 131. (b) Pettit, R.; Henery, J. OS 1970, 50, 21.
4. Rosenblum, M.; Gatsonis, C. JACS 1967, 89, 5074.
5. Brune, H. A.; Eberius, W.; Wolff, H. P. JOM 1968, 12, 485.
6. (a) Canziani, F.; Chini, P.; Quarta, A.; DiMartino, A. JOM 1971, 26, 285. (b) Pollock, D. F.; Maitlis, P. M. JOM 1971, 26, 407.
7. (a) Adams, C. M.; Schemenaur, J. E.; Crawford, E. S.; Joslin, S. A. SC 1992, 22, 1385. (b) Adams, C. M.; Joslin, S. A.; Crawford, E. S.; Schemenaur, J. E. OM 1993, 12, 656.
8. (a) Schmidt, E. K. G. AG(E) 1973, 12, 777. (b) Grubbs, R. H.; Grey, R. A. CC 1973, 76. (c) Grubbs, R. H.; Grey, R. A. JACS 1973, 95, 5765.
9. (a) Fitzpatrick, J. D.; Watts, L.; Emerson, G. F.; Pettit, R. JACS 1965, 87, 3254. (b) Marcincal, P.; Cuingnet, E. TL 1975, 1223.
10. Davis, R. E.; Simpson, H. D.; Grice, N.; Pettit, R. JACS 1971, 93, 6688.
11. (a) Mauldin, C. H.; Biehl, E. R.; Reeves, P. C. TL 1972, 2955. (b) Wilson, S. R.; Tofigh, S.; Misra, R. N. JOC 1982, 47, 1360. (c) Biehl, E. R.; Reeves, P. C. S 1974, 883.
12. Reeves, P. C. JOM 1981, 215, 215.
13. Brunet, J. C.; Cuingnet, E.; Gras, H.; Marcincal, P.; Mocz, A.; Sergheraert, C.; Tartar, A. JOM 1981, 216, 73.
14. Watts, L.; Fitzpatrick, J. D.; Pettit, R. JACS 1966, 88, 623.
15. (a) Paquette, L. A.; Stowell, J. C. JACS 1971, 93, 5735. (b) Meinwald, J.; Mioduski, J. TL 1974, 3839. (c) Meinwald, J.; Mioduski, J. TL 1974, 4137. (d) Masamune, S.; Nakamura, N.; Spadaro, J. JACS 1975, 97, 918. (e) Martin, H. D.; Hekman, M.; Rist, G.; Sauter, H.; Bellus, D. AG(E) 1977, 16, 406. (f) Wildi, E. A.; Carpenter, B. K. TL 1978, 2469. (g) Hasselmann, D.; Loosen, K. AG(E) 1978, 17, 606. (h) Whitman, D. W.; Carpenter, B. K. JACS 1980, 102, 4272. (i) Paquette, L. A.; Gugelchuk, M.; Hsu, Y.-L. JOC 1986, 51, 3864. (j) Warrener, R. N.; Wang, J.-M.; Weerasuria, K. D. V.; Russell, R. A. TL 1990, 31, 7069. (k) Mehta, G.; Reddy, S. H. K. SL 1993, 75.
16. Barborak, J. C.; Watts, L.; Pettit, R. JACS 1966, 88, 1328.
17. Barborak, J. C.; Pettit, R. JACS 1967, 89, 3080.
18. Paquette, L. A.; Wise, L. D. JACS 1967, 89, 6659.
19. Gree, R.; Park, H.; Paquette, L. A. JACS 1980, 102, 4397.
20. Roth, W. R.; Klärner, F. G.; Grimme, W.; Köser, H. G.; Busch, R.; Muskulus, B.; Breuckmann, R.; Scholz, B. P.; Lennartz, H. W. CB 1983, 116, 2717.
21. Paquette, L. A.; Hefferon, G. J.; Samodral, R.; Hanzawa, Y. JOC 1983, 48, 1262.
22. (a) Watts, L.; Fitzpatrick, J. D.; Pettit, R. JACS 1965, 87, 3253. (b) Burt, G. D.; Pettit, R. CC 1965, 517.
23. Pitt, I. G.; Russell, R. A.; Warrener, R. N. JACS 1985, 107, 7176.
24. Rebek, J., Jr.; Gaviña, F. JACS 1975, 97, 3453.
25. Grubbs, R. H.; Pancoast, T. A. JACS 1977, 99, 2382.
26. (a) Pruitt, P. L.; Biehl, E. R.; Reeves, P. C. JOM 1977, 134, 37. (b) Gist, A. V.; Reeves, P. C. JOM 1981, 215, 221.
27. Franck-Neumann, M.; Martina, D.; Brion, F. AG(E) 1978, 17, 690.

William A. Donaldson

Marquette University, Milwaukee, WI, USA

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