[38960-10-0]  · C10H10FeO2  · Allyldicarbonylcyclopentadienyliron  · (MW 218.04)

(allyl 1,3-dipole equivalent which undergoes [3 + 2] cycloaddition with electrophilic carbon2 and heteroatom3 dipolarophiles; reagent for the Lewis acid-promoted allylation of aldehydes and unactivated ketones;4 reacts with cationic oxyallyl iron complexes to give [3 + 3] cycloadducts5)

Physical Data: amber oil which decomposes at ~65 °C.

Solubility: sol all common organic solvents.

Preparative Methods: prepared under an inert atmosphere by the addition of excess Allyl Chloride to a degassed THF solution of Sodium Dicarbonylcyclopentadienylferrate at 0 °C.6 The mixture is stirred for 15 min, warmed to room temperature over 1 h, and concentrated in vacuo. The residue is extracted with pentane, filtered, then concentrated in vacuo to give Cp(CO)2Fe(CH2HC=CH2) in 96% crude yield.7

Purification: may be protonated with HBF4 to give the cationic propene complex, Cp(CO)2Fe(h2-H2C=CHMe)+BF4-, which is precipitated with Et2O and washed with a large volume of Et2O. Cp(CO)2Fe(CH2HC=CH2) is then regenerated with Et3N prior to use.7,8

Handling, Storage, and Precautions: all manipulations involving Cp(CO)2Fe(CH2HC=CH2) should be carried out under an inert atmosphere. Solvents should be thoroughly dried and degassed. The toxicity of allyldicarbonylcyclopentadienyliron has not been fully investigated. For comparative data, see Cp(CO)2FeI or Bis(dicarbonylcyclopentadienyliron) (Sigma-Aldrich Material Safety Data Sheets available).

Cycloaddition Reactions.

Allyldicarbonylcyclopentadienyliron (hereafter referred to as allyl-Fp) is a 1,3-dipole equivalent which undergoes [3 + 2] cycloaddition reactions with electron deficient alkenes (eq 1) to give high yields of the corresponding cyclopentannulation products.7 The reaction occurs in two steps. Initial attack of the g-carbon of Cp(CO)2Fe(CH2HC=CH2) on the electron-deficient carbon atom generates a zwitterionic iron alkene complex (1), which collapses to form the metalated cyclopentane product (see eq 2). Allyl-Fp also reacts with electrophilic heteroatom dipolarophiles such as N-tosyl isocyanates and N-sulfonylurethanes to give g-lactams and -sultams, respectively.3

Cyclohexenones react with allyl-Fp to give cis-fused hydrindane products.9 The reaction is promoted by the addition of Lewis acids such as Aluminum Bromide; electron-deficient cycloalkenones such as 1-ethoxycarbonylcyclohexenone undergo cyclopentannulation in the absence of added Lewis acid. These reactions are sensitive to steric effects at the 3-position of the cycloalkenone.

A variety of methods have been developed for demetalation of the cyclopentyl iron complexes.7 Esters are obtained by oxidation with Cerium(IV) Ammonium Nitrate (CAN) in ethanol under a carbon monoxide atmosphere (eq 2). Proteolytic demetalation with HCl has been employed to generate cyclopentanes; however, reversal of the cycloaddition is sometimes observed, and is a major drawback of this method.7,10

Metal-complexed alkenes have been shown to react with allyl-Fp to give bimetallic condensation products in moderate yields. For simple alkene complexes such as Cp(CO)2Fe(h2-H2C=CHR)+BF4- (R = H, Me, Ph), the reaction proceeds readily with little regiospecificity.10 Acrolein complexes (eq 3) react regiospecifically to give the product of Michael-type addition to the complexed enal.10 Sodium Iodide in acetone selectively removes the cationic iron group from the alkene.10 The alkyl-iron bond is selectively cleaved by treatment with HCl.10

A variety of cationic tricarbonyl tropyliumiron complexes add allyl-Fp to give bimetallic hydroazulene complexes via [3 + 2] cycloaddition in moderate to good yields.11 The cyclopentane ring in the product is cis-fused to the seven-membered ring, trans to the iron tricarbonyl unit. The product may be further functionalized by addition of electrophiles or nucleophiles.11c

In addition to [3 + 2] cycloadditions with two-carbon dipolarophiles, allyl-Fp undergoes a [3 + 3] cycloaddition reaction with cationic oxyallyl iron complexes.5 The oxyallyl complex is generated in situ by refluxing a CH2Cl2 solution of Nonacarbonyldiiron and an a,a-dibromo ketone. The yields for the reaction are moderate for tetrasubstituted, trisubstituted, or a,a-disubstituted ketones; monosubstituted dibromo ketones fail to give cyclic products.

Allylation of Aldehydes and Ketones.

Allyl-Fp also reacts with aldehydes and ketones in the presence of Lewis acids such as Boron Trifluoride Etherate.4 Under these conditions the cationic iron alkene complex does not undergo internal trapping to give a substituted tetrahydrofuran. Instead, the iron alkene complex is isolated, which may be demetalated with sodium iodide in wet acetone to give the homoallyl alcohol (overall isolated yields range from 50-98%). In this respect the reagent is reminiscent of allylsilane or -stannane reagents. However, the ability to isolate a homoallyl alcohol with iron coordinated to the alkene gives the potential for further elaboration which is not available with silanes and stannanes.

Finally, it must be mentioned that homologs of allyl-Fp, such as methallyl-,1 2-12 or 3-methoxyallyl-,13 and propargyldicarbonylcyclopentadienyliron,14 undergo many similar transformations. These related reagents allow for further versatility and unique reactivity.

Related Reagents.

Dicarbonyl(cyclopentadienyl)(isobutene)iron Tetrafluoroborate.

1. Rosenblum, M. ACR 1974, 7, 122.
2. Giering, W. P.; Rosenblum, M. JACS 1971, 93, 5299.
3. Giering, W. P.; Raghu, S.; Rosenblum, M.; Cutler, A.; Ehntholt, D.; Fish, R. W. JACS 1972, 94, 8251.
4. (a) Agoston, G. E.; Cabal, M. P.; Turos, E. TL 1991, 32, 3001. (b) Jiang, S.; Turos, E. TL 1991, 32, 4639.
5. Hegedus, L. S.; Holden, M. S. JOC 1985, 50, 3920.
6. (a) Eisch, J. J.; King, R. B. Organometallic Syntheses; Academic: New York, 1965; Vol. 1, p 161. (b) Abram, T. S.; Baker, R. Synth. React. Inorg. Metal-Org. Chem. 1979, 9, 471.
7. Abram, T. S.; Baker, R.; Exon, C. M.; Rao, V. B. JCS(P1) 1982, 285.
8. Note: Cp(CO)2Fe(CH2HC=CH2) has also been purified by short path distillation: Lee, M.-T.; Waterman, P. S.; Magnuson, R. H.; Meirowitz, R. E.; Prock, A.; Giering, W. P. OM 1988, 7, 2146.
9. Bucheister, A.; Klemarczyk, P.; Rosenblum, M. OM 1982, 1, 1679.
10. Lennon, P. J.; Rosan, A.; Rosenblum, M.; Tancrede, J.; Waterman, P. JACS 1980, 102, 7033.
11. (a) Rosenblum, M.; Watkins, J. C. JACS 1990, 112, 6316. (b) Watkins, J. C.; Rosenblum, M. TL 1985, 26, 3531. (c) Watkins, J. C.; Rosenblum, M. TL 1984, 25, 2097. (d) Genco, N.; Marten, D.; Raghu, S.; Rosenblum, M. JACS 1976, 98, 848.
12. Abram, T. S.; Baker, R.; Exon, C. M.; Rao, V. B.; Turner, R. W. JCS(P1) 1982, 301.
13. (a) Baker, R.; Keen, R. B.; Morris, M. D.; Turner, R. W. CC 1984, 987. (b) Baker, R.; Exon, C. M.; Rao, V. B.; Turner, R. W. JCS(P1) 1982, 295.
14. See, for example, Ref. 11(a) and references therein.

Todd L. Underiner

The Procter & Gamble Co., Cincinnati, OH, USA

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