[38333-35-6] · C13H10FeO4 · (h4-Benzylideneacetone)tricarbonyliron · (MW 286.07)
Physical Data: mp 88-89 °C.
Solubility: sol hexane, benzene, toluene, diethyl ether, THF, 1,2-dimethoxyethane, chloroform, ethyl acetate.
Form Supplied in: red crystals.
Analysis of Reagent Purity: IR, NMR.
Preparative Methods: conveniently prepared by either thermal2-4 or photochemical and thermal1,5,8a reaction of nonacarbonyldiiron with benzylideneacetone. The best procedure is heating a solution of benzylideneacetone in diethyl ether with an excess of nonacarbonyldiiron at reflux, which provides the (h4-benzylideneacetone)tricarbonyliron complex Fe(bda)(CO)3 in 81% yield (eq 1).6,7
Purification: chromatography on silica gel (ethyl acetate/petroleum ether, 1:9); recrystallization from hexane.
Handling, Storage, and Precautions: should be handled and stored under an inert gas atmosphere of argon or nitrogen. Use in a fume hood.
The Fe(bda)(CO)3 complex serves as a mild source of the tricarbonyliron fragment. The reaction of Fe(bda)(CO)3 and 1,3-dienes occurs at ca. 60 °C with smooth transfer of the tricarbonyliron moiety onto the diene ligand and affords the corresponding tricarbonyl(h4-1,3-diene)iron complexes in high yields (eq 2).5
The standard procedure for the synthesis of tricarbonyl(h4-diene)iron complexes involves thermal or photochemical reaction of the diene with Pentacarbonyliron, Nonacarbonyldiiron, or Dodecacarbonyltriiron. Thus tricarbonyliron transfer reagents offer a useful synthetic alternative for the preparation of the tricarbonyliron complexes of dienes which are sensitive towards heat or UV irradiation. For example, tricarbonyl(h4-8,8-diphenylheptafulvene)iron cannot be prepared by direct reaction of the free ligand with one of the carbonyliron compounds, but is obtained in high yield on transfer complexation with Fe(bda)(CO)3 (eq 3).2
The tricarbonyliron unit can also be used for the protection of labile diene systems in order to achieve selective transformations at other double bonds present in the molecule. Protection of the steroid B-ring diene system of ergosterol derivatives by the tricarbonyliron fragment provides the corresponding diene complex, which has been used for selective reactions at the 22,23-double bond (eq 4).8
A broad range of labile diene systems can be complexed using the Fe(bda)(CO)3 reagent and therefore stabilized by the tricarbonyliron fragment.1,9 These transfer reactions of metal fragments to diene ligands involving the benzylideneacetone ligand have been extended to the [Fe(CO)2PPh3] and the [Fe(CO)2P(OPh)3] moieties. Using this method the corresponding cycloheptadiene complexes are conveniently prepared in high yields (eq 5).4,10
The mechanism of the tricarbonyliron transfer reaction using Fe(bda)(CO)3 has been the subject of extensive investigations.1,5,10b,11 Low asymmetric inductions (<40% ee) in the complexation of prochiral dienes using optically active Fe(1-oxadiene)(CO)3 complexes have been obtained.12 Nucleophilic additions to Fe(bda)(CO)3 have also been studied.6,7,13
The photolytic reaction of pentacarbonyliron with an excess of cis-cyclooctene in hexane solution at -40 °C provides tricarbonylbis(h2-cis-cyclooctene)iron (Grevel's reagent) in 56% yield (eq 6).14
The solid compound can be handled at room temperature, but in solution the complex is labile at temperatures above -35 °C. The advantage of Grevel's reagent compared to Fe(bda)(CO)3 is that the tricarbonyliron transfer reaction occurs under extremely mild conditions at temperatures below 0 °C. Moreover, in contrast to Fe(bda)(CO)3,5 1,4-dienes can be complexed using Grevel's reagent with concomitant double bond migration to the conjugated diene system (eq 7).14a Useful applications of Grevel's reagent to organic synthesis have been described.15
More recently, (h4-1-aza-1,3-butadiene)tricarbonyliron complexes have been shown to represent very useful tricarbonyliron transfer reagents.16 The tricarbonyliron complexes of the 1-aza-1,3-butadienes can be conveniently prepared by thermal reaction with nonacarbonyldiiron.17,18 However, the yields of such complexes have been considerably increased by an ultrasound-promoted complexation of the 1-aza-1,3-butadienes at room temperature (e.g. 88% for the cinnamaldehyde/p-anisidine imine (eq 8).16
In contrast to the former reagents, the 1-azadiene complex is stable in the air for months. A smooth transfer of the tricarbonyliron fragment from the 1-azadiene complex to 1,3-dienes is effected in THF at reflux temperature and provides, for example, tricarbonyl(h4-1,3-cyclohexadiene)iron in 95% yield (eq 9).16 After the transfer the free 1-azadiene is recovered in more than 95% yield by crystallization.
The major advantage of the 1-azadiene in comparison with the two former reagents is that the corresponding free ligand can be employed in catalytic amounts. Thus the 1-aza-1,3-butadiene is a useful catalyst for the complexation of 1,3-dienes by nonacarbonyldiiron or pentacarbonyliron. The yield for the 1-azadiene-catalyzed complexation of 1,3-cyclohexadiene is quantitative based on the iron carbonyl source (eq 10).16
Immobilization of the 1-azadiene on a polystyrene resin provides a heterogeneous catalyst which is more easily separated from the reaction mixture and which can be resubmitted to the catalytic complexation.16 The mechanism of the 1-azadiene-catalyzed complexation has been investigated and it was found that condensation of cinnamaldehyde with o-anisidines provides even more efficient catalysts.19
Universität Karlsruhe, Germany