Pentacarbonyliron 1,4-diene complexation

The classical protocol for synthesis of iron-diene complexes starts from the homoleptic pentacarbonyliron complex. In a stepwise fashion, via a dissociative mechanism, two carbonyl ligands are displaced by the diene system. However, thermal dissociation of the first CO ligand requires rather harsh conditions (ca. 140 °C). For acyclic 1,3-dienes, the diene ligand adopts an s-cis conformation to form stable q4-complexes (Scheme 1.18). 

The catalytic complexation of cyclohexa-1,3-diene 2b is much superior to the classical procedures by direct reaction of the diene with pentacarbonyliron... 

A catalytic process for the complexation of cyclohexadiene with pentacarbonyliron using 0.125 equivalents of the 1-azabuta-l,3-diene in refluxing dioxane affords quantitatively the corresponding tricarbonyliron complex [55c]. Supported by additional experimental evidence, the mechanism shown in Scheme 1.23 has been proposed for the 1-azadiene-catalyzed complexation [52]. 

The first aspect is illustrated by the synthesis of tricarbonyl(ri4-l,3-diene)iron complexes from pentacarbonyliron in the presence of catalytic amounts of a 1-azabuta-... 

Alkyl-allyl complexes of isomeric systems can be interconverted and thus be used in isomerization of vinylcyclopropanes. Ethyl 4-azabicyclo[5.1.0]octa-2,5-diene-4-carboxylate (20) reacts with pentacarbonyliron to give complex 21, which photochemically rearranges to complex 23. Carbonylation of both products 21 and 23 leads to ethyl 9-oxo-2-aza-bicyclo[3.3.1]nona-3,7-diene-2-carboxylate (22). While complex 21 upon heating regenerates the starting material, complex 23 gives the isomeric product 24. In contrast to iron, with rhodium only the endo-complex 25 is formed. ... 

Vinylcyclopropanes can react with pentacarbonyliron to give two different types of addition products i) (diene)tricarbonyliron 7r-complexes as the result of ring opening, hydrogen shift, and complexation ii) cyclohexenones by photoinduced carbonyl insertions across the vinyl-cyclopropane system. 

Pentacarbonyliron releases its carbonyl ligands upon heating or exposure to UV light, and the resulting unsaturated species are known to react with various kinds of 1,3-dienes to form dieneiron tricarbonyl complexes. For example, (butadiene)tricar-bonyliron is prepared by the direct reaction of 1,3-butadiene with Fe(CO)5 at 130-140 °C in a closed system (eq (12)) [42]. 

The synthesis of 7-oxygenated carbazoles via the iron-mediated route requires the 2-methoxy-substituted iron complex salt 52 (Scheme 14). Complexation of the methoxycyclohexadienes 47 and 48 with pentacarbonyliron in the presence of catalytic amounts of l-(p-anisyl)-4-phenyl-l-azabuta-1,3-diene affords a mixture of the regioisomeric 1 -methoxy- and 2-methoxy-ri -cyclohexadieneiron complexes 49 and 50 [77, 78, 84]. Hydride abstraction by triphenylcarbenium tetrafluoroborate provides a mixture of regioisomeric ri -cyclohexadienyUron complexes 51 and 52. Separation of the 1-methoxy- and 2-methoxy-substituted complex salts 51 and 52 can be achieved by selective hydrolysis of 51 to the cyclohexadieno-ne—tricarbonyliron complex 53 [74]. 

Ghiral 1-aza-l,3-butadienes react with nonacarbonyldiiron in THF at RT using ultrasound, to yield a 5 1 mixture of the (S,R)-32 and R,R)-33 diastereoisomers, respectively. The crystal structure of 32 has been reported (Scheme 27). The free ligands of 1-aza-l,3-dienes are reported to be efficient catalysts for the complexation of 1,3-dienes with either pentacarbonyliron or nonacarbonyldiiron.In this respect, the heterodiene complexes serve as useful tricarbonyliron transfer reagents, to yield free 1-azabuta-1,3-dienes (Scheme 28). 

The tricarbonyliron fragment can be attached to a diene system by a variety of transfer reagents. Homoleptic carbonyliron complexes are the most simple ones and have been known for the longest time. The replacement of two carbonyl ligands in pentacarbonyliron by a diene ligand requires quite harsh conditions. A temperature of approximately 140 °C or irradiation is necessary to induce this dissociative process. (Scheme 4-101). ° ... 

Scheme 4-101. Formation of ( q -diene)iron complexes by reaction of pentacarbonyliron with 1,3-butadienes.

A solution of pentacarbonyliron (50.0 g, 33.6 mL, 255 mmol), l-(4-methoxyphenyl)-4-phenyl-l-azabuta-1,3-diene (7.70 g, 32.4 mmol), and cyclohexa-l,3 iene (31.3 g, 37.2 mL, 391 mmol) in dioxane (200 mL) is heated at reflux. During the reaction, a light flow of argon is directed through the apparatus. After a reaction time of 72 h, a solution of the azabutadiene catalyst (2.00 g, 8.43 mmol) and cyclohexa-1,3-diene (4.21 g, 5.0 mL, 52.5 mmol) in dioxane (50 mL) is added. The reaction mixture is stirred at reflux for 24 h. Caution Carbon monoxide is formed in the course of the reaction). The solvent is evaporated, pentane (200 mL) is added to the residue, and the solution is filtered over Celite to provide, after evaporation of the solvent, the tricarbonyliron complex as a yellow oil 50.2 g (89%). ... 

An asymmetric variant of this reaction can be carried out introducing a camphor-derived 1-azabutadiene ligand. Enantiomeric excess values of up to 86% can be achieved with this system to obtain planar chiral iron complexes. The photolytically induced reaction of pentacarbonyliron with prochiral cyclohexa-1,3-dienes can also be run enantioselectively using a chiral 1-azabutadiene catalyst. Quantitative yields and ee values up to 86% are possible under these conditions. Cyclic 1,4-dienes can also be complexed by pentacarbonyliron under concomitant rearrangement to the 1,3-diene... 

Tricarbonyl(Ti -cyclohexadienylium)iron tetrafluoroborate was described first by Fischer in 1960. The required tricarbonyl(T -cyclohexadiene)iron complex can be conveniently obtained in high yield from cyclohexa-1,3-diene and pentacarbonyliron using catalytic amounts of a 1-azabuta-1,3-diene (see Section 2.5.1). Hydride abstraction from the tricarbonyl(ri -cyclohexadiene)iron complex using triphenylmethyl tetrafluoroborate affords the Ti -dienyliumiron complex salt almost quantitatively (Scheme 4-1... 

Cyclohexadiene Series. Tricarbonyliron complexes are formed by reaction of Fe(CO)4 with dienes. Fe2(CO)9 is an important starting material for this reaction because it offers a convenient source of Fe(CO)4 generated under much milder conditions than from pentacarbonyliron (eq 2). 

Starting from a cisoid 1,3-diene, Fe2(CO)9 can yield, under mild thermal conditions, tricarbonyl(T -l,3-diene)iron complexes. Cyclohexa-1,4-dienes (readily available from the Birch reduction of substituted benzenes) often require prior conjugation, but conjugated 1,3-dienes (both cyclic and acylic) can be converted into tricarbonyliron complexes directly. Some of the earliest examples " of complexation of 1,3-dienes employed Fe2(CO)9 (eq 3, dates shown in parentheses). (Alternative reagents in the early days were Pentacarbonyliron or Dodecacarbonyltriiron.)... 

Acyclic Series. The first complex in the acyclic series was prepared from butadiene by the thermal method. Heating isoprene and pentacarbonyliron at high temperature, however, is inefficient due to competitive Diels-Alder dimerization. Despite the formation of some bis(diene) iron carbonyl complexes on prolonged irradiation, the photochemical method is superior in this case. Complexation of acyclic dienes by Fe(CO)3 is limited to those that can adopt a cisoid conformation, with the syn substitution pattern normally preferred. 2,4-Hexadienolc acid, for example, ean be conveniently complexed by a photolytic procedure. Trialkylsilyl-substituted dienes have also been complexed. ... 

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