Iron complexes, with ketene

In 1991, Thomas reported88a that the reaction between vinylketene complexes (221) and several phosphonoacetate anions generated vinylallene complexes (246), in some cases with extremely high stereoselectivity.88,89 This Wadsworth-Emmons type reaction occurs via attack by the phosphonoacetate carbanion at the ketene carbonyl carbon, and product ratios clearly depend on the steric bulk of the R and R substituents. The relative stereochemistry of the major isomers of 246 were determined by X-ray analysis. Upon oxidation of the vinylallene complexes with iron(III) chloride, a range of substituted furanones were isolated.8813,89... 

Many of the syntheses we have seen within this review depend on the carbonylation of a vinylcarbene complex for the generation of the vinylketene species. The ease of this carbonylation process is controlled, to some degree, by the identity of the metal. The electronic characteristics of the metal will clearly have a great effect on the strength of the metal-carbon double bond, and as such this could be a regulating factor in the carbene-ketene transformation. It is interesting to note the comparative reactivity of a (vinylcarbene)chromium species with its iron analogue The former is a fairly stable species, whereas the latter has been shown to carbonylate readily to form the appropriate (vinylketene)iron complex. 

In light of the above results it is interesting to note that the reaction of diphenylcyclo-propenone dimer spirolactone with ironenneacarbonyl yields a mixture of ring-opened vinyl carbene and -vinylketene complexes, and these interconvert under addition (or removal) of CO (equation 225) . A possible pathwav to vinylketene Fe-complexes, prepared earlier from cyclopropenes and ironcarbonyls " , may thus involve initial f -coordination, followed by ring cleavage to vinyl carbene and finally carbonylation to the ketene iron // -complexes. An analogous // -manganese complex is prepared similarly by the reaction of CpMn(CO),THF with 3,3-dimethylcyclopropene complex (equation 226) . ... 

The chemistry of iron vinylidene complexes is dominated by the electrophilicity of the carbon atom adjacent to the iron organometallic unit. While addition of water leads to an acyl complex (i.e., the reverse of the dehydration shown in equation 10), addition of an alcohol leads to a vinyl ether complex. Similarly, other iron vinyl complexes can be prepared by the addition of thiolate, hydride, or an organocuprate (Scheme 33). " The nucleophilic addition of imines gave enaminoiron intermediates that could be further elaborated into cyclic aminocarbenes. This methodology has been used to provide access to /3-lactams and ultimately penicillin analogs, and good diastereoselectivities were observed (6 1-15 1) (Scheme 34). 04 Iso, vinylidene complexes are intermediates in cyclizations of alkynyl irons with substituted ketenes, acid chlorides, and related electrophiles an example is shown (equation 11). These cyclizations led to the formation of a series of isolable and characterizable cyclic vinyl iron complexes. 

Vinylketone easily reacts with Fe2(CO)9 to afford an oxadiene 7r-complex in high yield as shown in Scheme 15.6. The 7r-complex reacts with an alkyllithium under nitrogen atmosphere to afford 1,4-diketone. However, under a carbon monoxide atmosphere vinylketene is obtained as shown in Scheme 15.6 [79,80]. The formation of the 1,4-diketone, as shown in Scheme 15.7 [80], is at first the alkylanion to the carbonyl carbon with the addition of alkyllithium to form iron carbene, and the metathesis of the iron carbene with C = C proceeds. On the other hand, under a carbon monoxide atmosphere, the metathesis of the iron carbene with C = O proceeds, the alkyl group of alkyllithium is eliminated as a carboxylic acid to form an iron carbene, and reacts with carbon monoxide to afford a vinyl ketene as shown in Scheme 15.7 [80]. 

New catalysts have heen recently developed for promoting the aldol-type addition of acetate-derived silyl ketene acetals with high efficiency 10-methylacridinium perchlorate (5 mol %), cationic mono- and dinuclear iron complexes (5 mol %), t-hutyldimethylsilyl chloride-indium(III) chloride (10 mol %), [l,2-henzenediolato(2—)-0,0 ]oxotitanium(20 mol%), phos-phonium salts (7 mol%), and trityl salts (5-20 mol%). ... 

Aldol reactions of aldehydes with cycloakanones were performed in ionic liquids and catalyzed by FeCl3-6H20 [32]. Mukaiyama aldol reactions of silylenol ethers with aldehydes can be carried out in aqueous media however, among several Lewis acidic catalysts investigated, iron compounds were not the optimal ones [33], If silyl ketene acetals are applied as carbon nucleophiles in Mukaiyama aldol reactions, cationic Fe(II) complexes give good results. As catalysts, CpFe(CO)2Cl [34] and [CpFe(dppe) (acetone)] BF4 [35] [dppe = l,2-bis(diphenylphosphano)ethane] were applied (Scheme 8.8). No diastereomeric ratio was reported for product 26a. 

Photolysis of hydroxy Fischer carbene complexes (96) (Scheme 20) in the presence of alcohols under several atmospheres of carbon monoxide gives low to moderate yields of a-hydoxy esters (97). It is proposed that the reactions proceed via ketenes formed from the liberated or complexed carbenes and CO. In some cases, acetals formed via thermal decomposition of the carbenes are the major products. Photolysis of iron porphyrin carbene complexes results in cleavage of the iron-carbon double bond, producing a four coordinate iron(II) porphyrin and the free carbene. The carbenes can be trapped in high yield with a variety of alkenes. 

Parallel chemical oxidation of deuterium labeled biphenylacetylene with metachloro-perbenzoic acid yielded the same product as was obtained in the enzymatic reaction. These observations exclude the possible oxidation of the acetylenic terminal C—H bond, because such a reaction would not be compatible with complete conservation of the deuterium atom. Thus, the formation of a ketene derivative was postulated (Scheme 5). However, the nature of the intermediate from which the ketene derivative is formed remains to be elucidated. An oxirene structure was suggested in parallel to alkene epoxidation however, oxirenes are very unstable species due to electronic and steric factors It is currently believed that a complex between the acetylenic n electrons and cytochrome P450 iron-oxene moiety is formed, leading to the formation of the ketene. ... 

The reaction of vinyl ketones with pentacarbonyliron affords the corresponding rj -oxadiene tricarbonyliron complex 34, which can easily be converted into the vinyl ketene iron tricarbonyl complexes 35 upon reaction with alkyllithium reagents under an atmosphere of carbon monoxide. ... 

The iron carbene complex (ri -CsHs) Fe(CO)(CH2) generated by the electron impact from (Ti -C5H5)Fe(C0)2CH20CH3 in the gas phase was fotmd to react with cyclohexene or n-donor bases such as NH3, CH3CN, or CD3CDO by displacement of ketene from the iron center (reaction 8.25). Methylene-carbon monoxide coupling was assumed to occur in the coordination sphere of iron during the reaction [51]. 

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