Iron vinylidenes

Experimental Procedure 3.1.2. Preparation of an Iron Vinylidene Complex by Electrophilic Abstraction CarbonyUrj -cyclopentadienyOldimethylvinylidene)-(triphenylphosphine)iron tetrafluoroborate [464J... 

The generation of four-membered ring systems can be accomplished by a cycloaddition process under photochemical conditions or with special substrates under thermal conditions. Iron-vinylidene complexes belong to such a class of special substrates where a thermal [2 + 2]-cycloaddition is possible. If imines are used, a hetero-[2 + 2]-cycloaddition with an iron-vinylidene complex leads to an iron-carbene complex attached to an azetidine ring system, as reported by Barrett and coworkers (Scheme 9.20) [46, 47]. The oxidation of these iron-carbene complexes leads to [3-lactams 27. Interestingly, the application of 2-thiazolines generates penam... 

Water also attacks the electrophilic a carbon of the ruthenium vi-nylidene complex 80. The reaction does not yield the ruthenium acyl complex, however, as is found for the reaction with the similar iron vinylidene complex [(i75-C5H5)(CO)2Fe=C=CHPh]+ (56), but rather 91 is the only isolated product (78). The mechanism for this transformation most reasonably involves rapid loss of H+ from the initially formed hydroxycarbene to generate an intermediate acyl complex (90). Reversible loss of triphenyl-phosphine relieves steric strain at the congested ruthenium center, and eventual irreversible migration of the benzyl fragment to the metal leads to formation of the more stable carbonyl complex (91) [Eq. (86)]. 

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. 

In some cases, vinylidene complexes undergo [2-t-2] reactions that are characteristic of Fischer and Schrock carbene complexes. However, these [2+2] reactions involving vinylidene complexes can result from nucleophilic addition at the central carbon, rather than a concerted [2+2] process. For example, the reaction of an imine with the iron-vinylidene complex in Equation 13.28 leads to formation the product of a [2+2] reaction between the carbon-nitrogen double bond and the carbon-carbon double bond. ° - This reaction is believed to occur by nucleophilic attack of the nitrogen at the central carbon, followed by ring closure at the p-carbon, instead of a concerted [2+2] process. 

Also, bi- and trimetallic cumulene complexes, such as M=C=M, M=M=C, M=M=M and M=C=C=M are known. Cationic ruthenium allenylidene complexes are used as catalysts for ring closing metathesis reactions. Nonlinear optical properties have been measured for the Group 6 cumulenylidene complexes. Also, cationic chromium or iron vinylidene complexes undergo [2-1-2] cycloaddition reactions across imines to give fi-lactams. This reaction is useful for the synthesis of j8-lactam antibiotics. ... 

One-electron oxidation of the vinylidene complex transforms it from an Fe=C axially symmetric Fe(ll) carbene to an Fe(lll) complex where the vinylidene carbon bridges between iron and a pyrrole nitrogen. Cobalt and nickel porphyrin carbene complexes adopt this latter structure, with the carbene fragment formally inserted into the metal-nitrogen bond. The difference between the two types of metalloporphyrin carbene, and the conversion of one type to the other by oxidation in the case of iron, has been considered in a theoretical study. The comparison is especially interesting for the iron(ll) and cobalt(lll) carbene complexes Fe(Por)CR2 and Co(Por)(CR2) which both contain metal centers yet adopt... 

The photochemistry of several of the iron porphyrin halocarbene complexes Fe(TPP)(=CXY) (CXY = CCF, CBr2, CCIF. CCl(CN) and the vinylidene complex Fe(TPP)=C=CAr2 (Ar = p-C6Fl4Cl) has been studied in degassed benzene... 

Acyl complexes can also result from the reaction of terminal alkynes with cationic, hydrated complexes of iron (Entry 4, Table 2.7) [47]. An electrophilic vinylidene complex is probably formed as intermediate this then reacts with water and tautomerizes to the acyl complex. 

Unusual iron-porphyrin vinylidene complexes were obtained from DDT [l,l-bis(4-chlorophenyl)-2,2,2-tricMoroethane] and Fe(tpp) [tpp = meso-tetraphenylporphinato (2-)] in the presence ofa reducing agent [10a, 264]. The derived N,N -vinylene-bridged porphyrin reacts with metal carbonyls [Fe3(CO)i2, Ru3(CO)i2] to break one or both N—C bonds with insertion of the vinylidene into an M—N bond. While the iron complex was formed in 90% yield, the reaction with Ru3(CO)i2 afforded three products, the vinylidene being formed in only 40% yield [265]. 

The copolymer of vinyl ferrocene (VF) and butadiene has also been reported in the literature for use as a binder for composite propellants. It does not require any burn-rate (BR) accelerator because of the presence of iron (Fe) in vinyl ferrocene which is converted to finely divided Fe203 (a well-known BR accelerator) during combustion. A few groups of scientists have also studied fluorocarbon polymers as binders for composite propellants because of their excellent compatibility with oxidizers and fuels coupled with high density. Accordingly, Kel-F elastomer (a copolymer of vinylidene fluoride and chlorotrifluoroethylene, trade name of 3M, USA) and Viton-A (copolymer of hexafluoropropylene and vinylidene fluoride, trade name of Du Pont, USA) have also been reported for this purpose. The structures of Kel-F 800 [Structure (4.13)] and Viton-A [Structure (4.14)] are ... 

Rapid development of this area followed the discovery of routes to these complexes, either by ready conversion of terminal alkynes to vinylidene complexes in reactions with manganese, rhenium, and the iron-group metal complexes (11-14) or by protonation or alkylation of some metal Recent work has demonstrated the importance of vinylidene complexes in the metabolism of some chlorinated hydrocarbons (DDT) using iron porphyrin-based enzymes (15). Interconversions of alkyne and vinylidene ligands occur readily on multimetal centers. Several reactions involving organometallic reagents may proceed via intermediate vinylidene complexes. 

Treatment of some iron-acyl complexes with trifluoromethanesul-phonic anhydride (TfzO) affords vinylidene derivatives directly (5 7,38). The reaction is envisaged as a nucleophilic attack on TfzO by the acyl, followed by deprotonation to the vinyl ether complex. A combination of an excellent leaving group (TfO-) with a good electron-releasing substituent on the same carbon atom facilitates the subsequent formation of the vinylidene ... 

The base used may be hydroxide, alkoxide, carbonate, alkyl lithium, or alumina (13, 20, 24, 41, 49). The reaction is the reverse of the vinylidene synthesis by protonation of the n-acetylide, and the two complexes form a simple acid-base system. For the iron complexes in Eq. (2), the pK has been measured at 7.78 (in 2 1 thf-H20) (24). 

Irradiation of mixtures of Ph2C=C=0 and iron carbonyls afforded 32, the first vinylidene complex to be characterized (7) ... 

Scheme 5. Synthesis and reactions of iron-porphyrin vinylidene complexes.

Acetylene-vinylidene rearrangements of silylacetylene-iron carbonyl complexes have been observed,537 while iron-acetylide hydride complexes of the type [Fe(H)(C=CR)(dmpe)2], where dmpe=l,2-bis(dimethylphosphino)ethane, have been found to react with anions to afford substituted alkenyl complexes. It has been proposed538 that a likely reaction course for this latter rearrangement involves initial protonation of the cr-bound acetylide ligand at the carbon (I to the metal centre to form a vinylidene complex. Metal-to-carbon hydride migration in this vinylidene complex with attack by the anion would then lead to the neutral complex (see Scheme 106). A detailed mechanistic investigation has been carried out539 on the novel metathetical... 

Iridium nanoparticles, preparation, 12, 82 Iridium(III) O-ligated complexes, preparation, 7, 315 Iridium polyhydrides, preparation and characteristics, 7, 405 Iridium pyrrolyl derivatives, reactivity, 7, 282 Iridium tetrahydrides, characteristics, 7, 407-408 Iridium trihydrides, preparation, 7, 405 Iridium vinylidenes, synthesis and characteristics, 7, 352 Iridium xyliphos complexes, properties, 7, 442 Iridoids, via Pauson-Khand reaction, 11, 360 Iron(arene) (cyclopentadienyl) cations, preparation and reactivity, 6, 166... 

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