Cyclobutadiene-Fe

Disubstituted (cyclobutadiene)Fe(CO)3 complexes in which the two substituents are different may exist as enantiomers. Racemic cyclobutadiene carboxylic acids or cyclobutadiene amine complexes of this type have been separated by classical resolution methodology234. These optically active (cyclobutadiene)Fe(CO)3 complexes are stable with respect to racemization at 120°C for 24 h. This stability contrasts with acyclic... 

Neutral (cyclobutadiene)Fe(CO)3 complexes undergo thermal and photochemical ligand substitution with phosphines, with alkenes such as dimethyl fumarate and dimethyl maleate and with the nitrosonium cation to generate the corresponding (cyclobutadiene)Fe(CO)2L complexes15. These types of complexes are presumably intermediates in the reaction of (cyclobutadiene)Fe(CO)3 complexes with perfluorinated alkenes and alkynes to generate the insertion products 266 or 267 respectively (Scheme 70)15,238. 

The reaction of (cyclobutadiene)metal complexes with X2 results in the oxidative decomplexation to generate either dihalocyclobutenes or tetrahalocyclobutanes. In comparison, substitution of (cyclobutadiene)MLn complexes 223 [MLn = Fe(CO)3, CoCp, and RhCp] with a variety of carbon electrophiles has been observed (equation 34)15. Electrophilic acylation of 1-substituted (cyclobutadiene)Fe(CO)3 complexes gives a mixture of regioisomers predominating in the 1,3-disubstituted product and this has been utilized for the preparation of a cyclobutadiene cyclophane complex 272 (equation 35)246. For (cyclobutadiene)CoCp complexes, in which all of the ring carbons are substituted, electrophilic acylation occurs at the cyclopentadienyl ligand. 

Deprotonation of (cyclobutadiene)Fe(CO)3 with methyl lithium or n-butyl lithium is not possible15, however lithiation is achieved by use of s-butyl lithium247, or by transmetalation of (chloromercurycyclobutadiene)Fe(CO)3. The metalated cyclobutadiene... 

Photolysis of substituted (cyclobutadiene)Fe(CO)3 complexes in the presence of alkynes affords substituted benzenes (Scheme 60). From the substituent patterns of the products, it is clear that this reaction cannot be accounted for by generation of the free hgand. The proposed mechanism involves initial loss of a carbon monoxide ligand and coordination of the alkyne. Insertion of the alkyne predominantly into the... 

Finally, iodo-substituted (cyclobutadiene)Fe(CO)3 complexes can undergo palladium-catalyzed animations as well as Stille reactions using aUcynylstannanes (Scheme 61). An iterative process allowed the diiron dumbbell complex to be prepared alternatively, tetraethynyl complexes could be prepared directly from the tetraiodo(cyclobutadiene)Fe(CO)3 complex. " ... 

The mechanism of these substitution reactions can be readily rationalized in a manner which completely parallels the accepted electrophilic mechanism of benzene and other aromatic systems. The electrophile, R", adds to the cyclobutadiene ligand to produce the 7r-allyl-Fe(CO)3 cationic intermediate (XVI) loss of a proton from this intermediate generates the substituted cyclobutadiene -Fe(CO)3 complex. We have previously isolated salts of the 7r-allyl-iron tricarbonyl cation (XVII), as well... 

The reduction of 3,4-dichlorocyclobutene (222) in the presence of metal carbonyls has been utilized to prepare the parent complex [223, MLn = Cr(CO)4, Mo(CO)3, W(CO)3, Fe(CO)3, Ru(CO)3 orCo2(CO)6] (equation 32) .Morerecently, reaction ofNi(CO)4 with 3,4-dihalocyclobutenes (X = Br or I) or with 222 in the presence of AICI3 produced the corresponding (cyclobutadiene)nickel dihalides . Methodology for the preparation of 1,2- or 1,3-disubstituted (cyclobutadiene)Fe(CO)3 complexes from 1,2- or 1,3-disubsli-tuted-3,4-dibromocyclobutenes has been presented - In turn, the substituted dibromo-cyclobutenes are prepared from squaric esters. The reaction of cz5-3,4-carbonyldioxycy-clobutene and substituted variants with l c2(CO)9 orNa2Fe(CO)4 also produces (cyclobu-tadiene)Fe(CO)3 complexes . Photolysis of a-pyrone generates 3-oxo-2-oxabicyclo [2.2.0]hex-5-ene (224) which undergoes photolysis with a variety of metal carbonyls to afford the parent cyclobutadiene complex 223 [MLn = CpV(CO)2, Fe(CO)3, CoCp. or RhCp] (equation 33) 2 0. 

As in the isoelectronic (h -arenelCrfCOlj and (h -cyclopentadienyOMnfCOlj species, the predominant result of irradiation of (h -cyclobutadiene)Fe(CO)j complexes is carbonyl loss, allowing simple photosubstitution " . In some cases, however, reaction of the cyclobutadiene moiety does take place ... 

For conjugated (diene)metal complexes, the 7r-molecular orbitals of cyclobutadiene consist of a degenerate eg pair (Figure 3). Because of this orthogonality, there is not a single electronically preferred conformei and barriers for rotation of the cyclobutadiene ligand are generally low . For (cyclobutadiene)Fe(CO)3, in the solid state, NMR spin relaxation data indicate that there are two inequivalent lattice sites. For each of these inequivalent lattice sites, barriers for rotation of the cyclobutadiene have been measured to be 3.63 and... 

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