Iron carbonyl-cyclobutadiene

In addition to benzene and naphthalene derivatives, heteroaromatic compounds such as ferrocene[232, furan, thiophene, selenophene[233,234], and cyclobutadiene iron carbonyl complexpSS] react with alkenes to give vinyl heterocydes. The ease of the reaction of styrene with sub.stituted benzenes to give stilbene derivatives 260 increases in the order benzene < naphthalene < ferrocene < furan. The effect of substituents in this reaction is similar to that in the electrophilic aromatic substitution reactions[236]. 

Reactions of acetylene and iron carbonyls can yield benzene derivatives, quinones, cyclopentadienes, and a variety of heterocycHc compounds. The cyclization reaction is useful for preparing substituted benzenes. The reaction of / fZ-butylacetylene in the presence of Co2(CO)g as the catalyst yields l,2,4-tri-/ f2 butylbenzene (142). The reaction of Fe(CO) and diphenylacetylene yields no less than seven different species. A cyclobutadiene derivative [31811 -56-0] is the most important (143—145). 

Catenated Organic Compounds of the Group IV Elements, 4,1 Conjugate Addition of Grignard Reagents to Aromatic Systems, 1, 221 Cyclobutadiene Metal Complexes, 4, 95 Cyclopentadienyl Metal Compounds, 2, 365 Diene-Iron Carbonyl Complexes, 1, 1... 

Cyclobutatropylium iron carbonyl complex 285 shows C=0 bands at 2095, 2049, and 2032 cm"1, compared to 2044 and 1980 cm"1 for the precursor tropylidene mixture 284 (78AJC1607). This shift to higher wave numbers is a result of increased back-bonding from iron to the cyclobutadiene ligand in the charged complex. 

The synthesis of the iron carbonyl adduct of cyclobutadiene carboxylic acid (212) has been reported.122 This synthesis involved photolysis of the pyrone (209c) in THF solution to yield the bicyclic lactone (213) in the presence of an excess of iron pentacarbonyl. The ester (212a) obtained from this process is light-sensitive, but hydrolysis yields a more stable acid (212b) in a maximum yield of 21%. 

As iron forms cyclobutadiene complexes preferentially in the (0) oxidation state, the best starting materials for these complexes are Fe(0) compounds such as the carbonyls. Cobalt, however, tends to form Co(I) (also cyclobutadiene complexes. The best known one (cyclopentadienyl)(tetra-phenylcyclobutadiene)cobalt (XXVIII), has been prepared from cyclo-pentadienylcobalt(I) derivatives and also from cobaltocene, a cobalt(II)... 

Further proof that cyclobutadiene liberated by ceric oxidation of the iron carbonyl complex is completely free of the metal comes from an ingenious new three-phase test for reactive intermediates which resembles the Paneth test for free radicals in its basic flow design. Thus, a cyclobutadiene-iron carbonyl complex bound to a support of polymer beads is treated as a suspension with Ce in the presence of separate polymer beads to which a trapping agent (maleimide) is bound. The resulting adduct is then liberated from the second polymer by hydrolysis for identification, its isolation... 

In cases where the diene is unstable, a dihalo starting material may be used with the iron carbonyl acting as a reducing agent. Both the trimethylene methane complex 10.5 (Scheme 10.3) and the cyclobutadiene complex 10.7 (Scheme 10.4) have been made in this way. The cyclobutadiene-iron complex 10.7 is a convenient storable form of this highly unstable diene. It can be liberated by oxidation and, if this is done in the presence of a dienophile, the Diels-Alder product is obtained. The Diels-Alder reaction with 2,5-dibromobenzoquinone gave the expected e do-product 10.8. An intramolecular photochemical [2-1-2] cycloaddition, followed by a Favorskii reaction, gave a cubane dicarboxylic acid 10.10. ... 

C31H2 eFeOP2, (Cyclobutadiene)(1,2-bis(diphenylphosphino)ethane-P,P )iron carbonyl, 46B, 934... 

Cp)Co(CO)2 to give (Cp)Co(alkadiyne) complexes in which the acetylene has cyclized to a tricyclic cyclobutadiene ring-size effects are discussed in cyclizations with iron carbonyls. 

Carboxylic acids, a-bromination of 55, 31 CARBOXYLIC ACID CHLORIDES, ketones from, 55, 122 CARBYLAMINE REACTION, 55, 96 Ceric ammonium nitrate [Ammonium hexa mtrocerate(IV)[, 55, 43 Chlorine, 55, 33, 35, 63 CHROMIUM TRIOXIDE-PYRIDINE COMPLEX, preparation in situ, 55, 84 Cinnamomtnle, a-phenyl- [2-Propeneni-tnle 2,3-diphenyl-], 55, 92 Copper(l) iodide, 55, 105, 123, 124 Copper thiophenoxide [Benzenethiol, copper(I) salt], 55, 123 CYCLIZATION, free radical, 55, 57 CYCLOBUTADIENE, 55, 43 Cyclobutadieneiron tricarbonyl [Iron, tn-carbonyl(r)4-l,3-cyclo-butadiene)-], 55,43... 

Comparison of the tropone ring geometry in iron complex 282 with that of the parent tropone shows that the double bond character of the carbonyl group and the bond between C-2 and C-3 in 282 is even higher, probably as a result of dominant contributions from two cyclobutadiene resonance... 

Thus the iron atom in (Tr-cyclopentadienvl)iron dicarbonyl bromide forms additional bonds with the halide ligands on palladium. Carbonyl substitution on the generated intermediate (XV) then occurs. In this case the cyclopentadienyl group acts as a bridge. In general, such a mechanism corresponds to an A-type substitution, where the halogens in (7T-tetraphenyl-cyclobutadiene)palladium dichloride function as nucleophiles. 

Cyclobutadiene-iron tricarbonyl is prepared through reaction of S,4-dichlorocydolmtene and diiron enneacarbonyl. In an analogous manner, one can prepare 1,2-diphenyl- 1,2,3,4-tetramethyl- and benzocyclobutadiene-iron tri-carbonyl complexes. Cyclobutadiene-iron tricarbonyl is aromatic" in the sense that it undergoes facile attack by electrophilic reagents to produce monosubstituted cydo-butadiene-iron tricarbonyl complexes. Functional groups in the substituents display many of their normal chemical reactions which can be used to prepare further types of substituted cyclobutadiene-iron tricarbonyl complexes. 

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