Cyclobutadiene complexes oxidation

The cyclobutadiene complex 1 can be prepared in enantiomerically pure form. When the complex is reacted with an oxidizing agent and a compound capable of trapping cyclobutadienes, the products are racemic. When the reaction is carried only to partial completion, the recovered complex remains enantiomerically pure. Discuss the relevance of these results to the following questions In oxidative decomposition of cyclobutadiene complexes, is the cyclobutadiene liberated from the complex before or after it has reacted with the trapping reagent ... 

Like its cyclobutadiene analog 39, the 7j4-l,3-diphosphacyclobutadiene complex 38 has high thermal stability, is stable to air, and generally has low reactivity. Reagents that normally release cyclobutadiene from transition metal-cyclobutadiene complexes by oxidative demetallation [i.e., HCN, (NH4)2Ce(N01)6, and alkali metals] do not react with compound 38. 

Thus cyclohutadiene. which was nonexistent at the time, was shown to be stabilized by complexation. Oxidation of the complex liberated free cyclobutadiene which was trapped by ethyl propynoate to give a cycloadduct. The experiments established that... 

Some highly substituted phospholium salts, e.g., (180), and phosphole oxides have been prepared by a ring-expansion reaction of aluminium chloride-cyclobutadiene complexes with dichlorophosphines. The [4 - - 2] dimer (181)... 

An elegant use of [4]annulene chemistry utilizes the stable tricarbonylcyclobutadiene iron complex as a protected cyclobutadiene.One advantage over the aluminum cyclobutadiene complex is the ability of the tricarbonylcyclobutadiene iron complex to be amenable to synthetic modifications of the four-membered ring. Typically, ceric ammonium nitrate (CAN or other oxidants) may be used to oxidize the iron and liberate free cyclobutadiene. An example of this methodology is shown in Scheme 2, as applied to the synthesis of (-i-)-asteriscanolide. Further examples of reactions utilizing this chemistry are shown in Table 1. 

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)... 

The most important methods for the preparation of cyclobutadiene complexes are oxidative addition reactions of dihalogenocyclobutenes to low-valent transition metal compounds [equations (8.27) and (8.28)], as well as reactions of acetylenes with coordination compounds. 

Iron(III) and Ce(IV) compounds readily oxidize cyclobutadiene complexes. In this way it is possible to obtain many organic compounds which are otherwise difficult to prepare. 

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. ... 

As we saw in Section 11.17, cyclobutadiene is antiaromatic and exceedingly difficult to prepare and study. Its successful preparation by Rowland Pettit (University of Texas) in 1965 demonstrated how transition-metal organometallic cbemistry can provide access to novel reactions and structures. His approach was to prepare cyclobutadiene as a transition-metal complex, then destabilize the complex to trigger its dissociation. The sequence for cyclobutadiene begins with the reaction of cis-3,4-dichlorocyclobutene with diiron nonacarbonyl [Fe2(CO)9]. The resulting iron-cyclobutadiene complex satisfies the 18-electron rale, is stable, and undo-goes a variety of reactions. Most importantly, oxidation with ceric ammonium nitrate (a source of Ce ) lowers the electron count Irom 18 to 16, causing the complex to dissociate and Ubo-ate free cyclobutadiene. 

A rhodium-catalyzed cycloisomerization reaction of triyne 137 to 141 involves cleavage of the C=C triple bond (Scheme 7.49) [68]. The following reaction pathway is proposed initially, oxidative cyclization produces the rhodacycle 138, which then undergoes reductive elimination. The rhodium cyclobutadiene complex 139 is thus generated, and then undergoes oxidative addition to produce the rhodacycle 140. This isomerization from 138 to 140 would reduce the steric congestion of the heUcal structure. Subsequently, a cycloaddition reaction between the rhodacycle and the pendant alkyne moiety takes place to afford 141. 

Within the cubane synthesis the initially produced cyclobutadiene moiety (see p. 329) is only stable as an iron(O) complex (M. Avram, 1964 G.F. Emerson, 1965 M.P. Cava, 1967). When this complex is destroyed by oxidation with cerium(lV) in the presence of a dienophilic quinone derivative, the cycloaddition takes place immediately. Irradiation leads to a further cyclobutane ring closure. The cubane synthesis also exemplifies another general approach to cyclobutane derivatives. This starts with cyclopentanone or cyclohexane-dione derivatives which are brominated and treated with strong base. A Favorskii rearrangement then leads to ring contraction (J.C. Barborak, 1966). 

The complex exhibits remarkable stabiUty, and the cyclobutadiene undergoes reactions without destmction of the ring (139). Cyclobutadieneiron tricarbonyl [12078-17-0] can be oxidized to generate cyclobutadiene in situ (140). 

NMR, 3, 542 oxidation, 3, 546 phosphorescence, 3, 543 photoelectron spectra, 3, 542 photolysis, 3, 549 reactions, 3, 543-555 with alkenes, 3, 50 with alkynes, 3, 50 with IH-azepines, 3, 552 with azirines, 3, 554 with cyclobutadiene, 3, 551 with cyclopropenes, 3, 550 with dimethylbicyclopropenyl, 3, 551 with heterocyclic transition metal complexes, 7, 28 29... 

The reaction of o-diphenylcyclobutadiene (generated in situ by oxidation of its iron tricarbonyl complex) with p-benzoquinone yields A as the exclusive product. With tetracyanoethylene, however, B and C are formed in a 1 7 ratio. Discuss these results, and explain how they relate to the question of the square versus rectangular shape of cyclobutadiene. 

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. 

Exercise 31-2 The cyclobutadiene iron complex, 10, has been prepared optically active, and when oxidized with Ce(IV) in the presence of tetracyanoethene gives a mixture of cyclobutadiene cycloadducts, all of which are optically inactive. 

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