Cyclopentadienone complexes preparation

While one of the first preparations of a cyclobutadiene-metal complex involved the cyclodimerization of diphenylacetylene in the presence of Fe(CO)5 at high temperature212, the thermal reaction of alkynes with Fe(CO)s gives predominantly cyclopentadienone complexes (Section IV.E.l.b). The cyclization of alkynes by a wide variety of metal complexes has been reported (Scheme 59)l 5-21 A—222... 

In support of this mechanism is the fact that stable cobaltacyclopentadienes 11.23, with an additional PPhs ligand can be prepared (Scheme 11.12). These cobaltacycles undergo further reactions with alkynes, alkenes, azides and diazo compounds to give a variety of cyclic products. Cyclobutadiene complexes 11.26, cyclopentadienone complexes 11.27 and arenes 11.28 can also be formed (Scheme 11.13). ... 

Dimerization with Insertion of One Molecule of CO. A number of tricarbonyliron complexes of substituted cyclopen-tadienones have been prepared from alkynes. The reaction was first described by Jones et al. " for FefCOJj and Ph-C=CH in the presence of Ni(CO)4. A number of other alkynes (PhC2Me, PhC2SiMc3, Mc3SiC2R, BrCgH4C2H, CF3C2CF3, PhC2Ph and a,(i)-diynes) give cyclopentadienone complexes (eq 30). ... 

Since the corresponding endoperoxide precursors are all too unstable for isolation, the diimide reduction constitutes an important chemical structure confirmation of these elusive intermediates that are obtained in the singlet oxygenation of the respective 1,3-dienes. However, the aza-derivative 14 and the keto-derivative 15 could not be prepared,17> because the respective endoperoxides of the pyrroles 18) and cyclopentadienones suffered complex transformations even at —50 °C, so that the trapping by the diimide reagent was ineffective. 

Thermal cyclization of alkynes with Fe(CO)5 proceeds predominantly with CO incorporation to afford (cyclopentadienone)Fe(CO)3 complexes, however small amounts of cyclobutadiene complexes can be isolated (see Section VI.B.)15. 1,6-FIeptadiyne and 1,7-octadiyne substrates 107 have been utilized to prepare bicyclo[3.3.0] and bicyclo[4.3.0] complexes 108 in excellent yield (equation 12)115, while 1,8-nonadiynes gave bicyclo [5.3.0] complexes in low yield. 

Many complexes of conjugated ketones are also known, such as the iron tricarbonyl complexes of substituted cyclopentadienones 30), although reaction with chromium hexacarbonyl occurs only if phenyl substituents are available for tt complexing 31). A common difficulty of preparing complexes of heterocyclics is the ability of the heteroatom to form o bonds with the metal. 

In the case of tt complexes of substituted cyclopentadienones, such as the iron tricarbonyl derivatives prepared by Weiss and H libel (30), qualitative molecular-orbital theory (20) predicted a considerable reduction of the ketonic carbonyl bond order. It was observed that the ketonic carbonyl frequency dropped by as much as 65 cm-1, in agreement with theory. A similar explanation can also be provided in terms of valence bond theory (Fig. 14). It has been suggested that n complexing of arenes such as benzene results in loss of aromaticity of the ring in contrast to the dicyclopentadienyl... 

Stable, isolable metallacycles are also obtained from reaction of complexes that serve as sources of the CpCo fragment (e.g. CpCo(PPh3)2) and alkynes. Upon carbonylation diese typically give high yields of cobalt-complexed cyclopentadienones. Direct reaction of CpCo(CO)2 with alkynes is similarly useful. The cycloaddition of di(t-butoxy)acetylene upon photolysis with CpCo(CO)2 is an example (Scheme 5). In all these systems the final complexes lack coordinated CO, and therefore amine oxides are not suitable reagents for liberating the stable cyclopentadienones. Tetra(t-butoxy)cyclopentadienone is accessible on a preparative scale via controlled electrochemical oxidation. Other oxidants such as Cr have been used as well in other systems. 

Although catalytic preparations of cyclopentadienones with other metal systems are known, chemose-lectivity is often a problem. For example, the reactions of 2-butyne, 3-hexyne and diphenylacetylene with [(CO)2RhCl]2 at 80 °C give mixtures of hexasubstituted benzenes, metal-complexed, tetrasub-stituted cyclopentadienones and quinones. An exception is the preparation of the stable tetrakis(trifluo-romethyl)cyclopentadienone from the alkyne the unusual electronic properties of the product make this result lack generality. 

The reaction between acetylenes and ruthenium carbonyls produces a series of n complexes with cyclic ligands which, as in the iron system, have either the metal or a CO group incorporated into the ring. Accordingly, 3-hexyne 536) and hexafluoro-2-butyne 90) react with Ru3(CO)i2 to give the (substituted cyclopentadienone)tricarbonylruthenium complexes with structures presumably comparable to those of the iron complexes (93-95). Although diphenylacetylene will not react directly with Ru3(CO)i2 to produce this type of complex 536), it can be prepared 90) by treating Ru3(CO)i2 with tetracyclone in benzene under reflux. 

A number of symmetrically substituted cyclopentadienone metal complexes have been prepared by metalcarbonyl-mediated dimerization of alkynes [23a, 24]. The (tetracyclopropylcyclopentadienone)tricarbonyliron complex 27 can easily be obtained as the major product by ironcarbonyl-mediated dimerization with CO insertion of dicyclopropylacetylene [25]. Upon treatment of the complex 27 with triethylamine N-oxide, the uncomplexed tetracyclopropylcy-clopentadienone 28 apparently is liberated however, in contrast to the kineti-cally sufficiently stabilized tetra-ferf-butylcyclopentadienone 19 (see Scheme 5)... 

Cyclopentadienones are reactive and versatile diene systems that can be used to construct polycyclic compounds and polymeric materials. The early preparation of cyclopentadienones via Rh-catalyzed [2+2+1] cycloaddition of two alkynes and CO often required a stoichiometric amount of a metal complex and a stepwise process, as shown in Scheme 2-18. The rhodium-pentadiene complex... 

Using this cycloaddition strategy, a series of differently substituted tricarbonyl (T] -cyclopentadienone)iron complexes has been prepared and tested as catalysts for the oxidation of alcohols to ketones. [2+2+1] Cycloaddition of 1,8-diyne derivatives promoted by pentacarbonyliron in an carbon monoxide atmosphere affords functionalized hydroazulenes as exemplified in Scheme 4-9. These compounds constitute a structural... 

Fig. 72. The preparation of cyclopentadienone-metal complexes using acetylenes, and...

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