Cyclopentadienone complexes

Low-temperature photochemical cyclization of alkynes bearing a bulky substituent, mediated by CpCo(CO)2, proceeds with CO insertion to give cyclopentadienone complexes. Higher reaction temperatures lead to cyclotrimerization. The intramolecular variant of this reaction gives the bicyclic cyclopentadienones 139 and 139 (equation 19)142. Cyclization of unsymmetrically substituted diynes with the chiral R CpCo(CO)2 (R = 8-phenylmenthyl) leads to the formation of a mixture of diastereomers modest diastere-oselectivity was found. 

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 refluxing decane, a mixture of Me3SiC=CC=CSiMc3 and Co(CO)2Cp gives cyclobutadiene (275a,b Scheme 59) and cyclopentadienone complexes (276), which could be protodesilylated with ethanolic Complexes 275 were... 

The reaction of two alkynes in the presence of pentacarbonyliron affords via a [2 + 2 + 1]-cycloaddition tricarbonyl(ri4-cyclopentadienone)iron complexes (Scheme 1.6) [5, 21-23]. An initial ligand exchange of two carbon monoxide ligands by two alkynes generating a tricarbonyl[bis(ri2-alkyne)]iron complex followed by an oxidative cyclization generates an intermediate ferracyclopentadiene. Insertion of carbon monoxide and subsequent reductive elimination lead to the tricarbonyl(T 4-cyclopentadienone)iron complex. These cyclopentadienone-iron complexes are fairly stable but can be demetallated to their corresponding free ligands (see Section 1.2.2). The [2 + 2 + l]-cycloaddition requires stoichiometric amounts of iron as the final 18-electron cyclopentadienone complex is stable under the reaction conditions. 

Trimethylazulene reacts with Ru3(CO),2 to give Ru6(CO)l7C and Me3C10H5Ru4(CO)9, which were identified by mass spectrometry (43). Tetracyclone reacts with Ru3(CO)l2 to give the cyclopentadienone complex (CXVII), which exhibits the molecular ion and fragments corresponding... 

Hexafluorobut-2-yne and Ru3(CO) 12 give the cyclopentadienone complex (CXVIII), and the molecular weight was determined by mass spectrometry... 

The reaction of cyclooctyne (14) with NiBr2 and Nil2 gave the corresponding cyclobutadiene-complexes in low yield, while with Ni(CO)4 the nickel(O) cyclopentadienone complex (76) was formed, whose thermal decomposition yielded the cyclopentadienone (77). In this case again a cyclooctyne Ni(0) complex (78) was postulated as an intermediate, but could not be isolated due to its instability l93). 

The unsymmetrical isomer 2,4-bis(pentafluorophenyl)-3,5-diphenyl-cyclopentadienone-TT-cyclopentadienylcobalt is the only product isolated from the reaction of (7t-C5H5)Co(CO)2 and CgHgCsCCeFs 161). However, all possible isomers of the rr-cyclopentadienone complex (XIX R, R = Me, CFg) are formed in the related reaction of (7t-C5Hs)Co(CO)2 and MeC=CCF3 68). Again, the unsymmetrical isomer is the major product, and it has been suggested that a strong preference for the formation of this isomer is due to attractive forces between hydrocarbon and fluorocarbon substituents in the transition state 68). 

A number of examples exist involving reactions of CpCo(CO)2 and silylated alkynes. Cycloaddition of MesSiC CH gives a 3 1 mixture of 2,5- and 2,4-bis(trimethylsilyl)cyclopentadienone complexes. The use instead of [C5Me5]Co(CO)2 reverses the regioselectivity to 1 3.7, presumably due to the increased steric crowding about cobalt (equation 9). This effect has yet to be exploited in synthesis. 

The use of complexes in which unsymmetrical disubstitution is present on the Cp ring leads to dia-stereoselection in the cycloaddition process, leading to chiral cyclopentadienone complexes. In the course of subsequent transformations stereochemical information has been shown to be transferable to the complexed cyclopentadienone ring (Scheme 7). ... 

Diphenylacetylene with Mo(CO)a (298) in a sealed tube at 160°-170°C produces, in addition to two cyclobutadiene complexes, a yellow compound with the empirical formula [C9(CgH5)gO]Mo(CO)2, the infrared spectrum and chemical properties of which suit the tetraphenyl-cyclopentadienone complex of structure (11). On the other hand, the interaction of 3-hexyne with (CH3CN)gMo(CO)3 yields only the alkyne complex (570). 

The formation of cyclopentadienone complexes is a general phenomenon in photoreactions of acetylenes and metal carbonyls compounds... 

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