Cyclopentadienones 4 + 3 cycloaddition reactions

SCS-MP2 and the new perturbative B2-PLYP density functional methods provide accurate reaction barriers and outperform MP2 and B3-LYP methods when applied to the 1,3-dipolar cycloaddition reactions of ethylene and acetylene.39 Phosphepine has been shown to catalyse the asymmetric 3 + 2-cycloaddition of allenes with a variety of enones (e.g. chalcones) to produce highly functionalized cyclopentenes with good enantiomeric excess.40 The AuPPh3SbF6 complex catalysed the intramolecular 3 + 2- cycloaddition of unactivated arenyne- (or enyne)-yne functionalities under ambient conditions.41 A review of the use of Rh(I)-catalysed 3 + 2-cycloadditions of diaryl-and arylalkyl-cyclopropenones and aryl-, heteroaryl-, and dialkyl-substituted alkynes to synthesise cyclopentadienones for use in the synthesis of natural products, polymers, dendrimers, and antigen-presenting scaffolds has been presented.42... 

The cycloaddition reactions of nitrile oxides with several substituted cyclopentadienones led to the formation of only one regioisomer the cyclopenta[2,3-d]isoxazol-4-one (6) structure for five 1 1 adducts was fully supported by X-ray analysis (79JHC731). 

Cycloaddition reactions of the C=N bond of azirines are common, e.g., Scheme 68. Azirines can also participate in [4 + 2] cycloadditions with cyclopentadienones, isobenzofurans, triazines, and tetrazines. 

When compounds (7) were heated with alkyne in excess, two types of complexes, both involving alkyne coupling, are formed. A compound with the stoichiometry Co2(CO)4(C4R2CO)2, formed mainly from terminal alkynes having one bulky substituent R, represents derivatives of Co2(CO)g where two CO groups at either metal are replaced by a cyclopentadienone ligand. This compound type represents one of the many instances where alkynes combine with CO in the presence of a transition metal fragment to yield mostly cyclopentadienones, often complexed to the metal this cycloaddition reaction is similar to the Pauson-Khand scheme except for the use of an alkyne in place on an alkene (see also Section 5.1.4 and Scheme 26). The reaction eventually proceeds further to liberate an arene. Thus, from the use of t-BuC=CH, the alkyne trimerization product 1,2,4-tri-f-Bu-benzene was isolated. 

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. 

Cycloaddition reactions of the C(3)=N bond of azirines are common (Scheme 45) <71AHC(13)45, B-83MI 101-03,84CHEC-I(7)47>. Azirines can participate in [4 + 2] cycloadditions with dienes including cyclopentadienones, isobenzofurans, triazines, and tetrazines. They also participate in 1,3-dipolar cycloadditions with azomethine ylides, nitrile oxides, mesoionic compounds, and diazomethane. Cycloadditions with heterocumulenes, benzyne, and carbenes are known. Azirines also participate in other pericyclic reactions, such as ene reactions. 

Ethers.—A new dienophile, bis-(4,4 -diphenylethynyl) ether (16), has been made" and utilized in a convenient preparation of a new series of bis-ethers (18) and (19) by cycloaddition reactions with cyclopentadienones (17). 

Cyclopentadienones are formed in a [2+2+1] cycloaddition reaction of diynes with homoleptic carbonyliron complexes. This method is described in more detail in Section 2.1.2. The majority of (diene)iron complexes are obtained by addition of... 

The further reaction patterns of 1,4-dicopper-l,3-butadienes 54 were expanded by investigation of the annulation of 54 with carbon monoxide. This reaction led to cycloaddition reaction and afforded expected cyclopentadienones 20 as well as the... 

Regitz et al. described [4 + 2] cycloaddition reactions of phosphaalkynes with cyclopentadienones or pyrones under subsequent exclusion of CO and CO2, respectively. In this way, alkyl-substituted phosphinines, such as 4, are synthetically accessible (Scheme 6.3) [25, 26]. 

Interestingly, in the inverse-electron-demand Diels-Alder reactions of oxepin with various enophiles such as cyclopentadienones and tetrazines the oxepin form, rather than the benzene oxide, undergoes the cycloaddition.234 236 Usually, the central C-C double bond acts as dienophile. Oxepin reacts with 2,5-dimethyl-3,4-diphenylcyclopenta-2,4-dienone to give the cycloadduct 6 across the 4,5-C-C double bond of the heterocycle.234 The adduct resists thermal carbon monoxide elimination but undergoes cycloreversion to oxepin and the cyclopenta-dienone.234... 

Harano and colleagues [48] found that the reactivity of the Diels-Alder reaction of cyclopentadienones with unactivated olefins is enhanced in phenolic solvents. Scheme 6.28 gives some examples of the cycloadditions of 2,5-bis-(methoxycar-bonyl)-3,4-diphenylcyclopentadienone 45 with styrene and cyclohexene in p-chlorophenol (PCP). Notice the result of the cycloaddition of cyclohexene which is known to be a very unreactive dienophile in PCP at 80 °C the reaction works, while no Diels-Alder adduct was obtained in benzene. PCP also favors the decarbonylation of the adduct, generating a new conjugated dienic system, and therefore a subsequent Diels-Alder reaction is possible. Thus, the thermolysis at 170 °C for 10 h of Diels-Alder adduct 47, which comes from the cycloaddition of 45 with 1,5-octadiene 46 (Scheme 6.29), gives the multiple Diels-Alder adduct 49 via decarbonylated adduct 48. In PCP, the reaction occurs at a temperature about 50 °C lower than when performed without solvent, and product 49 is obtained by a one-pot procedure in good yield. 

The fourfold cycloaddition of an excess of cyclopentadienone dendron 27 to the tetraethynyltetraphenylmethane 4 in diphenylether at 200°C affords dendri-mer 2 in 85% isolated yield, respectively (see Scheme 7). Dendrimer 2 corresponds to the second-generation polyphenylene dendrimer made by the divergent method [30]. It should be mentioned that while the addition of dendron 27 to the biphenyl core 9 takes two days the addition to the tetraphenylcore 4 takes one week. This can be explained by the higher mobiUty of the biphenylic core compared to the stiff tetrahedral core, which allows the proper orientation of the ethynyl functions for reactions with the bulky dendrons. 

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. 

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