Cyclopentadienones, cycloaddition with

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. 

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. 

The retro-Diels-Alder reaction of dimer 220 led to monomer 221, which underwent a [4 + 2]-cycloaddition with the cyclopentadienone formed in situ by dehydrogenation of 222. The crude product was subsequently treated with BBtj to afford (+)-chamaecypanone C (223) in 53% yield over two steps. Starting material 220 was readily accessible in three steps and 99% ee from 2-hydroxy-5-isopropylbenzaldehyde. In advanced studies, a rhodium-catalyzed dehydrogenation has been applied to the synthesis of chamaecypanone C analogs ]91]. 

Due to their increased reactivity, cyclic dienes are very useful in the Diels-Alder reaction. Several groups have recently reported interesting applications of functionalized cyclopentadienes. Pearson demonstrated that cyclopentadienones react with aryl alkynes to yield polysubstituted biaryl compounds. Highly functionalized biaryl 87 is available in high yield on reaction of cyclopentadienone 85 and electron-poor aryl alkyne 86. The mechanism of this reaction includes extrusion of carbon monoxide to yield the pentasubstituted benzene after the initial cycloaddition. ... 

Most recently, Waldmann and co-workers disclosed a cascade transformation to allow the highly diastereo- and enantioselective synthesis of structurally complex 5,5,5-tricyclic products with eight stereocenters, which initiated by copper-catalyzed aerobic oxidation of cyclopentadiene to cyclopentadienone followed by catalytic asynunetric double 1,3-dipolar cycloaddition with azomethine ylides (Scheme 18) [33]. 

The use of annulated 2,5-bis(trimethylsilyl)cyclopentadienone 93 in this reaction gave the hexacyclic cage compound 95 in high yield via consecutive [4-1-2]- and photochemical [2 + 2]-cycloaddition with p-benzoquinone (Scheme 16). ... 

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... 

Diels-Alder cycloaddition of 2/f-azirines 23 with cyclopentadienones provides 3//-azepines 25 in excellent yields by electrocyclic ring opening, with concomitant loss of carbon monoxide, of the initially formed, nonisolable cycloadducts 24, followed by a [1,5]-H shift in the resulting 2//-azepines.31 108... 

Azcpincs under acid conditions reportedly117-225 yield aniline derivatives although ring contraction to pyridines is more usual. Thus, highly substituted 3//-azepines, e.g. 28, with a vacant 7-position, formed by cycloaddition of 2//-azirines with cyclopentadienones, on heating in acetic acid isomerize rapidly to the correspondingly substituted anilines 29.117... 

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. 

When benzyne is generated in the absence of another reactive molecule it dimerizes to biphenylene.132 In the presence of dienes, benzyne is a very reactive dienophile and [4+2] cycloaddition products are formed. The adducts with furans can be converted to polycyclic aromatic compounds by elimination of water. Similarly, cyclopentadienones can give a new aromatic ring by loss of carbon monoxide. Pyrones give adducts that can aromatize by loss of C02, as illustrated by Entry 7 in Scheme 11.9. 

The gas-phase pyrolysis of vinylogous systems of isopropylidene amino-methylenemalonates (1280,1287, and 1290), prepared from the appropriate enaminone or dienaminone and Meldrum s acid in pyridine, was studied by McNab etal. at 500°C and 10 2 torr (87CC140). Flash vacuum pyrolysis of 1280 gave l//-azepinones (1283) in —60% yields, together with a small amount of cyclopentadienone dimer (1284). They suggested that the azepi-nones (1283) were formed by electrocyclization from dipolar intermediates (1282) produced from the methyleneketenes (1281) by hydrogen transfer (Scheme 54). Cycloaddition of 1282 yielded bicyclics (1285), which col-... 

Scheme 7). The two-fold Diels-Alder cycloaddition of an excess of tetra-phenylcyclopentadienone (13), which is regarded as a first-generation dendron, to 24 in refluxing o-xylene leads to the second-generation benzilic dendron 25. The dendron is isolated as a pale yellow amorphous powder in 91% isolated yield. The Knoevenagel condensation of 25 with 1,3-diphenylacetone (26) to the corresponding cyclopentadienone dendron 27 is achieved in dioxane and in the presence of tetrabutylammonium hydroxide as a base. Like 25, the cyclopentadi-... 

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. 

Such cycloadditions are dependent on the interactions of the azepine HOMO and the diene LUMO. Theoretical consideration of these orbitals reveals that bonding overlap is favourable for C-6—C-7 and C-4—C-5 additions and that, on the basis of secondary orbital interactions, the endo product is favored. Experimentally, however, it is found that additions are periselective and C-4—C-5 addition predominates in the cycloaddition of 1//-azepines with cyclopentadienones, isobenzofurans, tetra- and hexa-chlorocyclopentadienes, 1,2,4,5-tetrazines, a-pyrones and 3,4-diazacyclopentadienones (8lH(15)1569). 

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). 

We have described the cycloadditions of a variety of dienes, ranging from cyclo-pentadiene to cyclopentadienones with alkyl and aryl fulvenes80-82. In these cases, only the [4 + 2] cycloadducts across the 2 and 3 positions are observed. Similarly, 1,3-dipoles such as nitrones and nitrile oxides add in this fashion, as well. We discovered the first authentic [6 + 4] cycloaddition of a fulvene in 197083. The cycloadditions of tropone to fulvenes, which we originally suggested involved [6-fulvene + 4-tropone] cycloadditions, now appear to be [6-tropone + 4-fulvene] cycloadditions8... 

Chen C, XI C, Jiang Y, Hong X (2005) 1,1-cycloaddition of oxalyl dichloride with dialkenylmetal compounds formation of cyclopentadienone derivatives by the reaction of 1,4,-dilithio-l,3-dienes or zirconacyclopentadienes with oxalyl chloride in the presence of CuCl. J Am Chem Soc 127 8024-8025... 

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. 

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