Cyclopentadienone decarbonylation reaction

Decarboxylation.—The most important synthetic applications of photochemical decarboxylation reactions have been to the low-temperature formation of ben-zyne and cyclobutadiene from appropriate precursors.Interesting recent results in this area concern a decarbonylation reaction to form the elusive tetrahedrane skeleton. Irradiation of the cyclopentadienone (184) at low temperature gave the crystalline (m.p. 135 C) and air-stable tetra-t-butyltetra-hedrane (186), presumably by decarbonylation of the intermediate (185) (Scheme 52). ... 

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. 

Pyridinealdehyde yields pyridine and some dipyridyl, a reaction which proceeds without destruction of the heterocyclic ring 19). Ketones decarbon-ylate equally well. Benzophenone and dibenzoyl form diphenyl in high yields 2°). Several interesting eliminations have been found for cyclic and bi-cyclic ketones. In the case of cyclohexanone the decarbonylation accounts for only a small proportion of the reaction products. Benzoquinone decarbon-ylates to cyclopentadienone in good yields 20K Under the reaction conditions this compound dimerizes and decarbonylates again. 

Reaction with cyclopentadienones or their acetals also occurs readily hydrolysis of the acetals and decarbonylation also leads to cycloheptatrienes, while hydrogenation prior to the elimination produces cyclohepta-1,4-dienes 221 222) ... 

Conjugated diynes can function as dienophiles in Diels-Alder reactions, and adducts have been obtained in which one or both of the triple bonds participates. Most of the studies have involved the use of substituted cyclopentadienones, e.g. tetraphenylcyclopentadienone ( tetracyclone ), which form adducts that undergo decarbonylation to give aromatic hydrocarbons. Butadiyne itself reacts with tetracyclone to give hexaphenylquaterphenyl (221) while diphenylbutadiyne gives the mono(222)- and di(223)-adduct . Bis(4-biphenylyl)butadiyne gives the monoadduct 224 . 

Diene syntheses in which cyclopentadienone is the diene component give, as primary products, ketones whose decarbonylation is a radical reaction similar to those discussed above these Diels-Alder adducts contain a bridging carbonyl group which is readily eliminated as carbon monoxide when the... 

Reaction of hexa-2,4-diyne with Fe(CO)6 yields the cyclopentadienone complex (108), characterized crystallographically. Reaction of Fe(CNBu )6 with PhC=CPh yields (109), characterized crystallographically. The ion HFe(CO)4 reacts with acetylenes HC=CCOR to yield products of structure (110), although a crystal structure determination indicates some contribution from a -olefin- -acyl structure. Protonation of (110) yields Fe[t -HRC=C(H)(COR)](CO)4 via an intramolecular decarbonylation. - Photochemical reaction between... 

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