Decarbonylative cycloaddition

A wide range of alkyl/aryl 4H-thiochromen-4-ones are readily available from the nickel-catalyzed decarbonylative cycloaddition of thioisatins with alkynes in toluene at 50 °C (14OL5660).The reaction of 3-aryl-1-substituted prop-2-en-l-ones with NaSH in DMF at low temperature (—20°C) gave access to 2-aryl-4H-thiochromen-4-one-type compounds (14H(89)495). A palladium(II)-catalyzed dehydrogenative cyclization of aryl benzyl sulfoxides and iodobenzene-produced dibenzothiopyran S-oxides in moderate-to-good yields (140L4574). 

SCHEME 22.15 Decarbonylative cycloaddition of (thio)phthalic anhydrides with allenes. 

In this decarbonylative cycloaddition process, Ni(0) is thought to be responsible for CO elimination. Unsymmetrical substrates mostly lead to mixtures of regioisomeric isocoumarins terminal alkynes do not react due to their rapid Ni-induced oligomerization. Derivatives of maleic anhydride give rise to 2-pyrones. 

Azanickelacycle formation with partial replacement of the heterocyclic moiety substance by the nickel catalyst was also employed for the synthesis of indoles, an important class of heterocycles found in natural products and pharmaceuticals. Maizum et al. demonstrated a nickel-catalyzed decarbonylative cycloaddition by which anthranilic acid derivatives 39, which are readily available, react with alkynes to afford substituted indoles 41 (Scheme 12.17) [20]. The reaction is supposed to proceed via oxidative addition and decarbonylation to afford azanickelacycle 40, followed by alkyne insertion, 1,3-acyl migration, and reductive elimination, to afford A-pivaloyl-protected indole 41. Deprotected indole 42 was obtained as the final product upon workup, that is, treatment of the reaction crude reaction mixture with NaSMe in MeOH. 

Notably, the nickel-catalyzed decarbonylative cycloaddition affords a regioisomer opposite to that formed in the palladium-catalyzed Larock heteroannulation [21], For example, the nickel-catalyzed reaction of anthranilic acid derivatives 39 with l-trimethylsilyl-2-phenylacetylene (43) affords indole 44 with phenyl substituents at the 2-position [Scheme 12.18(a)], whereas the palladium-catalyzed reaction... 

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. 

Increasing use is being made of pyran syntheses based upon [4 + 2] cycloadditions of carbonyl compounds. The appropriate unsaturated aldehyde with ethyl vinyl ether yields 53 with peracids this affords an epoxide that undergoes ring contraction to the aldehyde 54 (Scheme 23) and rhodium catalyzed decarbonylation affords the required 3-alkylfuran with the optical center intact.116 Acetoxybutadiene derivatives add active carbonyl compounds giving pyrans that contract under the influence of acids to give... 

The photoreactions of a-dicarbonyl compounds are quite different in the vapor and condensed phases. In the vapor phase, carbon-carbon bond cleavage is the preferred mode of reaction but in the condensed phase, many of the observed reactions can be rationalized by a mechanism involving hydrogen abstraction. Internal hydrogen abstraction, when possible, is generally preferred over abstraction from the solvent. With the exception of diethyl oxalate, which undergoes photoreactions typical of an ester, only those compounds that are reasonably strained or can yield reasonably stable free radicals give decarbonylation products. In the presence of suitable substrates, cycloaddition reactions have also been observed. 

When heated, 5-arylfuran-2,3-diones (346) readily evolve carbon monoxide to give 3-aroyl-6-aryl-4-hydroxypyran-2-ones (75KGS1468). Based on a kinetic study of this thermal decarbonylation, it is proposed that the formation of pyranones involves an initial cheletropic cycloreversion. The benzoylketene so formed dimerizes through a [4 + 2]-cycloaddition to the pyranone (Scheme 107) (78JOU2245). 

The thermal intramolecular 2 + 2-cycloaddition of 4,4,-disubstituted-2,2/-bis(phenyl-ethynyl)biphenyls (1) yielded the intermediate l,2-diphenylcyclobuta[l]phenanthrenes (2), which could be trapped with 2,3,4,5-tetraphenylcyclopenta-2,4-dione (3) to produce the Diels-Alder adduct (4). Thermal decarbonylative ring opening of (4) gave 9,10,11,12,13,14-hexaphenylcycloocta[l]phenanthrenes (5) as the final product in 12-23% yield (Scheme l).1... 

The chemistry of pyrrol-1-ylbenzylidene pentacarbonyl chromium, molybdenum and tungsten complexes was investigated. Reaction with electrophilic alkenes gives l-(phenylcyclo-propyl)pyrroles Under photolytic decarbonylation conditions 2 + 2 cycloaddition products were obtained with nucleophilic alkenes, cyclic dienes and imines. <950M2522>... 

Di-(l-naphthylmethyl)sulphone forms an excimer but does not react to give an intramolecular cycloaddition product like the corresponding ether but rather fragments to give sulphur dioxide and (l-naphthyl)methyl radicals (Amiri and Mellor, 1978). I-Naphthylacetyl chloride has a very low quantum yield of fluorescence and this is possibly due to exciplex formation between the acyl group and the naphthalene nucleus (Tamaki, 1979). Irradiation leads to decarbonylation. It is known that acyl chlorides quench the fluorescence of aromatic hydrocarbons and that this process leads to acylation of the aromatic hydrocarbon (Tamaki, 1978a). The decarboxylation of anhydrides of phenylacetic acids [171] has been interpreted as shown in (53), involving... 

The thermal cycloaddition of phencyclone (85) with cycloheptatriene offers additional mechanistic insight into these reactions (Scheme 13). In this case a 57% yield of the corresponding endo [2 -i- 4] adduct was isolated. Further pyrolysis of this material at 170 C provided the decarbonylated pentacyclic material (86) along with an exo [6 h- 4] adduct (87). The course of this reaction can be contrasted with the thermolysis of compound (83). l ile neither of these examples is of great potential synthetic value, they do serve to illustrate some of the typical mechanistic themes which characterize these reactions. 

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