Cyclohexen decarbonylation

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 decarbonylations, which do not appear to be affected by light, are reasonably selective with aromatic aldehydes, yielding the expected product however, significant amounts of other products are obtained with non-aromatic substrates (e.g. cyclohexane-aldehyde gives methylcyclopentane and small amounts of n-hexane, as well as the expected cyclohexane and cyclohexen-4-al gives both cyclohexene and cyclohexane). Indeed, the unexpected products perhaps provided a major clue to an understanding of the reaction mechanism(s) involved. 

Figure 4. Visible spectral changes as a function of time (first hour) during decarbonylation of cyclohexen-4-al ( 0.5M) using Ru(TPP)(CO)(tBuPOH) in toluene at room temperature A, visible region using 10 4M Ru B, Soret region using...

However, formation of an R species, either free or within a radical-pair cage with the metal (14), is strongly favored in view of the methylcyclopentane noted in the cyclohexen-4-al decarbonylation, since the rearrangement shown in eq. 5, metal-assisted if necessary, seems plausible (15) ... 

The bridging methylidene ligand in 255 also reacts with the sulfur from cyclohexene sulfide to form a thioformaldehyde, which is in a bonding mode in 256 and triply bridging in 257. (Compounds 256 and 257 are interconvertible by the elimination and addition of CO.) One also has access to the thioformyl ligand in 258 by the further decarbonylation of 257 at 25°C with concomitant activation of a C—H bond, resulting in hydride migration (169). 

Aromatic aldehydes and ketones are reduced to the corresponding hydrocarbons in good yield by catalytic transfer reduction using cyclohexene or limonene as a donor, palladium-carbon as catalyst and a Lewis-acid promotor such as ferric chloride. The major competing reaction is decarbonylation, otherwise the reaction is straightforward and simply involves heating the catalyst, carbonyl compound, and donor under reflux for 3—5 h, furthermore the method is convenient and dispenses with elaborate equipment or potentially explosive hydro n. ... 

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