Cyclohexanone 5-hexenal formation

Phenylsulphine prepared in situ from phenylmethanesulphinyl chloride and triethyl-amine reacted with 1 -morpholinocyclohexene to form the addition product 169 having the enamine structure218. A similar experiment with phenylsulphine and 2-pyrrolidinocyclo-hexene gave only 2-phenylmethanesulphinyl cyclohexanone 170. The latter is most probably formed by hydrolysis of the corresponding enamine sulphoxide upon isolation. The reaction of sulphines with enamines is apparently a stepwise process involving the transient formation of the dipolar intermediate 171 which is stabilized by proton transfer, giving the enamine sulphoxide. 

The formation of 5-hexenal (reaction 18) is believed to be an intramolecular rearrangement since the addition of oxygen does not cause its suppression. At least in a methyl substituted cyclohexanone the analogous process has been shown to occur by the transfer of a hydrogen atom from the beta position to the carbonyl group before the fission of the six-membered carbocyclic ring (29) as only one of the two possible isomeric heptenals is formed. 

The photolysis of cyclohexanone has been studied by several workers. The formation of 5-hexenal, presumably by an intramolecular rearrangement as in the vapor phase by eq. 18, has been observed in the pure liquid (36) and in 1-octene solution (21). 

The reactions taking place in the vapour phase also occur in the condensed phase, and their mechanisms are probably similar. However, as may be expected on the basis of the results obtained for the gas phase photolysis, the formation of olefins, cycloparaffins, and CO is of less importance, while that of the saturated aldehydes is more important in the liquid phase or solution, where energy dissipation by collision is more efficient. The decarbonylation products were shown to be only of minor importance in the photolysis of liquid cyclopentanone and cyclohexanone . The unsaturated aldehyde was found to be the main product in the liquid-phase photolysis of cyclopentanone (methyl cyclohexanone . Unsaturated aldehydes were also identified in the photolysis products of other cyclic ketones in the liquid phase as well as in solution . ... 

Hydrogenation of ketones is less facile and less selective than the reduction of aldehydes. [H2Ru(TPPTS)4] proved to be an active catalyst in hydrogenation of 2-butanone, cyclohexanone, and benzylacetone (80 °C, 3.5 MPa H2) [34]. The same catalyst was also rather selective toward the formation of the saturated ketone in the hydrogenation of tr ns-4-hexen-3-one [Eq. (9)], yielding only 2-7% of 3-hexanol. Similar C=C/C=0 selectivity was found with the [(C5R5)RuCl(PTA)2j (R = H or CHj) catalysts, too [41]. 

The early studies of the photochemistry of cyclohexanone have been reviewed by Srinivasan (1964), Pitts and Wan (1966), Cundall and Davies (1967), and Wagner and Hammond (1968). The major products are identified as CO, cyclopentane, 1-pentene, and 5-hexenal, but the mechanisms of their formation remain ill-defined. Two alternative mechanisms have been discussed through the years (Calvert et al., 2008). The first invokes intramolecular rearrangements of the original excited ketone (e.g., see Shortridge and Lee, 1970), while the second implies formation of biradicals that are intermediates to the final products (e.g., Benson and Kistiakowski, 1942 Blacet and Miller, 1957). 

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