P-Isophorone

Isophorone usually contains 2—5% of the isomer P-isophorone [471-01-2] (3,5,5-trimethyl-3-cyclohexen-l-one). The term a-isophorone is sometimes used ia referring to the a,P-unsaturated ketone, whereas P-isophorone connotes the unconjugated derivative. P-lsophorone (bp 186°C) is lower boiling than isophorone and can be converted to isophorone by distilling at reduced pressure ia the presence of -toluenesulfonic acid (188). Isophorone can be converted to P-isophorone by treatment with adipic acid (189) or H on(Ill) acetylacetoate (190). P-lsophorone can also be prepared from 4-bromoisophorone by reduction with chromous acetate (191). P-lsophorone can be used as an iatermediate ia the synthesis of carotenoids (192). 

The catalytic oxidation of isophorone (259—261) or P-isophorone (262,263) to ketoisophorone [1125-21 -9] (2,6,6-trimethyl-2-cyclohexen-l,4-dione) has been reported. Ketoisophorone is a building block for synthesis in terpene chemistry and for producing compounds of the vitamin A and E series. 

With its high surface area and the accessibility to the amino groups, chitosan aerogel appeared as a good candidate to play the double role of support for metal complexes and organic base. Silica supported metallophthalocyanine are efficient catalysts for the oxidation of aromatic compounds [139]. The immobilization of hydrosoluble metallophthalocyanines (MPcS with M = Fe or Co) on chitosan aerogels afforded new heterogeneous catalysts for the aerobic oxidation of p-isophorone [140]. 

These materials are thus promising heterogeneous catalysts for the oxidation of p-isophorone to ketoisophorone. This novel method corresponds to several criteria of green chemistry and sustainable development (1) support from inexpensive renewables, (2) dioxygen as an oxidant, (3) combination of basic and oxidation sites in one solid material to avoid an addition of external base or other additives resulting in no wastes, and (4) easy separation of catalyst from reaction mixture and possibility of recycling. 

Most of the earlier studies described the oxidation of simple (electron-rich) cycloalkenes, such as cyclohexene euid cyclododecene. Here we report the catalytic behaviour of titania-silica aerogels in the oxidation of cycloalkenones. The model reactions are the epoxidation of a- and P-isophorone, depicted in scheme 1. 

The oxidation reactions were performed in a closed, mechanically stirred 100 ml glass batch reactor under Ar. For the epoxidation of a-isophorone, 0.2 g catalyst, 9 ml solvent, 7.2 mmol cumene (internal standard) and 77 mmol olefin were introduced into the reactor. The slurry was heated to the reaction temperature and the reaction stauted by adding 13.4 mmol t-butyl hydroperoxide (TBHP, ca. 3 M in isooctane) from a dropping funnel to the vigorously stirred slurry (n = 1000 min ). For the epoxidation of P-isophorone, 20 ml ethylbenzene solvent, 61 mmol P-isophorone, 7.2 mmol cumene and 5.6 mmol TBHP or ciunene hydroperoxide (CHP) were introduced into the reactor in this order. The solution was heated to 80 °C and... 

Preliminary experiments revealed that the selectivity of titania-silica aerogel in the epoxidation of P-isophorone was moderate. The selectivity related to the olefin converted was below 90 % at low temperature, and dropped rapidly at 80 °C or above. The most important side reactions were the formation of 3,5,5-trimethyl-2-cyclohexene-4-hydroxy-l-one (2) by ring opening of the epoxide (1), and the isomerization of P- to a-isophorone (3), as shown in Scheme 2. Epoxidation of 2 and 3, and the oxidation at the OH group of 2 to a dicarbonyl compound were slow and the amounts of these by-products were usually aroimd 1 % or less. 

Effect of basic treatment of the 20 wt% TiO2-80 wt% SiOg aerogel on the epoxidation of P-isophorone with TBHP at 80 °C... 

The epoxidation of two cycloalkenones, a- and P-isophorone, with alkyl hydroperoxides demonstrates that active and selective titania-silica aerogels can be prepared by the sol-gel method combined with extraction of the solvent with supercritical COg at low temperature. The key factors for obtaining high activity in the epoxidation of bulky cyclic olefins are the high Ti-distribution in the silica matrix, the mesoporous structure and high surface area. 

The epoxide selectivity is considerably lower in the other model reaction, the oxidation of P-isophorone. The acid-catalyzed side reactions could be suppressed by a treatment of the mixed oxide catalyst with a weakly basic salt prior to the reaction. The epoxide selectivity related to the olefin converted could be increased up to 94 % at 90 % peroxide conversion. 

Isophorone [14.268], [14.269] is an unsaturated cyclic ketone. It consists of a-isophorone [78-59-1] (3,5,5-trimethyl-2-cyclohexen-l-one), which contains about 1-3% of the isomer P-isophorone [471-01-2] (3,5,5-trimethyl-3-cyclohexen-l-one). Isophorone is a stable, water-white liquid with a mild odor that is miscible in all proportions with organic solvents. It dissolves many natural and synthetic resins and polymers, such as poly(vinyl chloride) and vinyl chloride copolymers, poly(vinyI acetate), polyacrylates, polymethacrylates, polystyrene, chlorinated rubber, alkyd resins, saturated and unsaturated polyesters, epoxy resins, cellulose nitrate, cellulose ethers and esters, damar resin (dewaxed), kauri, waxes, fats, oils, phenol-, melamine-, and urea-formaldehyde resins, as well as plant protection agents. However, isophorone does not dissolve polyethylene, polypropylene, polyamides. 

The 3-hydroxy-p end group is the most abundant chiral end group in carotenoids and is often called the zeaxanthin end group. Zeaxanthin (119-121) possesses two chiral centres at C(3) and C(3 ), making possible three optical isomers, namely the (3/ ,3 / )-isomer (119, most abundant in Nature) and the (35,3 5)-isomer (121) as well as the (3/ ,3 5)-isomer (120) which constitutes a meso-form. It is this optically inactive mixture of isomers which is usually obtained by synthesis of the so-called racemate. For the first total synthesis of racemic zeaxanthin (119-121) the strategy Ci9 + C2 + Ci9 = C4o was applied [15]. The Ci9-synthon 50 was synthesized starting from isophorone (51) which was converted into p-isophorone (52) (Scheme 12). 

A new strategy for the synthesis of xanthophylls was developed in the 1970s by Roche [77]. The cyclic moieties are all derived from the common precursor 6-oxoisophorone (68), which is obtained from inexpensive a-isophorone (69) in two steps. In the liquid phase in the presence of weak acids, or in the gas phase on nickel oxide catalysts [78,79], 69 is in equilibrium with p-isophorone (70), which may be separated from the higher-boiling starting material by fractional distillation (Scheme 21). 

A related migration of an endocyclic double bond was found prior to the hydro-formylation of P-isophorone acetal (Scheme 5.11) [71]. The exocyclic aldehyde was formed as a major product. 

Hydroformylation of P-isophorone gives a keto aldehyde that is an intermediate for the synthesis of 5- and s-damascone, both of which are valuable aroma compounds (Route I, Scheme 6.54) [151a,b]. The hydroformylation at 600 bar syngas pressure was carried out in a 75 g scale and yielded, besides some hydrogenation products, exclusively 4-formyl-3,3,5-trimethyl-cyclohexanone [151b]. 

Under these conditions, no isomerization into the thermodynamically more stable a-isophorone was observed. Remarkably, when the latter was subjected to the same reaction conditions, only hydrogenation took place, showing the general reactivity problems of enone hydroformylation [152]. Since the application of lower syngas pressure (100-150 bar) in the hydroformylation of P-isophorone also mainly yielded the hydrogenation product, the carbonyl group was protected as acetal (Route II) [152]. Surprisingly, under hydroformylation conditions (90 bar, 100 C), the exocyclic aldehyde was formed as the major product, obviously due to a prior Rh-mediated isomerization process. 

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