4- Hexen-2-one

Triple-bond participation was also observed in the formolysis of 64 (81). Tosylate 64 in formic acid at 60° in the presence of sodium formate gave a nearly quantitative yield of 5-methyl-4-hexen-3-one, 65. 

Table 3 summarizes the scope and limitation of substrates for this hydrogenation. Complex 5 acts as a highly effective catalyst for functionalized olefins with unprotected amines (the order of activity tertiary > secondary primary), ethers, esters, fluorinated aryl groups, and others [27, 30]. However, in contrast to the reduction of a,p-unsaturated esters decomposition of 5 was observed when a,p-unsaturated ketones (e.g., trans-chalcone, trans-4-hexen-3-one, tra s-4-phenyl-3-buten-2-one, 2-cyclohexanone, carvone) were used (Fig. 3) [30],... 

However, as illustrated in Scheme 2.112, the reaction of ylide 2-490 with 4-hexen-3-one (2-491) did not lead to the expected cycloheptene, but to the cyclopropane derivative 2-493 in 98% yield by a simple addition of the ylide to 2-491 to give 2-492... 

Hernandez and Kalck495,496 synthesized various Ru/tppts complexes and used them as catalysts in the hydrogenation of various cc,J3-unsaturated aldehydes and ketones in a two phase system. Using Ru(H)2(tppts)4 high conversions of cinnamaldehyde (97%) and high selectivity to cinnamylalcohol (95%) were observed under mild reaction conditions (40°C, 20 bar, 3h) 495 More forcing conditions (80°C, 35 bar, 16h) were needed for the hydrogenation of unsaturated a, 3-ketones such as 4-hexen-3-one or benzylideneacetone and only low chemo-selectivities were observed.495... 

However the catalyst can be easily reactivated in flow of o2/Ar at 350°C (compare runs 2 and 2.1, Table 1) using the procedure reported in the previous section. Catalytic activity can be restored also by a thermal treatment in flow of He (350°C, 15 h), and this suggests that strongly adsorbed produts could be responsible for catalyst deactivation. The amount of 4-hexen-3-one converted depends on the nature of the catalyst precursor and on its thermal pretreatment. Thus, over a non activated commercial Mgo (obtained by thermal decomposition of MgC03, surface area 17 m2/g), 0.5 moles of 4-hexen-3-one/mole Mgo are converted, while when the same Mgo was activated at 350°C (surface area 34 m2/g), 2 moles of 4-hexen-3-one/mole MgO are converted. Over a high surface area Mgo (prepared by thermal decomposition of Mg(OH)2r surface area 281 m2/g) up to 5 moles of 4-hexen-3-one/mole Mgo can be converted. Conversion of 4-hexen-3-one depends also on reaction temperature 250°C is found to be the best one, since both at higher and lower temperatures side reaction are favoured (runs 2.2 and 2.3, Table 1). Since different oxides were employed, the product distributions reported in Table 1 were measured in stationary conditions after 1 hour of reaction. 

This is the case as shown in Table 1, runs 4,5, provided that basic sites are free from adsorbed H20 and C02 (compare runs 3 and 4, Table 1). Notably, once again the ratio hexan-3-one/hexen-3-ols appears unaffected by the catalyst precursor and/or pretreatment (runs 3,4, Table 1). These observations suggest that in the transfer hydrogenation of 4-hexen-3-one, the substrate is coordinated on a weak acid site while propan-2-ol must be coordinated on an adjacent surface basic site [7,24]. This is confirmed by the lack of reduction products observed over Mg(OH)2 and MgCl2 (runs 8,9, Table 1). 

Hydrogen transfer reduction of 4-hexen-3-one catalyzed by doped MgO catalysts.3... 

The oldest syntheses of chrysanthemates are those starting from 2,5-dimethyl-2,4-hexadiene (238). There have been more papers on the use of rhodium or antimony to catalyze the addition of diazoacetate and chiral copper complexes to create asymmetry during the addition (see Vol. 4, p. 482, Refs. 219-222). The problem with this route is to avoid the use of diazo compounds. An old synthesis of Corey and Jautelat used the ylide addition of a sulfurane to a suitable precursor (in this case a C3 unit was added to methyl 5-methyl-2,4-hexadienoate, 239), and a recent paper gives details about the addition of ethyl dimethylsulfuranylideneacetate to 2,5-dimethyl-4-hexen-3-one (240). This led exclusively to the tran -isomer 241, from which ethyl trans-chrysanthemate (185, R = Et) was made. Other ylide additions are mentioned below. 

Kaspar et al. demonstrated the reduction of a,P-unsaturated ketones to allylic alcohols with /-PrOH in the gas phase over MgO as fixed bed catalyst at 250°C [7]. The MgO was formed in situ by heating Mg(OH)2 at 350°C in an air current during 4 hours. Regeneration of the catalyst was done in the same way. In a subsequent paper the chemoselective reduction of the carbonyl group of 4-hexen-3-one over various solid catalysts was reported [8]. MgO was found to show the highest chemoselectivity. However, as a result of its high basicity several side reactions were also observed. Doping of the MgO catalysts with HCl afforded solid catalysts with improved selectivity. 

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