Cyclohexane derived catalysts

MacMillan s organocatalyst, 56a, which was used typically for electrophilic activation, was seen also to be efficient in promoting conjugate addition via enamine formation (Scheme 2.52) [42]. The proof of the enamine pathway was furnished by extended NMR studies. Gellman and colleagues noted an interesting dependence of selectivity on the catalyst structure improved conversion and ee-value can occur with the spirocyclopentane derivative 56b, and by the addition of a catechol derivative as acid additive (Scheme 2.52). The cyclohexane-derived catalyst 56c was unreactive, however. 

Silver sulfate has been described as a catalyst for the reduction of aromatic hydrocarbons to cyclohexane derivatives (69). It is also a catalyst for oxidation reactions, and as such has long been recommended for the oxidation of organic materials during the deterrnination of the COD of wastewater samples (70,71) (see WASTES, INDUSTRIAL WATER, INDUSTRIAL WATERTTEATI NT). 

Oxygen compounds in crude oils are more complex than the sulfur types. However, their presence in petroleum streams is not poisonous to processing catalysts. Many of the oxygen compounds found in crude oils are weakly acidic. They are carboxylic acids, cresylic acid, phenol, and naphthenic acid. Naphthenic acids are mainly cyclopentane and cyclohexane derivatives having a carboxyalkyl side chain. 

The reductive amination of ketones can be carried out under hydrogen pressure in the presence of palladium catalysts. However, if enantiopure Q -aminoketones are used, partial racemization of the intermediate a-amino imine can occur, owing to the equilibration with the corresponding enam-ine [102]. Asymmetric hydrogenation of racemic 2-amidocyclohexanones 218 with Raney nickel in ethanol gave a mixture of cis and trans 1,2-diamino cyclohexane derivatives 219 in unequal amounts, presumably because the enamines are intermediates, but with excellent enantioselectivity. The two diastereomers were easily separated and converted to the mono-protected cis- and trans- 1,2-diaminocyclohexanes 220. The receptor 221 has been also synthesized by this route [103] (Scheme 33). 

Bifunctional catalysts have proven to be very powerful in asymmetric organic transformations [3], It is proposed that these chiral catalysts possess both Brpnsted base and acid character allowing for activation of both electrophile and nucleophile for enantioselective carbon-carbon bond formation [89], Pioneers Jacobsen, Takemoto, Johnston, Li, Wang and Tsogoeva have illustrated the synthetic utility of the bifunctional catalysts in various organic transformations with a class of cyclohexane-diamine derived catalysts (Fig. 6). In general, these catalysts contain a Brpnsted basic tertiary nitrogen, which activates the substrate for asymmetric catalysis, in conjunction with a Brpnsted acid moiety, such as urea or pyridinium proton. 

Takemoto and co-workers designed a small hbrary of thiourea cyclohexane-diamine derived catalysts for the Michael reaction of malonates to nitrolefins [15]. The authors observed an interesting trend in catalysis the reaction only proceeded enantioselectively and in decent yields when the catalyst possessed both thiourea... 

The asymmetric Mannich addition of carbon nucleophiles to imines catalyzed by the cyclohexane-diamine catalysts has developed significantly in the past decade. List and co-workers reported the asymmetric acyl-cyanantion of imines catalyzed by a cyclohexane-diamine catalyst [103], Using a derivative of Jacobsen s chiral urea catalyst, the authors optimized reaction conditions and obtained chiral iV-acyl-aminonitriles in high yield and enantioselectivities (Scheme 51). The scope of the reaction was explored with both aliphatic and aromatic imines, providing good to high selectivities for a variety of substrates. 

Wang and co-workers reported a novel class of organocatalysts for the asymmetric Michael addition of 2,4-pentandiones to nitro-olefms [131]. A screen of catalyst types showed that the binaphthol-derived amine thiourea promoted the enantiose-lective addition in high yield and selectivity, unlike the cyclohexane-diamine catalysts and Cinchona alkaloids (Scheme 77, Table 5). 

Benzo[6]thiophenes are converted into cyclohexane derivatives by treatment with hydrogen over a heated palladium catalyst.21,22 766 If the reaction tube is coupled directly to the injection port of a GLC... 

The cyclobutane derivative (55) gave an optical yield of 91% in the hydrogenation of a-acetamidocinnamic acid. The catalyst was here prepared in situ from [RhCl(l,5-hexadiene)]2. The corresponding 1,2-derivative of cyclopentane gave an optical yield of only 73% and the cyclopropane and cyclohexane derivatives gave 15% and 36% respectively.235 [RhCl(cod)2] in presence of the norbomadiene-based ligand NORPHOS (56) gave up to 96% optical yields with a-acetamidocinnamic acid as substrate.236... 

The direction of addition (syn or anti) with amine nucleophiles has been tested and fonnd to be dependent on the reaction conditions. The sitnation is clearest with the homochiral cyclohexane derivative (Scheme 15). In this example, a simple Pd catalyst gives a mixture of isomers owing to competitive syn and anti addition, while the polymer-supported catalyst produces only the (usual) anti addition. ... 

A further support for the mechanism outlined in Eq. 118 is that with Ni(0) catalysts a second type of [3+2]-cycloaddition can occur which involves the oxidative coupling of two alkenes coordinated at the nickel (one must be methylenecyclopropane). The initially formed nickelacyclopentane derivative may collapse to give a spiro[2.3]cyclohexane derivate or rearrange into a 4-methylenenickelacyclohexane derivate, which at the end of this catalytic cycle gives methylenecyclopentanes with a new substitution pattern by reductive elimination (see Eq. 78 and Scheme 8). 

Beside the cross aldol reaction, the Mannich reaction, too, has been the object of successful efforts using organocatalysis. The use of small organic molecules such as proline, cyclohexane diamine and Cinchona alkaloid-derived catalysts has proven extraordinarily useful for the development of asymmetric Mannich reactions in traditional polar solvents such as DMSO, DMP, DMF, etc. However, very few studies have been conducted so far in non-conventional solvents. 

The synthesis of enantiopure cyclohexane derivatives has been investigated by hydrogenating arenes by use of chiral auxiliaries bound either to the support of the catalyst or to the substrate, or by use of chiral phase-transfer reagents [15]. Although significant progress has been reported, enantioselectivity is still moderate (maximum 68 % e. e.). 

Raney nickel and platinum, palladium, and rhodium catalysts have been used to accomplish the hydrogenation of polycyclic aromatics. Hydrogenation of fused polycyclic arenes leads to the cis- or fran -substituted cyclohexane derivatives. The cis product is usually obtained again this can be understood in terms of the mechanism proposed for aromatic hydrogenation (vide supra). 

Third, the doublet and, especially, sextet models require very precise superimposing of the molecule on the catalyst lattice. We have found that the cyclohexane derivatives, in accordance with the sextet model, smoothly dehydrogenate only on the following metals nickel, cobalt, iridium, palladium, platinum, ruthenium, osmium, and rhenium, all of which crystallize in Al, A3 lattices with certain interatomic distances. These results extend to the alloys of these metals. The catalytic activity of rhenium for this reaction was predicted by the multiplet theory as this metal maintains the square of activity this prediction was realized experimentally in the laboratory of the author. Similar correlations take place in the exchange of cyclanes with deuterium. 

The syntheses of cyclohexane derivatives by 6-exo-cyclizations of 2-halo-1,7-octadienes and l-halo-l,6-heptadienes are well documented, yet the formation of seven-membered rings during these cyclizations has also been observed (Scheme 10). This type of ring closure to cyclohexane derivatives has been applied in various total syntheses of natural products and been further elaborated applying chiral ligands in the catalysts to enable an enantioselective control (Tables 8 and 9). 

The most studied catalytic system is the one derived from 1,2-diamino-cyclohexane-derived Schiff base, presented for the first time by North and Belokon in 1998 for the cyanosilylation of aldehydes. In contrast to many catalysts known to date that require more than 10 mol% of loading and low temperatures, the catalyst 18 employed here was efficient at 0.1 mol% for a complete conversion at room temperature (0.01 mol% for 80% conversion. Scheme 7.14). Enantiomeric excesses from 30 to 86% were obtained from aromatic aldehydes and lower 44-46% enantiomeric excesses were observed from aliphatic ones (propanal and pivalaldehyde). 

Benzene reacts with hydrogen gas and a suitable catalyst to give cyclohexane derivatives and, in some cases, cyclohexene or cyclohexadiene derivatives. 

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