Hydrogenation cyclohexanones from

Ir(cod)Cl]2 reacts with Q-diimines LL (derived from glyoxal and biacetyl) to yield cationic [Ir(cod)LL]+.523 If the reaction is carried out in the presence of SnCl2, then the pentacoordinate Ir(SnCl3)(cod)LL species results. The compounds are active catalysts in the homogeneous hydrogen transfer from isopropanol to cyclohexanone or to acetophenone followed by hydrogenation... 

Table 1 Hydrogen transfer from 2-propanol to 4-teri-butyl-cyclohexanone in the presence of different copper catalysts.

The photoreductions of a number of carbonyl compounds with either lowest mr or jiji triplet states in the presence of tributyltinhydride are reported24. The carbonyl compounds include cyclohexanone and acetone which possess nrr lowest-energy triplets, and 2-acetonaphthone, 1-naphthaldehyde and 2-naphthaldehyde which possess lowest-energy mr triplets. In the case of the two njr triplets, a simple mechanism is proposed which involves the abstraction of a hydrogen atom from the tributyltinhydride by the triplet state... 

The comments made about the diradical hypothesis with respect to the photochemistry of cyclopentanone are equally applicable to cyclohexanone. Since the formation of none of the products listed in reactions 15-18, and (15,29) is quenched by even 10-20 mm. of oxygen the existence of diradical intermediates in this system is subject to question. The alternative mechanism would be one that causes a concerted split of the ketone molecule in the excited state into two (in the cases of reactions 15 and 16) or three (reaction 17) molecular fragments. Both 16 and 17 are analogous to reactions 3 and 2 in the photochemistry of cyclopentanone and do not involve a shift of hydrogen atom from one... 

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 production of Cyclohexanone from phenol was simplified when selective hydrogenation with Pd catalysts was made possible ([see Eq. 21.3)]. In this process, phenol is completely converted in the gas phase at 140 to 170°C and 1 to 2 bar using a supported Pd catalyst containing alkaline earth oxides (e.g., Pd-CaO/A Os). The selectivity to Cyclohexanone is greater than 95%46. 

Under vigorous conditions (100°C and 1000 psig) in 25% aqueous sulfuric acid, the yield of cyclohexanol plus cyclohexanone from dimethyl-aniline was 90, 75, and 13% over 5% palladium-, 5% rhodium-, and 5% platinum-on-carbon, respectively (33). The decreasing yield parallels the decreasing tendency for migration. Reductive hydrolysis is favored by substitution on the nitrogen atom attributed in part to the relative difficulty of hydrogenating hindered enamines. 

The relative mobilities of the axial and equatorial hydrogen atoms from C(2) and C(6) positions of rigid cyclohexanones have been determined by several authors from the relative rate constants of hydrogen-deuterium exchange. Indeed, according to the principle of microscopic reversibility, the elementary rate constants for axial and equatorial proton detachment correspond to those of proton addition on the two faces of the enol or enolate (26). 

A particularly interesting extension of this work is offered by the observed enandoselective hydrogen abstraction from the prochiral cyclohexanone (47) on treatment with chiral lithium amide bases (Scheme 27). Thus, quenching the initially formed enolate afforded the asymmetric trimethylsilyl ether (48) which gave the chiral enone (49) in 65% enantiomeric excess on dehydrogenation. Further woilc in this area should provide valuable methodology for the formation of chiral a,3-unsaturated carbonyl systems. 

Some pyrazolate complexes were found to possess high catalytic activity for various hydrogen-transfer processes and to be of considerable value in synthesis. The heterodinuclear complexes [(CO)(H)(PPh,)2Ru( -pz)(/x-Cl)M(diolefin)] (M = Rh or Ir diolefin = cod or tfbb) catalyze the hydrogen transfer from isopropanol to cyclohexanone (101). The heterodinuclear complexes were shown to be more active catalysts than the mononuclear pyrazole compounds [Ru(H)(Cl)(Hpz)(PPh,)2] and [M(H)(Cl)(Hpz)(diolefin)] (101). 

Although outside the scope of the present chapter, another transformation of interest is the conversion of the fully hydrogenated product from phenol, namely cyclohexanol, to cyclohexanone in 100% yield by addition of a dichloromethane solution to bis(quinuclidine)bromine fluoroborate and silver fluoroborate in dichloromethane followed by reaction for 30 mins.at ambient temperature (ref.65). 

Some other examples of carbonyl compounds reduced by hydrogen transfer from alcohols arc benzaldehyde (36) on aluminium alkoxides. MgO, zeolites, and LDH s, cyclohexanone [264] over LDIFs or 4-tcrt-butyIcyclohexanonc [110] over calcined hydrotalcites. NaBHA zeolites and KF AI Oi. 

Aramendia. MA Borau. V Jimenez. C Marinas. JM Ruiz. JR Urbano. FJ. Catalytic hydrogen transfer from 2-propanol to cyclohexanone over basic Mg-AI oxides. Applied Catalysis A General, 2003 255, 301 308. 

Hydrogen transfer reactions are catalyzed by several iridium complexes, including the dimethyl sulfoxide (DMSO) complexes cis- and trans-[Ir(Cl)4(DMSO)2]", [Ir(Cl)3(DMSO)3] and [lr(H)-(Cl)2(DMSO)3], as well as the cyclooctadiene (cod) complexes [Ir(Cl)(cod)]2 and [Ir(3,4,7,8-Me4phen)(cod)], and tra 5-[Ir(Cl)(CO)(PPh3)2]. Vaska s complex catalyzes the conversion of p-methoxybenzoyl chloride to the corresponding aldehyde. The dimethyl sulfoxide iridium(III) complexes catalyze hydrogen transfer from propan-2-ol to unhindered cyclohexanones to yield cyclohexanols, while the cod complexes serve as catalysts in the transfer of hydrogen from propan-2-ol to alkenes, ketones and a,/3-unsaturated ketones. ... 

Bubble columns, in which the liquid is the continuous phase, are used for slow reactions. Drawbacks with respect to packed columns are the higher pressure drop and the important degree of axial and radial mixing of both the gas and the liquid, which may be detrimental for the selectivity in complex reactions. On the other hand they may be used when the fluids carry solid impurities that would plug packed columns. In fact, many bubble column processes involve a finely divided solid catalyst that is kept in suspension, like the Rheinpreussen Fischer-Tropsch synthesis, described by Kolbel [1], or the former I. G. Farben coal hydrogen process, or vegetable oil hardening processes. Several oxidations are carried out in bubble columns the production of acetaldehyde from ethylene, of acetic acid from C4 fractions, of vinylchloride from ethylene by oxychlorina-tion, and of cyclohexanone from cyclohexanol. 

Hydrogen circulation pump 2 Phenol vaporizer 3 Reactor 4 Cyclohexanone separator Figure 5.22 Flow diagram for the production of cyclohexanone from phenol... 

Fig. 60. The mechanism of hydrogen transfer from 2-propanol to cyclohexanone by the IrH2(Pz)(HPz)(PPh3)2 complex.

Secondary Amines.—The reduction of imines to the corresponding secondary amines can be effected by various methodologies. New additions are the sodium triacyloxyborohydrides (easily obtainable from sodium borohydride and AT-acyl derivatives of optically active amino-acids), which are used for the asymmetric reduction of cyclic imines. Also now available is a highly stereoselective reduction of N-benzylimines derived from substituted cyclohexanones, with alkali-metal borohydrides, in particular L-selectride. A fiuther addition is the first report of the reduction of aldimines by hydrogen transfer from propan-2-ol,... 

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