2-Cyclohexenone hydrogenation

Cyclohexenone has been prepared by dehydrohalogenation of 2-bromocyclohexanone, by the hydrolysis and oxidation of 3-chlorocyclohexene, by the dehydration of a-hydroxycyclohexa- ione, by the oxidation of cyclohexene with chromic acid or hydrogen peroxide in the presence of a vanadium catalyst, by I lie addition of acroleiti to ethyl acetoacctate followed by cycliza-lion, hydroly.sis, and decar])oxylation, by the reduction of N,N-dimelliyliiniline with sodium and ethanol itt liquid ammonia... 

A convenient synthesis of 3,3-dimethylcyclohexanone, a compound obtained otherwise with difficulty, involves hydrogenolysis of 5,5-dimethyl-l,3-cydohexanedione (55). The reduction is believed to go through 3,3-dimelhyl-cyclohexenone (24). Hydrogenation virtually ceases after absorption of 2 mol of hydrogen. 

Chiral fe-thiourea-type catalysts effectively provide the Baylis Hillman reaction with cyclohexenone and aldehydes.181 In several reactions, thiourea derivatives have been used as significant and specific catalyst because of their intermolecular hydrogen bonding ability (Scheme 74).182 186... 

Ru complex and (CH3)3COK [(S, R)-34B] is also an excellent catalyst for hydrogenation of the cyclic enone [111]. The allylic alcohol product is a useful intermediate for the synthesis of carotenoid-derived odorants and other bioactive ter-penes. Hydrogenation of 2-cyclohexenone in the presence of the (S,S)-DIOP-Ir catalyst gives (R)-2-cyclohexenol in 25% ee (Fig. 32.43) [137]. 

Irradiation of a mixture of dimethyl cyclobutene-1,2-dicarboxylate (372) and 3-methyl-2-cyclohexenone (373) gave the adduct (374), which was then pyrolyzed to give the diene (375). Hydrogenation afforded the keto diester (376). On the other hand, when the compound (374) was reduced by NaBH4, y-lactone (377) was... 

It is shown that the less substituted a-C—H bond of several tertiary amines added across the olefin bond of 2-cyclohexenone (Scheme 3). In the radical ion pair the proton of the less substituted hydrogen is transferred to the a-position of enones that leads to a predominance of products 23, 26, 29 and 32 (about 95% yield). 

Substituent effects on the -(aminoethyl)cyclohexenone photochemistry were carried out to study the relative kinetic acidities of the tertiary aminium radical47. The ease of the methylene hydrogen to be removed as H+ increased in the order of X = alkyl < Si(CH3)3 < C=CH (equation 13). 

When the enol ring is adjacent to a cyclic moiety, then it is possible to achieve very short hydrogen bonds, as in the structure of usnic acid, a natural product found in lichens. A low-temperature X-ray diffraction analysis of this compound showed two enol moieties, one in which a carbon-carbon bond of the enol was part of a cyclohexenone ring, and this had... 

The catalyst is also effective for the reduction of styrenes, ketones, and aldehydes. Cyclohexenone 16 was reduced to cyclohexanone 11 by transfer hydrogenation, and using a higher catalyst loading, styrene 17 was reduced to ethylbenzene 18. The elaboration of [Ir(cod)Cl]2 into the triazole-derived iridium carbene complex 19 provided a catalyst, which was used to reduce aUcene 20 by transfer hydrogenation [25]. 

If 2-methylenecyclobutanones, which themselves are not capable of this tautomerization and the following rearrangement, bear alkyl substituents with an a-hydrogen in the 3-position, still another rearrangement, a [1,3] shift, precedes the actual ring-enlargement step. This is important, because either 3-alkyl-2-methylene- or 2-alkylidenecyclobutanones or mixtures of both may be formed via [2 + 2] cycloaddition of allenes92 and can all be used as precursors for the same cyclohexenones. 

The push toward enaotiomerically-pure carbocyclic intermediates has led to the development of new methods for the enantiodifferentiation of inexpensive prochiral cyclic starting materials. For instance, Robert H. Morris of the University of Toronto recently reported (Organic Lett. 2005, 7, 1757) that a family of enantiomerically-pure Ru complexes originally developed for asymmetric transfer hydrogenation also mediate the enantioselective addition of malonatc to cyclohexenone. 

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