3- Cyclohexenol

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]. 

BINAP Ru catalyst and (lR,25 )-ephedrine (Scheme 8-53). This result is similar to that obtained when catalyzed by pure (R)-BINAP. In pure (R)-BINAP complex-catalyzed hydrogenation, (S )-2-cyclohexenol can also be obtained with over 95% ee. This means that in the presence of (R)-BINAP-Ru catalyst, (R)-cyclohexenol is hydrogenated much faster than its (S )-enantiomer. When ephedrine is present, (R)-BINAP-Ru will be selectively deactivated, and the action of (S -BINAP-Ru leads to the selective hydrogenation of (S)-2-cyclohexenol, leaving the intact (R)-2-cyclohexenol in high ee. 

Cyclohexene, bromination of, 47, 32 reaction with 1-butyl perbenzoate and cuprous bromide, 48,18 2-Cyclohexenol, 46, 32 2-Cyclohexen-I-ol, benzoate, 48, 18 2-Cyclohexenone, 45, 32 Cydohexyl allophanamide, 45, 72 Cyclohexylamine, 45, 85 reaction with 1-butyl hypochlorite, 46, 16... 

Using methods developed by Sharpless (68), Reich (69), and others, the optically active 4,4-dimethyl-2-cyclohexenol is prepared in excellent yield from the corresponding chiral selenide (eq. [19]). The (S)-4,4-dimethyl-3-p-methylphenylselenocyclohexanone, [a] 42.1° (e.e. 39%), was reduced with sodium borohydride to the (one) diastereomeric alcohol, [a] 11.0°, in quantitative yield and converted to the allylic alcohol, [a] — 17.7°, with an e.e. of 40%. 

In 1999, Sanjuan and co-workers [27] reported a very elegant type IIavRH oxidation of cyclohexene to give a mixture of cyclohexane-1, 2-diol, 2-cyclohexenol, and 2-cyclohexenone. The reaction is initiated by excitation of the zeolite-embedded 2,4,6-triphenylpyrylium cation to produce a hydroxy radical (steps 1 and 2... 

Complete enantiomer discrimination and asymmetric deactivation of the racemic XylBINAP-RuCl2(dmf) ( )-7b using DM-DABN as a chiral poison are shown to be effective in the kinetic resolution of 2-cyclohexenol (Scheme 8.9). Use of just a 0.5 molar amount of (5)-DM-DABN relative to ( )-7b gives enantiopure (S)-2-cyclohexenol, which is kinetically resolved in the same conversion as enantiopure 7b. Indeed, the relative rate of hydrogenation of (R)- versus (5)-2-cyclohexenol in the presence of only a 0.5 molar amount of (5)-DM-DABN relative to ( )-7b is significantly large (kf/kg = 102). The combination of ( )-7b with (S)-DM-DABN also gives 99.3% ee of (R)-methyl 3-hydroxybutanoate quantitatively... 

Oxidation of (15,47 )-4-/ert-butyldimethylsilyloxy-2-cyclohexenol (see p400) and subsequent silyl group removal gave 4-hydroxy-2-cyclopentenone (1) of known R configuration55. 

Because of the minimal steric difference between the substituents surrounding the alcohol moiety in simple 2-cyclohexenols, these are not easy to resolve by hydrolase-catalyzed reactions. However if a sterically bulky group can be temporarily introduced and then removed, such resolution will be facilitated. One example of this approach is the preparation of enantiomerically enriched 4-hydroxycyclohex-2-enone (64) (Scheme 4.25) [85]. A similar approach was used for the preparation of enantiomerically enriched cryptone (65) (Scheme 4.25) [86]. 

Mixed cyanocuprates (RCuCN)LL The earlier synthesis of 2-cyclohexenols (9, 329 320) has been extended to provide a general route to highly substituted 2-cyclo-hcxenols as shown in equation (I). The crucial step involves a regiospecific 1,4-uddition of a cyonocupralc to an a-cxo-mcthylcne epoxide.2"... 

When racemic 3-methyl-2-cyclohexenol is hydrogenated by the BINAP-Ru catalyst at 4 atm H2, trcms- and cis-3-methylcyclohexanol are produced in a 300 1 ratio (Scheme 33). The reaction with the (/ )-BINAP complex affords the saturated R,3R trans alcohol in 95% ee in 46% yield and unreacted S allylic alcohol in 80% ee with 54% recovery. 

On the other hand, if the HYD/I+HDO and the HDO/I ratios are very weak for 2,5-dihydrofuran (V), 0.04 and 0.1 respectively, these values increase with ethyl-2-propenyl ether, (III), up to 0.12 and 11. These results are similar too those obtained for 2-cyclohexenol, (VIII), which presents weak hydroge nation and HDO activities (Table 2). 

Hydrogenation of 2,4,4-trimethyl-2-cyclohexenone with rrans-RuCl2(tolbinap)(dpen) and (CH3)3COK under 8 atm of hydrogen gives 2,4,4-trimethyl-2-cyclohexenol quantitatively with 96% ee (Scheme 1.70) [256,275,276]. In this case, unlike in the reaction of aromatic ketones, the combination of the R diphosphine and S,S diamine most effectively discriminates the enantiofaces. The chiral allylic alcohol is a versatile intermediate in the synthesis of carotenoid-derived odorants and other bioactive terpens such as a-damascone and dihydroactinidiolide [277]. 

Suresh, Lee and coworkers demonstrated oxidation of cyclohexene catalyzed by Mn or Cu complexes using H202 in aqueous phase in a microreador (width = 200 pm and depth = 50 pm) [36], Water-soluble ionic liquid [bmim]BF4 was added (0.5%, v/v) to improve the solubility of cydohexene in the readionbuffer. With the use of a reduced Schiff base-Cu complex, 2-hydroxycyclohexanone was obtained as the maj or produd with 2 5 min residence time, whereas the bulk scale reaction gave 2-cyclohexenol as the major reaction produd. 

Build a model of 2-cyclohexenol with the stereochemistry shown in Figure 7.3 and confirm that it has the R configuration. 

Designating CONFIGURATIONS. Q (S)-2-ChlorobutaNE AND O (R)-2-CYCLOHEXENOL. Test yourself on the concepts in this figure at OrganicChemistryNow. 

---