Cyclohexanols 2,3-disubstituted

Another situation is observed when salts or transition metal complexes are added to an alcohol (primary or secondary) or alkylamine subjected to oxidation in this case, a prolonged retardation of the initiated oxidation occurs, owing to repeated chain termination. This was discovered for the first time in the study of cyclohexanol oxidation in the presence of copper salt [49]. Copper and manganese ions also exert an inhibiting effect on the initiated oxidation of 1,2-cyclohexadiene [12], aliphatic amines [19], and 1,2-disubstituted ethenes [13]. This is accounted for, first, by the dual redox nature of the peroxyl radicals H02, >C(0H)02 and >C(NHR)02 , and, second, for the ability of ions and complexes of transition metals to accept and release an electron when they are in an higher- and lower-valence state. 

Depending on the orientation of substituents, the disubstituted cyclohexanes can be either equatorial-axial or diequatorial. This is illustrated with 2-methyl cyclohexanol. It exists in two stereoisomeric forms and each of which has two conformations which are readily interconvertible. 

Steric effects also play a significant role, as is apparent in the monosubstituted cyclohexanols series (entries 7-9 in Table 13.3) and from comparison between linear and branched substrates (entry 1 vs. entry 3). This effect is so strong that in disubstituted cyclohexanols the reaction takes place at reasonable rates only when the —OH group is in axial conformation, as shown by the comparison of (-)-menthol (entry 16) and neomenthol (entry 17). This particular behavior could once more be exploited for synthetic purposes, in order to set up a kinetic resolution process. This has indeed been achieved in the one-pot hydrogenation of pennyroyal oil into menthol [67]. 

By this means cfj-l,2-disubstituted cyclohexanes can be prepared from trans-2-substituted cyclohexanols ... 

At high temperatures and In the presence of catalysts, PH3 reacts with butanol (200 C, 5 A molecular sieves) or cyclohexanol (300"C, hydrogen faujasites) to give the mono- and disubstituted phosphanes [101]. 

The 2 rule applies equally weU to molecules with three or more stereocenters. Here is a disubstituted cyclohexanol with three stereocenters, each marked with an asterisk ... 

Use of the appropriate alkylaluminum halide allows these ene reactions to be extended to aldehydes with less nucleophilic 1,2-disubstituted double bonds. Treatment of the 2-isomer 53 with 1 equivalent of Me2Aia for 2 h at 0 °C gives exclusively the cw-substituted ene adduct 55 in 75% yield while the E-isomer 54 gives mainly the rra w-substituted isomers 56 and 57 (see Figure 13). This type of ene reaction offers a promising route to 2-(l-alkenyl)cyclohexanols with... 

The domino Henry-Michael reaction of CH3NO2 with 7-oxo-hept-5-en-l-als RC0CH=CH(CH2)3CH=0, catalysed by the quinine-derived thiourea (288b) (<96% ee), followed by the tetramethyl guanidine (TMG)-catalysed retro-Henry-Henry reaction of the initially formed disubstituted cyclohexanols, led to a refinement of diastereoselectivity (to >99 1 dr) ... 

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