Influence des substituants axiaux en position 4 sur la stereochimie de la reduction, par les hydrures complexes, de cyclohexanones substituees

Inductive and Resonance Effects of Substituents on π-Face Selection

“…Small increases were found in NaBH 4 and LiAlH 4 reductions of 4-X-3-tertbutyl-and 4-X-4-tert-butylcyclohexanones, but in Grignard alkylation, the preference for equatorial addition is reversed by the C(4)-OCH 3 group in 3-tert-butylcyclohexanone. 155,156 In the series of 2-tertbutyl-4-X-cyclohexanones, 154 the increases of the axial preference induced in NaBH 4 reductions by the axial C(4) substituents are comparable to those induced by the equatorial substituents, see Figure 16A, with one exception of the 4-ax-Br group. The latter effect is illustrated in Scheme 47.…”

“…The effect of γ-substitution on the stereochemistry of nucleophilic additions to cyclohexanones was investigated in metal hydride reductions, organometallic alkylations, and sulfur ylide alkylations of 4-X-cyclohexanones, ,, 2-, 3-, and 4- tert -butyl-4-X-cyclohexanones, − 10-X- trans -decal-2-ones, ,,,, and 4-X- trans -decal-1-ones. , It is useful in this discussion to consider separately the effect of the equatorial and axial γ-substitution.…”

“…[150][151][152] This effect was examined in a more systematic way using a series of cyclohexanones and methylenecyclohexanes with polar substituents at C(3) that were sufficiently large to lock the conformation of the ring. 12 As with the polycyclic probes discussed earlier, electronegative β-substitution was found to increase the syn preference (the relative yield of axial approach) in several reactions, widely differing from the point of view of the transition-state polarization, geometry, and electron deficiency, The effect of γ-substitution on the stereochemistry of nucleophilic additions to cyclohexanones was investigated in metal hydride reductions, organometallic alkylations, and sulfur ylide alkylations of 4-X-cyclohexanones, 35,145,153 2-, 3-, and 4-tertbutyl-4-X-cyclohexanones, [154][155][156] 10-X-trans-decal-2ones, 138,139,141,142,157 and 4-X-trans-decal-1-ones. 112,157 It is useful in this discussion to consider separately the effect of the equatorial and axial γ-substitution.…”