Regioselectivity of oxiranes

Studies of the regioselectivity of oxirane/carbon dioxide copolymerisation showed the polar effect exerted by the ring substituent, but not the bulkiness, to be the factor determining the direction of ring opening [231,232]. In the case of propylene oxide/carbon dioxide copolymerisation, C g—O bond cleavage... 

As shown in Scheme 63, the regioselectivity of oxirane ring opening of 2,3-epoxy alcohols is increased in TMSNa-Lewis acid systems compared to conventional systems using sodium azide. It has been reported, however, that sodium azide supported on zeolite CaY facilitates the C-3 attack. 

However, when the reaction is performed [150] in the presence (a) of [BMIMjOTf (as ionic liquid medium and catalyst) or (b) of an acidic organocatalyst, exclusive C-3-aLkylation of indole and reversed regioselectivity of oxirane opening occurs giving rise to products 29. 

Regioselective anbd Chemoselective Synthesis of Halohydrins by Cleavage of Oxiranes with Metal Halides," Bonini. C. Righi, G. Synthesis, 1994, 225... 

In the case of Pt and Pd, the surface oxidation can also take place, but the resulting metal ions can be easily reduced in the presence of hydrogen. According to this fact, the observed difference in the regioselectivity of the rupture of the oxirane ring on Ni and Cu or Pt and Pd can be explained on the basis of the different stability of the metal ions in a hydrogen atmosphere.289... 

In this particular case, remote control by the heteroatom of the substituent is invoked to explain the regioselectivity of the elimination. Complexation of lithium with both oxygens fixes the basic carbon atom close to the -proton . A similar model is proposed to rationalize the reversal to regiospecific a-deprotonation proximate to the hydroxyl group for oxiranes of type 23, the basic site now being close to the a-proton (Scheme 9) ° . 

In conclusion, the regioselectivity of the deprotonation a/p competition) is related not only to the structure of the oxirane but also to the experimental conditions (nature of the base, solvent, temperature etc.). Thus, prediction must take into account all of these parameters and evaluate their relative importance. 

The effect of the BF3 activation on the regioselectivity of the ring opening of vinylic and acetylenic oxiranes is dramatic, as in these conditions the reaction occurs exclusively on the allylic " or the propargylic " position, and still with inversion (Scheme 40). This excellent regioselectivity allows the direct stereospecific preparation of homoallylic and homopropargylic alcohols in excellent yields. In the case of acetylenic oxiranes, a remarkable difference in the reactivity of cis and trans oxiranes has been evidenced, the former being more reactive. 

NATURE OF THE ACTIVE SITES IN THE REGIOSELECTIVE HYDROGENATION OF OXIRANES... 

Cleavage of oxetanes.1 Oxetanes are cleaved by this salt to the corresponding -y-phenylselenenylpropanol at room temperature, whereas tetrahydrofurans (oxo-lanes) are cleaved only at 60-100°. In each case the less-substituted carbon is attacked by the reagent. Cleavage of oxiranes with this reagent is not regioselective. 

Furthermore, in respect to the regioselectivity of the ring opening reaction of oxiranes, electronic as well as steric factors can play a role. These general considerations stimulate the use of zeolites and non zeolitic molecular sieves as heterogeneous catalysts for such rearrangement reactions in the liquid or in the gas phase in a slurry reactor and in a continuous fixed bed reactor, respectively. 

Li(t-BuO)3AlH-Et3B is a suitable reagent for the instantaneous and quantitative opening of oxiranes (Eq. 162), ° though it is less regioselective than Li(Et)3BH, which can be used well for the reduction of hindered and rearrangement-susceptible oxiranes (Eqs. 163 and 164). 

---