Allylic alcohols oxidation, mechanistic study

Rhodium catalysts have also been used with increasing frequency for the allylic etherification of aliphatic alcohols. The chiral 7r-allylrhodium complexes generated from asymmetric ring-opening (ARO) reactions have been shown to react with both aromatic and aliphatic alcohols (Equation (46)).185-188 Mechanistic studies have shown that the reaction proceeds by an oxidative addition of Rh(i) into the oxabicyclic alkene system with retention of configuration, as directed by coordination of the oxygen atom, and subsequent SN2 addition of the oxygen nucleophile. 

Selenium-mediated allylic oxidations producing allylic alcohols have been discussed above however, in some cases oxidation proceeds further to give the a, -unsaturated carbonyl compounds directly, or mixtures of alcoholic and ketonic products. That the regioselectivity observed in these allylic oxidation reactions closely resembles that found in classical selenium dioxide oxidations is in accord with initial formation of the intermediate allylic alcohol before in situ oxidation to the carbonyl compound. This process was studied by Rapoport and was explained mechanistically as an elimination of the intermediate allylic selenite ester via a cyclic transition state, analogous to Ssi (rather than 5n20 solvolysis (Scheme 21). Of the two possible transition states (78) and (79), the cyclic alternative (78) was preferred tecause oxidation exclusively yields trans aldehydes. 

A ternary soft Lewis-acid/hard Bronsted-base/hard Lewis-base catalytic system for the direct catal5rtic enantioselective addition of allyl cyanide 37 to ketones to give tertiary alcohols with a Z-olefin (38) has been developed by Shibasaki (Scheme 2.24). Mechanistic studies revealed that Cu(i)/chiral phosphine ligand 35 and Li(OC6H4-p-OMe) serve as a soft Lewis acid and a hard Bronsted base, respectively, allowing for deprotonative activation of 37 as the rate-determining step. The hard Lewis base, bis(phosphine oxide) 36,... 

The oxidation of benzylic alcohols was quantitative within hours and selective to the corresponding benzaldehydes, but the oxidation of allylic alcohols was less selective. The oxidation of aliphatic alcohols was slower but selective. In mechanistic studies considering oxidation of benzylic alcohols, similar to the oxidation of alkylarenes, a polyoxometalate-sulfoxide complex appears to be the active oxidant. Further isotope-labeling experiments, kinetic isotope effects, and especially Hammett plots showed that oxidation occurs by oxygen transfer from the activated sulfoxide and elimination of water from the alcohol. However, the exact nature of the reaction pathway is dependent on the identity of substituents on the phenyl ring. 

A systematic mechanistic study of the K2[0s02(0H)4]-catalysed and highly chemos-elective oxidation of allylic, electron rich/deficient benzylic, and heterocyclic alcohols with CAT using the LC-ESI-MS/MS method reveals the presence of imidotriooxoos-mium species which react further with alcohol to give the respective ketones. ... 

Extensive mechanistic studies have identified a bis-tita-nium complex [Ti(tartrate)(OR)2]2 as the active catalyst for asymmetric epoxidation of allylic alcohols by tertiary alkyl hydroperoxides, and they have suggested complex 3 as the transition state conformation (Figure 35.1). The proposed mechanism and transition state model allow for the prediction of stereochemical outcome. As depicted in Figure 35.2, the approach of the oxidant to the double bond occurs from the top face when d-(—)-DET or d-(-)-DIPT is used. When l-(+)-DET or l-(+)-DIPT is employed, the epoxide oxygen is added from the bottom face. ... 

SemmeUiack et al. [104] reported that the combination of CuCl and 4-hydroxy TEMPO catalyzes the aerobic oxidation of alcohols. However, the scope was limited to active benzyhc and allylic alcohols and activities were low (10 mol% of catalyst was needed for smooth reaction). They proposed that the copper catalyzes the reoxidation of TEMPO to the oxoammonium cation. Based on our results with the Ru/TEMPO system we doubted the validity of this mechanism. Hence, we subjected the Cu/ TEMPO to the same mechanistic studies described above for the Ru/TEMPO system [105]. The results of stoichiometric experiments under anaerobic conditions, Hammett correlations and kinetic isotope effect studies showed a similar pattern to those with the Ru/TEMPO system, i.e., they are inconsistent with a mechanism involving an oxoammonium species as the active oxidant. Hence, we propose the mechanism shown in Scheme 4.18 for Cu /TEM PO-catalyzed aerobic oxidation of alcohols. 

Asymmetric allylations of ArCHO with allyltrichlorosilane in CH2CI2 to form homoallylic alcohols has been Lewis base catalysed by chiral bisformamide-type catalysts (with ee < 83%) and by (f )-methyl p-tolyl sulfoxide. Mechanistic study of the latter reaction supports a dissociative pathway via an octahedral cationic complex with two sulfoxides. The greater stereoselectivity of A-oxide-catalysed allylations, compared to propargylations, has been explained by a simple electrostatic model that should enable design of suitable catalysts for both reactions. ... 

Mechanistic studies on the Sharpless asymmetric epoxidation, Eq. (8), where DIPT is diisopropyl tartrate, have been published.The rate law in CH2CI2 is first-order in substrate, catalyst, and oxidant, and shows an inverse second-order dependence on the inhibiting alcohol, in this case Pr OH. This is consistent with a mechanism in which both substrate and the peroxide displace Pr O to form a key intermediate in the reaction. [Differences in the selectivities of allylic and homoallylic alcohols in this reaction have been exploited to invert the expected enantioselectivity. ... 

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