Alcohols alcohol-catalyzed mechanism

Figure 4.5 Simplified mechanism of the racemization of sec-alcohols catalyzed by transition metal complexes.

When proton acids catalyze alcohol dehydration, the mechanism is El. ° The principal process involves conversion of ROH to ROHj and cleavage of the latter to R and H2O, though with some acids a secondary process probably involves conversion of the alcohol to an inorganic ester and ionization of this (illustrated for H2SO4) ... 

Catalytic amounts of chiral amino alcohols both catalyze the reactions of alkylzinc reagents with aldehydes and induce a high degree of enantioselec-tivity. Two examples are given below. Formulate a mechanism for this catalysis. Suggest transition structures consistent with the observed enantioselectivity. 

Recently, Oshima et al. developed the conversion of acid chlorides into the corresponding homoallylic alcohols catalyzed by in r(/ -prepared hydridozirconium allyl reagents (Scheme 41),147 147a The proposed mechanism suggests an initial hydride transfer from the zirconocene crotyl hydride species, in equlibrium with its Cp2Zr(l-alkene),147a to the acid chloride with subsequent allylation to afford the corresponding homoallylic alcohols. 

Scheme 3.11 Partial mechanistic scheme for the hydrogenation of aryl ketones to give the (S)-alcohol catalyzed by RuCI2((R)-binap)((R,R)-dpen)/KO Bu/H2 as based on the observed mechanism for RuH2((R)-binap)(NH2CMe2CMe2NH2).

Gabrielsson et al. reported the aerobic oxidation of alcohols catalyzed by a cationic Cp Ir complexes bearing diamine ligands such as bipyrimidine 10 (Scheme 5.8) [35], the mechanism of which is closely related to the Oppenauer-type oxidation mentioned above. In this reaction, the deprotonation of Ir hydrido species to afford Ir species, and the reoxidation of Ir to Ir by O2, are crucial. 

A possible mechanism for the P-alkylation of secondary alcohols with primary alcohols catalyzed by a 1/base system is illustrated in Scheme 5.28. The first step of the reaction involves oxidation of the primary and secondary alcohols to aldehydes and ketones, accompanied by the transitory generation of a hydrido iridium species. A base-mediated cross-aldol condensation then occurs to give an a,P-unsaturated ketone. Finally, successive transfer hydrogenation of the C=C and C=0 double bonds of the a,P-unsaturated ketone by the hydrido iridium species occurs to give the product. 

A possible mechanism for the oxidahon of secondary alcohols catalyzed by 38 is shown in Scheme 5.36. The first step of the reachon involves formahon of the... 

Finally, /i-hydrogen transfer is the key step in the Meerwein-Pondorf-Verley (MPV) reduction of ketones by alcohols, catalyzed by aluminium alkoxides and many other catalysts. In that case, competition is not an issue, since polymerization is usually not thermodynamically favourable. The accepted mechanism for this reaction is direct transfer of the hydride from alkoxide to ketone. 

Cyclopropylamines and cyclopropanols can be prepared from alkyknagnesium ha-lides. The reaction is catalyzed by titanium alcoholates and its mechanism includes the formation of a dialkoxytitanacyclopropane 270, which reacts with a carbonyl compound or nitrile (Scheme 22). The use of chiral titanium alcoholates allows the reaction to be performed with up to 78% ee (equation 171) . 

Reaction Mechanism. The amino alcohol-catalyzed reaction proceeds by the mechanism illustrated in Scheme 19. First, the reaction of equimolar amounts of ( —)-DAIB and dialkylzinc eliminates an alkane to... 

The amino alcohol-catalyzed enantioselective addition of dialkylzincs to aldehydes, detailed in Chapter 5 (27), is accomplished with polymer catalysts containing DAIB, a camphor-derived auxiliary, and other chiral amino alcohols (28). Reactions that involve matrix isolation of the catalyst not only result in operational simplicity but also greatly facilitate understanding of the reaction mechanism. In solution, the catalytic chiral alkylzinc alkoxide derived from a dialkylzinc and DAIB exists primarily as dimer (27) however, when immobilized, its monomeric structure can be maintained. 

The discussion to this point has focused entirely on the epoxidation of allylic (and homoallylic) alcohols catalyzed by the Ti(OR)2(tartrate) complex. The role of the olefin as a nucleophile toward the activated peroxide oxygen in this reaction has been established (see discussion of mechanism). If the olefin of the allylic alcohol is replaced by another nucleophilic group then, in principle, oxidation of that group may occur (Eq. 6A.6) [141 ]. 

Scheme 3.3 The possible oxidation mechanism of secondary alcohols catalyzed by Co(II) in PEG (Reprinted from Ref. [24], with permission from Elsevier)...

Scheme 1. Mechanism of alcohol oxidation catalyzed by TEMPO.

The mechanism for the Fischer esterification is shown in Figure 19.3. Sulfuric acid, hydrochloric acid, orp-toluenesulfonic acid is most often used as a catalyst. The mechanism will be easier to remember if you note the similarities to other acid-catalyzed mechanisms, such as the one for the formation of acetals in Figure 18.5. Also note that the steps leading from the tetrahedral intermediate to the carboxylic acid and alcohol starting materials and to the ester and water products are very similar. 

By analogy, it seemed plausible that alcohol racemizations catalyzed by the same type of ruthenium complexes involve essentially the same mechanism (Fig. 9.11), in which the active catalyst is first generated by elimination of HC1 by the added base. This 16e complex subsequently abstracts two hydrogens from the alcohol substrate to afford an 18e complex and a molecule of ketone. Reversal of these steps leads to the formation of racemic substrate. 

Although thus far a number of reports have been made on mthenium complex-catalyzed isomerization of alkenyl alcohols to saturated aldehydes or ketones, the mechanistic details of these reactions have not yet been fully elucidated. It has been generally accepted that isomerization of the alkenyl alcohol forming a carbonyl compound proceeds via an intermediate enol. However, developments in mechanistic studies have advanced recently, and a new mechanism which involves an intermediary mthenium alkenylalkoxide is proposed by Trost et al. for isomerization of allylic alcohols catalyzed by CpRuCl(PPh3)2 (Cp = [1]. Gmbbs et al. also proposed... 

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