Base-promoted pathway

The empirical observation that (—)-sparteine 55 is necessary for catalysis implicates a base-promoted pathway in the mechanism. In the first step, a palladium alk-oxide is formed after alcohol binding, followed by p-hydride elimination of the alkoxide to yield a ketone product. On the basis of a kinetic study of the enantio-selective oxidation of 1-phenylethanol, it was revealed that (—)-sparteine plays a dual role in the oxidative kinetic resolution of alcohols, as a ligand on palladium and an exogeneous base " ... 

Apart from the azirine pathway, a vinylnitrene 526 was postulated as a possible intermediate in the reaction (Scheme 14). The nitrene may be formed by a base-promoted elimination of the leaving group on the nitrogen and gives rise to the 277-azirine by electrocyclization (nitrene insertion) and, in view of current data, this pathway cannot be excluded. 

Experimental results of an unprecedented haloform-type reaction in which 4-alkyl-4-hydroxy-3,3-difluoromethyl trifluoromethyl ketones undergo base-promoted selective cleavage of the CO-CF3 bond, yielding 3-hydroxy-2,2-difluoro acids and fluoro-form, have been rationalized using DFT (B3LYP) calculations.159 The solvent-induced effects on the two pathways, introduced within the SCRF formalism through PCM calculations, do not reverse the predicted preference of the CO-CF3 over the CO-CF2 bond cleavage in the gas phase. 

General base promotion, though the likely pathway for the reactions in organic media discussed above, is not the only conceivable mechanism for addition of HZ. Specific base... 

In Fig. 7, the do values obtained for MgO, AI2O3 and MgyAlOx oxides were plotted as a function of the density of basic sites, nb (Table 1). It is observed that the initial deactivation correlates linearly with the density of basic sites. No correlation was found between the NH3 site density (n,. Table 1) and do. These results show that although Mg AlOx oxides promote the selfcondensation of acetone by both acid- and base-catalyzed pathways (Fig. 2), the initial deactivation rate is essentially related to the surface basic properties. This dependence of do with nb for MgyAlOx oxides probably reflects the fact that the acetone oligomerization rate decline is significantly higher when coke poisons very active basic Mg-0 pairs than when eliminates moderately active acidic Al-0 pairs. 

Base-promoted 1,4-dehydrochlorination of 3-(l-chloroethyl)indene leads to a pair of diastereomeric dienes (equation 10.82). How could compounds 56 and 57 be used to determine whether the reaction occurs by a syn or an anti pathway ... 

Stevens and colleagues extended the base-promoted reverse-Madelung reaction to a vinylogous version (Scheme 3, equation 1) [22]. Among the many 2-acyl-3-indole esters (and carboxylic acids) that were prepared are 5-7. This clever tactic has been used by Opatz [23, 24] and Webb [25] as summarized in Table 2. Scheme 4 depicts the reaction pathway and includes some of Opatz s examples (8-10). 

The high-temperature (185—250 °C) base-promoted (Bu OK-Bu OH) P-epimeriza-tion and y-hydrogen abstraction via homoenolate anions) pathway for the inter-conversion of camphor, exo-isocamphanone, and endo-isocamphanone has been traced with the aid of deuterium labelling studies and by the use of deuteriated solvents. ... 

Because all nucleophiles are also bases, nucleophilic substitution and base-promoted /3-elimmation are competing reactions. The ethoxide ion, for example, is both a nucleophile and a base. With bromocyclohexane, it reacts as a nucleophile (pathway shown in red) to give ethoxycyclohexane (cyclohexyl ethyl ether) and as a base (pathway shown in blue) to give cyclohexene and ethanol ... 

Base-promoted oxidation, which leads to oxidative dimerisation products, is another viable pathway for complexes based on pentan-2,4-dione bis-(S -alkylisothio-semicarbazones). It has been discovered [97, 98] that the mononuclear nickel(II) complex methylated at sulphur [Ni°(H2L63)]I l/2CH30H reacts in an ethanol/ ammonia mixture with dioxygen with formation of a dinuclear species formulated as [Ni2(L66)] (Eq. 2.39). 

---