Proelectrophiles reactions

With 1-alkoxyallenes as proelectrophiles, the palladium-catalysed asymmetric allylic alkylation proceeds with 1,3-dicarbonyl compounds as pronucleophiles with excellent regioselectivity good enantioselectivity (82-99% ee) was obtained with the Trost lig- and. The pH of the medium proved crucial for the reactivity and selectivity. By using the more acidic Meldrum s acids, the reactions required a co-catalytic amount of a Brpnsted acid, such as CF3CO2H. On the other hand, the less acidic 1,3-diketones failed to react under these conditions but the reaction proved to occur in the presence of the weaker benzoic acid, suggesting the need for general base catalysis. Indeed, a mixture of Et3N and PhCOiH proved to be optimal (93-99% ee). A mechanistic model to rationalize these results has been developed.88... 

There are, however, examples indicating that in ion molecule reactions between a protonated species (AH+) and benzene (B), two isomeric forms of the intermediate complex may exist (AH+)(B) and (A)(BH+) [74,286]. In the cases of water [287] and propene [74], quantum chemical calculations clearly indicate that the former corresponds to a n complex where A-H acts as a hydrogen bond donor towards the centre of the benzene ring, while the latter is a hydrogen bonded complex between the benzenium ion and A. In neither case has a barrier been located, but is probably rather low in both cases. The role of the n complex has still not been clarified, since direct downhill routes from the reactants to the a complex exist. It has been pointed out that n complex formation between a pro electrophile and the substrate may be important in solution and in biological systems for molecular recognition purposes. In such cases the proelectrophile is activated to form the actual electrophile subsequent to n complexation, thereupon giving rise to the a complex. This has been shown by quantum chemistry to provide a reasonable scenario for the reaction between HF and benzene, in which BF3 is ultimately required to promote ion formation of the HF/benzene tt complex [288]. 

For the purposes of this chapter, a cross-dehydrogenative-eoupling (CDC) reaction will be defined as the oxidative coupling of species that are nominally related to electrophiles proelectrophiles) with those nominally associated with being nucleophiles pronucleophiles), see Scheme 11.1. 

We must also deprioritize simple oxidation reactions where there is no added pronucleophile, such as oxidations at mildly activated positions without attendant coupling processes, since a major advantage of CDC reactions is that we can combine pronucleophiles and proelectrophiles clearly some of these studies provide mechanistic information as to the composite steps. While much of the related chemistiy is relevant to Af-demethylation reactions, " oxidative dehalogenations or other processes we do not cover those studies here since they are not aimed at (constructive) synthesis, which is the primary focus of CDC reactions. 

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