Ylides covalent activations

Whereas in Wmethyl-4,6-dimethylpyrimidinium ion the covalent addition takes place at C-6, it is assumed that 30 also undergoes covalent hydrazina-tion at C-6. However, the formation of dimer 32 shows the high sensitivity of C-2 in 30 for addition of nucleophiles, and it leads to the daring suggestion that it is the resonance-stabilized ylide 31 that probably is the active species undergoing addition at C-2 (Scheme III. 19). It was calculated (80UP1) that the reactivity at C-2 in the N-ylide 31 is greater than that at C-2 in the Waminopyrimidinium salt 30. 

The vast majority of organocatalytic reactions proceeds via covalent formation of the catalyst-substrate adduct to form an activated complex. Amine-based reactions are typical examples, in which amino acids, peptides, alkaloids and synthetic nitrogen-containing molecules are used as chiral catalysts. The main body of reactions includes reactions of the so-called generalized enamine cycle and charge accelerated reactions via the formation of iminium intermediates (see Chapters 2 and 3). Also, Morita-Baylis-Hillman reactions (see Chapter 5), carbene-mediated reactions (see Chapter 9), as well as asymmetric ylide reactions including epoxidation, cyclopropanation, and aziridination (see Chapter 10), and oxidation with the in situ generation of chiral dioxirane or oxaziridine catalysts (see Chapter 12), are typical examples. 

The covalent sidewall functionalization of SWCNTs has been conducted by a number of researchers. Highly reactive addends are needed for the sidewall functionalization reactions. The outer surface of SWCNTs is more active than the inner surface. There are several methods for covalent side-wall functionalization of SWCNTs, including fluorination, diazonium, radical chemistry, nitrene, azomethine ylides and dichlorocarbene, as shown in Table 4.2. The added functional groups include aryl, fluorine, pyrrolidine, aziridine and cyclopropane. Multiple analytical techniques can be used to... 

Recently, asymmetric induction mediated by external chiral ligands that are not covalently bonded to the reagent has attracted much attention, and it is believed that the information obtained from these studies will prove useful in developing a novel system for efficient catalytic asymmetric transformation. In order to explore the possibilities, a variety of reaction systems capable of effective asymmetric induction at a specific site in the course of a reaction have been devised, and several investigations have also been directed towards Wittig-type olefination. An early study using an optically active organic acid with stabilized ylides [22] was unfruitful, as discussed in the introductory section. 

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