Nitrogen ylides reactions

Cycloaddition of 2-nitrosopyridine 48 with nitrile oxides can give either di-A -oxides such as 49 or 3-mono-A -oxides such as 50 (93JHC287). In general, greater electron withdrawing character in the aromatic substituent appears to favor formation of the di-A -oxides. Sulfur ylides such as compound 51 are obtained from aryl isothiocyanates and l-amino-2-methylthiopyridinium iodides (84JCS(P1)1891) nitrogen ylides can be obtained from a similar reaction (86H(24)3363). 

A class of compounds in which a positively charged atom from group V or VI of the periodic table (c.g., N, O, S, P, As, Se) is bonded to a carbon atom having an unshared pair of electrons. Whereas there is only one canonical form for nitrogen ylides (R3N —CR2 ), because of pTT-dTT bonding, two canonical forms can be written for phosphorus Le., R3P=CR2 R3P —CR2 ) and sulfur ylides (R2S=CR2 R3S —CR2 ). A number of enzyme-catalyzed reactions have been reported to utilize ylide-based chemistry. For example, the ylide form of the... 

Formation of Nitrogen Ylide from Metal Carbene Complex and Subsequent Reactions 168... 

A closely related reaction is formation of nitrogen ylides from quaternary ammonium salts (see 7-7) ... 

Zwitterionic four-membered PN2Nb heterocycles were obtained in the reaction of ij5-phosphorus-nitrogen ylides with NbCl5 according to equation (40). 496... 

Carbenoid generation of nitrogen ylides represents a useful alternative to the widely employed base-promoted methodology.49 The reaction of aliphatic diazo compounds with tertiary amines was first investigated by Bamford and Stevens in 1952.50 The formation of a-benzyl-a-dimethyl-aminofluorene (99) from the reaction of diazofluorene (97) with ben-zyldimethylamine is consistent with a mechanism involving the generation of ammonium ylide 98 which then undergoes a [l,2]-benzyl shift. 

The reaction of triethylamine with dichlorocarbene produced diethyl-formamide (124) as the major product.59 This result is also consistent with -elimination from an initially formed nitrogen ylide 122 which... 

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. 

Reactions which formally involve the oxidation of azides have been reviewed by Boyer. Other oxidations with useful synthetic applications include two which start from nitrogen ylides. Sulfimides (50) derived from electron-deficient aromatic and heterocyclic amines are oxidized to the corresponding nitroso compounds by MCPBA. - This is a very useful method of preparation of some otherwise inaccessible nitroso compounds such as 2-nitrosopyridine and 1-nitrosoisoquinoline. They can be further oxidized, for example by ozone, to the nitro compounds. Phosphimides (51) are oxidized directly by ozone to the nitro compounds, although the nitroso compounds are intermediates. Isocyanates can also be oxidized to the corresponding nitro compounds, by dimediyldioxiraiK (1). ... 

Thiazoles can be quaternized at nitrogen by reaction with a range of alkylating agents. These salts can form an ylide by deprotonation at C-2. This thiazolium 2-ylide is markedly stable because of the ability of sulfur to stabilize an adjacent carbanion. The reaction is of considerable importance due to the occurrence of thiazolium-2-ylides as intermediates in classical biochemical (thiamine action) and chemical (Stetter reaction) processes (see Section 3.06.12). Desilylation at C-2 can lead to a thiazolium 2-ylide as well. Thus, the formation of this type of intermediate has been formulated as a key step along the reaction pathway involving a 2-trialkylsilylthiazole and C-electrophiles (Dondoni reaction, see Section 3.06.12.12). Thiazolium salts are also susceptible to be oxidized by a variety of oxidants (see Section 3.06.5.4.8). 

Phosphorus-based olefinations continue to be the most widely used methods for the synthesis of alkenes. An understanding of most facets of the mechanism of the Wittig reaction seems to have been achieved and this has been summarised in a substantive review. Some of the principles established in these mechanistic studies can be applied to phosphorus-based olefinations other than the Wittig reaction. However, substantive mechanistic studies of phosphine oxide-based and phosphonate-based oiefinations are urgently required. A combination of the variety of phosphorus-based methods and the improved understanding of their mechanisms now aliows a substantiai degree of controi of both reactivity and stereochemistry in olefin synthesis. However, studies are required of the applications of established structure-reactivity relationships in ylides and of the various carbon and nitrogen ylide-anions recently reported. 

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