Nitrenes ylides

On the other hand, poly(ethoxycarbonylimino-4-vi-nylpyridinium ylide) (Scheme 13) was prepared essentially by the same method from 1-ethoxycarbonylimino-pyridinium ylide, as described by Hafner [15] from the reaction of poly (4-vinylpyridine) with nitrene, generated from the pyrolysis of ethyl azidoformate. 

The synthesis of aziridines through reactions between nitrenes or nitrenoids and alkenes involves the simultaneous (though often asynchronous vide supra) formation of two new C-N bonds. The most obvious other alternative synthetic analysis would be simultaneous formation of one C-N bond and one C-C bond (Scheme 4.26). Thus, reactions between carbenes or carbene equivalents and imines comprise an increasingly useful method for aziridination. In addition to carbenes and carbenoids, ylides have also been used to effect aziridinations of imines in all classes of this reaction type the mechanism frequently involves a stepwise, addition-elimination process, rather than a synchronous bond-forming event. 

Several reviews on the synthesis of aziridines have been published in the previous year. These publications include a review on the silver catalyzed addition of nitrenes (among other intermediates such as carbene) across a double bond <06EJOC4313> a review on sulfur ylide addition to imines to form aziridines <06SL181> a review on nitrogen addition across double bonds <06ACR194> a general review on functionalization of a,p-unsaturated esters with some discussion of aziridination <06TA1465>... 

A novel synthesis of isothiazolidines involves sulfonium ylides, formed by the reaction of thietanes and nitrenes <06TL1109>. Exposure of A-(/ -tolylsulfonyl)imino)phenyliodinane 232 with excess of thietanes 231 (5 equiv) in the presence of a catalytic amount of Cu(II)... 

Nitrenes can be generated from many precursors such as azides, isocyanates, ylides, heterocycles, and nitro compounds.236,237 Amongst these, azides are the most convenient precursors since they are easily prepared and can be decomposed by heat, light or a suitable catalyst. Despite considerable endeavors, no one has yet provided a synthetically viable method to use azides as sources of nitrenes.237 The breakthrough of nitrene chemistry was the recognization of the value of A-arenesulfonyl iminoiodinanes (ArS02N=IPh) as nitrene precursors by Breslow and Mansuy. - They reported inter- and intramolecular C-H insertions by tosylimino phenyl-iodinane (TsN=IPh) in the presence of Mn(m) or Fe(m) porphyrins or [Rh2(OAc)4]. Subsequently, Muller... 

Nitrene intermediates are implicated in the formation of the pyrrole (64) from an azirinyl aldehyde and (65).67 Treatment of the nitrosouracil (66) with benzyl idene-ylides affords the corresponding theophylline derivatives.68 Hydrazonic halides and keto-ylides initially give betaines (67), which ring-close to (68) on heating.69... 

The presence of oxygen had no effect, outside of experimental error, on the transient hfetime in ethanol. On the basis of the solution and glassy matrix work the transient absorbing at 345 nm was assigned to the triplet nitrene. LFP of 25 in acetonitrile produced a longer hved transient (r > 150 ps) also absorbing near 345 nm, which may be the result of formulation of a nitrene-solvent ylide. 

In contrast to considerations of 50 years ago, today carbene and nitrene chemistries are integral to synthetic design and applications. Always a unique methodology for the synthesis of cyclopropane and cyclopropene compounds, applications of carbene chemistry have been extended with notable success to insertion reactions, aromatic cycloaddition and substitution, and ylide generation and reactions. And metathesis is in the lexicon of everyone planning the synthesis of an organic compound. Intramolecular reactions now extend to ring sizes well beyond 20, and insertion reactions can be effectively and selectively implemented even for intermolecular processes. 

So what is left to be accomplished During the current decade one can expect further asymmetric applications and catalyst designs for metathesis reactions, a maturing of chiral catalyst development for cyclopropanation and insertion with increasing synthetic applications, and decreased reliance on traditional Fischer carbenes in synthesis. Major changes remain for ylide applications, especially those that can be enantioselective, in catalytic carbene chemistry, and advances in nitrene chemistry that are comparable to those achieved over the years in carbene chemistry are in their infancy. 

Although not as common as the ylide derived from metal carbenes, the ylide-like species generated from metal nitrene or free nitrene has been attracting increasing attention in recent years. The overall transformation is parallel to that of metal carbene reactions. Progress in this direction is also covered in this chapter. 

In view of the chemistry of a metal carbene complex, it is reasonable to expect that nitrene or metal-complexed nitrene will similarly interact with Lewis base to generate ylide-like species, which then proceeds to undergo further transformations (Figure 7). 

One of the most interesting products obtained from the capture of singlet nitrene 16e is ylide 22e produced by photolysis of 15e in pyridine (Scheme 4). Ylide 22e has a very intense absorption band with maximum absorption at 390 nm. The pyridine-ylide method was successfully used by our group to probe the dynamics of the fluoro-substituted singlet arylnitrenes - " -" ... 

Other synthetic routes to benzazepines involving ring expansion of six-membered heterocycles include the action of diazomethane (77CPB321), sulfonium ylides (77H(7)37> or acyl halides (75T1991) on quaternary 3,4-dihydroisoquinolines that of sulfoxonium ylides on quaternary quinolines (74IJC(B)1238) and the zinc-acetic acid reduction of quaternary 1-acyltetrahydroisoquinolines (77BSF893). Photoaddition of acyl- or aryl-nitrenes to the exocyclic alkene bond of 2-methylene-1,2-dihydroquinolines results in ring expansion to... 

It was formerly considered that nitrenes attack only the carbons atom of thiophene (84CHEC-(4)74i). Since 1984, several S,N-ylides formed by the attack of a nitrene on the ring sulfur atom of thiophene have been prepared (89AHC(45)l5l). Thus, ethoxycarbonyl nitrene with polyhalogenothiophenes forms the S,N-ylide (46) in 44% yield (84CC190). The X-ray crystal structure reveals that the sulfur in such ylides is pyramidal. 

Electrophilic nitrogen compounds, such as arenesulfonyloxyamines, can convert alkenes to aziridines without the intervention of free nitrenes (80CC560). The ylide Ph2S+-NH adds stereospecifically to E and Z conjugated alkenes, and chiral sulfimides can transfer chirality to the aziridines formed (80T73). These reactions are often named aziridinations . 

The comparison of thiophene with thioethers on the one hand and with enol thioethers on the other, in regard to its behaviour towards conventional electrophiles, has been made in Section 3.02.2.3. Attack on carbon is the predominant mode of reaction (Section 3.14.2.4) reaction at sulfur is relatively rare (Section 3.14.2.5). Carbenes are known to act as electrophiles attack at both carbon and sulfur of thiophene has been reported. The carbene generated from diazomalonic ester by rhodium(II) catalysis attacks the sulfur atom of thiophene, resulting in an ylide. It has also been shown that the carbenoid species derived by thermolysis of such an ylide functions as an electrophile, attacking the a-carbon of a second molecule of thiophene (Section 3.14.2.9). Singlet nitrene is electrophilic. However, in contrast to carbenes, it invariably attacks only the carbon atom (Section 3.14.2.9). 

Thermal decomposition of ethyl azidoformate in the presence of 4-phenyl-l,2-dithiole-3-thione leads to the rare 1,2,3-dithiazine system (254) (76CJC3879). The reaction presumably involves initial attack of the nitrene at S-l, followed by a [1,2] rearrangement of the resulting ylide (Scheme 34). 

Ultraviolet irradiation of mesityl azide 288 in the presence of tetracyanoethylene has resulted in the isolation of the intermediate azomethine ylide 289 (from trapping of the aryl nitrene) together with its rearrangement product, the spiroazepine 290 (Scheme 36) <1997JOC3055>. Photolysis (at 313 nm) at low temperature of 1- and 2-azido-naphthalenes in an Ar matrix provided access to the novel seven-membered cyclic ketenimines 291 and 292, respectively <2004JA237>. 

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