Nitrenes carbenes

Certain diazo compounds can be photolyzed with UV light to generate highly reactive carbenes (Reaction 48). Similar to nitrenes, carbenes can insert into active C—H or... 

Following their investigations on nitrene, carbene, and oxo transfer reactions catalyzed by fluorinated silver tris(pyrazoyl)borate (see Chapter 6 on nitrene chemistry), Lovely et al. looked for a combined carbene transfer and [2,3]-sigmatropic rearrangement. On the basis of these mechanistic considerations, these authors showed that diazoacetates, indeed, reacted with allyl halides in the presence of this silver catalyst to give a-halo-y-unsaturated esters (Scheme 3.51).77... 

Natural products, synthesis via aziridine intermediates, 39, 181 New developments in the chemistry of oxazolones, 21, 175 Nitration of phenyl-substituted heterocycles, 58, 215 Nitrenes, carbenes and, intramolecular reactions of, 28, 231... 

Silver can mediate oxidation reactions and has shown unique reactivity. In a few cases, namely, nitrene-, carbene-, and silylene-transfer reactions, novel reactivity was found with homogeneous silver catalysts. Some of these reactions are uniquely facilitated by silver, never having been reported with other metals. While ligand-supported silver catalysts were extensively utilized in enantioselective syntheses as Lewis acids, disappointingly few cases were reported with oxidation reactions. Silver-catalyzed oxidation reactions are still underrepresented. Silver-based catalysts are cheaper and less toxic versus other precious metal catalysts. With the input of additional effort, this field will undoubtedly give more promising results. 

Methyltrioxorhenium has been found to be a universal catalyst for a number of [2-1-1] cycloaddition reactions, including nitrene, carbene, or oxo-atom addition to olefins <2001GC235>. Typically, to increase the chemical yield of the reaction, at least 5 equiv of an olefin is required. As with most nitrene transfer reactions, simple cyclic olefins such as cyclohexene produce a low chemical yield of aziridine. The authors assume that the intermediate of the reaction is a reactive rhenoxaziridine intermediate. 1,2-Dihydronaphthalene provides aziridine 28 in 43% chemical yield under these reaction conditions (Equation 11). 

Nitrene-carbene interconversions have been observed at elevated temperatures. For example, 2,6-dimethylphenyl azide was pyrolysed at 900°/0 02 mm to give an 8% yield of 2-vinyl-6-methyl-p Tidine (213) As with the nitrene-carbene interconversion, nitrogen... 

Platz, M.S., Leyva. E., Haider, K. (1991), Selected Topics in the Matrix Photochemistry of Nitrenes, Carbenes, and Excited Triplet States, Org. Photochem. (Padwa, A., Ed.) 11, 367. 

Pyridines are observed in some of these reactions, and the substituent pattern is that expected from nitrene-carbene interconversion Small yields of azepine have been recorded in the photolysis of 158 i ). The bicyclic intermediate 44 [Eq. (64)] has been postulated frequently over the last twenty years. A wavelength dependence of the formation of azepines has been observed i ) the yield of azepine increases with the energy of the light and with the concentration of the trapping agent (diethylamine). At the same time, the yields of competitive processes of the nitrene (carbazole formation from 2-biphenylylnitrene p-cyanophenyl-hydrazine from p-cyanophenylnitrene) decrease. The rate of the azepine-forming... 

Three major classes of photoreactive compounds have been used to label specific sites of proteins nitrenes, carbenes, and free radicals. Carbenes, generated by photolysis of diazo derivatives, are highly reactive and relatively nonselective in their target sites. Nitrenes, formed by photoactivation of azido derivatives, are less reactive than carbenes and more electrophilic, exhibiting a preference... 

Despite the fact that they can be much more reactive than nitrenes, carbene derivatives are less frequently cited in reports on photoaffinity labels. Two such derivatives of thyroid hormone have been described in which the amino group of the alanine side chain of 3,5,3 -triiodo-L-thyronine or 3,5,3, 5 -tetraiodo-L-thyronine was derivatized with 2-diazo-3,3,3-trifluoropropionate (246). Irradiation at 254 nm of these compounds with several cell lines yielded low incorporation of reagent (0.5-13.5%) into two forms of thyroid hormone receptor proteins. Low incorporation was also characteristic of early reports of photoaffinity labeling (e.g., 247) in which 0 -ethyl-2-diazomalonyl-cAMP and A, C)2. (ji(ethyl-2-diazomalonyl)-cAMP were used to generate carbenes which modify rabbit muscle phosphofructokinase. Similarly, only 3-14% of the potential sites of lactate dehydrogenase were labeled by the carbene derived from 3-(3//-diazirino)pyridine adenine dinucleotide (248). 

The reaction is analogous to part of the pathway proposed in Scheme 1, although evidence was also provided that the site of protonation of the cluster by methanol is the acyl oxygen atom, rather than the imido nitrogen, and that a nitrene-carbene reductive elimination then occurs, followed by tautomerization of the organic product to the final carbamate. 

Treatment of the /3-lactones (66) with MgBf2 in ether at room temperature brings about quantitative, stereospecific conversion into y-lactones by a novel dyotropic Wagner-Meerwein rearrangement this is a reaction of considerable synthetic potential. A number of additions e.g. of nitrenes, carbenes, and diazo-compounds) to diketen to give spiro-/3-lactones, which can be further transformed, have been described.For example, photolysis of acyl azides in the presence of diketen leads to pyrrolinone derivatives (67), presumably as shown.With phenyl(trichloromethyl)mercury, the spiro-lactone (68), in which... 

Recent advances in silver-catalyzed nitrene, carbene, and silylene-trans-fer reactions leading, particularly, to aziridines and silacyclopro-panes 06EJO4313. 

Wentmp, C. (2011) Nitrenes, carbenes, diradicals, and ylides. Interconversions of reactive intermediates. Acc. Chem. Res., 44, 393-404. 

The tetrazoles family of compounds represents an important source of nitrenes, carbenes and ylides when subjected to UV irradiation. Baskir et isolated 5-amino- and 5-(dimethyl)amino-2-methyl-... 

By far the main part of the studies involves aromatic azides, however. These are the object of unrelenting interest. During the period considered, some reference works have been published, including a wide-scope review on the detection and role of intermediates, another one on pyr-idineazides and an account on the relation between nitrenes, carbenes, diradicals and zwitterions in the thermal and photochemical reaction operating in the decomposition of aromatic and heterocyclic azides. A recent, nice example illustrates the variety of paths followed. Thus, tetra-zolo[l,5-a]quinoxaline and tetrazolo[5.1-c]quinazoline equilibrate thermally with the corresponding azides. 

---