Chemistry
Nitrene
Nitrene is a highly reactive intermediate in organic chemistry that contains a nitrogen atom with two unpaired electrons. It can be generated by various methods, including thermal decomposition of azides and photolysis of diazo compounds. Nitrenes are involved in a variety of reactions, including insertion into C-H bonds, cycloadditions, and rearrangements.
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8 Key excerpts on "Nitrene"
eBook - PDF- Axel G. Griesbeck, Jochen Mattay(Authors)
- 2004(Publication Date)
- CRC Press(Publisher)
13 Photogenerated Nitrene Addition to p -Bonds H.-W. Abraham Institut fu ¨r Chemie der Humboldt-Universita ¨t zu Berlin, Berlin, Germany 13.1. HISTORICAL REMARKS Nitrenes of the general formula R–N are reactive intermediates containing a monovalent nitrogen atom with a sextet of electrons in its outer shell. A variety of names such as azenes [1], imine radicals [2], imene [3], and imido intermediates (imidogens) [4] have all been used by various authors, however the term ‘‘Nitrene’’ is generally accepted in the literature. The R group may be an alkyl, aryl, acyl, a hetaryl, sulfonyl, phosphazyl, or an amino group. Nitrenes were first mentioned as reactive intermediates more than 100 years ago by Tiemann [5]. Nitrenes are isoelectronic to carbenes and this relationship heated up the investigation of Nitrenes themselves and their addition to olefins in parallel with carbenes in the 1960s. As a result there are a number of reviews dealing with the generation of and chemistry of Nitrenes [3a,4,6]. However, given all this study, only carbenes have been produced as stable entities, this development has yet to be reported for Nitrenes. 13.2. MECHANISTIC MODELS 13.2.1. Generation of Nitrenes In general, the reactive Nitrenes can be generated by thermolytic or photolytic elimination of stable compounds from suitable precursors such as heterocycles, ylides, and azides. The most commonly used method for the generation of Nitrenes is the thermolysis or photolysis of the corresponding 391 azides, however, not all Nitrenes are available by the thermolysis route. The decomposition temperature of azides is often high and a competing reaction of the azide or the intermediate Nitrene may take place. Therefore, azide photolysis is perhaps the most popular method for making Nitrenes [6d]. Nitrenes generated via the excited states of heterocyclic or azide precursors are formed in the ground state, therefore Nitrene generation is the only photochemical key step in Nitrene chemistry.
eBook - PDF- Samuel McManus(Author)
- 2012(Publication Date)
- Academic Press(Publisher)
The name Nitrene was proposed by analogy with the isoelectronic carbene intermediates but was objected to because the term had previously been used to designate azomethine ylides (Staudinger and Miescher, 1919; Hassall and Lippman, 1953), and the ending ene is reserved for olefins and aromatic hydrocarbons (IUPAC nomenclature rules). On the basis of this last objec-tion, the editors of Chemical Reviews and Chemical Abstracts Index insisted on the name Imidogen for NH and an imido intermediate or an imidogen for R—N (Abramovitch and Davis, 1964). The term Nitrene has, however, found almost universal acceptance (including its use in the biweekly indexes of Chemical Abstracts!) and is now in common usage. Its use will also be adopted here. The protonated or alkylated Nitrene, R—N + —R', is called a nitrenium ion. B. HISTORICAL BACKGROUND The parent compound NH is of great interest to the astrophysicists since it has been observed as a component of comet heads and tails and of the sun, and its spectral bands have been observed in the night sky spectrum. It has been suggested that some of the colors on Jupiter may be due to condensed reactive species such as (NH) n (from N + MH -> NH + M). The proposal that Nitrenes may be formed as reactive intermediates was first made by Tiemann (1891) in connection with the mechanism of the Lossen rearrangement. Stieglitz (1896) postulated their formation to account for the mechanisms of the Hofmann, Curtius, Lossen, and Beckmann rearrangements as well as for a number of rearrangements involving compounds of the type (C 6 H 5 ) 3 CNHX (Morgan, 1916; Stagner, 1916; Vosburgh, 1916). Curtius (1913) compared the intermediates formed in the decomposition of sulfonyl azides in aromatic hydrocarbons with those from ethyl diazoacetate. The rejection of the intermediacy of a monovalent nitrogen species in the Beck-mann rearrangement cast a simultaneous shadow on its postulated participa-tion in other reactions.
eBook - PDF- Metin Balcı(Author)
- 2021(Publication Date)
- Wiley-VCH(Publisher)
In many cases, these intermediates can be isolated. The intermediate can eliminate N 2 to produce aziridine derivatives. Alkyl azide CH 3 CH 3 + N N N R CH 3 CH 3 1,3-dipolar cycloaddition Δ CH 3 CH 3 N R –N 2 Aziridine R N N N 8.3.4 Nitrene Insertion Reactions Another significant reaction of Nitrenes is insertion reactions like carbenes undergo [131]. A Nitrene can insert into a carbon–hydrogen bond, forming an amine. While singlet Nitrene insertions processes are concerted, involving a three-center cyclic transition state, triplet Nitrenes form products because of multistage reactions. A singlet Nitrene reacts with retention of configuration. N R 2 C H R 1 R 3 + R 2 C H R 1 R 3 N R 2 C R 1 R 3 NHR * * R R Transition state Configuration retention Problems 461 Pyrolysis of two different optically active azides, an o-azidoalkylbenzene and an alkyl azidocarbonate, led in each case to cyclization at the asymmetric carbon atom to give an indoline and oxazolone derivatives [132]. The products were shown to be still optically active. It was concluded that the insertion occurs by one process involving a singlet-state Nitrene. For such insertion reactions to occur, the insertion center (C—H bond) must be close to the nitrogen atom and there must also be no other C—H bonds in the α-position. N 3 H Et Me Δ N H Et Me 50–60% O O N 3 H Et Me Δ O O N H Et Me 68% For a triplet Nitrene, the C–H insertion process proceeds in two steps. Because of its diradical nature, the triplet Nitrene first abstract hydrogen from the C—H bonds, forming two new radicals. The combination of radicals results in the product of insertion. N R R 2 C H R 1 R 3 + R 2 C R 1 R 3 R 2 C R 1 R 3 + HN R H N R Selectivity is observed in Nitrene inversion and Nitrenes prefer tertiary carbon atoms over secondary and primary ones in inversion. The example below clearly supports this. Insertion is observed more in tertiary carbon atoms than in secondary and primary ones.
eBook - ePub- Metin Balcı(Author)
- 2021(Publication Date)
- Wiley-VCH(Publisher)
[98] .8.3 Azides and Nitrenes
The first organic azide, phenyl azide, as an energy-rich compound, was discovered by Griess in 1864, and later Tiemann proposed in 1891 the formation of Nitrenes during decomposition of azides. After these findings, interest in this field has continued to grow. Nitrenes are nitrogen compounds that are analogous to carbenes. In Nitrenes, only one substituent is attached to the nitrogen atom, and there are two pairs of nonbonding electrons on the nitrogen atom. Because the nitrogen atom has six electrons in its outer shell, Nitrenes also fall into the class of electron-deficient compounds, like carbenes. Because of these electronic properties, Nitrenes act as electrophiles, like carbenes; they are unstable and react very quickly. A Nitrene has the following general formula.Nitrenes can have two different electronic configurations, like carbenes. The simplest Nitrenes have a linear structure. If the nitrogen atom's hybridization is sp, one of the hybrid orbitals is used to bond the R group attached to the nitrogen, while the other sp hybrid orbital has a pair of electrons. Because there are two p orbitals, the two remaining electrons occupy two degenerate orbitals according to Hund's rule; electrons fill these orbitals one by one. For this reason, Nitrenes in the ground state prefer the triplet configuration. In the singlet state, an electron pair populates one p orbital and the other one is empty.While the energy difference between singlet and triplet carbenes in hydrocarbons is about 8–10 kcal/mol (33−42 kJ/mol), this difference in Nitrenes is about 16–18 kcal/mol (63–75 kJ/mol) [99] . Moreover, calculations show that Nitrenes are more stable than carbenes, with an energy difference of 25–26 kcal/mol (105–109 kJ/mol) [100]
eBook - ePubOrganic Reaction Mechanisms 2008
An annual survey covering the literature dated January to December 2008
- A. C. Knipe(Author)
- 2011(Publication Date)
- Wiley(Publisher)
CHAPTER 4 Carbenes and Nitrenes M. G. MOLONY Department of Chemistry, University of OxfordReviewsStructure and ReactivityGenerationMetal-bound CarbenesCarbenes as ReagentsAddition and FragmentationInsertion and AbstractionRearrangementNitrenesNucleophilic CarbenesSilylenes and GermylenesReferencesThe resurgence of interest in the chemistry of carbene and Nitrene reactive intermediates, and in particular that of carbenes, continues apace, and the most successful strategies continue to be both the use of adjacent heteroatoms for electronic stabilization, and coordination to a suitable transition metal to assist carebene generation and control reactivity.ReviewsA comprehensive review has appeared which overviews synthetic methods for and the reactions of stable heterocyclic carbenes, of different ring sizes (3–6-membered) and containing different stabilizing heteroatoms (typically N, P, and S), and their metal complexes.1The synthesis of chiral bicyclic triazolium derived-carbenes (1) and (2) has been described;2 these nucleophilic carbenes find application as organic catalysts for asymmetric acyl anion (e.g. benzoin and Stetter reactions) and redox chemistry. Success in this regard has depended critically upon the nature of the N -aryl substituent on the triazole ring. Recent advances in the asymmetric Stetter reaction, in which the active species is the N -heterocyclic carbene (NHC) generated from thiamine diphosphate, have been reviewed.3Reviews on the generation and reactions of α-diazocarbonyl compounds4 and on recent developments in asymmetric cyclopropanations have appeared.5Structure and ReactivityThe structures, energies, and thermal stabilities of the singlet, triplet, and quintet states of Nitrene- and carbene-carrying naphthalenes (3) and (4) have been reported, as determined by DFT methods. For both the 1,2- and 3,2-isomers, singlet quinoidal diradicals were predicted to be the ground state, with the quintet state of the former lying 30 kcal mol−1 but the latter only 9 kcal mol−1 above the ground state. However, attempts to generate the ground-state singlet intermediates by photolysis of the corresponding azido-diazo compounds resulted only in the direct formation of azetes (5) and (6).6
eBook - ePubOrganic Reaction Mechanisms 2010
An annual survey covering the literature dated January to December 2010
- A. C. Knipe(Author)
- 2012(Publication Date)
- Wiley(Publisher)
Chapter 4 Carbenes and Nitrenes E. GrasLaboratoire de Chimie de Coordination, Centre National de la Recherche Scientifique, Toulouse, FranceReviews Generation, Structure, and Reactivity Carbenes in Coordination Chemistry Addition—Fragmentations Free Carbenes or Main Group Carbenoids Reactions Transition-metal-assisted Reactions Insertion—Abstraction Free Carbenes or Carbenoids Reactions Transition-metal-assisted Reactions Rearrangements Free Carbenes or Carbenoids Reactions Nucleophilic Carbenes—Carbenes as Organocatalysts Nitrenes Heavy-atom Carbene Analogues ReferencesReviews
The chemistry of carbenes and Nitrenes has been covered from the point of view of asymmetric aziridination.1 A retrospective review covering the years from 1972 to 1999 has appeared on the intermediacy of methylenecarbene in the course of flash vacuum pyrolysis generation of propanedienones.2 A review of dihalocarbene chemistry has featured syntheses and reactivities.3A short and instructive essay highlighting the work of Wanzlick at the origins of N-Heterocyclic Carbenes (NHCs) has been published.4 A detailed review on stable cyclic carbenes (other than diaminocarbenes) has appeared.5 The synthesis, structure, and reactivities of a range of cyclic carbenes are widely explained. NHC structures (mainly diaminocarbenes) have been thoroughly described with a view on catalytic applications.6
eBook - PDFOrganic Reaction Mechanisms 2015
An annual survey covering the literature dated January to December 2015
- A. C. Knipe(Author)
- 2019(Publication Date)
- Wiley(Publisher)
143 Var- ious simple arenes have been successfully aminated using aryl sulfonyl azides as Nitrene source under solvent-free conditions at 130 ∘ C. Another intermolecular Nitrene transfer approach has been reported for the synthesis of polysubstituted 2-aminopyrroles. 144 The approach comprises a formal 3 + 2- cycloaddition process via gold-catalysed Nitrene transfer from vinyl azides to ynamides. From a mechanistic viewpoint, the participation of a Nitrene or its corresponding 2H-azirine as key intermediate has been investigated, the obtained results supporting the 2H-azirine pathway. The reaction of triplet phenyl Nitrene with molecular oxygen has been studied in detail. 145 Under optimized reaction conditions, the phenyl Nitrene, smoothly generated by flash vacuum thermolysis, proved to react with molecular oxygen to furnish the nitroso O-oxide derivative, which further rearranges to nitrobenzene under light. Transition-Metal-Assisted Reactions The insertion of transition metal–imides (i.e. nitrenoids) into C–H bonds remains a tactic of choice for C–N bond formation. Using this tactic, the synthesis of quinolin- 8-ylmethanamines has been achieved under Ru(II)-catalysis from readily available 8-methylquinolines and aryl sulfonyl azides as nitrenoid source. 146 This method corresponds to the first example of [(p-cymene)-RuCl 2 ] 2 -catalysed C sp 3 –H bond intermolecular amidation reaction. The mechanism of diverse C–H insertion reactions has been explored computationally. Regarding C sp 2 –H activation reactions, the copper-catalysed amination of benzene 147 and the chemoselective gold-catalysed amination of mesitylene 148 have been studied by means of computational methods, and the results of both studies have supported a Nitrene addition pathway. A mechanistic study has also been led on the Rh(III)-catalysed C sp 2 –H activa- tion/cycloaddition of benzamide and diazo compounds.
eBook - ePubOrganic Reaction Mechanisms 2007
An annual survey covering the literature dated January to December 2007
- A. C. Knipe(Author)
- 2011(Publication Date)
- Wiley(Publisher)
CHAPTER 4Carbenes and NitrenesE. GRAS Laboratoire de Chimie de Coordination, Centre National de la Recherche Scientifique, Toulouse, FranceReviewsGeneration, Structure, and ReactivityMetal-bound CarbenesCarbenes as Ligands Carbenes as ReagentsAdditionInsertionRearrangementNucleophilic Carbenes - Carbenes as OrganocatalystsNitrenesHeavy-atom Carbene AnaloguesReferencesReviewsA short review in Japanese gave a general description of the chemistry of carbenes for beginners.1Fragmentations of alkoxychlorocarbenes via internal nucleophilic substitution (S N i) have been reviewed from both experimental and theoretical points of view;2 the influence of the alkyl group, the counterion, and the solvent on the reaction outcome is discussed. A review from the same group detailed some aspects of chlorodiazirines,3 with particular reference to the spectroscopy of alkylchlorocarbene and the reactivity at the carbon centre of the chlorodiazirine depending of the nature of the second substituent.A short review in Japanese described the preparation of long-lifetime triplet diphenylcarbenes.4 A more comprehensive review described the switch from stable triplet carbene to polycarbene systems from a synthetic point of view and gave some insight into the magnetic properties of these materials.5 Structurally, the carbene moiety was located either at the periphery of a dendritic core, on the side-chain of a polymeric string constituted of vinyl units, or on the polymeric chain of pyridine–copper complexes.Reactions of imidazoles leading to N -functionalized N -heterocyclic carbenes (NHC) were overviewed.6 Some catalytic applications of the complexes obtained with transition metals were depicted as were immobilizations of these complexes. A tutorial review introduced the coordination chemistry of NHCs bearing an anionic moiety on their side-chain.7
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