Nitrenes electronic structures

Finally, some diradicals can be made in situ by an internal hydrogen-transfer reaction from a suitable hydrogen donor to a carbene or nitrene. In benzene derivatives, this is a well-tested route to o-quinoid compounds, which are not biradicals, although a biradical valence structure probably makes a significant contribution to their electronic structure. However, if the donor and the carbene or nitrene are... 

CASSCF and CASPT2 calculations both overestimate the stability of the open-shell electronic structure of singlet nitrenes 8c-f by about 3 kcal/moP as in the case of parent The ring opening is also computed to require... 

With respect to the electronic structure, nitrenes are analogous (isoelectronic) to carbenes. As in the case of diazocompounds, photolysis as well as thermolysis of azides leads to the corresponding nitrenes. Some reactions are collected in the following scheme ... 

The addition of dichlorocarbenes also allows for further conversions as the existing substituents can be replaced. Normally, PhHgCCbBr is used as carbene source in this reaction, but it succeeds with other carbenes or carbene sources as well. The addition of dichlorocarbene markedly changes the electronic structure of the functionahzed nanotube. Metallic SWNT, for instance, turn into semiconductors, which is due to altered electronic transitions close to the Fermi level. What is more, the intensity of interband transitions decreases for both the original metalHc and the semiconducting species because the extended n-network is severely disturbed by the functionalization and the concomitant introduction of sp -carbon atoms. However, a completely different picture of the carbene or nitrene addition is... 

This chapter will discuss the use of theory to calculate the minimum-energy geometries of singlet and triplet nitrenes and the energy separations between these states. We will demonstrate how theoretical methods can now routinely be utilized to simulate the vibrational and electronic spectra of nitrenes and, thus, to assign the experimental spectra to one or both of these spin states. Finally, we will review the ways in which theory has explained how the electronic structures of nitrenes controls their reactivity. 

In the context of this review, the electronic structure and spectroscopy of the simplest nitrene, NH, are very important because they will be useful in the analysis of the more complicated nitrenes. The electronic structure of NH can be... 

CASSCF and CASPT2 calculations both overestimate the stability of the open-shell electronic structure of singlet nitrenes 62a-d by about 3 kcal/mol, as in the case of parent 149 101 pjj g opening is computed to require passage over a 2 to 3 kcal/mol lower energy barrier than reversion of the intermediates to the reactants. Therefore, cyclization is the rate-determining step in the ring-expansion reactions of cyano-substituted phenylnitrenes 62a-d to derivatives of 64 and 64 (Scheme 5.3). 

In the case of transient species with unpaired electrons such as free radicals, and the triplet states of carbenes or nitrenes, electron spin resonance (ESR) spectroscopy can provide unique evidence about the structure of the intermediate. Useful information about intermediates in reactions involving radical pair coupling can also be obtained by a technique known as chemically-induced dynamic nuclear polarization (CIDNP). However, detailed discussions of ESR and CIDNP are outside the scope of this book and for further information suitable text books on physical organic chemistry or the references given in the Further Reading section should be consulted. 

Although nitrenes are isoelectronic with carbenes there are important differences in their electronic structures, which accounts for their different reactivity. Chemically nitrenes behave similarly to carbenes, that is, they add to carbon-carbon double bonds and insert into C-H single bonds. In addition they isomerize to imines, abstract a hydrogen atom to form primary amino groups, and effect ring closures. 

Mahy JP, Battioni P, Mansuy D, Fisher J, Weiss R, Mispelter J, Morgenstem-Badarau 1, Gans P (1984) Iron porphyrin-nitrene complexes preparation from 1,1-dialkylhydiazines electronic structure from NMR, Mossbauer, and magnetic susceptibility studies and crystal structure of the [tetrakis(p-chlorophe-nyl) porphyrinato-(2,2,6,6-tetramethy 1-1 -piperidy 1) nitrene]iron complex. J Am Chem Soc 106 1699-1706... 

A series of aromatic nitrenes substituted by anionic r-donating groups have been studied regarding their electronic structure, ground state and stability. The results obtained have provided data indicative of the behaviour of such peculiar nitrenes, thus giving the opportunity to further understand, predict and tune the electronic properties of this class of reactive intermediates. 

Fusion of a benzene ring to another aromatic ring or rings changes the electronic structure of the modified nitrenes and related intermediates sufficiently enough to alter their chemistry, kinetics and thermodynamics. Thus photolysis of polynuclear aromatic azides in the presence of primary and secondary amines leads not to 3H-azepines, but to corresponding... 

In their seminal paper on the rhodium(ll)-catalyzed C—H insertion with PhI=NNs, the group of Muller reached the conclusion that the reaction proceeds through the concerted asynchronous insertion of a rhodium-bound nitrene species.This hypothesis was supported by a Hammett analysis (/9= —0.90 vs. tr+), the absence of ring-opened products in reactions involving cyclopropyl radical clocks, and the stereospecific C(sp )—H ami-nation of (Ji)-2-phenylbutane that occurs with complete retention of configuration. However, the very low yields obtained for these test reactions as well as the kinetic isotope effect measured for the reaction from (1,3-D2)-adamantane (KIE = 3.5 0.2) put this conclusion into question as these did not rule out the possible involvement of radicals that could undergo fast recombination. Nevertheless, this initial study already highlighted the discrepancies that could be observed between the carbene and nitrene chemistries in terms of mechanism. The electronic structure of nitrenes, contrary... 

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