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73.R. Sztyrbicka, M. M. Rahman, M. E. D’Aunoy, and P. B. Shevlin, J. Am. Chem. Soc. 1990, 112, 6712.

74.This reaction finds analogy in the work of M. Pomerantz and A. S. Ross, J. Am. Chem. Soc. 1975, 97, 5850 and references cited therein.

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76.(a) Christopher J. LaFrancois, Ph. D. Thesis, Auburn University, June, 1996.

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78.For reviews with many references see: (a) W. M. Jones, in Rearrangements in Ground and Excited States, Vol. 1., P. de Mayo, Ed., Academic, New York, 1980. (b) C. Wentrup, in Methoden der Organischen Chemie (Houben-Weyl), Vol. E19b, M. Regitz, Ed.,

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79.P. P. Gaspar, D. M. Berowitz, D. R. Strongin, D. L. Svoboda, M. B. Tuchler, R. A. Ferrieri, and A. P. Wolf, J. Phys. Chem. 1986, 90, 4691.

80.(a) S. Matzinger, T. Bally, E. V. Patterson, and R. J. McMahon, J. Am. Chem. Soc. 1996, 118, 1535. (b) M. W. Wong and C. Wentrup, J. Org. Chem. 1996, 61, 7022.

(c)P. R. Schreiner, W. L. Karney, P. v. R. Schleyer, W. T. Borden, T. P. Hamilton, and H. F. Schaefer, III, J. Org. Chem. 1996, 61, 7030. (d) C. J. Cramer, F. J. Dulles, and

D.E. Falvey, J. Am. Chem. Soc. 1994, 116, 9787.

81.F. Sevin, I. So¨kmen, B. Du¨z, and P. B. Shevlin, Tetrahedron Lett. 2003, 44, 3405.

82.(a) M. Jones, Jr. and A. H. Golden, J. Org. Chem. 1996, 61, 4460. (b) A. Kumar,

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83.C. M. Geise, C. M. Hadad, F. Zheng, and P. B. Shevlin, J. Am. Chem. Soc. 2002, 124, 355.

84.F. Zheng, M. L. McKee, and P. B. Shevlin, J. Am. Chem. Soc. 1999, 121, 11237.

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86.Y. Xie, P. R. Schreiner, P. v. R. Schleyer, and H. F. Schaefer, J. Am. Chem. Soc. 1997, 119, 1370.

87.W. Pan and P. B. Shevlin, J. Am. Chem. Soc. 1997, 119, 5091.

88.W. Pan, M. Balci, and P. B. Shevlin, J. Am. Chem. Soc. 1997, 119, 5035.

89.M. L. McKee and P. B. Shevlin, J. Am. Chem. Soc. 2001, 123, 9418.

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J. Chem. Phys. 1999, 110, 6091.

1. INTRODUCTION

Compounds containing neutral, monovalent nitrogen atoms are known as nitrenes.1 The parent structure, NH, is also called imidogen. Because most stable compounds of neutral nitrogen have a valence of 3, it is no surprise that nitrenes typically are very short lived, reactive intermediates. A short history of nitrenes has been presented by Lwowski2 who points out that they were first proposed by Tiemann in 18913 as transient intermediates in the Lossen rearrangement.

Nitrenes are commonly generated by decomposition of organic and inorganic azides, although other types of precursors are known.1 As azides are bound to many elements, many types of nitrenes are known or can be imagined. This chapter will be limited to only those nitrenes commonly encountered in organic chemistry.

The term ‘‘nitrene’’ is reminiscent of ‘‘carbene.’’ In fact, the two types of intermediates have more than a superficial resemblance. Carbenes and nitrenes have two bonds fewer than most stable compounds of carbon and nitrogen, respectively. As we will see, many of the properties of nitrenes are best appreciated on comparison with carbenes.

Nitrenes are involved or imagined in many useful transformations in classical synthetic organic chemistry.4 Physical organic chemists seek to understand the role of nitrenes in these reactions and how their structures control their reactivity. Biochemists sometime append azide groups to the natural ligands of biological macromolecules.5 Photolysis of the azide–biomolecule complex often leads to covalent attachment of the ligand to the biomolecule. This technique, invented by Singh et al.6 with carbenes, and later by Knowles and Bayley7 with nitrenes, has come to be known as photoaffinity labeling. Materials chemists also seek to attach probes to the surfaces of polymers using reactions of nitrenes.8 Industrial scientists use azide photochemistry and nitrenes in lithography and as photoresists.9 Thus, the chemistry of nitrenes attracts the interest of a diverse group of scientists and to understand their properties requires the multidisciplinary contributions of organic chemists, spectroscopists, kineticists, and theoreticians. The goal of this chapter is to interweave these contributions rather than to present the historical development of the field. Space does not permit a comprehensive review and coverage reflects the biases of the author.

1.1. Imidogen (NH)

The nitrene nitrogen atom of imidogen is sp hybridized. Both the NH bond and the sigma lone pair of electrons use nitrogen orbitals that are rich in 2s character.

INTRODUCTION 503

z

H

y

x

sp

σ N–H

In triplet imidogen, there are two electrons with parallel spins, singly occupying pure px and pz orbitals. This state is designated as 3 by spectroscopists.

Because of the high symmetry of the NH molecule, the singlet state of imidogen cannot be classified as either open or closed shell. The ‘‘closed-shell’’ and ‘‘openshell’’ singlets have exactly the same energy and form the two components of a doubly degenerate 1 state.10

H H

The triplet state of NH is 36 kcal/mol lower in energy than the singlet state.11 The triplet state is favored because, on the average, electrons with parallel spin spend less time in proximity with each other than electrons with antiparallel spin. Consequently, the Coulombic electron–electron repulsion in the triplet state is less severe than in the singlet state (cf. Borden, Chapter 22 in this volume.) As we will see in vinyland phenylnitrene, delocalization of an unpaired electron by conjugation dramatically stabilizes the singlet relative to the triplet states of nitrenes.

The singlet–triplet splitting of NH was determined experimentally by spectroscopy of neutral NH11 and by negative ion photoelectron spectroscopy (PES) of the NH anion.12 In the latter experiment, the anion NH is prepared in the gas phase and exposed to monochromatic ultraviolet (UV)-laser light. This photolysis leads to ejection of photoelectrons whose kinetic energies (EK) are analyzed. As the energy

INTRODUCTION 505

pair of electrons and the singly occupied sp2 orbital. Thus the HCH bond angle of triplet methylene expands to 135 .15

Triplet methylene is 9.05 kcal/mol lower in energy than singlet methylene even though both nonbonding electrons of the singlet state reside in the lower energy sp 2 orbital in the singlet state. The reason for this is that the two nonbonding electrons experience more severe Coulombic repulsion when they both reside in the same sp2 orbital than when they are placed in two separate orbitals. If the difference in Coulombic repulsion outweighs the sp2/pp orbital energy gap (which is the case in CH2), then the triplet becomes the ground state.

In NH, the ‘‘last’’ two electrons will reside in two degenerate pure p orbitals. The singlet state of NH still suffers from increased electron–electron repulsion, relative to the triplet, but unlike in 1CH2, does not benefit from placing two electrons into a lower energy orbital rich in 2s character. Thus, the singlet–triplet (ST) splitting of NH is much larger than that of CH2. This rule is general. In many cases, the ST splittings of carbenes are so small that the two states rapidly and reversibly interconvert. Relaxation of a singlet to the corresponding triplet nitrene, however, is always irreversible (when the triplet is the ground state) because of the large ST gap.

Simple MO pictures also explain the differences in the bond dissociation energies (BDE) of methane and ammonia, and explain different triplet carbene and nitrene reactivities toward hydrogen atom donors.10,16

The first BDE of ammonia is slightly larger (3.5 kcal/mol) than that of methane, but the second BDE of methane is substantially larger (13.3 kcal/mol) than that of ammonia.

In the first bond dissociation of CH4 and NH3 the hybridization of the central atom is changing from sp3 to sp2 in CH3 and approximately to sp2 in NH2. When .CH3 fragments to 3CH2 þ .H, there is little change in hybridization. However, when .NH2 fragments to NH þ .H, there is substantial rehybridization. The rehybridization of NH to sp stabilizes 3NH by lowering the energy of the nonbonding electron pair. This requirement makes the second BDE of ammonia less endothermic. The result is that triplet methylene reacts exothermically with methane to yield two methyl radicals, whereas the reaction of 3NH with methane to give .NH2 þ . CH3 is endothermic. Therefore triplet nitrenes are much less reactive toward hydrogen atom donors than triplet carbenes.