Phenylnitrene and Phenylcarbene

The development of the chemistry of aryl nitrenes has occurred primarily over the last 20 years, even though initial forays were made nearly a century ago. This slow development can be attributed to the fact that photolysis or pyrolysis of typical aryl azides, such as phenylnitrene 1, produces polymeric tar (Reaction 5.1). This is in sharp contrast to the rich chemistry of phenylcarbene (2), which exhibits insertion into C-H bonds, addition to Jt-bonds, reaction with oxygen, and so on.  

Instead of intermolecular chemistry, phenylnitrene (1) rapidly undergoes intramolecular rearrangement. Substituted azepine (3) is the product of both thermolysis and photolysis of phenyl azide in the presence of amine. On the other hand, substituted aniline (4) is obtained when phenyl azide is photolyzed with ethanthiol.  

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For example, the ring expansion reactions undergone by phenylnitrene and phenylcarbene manifested themselves for the first time by conspicuous IR bands at 1895 or 1815 cm that were observed after photolysis of matrix-isolated phenyl... 

DIFFERENCES BETWEEN PHENYLCARBENE AND PHENYLNITRENE AND THE RING EXPANSION REACTIONS THEY UNDERGO... 

In this chapter we describe experimental studies on the ring expansion reactions of phenylcarbene and phenylnitrene and the calculations that have been performed in order to try to explain the experimental results. Our aim is to show how theory can rationalize these observations and can also serve to stimulate additional experiments by predicting their outcome. We will attempt to demonstrate that an understanding of the fundamental differences between the electronic structures of phenylcarbene and phenylnitrene can explain the many differences in the chemistry of these reactive intermediates. 

Experiments. There are many parallels between the investigations — both experimental and theoretical — of the ring expansion of phenylnitrene and that of phenylcarbene. Huisgen and co-workers demonstrated in the late 1950s... 

Following the section on methylene, we present the chemistry of phenylcarbene and phenylnitrene and describe how computational chemistry helped detail why these two closely related molecules behave so differently. A discussion of tetram-ethyleneethane (TME) and oxyaUyl diradicals explores how theories of apparently simple molecules may be quite complicated. Next, we discuss the chemistry of... 

Kamey, W. L. Borden, W. T. Differences between Phenylcarbene and Phenylnitrene and the Ring Expansion Reactions They Undergo, in Advances in Carbene Chemistry Blinker, U. H., Ed. Elsevier Science Amsterdam, 2001 Vol. 3, p 205-251. 

Before discussing recent studies on the ring expansion reactions of phenylcarbene and phenylnitrene, we will describe some of the earlier experimental and theoretical work on these molecules. Our purpose here is to give a brief overview, in order to provide a context for the discussion of more recent results. For detailed descriptions of the chemistry of arylcarbenes and arylnitrenes, we refer the reader to the many excellent reviews in this area.1,2... 

A. Electronic Structures of the Lowest Singlet States of Phenylcarbene and Phenylnitrene... 

Figure 7. Comparison of the energetics of the ring expansions of phenylcarbene ( A -la) and phenylnitrene (1A2-lb), calculated at the CASPT2(8,8)/6-31 G //CASSCF(8,8)/6-31 G level.57-61 The numbers in parentheses represent corrections for the known deficiencies of CASPT2/6-31G in computing the energies of singlet phenylnitrene61 and singlet phenylcarbene.55 The small differences in the energies in Fig. 5 are a consequence of the difference between the basis sets used in the two sets of calculations.

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