Phenylcarbene phenylnitrene

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. 

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... 

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... 

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.

Wentrup s experimental work on carbene-to-nitrene rearrangements10 suggested that calculations on phenylnitrene (lb) and the isomeric pyridylcarbenes might provide some useful relative energies. Kemnitz et al. carried out calculations on lb and 3-pyridylcarbene (lc).77 The latter molecule was used to provide a link between the energies of not only phenylcarbene (la) and phenylnitrene (lb), but also between CH2 and NH. 

By furnishing both explanations and predictions, calculations have not only led to an understanding of experiments that have already been performed on la and lb, but also have motivated new ones. The study of the chemistry of phenylcarbene (la) and phenylnitrene (lb), particularly the ring expansion reaction that each undergoes, thus provides an excellent example of the synergy between calculations and experiments. 

Although AEst of phenylnitrene is small in comparison to that of imidogen, it is still very large compared to phenylcarbene. Thus, while singlet and triplet phenylcarbene interconvert rapidly, singlet to triplet intersystem crossing of phen-... 

Singlet phenylcarbene is a closed-shell species, but singlet phenylnitrene has an open-shell electron configuration. This observation explains why singlet... 

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