Phenylnitrenes

Singlet phenylnitrene, and hence /V,A -diethyl-3//-azcpin-2-amines, e. g. 102, can be generated by the thermolysis of A,-phenyl-Af,<9-bis(trimcthylsi]yl)hydroxylamine (100) in the presence of dialkylamines the reaction fails, however, with arylamines.210 Photofragmentation of the spiro oxaziridine 101 in diethylamine solution also produces the 3//-azepine 102,2,1 and an oxaziridine intermediate is probably involved in the formation, in low yield (1 %), of azepine 102 by the photolysis of A/,A( -diarylbenzoquinonc diimine A/,A/ -dioxides in benzene/die-thylamine solution.212... 

Phenylazide is comparable to benzenediazonium ion in the sense that it is prone to dediazoniation, forming phenylnitrene, which is a strong electrophile, particularly in the presence of acid (Scheme 10-70). This dediazoniation can be used to form diarylamines in solution (Takeuchi et al., 1986, and preceding papers) and in the gas phase (Attina et al., 1990 Attina and Ricci, 1992). 

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... 

Another example of the reversibility of the ring expansion — one of special relevance to this chapter — is found in the interconversion of the isomeric pyridylcarbenes, and their conversion to phenylnitrene (Scheme 4).10,11... 

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 2. Orbital occupancies for the nonbonding electrons and CASSCF(8,8)/6-31G optimized geometries of the lowest triplet and singlet states of phenylnitrene (1 b).61 Bond lengths in angstroms, and bond angles in degrees.

Table 2. Relative Energies (kcal/mol) of the Lowest Spin States of Phenylnitrene...

Our calculations on the ring expansion of the lowest singlet state of phenylnitrene ( A2-lb) to azacycloheptatetraene (3b) predict a two-step mechanism that is analogous to that for the rearrangement of la to 3a and which involves the bicyclic azirine intermediate 2b.61 The CASPT2 energetics are depicted in Fig. 5, and the CASSCF optimized geometries of the stationary points are shown in Fig. 6. 

Figure 5. Energetics of the ring exponsion of singlet phenylnitrene (1A2-lb), colculated at the CASPT2(8,8)/6-31 lG(2d,p)//CASSCF(8,8)/6-31G level.61...

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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