Phenylnitrene Ring expansion

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... 

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 6. CASSCF(8,8)/6-31G optimized geometries of stationary points in the ring expansion of singlet phenylnitrene ( Aj-lb).61 Bond lengths in angstroms, and bond angles in degrees. The ball-and-stick pictures are profiles that indicate the nonplanar nature of these species.

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.

As discussed in Section HI, calculations on the ring expansions of phenylcar-bene (la) and phenylnitrene (lb) suggest that singlet lb is thermodynamically... 

V. SUBSTITUENT EFFECTS ON THE RATES AND REGIOCHEMISTRY OF THE RING EXPANSION OF PHENYLNITRENE... 

Earlier, Dunkin and Thomson had observed that matrix-isolated triplet 10a did not undergo photochemical ring expansion.83 However, Morawietz and Sander have recently provided evidence for photochemical conversion of 310a and 310b to the corresponding fluorinated azirines (Scheme 19).48d This represents a rare instance where an azabicyclo[4.1.0]heptatriene, the putative intermediate in the ring expansion of a phenylnitrene, has actually been observed. 

Table 6. CASPT2/cc-pVDZ Barrier Heights (kcal/mol) for Ring Expansions of Fluorinated Singlet Phenylnitrenes °...

Table 7. Experimental Activation Parameters for Ring Expansion of Fluoro- and Methyl-Substituted Phenylnitrenes...

In order to test the validity of these qualitative expectations, CASPT2/6-31G calculations on the ring expansion reactions of ortho, meta, and para-cyano-phenylnitrene (12a-c) were performed.96 Fig. 15 summarizes the results. In all three cases, the ring expansions were computed to be nearly thermoneutral, with the first step rate-determining. 

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. 

Galbraith, J. M. Caspar, P. P Borden, W. T. What Accounts for the Difference between Singlet Phenylphosphinidene and Singlet Phenylnitrene in Reactivity toward Ring Expansion J. Am. Chem. Soc. 2002, 124, 11669-11674. 

Figure 11.3. Energetics of the ring expansion of singlet phenylnitrene calculated at the CASPT2 (8,8)/6-311G(2d,p)//CASSCF(8,8)/6-31G level. [Reproduced with permission from W. L. Karney and W. T. Borden, J. Am. Chem. Soc. 1997,119, 1378. Copyright 1997 American Chemical Society.]...

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... 

Fig. 8 CASPT2N/6-311G(2d,p) relative energies of species involved in the ring expansion of phenylnitrene (PN). Energies are for CASSCF/6-31G geometries and include ZPE corrections." ...

Table 9 (8/8)CASSCF/6-31G, CASPT2/6-31G and CASPT2/cc-PVDZ relative energies (kcal/mol) for species involved in the first step of the ring expansion of fluoro-substituted phenylnitrenes (Scheme 5). ...

Fig. 14 Relative energies (in kcal/mol) of species involved in the ring expansions of singlet fluoro-substituted phenylnitrenes calculated at the CASPT2/cc-pVDZ// CASSCF(8,8)/6-31G level, (a) Difluorinated phenylnitrenes. (b) Monofluorinated phenylnitrenes.

However, from elegant labelling studies by Wentrup and coworkers on the gas-phase thermal interconversion of phenylnitrene and 2-pyridylcarbene, it became evident that an alternative and equally viable intermediate in these nitrene ring expansions is the azacycloheptatrienylidene (216) or its tautomer, the cumulated system (217) (81AHC(28)23l). 

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