Azirines fluorination

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. 

Laser flash photolysis of a series of fluorinated aryl azides produces the transient spectra of the corresponding singlet nitrenes. ° With the exception of singlet o-fiuorophenylnitrene (39s), the rate of decay of the singlet nitrene was equal to the rate of formation of the reaction products, for example, didehydroazepines and triplet nitrenes. Values of fejsc and the Arrhenius parameters for azirine formation are summarized in Table 11.5. 

Curiously, the addition of a second orf/zo-fluorine substituent (i.e. in benzazirine 17d) raises the barrier to reversion to singlet nitrene 16d, relative to the mono orf/io-fluoro system (Fig. 14). This is partly due to steric hindrance by fluorine in the transition state for cyclization, but also to the stabilization of 17d by the fluorine attached directly to the azirine ring vide suprd)F The addition of the second fluorine substituent (benzazirine 17d) decreases the barrier to conversion of azirine 17d to ketenimine 18d slightly... 

Morawietz, J. Sander, W. Photochemistry of fluorinated phenyl nitrenes matrix isolation of fluorinated azirines, J. Org. Chem. 1996, 61,4351-4354. 

Fluorinations of other functionalities containing C = N bonds, such as 2//-azirines and imines, are included in this section. Certain 2Ff-azirines and imines yield a-fluoro ketones. With other 2Ff-azirines, /i,/(-difluoro amines are the main reaction products. In a strict sense, these reactions cannot be considered as examples of substitution of nitrogen-containing functional groups no substitution takes place when /S. -difluoro amines are the reaction products. However, these methods have been included. 

The addition of the second fluorine substiment (benzazirine 63d) decreases the barrier to conversion of azirine 63d to ketenimine 64d slightly (Figure 11.14), although the... 

A rare example of [3+3] retrosynthetic disconnection of fluorinated pyrazine ring was implemented by dimerization of fluorinated formamidine 1099 (Scheme 234) [670], Another example is dimerization of azirine derivative 1101 [671],... 

Disubstituted indoles are obtained by iron(II) chloride-catalyzed ring opening of 3-phenyl-2//-azirines and amination of an aromatic C-H bond (Scheme 4-354). Moderate to good yields are achieved by this method, which tolerates a number of functional groups like bromine, fluorine, nitro, methoxy, trifuoromethyl, tert-butyldimethylsilyloxy, alkenyl, and pivaloyl substituents. ... 

---