Azirines naphthylnitrenes

In principle, singlet 1-naphthylnitrene can cyclize to two distinct azirines 43 and 44, but 43 is clearly much lower in energy than isomer 44, which lacks aromaticity in either ring. One can therefore predict that 43 will be formed preferentially. 

Experimental observations support these views. Photolysis of 1-naphthylazide in the presence of diethylamine and tetramethylethylenediamine (TMEDA) yields azirine, but no ketenimine-derived adducts at ambient temperature. " In the presence of diethylamine but in the absence of TMEDA, good yields of 1-amino-naphthalene and 1,1 -azo-naphthalene, products attributable to the triplet nitrene are observed. Good yields of 46 are also achieved when the photolysis of 1-naphthylazide and diethylamine is performed at —60 °C in the absence of TMEDA. Presumably, lowering the temperature extends the hfetime of azirine 43 by reducing its rate of reversion to singlet 1-naphthylnitrene more than it retards the rate of its reaction with diethylamine. 

No transient absorption >350 nm is detected upon LFP of 1-naphthylazide. A band with absorption maxima at 370 nm is formed with a time constant of 2.8 ps after the laser pulse. The carrier of the 370-nm absorption reacts over >100 ps to form azonaphthalene. The carrier of the 370-nm absorption is identified as triplet 1-naphthylnitrene that has previously been characterized as a persistent species at 77 K by UV-vis (A,nmx = 367 nm) and EPR spectroscopy. Azirine 43, detected by TRIR spectroscopy must not absorb significantly >350 nm, a fact that was established later by the matrix isolation studies of Wentrup s and Rally s groups. Assuming a rapidly equilibrating mixture of azirine and nitrene, and given that kisc = 1 X 10 s (determined by Tsao by LFP at 77 K and assumed to have the same value at 298 then K = [singlet nitrene]/[azirine 43] = 0.038 at 298 K. 

The 2-naphthylnitrene story is similar. Two azirines (47 and 48) can be formed, but azirine 47 is preferred and this azirine will form ketenimine 48 only reluctantly. The azirine 47 has been detected as a persistent species by matrix isolation spec-... 

In a manner formally analogous to the rearrangement shown in Scheme 33, 2-quinolylcarbene and 1-isoquinolylcarbene rearrange thermally to 1-naph-thylnitrene and 2-naphthylnitrene, respectively (Scheme 34).202,223 9-Phen-anthridylnitrene affords the cyclic carbodiimide 169, which can be prepared almost pure by pyrolysis at 490°C, condensing the product at — 196°C. Compound 169 is stable up to about — 40°C, where dimerization occurs.2223 In this case, the fused azirine 170 (Scheme 34) is expected to be highly unstable due to complete loss of aromaticity, and no evidence for its formation has been found. 

The directions of ring expansion in condensed aromatic nitrenes are relatively easy to explain. The nitrene always moves toward the carbon atom with the highest electron density [e.g., 2-naphthylnitrene inserts into the 1,2-bond, not the 2,3-bond see Eqs. (52-61) and Scheme 41], This is to be expected for the addition of an electrophilic singlet nitrene to a double bond, and it indicates that an azirine is either an intermediate or a transition state in the ring expansion (see also Section VIII,H,2). 

With the assumption that the undetected precursors to triplet 1- and 2-naphthylnitrenes are the azirines seen in the low temperature experiments, we can reach useful conclusions about the photochemistry of polynuclear aromatic azides. First, unlike phenyl azide where the closed-shell singlet intermediate formed in room temperature irradiations is dehydroazepine [46, 49, 69], the intermediates formed from both 1- and 2-naphthyl azide are azirines. The difference in the chemistry of 1- and 2-naphthyl azides is traced to a difference in the lifetime of the respective azirines. The azirine from 2-naphthyl azide survives at least 200 times longer than does the azirine formed from 1-naphthyl azide. The increase in lifetime permits the bimolecular trapping reaction by diethylamine to compete with isomerization to the triplet nitrene in the case of the 2-naphthyl but not the 1-naphthyl azides. 

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