Nitrenes naphthylnitrenes

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. 

Carbenes 210 generated from the triazoloquinoline 209 by FVT rearrange into a seven-membered ring ketenimine 211, similar to carbodiimide 208. The ketenimine similarly rearranges to 1-naphthylnitrene 212 and nitrene derivatives 213, 214, 215, and 216 (Scheme 36) <2004JOC2033>. 

Such rearrangements are so rapid that it is usually difficult to exclude the possibility that a free nitrene was never present at all, that is, that migration takes place at the same time that the nitrene is formed " (see p. 1606). However, the rearrangement of naphthylnitrenes to novel bond-shift isomers has been reported. 

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). 

Quinolylcarbene and 1-isoquinolylcarbene undergo carbene-nitrene rearrangement to 1-naphthylnitrene and 2-naphthylnitrene, respectively.10,202 Both 2- and 4-pyrimidylcarbenes and 2-pyrazinylcarbene isomerize to 3-and 4-pyridylnitrenes, respectively, which then contract to cyanopyrroles.10... 

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. 

From the effect of solvent (Table 15) it is evident that the reactions discussed are nitrene reactions hydrogen-rich solvents suppress ring contraction and give rise to solvent dimer (bibenzyl) and/or a yellow nitrene dimer. The structure of the dimer is not known, but one possibility is shown in 144. A similar (colorless) dimer was obtained from 9-phenanthridylnitrene at 500 ° 7). Xhe two dimers formed from 137 and 141 in cyclohexane have nearly identical IR spectra. How could a hydrogen-rich solvent promote dimeriztion There is evidence from aryl azide decomposition in solution that amino radicals are formed first, and these dimerize and dehydrogenate as shown for 1-naphthylnitrene in [Eq. (48)] 82). 

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