Nitrenes, Diradicals, and Ylides. Ring Expansion and Ring Opening in 2-Quinazolylnitrenes

Abstract

Tetrazolo[1,5-a]quinazoline (9) is converted to 2-azidoquinazoline (10) on sublimation at 200 degrees C and above, and the azide-tetrazole equilibrium is governed by entropy. 2-Quinazolylnitrenes 11 and 27 and/or their ring expansion products 14 and 29 can undergo type I (ylidic) and type II (diradicaloid) ring opening. Argon matrix photolysis of 9/10 affords 2-quinazolylnitrene (11), which has been characterized by ESR, UV, and IR spectroscopy. A minor amount of a second nitrene, formed by rearrangement or ring opening, is also observed. A diradical (19) is formed rapidly by type II ring opening and characterized by ESR spectroscopy; it decays thermally at 15 K with a half-life of ca. 47 min, in agreement with its calculated facile intersystem crossing (19T --> 19OSS) followed by facile cyclization/rearrangement to 1-cyanoindazole (21) (calculated activation barrier 1-2 kcal/mol) and N-cyanoanthranilonitrile (22). 21and 22 are the isolated end products of photolysis. 21 is also the end product of flash vacuum thermolysis. An excellent linear correlation between the zero-field splitting parameter D (cm(-1)) and the spin density rho on the nitrene N calculated at the B3LYP/EPRIII level is reported (R2 = 0.993 for over 100 nitrenes). Matrix photolysis of 3-phenyltetrazolo[1,5-a]quinazoline (25) affords the benzotriazacycloheptatetraene 29, which can be photochemically interconverted with the type I ring opening product 2-isocyano-alpha-diazo-alpha-phenyltoluene (33) as determined by IR and UV spectroscopy. The corresponding carbene 37, obtained by photolysis of 33, was detected by matrix ESR spectroscopy.

9‐Azidoacridine and 9‐acridinylnitrene

“…The zero‐field splitting parameter | D | for the nitrene, 0.5196 cm −1 , is close to the value for 9‐anthracenylnitrene (0.47 cm −1 ) 34. The nitrene | D | values show an excellent correlation35 with the calculated spin densities on the nitrene–nitrogen (see Supporting Information, Fig. S6).…”

3-Pyridazinylnitrenes and 2-Pyrimidinylnitrenes

“…1) was clearly demonstrated to be a photochemical process 6 -not surprisingly, since a formal double bond is broken. There is no evidence for the formation of 36.…”

“…The D value fits our correlation with calculated spin densities (D -ρ correlation). 6 It is noted, however, that an additional, strong signal is present at 5962 G ( Figure 2). As reported previously, the same or a very similar signal is formed from 1,2,3-triazolo[1,5-a]pyridazine, and we assigned it to propargylene, HCCCH, formed by elimination of 2N 2 + HCN.…”

Carbenes and Nitrenes

“…Ion pairs of similar structure are thought to be generated in polar solvents upon fragmentation of oxychlorocarbene (79), chlorocarbonate (80), and chlorosulfite (81). 63 They all lead to similar mixtures of 2-chloro-5-norbornene (82) and 3-nortricyclyl chloride (83).…”

“…81 The reactions of this nitrene were characterized and included fragmentation to radical species (98), rearrangement to cycloheptatetraene (99), and ring opening to a new nitrene (100). …”