Communications to the Editor lti-Benzazirines. Intermediates in the Ring Contraction of Iminocyclohexadienylidenes and Arylnitrenes1Sir:Recent work on oxocarbenes,2 thioxocarbenes,3 and imi-nocarbenes4 has demonstrated that the Wolff rearrangements in these species are often preceded by isomerization to oxirenes, thiirenes, and l//-azirines, respectively. How-

Flash vacuum pyrolysis (FVP) of 1-(5-(13)C-5-tetrazolyl)isoquinoline 18 generates 1-((13)C-diazomethyl)isoquinoline 19 and 1-isoquinolyl-((13)C-carbene) 22, which undergoes carbene-nitrene rearrangement to 2-naphthylnitrene 23. The thermally generated nitrene 23 is observed directly by matrix-isolation ESR spectroscopy, but undergoes ring contraction to a mixture of 3- and 2-cyanoindenes 26 and 27 under the FVP conditions. The (13)C label distribution in the cyanoindenes was determined by (13)C NMR spectroscopy and indicates the occurrence of two parallel paths of ring contraction starting from 1-isoquinolylcarbene; path a via ring expansion to 3-aza-benzo[c]cyclohepta-1,2,4,6-tetraene 32 bifurcating to 2-naphthylnitrene 23 and 2-aza-benzobicyclo[3.2.0]heptatriene 39 (paths a1 and a2); and path b via ring closure of the carbene onto the ring nitrogen, yielding 1-aza-benzo[d]bicyclo[4.1.0]hepta-2,4,6-triene 34 and 3-aza-benzo[d]cyclohepta-2,3,5,7-tetraene 35. Product studies demand that the major path is route a1 via 2-naphthylnitrene 23, which then undergoes direct ring contraction to 1-cyanoindene; but the (13)C label distribution requires that the non-nitrene route b contributes significantly. The two reaction paths are modeled at the B3LYP/6-31G* level. The initially formed carbene 22 is estimated to carry chemical activation of some 40 kcal/mol. This allows both reaction channels to proceed simultaneously under low-pressure FVP conditions. FVP of 3-(5-tetrazolyl)isoquinoline 28 similarly generates 3-diazomethylisoquinoline 29 and 3-isoquinolylcarbene 30, which rearranges to 3- and 2-cyanoindenes 26 and 27.

Summary. 15N-labelling experiments and energy calculations on nitreno-azines and -diazines are in agreement and lead to the theory that both ring contraction and ring expansion in hetarylnitrenes can be one-step processes which are govcrned mainly by the energy differences between the reacting species and the products.In the preceding paper [l] we showed that ring expansions in aromatic carbenes are largely determined by the energy difference between the first reacting species and the product. We report here similar observations on ring contractions and ring expansions in heteroaromatic nitrenes.Calculations of total and binding energies for several nitrenes and isomeric carbenes by the CNDOj2 [Z] and extended Hiickel method [3] are presented in the Table. As it is known that 2-pyridylnitrene (1) and 2,7-diazatropylidene (2) interconvert completely in the gas-phase [4], a calculated energy difference of ca. 3.5 eV can be taken as a lower limit for the excess energy of the reacting nitrene. The calculated energies for 4-pyrimidylnitrene (5 a), 2,4,7-triazatropylidene (a), and pyrazinylnitrene (3) now indicate an energy barrier of ca. 7 eV between 4 and 3, and 2-4 eV between 5 and 4. These calculated energy differences are borne out by experiment, for we find that pyrazinylnitrene is indeed the most stable of the three isomers, and that 5 converts to 3 via 4. Total and binding energies of hetarylnitrenes and isomeric sfiecies a)Compound -E t o t (CND0/2) In cV. All calculations are for singlet niolecules; all C-C = C-N = 1.40 8 ; C-H = 1.1 A.Geometries are not minimized Figures given are qualitative; only energy differences are important.Tetrazolo [l, 5-alpyrazine (6)[5] and 5,7-dimethyltetrazolo[l, 5-clpyrimidine (7 b) [5] labelled with 15N on N ( l ) and N(3) were prepared from the corresponding chlorol )The full paper will be submitted to Helv.

tron of r5d-A is demonstrated by a relatively large coupling of cu 0 7 Hz bctween the CH, group and the eiocyclic proton. € g lor /?el the ' H NMK spectrum in (IDCI, shows a n A B X , sy\tem lor the methylidene proton ( A ) . the ring proton H-4 (H). and the methyl group at C -5 (X)): A = 2 18 ( X d . 6 24 (B). 7 21 ( A ) . J 4 u = O Y . J A x = 0 7 . J U x = 1.2 Hz (based on computer simulation and spin decoupling experiments1 The remainin?, signdls in O r ) wrrr at S=3.84(\. 3t((OCH,)).69?(d.J=R8Hz.ZH(arom)).:49(d.J=8XH1. 2H (arom ))[9] T L. Gdchrrsr. U P Prur.von. J. Chem Soc. Perkin I IY76. Y89.

The reaction of 4‐chloro‐2‐phenylquinazoline with K15NN2 has been studied by 15N‐NMR. spectroscopy. 15N‐chemical shifts in 5‐phenyl‐1 (3)‐[15N]‐tetrazolo[1,5‐c]quinazoline and ‐Nα(Nγ)‐[15N]‐4‐azido‐2‐phenylquinazoline are reported. The characteristic IR. absorption frequencies of the tetrazole group have been determined in a series of annelated 15N‐labelled compounds. From these studies and the chemistry of the labelled tetrazoles, it is concluded that all haloazines examined react with KN3 by the direct nucleophilic substitution mechanism. An addition of nucleophile‐ring opening‐ring closure (ANRORC) mechanism was not observed. The synthesis of several 15N‐labelled tetrazoloazines is described.