6,9-Di(tert-butyl)-1-methyltetrazolo[1,5-a]perimidine (1) has been synthesized from naphthalene in seven steps. The EPR spectra, recorded after irradiation of 1 in a butyronitrile matrix at 77 K (lambda = 351 nm) and in Ar and Xe matrixes at 4.6 K (lambda > or = 345 nm), showed a six-line, high-field signal (Delta m(S) = +/- 1), centered at 3350 G in butyronitrile, along with a half-field signal (Delta m(S) = +/- 2), which is characteristic for triplets. Simulation of the observed EPR spectra gave values for the zero-field splitting parameters of |D/hc|/cm(-1) = 0.0105, |E/hc|/cm(-1) = 0.0014 in butyronitrile and |D/hc|/cm(-1) = 0.0107, |E/hc|/cm(-1) = 0.0016 in Ar. These EPR parameters are consistent with the diradical 5,8-di(tert-butyl)-2-(N-methylimino)perimidine-1,3-diyl ((3)2) as source of the EPR spectra. Linearity of the Curie-Weiss plot and UB3LYP and (14/14)CASPT2 calculations of the singlet-triplet energy difference (DeltaE(ST) approximately 8-10 kcal/mol) indicate that the triplet is the ground state of 2, as predicted for such a nondisjoint diradical.

Deprotonation of the annulated tetrazolium salts 4, 6, 8, 10, and 12 with sodium or potassium hydride yields the alkylidenedihydrotetrazoles 5, 7, 9, 11, and 13, respectively. While 5a and b are unstable, even in solution at low temperatures, 7, 9, 11, and 13 form yellow oils that are distilled under high vacuum. − Irradiation of solutions of 7, 9, and 11 in [D8]toluene at −60°C yields, besides molecular nitrogen, annulated iminoaziridines that have an exocyclic CN double bond, i.e. 14, 16, and 18, respectively. In addition, an equal amount of the isomer 19 with the endocyclic CN double bond is formed from 11. On thermolysis, 14, 16, and 18 undergo [2 + 1] cycloreversion into methyl isocyanide and the cyclic imines 15, 17, and 20, respectively. By contrast, 19 rearranges thermally to yield 18. While the doubly bridged alkylidenedihydrotetrazole 13a affords only unidentified decomposition products on photolysis, its methyl homologue 13b is converted into the hexahydronaphthyridine 22 which is also formed on thermolysis. − Irradiation of 13b in a 2‐methyltetrahydrofuran or butyronitrile matrix at 77 K yields a triplet diradical showing a four‐line EPR spectrum centred at 3362 G and a half‐field transition (at 1669 G) with a hyperfine structure. The zero‐field splitting parameters |D‐hc| = 0.031 cm−1 and |E‐hc| = 0.0014 cm−1 are obtained by simulation of the EPR spectrum. The signal‐carrier is assigned the diazatrimethylenemethane structure 23 on the basis of the close similarity between its EPR spectrum and those of trimethylenemethane (28) and tris(N‐methylimino)methane (29). − Structural features are discussed that are responsible for the observed differences between the photochemical pathways.

RI-R2-H -(CH2)2-Me -(CH2)2-3b, 4b Me -(CH2)3-H -MqC-C&-CMq-H -(CH2)3-H -(CH&-Lithiation of the annulated tetrazoles 6a, c with butyllithium yields the N-lithiotetrazoles 7a, c which are allowed to react with alkyl halides. Alkylation at the a-carbon atom is observed in the reaction with methyl iodide (+ 6b, d), l-bromo-2chloroethane (7c + 14a), and 1,3-dibromopropane (+ 16, 17), while 1,2-dichloro-and 1,2-dibromoethane give other products, viz. 11 -13. Quaternisation of 6 with dimethyl sulphate affords mixtures of 1-methyl-(1) and 2-methyltetrazolium salts (8) (3:l-4:l) from which the hexafluorophosphates 1 .PF6 are obtained by crystallisation. Methyl triflate converts the a-azidonitriles 9 into the N-methylnitrilium triflates 10 which immediately undergo an intramolecular 1,3-dipolar cycloaddition to afford the 1-methyltetrazolium triflates 1 . F3CS03. Cyclisation of 16 by intramolecular N-alkylation furnishes the bisannulated tetrazolium bromide 3a . Br. Attempts to obtain the lower homologue 15, either from 14a or from 17, met with failure. Instead, 17 rearranges via 15b into 14b. The a-branched tetrazole 24 is synthesised from ethyl cyanoacetate and the bromide 18. Double cyclisation of 24 affords the bisannulated tetrazolium chloride 3b . C1. The analogous scheme, envisaged for the synthesis of the lower homologue 32, failed in the last step owing to the high strain of this system. Deprotonation of 1,4-substituted 5-alkyltetrazolium salts affords 5-alkylidenedihydrotetra~oles[~~~] which undergo a number of interesting reactions. Photoextrusion of molecular nitrogen yields diastereoselectively iminoaziridines in the E configuration[*]. Iminoaziridines are also formed on thermolysis, besides products derived from cycloadducts of fragments like methyl azide or 1,3-diazabutadienes to the exocyclic double bond of 5-alkylidenedihydrotetrazole~[~]. This double bond is extremely electron-rich and polari~ed[~] and hence even adds normally unreactive 1,3-dipoles such as alkyl azidesL6]. Electrophilic azides afford zwitterions, that are intermediates of non-concerted 1,3-dipolar cycloaddition reactions['], or 5-imino-l,4,5,6-tetrahydro-l,2,3,4tetrazines via [3 + 21 cycloaddition followed by extrusion of molecular nitrogen with concomitant ring expansion[*]. With the view of exploiting and extending the rich chemistry of 5-alkylidenedihydrotetrazoles, we embarked on the synthesis of a number of annulated tetrazolium salts of type 1 and 3 which yield the annulated alkylidenedihydrotetrazoles 2 and 4, respectively, on deprotonation[']. Three different routes were envisaged: Alkylation of known annulated tetrazoles, cyclisation by intramolecular N-alkylation, and intramolecular [3 + 21 cycloaddition of o-azidonitrilium salts. The results are reported here. . . N=N X-N=N 1 2 3 2 6 H 7 R -7 G Q 4 R a R \ I N=N \ I N=N X-2 1 3 4Annulated Tetrazoles f114,1s1) and hydrogenation of tetrazolo[ 1,5-a]pyridine The annulated tetrazoles 6a, c, and f, which were needed ( 6~' '~~) .The tetramethyl compound 6e was now obtained as starti...

A Perimidine-Derived Non-Kekule Triplet DiradicalSignificance: A triazaTMM triplet diradical is synthesized and characterized by EPR, confirming theoretical calculations from the authors that diradicals of this type would be high-spin. They generated the compound in several cryogenic matrices by photochemically eliminating dinitrogen. Comment:Many examples of high-spin organic diradicals inspired by TMM have been generated and observed, though generally at low temperatures. In this example, the rigidity of the naphthalene framework forces the nitrogen atoms attached to it farther apart, therefore favoring the triplet state over the singlet state. In the future, diradicals of this type may be used to form organic magnets, though the stability of the diradicals to oxygen and heat will have to be improved. e r a t e d i n a b u t y r o n i t r i l e m a t r i x a t 7 7 K a n d i n A r a n d X e m a t r i c e s a t 4 . 6 K . t r i m e t h y l e n e m e t h a n e ( T M M ) N R R N N R t r i a z a T M M -N 2