Preparation, reactivity, NMR properties and semiempirical MNDO/PM3 structural calculations of 2-azido- and 3-azido-selenophene

摘要

J. CHEM. SOC. PERKIN TRANS. 2 1994 Preparation, Reactivity, N MR Properties and Semiempirical MNDO/PM3 Structural Calculations of 2-Azido- and 3-Azido-selenophene Salo Gronowitz8 and Paolo Zanirato *nb a Division of Organic Chemistry 1, Chemical Center, University of Lund, S-221 00, Lund, Sweden Dipartimento di Cbimica Organica, 'A. Mangini; Universita di Bologna, Viale Risorgimento 4, 1-40736Bologna, Italy The preparation, by azido-transfer reaction, of 2-azido- 1 and 3-azido-selenophene 2 using the appropriate heteroarylithium derivatives and tosyl azide, followed by fragmentation of the intermediate triazene lithium salts is reported here. Different chemical reactivity and kinetic behaviour were observed for azides 1 and 2 in either 1,3-cycloaddition reactions, with (trimethyl- sily1)acetylene and trimethyl(vinyl)silane, or thermal decomposition.Compound 1 gives cyclo- A,A, adducts (silylated triazole or triazoline) ca. three times faster than compound 2. Both the elusive triazoline adducts undergo rapid ring-contraction, with extrusion of nitrogen, to give rise to the corresponding 1 -(selenophenyl)-2-(trimethylsilyl)aziridine (1b, 2b). Kinetic measurements of the unimolecular thermal decompositions afford distinct activation parameters: euro;, = 21.5 and 30.4 kcal mol-',t AS*= -10.7 and -0.9 cal mol-' K-l for 1 and 2, respectively. Experimental data, as a result of geometric and electronic disturbances exerted by the azido-group located at a-or p-positions of the selenophene ring, are qualitatively supported by measurement of 'H, 13C and 77Se N M R chemical shifts.The present experimental evidence and those previously obtained with related 2-azido- and 3-azido-thiophenes are corroborated with the determination of the structures and comparison between 13C substituent chemical shift (SCS) and charge distributions by using a semiempirical computational MNDO/PM3 method. The chemistry of aryl azides has been widely investigated mainly for their synthetic, biological and industrial applic- ations.' However, less attention has been devoted to the pre- paration and the chemistry of five-membered heteroaromatic azides, perhaps because suitable starting azides were not readily A available. Exceptions would be a few heteroaryl azides prepared tfrom heterocycles containing a diazotizable amino group or -N2 halogen derivatives prone to nucleophilic displacement by azido ion.2 In heteroaromatic five-membered rings, metallation (especi- Scheme 1 ally halogen-metal exchange with organolithium derivatives), followed by the reaction of the heteroaryllithium derivatives with opportune electrophiles offers a most convenient route to dependent on the nature of the heteroatom; Scheme 1, pathway many compound^,^ including the formation of carbon-nitrogen b.Therefore, the formal generation of a 2-positioned nitrene bonds4 In fact, we show that heteroaryl azides (and their benzo- generally results in the opening of the former thiophene ring condensed systems) may be achieved easily by reaction of the leading to the formation of the possible building-block 4-cyano appropriate heteroaryllithium with tosyl azide, followed by enethione intermediate A (X = S).fragmentation of the resulting triazene salt. Moreover, the In continuation of this research and in order to detect isolated heteroaryl azides may be easily reduced to the common patterns of behaviour, we now report the syntheses of corresponding amines. 2-azido-1 (2-ASe) and 3-azido-selenophene 2 (3-ASe)J and Recently we reported studies into the reactivity of isomeric their thermal kinetic and chemical behaviour in the presence a-and P-azidothiophenes and benzo-condensed derivatives,6 of silylated dipolarophiles. Thermal activation parameters and together with the preparation, by the same method, of the 'H, '3Cand 77Se NMR spectra are discussed in terms of diverse a-azides of benzofuran and 1 -methylindole.The general trend directing and activating effects produced by the azido-group that emerged from those studies indicated that the formal located at the a-or P-position of the selenophene ring. generation of nitrenes at the a-position of the five-membered Experimental data, are compared with the relative geometries rings generally results in the opening of the ring leading to the and charge distributions of both isomers, calculated theoretic- product. On the ally at MNDO/PM3 level. formation of a 4-cyano-l-hetero-l,3-diene other hand, normal arylnitrene behaviour was exhibited by j3-positioned nitrenes. On the basis of the observed chemical and kinetic reactivities, we suggested that the unimolecular 1Comparative investigations on the aromaticity, based mainly on decomposition and ring-opening of a-azidothiophene would NMR parameters and mesomeric dipole moment, of furan, thiophene, occur in a concerted manner and the process should be selenophene and tellurophene have been carried out.8 Furthermore, the correlation of their reactivity with NMR ('H, I3C and "Se) chemical shifts has been investigated and the directing effects of the selenophene t 1 kcal mol-' = 4.184 kJ mol-' ring have been reviewed.'O J. CHEM. SOC. PERKIN TRANS. 2 1994 N rSiMe3 1 la lb NLSiMe3 2 2a 2b Results and Discussion The azides 1and 2 were obtained by reaction of tosyl azide with the appropriate heteroaryllithium, prepared by using butyl- lithium for direct metallation of selenophene or by halogen- lithium exchange with 3-bromoselenophene, followed by fragmentation of the resulting triazene salts.Compound 1 is less stable than isomer 2 and must be stored in a freezer. Structures of azides 1 and 2 were confirmed by recording their IR, I3C NMR and exact mass spectral data. The mass spectra of these azides are characterized by the significant abundance of the molecular ions showing characteristic isotopic peaks at m/z = 171 and 173. The first mode of fragmentation m/z = 145, corresponding to the loss of the molecular nitrogen mass unit, is common and somewhat reflects the stability of the +corresponding azides.The subsequent C,H,NSe ' species, has been shown to undergo a different kind of fragmentation with elimination of hydrogen cyanide and acetylene (fragment ions at m/z 118 and 119) for azides 1 or 2, respectively. Analogous fragmentations have been observed with aryl azides but these cases generally exhibit contemporary loss of both acetylene and hydrogen cyanide from ArN".'' In our case, the specific formation of neutral HCN and H2C2 might be dependent on the relative elimination rates occurring with the different intermediate C4H3NSe+' and is diagnostic for the different pathways involved in the ion fragmentation mechanisms for selenophenel2 and compounds 1and 2; Scheme 2. The 'H NMR spectra display three typical quartet patterns which may be assigned, on the basis of the two vicinal coupling constants on the selenophene ring J45 = 5.9 and J3, = 3.9 Hz and the separated protons J,, = 1.4 Hz, to azide 1, while the azido group located in the 3-position of 2, gives rise to a vicinal coupling constant of J45 = 5.7 Hz and two separated proton coupling constants J25 = 2.8 and J2, = 1.5 Hz, typical for selenophene rings.g All protons of 1 and 2 are shifted upfield compared with those of selenophene, in particular, in the case of 1 the 3-and 5-positions (AdH = -0.67 and -0.53, respectively) on the ring, are shifted more upfield than the 4-position (AdH = -0.34).On the other hand, the proton located at the 2-position in 2 is more markedly affected (AdH = -0.77) than that of other positions (AdH = -0.26 and -0.17 for 4- and 5-H) (Table 1).In aromatic (or heteroaromatic) systems there is a reasonable correlation between AdH (or Ad,) NMR parameters and the nature of the substituent.I3 Empirically, it can be assumed that the electron densities produced (or modified) by the substituent on the single protons are reasonably well correlated with their observed diamagnetic or paramagnetic shifts. Thus, if a negative value of MH (diamagnetic), determined at paragonable magnetic anisotropy, is taken as an indication of increase in electron density of the proton systems examined, the values we found speak in favour of a major degree of conjugation by the azido group through the aposition of the 2-azidoselenophene 1 compared with the P-position of 3-mh =173 +-m/z = 145I-HCI+ m/z =93 m/z = 118 m/z = 173 Y m/z = 145 HCOSe+- GI+- -H2C2I +*Se$N m/z = 93 m/z = 119 Scheme 2 azidoselenophene 2.As might have been expected, a general concordance with the AdH trend, observed previously for both isomeric azidothiophenes occurred.6g In particular, all the positions of 1 are slightly shifted upfield, compared to 2- azidothiophene. On the other hand, the same comparison for 2, relative to 3-azidothiophene, allows the observation that the 2-and 5-positions appear shifted upfield, but the 4-position does not.* Additional information was gained by studying '3C NMR chemical shifts, especially with para-substituent chemical shifts (para-SCS), which are considered to be related to the total and local 7c charge density of the m~lecule.'~ As expected for nitrogen substituted carbon, both the carbon atoms carrying the azido group are shifted downfield with a larger downfield shift for 1than for 2.Moreover, all the ortho-like carbon atoms are shifted upfield and a large upfield shift was observed also for C-5 l6 (para-like) in compound 1 (Table 1). Thus, qualitative comparison between AdH and Adc for all positions of isomeric azidothiophenes and azidoselenophenes show a similar trend, and the slight differences observed between selenophene and thiophene azido-systems may be speculatively interpreted as a minor aromaticity of the selenophene, but with a greater ability to delocalize charges, with respect to the thiophene ring.I7 The resemblance observed from the SCS and AdH spectro-scopic data between the a-and P-azidoselenophenes and the isomeric azidothiophenes suggests that the delocalized chemical bond of the heteroaromatic nucleus undergoes comparable electrical effects (field/inductive plus resonance) by the canonical structures of the azido group.'* * Detailed 'H, 13C(and 77Se) NMR spectroscopic data of various 2-and 3-substituted thiophenes and selenophenes, including the Swain- Lupton empirical regression analysis, have been previously reported by Gronowitz et al.In addition, the correlation between the relative chemical shifts (AdH, Aamp; or Aamp;,) for the couple thiophene- selenophene have been carried J.CHEM. soc. PERKIN TRANS. 2 1994 Table 1 'H and 13C NMR chemical shifts for selenophene (Se), thiophene (Th) and their a-and P-aides" dH(AH") amp;(ACn) 2-H 3-H 4-H 5-H c-2 c-3 C-4 c-5 Se 8.10 7.33 131.0 129.8 2-N3Se 3-N3Se 7.33 (-0.77) 6.66 (-0.67) 6.99 (-0.34) 7.07 (-0.26) 7.57 (-0.53) 7.93 (-0.17) 146.8 (15.8) 113.1 (-17.9) 117.6 (-12.2) 138.8 (9.0) 128.7 (1.1) 123.9 (-5.9) 124.8(-6.2)131.2 (0.2) Th 7.36 7.13 124.9 126.4 2-N3Th 3-N3Th 6.78 (-0.58) 6.60(-0.53) 6.83 (-0.30) 6.81 (-0.32) 6.90 (-0.46) 7.28(-0.08) 143.2 (18.3) 110.4(-14.5) 119.8 (-6.6) 138.3(11.9) 127.3 (0.9) 121.2(-5.2) 116.0(-8.9) 127.2(2.3) ~ In order to obtain a uniform and reliable basis for these comparison, we used as far as possible 'H and 13C NMR spectral data obtained from solutions (ca.lo, w/w) in deuteriochloroform; 6, values relative to TMS and 6, relative to CDC1, (76.9 ppm with respect to TMS). (AH,,) and (Acn) = 6H(substrate) Or 6C(substrate)l -amp;(reference) Lor 6C(refercncc)l* With the aim of correlating the NMR spectroscopic data with the ground state charge distributions on the various carbon atoms of azides 1and 2, and the parent selenophene, we carried out semiempirical theoretical calculations. Computational MNDO/PM3 molecular geometries of the selenophene and their azido derivatives 1 and 2, were determined by energy minimization using the keyword PRECISE, to allow the lowest gradient possible. We found that the comparison between the structure of the selenophene, determined by this method, and those determined by microwave spectra,lg generally showed an accuracy of the geometrical parameters to less than 2 1.The optimized geometry of selenophene is planar and the nearly colinear azido group showed preferential CCNN cis-planar conformation for 2 and SeCNN trans-planar conformation for 1. Simple HMO-type calculations illustrating geometries and charge distributions have been performed with phenyl azide,lsb but to our knowledge similar treatment of five-membered heteroaryl azides is actually lacking. Of particular interest is the conjugative effect of the azido group located at the a-or P-position on the selenophene portion of the molecules and the x-electron populations with respect to the reaction mechanism pathways whose intermediates I and I1 are depicted on Scheme 3.Charge distribution and bond length data, reported in Fig. 1, suggest that the ground-state structure of ol-azide 1 is close to the canonical structure I, characterized by a greater delocalized n-system due to conjugation with the azido group, unlike P-azides 2 with structure 11. Conformations were made from the above reported geometrical data together with the observation that the Se-C, bond length of 1 is significantly elongated (1.8898A) with respect to that of P-azido derivative 2 (1 3788 A), and conversely, the C-N bond has less double bond character which is indicated by the slight elongation (1.424 24 A) with respect to the C-N bond of derivative 1 (I .397 45 A).The underlying semiempirical calculation includes electronic charge distribution on the various carbons of the selenophene ring, which can be correlated with the sum of the carbon shifts (SCS) produced by the substituent azido group. An illustration of these correlations was obtained by plotting the SCS values determined, for each carbon atom of 1 and 2 against the difference in charge with unsubstituted selenophene. Evidence for satisfactory linear correlations was found for 1 (r = 0.979 with slope rn = 121.8, plot 1) and 2 (r = 0.919 with slope rn =99.3). In the light of these findings, it might be concluded that the degree of polarization of the azido group appears to be directly dependent on the degree of its interaction by resonance with the heteroaromatic substrate.The electron-charge distri- butions are consistent with contributions from the expected structures where the effects appear more pronounced at the a-position than the P-position of the heteroaryl ring (I and 11, respectively) (Scheme 3). A similar conclusion was reached by the consideration of Arrhenius activation parameters, determined from the first- I I1 Scheme 3 1.44666 -0.09763 -0.06386 1.44803 -0.10575 1.365350 bsol; 1.37795 -0.2433!3 -0.1 0758 -0.26218 (x1.266751 1.88992 Se 1.8md N =N =N 0,15427 0.21438 1.41633 CNN = 122"500 1.126247 SeC NN N tram 1.12206 1.26867 1 1.42424 bsol;/N=N-N -0.23272 -0.36156 0.18261 CNN = 122"029 CCNNN cis Fig.1 Significant geometric data determined by MNDO/PM3, conformation and charge distribution for selenophene and their isomeric azido derivatives order rate constants of thermolyses of azidoselenophenes 1and 2. Thermal decompositions were carried out in an inert solvent, such as p-chlorotoluene, at concentration ranges within 2-50 mmol dmP3 by measuring the variation in intensity of the strong IR asymmetric stretching band (ca. 2100 cm-') of the azido- group as a function of time. The results, collected in Table 2, show first-order kinetic rates, in agreement with the most important step in decomposition of azides, which is the formation of nitrene and nitrogen. However, a unimolecular process may also arise from a concerted rearrangement with elimination of nitrogen.In this case the activation parameters should be characterized by lower energy and larger negative entropy of activation. In fact, the calculated activation parameters (E, =21.5 kcal mol-', ASs = -10.7 cal mol-' IC') for 1 are remarkably different from E, = 30.4 kcal mol-' and ASt = -0.9 cal mol-' K-' for 2, and both are comparable with those previously reported for the thermolyses of corresponding Table 2 First-order rate constants and activation parameters for the thermolyses of the selenophenyl azides 1 and 2 in p-chlorotoluene AS'jcal hide T/"C kilo4 s-' E,/kcal mol-' mol-' K-' 1 32.0 0.43 f 0.01 1 52.0 4.36 f 0.07 21.5 f 0.1 -10.7 f 1.0 1 75.0 34.30 f 0.11 2 99.0 0.15 f 0.01 2 122.0 2.31 k 0.06 30.4 k 0.1 -0.9 f 0.6 2 139.5 8.67 k 0.12 2-azido- and 3-azido-thiophene (E, = 22.6 and 30.6 kcal mol-', ASs = -8.2 and -0.7 cal mol-' K-', respectively).6g Our experimental findings, obtained from the present study of thermal reactivity of azidoselenophenes, as well as previously observed azidothiophenes, are interpreted in terms of different reorgahization of the azido group by conjugation of the 7c-electron involved in the a-or P-position of the five-membered heteroaryl systems.This is reflected in the preferred conjugation of the carbon-bonded nitrogen atom in structure I, characterized by the C-N bond with more double bond character than that of structure 11. Thus, the bent structure which results, favoured by conjugation of the azido-group in compound 1, should enhance the reactivity of its 1,3- cycloaddition to carbon-carbon double (or triple) bonds.As expected, the azidoselenophene 1 reacts ca. three times faster than 2 with neat (trimethylsily1)acetylene or trimethyl(viny1)- silane (TMVS) affording the corresponding triazoles la and 2a, or aziridines lb and 2b, respectively. Similarly to that previously observed for 1,3-~ycloadditions to silylated dipolarophiles, chosen as a probe for the reactions with some heteroaryl azides,6d*f*g aziridines lb and 2b presum-ably arise from a primary triazoline adduct which undergoes ring-contraction with expulsion of nitrogen. Structural assign- ments of all new compounds la, 2a, lb and 2b were based on IR, 'H NMR and exact mass spectroscopic data (or elemental analyses).The '3C NMR spectroscopy provided evidence for the orientation of additions of azides 1 and 2 to the terminal alkyne by recording the off-resonance proton decoupled spectra, which displayed a doublet at 6 127.98 (J 191.7 Hz) and 6 127.79 (J 193.0 Hz), assignable to C-5, and a singlet at 6 148.12 and 143.38, assignable to C-4 of the triazole rings la and 2a, respectively. As might be expected, according to the lower nucleophilic nature of an alkyne with respect to the corresponding alkene,20 reactions of azides 1 and 2 with (trimethylsily1)acetylene were slightly slower than those with TMVS. In the light of these findings together with our previous studies on the reactivity of the heteroaryl azides, especially azidothiophenes, with silylated olefins, it might be concluded that the rate of 1,3-~ycloadditions is strongly dependent on the degree of polarization of the azido-group, as might be expected for concerted reactions initiated by the junction of the terminal azido-nitrogen to the nucleophilic a-carbon of the terminal alkene (or alkyne).The degree of polarization of the azido- group appears to be directly dependent on the degree of its interaction by resonance with the heteroaromatic substrate. In agreement with this consideration, the greater reactivity of the 2-azido-derivatives, compared with the 3-azido-derivatives, supports a more effective conjugation for the azido group on the a-position than on the P-position of the selenophene ring.J. CHEM. soc. PERKIN TRANS. 2 1994 previously reported for related heterocycles. '-'The metall- ations were made by reaction with butyllithium and the regioselectivity was obtained by direct reaction with seleno- phene for compound 1 and by metal-halogen exchange with 3-bromoselenophene for compound 2. (Trimethylsily1)acetylene and TMVS were purchased from Aldrich Chimica Italiana. Tosyl azide,2 ' 2,4,5-tribromoseleno-phene and 3-bromoselenophene 22 were prepared as described in the literture. IR spectra were recorded with a Perkin-Elmer Model 298 spectrophotometer. 'H, I3C and 77Se NMR data for compounds 1 and 2 were obtained with Varian XL 300 or Varian Gemini 200 instruments for solutions in CDCl,; Jvalues are given in Hz and long-range coupling constants were not determined.Mass spectra were recorded on a JEOL SX-C02 instrument. 2-Azidoselenophene 1.-A solution of selenophene (0.066 mol) in dry diethyl ether (100 an3)was added with stirring under nitrogen at room temp. to butyllithium (2.2 mol dm-, in cyclohexane; 33 cm3). The reaction mixture was stirred and heated under reflux for an additional 10 min, after which it was cooled to -70 "C and added dropwise to a solution of tosyl azide (0.07 mol) in dry diethyl ether (100 an3).After the addition was complete the resulting mixture was stirred and allowed to reach 0 OC within 5 h. The pale-yellow triazene salt which had formed was rapidly filtered off and suspended in pentane.The suspension was treated at 0 OC with a solution of tetrasodium pyrophosphate (0.07 mol) in water (200 cm3) and after 10 min with a solution of ammonium chloride (0.07 mol) in water (100 cm3). The yellow pentane layer, which had formed, was collected and the excess of solvent eliminated under vacuum to give a residue which was chromatographed on a 'Florisil' column using pentane as eluent. Chromatography gave the title azide 1 (0.022 mol, 33) as an unstable oil; v,,,/cm-' 3100, 3080, 2100 (-N3), 1500, 1220 and 670; 6H(300 MHz; CDCl3)7.57(1H,dd,J1.4and5.9,5-H),6.99(1H,dd,J3.9and 5.9, 4-H) and 6.66 (1 H, dd, J 1.4 and 3.9, 3-H); 6,(300 MHz; CDCl3) 146.80 (C-2, s), 128.70 (C-4, J 165.5), 124.86 (C-5, J 190.2) and 117.60 (C-3, J 165.8); m/z 173 (M', 22.5), 145 (69.5,M -N2), 118(68.0),93(30.5),84(33.5),64(100.0)and 39 (56.5) (Found: M', 172.9490.C,H,N,Se requires M, 172.9492). 3-Azidoselenophene 2.-A solution of 3-bromoselenophene (0.044 mol) in dry diethyl ether (80 cm3) was added with stirring under nitrogen at -70 "C to butyllithium (2.2 mol dm-, in cyclohexane; 20 cm3). The reaction mixture was stirred for an additional 10 min, after which it was added dropwise to a solution of tosyl azide (0.044 mol) in dry diethyl ether (100 cm3). After the addition was complete the resulting mixture was stirred and allowed to reach 0 "C within 5 h. The pale-yellow triazene salt which had formed was filtered off rapidly and suspended in pentane.The suspension was treated at 0 "C with a solution of tetrasodium pyrophosphate (0.07 mol) in water (200 cm3). The yellow pentane layer was collected and the excess of solvent eliminated under vacuum to give a residue which was chromatographed on a 'Florisil' column using pentane as eluent. Chromatography gave the title azide 2 (0.023 mol, 52) as an oil; v,,,/cm-' 2980, 2100 (-N,), 1530, 1370, 1200 and 750; 6,(300 MHz; CDCl,) 7.93 (1 5-H, dd, J 2.7 and 5.6), 7.33 (1 2-H, dd, J 1.5 and 2.7) and 7.07 (1 4-H, dd, J 1.5 and 5.6); dC(300 MHz; CDCl,) 138.81 (C-3, s), 131.19 (C-5, J 189.2), 123.88 (C-4, J 167.7) and 113.10 (C-2, J 186.2); m/z 173 (M', 41.3), 145 (92.5, M -N2), 119 (loo), 93 (75.0), 80 (18.5), 64 Experimental (7.2), 52 (14.6) and 39 (9.5) (Found: M+, 172.9493).Materials.-2-Azido, 1 and 3-azido-selenophene 2 were Reactions of Azides 1 and 2 with (Trimethylsi1yl)acetylene and prepared by reacting the corresponding heteroayllithium Trimethyl(viny1)silane (TMVS) at 25 "C.-Generalprocedure. A derivative with tosyl azide mostly following the procedure solution of the azidoselenophene (0.5 mol dm-3) in neat J. CHEM. soc. PERKIN TRANS. 2 1994 (trimethylsily1)acetylene or TMVS was allowed to react in a sealed tube at 25deg;C and in the dark until TLC showed the absence of the starting azide. The residue obtained after careful elimination of the excess of silylated alkyne (or alkene) under vacuum consisted of almost pure triazoles la, 2a and aziridines lb, 2b which were characterized.Approximate reaction times at 25 "C and product yields for the reactions of azides 1 and 2 with silylated alkyne (or alkene) are reported in parentheses. The following new triazoles la, 2a and aziridines lb, 2b were obtained. 1-(Selenophen-2-yl)-4-(trimethylsilyl)-l,2,3-triazole la (4 days, 83), m.p. 42-43 "C; v,,/cm-' 3120, 2960, 1250, 845 (SiMe,), 760 and 680; 6amp;00 MHz; CDCl,) 7.87 (1 H, s, 5'-H), 7.81 (1 H, dd, J 1.5 and 5.9,5-H), 7.29 (1 H, dd, J 1.3 and 4.0, 3-H), 7.21 (1 H, dd, J4.0 and 5.9, 4-H) and 0.36 (9 H, s); dc(200 MHz; CDCl3) 148.12 (C-4', s), 130.87 (C-2, s), 128.65 (C-5,d, J193.6), 128.57(C-4,d, J176.3), 127.79(C-5',d, J193.0), 119.20 (C-3, d, J 167.2) and -0.70 (9, J 119.4); m/z 243 (18.6, M -N2), 229 (15.2), 228 (loo), 177 (8.1), 138 (10.2), 118 (lO.l), 93 (10.3), 83 (90.2), 80 (5.8), 73 (98.5), 45 (44.9) and 43 (70.7) (Found: C, 39.95; H, 4.8; N, 15.5.C,H,,N,SeSi requires C, 40.0; H, 4.85; N, 15.55). 1-(Selenophen-3-yl)-4-(trimethylsilyl)-1,2,3-triazole 2a (1 6 days, 87), m.p. 72-73 "C; v,,,/cm-' 3120-3100, 2960, 1260 and 855 (SiMe,); 6H(200 MHz; CDC1,) 8.14 (1 H, dd, J 1.6 and 2.8, 2-H), 8.11 (1 H, dd, J2.8 and 5.5, 5-H), 7.86 (1 H, s, 5'-H), 7.78 (1 H, dd, J 1.6 and 5.5, 4-H) and 0.36 (9 H, s); dc(200 MHz; CDCl3) 143.38 (C-4', s), 138.07 (C-3, s), 132.37 (C-5,d, J190.7), 127.98(C-5',d, J191.7), 124.62(C-4,d, J173.0), 119.18 (C-2, d, J 188.7) and -0.67 (4, J 119.4), m/z 271 (M', 0.9), 243 (14.8, M -NZ), 228 (loo), 224 (17.0), 200 (7.3), 186 (4.1), 177 (4.1), 138 (6.2), 123 (8.1), 93 (6.3), 84 (26.8), 73 (36.6), 45 (22.4) and 43 (30.2) (Found: C, 40.0; H, 4.85; N, 15.5).1-(Selenophen-2-yl)-2-(trimethylsilyl)aziridine lb (3 days, 77), as an oil; vmax/cm-' 2960, 1250, 850 (SiMe,) and 750; 6,(200 MHz; CDCl,) 7.35 (1 H, dd, J 1.3 and 5.9, 5-H), 6.92 (1 H, dd, J3.8 and 5.9, 4-H), 6.51 (1 H, dd, J 1.3 and 3.8, 3-H), 2.30 (1 H, dd, J 1.4 and 5.2), 2.21 (1 H, dd, J 1.4 and 7.9), 1.41 (1 H, dd, J 5.2 and 7.9) and 0.12 (9 H, s); m/z 245 (M', 4.0), 230 (0.8, M -CH,), 177 (0.4), 145 (3.5), 118 (2.4), 100 (10.0), 85 (12.2), 73 (loo), 59 (24.7) and 45 (35.4) (Found: M', 245.0139. C,H,,NSeSi requires M, 245.0139). 1-(Selenophen-3-yl)-2-(trimethylsilyl)aziridine 2b (11 days, 82), as an oil; v,,,/cm-' 2965, 1260, 850 (SiMe,) and 760; 6amp;00 MHz; CDCl,) 7.79 (1 H, dd, J 2.6 and 5.6, 5-H), 7.18 (1 H, dd, J 1.5 and 5.6,4-H), 6.93 (1 H, dd, J 1.5 and 2.6, 3-H), 2.12 (1 H, dd, J 1.3 and 5.0), 2.07 (1 H, dd, J 1.3 and 7.7),.1.26 (1 H,dd, J5.0and7.7)andO.ll (9H,s);m/z245(M+,9.9), 230 (5.2, M -CH,), 189 (7.8), 146 (6.2), 131 (2.2), 107 (5.2), 93 (1.4), 73 (loo), 59 (8.9) and 45 (12.5) (Found: M+, 245.0139). Rates of Decompositionof Azides 1 and 2.-A solution of the azide (0.05 mol dm-,) in p-chlorotoluene (ca.10 cm3) was allowed to react in a thermostatic bath at the appropriate range of temperature (32.0-75.0 "C for azide 1 or 99.0-139.5 "C for 2). The rates of decomposition of the azides as a function of time were determined by IR spectroscopic measurement of the neat N, band (ca.2100 cm-l) of spaced aliquots of solution (0.5 cm3). Results are summarized in Table 2. 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