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J. Org. Chem. 2000, 65, 3612-3619
Cheletropic Decomposition of Cyclic Nitrosoamines Revisited: The Nature of the Transition States and a Critical Role of the Ring Strain† Gennady V. Shustov and Arvi Rauk* Department of Chemistry, The University of Calgary, 2500 University Drive, N.W., Calgary, Alberta, Canada T2N 1N4 Received November 5, 1999
The cheletropic decompositions of 1-nitrosoaziridine (1), 1-nitroso-∆3-pyrroline (2), 7-nitroso-7azabicyclo[2.2.1]hepta-2,5-diene (3), and 6-nitroso-6-azabicyclo[2.1.1]hexa-4-ene (4) have been studied theoretically using high level ab initio computations. Activation parameters of the decomposition of nitrosoaziridine 1 were obtained experimentally in heptane (∆Hq298 ) 18.6 kcal mol-1, ∆Sq298 ) -7.6 cal mol-1 K-1) and methanol (20.3 kcal mol-1, 0.3 cal mol-1 K-1). Among employed theoretical methods (B3LYP, MP2, CCD, CCSD(T)//CCD), the B3LYP method in conjunction with 6-31+G*, 6-311+G**, and 6-311++G(3df,2pd) basis sets gives the best agreement with experimental data. It was found that typical N-nitrosoheterocycles 2-4 which have high N-N bond rotation barriers (>16 kcal mol-1) extrude nitrous oxide via a highly asynchronous transition state with a planar ring nitrogen atom. Nitrosoaziridine 1, with a low rotation barrier ( 4 (12) kcal mol-1. The relative strain energies increase in the same order: 2 (0 kcal mol-1) , 3 (39 kcal mol-1) < 4 (52 kcal mol-1). Comparison of the relative energies of 2-4 and their transition states on a common scale where the energy of nitrosopyrroline 2 is assumed as reference indicates that the thermal stability of the cyclic nitrosoamines toward cheletropic decomposition is almost entirely determined by the ring strain. Introduction
Scheme 1
Secondary N-nitrosoamines are notorious carcinogens and mutagens that can be seriously hazardous to human health.1a-e These compounds can enter from a polluted environment1d or they can be formed in vivo in a reaction of nitrous acid with a secondary amine. For instance, nitrite-cured food2 can be a source of nitrous acid, and NH-heterocycles frequently used as drug components can play the role of the amine reactant. Carcinogenic nitrosoamines undergo metabolic activation, the key step of which is R-hydroxylation catalyzed by the cytochrome P450 dependent enzyme system.1e However, some nitrosoheterocycles are thermally unstable to cheletropic decomposition,3 and may be transformed to the relatively harmless products, N2O and alkene, by a nonenzymatic route (Scheme 1). N-Nitroso-substituted aziridines (such † Dedicated to Professor Remir G. Kostyanovsky on the occasion of his 65th birthday. * To whom correspondence should be addressed. Phone: (403) 2206247. Fax: (403) 289-9488. E-mail:
[email protected]. (1) (a) Hasegawa, R.; Futakuchi, M.; Mizoguchi, Y.; Yamaguchi, T.; Shirai, T.; Ito, N.; Lijinsky, W. Cancer Lett. 1998, 123, 185. (b) Lijinsky, W. ACS Symposium Series 1994, 553, 250. (c) Lijinsky, W. Chemistry and Biology of N-Nitroso Compounds; Cambridge University Press: Cambridge, U.K., 1992. (d) Fishbein, L. Potential Industrial Carcinogens and Mutagens; Elsevier Scientific Publishing Company: New York, 1979. (e) Bellec, G.; Goasduff, T.; Dreano, Y.; Menez, J.-F.; Berthou, F. Cancer Lett. 1996, 100, 115. (2) Cornforth, D. In Nitric Oxide. Principles and Actions; Lancaster, J., Jr., Ed.; Academic Press: San Diego, 1996; pp 259-287. (3) (a) Woodward, R. B.; Hoffmann, R. Angew. Chem., Int. Ed. Engl. 1969, 8, 781. (b) Mock, W. L. In Pericyclic Reactions; Marchand, A. P., Lehr, R. E., Eds; Academic Press: New York, 1977; Vol. II.
as 1) and larger unsaturated heterocycles (like 2-6) are candidates for this alternative decomposition pathway but not all undergo it. In principle, an “orbital symmetry allowed” pathway can always be found in a cycloelimination reaction, with a predictable alteration of the stereochemical course
10.1021/jo9917341 CCC: $19.00 © 2000 American Chemical Society Published on Web 05/16/2000
Cheletropic Decomposition of Cyclic Nitrosoamines Chart 1
depending on the number of endocyclic π-bonds.3 However, while stereochemical integrity is well maintained in a few cases, such as SO2 elimination from cyclic unsaturated sulfones, interpretation of most cheletropic reactions in terms of orbital symmetry control is problematic. Three-membered rings appear to decompose readily by a “forbidden” pathway, the situation is less clear for other leaving groups, and activation barriers for “forbidden” pathways are often just a few kcal mol-1 higher than “allowed” pathways.3b Nitrous oxide elimination from compounds such as nitrosoheterocycles 1-6 is a case in point. Previous experimental studies4-8 of the thermal stability of compounds 1, 2, 5, 6 suggests that not all such classes of nitrosoheterocycles are able to undergo fast cheletropic fragmentation. Rapid decomposition was observed only for nitrosoaziridine 14a,b and its derivatives,4c,d and for 7-nitroso-7-azabenznorbornadiene 55 (benzo derivative of 3), at a temperature below 0 °C and at 45 °C, respectively. On the other hand, 1-nitroso-∆3-pyrroline (2)6 and 9-nitroso-9-azabicyclo[4.2.1]nona-2,4,7-triene (6)7 resist the cycloelimination of N2O. Nitrosopyrroline 2 was less than half decomposed by heating for 1 h at 200 °C.6 From the main products of thermolysis6 of 2, pyrrole, and a polymer, one can conclude that the decomposition does not proceed by the cheletropic pathway. Similarly, bicycle 6 gave7 a polymer and only traces of N2O after 30 min at 198 °C. Attempts to rationalize the nature of transition states for cheletropic fragmentation of cyclic nitrosoamines such as 1, 2, 5, and 6 have a long history.3-8 Woodward and Hoffmann3a proposed two alternative cheletropic transition states for nitrosoaziridine 1, one with a planar configuration in which the departure of the NNO fragment proceeds by a nonlinear pathway in the plane of the three-membered ring (Chart 1a), and an alternative (4) (a) Bumgardner, C. L.; McCallum, K. S.; Freeman, J. P. J. Am. Chem. Soc. 1961, 83, 4417. (b) Rundel, W.; Mu¨ller, E. Chem. Ber. 1963, 96, 2528. (c) Clark, R. D.; Helmkamp, G. K. J. Org. Chem. 1964, 29, 1316. (d) Berlin, K. D.; Williams, L. G.; Dermer, O. C. Tetrahedron Lett. 1968, 873. (5) Carpino, L. A.; Barr, D. E. J. Org. Chem. 1966, 31, 764. (6) McGregor, S. D.; Lemal, D. M. J. Am. Chem. Soc. 1966, 88, 2858. (7) Mock, W. L.; Isaac, P. A. H. J. Am. Chem. Soc. 1972, 94, 2749. (8) Freeman, J. P.; Graham, W. H. J. Am. Chem. Soc. 1967, 89, 1761.
J. Org. Chem., Vol. 65, No. 12, 2000 3613
mode which requires prior rotation about the N-N bond (Chart 1b). Freeman and Graham8 and, subsequently, Mock and Isaac3b,7 believed that the ability of a cyclic nitrosoamine to adopt the antiperiplanar (ap) conformation about the N-N bond with a pyramidal ring nitrogen (Chart 1c) is a key factor for the extrusion of N2O. In their opinion, the ap conformation is most appropriate for the molecular orbital correlation between the initial nitrosoheterocycle and nitrous oxide. Adoption of such a conformation for the unstrained nitrosoamines 2 and 6 would entail an energy cost of ca. 23 kcal mol-1, the estimated value3b,7 of the barrier for rotation about the N-N bond in 2 and 6. This places a minimum value for the barrier hindering cheletropic decomposition of unstrained cyclic nitrosoamines. However, the ap conformation should be accessible for nitrosoaziridine 1,3b,7 which could have a pyramidal configuration of the ring nitrogen atom and a low rotation barrier due to the weakened nN-π*NO conjugation. Mock and Isaac3b,7 considered the influence of the ring strain on the rate of the cycloelimination of N2O from nitrosoaziridine 1, but regarded the stereoelectronic factor to be dominant. Later, the assumptions about the pyramidal configuration of nitrosoaziridines and their relatively low barriers for the rotation (ca. 4 kcal mol-1)9c and inversion (ca. 8 kcal mol-1)9c were confirmed experimentally9a-c and theoretically.9c,d However, there are no such data for compounds 2-6 in the literature. No theoretical studies of the structure and energy of transition states for the extrusion of N2O from nitrosoheterocycles that are formally capable of cheletropic fragmentation have been reported. We undertook the current high level computational study of cyclic nitrosoamines 1-4 in order to answer two questions: Is the ability to adopt the ap conformation about the N-N bond a crucial factor for the cheletropic decomposition of nitrosoheterocycles? What is the role of ring strain in influencing the propensity of nitrosoheterocycles to undergo cheletropic fragmentation? In answer to the first question, the structures of the ground states of nitrosoheterocycles 1-4 were studied, and their rotation and inversion barriers were calculated. Then the structures and energetics of the transition states for cheletropic decomposition were determined. An answer for the second question was obtained from calculations of isodesmic reactions, in which the leaststrained nitrosopyrroline 2 was used as the reference compound. In the experimental part, we measured kinetics of the decomposition of nitrosoaziridine 1 in heptane and methanol within the temperature range from 10 to 36 °C in order to obtain values of the activation energy and enthalpy. These values were needed for verification of theoretical data and choosing the most economical and, at the same time, accurate method for calculation of the activation barriers for the cheletropic fragmentation of cyclic nitrosoamines. (9) (a) Bandmann, H.; Heymanns, P.; Siem, C.; Rademacher, P. Angew. Chem., Int. Ed. Engl. 1984, 23, 363. (b) Rademacher, P.; Irsch, G.; Sicking, W.; Wurthwein, E.-U. J. Mol. Struct. 1989, 197, 291. (c) Shustov, G. V.; Kachanov, A. V.; Kadorkina, G. K.; Kostyanovsky, R. G.; Rauk, A. J. Am. Chem. Soc. 1992, 114, 8257. (d) Kirste, K.; Rademacher, P. J. Mol. Struct. 1981, 73, 171.
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Table 1. Potential Energy Barriersa (kcal mol-1) for the Rotations about the N-N Bond and the Inversion of the Ring Nitrogen in Nitrosoheterocycles 1-4 compound theoret level 1 B3LYP/6-31+G* B3LYP/6-311+G** 2 B3LYP/6-31+G* B3LYP/6-311+G** a
ring N inversion (TSinv)
syn rotation about N-N bond (sp-TSrot)
anti rotation about N-N bond (ap-TSrot)
5.8 5.8
8.8 8.7
5.1 4.8
0.0 0.0
29.4 29.0
24.4 23.8
compound theoret level 3 B3LYP/6-31+G* B3LYP/6-311+G** 4 B3LYP/6-31+G* B3LYP/6-311+G**
ring N inversion (TSinv)
syn rotation about N-N bond (sp-TSrot)
anti rotation about N-N bond (ap-TSrot)
0.7 0.6
27.7 26.8
18.7 18.2
2.8 2.7
25.3 24.8
16.7 16.3
Relative to the ground states, including 0.98 × ∆ZPVE.
Table 2. Relative Energiesa (kcal mol-1) of the Initial Compound, Products, and Transition States for the Cheletropic Decomposition of 1-Nitrosoaziridine (1) species
B3LYP/Ab
B3LYP/Bb
B3LYP/Cb
MP2/Ab
MP2/Bb
CCSD(T)/A//CCD/Ab
1 sp-TSlin-1 ap-TSlin-1 TSnonlin-1 CH2dCH2 + N2O
0.0 20.9 42.1 49.0 -27.2
0.0 20.4 41.3 47.8 -31.2
0.0 20.4 41.7 48.4 -32.3
0.0 25.8 53.5 46.3 -35.0
0.0 26.9 54.5 43.6 -36.7
0.0 22.6 46.8 47.8 -27.3
a Including 0.98 × ∆ZPVE and 0.967 × ∆ZPVE for the B3LYP and MP2 [CCSD(T)] calculations, respectively. b Basis set “A” is 6-31+G(d), “B” is 6-311+G(d,p), and “C” is 6-311++G(3df,2pd).
Table 3. Calculated Activation Parametersa for the Cheletropic Decomposition of 1-Nitrosoaziridine (1) via the Lowest Energy Transition State sp-TSlin-1 level
∆Sq
∆Hq
∆Gq
B3LYP/6-31+G* B3LYP/6-311+G** B3LYP/6-311++G(3df,2pd) MP2/6-31+G* MP2/6-311+G** CCSD(T)/6-31+G*//CCD/6-31+G*
2.7 2.5 2.8 3.8 5.8 2.7
21.1 20.6 20.6 26.2 27.2 22.1
20.3 19.9 19.8 25.0 25.5 21.3
a
Table 4. Experimental Rate Constants and Activation Parametersa for the Cheletropic Decomposition of 1-Nitrosoaziridine (1) T (K) 283 285 287 289 291 293 295 297 299 301 303 305 307 309
At 298 K; ∆Sq in cal mol-1 K-1, ∆Hq and ∆Gq in kcal mol-1.
Computational Methods All ab initio calculations presented here were performed with the Gaussian 9410a and Gaussian 9810b systems of programs. The geometry optimizations were carried out at various levels of theory including the B3LYP, MP2, and CCD methods with the internal 6-31+G*, 6-311+G**, and 6-311++G(3df,2pd) basis sets, using procedures implemented in the Gaussian molecular orbital packages. The CCD/6-31+G* energies were refined by single-point calculations at the CCSD(T)/631+G* level of theory. The methods and basis sets used for the calculations of each specific reaction system are shown in Tables 1-6. Vibrational frequency calculations were performed on all stationary points at the B3LYP (10) (a) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Gill, P. M. W.; Johnson, B. G.; Robb, M. A.; Cheeseman, J. R.; Keith, T.; Petersson, G. A.; Montgomery, J. A.; Raghavachary, K.; Al-Laham, M. A.; Zakrewski, U. G.; Ortiz, J. V.; Foresman, J. B.; Cioslowski, J.; Stefanov, B. B.; Nanayakkara, A.; Challacombe, M.; Peng, C. Y.; Ayala, P. Y.; Chen, W.; Wong, M. W.; Anders, J. L.; Replogle, E. S.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Binkley, J. S.; Defrees, D. J.; Baker, J.; Stewart, J. P.; Head-Gordon, M.; Gonzalez, C.; Pople, J. A. Gaussian 94, Revisions D.2 and E.2, Gaussian, Inc., Pittsburgh, PA, 1995. (b) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.; Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.; Ortiz, J. V.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Andres, J. L.; Gonzalez, C.; Head-Gordon, M.; Replogle, E. S.; Pople, J. A. Gaussian 98, Revisions A.3 Gaussian, Inc., Pittsburgh, PA, 1998.
283 285 287 289 291 293 295 297 299 301 303 305
b
104 × k (s-1)
∆GqT (kcal mol-1)
In Heptane 4.76 ( 0.05 20.83 6.51 ( 0.06 20.80 7.60 ( 0.04 20.86 10.0 ( 0.1 20.86 11.96 ( 0.05 20.90 17.4 ( 0.3 20.83 19.99 ( 0.07 20.90 23.0 ( 0.1 20.96 30.2 ( 0.1 20.94 36.8 ( 0.3 20.97 44.0 ( 0.8 21.00 57.4 ( 0.9 20.98 76.8 ( 0.5 20.95 86.5 ( 0.9 21.01 Eab ) 19.2 ( 0.6, ln Ab ) 26.6 ( 1.1, ∆Hq ) 18.6 ( 0.6, ∆Sq ) -7.6 ( 2.2 In Methanol 12.3 ( 0.1 20.29 18.34 ( 0.07 20.22 22.2 ( 0.3 20.25 29.5 ( 0.4 20.23 36.5 ( 0.3 20.26 41.0 ( 0.5 20.33 59.5 ( 0.4 20.26 86 ( 2 20.18 103.0 ( 0.7 20.21 122 ( 2 20.25 148 ( 3 20.27 184 ( 3 20.28 Eab ) 20.9 ( 1.1, ln Ab) 30.6 ( 2.0, ∆Hq ) 20.3 ( 1.1, ∆Sq ) 0.3 ( 3.9
a At 298K; ∆Sq in cal mol-1 K-1, E , and ∆Hq in kcal mol-1. a As estimated from a ln k versus [(1/T), K-1] plot.
and MP2 levels, and transition structures were characterized by a single imaginary frequency, whereas initial compounds and products had none. Zero-point corrections (∆ZPVE) were scaled by 0.98 for the B3LYP and by 0.967 for the MP2 level.11 In the cases of the MP2, CCD, and CCSD(T) calculations, the frozen core approximation was used. Total energies and ZPVEs of all species are given in Tables S-1 to S-5 of the Supporting Information. (11) Scott, A. P.; Radom, L. J. Phys. Chem. 1996, 100, 16502.
Cheletropic Decomposition of Cyclic Nitrosoamines
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Table 5. Relative Energiesa (kcal mol-1) of the Initial Compound, Products, and Transition State for the Cheletropic Decomposition of 1-Nitroso-∆3-pyrroline (2) species
B3LYP/ 6-31+G*
B3LYP/ 6-311+G**
MP2/ 6-31+G*
2 TS-2 SP-2b cis-butadiene + N2O
0.0 58.2 74.4 4.4
0.0 56.8 72.7 0.4
0.0 63.8 84.7 1.3
a Including 0.98 × ∆ZPVE and 0.967 × ∆ZPVE for the B3LYP and MP2 calculations, respectively. b A second-order saddle point obtained under Cs symmetry constraint.
Table 6. Relative Energiesa (kcal mol-1) of the Initial Compounds, Products, and Transition States for the Cheletropic Decomposition of 7-Nitroso-7-azabicyclo[2.2.1]hepta-2,5-diene (3) and 6-Nitroso-6-azabicyclo[2.1.1]hex-4-ene (4) species
B3LYP/ 6-31+G*
B3LYP/ 6-311+G**
3 TS-3 benzene + N2O 4 TS-4 cyclopentadiene + N2O
0.0 18.3 -71.8 0.0 12.6 -49.3
0.0 17.6 -74.3 0.0 12.0 -53.5
a
Including 0.98 × ∆ZPVE.
The intrinsic reaction coordinate (IRC)12 path was traced in order to check the B3LYP/6-31+G* energy profiles for all cheletropic fragmentations. Both singlet and triplet stabilities of the RB3LYP and RHF solutions for the B3LYP and MP2 transition structures have been checked.13 Heat capacities and entropy corrections were made using scaled B3LYP and MP2 frequencies and standard statistical procedures within the rigid rotatorharmonic oscillator model14 to determine enthalpies and free energies for the cheletropic decompositions of nitrosoaziridine 1 at 289K. Throughout the text, bond lengths are in angstroms and bond angles in degrees.
Experimental Section 1-Nitrosoaziridine (1). A solution of ClNO (0.0327 g, 0.5 mmol) in absolute heptane (1 mL) was added to aziridine (0.0431 g, 1 mmol) in absolute heptane (4 mL) with cooling (-78 °C) and stirring. After 15 min at -40 to -30 °C, the precipitate was filtered at the same temperature. The resulting solution was kept at -78 °C and was used for preparation of heptane and methanol solutions with an initial concentration of 2 × 10-2 M of 1 for kinetic measurements. Kinetics. The progress of the decomposition of nitrosoaziridine 1 was followed at 454 nm in a thermostatically controlled spectrophotometer cell ((0.1 °C, 1 cm). Rate constants were extracted from a single-exponential trace of the change in absorbance vs time. Activation parameters were obtained from the slope and intercept on the ordinate of the best straight line plot of ln k vs T-1. The rate constants and activation parameters are given in Table 4 along with the error margins at the 95% confidence limit.
Results and Discussion Structure and Intramolecular Dynamics of Cyclic Nitrosoamines 1-4. The ring nitrogen atom is (12) (a) Gonzalez, C.; Schlegel, H. B. J. Phys. Chem. 1990, 94, 5523. (b) Gonzalez, C.; Schlegel, H. B. J. Phys. Chem. 1991, 95, 5853. (13) (a) Schlegel, H. B.; McDouall, J. J. W. In Computational Advances in Organic Chemistry: Molecular Structure and Reactivity; O ¨ gretir, C., Csizmadia, I. G., Eds.; Kluwer: Dordrecht, 1991; p 167. (b) Chen, W.; Schlegel, H. B. J. Chem. Phys. 1994, 101, 5957. (14) McQuarrie, D. A. Statistical Thermodynamics; Harper & Row: New York, 1973.
Figure 1. B3LYP/6-31+G* structures of nitrosoaziridine 1 and its transition states for the cheletropic fragmentation and graphic representations of the potential energy curves for the linear (a,b) and nonlinear pathways (c) of this fragmentation derived with the IRC-B3LYP/6-31+G* calculations. The IRC relative energies are not ZPVE corrected.
pyramidal in optimized structures of the ground states of nitrosoheterocycles 1, 3, and 4 (Figures 1, 3, 4, S-1, S-3, S-4, Table S-6). For nitrosopyrroline 2, the B3LYP method predicts a planar configuration (Figures 2,S-2), while the MP2 method yields a slightly pyramidal one. The out-of-plane angle (angle R in Figures 1-4, S-1 to S-4, Table S-6) between the N-N bond and the C1N1C2 plane is a good measure of the pyramidality of the ring nitrogen. For the MP2 structure of nitrosopyrroline 2, R ) 10.4° and for the B3LYP structure R ) 0.0°. Nitrosoaziridine 1 is the most nonplanar (R ) ca. 54°) and bicycles 4 (R ) ca. 36°) and 3 (R ) ca. 30°) are less so. Despite the high pyramidality, a synclinal (sc) conformation about the N-N bond (Chart 2a) was found to be the ground state for nitrosoheterocycles 1, 3, and 4 apparently driven by the requirement to maximize the nN-π*NO conjugation at the pyramidal ring nitrogen atom. The B3LYP method, using the 6-31+G* and 6-311+G** basis sets, was used to determine the barriers to rotation about the N-N bond via syn- and antiperiplanar transition states (ap- and sp-TSrot) (Chart 2b,c, Figures S-1 to S-4). The values for cyclic nitrosoamines 1-4 are listed in Table 1. For the pyramidal compounds 1, 3, and 4 transition states for the ring nitrogen inversion (TSinv) (Figures S-1, S-3, S-4) were also calculated. As one can see from Table 1, the highly pyramidal nitrosoaziridine 1 and the planar nitrosopyrroline 2 represent two extreme cases. The aziridine is characterized by quite low and similar inversion and rotation barriers. The B3LYP method gives a rather different ratio of these barriers for 1 in comparison with the HF/ 6-31G* and MP2/6-31+G**//HF/6-31G* methods used earlier.9c However, it is important that all methods predict the stereochemical flexibility for nitrosoaziridine
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Figure 2. B3LYP/6-31+G* structures of nitrosopyrroline 2, its second-order saddle point, and transition state for the cheletropic fragmentation and graphic representation of the potential energy profile for this fragmentation derived with the IRC-B3LYP/6-31+G* calculations. The IRC relative energies are not ZPVE corrected.
Shustov and Rauk
Figure 4. B3LYP/6-31+G* structures of nitrosobicycle 4 and its transition state for the cheletropic fragmentation and graphic representation of the potential energy profile for this fragmentation derived with the IRC-B3LYP/6-31+G* calculations. The IRC relative energies are not ZPVE corrected.
Chart 2
Figure 3. B3LYP/6-31+G* structures of nitrosobicycle 3 and its transition state for the cheletropic fragmentation and graphic representation of the potential energy profile for this fragmentation derived with the IRC-B3LYP/6-31+G* calculations. The IRC relative energies are not ZPVE corrected.
1. In contrast, the nitrosoamine group in nitrosopyrroline 2 is relatively rigid, with rotation barriers > 23 kcal mol-1.
The calculated potential barriers for the internal rotation in nitrosobicycles 3 and 4 proved to be closer to that of the planar nitrosopyrroline 2 than to the pyramidal nitrosoaziridine 1 (Table 1). These compounds have relatively high rotation barriers (>16 kcal mol-1) and extremely low inversion barriers( 4 (12). The relative energies of nitrosoheterocycles 2-4, their transition states, and products of the nitrous oxide extrusion are shown in Figure 5 on a common scale where the energy of the ground state of nitrosopyrroline 2 is assumed as zero. As one can see from Figure 5, the thermal stability of the cyclic nitrosoamines in the channel of the cheletropic decomposition is almost entirely determined by the ring strain of the starting material since the energies of the transitions structures are almost the same.
planar ring nitrogen atom. The aziridine system is an exception (see below). 1-Nitrosoaziridine with the highly pyramidal and stereochemically mobile NNO group represents a special case owing to special properties of the three-membered ring. Besides the low rotation barriers (
