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17744375743
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-
note
-
The B3LYP/6-31G*-calculated Arrhenius activation energies for the rotation of the pyrazolyl groups are 2.76 kcal/mol (1a(NH) → 1a(NN)) and 0.52 kcal/mol (1a′(NH) → 1a′(NN)) at 298 K, and those for rotation of the azide group are 4.31 kcal/mol (1a(NN) → 1a′(NN)) and 3.68 kcal/mol (1a(NH) → 1a′(NH); full details are given in the Supporting Information). Thus, the four conformations of azide 1a equilibrate rapidly at room temperature.
-
-
-
-
39
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17744371106
-
-
note
-
32a.
-
-
-
-
40
-
-
17744364457
-
-
note
-
s symmetry, so only this conformation is considered here.
-
-
-
-
41
-
-
17744371305
-
-
note
-
Note that the CASSCF dipole moment of 2a(NN) is much higher than that of 2a(NH) (4.3 vs 1.7 D). Thus, the energy difference between the two conformers may be strongly attenuated in polar solvents, e.g. ethanol.
-
-
-
-
42
-
-
17744383543
-
-
note
-
The extent of this effect depends quite strongly on the size and the nature of the active space in the CASSCF calculations. Removing the σ lone pair on the nitrene N atom from the active space does not affect the energy difference between the two conformers in the triplet state but reduces that difference in the singlet state from 6.4 to 4.5 kcal/mol (and increases the S/T gap in the H-bonded conformer to 20.4 kcal/mol, only 1 kcal/mol less than in the other conformer).
-
-
-
-
43
-
-
17744394245
-
-
note
-
The transition states were found to be stationary points in a saddle point search. Unfortunately, we were unable to perform frequency calculations at the CASSCF(14,12) level to ascertain the nature of these stationary points and to obtain vibrational data for thermal corrections. However, the transition state found for the triplet nitrene looks very similar to that found (and characterized) by B3LYP, so we have reason to be confident that we have actually located true saddle points also by CASSCF.
-
-
-
-
44
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-
17744398393
-
-
note
-
To convince ourselves that we had not missed a secondary minimum, we carried out relaxed scans of the dihedral angle, the results of which are summarized in Figure S1 of the Supporting Information.
-
-
-
-
45
-
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17744376749
-
-
note
-
12b equilibrate rapidly at room temperature.
-
-
-
-
46
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0033550518
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Hrovat, D. A.; Duncan, J. A.; Borden, W. T. J. Am. Chem. Soc. 1999, 121, 169.
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-
-
17744362251
-
-
note
-
a, corresponds to an enthalpy difference (augmented by RT for a unimolecular reaction).
-
-
-
-
48
-
-
17744375336
-
-
note
-
One may argue that these differences between the parent system and the dimethyl derivative come about by an error in the calculation of the singlet/ triplet gap. However, to explain a 5 kcal/mol difference in activation energies would require the relative S/T gap of the parent system and the dimethyl derivative to be in errror by a similar amount. In the absence of H-bonds, the S/T gaps of the two compounds are calculated to be about the same (21 ± 0.2 kcal/mol), which is reasonable. Thus we believe that the differences in exothermicities and activation barriers for cyclization to azirines are not artifactual.
-
-
-
-
50
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-
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Photoinduced imermolecular H-abstractions of triplet nitrenes are known, cf.: Levya, E.; Munoz, D.; Platz, M. S. J. Org. Chem. 1989, 54, 5938.
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17744371616
-
-
note
-
The primary product of hydrogen abstraction from the methyl group is biradical 18. This turns out to be an interesting species, because it has an open-shell singlet ground state that is almost degenerate with the corresponding triplet state. The corresponding closed-shell singlet state (which is what is obtained in a DFT calculation) corresponds to a polyeneimine and lies vertically about 22 kcal/mol above the open-shell singlet ground state of 17. However, on geometry optimization that state ends up only ca. 5 kcal/mol above that state (all CASSCF/CASPT2 results). We did not succeed in finding a transition state for the cyclization of the open-shell singlet state to dihydroquinoxaline 8 (all attempts to shorten the distance betweeen the two radical centers resulted in collapse to 8), so we assume that this process is essentially barrierless.
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