One-Bond, Two-Bond, and Three-Bond Mechanisms in Thermal Deazetizations of 2,3-Diazabicyclo[2.2.2]oct-2-enes, trans-Azomethane, and 2,3-Diazabicyclo[2.2.1]hept-2-ene

Abstract

The thermal deazetizations of a series of substituted 2,3-diazabicyclo[2.2.2]oct-2-enes and some simpler model systems have been studied using the UB3LYP/6-31G(d) and CASPT2 methods, with a variety of active spaces. A fused cyclopropane exerts unique control on the mechanism and rate of deazetization. When the Walsh sigma-orbitals are appropriately aligned in an exo orientation, a pericyclic three-bond cleavage occurs. For an endo-fused cyclopropane, sequential one-bond cleavages occur to take advantage of orbital overlap with the Walsh orbitals. In systems lacking strongly directing substituents, concerted two-bond cleavage pathways to form diradical intermediates have a small enthalpic advantage (DeltaH(0K)++) over sequential one-bond cleavage pathways. However, the one-bond mechanism has an entropic advantage over the two-bond; consequently, at 400-500 K where decomposition occurs, one-bond and two-bond diradical cleavages both occur simultaneously. The thermal decompositions of trans-azomethane and 2,3-diazabicyclo[2.2.1]hept-2-ene are also studied, and the results are compared to extensive computational studies in the literature. Comparisons of UB3LYP, CASSCF, and CASPT2 surfaces for these reactions are made.

A Theoretical Study of Some Bicyclic Azoalkanes

“…It has been known that the DFT calculation for open shell system is much safer than the MP2 calculation with respect to the spin contamination arising from electronic states with higher multiplicities. [21][22][23] According to the result of recent theoretical study, 8 the UB3LYP and CASPT2 calculations show excellent agreement in predicting the energetics of DBO and DBH systems. Thus, it is clear that the B3LYP (UB3LYP) method with standard 6-311G(d) basis set is good choice to predict the molecular properties for much larger DBH and DBO derivatives with fused rings.…”

“…[1][2][3][4] In particular, deazetization (nitrogen loss) of azoalkanes is known to be one of the important routes of formation of photochemically active azo radicals and diradicals. [5][6][7][8] Extensive studies for verifying the mechanisms of such reactions have been reported. [1][2][3][4][5][6][7][8][9][10][11][12][13] The electronic structure of azo chromophores is also believed to play a key role in determining the reaction mechanism and photochemical properties of many azoalkanes.…”

“…[5][6][7][8] Extensive studies for verifying the mechanisms of such reactions have been reported. [1][2][3][4][5][6][7][8][9][10][11][12][13] The electronic structure of azo chromophores is also believed to play a key role in determining the reaction mechanism and photochemical properties of many azoalkanes. [5][6][7][8] The relative energies of their ground and excited electronic states appear to be useful in explaining some reaction mechanisms involving azo species.…”

“…[1][2][3][4][5][6][7][8][9][10][11][12][13] The electronic structure of azo chromophores is also believed to play a key role in determining the reaction mechanism and photochemical properties of many azoalkanes. [5][6][7][8] The relative energies of their ground and excited electronic states appear to be useful in explaining some reaction mechanisms involving azo species. 6,[14][15][16] In addition, the processes like intersystem crossing (ISC) between the states of different spin multiplicities may become important in interpretation of their reaction mechanisms.…”

Inquiry of the electron density transfers in chemical reactions: a complete reaction path for the denitrogenation process of 2,3-diazabicyclo[2.2.1]hept-2-ene derivatives

“…3,18,20 This reaction pathway has been predicted to be endothermic when the system reaches the DBH-diyl diradical. At the same time, DBH-diyl has been proposed to be the intermediate species for the formation of the closed-shell bicycle[2.1.0]pentane (BCP-DBH).…”