Isomerization and fragmentation pathways of 1,2‐azaborine

Abstract

The generation of 1,2-azaborine (4), the BN-analogue of ortho-benzyne, was recently achieved by elimination of tert-butyldimethylchlorosilane under the conditions of flash vacuum pyrolysis. The present investigation identifies by computational means pathways for the thermal isomerization and fragmentation of 1,2-azaborine. The computations were performed using single reference (hybrid/density functional, second order Møller-Plesset perturbation, and coupled cluster theories) as well as multiconfiguration methods (complete active space SCF based second order perturbation theory, multireference configuration interaction, and multiconfiguration coupled electron pair approximation) with basis sets up to polarized triple-ζ quality. The 1,2-azaborine is, despite the distortion of its molecular structure, the most stable C4H4BN isomer investigated. The formation of BN-endiyne isomers is highly unfavorable as the identified pathways involve barriers close to 80 kcal mol(-1). The concerted fragmentation to ethyne and 2-aza-3-bora-butadiyne even has a barrier close to 120 kcal mol(-1). The fragmentation of BN-enediynes has energetic requirements similar to enediynes.

Recent Developments in Azaborinine Chemistry

“…As expected, such a “free” (B‐ and N‐unsubstituted) azaborinine proved to be quite reactive, spontaneously binding dinitrogen ( 26 ) and carbon dioxide under matrix conditions (Scheme ). The fragmentation and isomerization of 1,2‐azaborine was also studied computationally . Among the various possible isomers examined, the 1,2‐azaborine form was the most thermodynamically stable.…”

Direct Spectroscopic Identification of BN‐Arynes and Subtle Steric Effects on Nitrogen Fixation**

“…Note that we have no spectroscopic evidence for the formation of the expected boryl nitrene 4 by IR spectroscopy. The parent nitreno borole 4’ , i. e. without dibenzo annulation, was computed to have a triplet ground state [21] . As triplet species should be detectable even in low concentration by ESR spectroscopy, we investigated the photolysis of 2 in methyl cyclohexane solid solution at 4 K. No triplet signal was observed, indicating that the triplet boryl nitrene does not form in detectable amounts from 2 .…”

Multireference Electron Correlation Methods: Journeys along Potential Energy Surfaces

“…Pyrrole was also used as an example to demonstrate geometry optimization with XMS-CASPT2 . Single-state CASPT2 optimizations (including ground and excited states) have been shown to perform well for bond formation/breaking or energy transfer problems for a variety of chemical systems. − Furthermore, CASPT2 geometries are often used to benchmark other computational methods, including those generated under extreme conditions or not observed in experiment. , These studies include optimizations along the reaction pathway for the F 2 + CH 3 SCH 3 reaction; , ground-state optimization of the hydromethoxy radical; geometries and excitation energies for RPSB models for benchmarking the performance of CASSCF, CC2, MP2, and DFT; the potential energy landscape search of singlet fission material candidate tetracyanoquinodimethane bithiophene (QOT2); and the equilibrium geometry, frequency calculation, and reaction pathway for the ring-opening reaction of azaborine . The analytical nuclear gradient for FIC-CASPT2 has been used for the copper corrole complex (Figure ), where a saddled geometry was obtained for the S 0 state while the S 1 state is planar.…”

“…311−313 Furthermore, CASPT2 geometries are often used to benchmark other computational methods, including those generated under extreme conditions or not observed in experiment. 314,315 These studies include optimizations along the reaction pathway for the F 2 + CH 3 SCH 3 reaction; 311,312 groundstate optimization of the hydromethoxy radical; 315 geometries and excitation energies for RPSB models for benchmarking the performance of CASSCF, CC2, MP2, and DFT; 316 the potential energy landscape search of singlet fission material candidate tetracyanoquinodimethane bithiophene (QOT2); 313 and the equilibrium geometry, frequency calculation, and reaction pathway for the ring-opening reaction of azaborine. 314 The analytical nuclear gradient for FIC-CASPT2 has been used for the copper corrole complex (Figure 4), 203 where a saddled geometry was obtained for the S 0 state while the S 1 state is planar.…”

“…314,315 These studies include optimizations along the reaction pathway for the F 2 + CH 3 SCH 3 reaction; 311,312 groundstate optimization of the hydromethoxy radical; 315 geometries and excitation energies for RPSB models for benchmarking the performance of CASSCF, CC2, MP2, and DFT; 316 the potential energy landscape search of singlet fission material candidate tetracyanoquinodimethane bithiophene (QOT2); 313 and the equilibrium geometry, frequency calculation, and reaction pathway for the ring-opening reaction of azaborine. 314 The analytical nuclear gradient for FIC-CASPT2 has been used for the copper corrole complex (Figure 4), 203 where a saddled geometry was obtained for the S 0 state while the S 1 state is planar.…”