A DFT study on the thermal reaction mechanisms of fluorobutanesulfonyl azide with pyrazine under solvent free condition

The thermal reaction mechanisms of fluorobutanesulfonyl azide with pyrazine under solvent free condition have been studied by density functional theory (DFT) in this paper. Four possible reaction channels including two stepwise channels (channels 1 and 2) and the other two concerted channels (channels 3 and 4) are shown. The calculated results indicate that the stepwise channels (channels 1 and 2) should be easier to occur than the concerted channels (channels 3 and 4), and one of the stepwise channels (channel 1) is the most energy favorable reaction path among all the four possible channels, thus, the main product should be the pyrazinium N-fluorobutanesulfonyl ylide, which is consistent with the experimental result. Moreover, the highest energy barrier of channel 1 is 35.10 kcal/mol at the B3LYP/6-31G (d, p) level (which is 33.97 kcal/mol at the B3LYP/6-311++G (2d, 2p) level), this should be not a very high energy barrier for the experimental temperature (418 K). Furthermore, there are very small differences between the calculated results of two different levels. Therefore, it should be reliable that the stepwise reaction channel is the main reaction path in this reaction.