BF3 Mediated [1,5]-Hydride Shift Triggered Cyclization: Thioethers Join the Game

2-(2-(Benzylthio)benzylidene)malonates can undergo the 1,5-hydride shift triggered cyclization resulting in thiachromanes in 45-84 % yield. Boron trifluoride as a reaction promotor is the key to success. DFT calculations revealed that the reaction proceeds via a chelate BF2 complex, which was confirmed by NMR-analysis. Experimental and theoretical comparison of nitrogen, sulfur and carbon analogues revealed that sulfur derivatives have the highest activation barrier for the hydride transfer due to the unfavorable transition state geometry, which explains the lack of previous reports on 1,5-hydride shifts in (alkylthio)styrenes.

Automated summary

In this study, researchers discovered that 2-(2-(benzylthio)benzylidene)malonates can undergo controlled 1,5-hydride shift triggered cyclization, resulting in the formation of thiachromanes. Boron trifluoride played a key role as a reaction promotor. Experimental and theoretical comparison revealed that sulfur derivatives have the highest activation barrier for hydride transfer due to unfavorable transition state geometry, explaining the lack of previous reports on 1,5-hydride shifts in (alkylthio)styrenes.