A molecular electron density theory study on the [3+2] cycloaddition reaction of thiocarbonyl ylides with hetaryl thioketones

作者：Asr Afsaneh; Aghaie Mehran; Emamian Saeedreza^*; Aghaie Hossein

来源：New Journal of Chemistry, 2018, 42(14): 11819-11830.

DOI：10.1039/c8nj02021c

Utilizing the newly proposed molecular electron density theory (MEDT), a detailed study was performed on the [3+2] cycloaddition (32CA) reaction of thiocarbonyl ylide TCY 2 with thioketone THK 3 at the B3LYP/6-31G(d) computational level. In the gas phase, this reaction displays a very low total electronic activation energy of 1.6 kcal mol(-1) arising from the pseudo(di)radical (pdr) character of TCY 2, which was distinguished by electron localization function (ELF) analysis over the ground-state electronic structure (GES) of this species. Further exploration of the relative Gibbs free energies in the presence of THF at -40 degrees C indicates that, in excellent agreement with the experimental findings, the C1-C4 regioisomeric channel is kinetically preferred over the C1-S5 channel, leading to the formation of cycloadduct CA 4 as the sole product. Meanwhile, due to the high global nucleophilicity index of TCY 2 and the high global electrophilicity index of THK 3, a highly polar 32CA reaction between the reagents is expected; however, this pdr-type 32CA reaction displays non-polar character. The exclusive C1-C4 regioselectivity can be explained via probing molecular electrostatic potential (MEP) maps, which portray enormous electrostatic repulsion between two interacting fragments of the transition state (TS) involved in the unfavorable C1-S5 regioisomeric channel. An ELF analysis of the most relevant points located along the intrinsic reaction coordinate (IRC) profile of the TS connecting the separate reagents to CA 4 establishes a non-concerted two-stage one-step molecular mechanism for the non-polar pdr-type 32CA reaction of TCY 2 with THK 3.

• 出版日期2018-7-21