Regioisomerism in the synthesis of a chiral aminotetralin drug compound: unraveling mechanistic details and diastereomer-specific in-depth NMR investigations

Quaternized chitin used as chiral stationary phase for HPLC and the high enantioseparation of 1,2,3,4-tetrahydro-1-naphthalenamine racemates

Quaternized chitin (QC) with different degrees of substitution (DSs) and molecular weight (M_w) were homogeneously synthesized. Eight novel chiral stationary phases (CSPs) for HPLC were prepared by coating the QC on 3-aminopropyl silica gel, which were firstly used to separate 1,2,3,4-tetrahydro-1-naphthalenamine (THNA) racemates. Enantioseparation capability of the CSPs was evaluated and the influence factors including DS and M_w of QCs were explored respectively. The results demonstrated that the successful separation of THNA enantiomers was obtained by all the new CSPs of the chitin derivatives. Resolution (R_s) increased from 1.12 to 1.58 with the increase of DS of QC from 0.40 to 0.62, while the R_s decreased with the reduction of M_w of the products from 2.8 × 10⁵ to 9.7 × 10⁴. The maximum R_s is 2.29. A simple pathway for the fabrication of novel CSPs of cationic chitin derivatives is developed, which has potential application for the separation of THAN racemates.

Modeling a halogen dance reaction mechanism: A density functional theory study

Since the discovery of the halogen dance (HD) reaction more than 60 years ago, numerous insights into the mechanism have been unveiled. To date however, the reaction has not been investigated from a theoretical perspective. Density functional theory (DFT) was used to model the potential energy surface linking the starting reagents to the lithiated products for each step in the mechanism using a thiophene substrate. It was found that the lithium-halogen exchange mechanism is critical to understand the HD mechanism in detail and yielded the knowledge that SN 2 transition states (TS) are favored over the four-center type for the lithium-bromine exchange steps. The overall driving force for the HD is thermodynamics, while the kinetic factors tightly control the reaction path through temperature. The SN 2 lithium-bromide TS are barrierless, except the second, which is the limiting step. Finally, the model for the HD is discovered to be a pseudo-clock type, due to a highly favorable bromide catalysis step and the reformation of 2-bromothiophene. © 2016 Wiley Periodicals, Inc.