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

Abstract

2-(2-(Benzylthio)benzylidene)malonates can undergo the 1,5hydride 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 nitro-gen, 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.

Synthesis and Biological Activity of Silicon-Containing Ethylsulfonylpyridine Insecticides

“…General Synthetic Procedures for Intermediate 1. To a stirred solution of 4-bromo-2fluorobenzaldehyde (2.04 g, 5 mmol) and EtSH (0.8 mL, 5 mmol) in DMSO (15 mL), K 2 CO 3 (3.04 g, 11 mmol) was added, and the reaction mixture then was heated to 100 °C for 4 h. 37 The reaction mixture was cooled to room temperature and filtered, and then water was added and the aqueous layer extracted with ethyl acetate. The combined organic phases were washed with brine, dried with Na 2 SO 4 , and concentrated under reduced pressure to give 1 as a white solid in 79% yield, which was used in the next step without purification.…”

“…To a stirred solution of 4-bromo-2-fluorobenzaldehyde (2.04 g, 5 mmol) and EtSH (0.8 mL, 5 mmol) in DMSO (15 mL), K 2 CO 3 (3.04 g, 11 mmol) was added, and the reaction mixture then was heated to 100 °C for 4 h . The reaction mixture was cooled to room temperature and filtered, and then water was added and the aqueous layer extracted with ethyl acetate.…”

C(sp3)–H cyclizations of 2-(2-vinyl)phenoxy-tert-anilines

“…with a sulphur analogue), leading to thiochromanes. 35 Although the heteroatom has an essential role in the stabilization of the carbocation formed by the hydride transfer, recently the carbo-version of the reaction was developed. The [1,5]-hydride transfer/cyclization sequences of appropriately substituted aliphatic or benzylic substrates afford various tetrahydronaphthalene systems.…”

Photoinduced [1,5]-hydride shift triggered cyclization

“…8 Such reactions are typically high yielding and reliable, and have granted successful creation and introduction of a novel synthetic antibiotic class – spiropyrimidinetriones, 9 and Zoliflodacin drug, in particular. 10 Based on the nature of hydride donor and acceptor fragments, the reaction could be initiated thermally, 11 under the action of catalytic, 12 substoichiometric 13 and stoichiometric amounts 14 of Lewis acid. In the ideal case, when the hydrogen is shifted from the α-position of the amino-substituent to a strong acceptor group such as Meldrum's acid derivatives, the reaction could be initiated at room temperature in hexafluoroisopropanol via hydrogen bond catalysis, 15 while for complicated reactions the required conditions could be as harsh as heating with scandium triflate in mesitylene.…”