Catalytic Activation of a Carbon–Chloride Bond by Dicationic Tellurium-Based Chalcogen Bond Donors

Abstract

Noncovalent interactions such as halogen bonding (XB) and chalcogen bonding (ChB) have gained increased interest over the last decade. Whereas XB-based organocatalysis has been studied in some detail by now, intermolecular ChB catalysis only emerged quite recently. Herein, bidentate cationic tellurium-based chalcogen bond donors are employed in the catalytic chloride abstraction of 1 chloroisochroman. While selenium-based ChB catalysts showed only minor activity in this given benchmark reaction, tellurium-based variants exhibited strong activity, with rate accelerations of up to 40 relative to non-chalogenated reference compounds. In general, the activity of the catalysts improved with weaker coordinating counterions, but tetrafluoroborate took part in a fluoride transfer side reaction. Catalyst stability was confirmed via a fluoro-tagged variant.

Chalcogen Bonding Catalysis: Tellurium, the Last Frontier?

“…[64] In this regard, the first chiral tellurium catalyst was just described in the literature (see Scheme 1: catalyst 4-Te 2 ), although it was not efficient in an asymmetric version of the Povarov reaction. [46] As noted by the authors, the use of less flexible chiral chains on tellurium should be beneficial for better asymmetric induction. Telluronium salts also offer some possibilities for ChB asymmetric catalysis, by using either chiral counterions or the central chirality of the tellurium atom.…”

“…Another aspect that will attract much interest is asymmetric catalysis [64] . In this regard, the first chiral tellurium catalyst was just described in the literature (see Scheme 1: catalyst 4‐Te 2 ), although it was not efficient in an asymmetric version of the Povarov reaction [46] . As noted by the authors, the use of less flexible chiral chains on tellurium should be beneficial for better asymmetric induction.…”

“…The chloride abstraction from 1‐chloroisochroman was also catalysed by the dicationic tellurium derivatives, resulting in its transformation to product 2 (Scheme 1). [46] The best catalytic activity was again observed with the BArF catalyst 2‐Te 2 (BArF) , followed by 2‐Te 2 (OTf) whereas the product was not formed when 2‐Te 2 (BF 4 ) was used as catalyst. In the latter case, decomposition of BF 4 − occurred to give BF 3 and F − which reacted with tert ‐butyldimethylsilyl (TBS) to produce TBSF (observed by 19 F NMR).…”

“…Forming The less-coordinating BArF anion was introduced through anion metathesis starting from 2-Te 2 (BF 4 ) to give 2-Te 2 (BArF) in moderate yield. This synthetic sequence further allowed two structural modifications of the catalysts: the introduction of a fluorine atom on the central aryl ring (catalysts 3-Te 2 ) with the aim to study the catalyst stability by 19 F NMR [46] and the installation of chiral groups as the second Te substituents (catalysts 4-Te 2 ) with the objective to use these tellurium derivatives in asymmetric catalysis (Scheme 2B). [47] Catalysts 2-Te 2 were first employed in a nitro-Michael reaction between 5-methoxyindole 5 and E-β-nitrostyrene 6 to give the 3-substituted indole 7 (Scheme 3).…”

N-Coordinated tellurenium(ii) and telluronium(iv) cations: synthesis, structure and hydrolysis

“…Noteworthily, the tellurium cations of both types have gained increasing attention with regard to their utilization in catalysis or ion recognition. 12…”