Tuning the Halogen Bonding Strength of Cyclic Diaryliodonium Salts

Abstract

Diaryliodonium(III) salts have recently received increasing interest as a new class of strong halogen bonding noncovalent organocatalysts. Even though this utilization of their Lewis acidity has only been investigated in few studies, their high potential in comparison to classical monovalent iodine based XB donors has become very apparent. So far, only acyclic and cyclic five‐membered core structures have been used, and titration studies have shown the latter to be superior in terms of binding constants towards Lewis bases. Herein, we now compare the Lewis acidity and activity of these five‐membered iodolium salts with those of six‐membered iodininium derivatives. X‐Ray structural analyses, ITC measurements and the reaction kinetics of a Ritter‐type solvolysis reaction in wet acetonitrile (a typical halogen bond donor benchmark reaction) all demonstrate that iodolium salts are stronger halogen bond donors and catalysts than iodininium salts. Subsequently, we were able to improve the activity of both core structures significantly by introducing electron‐withdrawing substituents.

Nature of the Hydrogen Bond Enhanced Halogen Bond

“…As expected, a similar linear correlation as ∆E int was found for this energy contributor (R 2 = 0.94, Figure 4). In agreement with recent works on the enhancement of the Lewis acidity in halogen donors [29,30], our EDA data confirms that the interaction energy between the XB donor 4 and the chloride anion increases when electron-withdrawing groups are placed in the para-position to the iodine atom. Thus, an increase of up to ca.…”

N‐Heterocyclic Iod(az)olium Salts – Potent Halogen‐Bond Donors in Organocatalysis

“…Although these iodolium salts only act as monodentate XB‐donors, their performance in most reactions is comparable to bidentate iodine(I) derivatives, underlining their high potential. Thiophene‐based bidentate iodolium salt 5 as well as perfluorinated iodolium salt 6 were recently described by Huber and co‐workers, which so far show the highest reactivity among all literature‐described iodine(I) and iodine(III) XB‐donors [34,35] …”

“…Following the high performance of our N‐heterocyclic iod(az)olium salts in the Ritter‐type solvolysis of benzhydryl chloride ( 12 ), we were eager to test the much more challenging activation of α‐methylbenzyl chloride ( 14 ), of which only the easier bromide variant has been studied [35] . In this reaction no conversion of the starting material 14 was observed using 4 a , whereas with the best non‐methylated derivative, pyrazole 8 a , 18 % of the acetamide 15 was obtained after three days (Figure 6).…”

Iodine(III)‐Based Halogen Bond Donors: Properties and Applications

“…Under slightly modified conditions (preparative instead of NMR experiments), they tested their compound 43 against the typical iodolium 17a OTf in the solvolysis of benzhydryl chloride and bromide and reported a significant increase in activity. Shortly thereafter, Huber and coworkers published their investigation on the tunability of cyclic diaryliodonium salts using the Ritter ‐type solvolysis of α‐methylbenzyl bromide as a benchmark reaction [101] . In a direct comparison, the simple iodolium derivative 17a OTf was found to be superior in terms of XB‐based activity compared to the equivalent, unfunctionalized, six‐membered iodininium counterpart 45 a , which was basically inactive.…”

“… New XB donors by Nachtsheim [100] and Huber [101] , which have been shown to be more active, with the exception of 45 a , than iodolium 17a OTf in Ritter‐type solvolysis reactions similar to the one depicted in Scheme 8.…”