I⋅⋅⋅Br‐idging: Benzhydryl bromide can be activated by novel halogen‐bond donors and subsequently undergoes a Ritter‐like reaction with acetonitrile (see scheme). Comparative experiments with non‐iodinated reference compounds and tests with added acids indicate that halogen bonds are very likely the basis for this effect. The activation seems to be applicable to other substrates as well.
I(n)organocatalysis: Neutral multidentate halogen‐bond donors (halogen‐based Lewis acids) catalyze the reaction of 1‐chloroisochroman with ketene silyl acetals. The organocatalytic activity is linked to the presence (and number as well as orientation) of iodine substituents. As hidden acid catalysis can be ruled out with high probability, this case constitutes strong evidence for halogen‐bond based organocatalysis. TBS=tert‐butyldimethylsilyl.
Using a prototypical Diels-Alder reaction as benchmark, we show that dicationic halogen-bond donors are capable of activating a neutral organic substrate. By various comparison experiments, the action of traces of acid or of other structural features of the halogen-bond donor not related to halogen bonding are excluded with high certainty.
Chalcogen bonding is a little explored noncovalent interaction similar to halogen bonding. This manuscript describes the first application of selenium‐based chalcogen bond donors as Lewis acids in organic synthesis. To this end, the solvolysis of benzhydryl bromide served as a halide abstraction benchmark reaction. Chalcogen bond donors based on a bis(benzimidazolium) core provided rate accelerations relative to the background reactivity by a factor of 20–30. Several comparative experiments provide clear indications that the observed activation is due to chalcogen bonding. The performance of the chalcogen bond donors is superior to that of a related brominated halogen bond donor.
We have conducted isothermal calorimetric titrations to investigate the halogen-bond strength of cationic bidentate halogen-bond donors toward halides, using bis(iodoimidazolium) compounds as probes. These data are intended to aid the rational design of halogen-bond donors as well as the development of halogen-bond-based applications in solution. In all cases examined, the entropic contribution to the overall free energy of binding was found to be very important. The binding affinities showed little dependency on the weakly coordinating counteranions of the halogen-bond donors but became slightly stronger with higher temperatures. We also found a marked influence of different solvents on the interaction strength. The highest binding constant detected in this study was 3.3 × 10(6) M(-1).
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