Iodine(III) Derivatives as Halogen Bonding Organocatalysts

Abstract

Hypervalent iodine(III) derivatives are known as versatile reagents in organic synthesis, but there is only one previous report on their use as Lewis acidic organocatalysts. Herein, we present first strong indications for the crucial role of halogen bonding in this kind of catalyses. To this end, the solvolysis of benzhydryl chloride and the Diels-Alder reaction of cyclopentadiene with methyl vinyl ketone served as benchmark reactions for halide abstraction and the activation of neutral compounds. Iodolium compounds (cyclic diaryl iodonium species) were used as activators or catalysts, and we were able to markedly reduce or completely switch off their activity by sterically blocking one or two of their electrophilic axes. Compared with previously established bidentate cationic halogen bond donors, the monodentate organoiodine derivatives used herein are at least similarly active (in the Diels-Alder reaction) or even decidedly more active (in benzhydryl chloride solvolysis).

Digging the Sigma‐Hole of Organoantimony Lewis Acids by Oxidation

“…Altogether,t he catalytic activity of these antimony compounds correlates well with their experimentally determined Lewis acidity as well as with the computed LUMO energy.Alast important point of discussion concerns the unusually elevated activity of 2 in all three reactions,a nd especially in the Ritter-like reaction involving diphenylbromomethane.G iven that the antimonycentered Lewis acidity in 2 is much lower than that in 5 and 6, we believe that the commendable performance of this stibine results in part from the p-acidic properties of the C 6 F 5 substituents,a sseen in the structure of Ph 3 PO!Sb(C 6 F 5 ) 3 . [22] Our analyses indicate that this Lewis acidity increase originates from alowering of the antimony-centered s*o rbital as well as ad eepening of the s-hole.T hese two effects,w hich respectively capture the covalent and electrostatic nature of the interaction formed between the antimony Lewis acid and the incoming Lewis base,are manifested in the binding constants obtained when these organoantimony compounds complex with Ph 3 PO.T he same effects also readily enhance their catalytic properties in transfer hydrogenation and Ritter-like reactions when the antimony is in the + Vs tate.F inally,t he work described herein illustrates the duality that connects the orbital-based and s-hole-based descriptors of Lewis acidity in aw ay that mirrors the continuum existing between covalent and ionic bonding extremes. [21] Ther esults presented herein show that the Lewis acidity of pnictogen bond donor based on organoantimony is notably enhanced by oxidation to the + Vs tate,aconclusion that parallels that drawn in the case of iodine(III) halogen bond donors.…”

Preorganization: A Powerful Tool in Intermolecular Halogen Bonding in Solution

“…As first test reaction, we focused on the Diels Alder reaction of cyclopentadiene ( 17 ) and methyl vinyl ketone ( 18 , Scheme ) . Moreover, Diels Alder reactions are powerful tools in organic synthesis, as large ring systems with several stereogenic centers can be formed in a single step .…”

Synergic Interplay Between Halogen Bonding and Hydrogen Bonding in the Activation of a Neutral Substrate in a Nanoconfined Space

“…In this regard, the potential application of XB donor derivatives in organocatalysis is attracting more interest in the scientific community. Thus, for example, the XB interaction has been exploited for the activation of carbonyl compounds, of substrates by halides abstraction, and of neutral substrates …”