Abstract
Enantiopure halogenated molecules are of tremendous importance as synthetic intermediates in the construction of pharmaceuticals, fragrances, flavours, natural products, pesticides, and functional materials. Enantioselective halofunctionalizations remain poorly understood and generally applicable procedures are lacking. The applicability of chiral trans-chelating bis(pyridine)iodine(i) complexes in the development of substrate independent, catalytic enantioselective halofunctionalization has been explored herein. Six novel chiral bidentate pyridine donor ligands have been designed, routes for their synthesis developed and their [N–I–N]+-type halogen bond complexes studied by 15N NMR and DFT. The chiral complexes encompassing a halogen bond stabilized iodenium ion are shown to be capable of efficient iodenium transfer to alkenes; however, without enantioselectivity. The lack of stereoselectivity is shown to originate from the availability of multiple ligand conformations of comparable energies and an insufficient steric influence by the chiral ligand. Substrate preorganization by the chiral catalyst appears a necessity for enantioselective halofunctionalization.
Highlights
A reactive halenium ion can be stabilized by a three-center, four-electron halogen bond in a [bis( pyridine)halogen(I)]-type complex,17 which allows the rational modulation of halenium ion reactivity
Halonium ions,§ including even chloronium ions,19 are stable in solution and can be experimentally studied.17,20–22 [Bis( pyridine)iodine(I)] tetrafluoroborate was introduced as a mild halogen transfer and oxidation reagent by Barluenga,23 and the fundaments for the
(1,2-Bis( pyridine-2-ylethynyl)benzene) [1] was prepared according to a previously established protocol,21 whereas ligands 2–7 were synthesized following the reaction routes shown in Schemes 1–4
Summary
A reactive halenium ion can be stabilized by a three-center, four-electron halogen bond in a [bis( pyridine)halogen(I)]-type complex, which allows the rational modulation of halenium ion reactivity. In such complexes, halonium ions,§ including even chloronium ions, are stable in solution and can be experimentally studied.17,20–22 [Bis( pyridine)iodine(I)] tetrafluoroborate was introduced as a mild halogen transfer and oxidation reagent by Barluenga, and the fundaments for the. A reactive halenium ion can be stabilized by a three-center, four-electron halogen bond in a [bis( pyridine)halogen(I)]-type complex, which allows the rational modulation of halenium ion reactivity.. § The hypocoordinate (6e) X+ is denoted as halenium, whereas the halogen of the hypercoordinate (10e) [N−X−N]+ complex as halonium ion The former is formally a halogen bond donor (Lewis acid) that may simultaneously interact with two halogen bond acceptors (Lewis bases) and thereby form a 3-center, 4-electron halogen bonded complex. We synthesized a series of chiral analogues of 1 (Fig. 1) These do not suffer from ligand scrambling, and may influence the halenium transfer process with both chiral pyridines even upon dissociation of the [N–I–N]+ bond. Ligands 8–9 have originally been introduced by Johnston and coworkers, and were reported to provide enantioselectivity in halonium transfer when using N-iodosuccinimide as iodine-source in the presence of acid, and were here synthesized as positive controls
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