Control of stereogenic oxygen in a helically chiral oxonium ion

Abstract

The pyramidal inversion of trisubstituted nitrogen, phosphorus and sulfur compounds and its impact on configurational stability is a fundamental and well-recognized stereochemical phenomenon that is widely exploited. In contrast, the chemistry of oxonium ions – compounds bearing three substituents on a positively charged oxygen atom – is poorly developed and there are few applications in synthesis beyond their existence as reactive intermediates. There are no examples of configurationally stable oxonium ions in which the oxygen atom is the sole stereogenic centre, likely due to the low barrier to oxygen pyramidal inversion, and the perception that all oxonium ions are highly reactive. Here we describe the design, synthesis and characterization of a helically chiral triaryloxonium ion in which inversion of the oxygen lone pair is prevented through geometric restriction to enable it to function as a determinant of configuration. A combined synthesis and quantum calculation approach delineate design principles that enable configurationally stable and room-temperature isolable salts to be generated. We show that the barrier to inversion is >110 kJ mol-1 and outline a process for resolution. This constitutes the only example of a chiral non-racemic and configurationally stable molecule in which the oxygen atom is the sole stereogenic centre.

Deciphering the chemical bonding of the trivalent oxygen atom in oxygen doped graphene

“…These ndings have encouraged the exploration of a plethora of new analogous oxonium ions. [6][7][8][9][10][11][12][13] However, it should be emphasized that, to the best of our knowledge, in all the molecules containing trivalent oxygen, O is positively charged and tetrahedral due to the sp 3 hybridization. There are only three exceptions to this rule: Al 3 O, Tl 3 O and Ga 3 O.…”

Harnessing triaryloxonium ions for aryne generation

“…A methyl group is employed as the ortho-blocking group due to its small size and the availability of precursors. We have demonstrated that helically chiral triaryloxonium ions can be generated by intramolecular O-arylation of a diphenylether derivative 35 with a diazonium salt, and we rationalized that this strategy offered a viable route to suitable oxonium ions for aryne generation 36 .…”