Catalysis based on reversible covalent interactions of organoboron compounds

CONSPECTUS: An Account of the development of organoboron-catalyzed methods for chemo- or regioselective activation of pyruvic acids, diols, and carbohydrate derivatives is presented. These methods are based on reversible, covalent interactions that have been exploited extensively in host-guest chemistry, but were comparatively underutilized in catalysis. Important differences between the established properties of organboron compounds in molecular recognition and their behavior as catalysts emerged over the course of this work: for instance, borinic acids, which have largely been ignored in molecular recognition, proved to be a particularly useful class of catalysts. Nonetheless, the high selectivity that has enabled applications of organoboron compounds in molecular recognition (e.g., the selective binding of cis-1,2-diol groups in carbohydrates) also appears to play a key role in the outcomes of catalytic reactions. This research program began as a modest, narrowly defined project aimed at developing direct aldol reactions based on established interactions between pyruvic acids and boronic acids. While this goal was achieved, it was unexpected observations related to the nature of the nucleophile in this transformation (a putative tetracoordinate boron enolate) that attracted our attention and pointed toward broader applications in the catalyst-controlled, regioselective functionalization of polyols. This line of research proved to be fruitful: diarylborinic-acid-based precatalysts were found to promote efficient monoalkylations, sulfonylations, and alkylations of a range of diol substrates, as well as cis-1,2-diol motifs in pyranoside-derived triols. Extension of this chemistry to glycosyl donors as electrophiles enabled the regioselective, catalyst-controlled synthesis of disaccharides from readily accessible feedstocks, and was also employed to modify the oligosaccharide component of a complex, glycosylated natural product. Mechanistic studies have played an important role in our efforts to optimize catalyst activity and expand substrate scope for this class of transformations. For instance, it was kinetic studies of the sulfonylation of diols that motivated us to investigate heteroboraanthracene-derived borinic acids as catalysts, despite their low affinity for these substrates. Likewise, preliminary studies suggesting an SN2-type pathway for organoboron-catalyzed glycosylations were instrumental in our development of a method for selective formation of β-2-deoxyglycosides. Details of these mechanistic studies, along with prospects for applying catalyst-controlled glycosylation in oligosaccharide synthesis and natural product glycorandomization, are discussed.