Allylboronic esters react readily with carbonyls and imines (π-electrophiles), but are unreactive toward a range of other electrophiles. By addition of an aryllithium, the corresponding allylboronate complexes display enhanced nucleophilicity, enabling addition to a range of electrophiles (tropylium, benzodithiolylium, activated pyridines, Eschenmoser’s salt, Togni’s reagent, Selectfluor, diisopropyl azodicarboxylate (DIAD), MeSX) in high regio- and stereocontrol. This protocol provides access to key new functionalities, including quaternary stereogenic centers bearing moieties such as fluorine and the trifluoromethyl group. The allylboronate complexes were determined to be 7 to 10 orders of magnitude more reactive than the parent boronic ester.

High nucleophilic reactivity and functional group tolerance are opposing properties of organometallic compounds. While organolithium compounds are highly nucleophilic and, therefore, react with most electrophiles, organosilicon compounds are on the low-reactivity end of the nucleophilicity scale. They tolerate numerous electrophilic functional groups and often require (Lewis) base activation in order to react. Organoboron compounds, which are key intermediates in organic synthesis, are positioned in between. While some of them undergo non-catalysed C–C bond forming reactions with various electrophiles, others tolerate a variety of functional groups and are most valuable substrates for Pd-catalysed cross-couplings (Suzuki–Miyaura reactions). As it is difficult for the preparative chemist to decide which functional groups are tolerated and which do react with organoboron reagents in the absence of a catalyst, we established a quantitative reactivity scale for representatives of the most important classes of organoboron reagents and we applied these data for predicting the feasibility of transition metal-free C–C bond forming reactions.