Synthetic access to electron-precise boron chains is hampered by the preferential formation of nonclassical structures. The few existing strategies for this involve either strongly reducing reagents or transition-metal catalysts, both with distinct disadvantages. The synthesis of new furyl- and thienyl-substituted diborenes is presented, along with their direct hydroboration with catecholborane (CatBH) to form a new electron-precise B-B bond and a B3 chain. The reaction is diastereoselective and proceeds under mild conditions without the use of strong reducing agents or transition-metal catalysts commonly used in B-B coupling reactions.

Abstract:The reactions of a diborene with elemental selenium or tellurium are shown to afford a diboraselenirane or diboratellurirane, respectively. These reactions are reminiscent of the sequestration of sub-valent oxygen and nitrogen in the formation of oxiranes and aziridines; however, such reactivity is not known between alkenes and the heavy chalcogens. While carbon is too electronegative to affect the reduction of elements of lower relative electronegativity, the highly reducing nature of the B=B double bond enables reactions with Se 0 and Te 0 . The capacity of multiple bonds between boron to donate electron density is highlighted in reactions where diborynes behave as nucleophiles, attacking one of the two Te atoms of diaryltellurides, forming salts consisting of diboratellurenium cations and aryltelluride anions.The energy stored in small, highly strained cyclic molecules has made them an integral part of modern synthetic chemistry. Since this "strain energy" increases with decreasing ring size, it is greatest for three-membered rings, and when these rings are heterocyclic the charge-asymmetry induced in the molecule provides sites ready for reaction. Accordingly, an enormous amount of research has gone into both the synthetic paths to, and reactions of, members of this class of compounds, most prominently oxiranes (C2O rings) and aziridines (C2N rings). The most common route to these materials is the oxidation of olefins using, in the case of oxirane formation, subvalent oxygen species such as O2, peroxides, peroxyacids, and ozone, or with reagents that impart a degree of electron deficiency to an oxygen atom, such as chlorite or iodosylbenzene. [1] Aziridination of olefins is most frequently accomplished through the in situ generation of nitrenes from azides or other electron deficient nitrogen sources such as iodinanes, hydroxylamines, and hydrazines. [1a,2] These alkene-oxidations are made possible by the relatively high electronegativity of oxygen and nitrogen, (χPauling = 3.44 and 3.04, respectively), relative to carbon (χPauling = 2.55).Thiiranes (C2S rings) are comparatively less common, and though examples of the direct addition of elemental sulfur to alkenyl double bonds are not unknown, [3] their syntheses are more likely than their first row neighbors to involve non-redox routes. [4] The similar electronegativities of carbon and sulfur (χPauling = 2.58) decreases the thermodynamic driving force for alkene oxidation, further exemplified by the noted willingness of thiiranes to thermally extrude atomic sulfur [5] and by their utility as sulfur atom transfer reagents. [6] Three-membered heterocycles featuring heavier chalcogens (Se and Te) are even less prevalent. Though seleniranes have been proposed as reactive intermediates in a handful of transformations, [7] the isolated examples of these compounds are few and none have been crystallographically verified. [8] To date, there are no known examples of telluriranes. The heavy chalcogens have roughly equal (χSe = 2.55) or smaller (χTe = 2.10...

Boron–boron J coupling constants provide new insight into the nature of the boron–boron triple bond.

The syntheses of sulfur- and selenium-bridged cyclic compounds containing boron stabilized by N-heterocyclic carbenes (NHCs) have been achieved by the reductive insertion of elemental chalcogens into boron-boron multiple bonds. The three pairs of bonding electrons between the boron atoms in the triply bonded diboryne enabled six-electron reduction reactions, resulting in the formation of [2.2.1]-bicyclic systems wherein bridgehead boron atoms are spanned by three chalcogen bridges. A similar reaction using a diborene (boron-boron double bond) resulted in the reductive transfer of both pairs of bonding electrons to three sulfur atoms, yielding a NHC-stabilized trisulfidodiborolane. The demonstration of these six- and four-electron reductions lends support to the presence of three and two pairs of bonding electrons between the boron atoms of the diboryne and diborene, respectively, a fact that may be useful in future discussions on bond order.