Highly Strained Heterocycles Constructed from Boron–Boron Multiple Bonds and Heavy Chalcogens

Abstract

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...

Formation and Reactivity of Electron‐Precise B−B Single and Multiple Bonds

“…In contrast, the oxidation of the dppm-stabilized cis-diborene 54 b occurs at much higher potential [E 1/2 = À0.51 V],i mplying ac onsiderably less-electron-rich system. [101] TheB À B bonds were lengthened with respect to the diborene starting materials,but nonetheless shorter than typical single bonds in base-stabilized diboranes [70 a 1.707(3) ; 70 b 1.713(5) ], following the pattern observed for related carbon C 2 E analogues such as thiiranes.T he fact that these compounds are stable,w hereas none of the few reported carbon-based seleniranes has been structurally characterized, and telluriranes are unknown, is ascribed to the lower electronegativity of boron and the higher reducing power of diborenes as compared to alkenes. [65] As well as being oxidized by [C 7 H 7 ][BAr [99] Thes ame reaction in the presence of dimethyldisulfide provided the corresponding trisulfadiborolane.T he plausibility of this mechanism was recently established by our group,byobtaining arelated 5-membered heterocycle,c ompound 69,b yt he insertion of elemental sulfur into isolated diborene 49 b,w hich represents af ourelectron oxidation of the diborene fragment (Scheme 18 b).…”

“…[101] Ap roposed nucleophilic attack of the BBu nit at one tellurium atom was found to yield ionic species containing ac ationic,t hree-membered diboraphenyltellurirenium moiety with an aryltelluride counterion (76 a,b;S cheme 21). [101] Ap roposed nucleophilic attack of the BBu nit at one tellurium atom was found to yield ionic species containing ac ationic,t hree-membered diboraphenyltellurirenium moiety with an aryltelluride counterion (76 a,b;S cheme 21).…”

Seleniranium and Telluriranium Salts

“…Telluriranium salts are so far completely unknown. Here, we note the synthesis of boron–chalcogen three‐membered ring compounds: diboraseleniranes, diboratelluriranes, and diboratellurirenium salts …”

Increasing the Reactivity of Diborenes: Derivatization of NHC‐Supported Dithienyldiborenes with Electron‐Donor Groups

“…[1] As ar esult, diborenes, unlike alkenes, are excellent reducinga gents, capable of acting as either i) four-electron reducinga gents towards elemental sulfur,b yf ully cleaving the B=Bd ouble bond and forming aB 2 S 3 heterocycle, [19] or ii)t woelectron reducing agentst owards the heaviere lemental chalcogens by forming three-membered B 2 Er ings (E=Se, Te ), [20] or even iii)o ne-electronr educing agents towards the tropylium cation and 1-mesityl-2,3,4,5-tetraphenylborole (MesBC 4 Ph 4 )t o form diborene radical cations. [10], [11] The substitution pattern of the diborene core plays as ignificant role in tuning this redox reactivity (Figure 2).…”