Transformations of Organoboranes at Elevated Temperatures

Abstract

Trialkylboranes with identical or different groups on the boron are stable to alkyl exchange at room temperature. Mixed trialkylboranes only undergo disproportionation in the presence of catalysts such as aluminum trialkyls and boron‐hydrogen compounds. At temperatures above 100 °C, organoboranes such as trialkylboranes and B‐alkylboracyclanes isomerize as a result of migration of the boron along the aliphatic hydrocarbon radical via dehydroboration‐hydroboration equilibria. Alkyl, cycloalkyl, as well as aralkyl and certain arylboranes can be converted irreversibly by pyrolysis at 200–300 °C into other organoboranes (methylboranes and boron‐heterocycles of various structures).

Hydroboration kinetics: Unusual kinetics for the reaction of 9-borabicyclo[3.3.1]nonane with representative alkenes

“…(29). In contrast, (Sia2BH)2 undergoes isomerization at 750C (34), and dicyclohexylborane dimer is reported to decompose at 180-200°C, yielding cyclohexene and polymeric borane (35).…”

Toward product control in ring‐opening oligomerization of 9H‐9‐borafluorenes

“…To explain the fact that difficult substituent exchange between trialkyl boranes can be efficiently catalyzed by organo(hydro)boranes, diborane intermediates involving the BHB‐supported BCB bridge were proposed by Köster . The structural similarity between the proposed diborane intermediate and G was taken as solid confirmation of the Köster's 50‐year‐old postulate.…”

“…[3] To explain the fact that difficult substituent exchange between trialkyl boranes can be efficiently catalyzed by organo(hydro)boranes, diborane intermediates involving the BAHAB-supported BACAB [4][5][6] bridge were proposed by K€ oster. [7,8] The structural similarity between the proposed diborane intermediate and G was taken [3] as solid confirmation of the K€ oster's 50-year-old postulate. However, as shown below, the reactivity of 9-borafluorene with constrained, antiaromatic borole ring is so different from normal boranes without antiaromatic ring constraint that both G and H are actually formed through aryl exchange involving double BACAB bridged intermediates, which is in sharp contradiction with the K€ oster's postulate that is valid for normal boranes.…”

Katalytische Friedel‐Crafts‐C‐H‐Borylierung elektronenreicher Arene: starke Reaktionsbeschleunigung durch Versetzen mit Alkenen

“…[4,5] Bedeutende Fortschritte wurden von Vedejs et al [6] und Ingleson et al [7] unter stçchiometrischer Verwendung positiv geladener Borreagentien [8] erzielt. [9,10] Beide Gruppen entwickelten auch katalytische Va rianten mit Aktivatoren wie Bis(trifluormethansulfonyl)imid (HNTf 2 ), [9d,e,11] [Ph 3 C] + [B(C 6 F 5 ) 4 ] À [9d,e] oder [Et 3 Si] + [HCB 11 H 5 Br 6 ] À , [12] und die so erzeugten Borelektrophile treten danach in selbsttragende Katalysezyklen ein. Chernichenko,Repo et al entwickelten jüngst eine elektrophile C-H-Borylierung auf dieser Grundlage.…”

“…[12] Weitere Optimierungen offenbarten, dass die Reaktion entweder unverdünnt oder, zum Preis geringfügig längerer Reaktionszeiten, in aromatischen oder halogenierten Lçsungsmitteln durchführbar ist (Tabelle 1, Nr. [8][9][10][11]. Erhitzen auf 120 8 8Cb rachte eine beträchtliche Reaktionsbeschleunigung mit sich, und vollständiger Umsatz wurde nun innerhalb von 24 he rreicht.…”