Tris(ortho‐carboranyl)borane: An Isolable, Halogen‐Free, Lewis Superacid

Abstract

The synthesis of tris(ortho-carboranyl)borane (BoCb 3 ), a single site neutral Lewis superacid, in one pot from commercially available materials is achieved. The high fluoride ion affinity (FIA) confirms its classification as a Lewis superacid and the Gutmann-Beckett method as well as adducts with Lewis bases indicate stronger Lewis acidity over the widely used fluorinated aryl boranes. The electron withdrawing effect of ortho-carborane and lack of pi-delocalization of the LUMO rationalize the unusually high Lewis acidity. Catalytic studies indicate that BoCb 3 is a superior catalyst for promoting CÀ F bond functionalization reactions than tris(pentafluorophenyl)borane [B(C 6 F 5 ) 3 ].

An easy-to-perform evaluation of steric properties of Lewis acids

“…However, B{2,4,6-(CF 3 ) 3 C 6 H 2 } 3 and the related borane B{2,6-(CF 3 ) 2 C 6 H 3 } 3 are unknown. Examples for synthetically accessible sterically highly demanding Lewis acids are B{OC(CF 3 ) 3 } 3 (FIA = 423 kJ mol −1 ) 34 and B(1,2-C 2 B 10 H 11 ) 3 (FIA = 605 kJ mol −1 ) 35 with a buried volume of 71.9 % V Bur . For aluminum, the Lewis acids Al{OC(CF 3 ) 3 } 3 (FIA = 543 kJ mol −1 ; 56.8 % V Bur ) and 36 a Al{N(C 6 F 5 ) 2 } 3 (FIA = 555 kJ mol −1 ; 68.1 % V Bur ), 36 b and the recently reported Al(OTeF 5 ) 3 (FIA = 591 kJ mol −1 ; 51.5 % V Bur ) 36 c provide excellent combinations of steric bulk and high Lewis-acidity (see also the ESI, Fig.…”

“…29f,g SambVca 2.1 was used to determine %V Bur for several Lewis acids for a radius R= 3.50 Å. The latter radius is typically used for the determination Please do not adjust margins Please do not adjust margins 605 kJ/mol) 35 with a buried volume of 71.9%V Bur . For aluminum, the Lewis acids Al{OC(CF 3 ) 3 } 3 (FIA = 543 kJ/mol; 56.8%V Bur ), 36a Al{N(C 6 F 5 ) 2 } 3 (FIA = 555 kJ/mol; 68.1%V Bur ), 36b and the recently reported Al(OTeF 5 ) 3 (FIA = 591 kJ/mol; 51.5%V Bur ) 36c provide excellent combinations of steric bulk and high Lewis-acidity (see also ESI, Figure S5).…”

Effects of surface acidity on the structure of organometallics supported on oxide surfaces

“…Part of this reason is related to the need for stronger Lewis acids that bind to -OH groups on surfaces with sufficient binding energy to form a bridging RE-OHÁ Á ÁLA site (E = surface element). Lewis acids stronger than Al(OR F ) 3 , which are becoming available, 172 would be useful for this purpose. An alternative is to react oxides containing stronger Brønsted acid -OH groups with Al(OR F ) 3 (PhF) to enhance acidity.…”

B(9)-OH-o-Carboranes: Synthesis, Mechanism, and Property Exploration

“…Under the assistance of a directing group, the B(4)–H hydroxylation of o -carboranes was achieved through Rh-catalyzed B–H bond activation (Scheme c) . Considering the great importance of carboranes in biomedicine, photochemistry, supramolecular chemistry, coordination chemistry, and materials chemistry, searching for a simple and efficient synthetic method of hydroxyl carboranes is still highly desired.…”