Synthesis and Structure of a Carbene‐Stabilized π‐Boryl Anion

Abstract

Attack with π electrons: Reduction of a chloroborole coordinated by an N‐heterocyclic carbene results in the formation of a carbene‐stabilized borole monoanion (see scheme; Mes=mesityl), the molecular structure of which has been determined by X‐ray analysis. Computational and reactivity studies of this boracycle confirm the presence of a π‐nucleophilic boron atom, which represents a rare example in the chemistry of boryl anions.

The Doubly Base‐Stabilized Diborane(4) [HB(μ‐hpp)]2 (hpp = 1,3,4,6,7,8‐hexahydro‐2H‐pyrimido[1,2‐a]pyrimidinate): Synthesis by Catalytic Dehydrogenation and Reactions with S8 and Disulfides

“…[3,4] Braunschweig et al stabilized a "π-boryl anion" with the aid of an N-heterocyclic carbene. [5,6] A number of complexes with borane, boryl, borylene and also boride ligands were synthesized, showing exciting electronic properties and a great potential for several applications. [7] For example, boryl ligands were intensively applied in metal-catalysed borylation reactions.…”

Reactivity of a Cl-boratabenzene Pt(ii) complex with Lewis bases: generation of the kinetically favoured Cl-boratabenzene anion

“…Although the chloroboratabenzene moiety is formally a nucleophilic substrate because of the anionic charge, the boron atom in borabenzene is known to remain electrophilic. 17 If an excess of L ligand is 80 available, it can undergo an associative substitution via the same intermediate speculated by Fu for the formation of Rboratabenzene adducts from 1-PMe3-borabenzene in presence of organic nucleophiles. 18 The ΔE ‡ for the transition state observed in Scheme 2 was calculated to be 27 kcal.mol -1 for L = Py.…”

Insights into the Formation of Borabenzene Adducts via Ligand Exchange Reactions and TMSCl Elimination from Boracyclohexadiene Precursors

“…7 On the other hand some research has been aiming at increasing the strength of these interactions in order to stabilize very reactive Lewis acid sites from further transformations. Such strategy has been recently devised in the work of Braunschweig to stabilize the borolyl anion ( Figure 1A), 8 of Piers to synthesize stable boraanthracene adducts ( Figure 1B), 9 and of Bertrand to generate the highly reactive borylene fragment. Even when generated by flash thermolysis and condensed at 10 K in an argon matrix, the basefree borabenzene moiety has never been isolated or observed.…”

“…The latter result is not surprising since NHCs are known to stabilize Lewis acid orbitals on boron, notably in the generation of highly reactive fragments such as the borolyl anion and boraanthracene. 8,9 It is well documented that boratabenzene species possess -accepting capabilities by the presence of an electrophilic pz orbital on boron, notably evidenced by the planarization of the nitrogen atom on amidoboratabenzene species. 17 η 6 -Borabenzene transition metal complexes also exhibit some level of -acidity as shown by the coplanarity between the borabenzene plane and the conjugated system of the boron coordinated 3-(dimethylamino)acrolein in a chromium(0) borabenzene complex.…”