Monobenzofused 1,4‐Azaborines: Synthesis, Characterization, and Discovery of a Unique Coordination Mode

Abstract

We report the first general synthesis of boron-substituted monobenzofused 1,4-azaborines using ring-closing metathesis of an enamine-containing diene as a key synthetic strategy. As part of our investigations, we discovered that the B-C3 moiety in a 1,4-azaborine can serve uniquely as a η(2)-L-type ligand. This functionality is exemplified by two κ(2)-N-η(2)-BC Pt complexes of a boron-pyridyl-substituted monobenzofused-1,4-azaborine. Single-crystal X-ray diffraction analysis of the Pt complexes shows a strong structural contribution from the iminium resonance form of the monobenzofused-1,4-azaborine ligand. We also demonstrate that a palladium(0) complex supported by a 1,4-azaborine-based phosphine ligand can catalyze hydroboration of 1-buten-3-yne with unique selectivity. In view of the importance of arene-metal π-interactions in catalytic applications, this work should open new opportunities for ligand design involving the 1,4-azaborine motif as an arene substitute.

Synthesis, structure and aromaticity of carborane-fused carbo- and heterocycles

“…The reaction of o -carboranyl triflate ( 13 ) with lithium N -allylmethylamide afforded the corresponding amine 14 as a colorless liquid in 53% yield. 11 Ruthenium-catalyzed olefin isomerization 12 and cage carbon iodination gave compound 16 as a light yellow solid in 50% yield. UV irradiation of 16 produced carborane-fused pyrrole ( 17 ) as colorless crystals in 30% yield ( Scheme 4 ).…”

Mechanism and Origin of the Stereoselectivity in the Palladium‐Catalyzed trans Hydroboration of Internal 1,3‐Enynes with an Azaborine‐Based Phosphine Ligand

“…Notably,b ased on the work reported by Liu et al,aunique k 1 -P-h 2 -BC coordination between the Pd atom and the ligand attracted our attention. [8,9] We envisagedt hat this coordination might be crucial to the trans hydroboration of the enyne, and initiated ac omplementary theoretical investigation to unveil the details.…”

Site-Selective and Stereoselective trans-Hydroboration of 1,3-Enynes Catalyzed by 1,4-Azaborine-Based Phosphine–Pd Complex

“…In a recent example, we disclosed that the 1,4-azaborine-based pyridine ligand A (Scheme 1) exhibits a κ 2 N - η 2 -BC coordination with group 10 transition metals and that the phosphine derivative B in conjunction with Pd could uniquely catalyze the hydroboration of a terminal enyne in a trans -selective fashion. 7 In contrast to B , the corresponding carbonaceous phosphine ligand isostere C behaves more similarly to a monodentate phosphine such as PPh 3 in terms of hydroboration selectivity and yield, producing preferentially allenes via 1,4-hydroboration; bisphosphine ligands such as 1,2-bis(diphenylphosphino)ethane (dppe) furnish syn -hydroboration products exclusively. 8 Despite this promising preliminary result, our trans -hydroboration reaction still needed optimization with regard to yield and selectivity.…”

“…Our original synthesis of Senphostype ligands via ring-closing metathesis 7 was unfortunately not amenable to scale-up 15 and ready modification of the C(3) position, which we believe could play an important role due to its proximity to the catalytically active Pd metal. This synthetic limitation significantly hampered our ability to develop a general and versatile trans -hydroboration protocol.…”