Aromatic 1,2‐Azaborinin‐1‐yls as Electron‐Withdrawing Anionic Nitrogen Ligands for Main Group Elements
Felix Lindl, Anna Lamprecht, Merle Arrowsmith, Eugen Khitro, Anna Rempel, Maximilian Dietz, Tim Wellnitz, Guillaume Bélanger‐Chabot, Andreas Stoy, Valerie Paprocki, Dominik Prieschl, Carsten Lenczyk, Jacqueline Ramler, Crispin Lichtenberg, Holger Braunschweig
Abstract
Abstract The 2‐aryl‐3,4,5,6‐tetraphenyl‐1,2‐azaborinines 1‐EMe 3 and 2‐EMe 3 (E=Si, Sn; aryl=Ph ( 1 ), Mes (=2,4,6‐trimethylphenyl, 2 )) were synthesized by ring‐expansion of borole precursors with N 3 EMe 3 ‐derived nitrenes. Desilylative hydrolysis of 1‐ and 2‐SiMe 3 yielded the corresponding N‐protonated azaborinines, which were deprotonated with n BuLi or MN(SiMe 3 ) 2 (M=Na, K) to the corresponding group 1 salts, 1‐M and 2‐M . While the lithium salts crystallized as monomeric Lewis base adducts, the potassium salts formed coordination polymers or oligomers via intramolecular K⋅⋅⋅aryl π interactions. The reaction of 1‐M or 2‐M with CO 2 yielded N‐carboxylate salts, which were derivatized by salt metathesis to methyl and silyl esters. Salt metathesis of 1‐M or 2‐M with methyl triflate, [Cp*BeCl] (Cp*=C 5 Me 5 ), BBr 2 Ar (Ar=Ph, Mes, 2‐thienyl), ECl 3 (E=B, Al, Ga) and PX 3 (X=Cl, Br) afforded the respective group 2, 13 and 15 1,2‐azaborinin‐2‐yl complexes. Salt metathesis of 1‐K with BBr 3 resulted not only in N‐borylation but also Ph‐Br exchange between the endocyclic and exocyclic boron atoms. Solution 11 B NMR data suggest that the 1,2‐azaborinin‐2‐yl ligand is similarly electron‐withdrawing to a bromide. In the solid state the endocyclic bond length alternation and the twisting of the C 4 BN ring increase with the sterics of the substituents at the boron and nitrogen atoms, respectively. Regression analyses revealed that the downfield shift of the endocyclic 11 B NMR resonances is linearly correlated to both the degree of twisting of the C 4 BN ring and the tilt angle of the N‐substituent. Calculations indicate that the 1,2‐azaborinin‐1‐yl ligand has no sizeable π‐donor ability and that the aromaticity of the ring can be subtly tuned by the electronics of the N‐substituent.