Syntheses And Reactivity Of Boron-Nitrogen Conjugated Heterocycles

Borylenes (R-B:) are key intermediates in boron chemistry. This dissertion has been focused on the syntheses and reactivity of N-heterocyclic borylene intermediates by employment of π-conjugated nitrogen-containing ligands, which were expected to stabilize borylenes by lone-pair conjugation with boron empty orbitals and extended π systems. The results have established the basis for understanding the stability and reactivity of elusive N-heterocyclic borylenes. The experimental results are summarized as follows:1. β-diketiminate boron monochloride HC[(CBu’)(NAr)]2BHCl (1, Ar2,6-Me2C6H3), three β-diketiminate boron difluorides HC[(CBu’)(NAr)]2BF2(4, Ar=2,6-Me2C6H3;4’, Ar=2,6-Pr’2C6H3;4", Ar=o-MeC6H4) and β-ketoiminate boron dibromide HC[(CBu’)(NAr)][(CBu’)O]BBr2(6,2,6-Pr’2C6H3) have been prepared and fully characterized. Reduction of these boron halides with alkali metals yielded the ligand C-N bond cleavage products [C(H)(Bu’)CHC(Bu’)NAr]BNAr-Li(THF)2(2) and [C(Bu’)CHC(Bu’)NAr]B=NAr (5,5’), and the intermolecular C-C coupling product {HC[(CBu’)(NAr)][(CBu’)O]BBr}2(7), respectively. The C-N bond cleavage is very likely due to the formation of the N-heterocyclic borylene and boryl anion intermediates, which are supported by DFT calculations. The calculations indicated that the N-heterocyclic borylene intermediate could accept an electron to form nucleophilic radical anion, which cleaves the C-N bond with a very low activation barrier. The single-electron reduction of5by Sml2in the presence of one equivalent of methanol afforded the persistent π-conjugated1,2-azaborolyl radical{[C(But)CHC(But)NAr]BNHAr}’(13), which has been characterized by EPR spectroscopy.2. Boron dichloride [C(But)CHC(But)NAr]BCl2(15, Ar=2,6-Me2C6H3) supported by the α,β-unsaturated imine ligand was prepared and charcaterized. Reduction of15with two equivalents of potassium gave the first B-halide substituted 1,2-azaborolyl anion [C(But)CHC(But)NAr]BCl-K(THF)(16), which was structurally characterized. This anion underwent salt elimination reaction with1,3,4,5-tetramethyimidazoiylidene (NHC), an aryl azide and H2O, affording the NHC-azaborole adduct [C(But)CHC(But)NAr]B(NHC)(17), the iminoborane [C(But)CHC(But)NAr]B=NAr (5) and the diboroxane {[CH(But)CHC(But)NAr]B}2O (18), respectively, indicating that16can be used as a source of the elusive1,2-azaborole diradical{[C(But)CHC(But)NAr]’B}’.3. N,C-coordinated four-membered β-diketiminate boron dichloride [CH(CBut)(NAr)](CBut)(NAr)BCl2(19, Ar=2,6-Me2C6H3) was synthesized, which underwent isomerization upon heating at40℃to form thermodynamically stable N,N-coordinated six-membered β-diketiminate boron complex HC[(CBu’)(NAr)]2BCl2(20). Reactions of19with aryl and amide lithium salts have been studied. With sterically hindered lithium salts MesLi (Mes=2,4,6-Me3C6H2) and LiN(SiMe3)2, C-H bond activation was observed, resulting in cyclopropanation of one of the β-But groups and affording HC[(CBut)(NAr)][C(C4H8)(NAr)]BCl (21) and HC[(CBut)(NAr)][C(C4H8)(NAr)]BN(SiMe3)2(22), while with less hindered lithium salts LiNEt2and PhLi, nucleophilic substitution on the boron atom took place.