Chiral 2,3-dihydro-1H-1,3,2-diazaboroles

Article abstract of DOI:10.1002/1099-0682(200209)2002:9<2438::AID-EJIC2438>3.0.CO;2-W

A series of differently substituted chiral 2,3-dihydro-1H-1,3,2-diazaboroles has been prepared by various methods. 2-Bromo-1-tert-butyl-3-[(S)-1-phenylethyl]-2,3-dihydro-1H-1,3,2-diazaborole (3a), 2-bromo-1,3-di[(S)-l-phenylethyl]-2,3-dihydro-1H-1,3,2-diazaborole (3b) and 2-bromo-1,3-di[(S)-1-cyclohexylethyl]-2,3-dihydro-1H-1,3,2-diazaborole (3c) were formed from the reaction of the corresponding 1,4-diazabutadienes and boron tribromide and the subsequent reduction of the resulting borolium salts [R¹N^a = CH-CH= N^b(R²)BBr₂] Br(N^a-B) [2a: R¹ = tBu, R² = CH(Me)Ph; 2b: R¹ = R² = CH(Me)Ph; 2c: R¹ = R² = CH(Me)(cC₆H₁₁)] with sodium amalgam. Treatment of (S)-3a with LiAlH₄ or methyllithium afforded 1-tert-butyl-2-hydro-3-[(S)-1-phenylethyl]-2,3-dihydro-1H-1,3,2-diazaborole [(S)-6] and 1-tert-butyl-2-methyl-3-[(S)-1-phenylethyl]-2-3-dihydro-1H-1,3,2-diazaborole [(S)-7], respectively. Aminolysis of the BBr bond of (S)-3a by tertbutylamine or (S)-1-phenylethylamine gave the corresponding 2-tert-butylamino- and 2-[1-phenylethylamino]-2,3-dihydro-1H-1,3,2-diazaboroles (S)-8 and (S,S)-9, respectively. Similarly, (S,S)-3b and (S,S)-3c were reacted with tert-butylamine to furnish the 2-tert-butylamino-2,3-dihydro-1H-1,3,2-diazaborole derivatives (S,S)-11 and (S,S)-12, respectively. The 2-trimethylstannyl-2,3-dihydro-1H-1,3,2-diazaboroles (S,S)-10 and (S,S)-14 were accessible from 3b or 3c and trimethylstannyllithium. The transformation of achiral 2-bromo-1,3-di-tert-butyl-2,3-dihydro-1H-1,3,2-diazaborole into the chiral (S)-2-[1-phenylethylamino] and (S)-2-[1-cyclohexyl-ethylamino] derivatives (S)-15 and (S)-16 was effected by aminolysis with enantiomerically pure (S)-1-phenylethylamine or (S)-1-cyclohexylethylamine. The novel compounds were characterized by ¹H, ¹¹B, ¹³C, and ¹¹⁹Sn NMR spectroscopy as well as mass spectrometry and determination of the optical rotation. The molecular structure of compound 3c was confirmed by X-ray structural analysis. Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002.