1-Tert-butyl-2-methyl-1,2-azaborolyllithium (LiAb, 1) and TlCl equilibrate at -75 Deg in THF under action of light to TlAb (2) and LiCl. Compd. 2 decomps. above -50 Deg with elimination of thallium and the neutral isomers 1-tert-butyl-2,5-dihydro-2-methyl-1H-1,2-azaborole and 1-tert-butyl-2,3-dihydro-2-methyl-1H-1,2-azaborole. While 1 reacts with Ph2PCl in only very small amts. to give the expected Ph2P-substituted azaborole ring 3, TlAb (2) forms this compd. by reaction with Ph2PCl in high yields. The yellow oily 3 is characterized by 1H-, 11B-, 13C-, and 31P-NMR data. The Me3Si-substituted deriv. 1-tert-butyl-3-(diphenylphosphinyl)-2,3-dihydro-2-methyl-3-(trimethylsilyl)-1H-2,2-azaborole (5), available via the Li salt of 3 and ClSiMe3, forms yellow crystals which could be used for an x-ray structure anal. As expected, the Ph2P and Me3Si groups substitute the boron neighboring C3 atom of the planar ring framework. The crystal structure of 5 shows the two expected enantiomers.