Alkali metal reduction of 2-halogeno- and 2-thiolato-2,3-dihydro-1H-1,3,2-diazaboroles

The reduction of 2-bromo-1,3,2-diazaborole tBuNCHCH(tBu)BBr (4c) with a potassium mirror in 1,2-dimethoxyethane afforded a non-separable 1 ∶ 2 ∶ 1 mixture of the compounds tBuNCHCHN(tBu)BH (5), tBuNCHCHN(tBu)BOCH3 (6) and {tBuNCHCHN(tBu)B}2O (7). Reaction of 4c with a potassium–sodium alloy in N,N,N′,N′-tetramethylethylenediamine led to 5 as the major product. 1,3,2-Diazaboroles tBuNCHCHN(tBu)BNMe2 (8) and {tBuNCHCHN(tBu)B}2 (9) were spectroscopically identified as minor products. The treatment of 4c with potassium–sodium alloy in toluene solution in the presence of [15]crown-5 yielded a 1 ∶ 1 mixture of 5 and the benzyl derivative tBuNCHCHN(tBu)BCH2Ph (12). The same reaction in toluene-d8 produced the deuterated species 5-d1 and 12-d7. tBuNCHCHN(tBu)BCH3 (17) and 1,4-diazabutadiene (tBuNCH)2 (18) resulted from the treatment of tBuNCHCHN(tBu)BSCH3 (15) with potassium–sodium alloy in n-hexane. In contrast to this, compound 9 was obtained as the main product of the reduction of tBuNCHCHN(tBu)BStBu (16) under similar conditions. The reduction of the 1-bromo-2-tert-butyl-1,2-dihydro[1,3,2]diazaborolo[1,5-a]pyridine (19) smoothly produced the respective diborane(4) derivative (20) which was subjected to X-ray diffraction analysis.