Structural requirements for the formation of the B2ElVIN2 heterocycles

The reactions of the bis(diorganoboryl) chalcogenides (R2B)2ElVI [R = Et, ElVI = O (9); R2 = 1,5‐C8H14 (1, ElVI = O), (2, ElVI = S)] with 3‐methyl‐, 3,5‐dimethyl‐, 3‐methyl‐5‐phenyl‐, 3,5‐diphenyl‐, and 3,5‐di‐tert‐butylpyrazole [mPz, m2Pz, mpPz, p2Pz, and (tb)2Pz, respectively] have been investigated. mPz reacts with 2 to form the heterocycle m5, which due to the interaction of the methyl substituent with the carbon skeleton of 2 is unstable and readily rearranges to the novel heterocycle 7 (X‐ray analysis) when dissolved in toluene at room temperature. In contrast the analogues m4 (El = O), formed from mPz and 1, is stable even when heated at 100°C. The reaction of 1 with m2Pz and mpPz does not produce stable heterocycles of the type 4, instead the 1:1 adducts m2Pz‐1 and mpPz‐1 are formed in which the pyrazole fluctuates between the two boron atoms of 1. The diboryl oxide 1 showed only weak interactions with mpPz and none with p2Pz and (tb)2Pz. With 9 the pyrazoles m2Pz, mpPz, and p2Pz from readily the corresponding heterocycles 10, however no reaction was observed with (tb)2Pz (1H‐, 11B‐, 13C‐NMR).