Structural requirements for the formation of the B2XN2 heterocycles.
The reactions of the bis(diorganoboryl) chalcogenides (R2B)2X (I; R = Et, X = O; R2 = 1,5-cyclooctanediyl, X = O, 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, resp.] have been investigated. MPz reacts with I (R2 = 1,5-cyclooctanediyl, X = S) (II) to form a heterocycle, which due to the interaction of the Me substituent with the C skeleton of II is unstable and readily rearranges to the novel heterocycle III (x-ray anal.) when dissolved in toluene at room temp. In contrast, the analogous heterocycle formed from mPz and I (R2 = 1,5-cyclooctanediyl, X = O) is stable even when heated at 100 Deg. The reaction of I (R2 = 1,5-cyclooctanediyl, X = O) with m2Pz and mpPz produces 1:1 adducts in which the pyrazole fluctuates between the two B atoms of I. I (R2 = 1,5-cyclooctanediyl, X = O) showed only weak interactions with mpPz and none with p2Pz and (tb)2Pz. With I (R = Et, X = O), the pyrazoles m2Pz, mpPz, and p2Pz form the corresponding heterocycles; however, no reaction was obsd. with (tb)2Pz (1H, 11B, 13C NMR).
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