Hydroboration of 2,5-diallylpyrrole by using a mixture diethylborane/triethylborane (Et2BH/Et3B) leads to the first tricyclic N-pyrrolylborane 3 which possesses an exceptionally stable B-N bond. In accordance with the pronounced Lewis-acidic character of 3, it is readily converted into adducts (6) or berates (9). The crystal structure of 3 was determined by X-ray analysis (triclinic, space group
, Z = 8) showing a fairly long B-N bond (mean value 144 pm), the first example of a diorgano N-pyrrolyl borane in which the pyrrole ring is not twisted against the C2BN-plane. Nuclear magnetic shielding (B-11, C-13, N-14) was calculated using the GIAO procedure, BN(pp)pi bonding is discussed on the basis of MO calculations. (C) 1998 Elsevier Science S.A.