Aromatic amine--haloboranes : evidence for trigonal boronium cations

In the last twenty years Lewis adducts of boron trihalides and mixed boron trihalides have been examined using nuclear magnetic resonance spectroscopy. The donor to which the boron trihalide attaches has generally been some type of amine (strong donor), though oxygen, sulfur, phosphorous, and other donors ahve also been studied. The boron trihalide portion serves as the Lewis acid. Various nuclei (1H, 13C, 15N, 11B, 19F) have been examined in such boron trihalide adducts and the general trend in signal position in all 5 nuclei observed is (in order of increasing downfield shift) BF3 The acid strength of the boron trihalide has generally been assumed to be the principal factor determining this order and many physical studeis confirm this order to be BF