Boron trihalide adducts of tetramethylurea (1) and of 1,1-bis(dimethylamino)ethylene (2) show similar complexation shifts and restricted rotation about the central C—N bonds. Rotation becomes more hindered as the Lewis acid strength increases in the series BF3 < BCl3 < BBr3. A large high-field complexation shift of the CH2 protons of 2 corresponds to loss of the olefinic structure on formation of the BF3 adduct.
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