The reaction of (isopropyloxy)bis(2-thienyl)borane (1d) with Grignard reagents leads to methylbis(2-thienyl)borane (1e) and to phenylbis(2-thienyl)borane (1f). Treatment of mono-lithiated heterocycles with dichloroboranes yields the bis(2-heteroaryl)boranes 1b, 1c, and 3a, which are dibrominated with NBS to give the 5,5'-dibromo products 1b', 2a', and 3a'. NMR spectroscopic and MS data confirm the composition of the compounds. The [2 + 2] cyclization of the dibromo derivatives yields, in the case of 2a', the macrocycle 4b, as confirmed by HR-MS data. ((C) Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002).