Abstract The exothermic reaction of BX 3 (X= F, Cl, Br) with 1, 1, 3, 3-tetramethyl-1, 3disilacyclobutane (I) gave the ring-cleavage product XMe 2 SiCH 2 Me 2 SiCH 2 BX 2 (II) in almost quantitative yield. The order of activity was BBr 3> BCl 3> BF 3. Heating compounds of structure II at 180 C for 40 h did not affect IIa (X= F) but caused a complete rearrangement of IIc (X= Br) to BrMe 2 SiCiH 2 MeBrSiCiH 2 BMeBr (IIIc). Compound IIb (X= Cl) was ...