Kinetic Studies on the Rearrangements of B-Monochloro Derivatives of closo-2,4-C2B5H7 and on the Rearrangement of 5,6-Cl2-closo-2,4-C2B5H5. Characterization of All B,B'-Cl2-closo-2,4-C2BsHs Isomers

The rearrangement patterns for two B-monochloro derivatives of closo-2,4-C2B5H7 suggest that a triangular-face-rotation mechanism for such a reaction in this set of compounds is highly unlikely. The relative stabilities of the B,B'-dichloro derivatives of this cage carborane system have been determined from a rearrangement rate study on one of the isomers; a correlation of these B,B-Cl2-C2B5H5 isomer stabilities with the results from the thermal equilibration of the B-monochloro set of isomers leads to the suggestion that, with the exception of the 1,3-C12 isomer, a (substituent) positional additivity effect is operating. An electronic interaction through the cage between a C1 located in an apical position and a C1 located at the unique equatorial 3-position may account for the stability of the 1,3-C12 isomer. All of the B,B'-Cl2-closo-2,4-C2B5H5 isomers are characterized for the first time. A slow side reaction observed during the rearrangement reactions involves intermolecular chlorine exchange; the rate of this halogen exchange is considerably reduced when the rearrangements are carried out at reduced pressures.