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The Gas-Phase Rearrangement of HCCBCCH to a Trans Pentacyclic Isomer. A Joint Experimental and Theoretical Study

A theoretical study has reported that CCBCC will rearrange to planar cyclo-C4B if the excess energy of CCBCC is ≥ 67 kJ mol−1 [calculations at the CCSD(T)/aug-cc-pVTZ//B3LYP/6-31G(d) level of theory]. Cyclo-C4B lies only 4.5 kJ mol−1 above CCBCC. Although we were unable to prepare an ionic precursor (of CCBCC) in sufficient yield to allow the production of neutral CCBCC by neutralisation–reionisation, the analogue HCCBCCH can be prepared by one-electron vertical (Franck–Condon) reduction of cation [HCCBCCH]+. Some of the HCCBCCH neutrals formed in this way are stable for the microsecond duration of the +NR+ experiment, whereas others rearrange. Calculations indicate that energised HCCBCCH may rearrange over a barrier of 203 kJ mol−1 to cyclo-C4H2B, which lies 105 kJ mol−1 above HCCBCCH. The cyclic isomer is not planar like cyclo-C4B, but has trans geometry. The cyclic species is formed with an excess energy of 98 kJ mol−1, and may undergo facile and degenerate formation of the alternative trans structure through a planar C4H2B transition state (+17 kJ mol−1). If the cyclic isomer has an excess energy of ≥ 133 kJ mol−1, it may ring open to give an open chain C4H2B isomer containing a terminal B atom.