mu-H-bridged bicyclo[3.3.3]undecyl cations. Theoretical calculations of physical and chemical properties

mu-H-Bridged carbocations 1 and 2, which are still unknown experimentally, are structures with fascinating possibilities as intermediates for the synt hesis of very strained in-bicyclic and tricyclic alkanes and alkenes. They are also expected to possess record high pK(a) values. In conjunction with our experimental program to try to prepare 1 and 2, we have carried out ab initio calculations on these structures and various reference compounds, wi th the aim of assessing just how stable these cations might be, and what ph ysical and chemical properties they might possess. The results of these stu dies confirm that 1 and 2 should be viable species; they have energies simi lar to those of the conventional out-cations, and the mu-H bond distances a re not very different from those calculated for known mu-H cations. The cal culated H-1 NMR chemical shift for the mu-H of 1 is -12 +/- 0.5 and -9.5 +/ - 0.5 for 2, both values considerably more negative than in known mu-H cati ons. The possible effect of large amplitude motions of the mu-H on the H-1 NMR chemical shift was investigated and not found to be significant. The pK (a)s for 1 and 2 are estimated to be 17-18 with respect to an alkene conjug ate base, a virtually unimaginable size for a formally alkyl cation. The pa per also discusses the possibility of the mu-H being involved in the acid-c onjugate base chemistry, since this is shown to be a hugely exothermic reac tion. Finally, the alkenes and alkanes associated with cations 1 and 2 have been calculated, the in-alkene 7 from cation 1 is shown, for example, to p ossess a large steric strain, yet this structure should be accessible via d eprotonation of 1 with a strong base.