Proton-induced intra-complex hydride transfer involving bicyclo[2.2.2]octane units as a rigid spacer and as a carbocation precursor

Matthias C, Bredenkötter B, Kuck D. Proton-induced intra-complex hydride transfer involving bicyclo[2.2.2]octane units as a rigid spacer and as a carbocation precursor. In: International Journal of Mass Spectrometry. INTERNATIONAL JOURNAL OF MASS SPECTROMETRY. Vol 228. ELSEVIER SCIENCE BV; 2003: 321-339.An alpha,omega-diphenylalkane bearing a tert-butyl group at one of the aromatic rings and a rigid and, ostensibly, inert alicyclic spacer unit (bicyclo[2.2.2]octa-1,4-diyl) within the aliphatic chain has been synthesized and subjected to protonolysis under CI(CH4) conditions. The metastable [M + H](+) ions of this hydrocarbon, 1-benzyl-4-(4-t-butylbenzyl)bicyclo[2.2.2]octane, undergo predominantly loss of isobutane via intermediate ion/neutral complexes. By contrast to expectation and to the straightforward interpretation of the results, and at variance to the fragmentation behavior of many related tent-butyl-substituted alpha,omega-diphenylalkanes lacking such a spacer, the intra-complex hydride transfer reaction leading to the isobutane loss involves the bicyclo[2.2.2]octane unit as the hydride donor. This has been revealed by careful deuterium labeling of not only one of the benzylic methylene groups but both of them. Notwithstanding these unexpected findings, it is also demonstrated that the bicyclo[2.2.2] octane unit can be used to generate ion/neutral complexes that contain the bicyclo[2.2.2]oct-1-yl cation solvated to an alpha,omega-diphenylalkane. In this case, well-behaved intra-complex hydride transfer has been observed between the bicyclic carbocation and alkylbenzene. (C) 2003 Elsevier Science B.V All rights reserved