1,4-Hydride shifts in ortho-alkyl-substituted di(1-adamantyl)benzyl cations: An NMR spectroscopic and X-ray crystallographic study

When carbocations are formed from ortho-alkyl-substituted phenyldi(1-adamantyl)-methanols in the anti conformation, the alkyl group being isopropyl or ethyl, rapid 1,4-hydride transfer from the alkyl group to the charged carbon occurs, giving either styrene (o-isopropyl) or secondary (o-ethyl) derivatives. C-13 and H-1 NMR spectroscopy and, in one case, single crystal X-ray crystallography show that all products are the syn rotamers, with the benzylic hydrogen of the diadamantylmethyl group oriented towards the ortho substituent. Even in the presence of triethylsilane no anti isomer is formed; instead, the rearranged carbocation is wholly or partially reduced by hydride transfer from the silane.