Unstable Compounds. Synthesis and Experimental and Computational Study of the Chemical Behavior of 9--9-xanthydrol

Article abstract of DOI:10.1021/jo00280a026

The title alcohol (1) was synthesized in good yield by a novel umpolung method involving reaction of the lithium dianion of 9-xanthone with tropylium ion.It was prepared to see if its derived carbenium ion might undergo spontaneous α fragmentation to form tropylium ion and singlet-state xanthylidene.In contrast to expectation, treatment of 1 with Broensted and Lewis acids under a variety of experimental conditions produced 9-benzylidenexanthene (8) as the exclusive dehydration product.Stable ion and trapping studies established that the 9-benzyl-9-xanthyl cation was the immediate precursor of 8.When this cation was generated from 1 or 8 and trapped with triethylsilane-d₁, 9-benzylxanthene-9-d₁ was produced with primary deuterium kinetic isotope effects of 1.27 +/- 0.07 and 1.28 +/- 0.07, respectively.The energy surfaces leading to contraction of the cycloheptatrienyl ring to the phenyl ring were explored by molecular mechanics and MNDO calculations.The computational results indicated that the most probable mechanism involved the intervention of a phenonium ion intermediate but not a tropylium ion.It was estimated that the activation energy of the reaction pathway leading to the observed ring-contracted product was ca. 31.0 kcal/mol lower than that of the unobserved α-fragmentation route (40.1 vs 71.1 kcal/mol).