A n e w titanium(ii1) mediated [(C5H5)2TiCl] reduction of 2,3-epoxy alcohols to alk-1-en-3-01s is described.Transition metal mediated reactions provide new dimensions for selectivity in organic synthesis. On examining the titanium(m) induced homolysisl.2 of the C-0 bond of 2,3-epoxy alcohol (1) with the possibility of enhancing its utility, we observed remarkable regioselectivity ; deoxygenation of (1) occurred exclusively from the least substituted carbon end to produce terminal alkenic alcohol (2) [equation (l)] in high yield.The high regioselectivity observed may be due to the participation of the hydroxy group in the transition state. Accordingly, a workable mechanism has been postulated for the deoxygenation of 2,3-epoxy alcohols with (CsH5)2TiC1, which is depicted in Scheme 1. The initial step is the formation of TiI*I-alkoxide (3) and HC1 which consumes a stoicheiometric amount of (C5H5)2TiCl. Compound (3) reacts in the usual way3.4 with (C5HS)zTiCl to produce the radical (4) which subsequently participates intramolecularly with the unfilled d-orbital of Ti111 to-form (S), an intermediate proposed by

The structure of the bisannelated semibullvalene 11, which was originally predicted to be in a symmetrical homoaromatic ground state, was investigated by ultralow-temperature diffraction experiments down to 15 K and by related HF calculations. High-resolution X-ray data taken at 15 K (sin θ/λ max ) 1.074 Å -1 ) provided a detailed electron density distribution that allowed a full topological analysis. The experimental and HF electron densities for 11 are in excellent accord. The electron density regions between C(2,8) clearly reveal a "normal cyclopropyl σ-bond" and very sparse density between C(4,6) corresponding with little or no interaction between these latter two centers. These results certainly confirm that in the solid state 11 is not homoaromatic. The experimental data at 15 K provide limiting values of structural parameters for a single tautomer of 11. In particular, the C(2,8) and C(4,6) distances are extremely useful base values for a bisannelated semibullvalene with an exceptionally low activation barrier to the Cope rearrangement.

The synthesis and study of a novel bisannelated semibullvalene that is calculated to be a neutral homoaromatic ground state molecule is presented. Through variable-temperature X-ray, 13 C solution-phase NMR, and 13 C solid-state CP-MAS NMR studies and by a modified Saunders' isotopic perturbation method, 1,5-dimethyl-2,4,6,8-semibullvalenetetracarboxylic dianhydride is shown to be a highly fluxional molecule that is not homoaromatic in the solution or solid states. The barrier to the Cope rearrangement of this dianhydride, in solution, is estimated (from 125 MHz 13 C NMR data) to be less than 14 kJ mol -1 .

Dedicated to Professor Jack D. Dunitz. K. H. has known Jack for about 55 of his 80 years beginning when we were both post-doctoral fellows at Caltech. Those were fun times!The bridged homotropilidines have been of interest for decades because their molecules offer the potential for homoaromaticity. Although many of these have been shown not to be homoaromatic, the energy differences of the delocalized (homoaromatic) forms and the localized (nonhomoaromatic) ones, and the barriers to the interconversion of the localized forms via a Cope rearrangement, have been found to vary greatly. The title compound is a strong candidate for homoaromaticity, and, since the structures of the possible localized and delocalized forms could differ significantly, we have carried out an electron-diffraction investigation of it augmented by quantum-mechanical calculations with different basis sets at several levels of theory. Three models were explored: one representing a localized form of C s symmetry, one a delocalized form of C 2v symmetry, and one a 2 : 1 mixture of the localized/delocalized forms. Although none of the models could be ruled out, the experimental evidence slightly favors the C s form. These results are consistent with those from the DFT B3PW91 calculations with basis sets ranging from 6-31G(d) to cc-pVTZ, which, surprisingly, predict essentially equal thermally corrected free energies for each. The results are discussed.Introduction. ± The molecular structures of the bridged homotropilidines, of which bullvalene and semibullvalene (1) are important examples, have been of great interest for well over three decades. One reason for this interest is that the molecules might be homoaromatic [1] as a consequence of a −negative× barrier to a Cope rearrangement [2] as illustrated below for the skeleton of semibullvalene (1). The homoaromatic, or delocalized, form of the molecule, 1b, is expected to have C 2v symmetry, and the nonhomoaromatic, or localized, forms 1a and 1c should have C s symmetry. The most obvious structural differences between the homoaromatic and localized forms lie in the nearest-neighbor distances between atoms in the six-membered ring of 1b, and the corresponding atoms in 1a and 1c: in 1b the C 2 ÀC 3 , C 3 ÀC 4 , C 6 ÀC 7 , and C 7 ÀC 8 distances should be characteristic of C,C aromatic bonds (ca. 1.4 ä), while the C 2 ÀC 8 and C 4 ÀC 6 distances are expected to be elongated (ca. 2 ä), but in 1a (1c) their values as double, single, and nonbonds are anticipated [3].There is abundant experimental and theoretical evidence that the ground state of semibullvalene itself is not homoaromatic (for a summary of current information about

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