| Abstract
| - The halocyclization reaction of 4-penten-1-ol mediated by various bis(2-substituted pyridine) and(2,6- disubstituted pyridine)bromonium triflates (P2Br+OTf-) was investigated to determine theinfluence of the substituents on the mechanism of reaction. In all cases, the reaction proceeds viaa two-step process where the starting P2Br+ reversibly dissociates to a reactive monosubstitutedPBr+, which then is captured by 4-penten-1-ol to form halocyclized product (2-bromomethyltetrahydrofuran). The dissociation rate constant of P2Br+ (kd) is sensitive to the steric bulk at the 2-and 6-positions, and in the case of the 2,6-dicyclohexylpyridine or 2,6-dicyclopentylpyridine, theP2Br+ species are too unstable to isolate. The partitioning ratio of the reactive intermediate (PBr+)between reversal and product formation (k-d/k2) is not particularly sensitive to the nature of thepyridine, the limiting values being 3−7 except in the case of bis(2(−)-menthylpyridine)bromoniumtriflate where the k-d/k2 ratio is ∼80. The reaction of 4-penten-1-ol and its OD isotopomer withbis(lutidine)bromonium triflate was investigated to determine the deuterium kinetic isotope effect(dkie) on the bromocyclization reaction. The (k-d/k2)H/D ratio is 1.0, indicating that the rate-limitingstep for the bromocyclization is probably formation of a PBr+−4-penten-1-ol complex which doesnot involve substantial changes in the bonding of the OH. The cyclization of 4-penten-1-ol and4-pentenoic acid mediated by bis(2(−)-menthylpyridine)bromonium triflate produces an enantiomericexcess in the cyclized products of only 2.4% and 4.8% respectively.
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