Aerobic Oxidation Of Substituted Cyclopentadienes To Pyrylium Cations In Solution

Pyrylium salts are a type of heterocyclic compound with a positive charge character at a and y positions. Because of this property, they not only have played an important role as versatile synthetic intermediates for complex heterocyclic skeletons, but also opened up broad prospects for practical applications in Q-switchers , laser dyes, sensors and recently in photocatalysis and gene-transfer etc.Very often, pyrylium cations are obtained by a lengthy stepwise manner hinging on the formation and cyclization of a 1,5-diketone, which follows the mechanism of condensation-dehydrogenation typically. In contrast, the preparation from a ring oxidation is a new approach. In our previous work, we discovered the formation of pyrylium salts through the reaction of phenyl substituted cyclopentadienes with silver(I) perchlorate, where the Ag(I) ion most likely fouctioned as both an oxidant and a Lewis acid catalyst. However, the high price of silver(I) perchlorate, low yield of pyrylium salts and the special request for reactive environment restricted its application as a reactant in this approach. Recently, the use of molecular oxygen as terminal oxidant has received considerable attention for both economic and environmental benefits. With the aim to examine whether molecular oxygen can be used in place of silver (I) ion in the cyclopentadiene-ring oxidation, and to investigate the mechanism and generality of this novel synthetic reaction, we have targeted some cyclopentadiene derivatives as our investigated molecules. The main points of this dissertation are as follows:1. As precursor compounds, a series of cyclopentadiene derivatives were synthesized, including 1,2,4-triphenyl-cyclopentadiene, 1,2,3,4-tetraphenyl-cyclopentadiene, 1,2,3,4,5-pentap -henyl-cyclopentadiene, 5-methyl- and 5-ethyl-1,2,3,4-tetraphenyl-cyclopentadiene, 1,2,3,4,5-pentamethyl-cycyclopentadiene, 1,2,3,4-tetrachloro-cyclopentadiene, 2,3,4,5-tetraphen yl cyclopentadienyl-1-diol, 2,3,4,5-tetraphenyl cyclopentadienyl-1-methyl ether, et al. Some effective improvements had been made during the course of syntheses.2. The reactions of cyclopentadiene derivatives with triplet molecular oxygen (i.e. ground state molecular oxygen) in organic solvents under the assistance of protic acids were explored. The structures of the forming pyrylium salts were identified by NMR, ESI-MS, IR, UV-Vis and Single Crystal-XRD. The reaction results indicated that the cyclopentadiene-rings with electron-donating substituents could be easily oxidized to pyrylium cations, but the...