Cyclodextrin induced switch between heterolytic and homolytic dediazoniation mechanisms

摘要

We have investigated the kinetics and mechanism of dediazoniation of 4-nitrobenzenediazonium (4NBD) tetrafluoroborate in the presence of α-cyclodextrin, α-CD, and γ-cyclodextrin, γ-CD, under acidic (HCl, pH=2) conditions by employing a combination of spectrometric and chromatographic techniques. In the absence of CDs, dediazoniation follows first-order kinetics, with t _(1/2)=22,000s at T=60°C, but addition of small amounts of either α-CD or γ-CD leads to more rapid but not first-order kinetics with t _(1/2)~400s when [α-CD]=20 [4NBD] or [γ-CD]=15 [4NBD]. Analyses of reaction mixtures by HPLC indicate that three main dediazoniation products are formed depending on the particular experimental conditions. These are 4-nitrophenol, ArOH, nitrobenzene, ArH, and 4-nitrochlorobenzene, ArCl. In the absence of CDs, the main dediazoniation product is the substitution product ArOH, but on increasing the concentration of CD, the reduction product ArH becomes predominant at the expense of ArOH, indicating that a switch between the heterolytic and homolytic reaction mechanisms take place under acidic conditions, where little significant ionization of the OH groups of the CDs takes place (pK _a≈12). Addition of the surfactant sodium dodecylsulfate, SDS, blocks the CD cavity inhibiting 4NBD dediazoniation and decreasing the yields of ArH with a concomitant increase in that of ArOH, suggesting that 4NBD ions form an inclusion complex prior to reacting with the OH groups of the CDs. This O-coupling reaction leads to the formation of a highly unstable Z-diazo ether adduct that cannot isomerize to the much more stable E-isomer because of the geometric restrictions imposed by the CD cavity, splitting homolitically.