乙醯丙酮於溶劑中烯醇與酮的異構化反應動力學與機制

Abstract

β-二羰基化合物，也稱為1, 3-二羰基化合物，在合成、配位化學及生物化學上有許多應用。乙醯丙酮 (AA) 為最簡單形式的β-二羰基化合物，在不同溶劑中以各自的平衡常數與反應速率進行烯醇與酮式間的異構化反應。此實驗透過紫外線吸收光譜法與核磁共振探討AA在水、甲醇、乙醇、乙腈及它們的氘化溶劑中烯醇與酮式間異構化反應的動力學。實驗結果呈現AA的異構化反應在不涉及同位素交換時為一級反應動力學，且各溶劑中異構化速率為水 > 醇類 > 乙腈。此外在CH3CN與醇類中，鹼催化效應分別為269及1715 s−1 M−1；酸催化效應僅在CH3CN中較為顯著，且值比鹼催化效應弱，為4.19 s−1 M−1；在CH3CN與醇類中，水催化效應分別為1.69×10−4及1.51×10−3 s−1 M−1。為了了解受同位素交換影響的偏離一級反應動力學的行為，我們分別在沒受鹼與受鹼催化的條件下提出動力學模型去模擬異構化反應中AA各形式的變化。模擬結果呈現AA異構化反應伴隨著同位素交換，且可透過溶劑形成的架橋與作為反應中間物的烯醇陰離子加速反應。此外，烯醇陰離子可以在沒有異構化反應下作同位素交換。最後AA於水及甲醇中異構化反應的動力學同位素效應 (kinetic isotope effect, KIE, kH/kD) 分別為6.15 ± 0.39與6.35 ± 0.17。

β-diketones, also known as 1,3-diketones, are compounds have a variety of applications on synthesis, coordination chemistry, and biochemistry. Acetylacetone (AA) is the simplest form of β-diketones, tautomerizing between enol form and keto form with an equilibrium constant depending on solvents. In this work, the enol-keto tautomerization of acetylacetone (AA) in water, methanol, ethanol, acetonitrile, and their deuterated solvents was investigated using ultraviolet (UV) spectroscopy and nuclear magnetic resonance (NMR). The results reveal that AA tautomerization follows first order kinetics when isotope exchange is not involved. The tautomerization rates of AA in solvents are water > alcohols > acetonitrile. In addition, base catalytic effects in CH3CN and alcohols are determined to be 269 and 1715 s−1 M−1, respectively. Acid catalytic effects are insignificant in all studied solvents except in CH3CN, where a weak catalytic effect of 4.19 s−1 M−1 is observed. Water catalytic effects are 1.69×10−4 and 1.51×10−3 s−1 M−1 in CH3CN and alcohols, respectively. To account for the non-exponential behavior due to the isotope exchange, two kinetic models were proposed to simulate the concentration profiles under both base-free and base-catalyzed conditions. The results suggest that AA tautomerization and isotope exchange proceed concurrently, facilitated by the solvent bridge and the enolate intermediate. Moreover, the enolate form enables isotope exchange in the absence of tautomerization. Finally, kinetic isotope effects (KIEs, kH/kD) of AA tautomerization in water and methanol are determined to be 6.15 ± 0.39 and 6.35 ± 0.17, respectively.