| Abstract
| - The Leitner and Fürstner groups reported (Fürstner, A.; Koch, D.; Langemann, K.; Leitner, W.; Six, C. AngewChem., Int. Ed. Engl.1997, 36, 2466) on the ring closing metathesis (RCM) of a 16-membered diene dissolvedin supercritical CO2 (scCO2). The authors found that the cyclic product, indicative of an intramolecular RCMevent, was formed in excellent yield when the CO2 density was high, but oligomers were formed by anacyclic diene metathesis (ADMET) pathway at lower CO2 densities. These results suggest that changes in theCO2 density lead to changes in the intra- vs intermolecular interactions between the 16-membered dienedissolved in scCO2. To assess this issue in more detail, we have prepared 6-(1-pyrenyl)hexyl-11-(1-pyrenyl)undecanoate [1-Py(CH2)10COO(CH2)61-Py] in which we replaced the terminal alkenes of Letiner and Fürstner'soriginal diene with the fluorophore pyrene. We have studied the pyrene excimer formation of 1-Py(CH2)10COO(CH2)61-Py when it is dissolved in five organic solvents (cyclohexane, dichloromethane, ethanol,acetonitrile, and dimethyl sulfoxide) and supercritical carbon dioxide (scCO2) to determine how the tail segmentsinteract with each other. The result show that the excimer formation mechanism is completely different when1-Py(CH2)10COO(CH2)61-Py is dissolved in scCO2 or organic liquids. In liquids, excimer formation is purelydynamic in nature, there are two formation pathways to the excimer, and all the rates can be understood withthe help of Kamlet−Taft linear solvent energy relationships. In scCO2, we found that the 1-Py(CH2)10COO(CH2)61-Py excimer-to-monomer intensity ratio (E/M) correlates directly with (1) the observed RCM yieldfor Leitner and Fürstner's original 16-membered diene and (2) the solvent refractive index function. Thesteady-state and time-resolved fluorescence of 1-Py(CH2)10COO(CH2)61-Py dissolved in scCO2 show thatthere are two excimers that form in scCO2 and their relative contributions change in a systematic way withchanges in the CO2 pressure/density. Interestingly, the typical dynamically formed excimer species that emitsat 470−480 nm (E1) forms within 2 ns of optical excitation; however, it is not the dominant species at lowCO2 densities. E1 is equivalent to the species that goes on to form the RCM product in Leitner and Fürstner'soriginal reaction. The second excimer (E2) emits in the 410−440 nm region. E2 is associated withintermolecular preassociated forms of the pyrene residues within a collection of 1-Py(CH2)10COO(CH2)61-Pymolecules, and this species dominates at low CO2 densities. E2 is equivalent to the species that goes on toform the oligomeric product in the original Leitner and Fürstner reaction. As the CO2 density increases, theE1 excimer contribution increases relative to the E2 excimer contribution. The combination of the fluorescenceand reaction outcome results are used to explain Leitner and Fürstner's previous density-dependent RCMyields.
|
