PCCPs
Motivated by a desire to develop a new class of strong acid catalysts that would be easy to synthesize, trivial to derivatize, and structurally distinct, we showed that pentacarboxycyclopentadienes or “PCCPs” were a viable aromatic ion-based option (Science 2016, 351, 961). The parent compound—the penta methyl ester—was originally made by Otto Diels. We found that a simple transesterification of this readily available material could furnish chiral derivatives that were highly effective for asymmetric catalysis. For example, we showed that the PCCP acid derived from menthol was an effective catalyst for enantioselective Mannich and oxocarbenium aldol reactions.
Simple Access to PCCP Derivatives
To maximize the utility of these species, we devised simple procedures to convert the Diels PCCP, which can be made on large scale (Synthesis 2019, 51, 1135), into a variety of derivatives including chiral esters, amides, and highly stabilized fluorinated species (Org. Lett. 2017, 19, 4227).
Enantioselection via a Helically Chiral Anion
We found that chiral PCCPs could catalyze the enantioselective inverse electron-demand Diels-Alder cycloaddition of oxocarbenium ions (J. Am. Chem. Soc. 2018, 140, 3523). Working with the group of Prof. Mathew Vetticatt at SUNY-Binghamton, we discovered that the asymmetric induction occurs by way of a helically chiral PCCP anion. Among the several key organization elements implicated by our study was a C–H to aryl anion interaction between the vinyl ether component and the cyclopentadienyl anion ring.
Cationic Polymerization Under Ambient Conditions
In collaboration with Brett Fors and his group, we have found that PCCPs initiate controlled cationic polymerizations of vinyl ethers, even in the presence of impurities such as water (J. Am. Chem. Soc. 2019, 141, 10605). With the addition of a three-point hydrogen bond donor, the rates of polymerization are greatly increased but without decreasing robustness to impurities, leading to the ability to make high molecular weight polymers (Angew. Chem. Int. Ed. 2021, 60, 4535). Recently, this chemistry has been applied to the synthesis of high molecular weight poly(2,3-dihydrofuran) (J. Am. Chem. Soc. 2022, 144, 15727) and to the development of a moisture-tolerant cationic RAFT polymerization (Polymer Chem. 2022, 13, Advance Article.