The Origin of Anion−π Autocatalysis

Abstract

The autocatalysis of epoxide-opening ether cyclizations on the aromatic surface of anion−π catalysts stands out as a leading example of emergent properties expected from the integration of unorthodox interactions into catalysis. A working hypothesis was proposed early on, but the mechanism of anion−π autocatalysis has never been elucidated. Here, we show that anion−π autocatalysis is almost independent of peripheral crowding in substrate and product. Inaccessible asymmetric anion−π autocatalysis and sometimes erratic reproducibility further support that the origin of anion−π autocatalysis is more complex than originally assumed. The apparent long-distance communication without physical contact calls for the inclusion of water between substrate and product on the catalytic aromatic surface. Efficient anion−π autocatalysis around equimolar amounts but poor activity in dry solvents and with excess water indicate that this inclusion of water requires high precision. Computational models suggest that two water molecules transmit dual substrate activation by the product and serve as proton shuttles along antiparallel but decoupled hydrogen-bonded chains to delocalize and stabilize evolving charge density in the transition state by "anion−π double bonds". This new transition-state model of anion−π autocatalysis provides a plausible mechanism that explains experimental results and brings anion−π catalysis to an unprecedented level of sophistication.

Anion‐(π)n‐π Catalytic Micelles

“…A complementary 3.4-fold increase of catalysis, rather than decrease of autocatalysis, in Triton X-100 micelles demonstrated that the less hydrophobic substrate 14 stays near the surface of the Triton micelles to benefit from the known [27,48] contributions of water to catalyze ether cyclizations (Figure 3h, * vs !). In clear contrast, anion-(π) n -π autocatalysis as in TS2 excels with two molecules of water [47] sandwiched between substrate and product on π surfaces in apolar environment with the more hydrophobic 13 deep within micelles but suffers in the presence of excess of water with the less hydrophobic 14 at the surface of NDI micelles 12. The depth of micelle penetration thus accounts for the maximal activity reached with substrate 13, sufficiently hydrophobic to interface for anion-(π) n -π autocatalysis in the best organized, least hydrated core of the most π-acidic NDI micelles 12.…”

“…Anion-π autocatalysis in particular is best in apolar solvents with two molecules of water sandwiched between substrate and product like in a reversed mini-micelle (Figure 3i). [47] Best activity with tethers that are both long and hydrophobic implied that anion-(π) n -π autocatalysis occurs deep inside the micelles, with the respective interfacers intercalating into nearby stacks with tethers long enough to place substrate, product and two molecules of water on top of the same polarizable π-stack, as outlined in TS2 (Figure 3i). This interpretation was supported by a drop of the record ae mtm = 5.0 for 13 to ae mtm = 1.5 for the more hydrophilic substrate 14 with unchanged DAN interfacers (Table 1, entry 5).…”

Electric field–assisted anion-π catalysis on carbon nanotubes in electrochemical microfluidic devices

“…3, A and B ). In the presence of autocatalysis, addition of 25 mol % product 7 at the beginning of the reaction would accelerate the conversion of substrate 6 on suspended MWCNTs ( 52 ). The fivefold deceleration found instead thus confirmed the presence of product inhibition caused by pyrene interfacers as outlined for 10 in VII ( Fig.…”

“…Epoxide-opening ether cyclizations on π-acidic surfaces are interesting because they do not require additional activating groups and show autocatalytic behavior. This is true already for the cyclization of monomers such as 4 into THF 5 (52). Here, epoxideopening ether cyclizations are used as benchmark reactions to elaborate on electric field-induced anion-π catalysis on carbon nanotubes in electrochemical microfluidic reactors.…”

Anion‐(π)n‐π Catalytic Micelles

“…A complementary 3.4‐fold increase of catalysis, rather than decrease of autocatalysis, in Triton X‐100 micelles demonstrated that the less hydrophobic substrate 14 stays near the surface of the Triton micelles to benefit from the known [27,48] contributions of water to catalyze ether cyclizations (Figure 3h, ○ vs ▿). In clear contrast, anion‐(π) n ‐π autocatalysis as in TS2 excels with two molecules of water [47] sandwiched between substrate and product on π surfaces in apolar environment with the more hydrophobic 13 deep within micelles but suffers in the presence of excess of water with the less hydrophobic 14 at the surface of NDI micelles 12 . The depth of micelle penetration thus accounts for the maximal activity reached with substrate 13 , sufficiently hydrophobic to interface for anion‐(π) n ‐π autocatalysis in the best organized, least hydrated core of the most π‐acidic NDI micelles 12 .…”