5/7/2023 0 Comments Ion bonding gold on skin![]() ![]() For example, it may impact the proton-transfer process via proton shuttling. This long-range electrostatic attraction may exhibit limited interaction with most atoms or groups, but if the transition-state intermediate contains an active proton (e.g., O–H δ+, N–H δ+), which has the smallest mass among all atoms and is usually highly charged, then a counterion may have significant impact ( Figure 1c). The interaction between a pairing counterion (X –) and the corresponding transition structure (TS2) is a long-range electrostatic attraction ( Figure 1b), which is usually relatively weak. Figure 1īut the above statement does have an exception. Therefore, in general, a catalyst that contains a weakly coordinating counterion (low affinity between M + and X –) will exhibit high reactivity. Our hypothesis is that in many cases the counterion acts as a spectator and has limited influence on the structures of the transition state ( Figure 1b), so the difference between Δ G ⧧ T2 and Δ G ⧧ T1 is mainly determined by the affinity between M + and X –. Thus, compared to the reaction of “free” M +, additional energy will be needed (Δ G ⧧ TS2 > Δ G ⧧ TS1) to overcome the Coulombic attraction. (5) When the paired cationic metal (M +X –) interacts with RC 1 and RC 2 to produce TS2, there will be a charge separation between M + and X – during the formation of TS2. (4) In low dielectric constant solvents, a cationic metal complex will exist as an ion pair rather than “free” ions. In most theoretical treatments of cationic gold catalysis, or transition-metal catalysis in general, cationic metals are treated as “free” ions (M + in Figure 1a). In a simplified representation of cationic metal-catalyzed reactions ( Figure 1), a cationic metal catalyst (M +) will somehow complex or connect to reactants (RC 1, RC 2, etc.) to form the corresponding transition state (TS1), which in turn, leads to an intermediate or product. Herein, we report a quantitative treatment of relevant physical properties of counterions that affect their reactivity in cationic gold catalysis. We posited that one important barrier for a rational understanding of counterion effect in cationic gold catalysis or cationic catalysis in general is the lack of a quantitative description of relevant physical properties of counterions. The selection of counterion is still empirical, and chemists test different counterions during the process of reaction condition optimization. (3j) However, a quantitative analysis of counterion effects in gold catalysis is still elusive. (3) For example, Maier and co-workers studied counterion effects in gold(I)-catalyzed hydroalkoxylation of alkynes, (3d) Echavarren and co-workers did the same in gold(I)-catalyzed intermolecular cycloadditions, (3e) whereas Bandini, Macchioni, and co-workers reported the effects of counterion in gold-catalyzed dearomatization of indoles with allenamides. Cationic gold catalysis (1) is a case in point much effort has been dedicated to understanding the effects of ligands in gold catalysis, (1d,2) but only recently have chemists begun to investigate the effects of counterion in gold-catalyzed reactions. ![]() However, the selection of the counterion is, still, mostly empirical. Reactions that involve cationic species are among the most important transformations in organic synthesis counterions often play a significant role in the efficiency of these reactions. ![]()
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