The Origins of the Differences between Alkyne Hydroalkoxylations Catalyzed by 8‐Quinolinolato‐ and Dipyrrinato‐Ligated RhI Complexes: A DFT Mechanistic Study

다국어 초록 (Multilingual Abstract)

Hydroalkoxylations of terminal alkynes with methanol, catalyzed by 8‐quinolinolato‐ and dipyrrinato‐ligated RhI complexes, afford anti‐Markovnikov (Z)‐ and (E)‐enol ethers, respectively. Herein we performed a DFT study to gain insight into the mechanisms of the two reactions, aiming at understanding why and how the two ligands differ. Of the two possible mechanisms, namely, Cβ‐HT and Rh‐HT, characterized by a RhI Fisher carbene and vinyl RhIII–H hydride complex, respectively, the hydroalkoxylation with 8‐quinolinolato prefers the Cβ‐HT mechanism, while that with dipyrrinato favors the Rh‐HT mechanism. The root cause for the mechanistic difference is that the O atom in 8‐quinolinolato, due to its large electronegativity and exposure, is more favorable than the N atom in dipyrrinato in stabilizing the protic hydrogen as methanol undergoes a 1,3‐addition, thus the addition in the former is much easier than that in the latter. Consequently, the addition in the hydroalkoxylation with dipyrrinato controls the reaction selectivity, giving the (E)‐enol ether. In contrast, the selectivity of the hydroalkoxylation with 8‐quinolinolato is determined by the RhI Fisher carbene to undergo a 1,2‐hydrogen transfer, leading to the (Z)‐enol ether. Furthermore, a DMA solvent molecule substantially facilitates the methanol addition in the reaction with dipyrrinato but not in the reaction with 8‐quinolinolato.
The hydroalkoxylations of terminal alkynes with methanol, catalyzed by 8‐quinolinolato‐ and dipyrrinato‐ligated RhI complexes, have different mechanisms. This is because the 8‐quinolinolato ligand greatly facilitates methanol addition to a RhI–vinylidene intermediate, compared with the dipyrrinato ligand, resulting in different selectivity‐determining steps.