et al
To account for the distinct regioselectivity, Ravikumar group postulated possible reaction mechanisms that are summarized in Scheme 2. [Cp*RhCl2]2 firstly reacts with Ag2CO3 to generate active catalyst Cp*RhCO3 (cat ), which is followed by the ligand exchange with cyclopropanol R’ to afford alkoxide compound I . FromI , there exist two possible pathways (path a or path b). In path a, β -carbon elimination would take place to afford alkyl-rhodium homoenolate II . Then, with the participant of another molecule of cyclopropanol, the successive ligand exchange andβ -carbon elimination occur to give dialkyl-rhodium speciesIV . In contrast, the ligand exchange with the second molecule of cyclopropanol prior to β -carbon elimination would yield species IV through compound II’ (path b). Finally, reductive elimination from speciesIV leads to 1,6-diketone P . It should be noted that the β -hydride eliminated product P’ could not be obtained in this reaction. As shown in Scheme 2, in the absence of [Cp*RhCl2]2, cyclopropanol would undergo the ring-opening to give monoketone product catalyzed by Ag2CO3.
SCHEME 2 Possible reaction mechanisms proposed by Ravikumar’s group
Although the plausible mechanistic pathway has been proposed by the Ravikumar group, some key issues still need to be addressed: (1) In this experiment, the role of the catalyst [Cp*RhCl2]2 and additive Ag2CO3 is still unclear; (2) It is unclear which steps determine the selectivity and what are the origins of that observation in this experiment; (3) The β -hydride eliminated product was not obtained in reaction A – it would be interesting to investigate the corresponding factors. To address these questions, a theoretical investigation of the detailed reaction mechanisms is needed. Herein, we report our detailed density functional theory (DFT) calculations on the reaction mechanisms, in order to gain insight into the interesting experimental observations and distinct selectivity. We expect that this work will help to understand the detailed reaction mechanisms and to design new related reactions.