Mechanism Study On Pd-Catalyzed Coupling Reaction Of 2-Bromobenzyl Bromide,Carbon Monoxide,and Tributylallylstannane

The organic synthesis of 2-naphthols has played an important role in recent several years due to their importance in synthetic applications. 2-naphthols can be used as universal and pivotal organic synthetic raw materials for the preparation of several bactericide materials, natural products, dye intermediates, and bioactive compounds. Over the past several decades, Mono- and poly-substituted-2-naphthols can be procured through the intramolecular cyclization reactions, for example, electrophilic cyclization, oxidative cyclization, intramolecular Aldol condensation,and Dieckmann condensation, etc. However, its application in organic synthesis is limited to some extent. For instance, these methods commonly involve harsh reaction conditions, expensive substrates,low yields, and multistep synthesis reactions.In this article, the detailed reaction mechanism for Pd(0)-catalyzed coupling reaction of 2-bromobenzyl bromide, carbon monoxide, and tributylallylstannane leads to the formation of2-naphthol, which has been studied with the aid of DFT calculationsunder the experimental conditions. The main contents are displayed as follows:In the first portion, we briefly describe the development and application of the Pd-catalyzed Stille cross-coupling reaction, Heck cross-coupling reaction, and carbonylative reaction. Moreover, the research background and significance of the synthesis of 2-naphthol have been introduced.In the second portion, we simply summarize the theory foundation involved in this article, such as density functional theory, solvent effect and transition state theory.In the third portion, a reaction mechanism of the formation of2-naphthol are researched in detail by calculating a series of reasonable structures of intermediates and transition states. The calculation results show that the monophosphine pathways are more favorable than bisphosphine pathways for the entire catalytic cycle.Two possible mechanisms have been proposed and discussed in the whole reaction. By comparing the energetics of the mechanisms 1and 2, it is found that mechanism 1 is a feasible reaction route for the formation of 2-naphthol. The preferred mechanism can be divided into two portions containing the carbonylative Stille coupling reaction(oxidative addition, transmetalation, carbonylation addition,carbonylation insertion, reductive elimination) and the intramolecular Heck reaction(oxidative addition, migratory insertion, ?-hydride transfer, reductive elimination). It is found that carbonylative Stille coupling reaction involves two kinds of pathways(path 1 and path 2)depending on different sequence of the carbonylation addition and insertion versus transmetalation. Both path 1 and path 2 possiblyexist in Me CN solvent due to the close free energies(33.7 vs. 28.8kcal/mol in gas phase and 29.8 vs. 29.9 kcal/mol in solution). The transmetalation step of the carbonylative Stille coupling reaction is the rate-determining step for the whole catalytic cycle. The intramolecular Heck reaction is a very facile portion compared with carbonylative Stille coupling reaction. The key steps are the oxidative addition of A14 and the migration of Br Pd(PH3). Finally, the product2-naphthol is formed by the ring closure. In addition, to authenticate the solvent effect, the solvation model density(SMD) was used to account for the gas-phase-optimized structures. These results show that the solvent effect does not change the general trends of the reaction potential energy surfaces. The theoretical results should have provided valuable reaction mechanism and help us to further investigate other transition-metal catalyzed carbon-carbon coupling reactions.