Mechanism and Origins of Diastereo‐ and Regioselectivities of Palladium‐Catalyzed Remote Diborylative Cyclization of Dienes via Chain‐Walking Strategy

Abstract

Density functional theory calculations have been performed to investigate the palladium‐catalyzed remote diborylative cyclization of dienes. The computations reveal that the reaction proceeds through a rarely explored Pd(II)/Pd(IV) catalytic cycle, and the formal σ‐bond metathesis between the alkylpalladium intermediate and B2pin2 occurs via the pathway of the B−B oxidative addition/C−B reductive elimination involving the high‐valent Pd(IV) species. The diastereoselectivity is determined by the migratory insertion into the Pd−C bond, which is mainly due to the combination of the torsional strain effect, steric repulsion and C−H—O hydrogen‐bonding interaction. The steric hindrance around the reacting carbon group in the C−B reductive elimination turns out to be a key factor to provide the driving force of the chain walking of the Pd center to the terminal primary carbon position, enabling the experimentally observed remote regioselectivity.

Conformational Isomerization as a Key Selectivity-Determining Step

“…In 1997, Morokuma and coworkers reported on the DFT calculation of chain-walking ethylene polymerization catalyzed by a palladium catalyst with a simpli ed α-diimine ligand and suggested that the energy barriers for the alkene rotation process are less than 1 kcal/mol. 19 While many palladium-catalyzed organic transformations via chain walking have been reported recently and DFT calculations have been conducted for some of these reactions, [20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35] the transition states of the alkene rotation process have not often been located in the calculation. [30][31][32][33][34][35] As a rare example, Baudoin, Clot and coworkers reported a palladium-catalyzed arylation of esters via metal migration and DFT calculations on the reaction to show that metal migration proceeds smoothly because the energy barrier for alkene rotation is higher than that for β-hydride elimination/insertion but lower than that for rate-determining reductive elimination.…”

“…To gain an understanding of the mechanism, we explored the theoretical investigation of the chain walking process of alkylpalladium species possessing a 1,10-phenanthroline ligand. It should be noted that DFT calculations on the chain walking mechanism of the 1,10-phenanthroline palladium have partly been reported by other groups, 27,29 but complete analysis including the structures and modes of the imaginary vibration at the transition states as well as the possible pathways involving all of the stereoisomers was not conducted. Therefore, entirely novel ndings and conclusions were drawn from our investigations.…”

Conformational Isomerization as a Process to Determine Selectivity over Reaction Pathways: Effect of Alkene Rotation in Chain Walking via Cis Alkene Intermediates

“…Our results showed that a conformational isomerization process can be a process to determine selectivity over reaction pathways in bond-forming/-cleaving reactions. It should be noted that DFT calculations on the chain walking mechanism of the 1,10-phenanthroline palladium have partly been reported by other groups, , but complete analysis including the structures and the modes of the imaginary vibration at the transition states, the processes associated with the metal migration such as alkene exchange and β-hydrogen exchange for each intermediate, and the possible pathways involving all the stereoisomers of alkyl complexes and cis/trans alkene intermediates was not conducted. Therefore, entirely novel conclusions were drawn from our investigations.…”

“…In 1997, Morokuma and co-workers reported on the DFT (density functional theory) calculation of the chain-walking ethylene polymerization catalyzed by a palladium catalyst with a simplified α-diimine ligand and suggested that the energy barriers for the alkene rotation process are less than 1 kcal/mol . While many palladium-catalyzed organic transformations via chain walking have been reported recently and DFT calculations have been conducted for some of these reactions, − the transition states of the alkene rotation process have not often been located in the calculation. − As a rare example, Baudoin, Clot and co-workers reported a palladium-catalyzed arylation of esters via metal migration and conducted a detailed DFT calculation on the reaction to show that the metal migration proceeds smoothly because the energy barriers for the alkene rotation and the more facile β-hydride elimination/insertion are lower than rate-determining reductive elimination. , …”

Fifty Shades of Phenanthroline: Synthesis Strategies to Functionalize 1,10-Phenanthroline in All Positions

“…To see just how crucial phenanthroline is today in catalysis, we need look no further than recent articles where phen and phen sub are used to design new molecular catalysts, e.g. , with copper (oxidation catalysts), , iron (CO 2 reduction, C–H bond activation), , nickel (reductive couplings, C–S cross-coupling), Pd (C–H bond activation, borylation, defluorinative coupling), and many more whose listing is out of the scope of this Review.…”