A DFT Study on the Mechanism and Origin of Regioselectivity in the Rhodium/Diene‐Catalyzed Ring‐Opening Reactions of C1‐Substituted Oxabenzonorbornadienes with Arylboronic Acids

Abstract

The mechanism and the origin of the regioselectivity of the rhodium/diene-catalyzed ring-opening reactions of C1-substituted oxabenzonorbornadienes with arylboronic acids has been examined using hybrid density functional theory (DFT). The overall mechanism for the reaction is considered as a four-step reaction including transmetalation, carborhodation, β-oxygen elimination, and protonolysis. The free energy profiles show that the rate-determining step is involved in the protonolysis step which differs from previous mechanistic studies on metalcatalyzed ring-opening reactions of oxabenzonorbornadienes. Additionally, it is theorized the β-oxygen elimination step is the origin of regioselectivity. DFT calculations predict the formation of the 1,2,4-trisubstituted naphthalene framework is selective over the 1,1,2-trisubstituted product, in agreement with experimental observations.

The Mechanism and Origin of Enantioselectivity in the Rhodium-Catalyzed Asymmetric Ring-Opening Reactions of Oxabicyclic Alkenes with Organoboronic Acids: A DFT Investigation

“…Following migratory insertion (Figure S3), successive β-oxygen elimination will take place to give the C–O cleaved intermediate IN x 3 ( x = a–d). In our proposed mechanism, insertion of the Rh­(I) complex into the bridgehead C–O bond with retention of configuration gives the Rh­(I) alkoxide IN x 3 ( x = a–d) via a distorted five-membered-ring transition state TS x 2–3 ( x = a–d) (Figure S4), similar to that described by previously reported Rh/heterobicyclic alkenes DFT investigations. , Transition states TSa 2–3 and TSc 2–3 , where the two phenyl groups are cis to the β-oxygen, are lower in activation energy compared to TSb 2–3 and TSd 2–3 , where the two tert -butyl groups are cis to the β-oxygen. Understandably, the significant increase in energy can be accredited to the large steric hindrance from the bulky tert -butyl groups and the oxygen moiety.…”

“…On the other hand, exo attack begins with a reaction between the Rh center and the nucleophile, typically an organometallic reagent . Coordination of the Rh complex to the more accessible exo -face will result in the delivery of the nucleophile via an internal nucleophilic attack (Figure b); however, endo -selective internal nucleophilic attack has been reported (Figure c). , Recently, Pounder et al calculated the difference in activation energy for exo and endo migratory insertion of an aryl group across an bicyclic alkene using a phosphine-ligand-free Rh catalyst, suggesting that both pathways are accessible at room temperature (Figure b,c) . Nevertheless, C–O bond cleavage via the endo -carborhodated intermediates seems doubtful, as the syn -coplanar requirement of the Rh–C and C′–O bonds cannot be met for β-oxygen elimination (Figure d).…”

“…34a,b Recently, Pounder et al calculated the difference in activation energy for exo and endo migratory insertion of an aryl group across an bicyclic alkene using a phosphine-ligandfree Rh catalyst, suggesting that both pathways are accessible at room temperature (Figure 3b,c). 35 Nevertheless, C−O bond cleavage via the endo-carborhodated intermediates seems doubtful, as the syn-coplanar requirement of the Rh−C and C′−O bonds cannot be met for β-oxygen elimination (Figure 4d). Moreover, endo-carborhodated intermediates are unable to undergo β-hydride elimination, as this would produce a double bond at the bridgehead of OA, violating Bredt's rule (Figure 3d).…”

“…37 S4), similar to that described by previously reported Rh/heterobicyclic alkenes DFT investigations. 21,35 Transition states TSa 2−3 and TSc 2−3 , where the two phenyl groups are cis to the β-oxygen, are lower in activation energy compared to TSb 2−3 and TSd 2−3 , where the two tert-butyl groups are cis to the β-oxygen. Understandably, the significant increase in energy can be accredited to the large steric hindrance from the bulky tertbutyl groups and the oxygen moiety.…”

Iridium-catalyzed hydroacylation reactions of C1-substituted oxabenzonorbornadienes with salicylaldehyde: an experimental and computational study

“…Electron-donating groups (EDGs) led to naphthol compounds 9 , while electron-withdrawing groups (EWGs) led to the anticipated ring-opened 1,1,2-trisubstituted naphthalene framework 10 [ 61 ]. On the other hand, Edmunds and co-workers described a ring-opening reaction of C 1 -substituted OBDs 5 with arylboronic acids that was catalyzed by rhodium/diene to afford the 1,2,4-trisubstituted naphthalene framework 8 with complete regio- and stereocontrol ( Scheme 1 ) [ 62 – 63 ].…”