Mechanistic Study of Chemoselectivity in Ni-Catalyzed Coupling Reactions between Azoles and Aryl Carboxylates

Itami et al. recently reported the C–O electrophile-controlled chemoselectivity of Ni-catalyzed coupling reactions between azoles and esters: the decarbonylative C–H coupling product was generated with the aryl ester substrates, while C–H/C–O coupling product was generated with the phenol derivative substrates (such as phenyl pivalate). With the aid of DFT calculations (M06L/6-311+G­(2d,p)-SDD//B3LYP/6-31G­(d)-LANL2DZ), the present study systematically investigated the mechanism of the aforementioned chemoselective reactions. The decarbonylative C–H coupling mechanism involves oxidative addition of C­(acyl)–O bond, base-promoted C–H activation of azole, CO migration, and reductive elimination steps (C–H/Decar mechanism). This mechanism is partially different from Itami’s previous proposal (Decar/C–H mechanism) because the C–H activation step is unlikely to occur after the CO migration step. Meanwhile, C–H/C–O coupling reaction proceeds through oxidative addition of C­(phenyl)–O bond, base-promoted C–H activation, and reductive elimination steps. It was found that the C–O electrophile significantly influences the overall energy demand of the decarbonylative C–H coupling mechanism, because the rate-determining step (i.e., CO migration) is sensitive to the steric effect of the acyl substituent. In contrast, in the C–H/C–O coupling mechanism, the release of the carboxylates occurs before the rate-determining step (i.e., base-promoted C–H activation), and thus the overall energy demand is almost independent of the acyl substituent. Accordingly, the decarbonylative C–H coupling product is favored for less-bulky group substituted C–O electrophiles (such as aryl ester), while C–H/C–O coupling product is predominant for bulky group substituted C–O electrophiles (such as phenyl pivalate)

Mechanistic Study on Ligand-Controlled Rh(I)-Catalyzed Coupling Reaction of Alkene-Benzocyclobutenone

Recently, Dong’s group [<i>Angew. Chem., Int. Ed.</i> <b>2012</b>, <i>51</i>, 7567–7571; <i>Angew. Chem., Int. Ed.</i> <b>2014</b>, <i>53</i>, 1891–1895] reported the ligand-controlled selectivity of Rh-catalyzed intramolecular coupling reaction of alkene-benzocyclobutenone: the direct coupling product (i.e., fused-rings) was formed in the DPPB-assisted system (DPPB = PPh₂(CH₂)₄PPh₂), while the decarbonylative coupling product (i.e., spirocycles) was generated in the P­(C₆F₅)₃-assited system. To explain this interesting selectivity, density functional theory (DFT) calculations have been carried out in the present study. It was found that the direct and decarbonylative couplings experience the same C­(acyl)–C­(sp²) activation and alkene insertion steps. The following C–C reductive elimination or β-H elimination–decarbonylation–reductive elimination leads to the direct or decarbonylative coupling reaction, respectively. The coordination features of different ligands were found to significantly influence C–C reductive elimination and decarbonylation step. The requisite phosphine dissociation of DPPB ligand from Rh center for the decarbonylation step is disfavored, and thus, the reductive elimination and direct coupling reaction are favored therein. By contrast, a free coordination site is available on the Rh center in the P­(C₆F₅)₃-assisted system, facilitating the decarbonylation process together with the generation of related decarbonylative coupling product

Population resequencing and coverage statistics.

(XLS)</p

Construction of ultra-high-density genetic linkage map of a sorghum-sudangrass hybrid.

Also available on protocols.io. http://dx.doi.org/10.17504/protocols.io.14egn2226g5d/v1. (DOC)</p

Heat maps for 10 linkage groups of the density genetic map for the sorghum-sudangrass hybrid.

Ten heat maps are shown from chromosome 1 to chromosome 10, in which markers are listed alphabetically by row and column. Different colors indicate the strength of linkage: yellow represents weak links, whereas red represents strong links. (TIF)</p

Statistics of population resequencing in sorghum-sudangrass hybrid.

(XLS)</p

Characteristics of genetic linkage groups of sorghum-sudangrass hybrid.

Characteristics of genetic linkage groups of sorghum-sudangrass hybrid.</p

Statistics of the existing map of sorghum-sudangrass hybrid.

Statistics of the existing map of sorghum-sudangrass hybrid.</p

Statistics of parents and F₂ population aa × bb marker with 9× 3×.

(XLS)</p

Segregation distortion for SNP markers.

Segregation distortion for SNP markers.</p