The detailed mechanism for the diboration of aldehydes catalyzed by (NHC)Cu(boryl) complexes (NHC = N-heterocyclic carbene) was studied with the aid of DFT by calculating the relevant intermediates and transition states. The results show that the catalyzed diboration occurs through aldehyde insertion into Cu-B to give a Cu-O-C(boryl) species followed by sigma-bond metathesis with a diboron reagent. It is the "electron-richness", that is, the nucleophilicity of the Cu-boryl bond, which gives rise to a small insertion barrier and determines the direction of insertion. The results of our calculations also explain the formation of the product, observed experimentally, from the stoichiometric reaction of (IPr)Cu-Bpin (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) with mesitylaldehyde. In the absence of a diboron reagent, the insertion intermediate having a Cu-O-C(boryl) linkage isomerizes to the thermodynamically preferred Cu-C-O(boryl) isomer via a boryl migration to the metal-bonded oxygen through an S(E)2-like transition state. We have also studied the catalyzed diboration of 2-pyridinecarboxaldehyde, which gives the unexpected reductive coupling product 1,2-di-2-pyridyl-1,2-bis(pinacolboroxy)ethane. The insertion intermediate, which contains a coordinated pyridyl group, isomerizes easily to a 1,2-dihydropyridine form, preventing its metathesis with a diboron reagent to give the expected diboration product as observed for other aldehyde substrates.
Detailed mechanisms of the diboration of the acyclic α,β-unsaturated carbonyl compounds acrolein, methyl acrylate, and dimethyl fumarate (DMFU) catalyzed by Pt(0) complexes were studied with the aid of density functional theory by calculating the relevant intermediates and transition states. For acrolein and methyl acrylate, the results show that the catalyzed diboration occurs via oxidative addition of the diboron reagent to the Pt(0) complex having diimine and acrolein (or methyl acrylate) as the ligands, 1,4-conjugate addition of a Pt–B bond to acrolein/methyl acrylate to give an O-bound boron enolate intermediate containing a Pt–C–C═C–O–B linkage, and subsequent acrolein/methyl acrylate coordination to the Pt(II) center followed by reductive elimination to obtain the 1,4-diboration product of acrolein/methyl acrylate, i.e., the β-boryl-substituted O-bound boron enolate. For acrolein, the 1,4-diboration product is the final product, whereas for methyl acrylate, the 1,4-diboration product then isomerizes to the experimentally observed and thermodynamically favored 3,4-addition product, i.e., the β-boryl-substituted C-bound boron enolate, via a 1,3-shift of the O-bonded boryl group. Slightly different from what we have seen in the catalyzed diboration of acrolein/methyl acrylate, the catalyzed diboration of DMFU takes place through oxidative addition of the diboron reagent to the Pt(0) complex having DMFU and diimine as the ligands, 1,6-conjugate addition of both of the two Pt–B bonds to the coordinated DMFU ligand to give a 1,6-addition intermediate containing BegO–C(OMe)═C–C═C(OMe)–OBeg (eg = ethyleneglycolato = −OCH2CH2O−) as a ligand, and then isomerization via two consecutive 1,3-shifts of the two O-bonded boryl groups to produce the experimentally observed 3,4-diborated diastereomeric products.
The catalytic activity of a series of ruthenium(II) complexes in azide-alkyne cycloadditions has been evaluated. The [Cp*RuCl] complexes, such as Cp*RuCl(PPh 3) 2, Cp*RuCl(COD), and Cp*RuCl(NBD), were among the most effective catalysts. In the presence of catalytic Cp*RuCl(PPh 3) 2 or Cp*RuCl(COD), primary and secondary azides react with a broad range of terminal alkynes containing a range of functionalities selectively producing 1,5-disubstituted 1,2,3-triazoles; tertiary azides were significantly less reactive. Both complexes also promote the cycloaddition reactions of organic azides with internal alkynes, providing access to fully-substituted 1,2,3-triazoles. The ruthenium-catalyzed azide-alkyne cycloaddition (RuAAC) appears to proceed via oxidative coupling of the azide and alkyne reactants to give a six-membered ruthenacycle intermediate, in which the first new carbon-nitrogen bond is formed between the more electronegative carbon of the alkyne and the terminal, electrophilic nitrogen of the azide. This step is followed by reductive elimination, which forms the triazole product. DFT calculations support this mechanistic proposal and indicate that the reductive elimination step is rate-determining.
The reactions of the 16-electron half-sandwich complex CpCo(S2C2B10H10) (1) (Cp: cyclopentadienyl) with sulfonyl azides (p-toluenesulfonyl azide, TsN3; methanesulfonyl azide, MsN3) in refluxing dichloromethane or at ambient temperature lead to imido-bridged adducts CpCo(S2C2B10H10) (NSO2R) (2a, R = 4-MePh; 2b, R = Me) which can convert to the tetraazadiene cobalt complexes CpCoN4(SO2R)2 (3a, R = 4-MePh; 3b, R = Me) in the presence of excess azide if heated. The reactions of 1 with acyl azides (methyl azidoformate and benzoyl azide) lead to CpCo(S2C2B10H10)(CONR) (4a, R = OMe; 4b, R = Ph) with a newly-generated five-membered metallacyclic ring Co–S–N–C–O. Complexes 2a and 2b show further reactivity toward alkynes to give rise to the insertion products CpCo(S2C2B10H10)(R1C═CR2) (NSO2R) (R1 = COOMe, R2 = H, R = 4-MePh, 5a, R = Me, 5b; R1 = R2 = COOMe, R = 4-MePh, 6a, R = Me, 6b; R1 = COOMe, R2 = Ph, R = 4-MePh, 8a, R = Me, 8b) formed by alkyne addition to a Co–S bond to generate a Co–C–C–S four-membered ring and CpCo(S2C2B10H10)(R1C═CR2NSO2R) (R1 = H, R2 = Ph, R = 4-MePh, 7a, R = Me, 7b; R1 = COOMe, R2 = Ph, R = 4-MePh, 9a, R = Me, 9b) formed by alkyne insertion into a Co–N bond to generate a Co–C–C–N–S five-membered ring. In the case of PhC≡CCO2Me, the products with insertion into both Co–S and Co–N bonds are isolated. Interestingly, if tert-butylacetylene is used, CpCo(S2C2B10H10)(R1R2C═CNSO2R) (R1 = tBu, R2 = H, R = 4-MePh, 10a, R = Me, 10b) are generated by insertion of terminal carbon into a Co–N bond to form four-membered ring Co–C–N–S. The insertion pathways of these reactions have been discussed on the basis of DFT calculations. All the new complexes were fully characterized, and X-ray structural analyses were performed for 2a, 3a, 3b, 4a, 4b, 5a, 6a, 7a, 7b, 8a, 9a, 9b, and 10b.
Molybdenum clusters consisting of 2-55 atoms were investigated using density functional theory calculations with a plane-wave basis set. The results show that the linear and planar molybdenum clusters have a strong tendency to form dimers. This tendency results in the formation of alternate short and long bonds within a linear cluster, in which the strength of these short bonds is covalent. Therefore, the linear and planar Mo clusters exhibit significant nonmetallic characteristics. Furthermore, the linear and planar Mo clusters show a strong even-odd effect in binding energy with the even-numbered clusters being more stable than their neighboring odd-numbered clusters. On the other hand, the even-odd effect in the energy gap between the highest occupied and the lowest unoccupied molecular orbitals, i.e., the HOMO-LUMO energy gap, for the linear and the planar clusters is different. The odd-numbered linear clusters and even-numbered planar clusters have larger HOMO-LUMO energy gaps than their corresponding neighboring clusters.
An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.
A new rhodium-catalyzed decarbonylated coupling reaction of ethyl benzo[h]quinoline-10-carboxylate and organoboron compounds that occurs through chelation-assisted sp2 C–COOEt bond activation was described. In this system CuCl played a very important role, and a five-membered rhodacycle was also involved as a key intermediate. Various functionalities were compatible in the reaction, and the desired products were obtained in good to excellent yields. DFT calculations on the mechanisms of this reaction using a Rh(I) model catalyst have also been carried out.
A rhodium-catalyzed [6 + 2] cycloaddition of internal alkynes with cycloheptatriene is described. A series of substituted alkynes were cycloadded to cycloheptatriene through a [6 + 2] addition to give a variety of substituted bicyclic compounds in excellent yields. The optimal catalytic system for these transformations was a [Rh(COD)Cl]2 (5.0 mol %) catalyst in combination with CuI (10 mol %) and PPh3 (10 mol %). The proposed mechanism for this system includes an initial oxidative coupling reaction between the coordinated cycloheptatriene and the internal alkyne, followed by a [1,3]-shift of the Rh metal center and a reductive elimination from the Rh(III)–allyl complex to give the final product. Calculations using a model Rh(I) catalyst were also carried out to further understand this mechanism.
Light-induced segregation limits the practical application of mixed halide perovskites in solar cells. Herein, halide segregation is evaluated by a data-driven approach with constructing a bandgap database of 53,361 mixed ABX3 [where A = Cs, formamidinium (FA) or methylammonium (MA); B = Pb or Sn; X = Br, Cl, or I] perovskites. A transfer learning strategy was employed to fine-tune the parameters of a Graph Neural Network model using experimental and density functional theory (DFT)-calculated bandgaps. This approach accelerated the construction of a unique database, distinguishing it from others primarily focused on ABX3 perovskite element substitution. The database is characterized by continuously varying compositions and accurate bandgaps. It was utilized to calculate the free energy of 20,688 mixed iodine-bromine perovskites and generate corresponding phase diagrams for predicting their light-induced segregation behavior. It is found that the bandgap increases with decreasing ionic radii at the A-site and X-site. This composition-dependent bandgap difference drives halide segregation. Moreover, using a higher Cs content at the A-site, rather than MA, reduces this bandgap difference, enhancing photostability. The proposed data-driven strategy can facilitate the targeted design of novel perovskites with mixed compositions and the investigation of halide perovskite segregation.
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