Computational Mechanistic Study of C–C Coupling of Methanol and Allenes Catalyzed by an Iridium Complex

Density functional theory calculations have been performed to understand the mechanism of the C–C couplings of methanol with allenes (e.g., 1,1-dimethylallene (<b>2all</b>)) catalyzed by an iridium complex (<b>1cat</b>). The study leads us to propose the following mechanism for the reaction. The iridium complex first needs to be activated via methanolysis to generate the active catalyst (an iridium alkoxide complex). Starting from the active catalyst, the catalytic cycle for the C–C coupling includes four steps: β-hydrogen elimination to give formaldehyde and an iridium hydride complex, allene hydrometalation to afford a (η³-π-allyl)­iridium intermediate, addition of formaldehyde to the (η³-π-allyl)iridium intermediate to produce a homoallylic iridium alkoxide complex, and methanolysis of the formed homoallylic iridium alkoxide complex to deliver the final coupling product <b>3alc</b> and regenerate the active catalyst. The regioselectivity exclusively producing the alcohol <b>3alc</b> with an all-carbon quaternary center is due to the Ir–CMe₂ bond being weaker than the Ir–CH₂ bond and the steric effect between the methyl groups of the allene substrate and the C,O-benzoate ligand of the catalyst. The replacement of the middle hydrogen of the η³-π-allyl moiety of <b>1cat</b> with a F, Cl, Me, or OMe group (F and OMe groups in particular) can benefit the catalyst activation both kinetically and thermodynamically. The possibility of using <b>1cat</b> for the coupling of allene (<b>2all</b>) with amine (CH₃NH₂) was also explored. The allene coupling with amine is energetically less favorable than the coupling with methanol but could be experimentally achievable. Because the barrier for the activation of <b>1cat</b> by amine (34.0 kcal/mol) could be too high, we proposed to lower the barrier by replacing the middle hydrogen atom of the η³-π-allyl moiety in <b>1cat</b> with a F or OMe group

A Thorough DFT Study of the Mechanism of Homodimerization of Terminal Olefins through Metathesis with a Chelated Ruthenium Catalyst: From Initiation to <i>Z</i> Selectivity to Regeneration

Density functional theory (DFT) calculations (B3LYP, M06, and M06-L) have been performed to investigate the mechanism and origins of <i>Z</i> selectivity of the metathesis homodimerization of terminal olefins catalyzed by chelated ruthenium complexes. The chosen system is, without any simplification, the experimentally performed homocoupling reaction of 3-phenyl-1-propene with <b>1cat</b>, a pivalate and N-heterocyclic carbene (NHC) chelated Ru precatalyst. The six-coordinate <b>1cat</b> converts to a trigonal-bipyramidal intermediate (<b>3</b>) through initial dissociation and isomerization. The metathesis reaction of complex <b>3</b> with 3-phenyl-1-propene occurs in a side-bound mechanism and generates the trigonal-bipyramidal Ru–benzylidene complex <b>6</b>. Complex <b>6</b> is the active catalyst for the subsequent side-bound metathesis with 3-phenyl-1-propene, which forms metallacyclobutanes that lead to the (<i>Z</i>)- and (<i>E</i>)-olefin homodimers. The transition states of cycloreversion leading to the (<i>Z</i>)- and (<i>E</i>)-olefins differ in energy by 2.2 kcal/mol, which gives rise to a calculated <i>Z</i> selectivity that agrees with experimental results. The <i>Z</i> selectivity stems from reduced steric repulsion in the transition state. The regeneration of complex <b>6</b> occurs along with the formation of the gaseous byproduct ethylene, whose evolution drives the overall reaction. As our results indicate, the chelating ligands are crucial for this new class of Ru catalysts to achieve <i>Z</i>-selective olefin metathesis, because they direct olefin attack, differentiate energies of the transition states and intermediates, and support the complexes in certain coordination geometries

Does the Ruthenium Nitrato Catalyst Work Differently in <i>Z</i>‑Selective Olefin Metathesis? A DFT Study

In the new class of N-heterocyclic carbene (NHC) chelated ruthenium catalysts for <i>Z</i>-selective olefin metathesis, the nitrato-supported complex <b>3cat</b> appears distinct from all the other carboxylato-supported analogues. We have performed DFT calculations (B3LYP and M06) to elucidate the mechanism of <b>3cat</b>-catalyzed metathesis homodimerization of 3-phenyl-1-propene. The six-coordinate <b>3cat</b> transforms via initial dissociation and isomerization into a trigonal-bipyramidal intermediate (<b>5</b>), from which two consecutive metathesis reactions via the side-bound mechanism lead to (<i>Z</i>)-PhCH₂CHCHCH₂Ph (major) and (<i>E</i>)-PhCH₂CHCHCH₂Ph (minor). In the overall mechanism, <b>3cat</b> functions similarly to the pivalate-supported analogue <b>1cat</b>. The substitution of a smaller nitrato group does not change the side-bound olefin attack mechanism for either the initiation or homocoupling metathesis. The chelation of the NHC ligand causes this class of Ru catalysts to favor the side-bound over the bottom-bound mechanism. The calculated energetics corroborate the experimental observation that <b>3cat</b> is somewhat more active than <b>1cat</b> in catalyzing the homodimerization of 3-phenyl-1-propene

Mechanism and Origins of <i>Z</i> Selectivity of the Catalytic Hydroalkoxylation of Alkynes via Rhodium Vinylidene Complexes To Produce Enol Ethers

We report the first theoretical study of transition-metal-catalyzed hydroalkoxylation of alkynes to produce enol ethers. The study utilizes density functional theory calculations (M06) to elucidate the mechanism and origins of <i>Z</i> selectivity of the anti-Markovnikov hydroalkoxylation of terminal alkynes with a Rh­(I) 8-quinolinolato carbonyl chelate (<b>1cat</b>). The chosen system is, without any truncation, the realistic reaction of phenylacetylene and methanol with <b>1cat</b>. Initiation of <b>1cat</b> through phenylacetylene substitution for carbonyl generates the active catalyst, a Rh­(I) η²-alkyne complex (<b>3</b>), which tautomerizes via an indirect 1,2-hydrogen shift to the Rh­(I) vinylidene complex <b>4</b>. The oxygen nucleophile methanol attacks the electrophilic vinylidene C_α, forming two stereoisomeric Rh­(I) vinyl complexes (<b>15</b> and <b>16</b>), which ultimately lead to the (<i>Z</i>)- and (<i>E</i>)-enol ether products. These complexes undergo two ligand-mediated proton transfers to yield Rh­(I) Fischer carbenes, which rearrange through a 1,2-β-hydrogen shift to afford complexes with π-bound product enol ethers. Final substitution of phenylacetylene gives (<i>Z</i>)- and (<i>E</i>)-PhCHCHOMe and regenerates <b>3</b>. The anti-Markovnikov regioselectivity stems from the Rh­(I) vinylidene complex <b>4</b> with reversed C_α and C_β polarity. The stereoselectivity arises from the turnover-limiting transition states (TSs) for the Rh­(I) carbene rearrangement: the <i>Z</i>-product-forming <b>TS24</b> is sterically less congested and hence more stable than the <i>E</i>-product-forming <b>TS25</b>. The difference in energy (1.2 kcal/mol) between <b>TS24</b> and <b>TS25</b> gives a theoretical <i>Z</i> selectivity that agrees well with the experimental value. Calculations were also performed on the key TSs of reactions involving two other alkyne substrates, and the results corroborate the proposed mechanism. The findings taken together give an insight into the roles of the rhodium–quinolinolato chelate framework in directing phenylacetylene attack by trans effect, mediating hydrogen transfers through hydrogen bonding, and differentiating the energies of key TSs by steric repulsion

How Does an Earth-Abundant Copper-Based Catalyst Achieve Anti-Markovnikov Hydrobromination of Alkynes? A DFT Mechanistic Study

The first catalytic hydrohalogenation of alkynes was recently achieved using a copper­(I) N-heterocyclic carbene (NHC) complex, and the reaction was found to be syn and anti-Markovnikov selective. The present work is a density functional theory (DFT) computational study (B3LYP and M06) on the detailed mechanism of this remarkable catalytic reaction. The reaction begins with a phenoxide additive turning over the precatalyst (NHC)­CuCl into (NHC)­Cu­(OAr), which subsequently transmetalates with the hydride source Ph₂SiH₂ to deliver the copper­(I) hydride complex (NHC)­CuH. (NHC)­CuH undertakes hydrocupration of the substrate RCCH via alkyne coordination and subsequent migratory insertion into the Cu–H bond, forming (<i>E</i>)-(NHC)­Cu­(CHCHR). The migratory insertion step determines the syn selectivity because it occurs by a concerted pathway, and it also determines the anti-Markovnikov regioselectivity that arises from the charge distributions across the Cu–H and CC bonds. The brominating agent (BrCl₂C)₂ uses the bromonium end to attack the Cu-bound vinylic carbon atom of (<i>E</i>)-(NHC)­Cu­(CHCHR), leading to the final (<i>E</i>)-alkenyl bromide product (<i>E</i>)-RHCCHBr, as well as the copper­(I) alkyl complex (NHC)­Cu­(CCl₂CBrCl₂), which undergoes β-bromide elimination to give the catalyst precursor (NHC)­CuBr for the next cycle. (NHC)­CuBr reacts with the phenoxide to regenerate the active catalyst (NHC)­Cu­(OAr). The computational results rationalize the experimental observations, reveal new insights into the mechanism of the Cu­(I)-catalyzed hydrobromination of alkynes, and have implications for other catalytic functionalization reactions of alkynes involving active [Cu]–H intermediates

How Does an Earth-Abundant Copper-Based Catalyst Achieve Anti-Markovnikov Hydrobromination of Alkynes? A DFT Mechanistic Study

The first catalytic hydrohalogenation of alkynes was recently achieved using a copper­(I) N-heterocyclic carbene (NHC) complex, and the reaction was found to be syn and anti-Markovnikov selective. The present work is a density functional theory (DFT) computational study (B3LYP and M06) on the detailed mechanism of this remarkable catalytic reaction. The reaction begins with a phenoxide additive turning over the precatalyst (NHC)­CuCl into (NHC)­Cu­(OAr), which subsequently transmetalates with the hydride source Ph₂SiH₂ to deliver the copper­(I) hydride complex (NHC)­CuH. (NHC)­CuH undertakes hydrocupration of the substrate RCCH via alkyne coordination and subsequent migratory insertion into the Cu–H bond, forming (<i>E</i>)-(NHC)­Cu­(CHCHR). The migratory insertion step determines the syn selectivity because it occurs by a concerted pathway, and it also determines the anti-Markovnikov regioselectivity that arises from the charge distributions across the Cu–H and CC bonds. The brominating agent (BrCl₂C)₂ uses the bromonium end to attack the Cu-bound vinylic carbon atom of (<i>E</i>)-(NHC)­Cu­(CHCHR), leading to the final (<i>E</i>)-alkenyl bromide product (<i>E</i>)-RHCCHBr, as well as the copper­(I) alkyl complex (NHC)­Cu­(CCl₂CBrCl₂), which undergoes β-bromide elimination to give the catalyst precursor (NHC)­CuBr for the next cycle. (NHC)­CuBr reacts with the phenoxide to regenerate the active catalyst (NHC)­Cu­(OAr). The computational results rationalize the experimental observations, reveal new insights into the mechanism of the Cu­(I)-catalyzed hydrobromination of alkynes, and have implications for other catalytic functionalization reactions of alkynes involving active [Cu]–H intermediates

DFT Mechanistic Study of Functionalizations of ω‑Ene-Cyclopropanes and Alkylidenecyclopropanes via Allylic C–H and C–C Bond Cleavage Facilitated by a Zirconocene Complex

A DFT mechanistic study has been performed to understand the [Zr]­C₄H₈ (Zr = ZrCp₂)-mediated transformations of ω-ene-cyclopropane (ω-ene-CP) and alkylidenecyclopropane (ACP) to acyclic compounds. The transformations proceed via allylic C–H bond activation, hydride transfer, C–C bond cleavage of the three-membered ring, and additions of electrophiles. The energetic results indicate that, among the possible pathways, the one leading to the experimental products is most energetically favorable, rationalizing the selectivity of the reactions. The Zr-walk takes place via allylic C–H bond activation followed by hydride transfer, completing a 1,3-hydrogen transfer. In comparison, the Pd-walk involved in the Pd-catalyzed Heck-type relay coupling reactions proceeds via migratory insertion followed by β-H elimination, resulting in a 1,2-hydrogen transfer. The difference is due to the fact that the [Zr] active species does not have a Zr–H or Zr–C bond for CC bond migratory insertion, while the Pd–H or Pd–C bond in [Pd] active intermediates enables such an insertion. In addition, the preference of Pd­(II) over Pd­(IV) disfavors the allylic C–H bond activation involved in the Zr-walk process. We further explored if the three-membered ring in the ω-ene-CP and ACP could be enlarged to four- or five-membered rings for similar transformations. The energetic results indicate that it is promising to enlarge a three-membered to a four-membered ring, but the extension to a five-membered ring is inferior because of the endergonic ring-opening with somewhat high barrier

Density Functional Theory Mechanistic Study of the Reduction of CO₂ to CH₄ Catalyzed by an Ammonium Hydridoborate Ion Pair: CO₂ Activation via Formation of a Formic Acid Entity

Density functional theory computations have been applied to gain insight into the CO₂ reduction to CH₄ with Et₃SiH, catalyzed by ammonium hydridoborate <b>1</b> ([TMPH]⁺[HB­(C₆F₅)₃]⁻, where TMP = 2,2,6,6-tetramethylpiperidine) and B­(C₆F₅)₃. The study shows that CO₂ is activated through the concerted transfer of H^δ+ and H^δ− of <b>1</b> to CO₂, giving a complex (<b>IM2</b>) with a well-formed HCOOH entity, followed by breaking of the O–H bond of the HCOOH entity to return H^δ+ to TMP, resulting in an intermediate <b>2</b> ([TMPH]⁺[HC­(O)­OB­(C₆F₅)₃)]⁻), with CO₂ being inserted into the B–H bond of <b>1</b>. However, unlike CO₂ insertion into transition-metal hydrides, the direct insertion of CO₂ into the B–H bond of <b>1</b> is inoperative. The computed CO₂ activation mechanism agrees with the experimental synthesis of <b>2</b> via reacting HCOOH with TMP/B­(C₆F₅)₃. Subsequent to the CO₂ activation and B­(C₆F₅)₃-mediated hydrosilylation of <b>2</b> to regenerate the catalyst (<b>1</b>), giving HC­(O)­OSiEt₃ (<b>5</b>), three hydride-transfer steps take place, sequentially transferring H^δ− of Et₃SiH to <b>5</b> to (Et₃SiO)₂CH₂ (<b>6</b>, the product of the first hydride-transfer step) to Et₃SiOCH₃ (<b>7</b>, the product of the second hydride-transfer step) and finally resulting in CH₄. These hydride transfers are mediated by B­(C₆F₅)₃ via two S_N2 processes without involving <b>1</b>. B­(C₆F₅)₃ acts as a hydride carrier that, with the assistance of a nucleophilic attack of <b>5</b>–<b>7</b>, first grabs H^δ− from Et₃SiH (the first S_N2 process), giving HB­(C₆F₅)₃^–, and then leave H^δ− of HB­(C₆F₅)₃^– to the electrophilic C center of <b>5</b>–<b>7</b> (the second S_N2 process). The S_N2 processes utilize the electrophilic and nucleophilic characteristics possessed by the hydride acceptors (<b>5</b>–<b>7</b>). The hydride-transfer mechanism is different from that in the CO₂ reduction to methanol catalyzed by N-heterocyclic carbene (NHC) and PCP-pincer nickel hydride ([Ni]­H), where the characteristic of possessing a CO double bond of the hydride acceptors is utilized for hydride transfer. The mechanistic differences elucidate why the present system can completely reduce CO₂ to CH₄, whereas NHC and [Ni]H catalysts can only mediate the reduction of CO₂ to [Si]­OCH₃ and catBOCH₃, respectively. Understanding this could help in the development of catalysts for selective CO₂ reduction to CH₄ or methanol

Noncovalent Molecular Heterojunction: Structure Determination and Property Characterization using Scanning Tunneling Microscopy/Spectroscopy and Theoretical Calculations

Noncovalent molecular heterojunctions (MHJs) formed by the stacking of p-type (electron donor) and n-type (electron acceptor) compounds are essentially important for various optoelectronics as well as molecular devices. Herein we report the construction of the fluorinated copper­(II) phthalocyanine (F₁₆CuPc; n-type, acceptor)–polymer NTZ12 (p-type, donor) MHJ via annealing of the film of F₁₆CuPc and NTZ12. Scanning tunneling microscopy and density functional theory calculations validate the formation of the F₁₆CuPc–NTZ12 MHJ at the single molecular level. The constructed MHJ shows a distinctive rectifying effect in the statistical data of scanning tunneling spectroscopy because of the different barriers in two directions of electron tunneling. In the proof-of-concept photocurrent tests, the F₁₆CuPc–NTZ12 MHJ produces much higher current under radiation than in the dark due to its well-organized donor–acceptor interface formed by annealing, whereas the current of the unannealed samples shows almost no response to radiation. The unveiled efficient construction approach, structure, and electronic properties of the MHJ in this study could greatly help the development of molecular devices. More importantly, our results provide direct evidence at the single molecular level for probing the intrinsic mechanism of improving the performances of varied photovoltaic cells by heating treatment. This mechanism is not completely understood as yet; there is a lack of understanding especially at the molecular level. The results in this paper fill this gap very well and, thus, are of significant importance for the development of related organic devices

Depolymerization of Oxidized Lignin Catalyzed by Formic Acid Exploits an Unconventional Elimination Mechanism Involving 3c–4e Bonding: A DFT Mechanistic Study

A DFT study has been performed to gain insight into the formic-acid-catalyzed depolymerization of the oxidized lignin model (<b>1</b>^<b>ox</b>) to monoaromatics, developed by Stahl <i>et al.</i> (<i>Nature</i> <b>2014</b>, <i>515</i>, 249–252). The conversion proceeds sequentially via formylation, elimination, and hydrolysis. Intriguingly, the elimination process exploits an unconventional mechanism different from the known ones such as E2 and E1cb. The new mechanism is characterized by passing through an intermediate stabilized by a proton-shared 3c–4e bond (HCOO^⊖···H^⊕···^⊖OC^α) and by shifting the 3c–4e bond to the 3c–4e HCOO^⊖···H^⊕···^⊖OOCH bond in the joint leaving group that is originally a regular H-bond (HCOO–H···OOCH−). According to these characteristics, as well as the important role of the original HCOO–H···OOCH– bond, we term the mechanism as E1H-3c4e elimination. The root-cause of the E1H-3c4e elimination is that the poor leaving formate group is less competitive in stabilizing the negative charge resulted from H^β abstraction by the HCOO^– base than the nearby carbonyl group (C^αO) that can utilize the negative charge to form a stabilizing 3c–4e bond with a formic acid molecule. In addition, the study characterizes versatile roles of formic acid in achieving the whole transformation, which accounts for why the HCO₂H/NaCO₂H medium works so elegantly for <b>1</b>^<b>ox</b> depolymerizaion