Density functional theory (DFT) computations (BP86 and M06-L) have been utilized to elucidate the detailed mechanism of a palladium-catalyzed reaction involving pyridine-type nitrogen-donor ligands ...that significantly expands the scope of C(sp3)–H activation and arylation. The reaction begins with precatalyst initiation, followed by substrate binding to the Pd(II) center through an amidate auxiliary, which directs the ensuing bicarbonate-assisted C(sp3)–H bond activation producing five-membered-ring cyclopalladate(II) intermediates. These Pd(II) complexes further undergo oxidative addition with iodobenzene to form Pd(IV) complexes, which proceed by reductive C–C elimination/coupling to give final products of arylation. The base-assisted C(sp3)–H bond cleavage is found to be the rate-determining step, which involves hydrogen bond interactions. The mechanism unravels the intimate involvement of the added 2-picoline ligand in every phase of the reaction, explains the isolation of the cyclopalladate intermediates, agrees with the observed kinetic hydrogen isotope effect, and demonstrates the Pd(II)/Pd(IV) redox manifold.
While limited choice of emissive organic linkers with systematic emission tunability presents a great challenge to investigate energy transfer (ET) over the whole visible light range with designable ...directions, luminescent metal‐organic frameworks (LMOFs) may serve as an ideal platform for such study due to their tunable structure and composition. Herein, five Zr6 cluster‐based LMOFs, HIAM‐400X (X=0, 1, 2, 3, 4) are prepared using 2,1,3‐benzothiadiazole and its derivative‐based tetratopic carboxylic acids as organic linkers. The accessible unsaturated metal sites confer HIAM‐400X as a pristine scaffold for linker installation. Six full‐color emissive 2,1,3‐benzothiadiazole and its derivative‐based dicarboxylic acids (L) were successfully installed into HIAM‐400X matrix to form HIAM‐400X‐L, in which the ET can be facilely tuned by controlling its direction, either from the inserted linkers to pristine MOFs or from the pristine MOFs to inserted linkers, and over the whole range of visible light. The combination of the pristine MOFs and the second linkers via linker installation creates a powerful two‐dimensional space in tuning the emission via ET in LMOFs.
Tunable energy transfer with designable direction, from second linkers to pristine MOFs or from pristine MOFs to second linkers, was achieved in the whole visible spectrum via installing color‐full emissive second linkers into the full‐color emissive pristine MOFs.
Reported is the enantioselective synthesis of tetracyclic indolines using silver(I)/chiral phosphoric acid catalysis. A variety of alkyne‐tethered indoles are suitable for this process. Mechanistic ...studies suggest that the in situ generated silver(I) chiral phosphate activates both the alkyne and the indole nucleophile in the initial cyclization step through an intermolecular hydrogen bond and the phosphate anion promotes proton transfer. In addition, further modifications of the cyclization products enabled stereochemistry–function studies of a series of bioactive indolines.
Duality: Highly enantioselective synthesis of tetracyclic indolines was realized using cooperative silver(I)/chiral phosphoric acid catalysis. Mechanistic studies identified the dual role of the chiral phosphate. M.S.=molecular sieves.
Density functional theory calculations have been performed to investigate the mechanism of the reactions of amines with primary alcohols to produce amides, catalyzed by the pincer complex Ru(II)-PNN ...(PNN = 2-(di-tert-butylphosphinomethyl)-6-diethylaminomethyl)pyridine). The results lead us to propose a catalytic cycle that includes four stages: (stage I) alcohol dehydrogenation to aldehyde, (stage II) coupling of aldehyde with amine to form hemiaminal, (stage III) hemiaminal dehydrogenation to amide, and (stage IV) catalyst regeneration via H2 elimination of the trans Ru dihydride complex produced in the two dehydrogenation stages. Both of the dehydrogenation reactions proceed via the bifunctional double hydrogen transfer mechanism rather than the β-H elimination mechanism. The selectivity of amide over ester is determined by the coupling stage in which the aldehyde∧amine coupling to give hemiaminal is more favorable than aldehyde∧alcohol coupling to give hemiacetal. The competition between dehydrogenation and dehydration of hemiaminal governs the selectivity of amide over imine. Three alternative mechanisms without involving hemiaminal or hemiacetal have also been taken into consideration. One of them is less favorable than the pathway involving hemiaminal, and the other two are unlikely, although they have been shown to operate in other catalytic systems. The mechanistic difference is that alcohol dehydrogenation in the present system takes place via bifunctional double hydrogen transfer, whereas it prefers the β-H elimination mechanism in the other systems. The different dehydrogenation mechanisms are attributed to the different ways in which the catalytically active species are generated. In the current system, the catalyst is the catalytically active species itself, requiring no further activation, and its bifunctional active site makes the dehydrogenation follow the double hydrogen transfer mechanism. By contrast, the catalysts in the other systems need to be activated in situ to generate the active species that have vacant coordinate sites suitable for the β-H elimination dehydrogenation.
Density functional theory computations have elucidated the detailed mechanism and intriguing selectivities of C(sp3)–H activation and arylation of aldehydes and ketones promoted by palladium–amino ...acid cooperative catalysis. The amino acid cocatalyst takes up the carbonyl substrate by a condensation reaction to form an imine–acid, which acts as a transient directing reagent and metathesizes with Pd(OAc)2 (the precatalyst) to initiate active Pd(II) complexes. The reaction then proceeds through C–H bond activation, oxidative addition of Pd(II) by iodobenzene, and reductive elimination from Pd(IV) completing C–C bond formation, followed by ligand exchange to regenerate the active Pd(II) catalyst and release the arylated imine–acid which continues on hydrolysis to give the final product and regenerate the amino acid cocatalyst. The C–H activation step via concerted metalation–deprotonation (CMD), which is rate- and selectivity-determining, favors palladacyclic transition states with a minimum chelate ring strain and an optimal Pd(d)/C–H(σ) orbital interaction. This finding reveals the origins of the regioselectivities that favor (1) the benzylic C(sp3)–H over ortho-phenyl C(sp2)–H activation for aromatic aldehydes and (2) the β-primary C(sp3)–H over γ-primary C(sp3)–H activation for aliphatic ketones. Incorporation of a chiral amino acid into the catalyst allows for enantioselective benzylic C(sp3)–H arylation of aromatic aldehydes, and the enantioselectivity arises from steric and torsional strains that discriminate between the diastereomeric transition states. The computational results demonstrate rich experimental–theoretical synergy and provide useful insights for the further development of C–H functionalization and metal–organic cooperative catalysis.
In the new class of N-heterocyclic carbene (NHC) chelated ruthenium catalysts for Z-selective olefin metathesis, the nitrato-supported complex 3cat appears distinct from all the other ...carboxylato-supported analogues. We have performed DFT calculations (B3LYP and M06) to elucidate the mechanism of 3cat-catalyzed metathesis homodimerization of 3-phenyl-1-propene. The six-coordinate 3cat transforms via initial dissociation and isomerization into a trigonal-bipyramidal intermediate (5), from which two consecutive metathesis reactions via the side-bound mechanism lead to (Z)-PhCH2CHCHCH2Ph (major) and (E)-PhCH2CHCHCH2Ph (minor). In the overall mechanism, 3cat functions similarly to the pivalate-supported analogue 1cat. 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 3cat is somewhat more active than 1cat in catalyzing the homodimerization of 3-phenyl-1-propene.
DFT calculations have been combined with experiments to study the mechanism of γ-C(sp3)–H arylation of aliphatic amines promoted by palladium–glyoxylic acid cooperative catalysis, with a focus on ...the role of silver(I) additives. Glyoxylic acid (the cocatalyst) uses its aldehyde functionality to react with the amine substrate to form an iminoacetic acid. This acid acts as a transient directing reagent and metathesizes with Pd(OAc)2 (the precatalyst) to give a Pd(II)–diiminoacetate five-membered chelate, which has been shown computationally as the catalyst resting state and which has been experimentally synthesized and characterized. C(sp3)–H activation from the Pd(II)–diiminoacetate complex or its mononuclear derivatives would face a high kinetic barrier (>30 kcal/mol) arising mainly from breaking a stable five-membered N,O-chelate ring. The crucial role of the silver(I) carboxylate additive is in reacting with the Pd(II)–diiminoacetate complex to provide a heterodimeric Pd(II)–Ag(I) complex supported by bridging chelators and intermetallic Pd–Ag interaction, which would lead to a C(sp3)–H activation transition state with a considerably lower barrier (∼25 kcal/mol). The Pd(II)–Ag(I) complex has been detected by mass spectrometry, which provides the first experimental evidence of a Pd–Ag-containing active species in Pd-catalyzed C–H activation reactions using Ag(I) additives. After C(sp3)–H activation, the reaction proceeds through oxidative addition of Pd(II) and reductive elimination from Pd(IV) completing C–C formation, followed by ligand exchange to regenerate the catalyst resting state and release the arylated iminoacetic acid which continues on hydrolysis to give the final amine product and regenerate the glyoxylic acid cocatalyst. The computational and experimental findings taken together provide new mechanistic insight into the broad range of palladium-catalyzed C–H activation reactions that use silver(I) additives.
We present a detailed DFT mechanistic study on the first Ni-catalyzed direct carbonyl-Heck coupling of aryl triflates and aldehydes to afford ketones. The precatalyst Ni(COD)
2
is activated with the ...phosphine (phos) ligand, followed by coordination of the substrate PhOTf, to form Ni(phos)(PhOTf) for intramolecular PhOTf to Ni(0) oxidative addition. The ensuing phenyl-Ni(
ii
) triflate complex substitutes benzaldehyde for triflate by an interchange mechanism, leaving the triflate anion in the second coordination sphere held by Coulomb attraction. The Ni(
ii
) complex cation undergoes benzaldehyde C&z.dbd;O insertion into the Ni-Ph bond, followed by β-hydride elimination, to produce Ni(
ii
)-bound benzophenone, which is released by interchange with triflate. The resulting neutral Ni(
ii
) hydride complex leads to regeneration of the active catalyst following base-mediated deprotonation/reduction. The benzaldehyde C&z.dbd;O insertion is the rate-determining step. The triflate anion, while remaining in the second sphere, engages in electrostatic interactions with the first sphere, thereby stabilizing the intermediate/transition state and enabling the desired reactivity. This is the first time that such second-sphere interaction and its impact on cross-coupling reactivity has been elucidated. The new insights gained from this study can help better understand and improve Heck-type reactions.
We present a detailed DFT mechanistic study on the first Ni-catalyzed direct carbonyl-Heck coupling of aryl triflates and aldehydes to afford ketones.
The reactions of Ln(NO3)3 (Ln = La, Er) with 1,4-phenylendiacetic acid (H2PDA) under hydrothermal conditions produce isostructural lanthanide coordination polymers with the empirical formula ...Ln2(PDA)3(H2O)·2H2O. The extended structure of Ln2(PDA)3(H2O)·2H2O consists of Ln-COO triple helixes cross-linked through the −CH2C6H4CH2− spacers of the PDA anions, showing 1D open channels along the crystallographic c axis that accommodate the guest and coordinated water molecules. Evacuation of Er2(PDA)3(H2O)·2H2O at room temperature and at 200 °C, respectively, generates Er2(PDA)3(H2O) and Er2(PDA)3, both of which give powder X-ray diffraction patterns consistent with that of Er2(PDA)3(H2O)·2H2O. The porosity of Er2(PDA)3(H2O) and Er2(PDA)3 is further demonstrated by their ability to adsorb water vapor to form Er2(PDA)3(H2O)·2H2O quantitatively. Thermogravimetric analyses show that Er2(PDA)3 remains stable up to 450 °C. The effective pore window size in Er2(PDA)3 is estimated at 3.4 Å. Gas adsorption measurements indicate that Er2(PDA)3 adsorbs CO2 into its pores and shows nonporous behavior toward Ar or N2. There is a general correlation between the pore size and the kinetic diameters of the adsorbates (CO2 = 3.3 Å, Ar = 3.40 Å, and N2 = 3.64 Å). That the adsorption favors CO2 over Ar is unprecedented and may arise from the combined differentiations on size and on host−guest interactions.
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 Z selectivity of the anti-Markovnikov hydroalkoxylation of terminal alkynes with a Rh(I) 8-quinolinolato carbonyl chelate (1cat). The chosen system is, without any truncation, the realistic reaction of phenylacetylene and methanol with 1cat. Initiation of 1cat through phenylacetylene substitution for carbonyl generates the active catalyst, a Rh(I) η2-alkyne complex (3), which tautomerizes via an indirect 1,2-hydrogen shift to the Rh(I) vinylidene complex 4. The oxygen nucleophile methanol attacks the electrophilic vinylidene Cα, forming two stereoisomeric Rh(I) vinyl complexes (15 and 16), which ultimately lead to the (Z)- and (E)-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 (Z)- and (E)-PhCHCHOMe and regenerates 3. The anti-Markovnikov regioselectivity stems from the Rh(I) vinylidene complex 4 with reversed Cα and Cβ polarity. The stereoselectivity arises from the turnover-limiting transition states (TSs) for the Rh(I) carbene rearrangement: the Z-product-forming TS24 is sterically less congested and hence more stable than the E-product-forming TS25. The difference in energy (1.2 kcal/mol) between TS24 and TS25 gives a theoretical Z 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.
PDF
