Facile and Reversible 1,3-Dipolar Cycloaddition of Aryl Ketonitrones to Platinum(II)-Bound Nitriles: Synthetic, Structural, and Theoretical Studies

DFT calculations in solution systems: solvation energy, dispersion energy and entropy

DFT calculations of reaction mechanisms in solution have always been a hot topic, especially for transition-metal-catalyzed reactions. The calculation of solvation energy is performed using either the polarizable continuum model (PCM) or the universal solvation model SMD. The PCM calculation is very sensitive to the choice of atomic radii to form a cavity, where the self-consistent isodensity PCM (SCI-PCM) has been recognized as the best choice and our IDSCRF radii can provide a similar cavity. Moving from a gas-phase case to a solution case, dispersion energy and entropy should be carefully treated. The solvent-solute dispersion is also important in solution systems, and it should be calculated together with the solute dispersion. Only half of the solvent-solute dispersion energy from the PCM calculation belongs to the solute molecules to maintain a thermal equilibrium between a solute molecule and its cavity, similar to the treatment of electrostatic energy. Relative solute dispersion energy should also be shared equally with the newly formed cavity. The entropy change from a gas phase to a liquid phase is quite large, but the modern quantum chemistry programs can only calculate the gas-phase translational entropy based on the idea-gas equation. In this review, we will provide an operable method to calculate the solution translational entropy, which has been coded in our THERMO program.

New light on an old debate: does the RCN-PtCl2 bond include any back-donation? RCN←PtCl2 backbonding vs. the IR νC[triple bond, length as m-dash]N blue-shift dichotomy in organonitriles-platinum(ii) complexes. A thorough density functional theory - energy decomposition analysis study

For a series of organonitrile [RCN (R = Me, CF₃, Ph, CH₃Ph, CF₃Ph)] ligands, the nature of the N-Pt bond in the related cis-/trans-(RCN)₂PtCl₂ complexes has been computationally investigated by Density Functional Theory. A fragment based bond analysis has been performed in the canonical Kohn-Sham molecular orbitals framework, and it has been ultimately assessed that this bond is characterized both by N→Pt σ and by N←Pt π contributions. Voronoi Deformation Density charges further confirms the occurrence of N←Pt π interactions. Moreover, the Energy Decomposition Analysis-Natural Orbital for Chemical Valence (EDA-NOCV) method shows that the strength of the N←Pt π interaction is not negligible by contributing to about 30-40% of the total orbital interaction. Finally, the well-known ν_{C[triple bond, length as m-dash]N} blue-shift occurring upon coordination to Pt^II, has been thoroughly investigated by exploiting the EDA-NOCV and by evaluating ν_{C[triple bond, length as m-dash]N} and force constants. The origin of the ν_{C[triple bond, length as m-dash]N} blue-shift in these systems has been discussed on the basis of the CN bond polarization. N←Pt π backbonding causes only a systematic decrease of the observed ν_{C[triple bond, length as m-dash]N} blue-shift when compared to the one calculated for RCN-X (X = H⁺, alkaline, Lewis acids) herein reported (X = purely σ acceptors).

Re(CO)3-Templated Synthesis of α-Amidinoazadi(benzopyrro)methenes

α-Amidinoazadi(benzopyrro)methenes were synthesized using the Re(CO)₃ unit as a templating agent. The products of these template reactions are six-coordinate rhenium complexes, with a facial arrangement of carbonyls, a noncoordinating anion, and a tridentate α-amidinoazadi(benzopyrro)methene ligand. The tridentate ligand shows the conversion of one diiminoisoindoline sp² carbon to a sp³ carbon, which has been seen in the "helmet" and bicyclic phthalocyanines. The bidentate diiminoisoindoline fragment tilts out of the plane of coordination. Five examples of α-amidinoazadi(benzopyrro)methenes produced from these reactions using different nitrile solvents, including the nitrile activation of acetonitrile, propionitrile, butyronitrile, cyclohexanecarbonitrile, and benzonitrile.

Nucleophilic Addition of Ketones To Acetylenes and Allenes: A Quantum-Chemical Insight

A CBS-Q//B3 based study has been carried out to elucidate the mechanism of the KOH/DMSO superbase catalyzed ketones nucleophilic addition to alkyl propargyl and alkyl allenyl ethers yielding, along with (Z)-monoadducts, up to 26% of unexpected (E)-diadducts. The impact of different substrates (alkynes versus allenes) on the reaction mechanism has been discussed in detail. Along with the model reaction of acetone addition to propyne and allene, the addition of acetone and acetophenone to methyl propargyl and methyl allenyl ethers is considered. The limiting reaction stage of the starting ketone carbanion addition to propargyl and allenyl systems occurs with activation energies typical for vinylation of ketones. In contrast, the addition of intermediate α-carbanions to the terminal position of methyl allenyl ether is associated with unusually low activation barriers. The results obtained explain the composition of the reaction products and indicate the participation of mainly the allene form in the reaction.

Copper(I)-Catalyzed 1,3-Dipolar Cycloaddition of Ketonitrones to Dialkylcyanamides: A Step toward Sustainable Generation of 2,3-Dihydro-1,2,4-oxadiazoles

Cu^I-catalyzed cycloaddition (CA) of the ketonitrones, Ph₂C=N⁺(R')O^- (R' = Me, CH₂Ph), to the disubstituted cyanamides, NCNR₂ (R = Me₂, Et₂, (CH₂)₄, (CH₂)₅, (CH₂)₄O, C₉H₁₀, (CH₂Ph)₂, Ph(Me)), gives the corresponding 5-amino-substituted 2,3-dihydro-1,2,4-oxadiazoles (15 examples) in good to moderate yields. The reaction proceeds under mild conditions (CH₂Cl₂, RT or 45 °C) and requires 10 mol % of [Cu(NCMe)₄](BF₄) as the catalyst. The somewhat reduced yields are due to the individual properties of 2,3-dihydro-1,2,4-oxadiazoles, which easily undergo ring opening via N-O bond splitting. Results of density functional theory calculations reveal that the CA of ketonitrones to Cu^I-bound cyanamides is a concerted process, and the copper-catalyzed reaction is controlled by the predominant contribution of the HOMO_dipole-LUMO_{dipolarophile} interaction (group I by Sustmann's classification). The metal-involving process is much more asynchronous and profitable from both kinetic and thermodynamic viewpoints than the hypothetical metal-free reaction.

PdII-mediated integration of isocyanides and azide ions might proceed via formal 1,3-dipolar cycloaddition between RNC ligands and uncomplexed azide

The generation of (tetrazolate)Pd^II complexes via the integration of (isocyanide)Pd^II precursors with uncomplexed azides and the verification of plausible reaction mechanisms.

Regio- and Stereoselective 1,3-Dipolar Cycloaddition of Cyclic Azomethine Imines to Platinum(IV)-Bound Nitriles Giving Δ(2)-1,2,4-Triazoline Species

The complex trans-[PtCl4(EtCN)2] (14) reacts smoothly at 25 °C with the stable cyclic azomethine imines R(1)CH═N(a)NC(O)CH(NHC(O)C6H4R(3))C(b)H(C6H4R(2))((a-b)) [R(1)/R(2)/R(3) = p-Me/H/H (8); p-Me/p-Me/H (9); p-Me/p-MeO/H (10); p-Me/p-Cl/p-Cl (11); p-MeO/p-Me/H (12); p-MeO/p-Cl/m-Me (13)], and the reaction proceeds as stereoselective 1,3-dipolar cycloaddition to one of the EtCN ligands accomplishing the monocycloadducts trans-[PtCl4(EtCN){N(a)═C(Et)N(b)C(O)CH(NHC(O)C6H4R(3))CH(C6H4R(2))N(c)C(d)HR(1)}])((a-d;b-c)) [R(1)/R(2)/R(3) = p-Me/H/H (15); p-Me/p-Me/H (16); p-Me/p-MeO/H (17); p-Me/p-Cl/p-Cl (18); p-MeO/p-Me/H (19); p-MeO/p-Cl/m-Me (20)]. Inspection of the obtained and literature data indicate that the cycloaddition of the azomethine imines to the C≡N bonds of HCN and of Pt(IV)-bound EtCN has different regioselectivity leading to Δ(2)-1,2,3-triazolines and Δ(2)-1,2,4-triazolines, respectively. Platinum(II) species trans-[PtCl2(EtCN){N(a)═C(Et)N(b)C(O)CH(NHC(O)C6H4R(3))CH(C6H4R(2))N(c)C(d)HR(1)}]((a-d;b-c)) [R(1)/R(2)/R(3) = p-Me/H/H (21); p-Me/p-Me/H (22); p-Me/p-MeO/H (23); p-Me/p-Cl/p-Cl (24); p-MeO/p-Me/H (25); p-MeO/p-Cl/m-Me (26)] were obtained by a one-pot procedure from 14 and 8-13 followed by addition of the phosphorus ylide Ph3P═CHCO2Me. Δ(2)-1,2,4-Triazolines N(a)═C(Et)N(b)C(O)CH(NHC(O)C6H4R(3))CH(C6H4R(2))N(c)C(d)HR(1(a-d;b-c)) [R(1)/R(2)/R(3) = p-Me/H/H (27); p-Me/p-Me/H (28); p-Me/p-MeO/H (29); p-Me/p-Cl/p-Cl (30); p-MeO/p-Me/H (31); p-MeO/p-Cl/m-Me (32)] were liberated from 21-26 by the treatment with bis(diphenylphosphyno)ethane (dppe). Platinum(II) complexes 21-26 were characterized by elemental analyses (C, H, N), high-resolution electrospray ionization mass spectrometry (ESI-MS), and IR and (1)H and (13)C{(1)H} NMR spectroscopies and single crystal X-ray diffraction in the solid state for 25·CH3OH, 26·(CHCl3)0.84. The structure of 26 was also determined by COSY-90 and NOESY NMR methods in solution. Quantitative evaluation of several pairs of interproton distances obtained by NMR and X-ray diffraction agrees well with each other and with those obtained by the MM+ calculation method. Platinum(IV) complexes 15-20 were characterized by (1)H NMR spectroscopy. Metal-free 6,7-dihydropyrazolo[1,2-a][1,2,4]triazoles (27-32) were characterized by high-resolution ESI-MS and IR and (1)H and (13)C{(1)H} NMR spectroscopies and single crystal X-ray diffraction for 29·CDCl3. Theoretical density functional theory calculations were carried out for the investigation of the reaction mechanism, interpretation of the reactivity of Pt-bound and free nitriles toward azomethine imines and analysis of the regio- and stereoselectivity origin.

A case study of the mechanism of alcohol-mediated Morita Baylis-Hillman reactions. The importance of experimental observations

The mechanism of the Morita Baylis-Hillman reaction has been heavily studied in the literature, and a long series of computational studies have defined complete theoretical energy profiles in these reactions. We employ here a combination of mechanistic probes, including the observation of intermediates, the independent generation and partitioning of intermediates, thermodynamic and kinetic measurements on the main reaction and side reactions, isotopic incorporation from solvent, and kinetic isotope effects, to define the mechanism and an experimental mechanistic free-energy profile for a prototypical Morita Baylis-Hillman reaction in methanol. The results are then used to critically evaluate the ability of computations to predict the mechanism. The most notable prediction of the many computational studies, that of a proton-shuttle pathway, is refuted in favor of a simple but computationally intractable acid-base mechanism. Computational predictions vary vastly, and it is not clear that any significant accurate information that was not already apparent from experiment could have been garnered from computations. With care, entropy calculations are only a minor contributor to the larger computational error, while literature entropy-correction processes lead to absurd free-energy predictions. The computations aid in interpreting observations but fail utterly as a replacement for experiment.

Zinc(ii)-mediated generation of 5-amino substituted 2,3-dihydro-1,2,4-oxadiazoles and their further ZnII-catalyzed and O2-involving transformations

Zn^II-activated cyanamides NCNR₂ react with the acyclic N-alkyl ketonitrones Ph₂CN⁺(O⁻)R′ achieving uncomplexed 2,3-dihydro-1,2,4-oxadiazoles.

Novel (cyanamide)ZnIIcomplexes and zinc(ii)-mediated hydration of the cyanamide ligands

The Zn^II-mediated hydration of cyanamides does not require any co-catalyst.

Dialkylcyanamides are more reactive substrates toward metal-mediated nucleophilic addition than alkylcyanides

The dialkylcyanamide complexes Q[PtCl3(NCNR2)] (Q = Ph3PCH2Ph, R2 = Me21, Et22, C5H103, C4H8O 4; Q = NMe4, R2 = Me25; Q = NEt4, R2 = Me26) were synthesized either by dissolving Q2[Pt2(μ-Cl)2Cl4] in neat NCNR2 (1-4) or by substitution of a NCNR2 ligand with Cl(-) in [PtCl2(NCNR2)2] by its treatment with QCl (5, 6). Nucleophilic addition of dibenzylhydroxylamine, HON(CH2Ph)2, to 1-6 results in the formation of the complexes Q[PtCl3{NHC(NR2)ON(CH2Ph)2}] (Q = Ph3PCH2Ph, R2 = Me2, 7; Et2, 8; C5H10, 9; C4H8O, 10; Q = Me4N, R2 = Me211; Q = Et4N, R2 = Me2, 12) that further convert at room temperature in the solid state (1-24 h) or in a solution (0.5-2 h) to the imine complexes Q[PtCl3{N(CH2Ph)=C(H)Ph}] (Q = Ph3PCH2Ph, 13; Me4N, 14; Et4N, 15) and the corresponding dialkylureas H2NC(=O)NR2. The competitive reactivity study of the nucleophilic addition of HON(CH2Ph)2 to (Ph3PCH2Ph)[PtCl3(NCR')] (R' = Ph, NR2, CH2Ph) indicated that the reactivity of the coordinated NCNR2 is comparable to NCPh, while NCCH2Ph appeared to be much less reactive than the former two ligands. Compounds 1-6 and 13 were fully characterized by elemental analyses (C, H, N), high resolution ESI-MS, IR, and (1)H and (13)C{(1)H} NMR spectroscopy. The structure of 1 was additionally verified by a single-crystal X-ray diffraction.

Tetra-butyl-ammonium 2-[2,5-dimethyl-3-(4-nitro-phen-yl)-2,3-dihydro-1,2,4-oxadiazo-lium-4-yl]nona-hydro-closo-deca-borate

The title ionic compound, C16H36N(+)·C10H20B10N3O3(-), consists of a tetra-butyl-ammonium cation and a closo-deca-borate cluster anion, which is bound to the substituted 2,3-dihydro-1,2,4-oxadiazole ring through a B-N bond [1.540 (2) Å]. The distances between connected B atoms in the deca-borate cluster range from 1.689 (2) to 1.844 (2) Å. The 2,3-dihydro-1,2,4-oxadiazole ring adopts an envelope conformation with the N atom as the flap atom.