Chalcogen- and Halogen-Bond-Donating Cyanoborohydrides Provide Imine Hydrogenation.

Sulfonium, selenonium, telluronium, and iodonium cyanoborohydrides have been synthesized, isolated, and fully characterized by various methods, including single-crystal X-ray diffraction (XRD) analysis. The quantum theory of atoms in molecules' analysis based on the XRD data indicated that the hydride···σ-hole short contacts observed in the crystal structures of each compound have a purely noncovalent nature. The telluronium and iodonium cyanoborohydrides provide a significantly higher rate of the model reaction of imine hydrogenation compared with sodium and tetrabutylammonium cyanoborohydrides. Based on the NMR and high-resolution electrospray ionization mass spectrometry data indicating that the reaction progress is accompanied by the cation reduction, a mechanism involving intermediate formation of elusive onium hydrides has been proposed as an alternative to conventional electrophilic activation of the imine moiety by its ligation to the cation's σ-hole.

Influence of Coordination to Silver(I) Centers on the Activity of Heterocyclic Iodonium Salts Serving as Halogen-Bond-Donating Catalysts.

Kinetic data based on 1 H NMR monitoring and computational studies indicate that in solution, pyrazole-containing iodonium triflates and silver(I) triflate bind to each other, and such an interplay results in the decrease of the total catalytic activity of the mixture of these Lewis acids compared to the separate catalysis of the Schiff condensation, the imine-isocyanide coupling, or the nucleophilic attack on a triple carbon-carbon bond. Moreover, the kinetic data indicate that such a cooperation with the silver(I) triflate results in prevention of decomposition of the iodonium salts during the reaction progress. XRD study confirms that the pyrazole-containing iodonium triflate coordinates to the silver(I) center via the pyrazole N atom to produce a rare example of a pentacoordinated trigonal bipyramidal dinuclear silver(I) complex featuring cationic ligands.

Xenon Derivatives as Aerogen Bond-Donating Catalysts for Organic Transformations: A Theoretical Study on the Metaphorical "Spherical Cow in a Vacuum" Provides Insights into Noncovalent Organocatalysis.

Computations indicate that cationic and noncharged xenon derivatives should exhibit higher catalytic activity than their iodine-based noncovalent organocatalytic congeners. Perfluorophenyl xenonium(II) is expected to demonstrate the best balance between catalytic activity and chemical stability for use in organocatalysis. Comparing its catalytic activity with that of isoelectronic perfluoroiodobenzene indicates that the high catalytic activity of cationic noncovalent organocatalysts is predominantly attributed to the electrostatic interactions with the reaction substrates, which cause the polarization of ligated species during the reaction progress. In contrast, the electron transfer and covalent contributions to the bonding between the catalyst and substrate have negligible effects. The dominant effect of electrostatic interactions results in a strong negative correlation between the calculated Gibbs free energies of activation for the modeled reactions and the highest potentials of the σ-holes on the central atoms of the catalysts. No such correlation is observed for noncharged catalysts.

Halonium, chalconium, and pnictonium salts as noncovalent organocatalysts: a computational study on relative catalytic activity.

This theoretical study sheds light on the relative catalytic activity of pnictonium, chalconium, and halonium salts in reactions involving elimination of chloride and electrophilic activation of a carbonyl group. DFT calculations indicate that for cationic aromatic onium salts, values of the electrostatic potential on heteroatom σ-holes gradually increase from pnictogen- to halogen-containing species. The higher values of the potential on the halogen atoms of halonium salts result in the overall higher catalytic activity of these species, but in the case of pnictonium and chalconium cations, weak interactions from the side groups provide an additional stabilization effect on the reaction transition states. Based upon quantum-chemical calculations, the catalytic activity of phosphonium(V) and arsenonium(V) salts is expected to be too low to obtain effective noncovalent organocatalytic compounds, whereas stibonium(V), telluronium(IV) and iodonium(III) salts exhibit higher potential in application as noncovalent organocatalysts.

Sulfonium and Selenonium Salts as Noncovalent Organocatalysts for the Multicomponent Groebke-Blackburn-Bienaymé Reaction.

Sulfonium and selenonium salts, represented by S-aryl dibenzothiophenium and Se-aryl dibenzoselenophenium triflates, were found to exhibit remarkable catalytic activity in the model Groebke-Blackburn-Bienaymé reaction. Kinetic analysis and density functional theory (DFT) calculations indicated that their catalytic effect is induced by the ligation of the reaction substrates to the σ-holes on the S or Se atom of the cations. The experimental data indicated that although 10-fold excess of the chloride totally inhibits the catalytic activity of the sulfonium salts, the selenonium salt remains catalytically active, which can be explained by the experimentally found lower binding constant of the selenonium derivative to chloride in comparison with the sulfonium analogue. Both types of salts exhibit lower catalytic activity in the model reaction than dibenziodolium species.

Diaryliodoniums as Hybrid Hydrogen- and Halogen-Bond-Donating Organocatalysts for the Groebke-Blackburn-Bienaymé Reaction.

Dibenziodolium and diphenyliodonium triflates display high catalytic activity for the multicomponent reaction that leads to a series of imidazopyridines. Density functional theory (DFT) calculations indicate that both the salts can play the role of hybrid hydrogen- and halogen-bond-donating organocatalysts, which electrophilically activate the carbonyl and imine groups during the reaction process. The ortho-H atoms in the vicinal position to the I atom play a dual role: forming additional noncovalent bonds with the ligated substrate and increasing the maximum electrostatic potential on the σ-hole at the iodine atom owing to the effects of polarization. Dibenziodolium triflate exhibits higher catalytic activity, and the results obtained from 1H nuclear magnetic resonance (NMR) titrations, in conjunction with those from DFT calculations, indicate that this could be explained in terms of the additional energy required for the rotation of the phenyl ring in the diphenyliodonium cation during ligation of the substrate.

Predicting the catalytic activity of azolium-based halogen bond donors: an experimentally-verified theoretical study.

This report demonstrates the successful application of electrostatic surface potential distribution analysis for evaluating the relative catalytic activity of a series of azolium-based halogen bond donors. A strong correlation (R2 > 0.97) was observed between the positive electrostatic potential of the σ-hole on the halogen atom and the Gibbs free energy of activation of the model reactions (i.e., halogen abstraction and carbonyl activation). The predictive ability of the applied approach was confirmed experimentally. It was also determined that the catalytic activity of azolium-based halogen bond donors was generally governed by the structure of the azolium cycle, whereas the substituents on the heterocycle had a limited impact on the activity. Ultimately, this study highlighted four of the most promising azolium halogen bond donors, which are expected to exhibit high catalytic activity.

Iodonium salts as efficient iodine(iii)-based noncovalent organocatalysts for Knorr-type reactions.

Hypervalent iodine(iii)-derivatives display higher catalytic activity than other aliphatic and aromatic iodine(i)- or bromine(i)-containing substrates for a Knorr-type reaction of N-acetyl hydrazides with acetyl acetone to give N-acyl pyrazoles. The highest activity was observed for dibenziodolium triflate, for which 10 mol% resulted in the generation of N-acyl pyrazole from acyl hydrazide and acetyl acetone typically at 50 °C for 3.5-6 h with up to 99% isolated yields. 1H NMR titration data and DFT calculations indicate that the catalytic activity of the iodine(iii) is caused by the binding with a ketone.

3-Dialkylamino-1,2,4-triazoles via ZnII-Catalyzed Acyl Hydrazide-Dialkylcyanamide Coupling.

Zinc(II)-catalyzed (10 mol % ZnCl2) coupling of acyl hydrazides and dialkylcyanamides in ethanol leads to 3-dialkylamino-1,2,4-triazoles (76-99%; 17 examples). This reaction represents a novel, straightforward, and high-yielding approach to practically important 3-NR2-1,2,4-triazoles, which utilizes commercially available and/or easily generated substrates. Seventeen new 3-NR2-1,2,4-triazoles were characterized by HRESI+-MS and IR, 1H, and 13C{1H} NMR spectroscopies and five species additionally by single-crystal X-ray diffraction (XRD). The ZnII-catalyzed reaction proceeds via initial generation of the [Zn{RC(=O)NHNH2}3](ZnCl4) complexes (exemplified by isolation of the complex with R = Ph, 76%; characterized by HRESI+-MS, IR, CP-MAS TOSS 13C{1H} NMR, and XRD). Electronic effects of substituents at the acyl hydrazide moiety do not significantly affect the reaction rate and the yield of the target triazoles, whereas the steric hindrances reduce the reaction rate without affecting the yield of the heterocycles.

Metal-Involving Synthesis and Reactions of Oximes.

This review classifies and summarizes the past 10-15 years of advancements in the field of metal-involving (i.e., metal-mediated and metal-catalyzed) reactions of oximes. These reactions are diverse in nature and have been employed for syntheses of oxime-based metal complexes and cage-compounds, oxime functionalizations, and the preparation of new classes of organic species, in particular, a wide variety of heterocyclic systems spanning small 3-membered ring systems to macroheterocycles. This consideration gives a general outlook of reaction routes, mechanisms, and driving forces and underlines the potential of metal-involving conversions of oxime species for application in various fields of chemistry and draws attention to the emerging putative targets.

Bifunctional reactivity of amidoximes observed upon nucleophilic addition to metal-activated nitriles.

Treatment of the aromatic nitrile complexes trans-[PtCl2(RC6H4CN)2] (R = p-CF3 NC1, H NC2, o-Cl NC3) with the aryl amidoximes p-R'C6H4C(NH2)=NOH (R' = Me AO1, H AO2, Br AO3, CF3 AO4, NO2 AO5) in all combinations, followed by addition of 1 equiv of AgOTf and then 5 equiv of Et3N, leads to the chelates [PtCl{HN=C(RC6H4)ON=C(C6H4R'-p)NC(RC6H4)═NH}] (1-15; 15 examples; yields 71-88% after column chromatography) derived from the platinum(II)-mediated coupling between metal-activated nitriles and amidoximes. The mechanism of this reaction was studied experimentally by trapping and identification of the reaction intermediates, and it was also investigated theoretically at the DFT level of theory. The combined experimental and theoretical results indicate that the coupling with the nitrile ligands involves both the HON and monodeprotonated NH2 groups of the amidoximes, whereas in the absence of the base, the NH2 functionality is inactive toward the coupling. The observed reaction represents the first example of bifunctional nucleophilic behavior of amidoximes. The complexes 1-16 were characterized by elemental analyses (C, H, N), high-resolution ESI(+)-MS, FTIR, and (1)H NMR techniques, whereas unstable 17 was characterized by HRESI(+)-MS and FTIR. In addition, 8·C4H8O2, 12, and 16·CHCl3 were studied by single-crystal X-ray diffraction.

Click-Type PtII -Mediated Hydroxyguanidine-Nitrile Coupling Provides Useful Catalysts for Hydrosilylation Cross-Linking.

The nitrile complexes trans-[PtCl2 (RCN)2 ] (R=Et (NC1), tBu (NC2), Ph (NC3), p-BrC6 H4 (NC4)) and cis-[PtCl2 (RCN)2 ] (R=Et (NC5), tBu (NC6), Ph (NC7)) react with 1 equiv of the hydroxyguanidine OC4 H8 NC(=NOH)NH2 (HG) furnishing the mono-addition products trans- and cis-[PtCl2 (RCN){NH=C(R)ON=C(NH2 )NC4 H8 O}] (1-4 and 9-11; 7 examples; 54-74 % yield). Treatment of any of the nitrile complexes NC1-NC7 with HG in a 1:2 molar ratio generated the bis-addition products trans- and cis-[PtCl2 {NH=C(R)ON=C(NH2 )NC4 H8 O}2 ] (5-8 and 12-14; 7 examples; 69-89 % yield). The PtII -mediated coupling between nitrile ligands and HG proceeds substantially faster than the corresponding reactions involving amid- and ketoximes and gives redox stable products under normal conditions. Complexes 1, 6⋅4 CH2 Cl2 , 7⋅4 CH2 Cl2 , 8⋅2 CH2 Cl2 , and NC4 were studied by X-ray crystallography. Platinum(II) species 1-3, 10, 11, and especially 9, efficiently catalyze the hydrosilylation cross-linking of vinyl-terminated poly(dimethylsiloxane) and trimethylsilyl-terminated poly(dimethylsiloxane-co-ethylhydrosiloxane) giving high-quality thermally stable silicon resins with no structural defects. The usage of these platinum species as the catalysts does not require any inhibitors and, moreover, the complexes and their mixtures with vinyl- and trimethylsilyl-terminated polysiloxanes are shelf-stable in air.

Zinc(II)-mediated nitrile-amidoxime coupling gives new insights into H⁺-assisted generation of 1,2,4-oxadiazoles.

The cyanamides Me2NCN (1a), OC4H8NCN (1b), and PhC(═O)N(H)CN (1c) and the conventional nitriles PhCN (1d) and EtCN (1e) react with 1 equiv of each of the amidoximes R'C(═NOH)NH2 (R' = Me (2a), Ph (2b)) in the presence of 1 equiv of ZnCl2 producing the complexes [ZnCl2{HN═C(R)ON═C(R')NH2}] (R/R' = NMe2/Me (3a), NMe2/Ph (3b), NC4H8O/Me (3c), NC4H8O/Ph (3d), N(H)C(═O)Ph/Me (3e), N(H)C(═O)Ph/Ph (3f), Ph/Me (3g), Ph/Ph (3h), Et/Ph (3j)) with the chelate ligands originating from the previously unreported zinc(II)-mediated nitrile-amidoxime coupling. Addition of 1 equiv of p-TolSO3H to any of one 3a-h, 3j results in the ligand liberation and formation of the iminium salts [H2N═C(R)ON═C(R')NH2](p-TolSO3) ([4a-j](p-TolSO3)), which then at 20-65 °C spontaneously transform to 1,2,4-oxadiazoles (5a-j). As a side reaction, cyanamide derived species [4a-f](p-TolSO3) undergo Tiemann rearrangement to produce the substituted ureas R'NHC(═O)NH2 (R' = Me (6a), Ph (6b)) and RC(═O)NH2 (R = NMe2 (6c), NC4H8O (6d), N(H)C(═O)Ph (6e)), whereas phenyl and ethyl cyanide derivatives besides their transformation to the oxadiazoles undergo hydrolysis to the parent amidoxime R'C(═NOH)NH2 (R' = Me (2a), Ph (2b)) and the carboxamides RC(═O)NH2 (R = Ph (6f), Et (6g)). All new obtained compounds were characterized by HRESI-MS, IR, ATR-FTIR, (1)H NMR, CP-MAS TOSS (13)C NMR, elemental analyses (C, H, N), and single crystal X-ray diffraction for seven species (3a-e, [4b](p-TolSO3), and [4d](p-TolSO3)). Two previously unknown heterocycles 5c and 5e were isolated and characterized by elemental analyses (C, H, N), HRESI-MS, IR, (1)H and (13)C{(1)H} NMR. The observed conversion of [4a-j](p-TolSO3) to the 1,2,4-oxadiazoles uncovers the mechanism of the previously reported H(+)-assisted generation of these heterocycles (Augustine; et al. J. Org. Chem. 2009, 74, 5640).

Di-chlorido-[N-(N,N-di-methyl-carbamimido-yl)-N',N',4-tri-methyl-benzohydrazonamide]-platinum(II) nitro-methane hemisolvate.

In the title compound, [PtCl2(C13H21N5)]·0.5CH3NO2, the Pt(II) atom is coordinated in a slightly distorted square-planar geometry by two Cl atoms and two N atoms of the bidentate ligand. The (1,3,5-tri-aza-penta-diene)Pt(II) metalla ring is slightly bent and does not conjugate with the aromatic ring. In the crystal, N-H⋯Cl hydrogen bonds link the complex mol-ecules, forming chains along [001]. The nitromethane solvent molecule shows half-occupancy and is disordered over two sets of sites about an inversion centre.

Amidrazone complexes from a cascade platinum(II)-mediated reaction between amidoximes and dialkylcyanamides.

The aryl amidoximes R'C6H4C(NH2)═NOH (R' = Me, 2a; H, 2b; CN, 2c; NO2, 2d) react with the dialkylcyanamide platinum(II) complexes trans-[PtCl2(NCNAlk2)2] (Alk2 = Me2, 1a; C5H10, 1b) in a 1:1 molar ratio in CHCl3 to form chelated mono-addition products [3a-h]Cl, viz. [PtCl(NCNAlk2){NH═C(NR2)ON═C(C6H4R')NH2}]Cl (Alk2 = Me2; R' = Me, a; H, b; CN, c; NO2, d; Alk2 = C5H10; R' = Me, e; H, f; CN, g; NO2, h). In the solution, these species spontaneously transform to the amidrazone complexes [PtCl2{NH═C(NR2)NC(C6H4R')NNH2}] (7a-h; 36-47%); this conversion proceeds more selectively (49-60% after column chromatography) in the presence of the base (PhCH2)3N. The observed reactivity pattern is specific for NCNAlk2 ligands, and it is not realized for conventional alkyl- and arylnitrile ligands. The mechanism of the cascade reaction was studied by trapping the isocyanate intermediates [PtCl(NCO){NH═C(NR2)NC(C6H4R')NNH2}] (5a-h) and also by ESI-MS identification of the ammonia complexes [PtCl(NH3){NH═C(NR2)NC(C6H4R')NNH2}](+) ([6a-h](+)) in solution. The complexes [3a]Cl, [3c-h]Cl, 5a-h and 7a-h were characterized by elemental analyses, high resolution ESI-MS, IR, and (1)H NMR techniques, while 5b, 5d, 5g, 7b, and 7e were also studied using single-crystal X-ray diffraction.

Amidoximes provide facile platinum(II)-mediated oxime-nitrile coupling.

The nucleophilic addition of amidoximes R'C(NH(2))═NOH [R' = Me (2.Me), Ph (2.Ph)] to coordinated nitriles in the platinum(II) complexes trans-[PtCl(2)(RCN)(2)] [R = Et (1t.Et), Ph (1t.Ph), NMe(2) (1t.NMe(2))] and cis-[PtCl(2)(RCN)(2)] [R = Et (1c.Et), Ph (1c.Ph), NMe(2) (1c.NMe(2))] proceeds in a 1:1 molar ratio and leads to the monoaddition products trans-[PtCl(RCN){HN═C(R)ONC(R')NH(2)}]Cl [R = NMe(2); R' = Me ([3a]Cl), Ph ([3b]Cl)], cis-[PtCl(2){HN═C(R)ONC(R')NH(2)}] [R = NMe(2); R' = Me (4a), Ph (4b)], and trans/cis-[PtCl(2)(RCN){HN═C(R)ONC(R')NH(2)}] [R = Et; R' = Me (5a, 6a), Ph (5b, 6b); R = Ph; R' = Me (5c, 6c), Ph (5d, 6d), correspondingly]. If the nucleophilic addition proceeds in a 2:1 molar ratio, the reaction gives the bisaddition species trans/cis-[Pt{HN═C(R)ONC(R')NH(2)}(2)]Cl(2) [R = NMe(2); R' = Me ([7a]Cl(2), [8a]Cl(2)), Ph ([7b]Cl(2), [8b]Cl(2))] and trans/cis-[PtCl(2){HN═C(R)ONC(R')NH(2)}(2)] [R = Et; R' = Me (10a), Ph (9b, 10b); R = Ph; R' = Me (9c, 10c), Ph (9d, 10d), respectively]. The reaction of 1 equiv of the corresponding amidoxime and each of [3a]Cl, [3b]Cl, 5b-5d, and 6a-6d leads to [7a]Cl(2), [7b]Cl(2), 9b-9d, and 10a-10d. Open-chain bisaddition species 9b-9d and 10a-10d were transformed to corresponding chelated bisaddition complexes [7d](2+)-[7f](2+) and [8c](2+)-[8f](2+) by the addition of 2 equiv AgNO(3). All of the complexes synthesized bear nitrogen-bound O-iminoacylated amidoxime groups. The obtained complexes were characterized by elemental analyses, high-resolution ESI-MS, IR, and (1)H NMR techniques, while 4a, 4b, 5b, 6d, [7b](Cl)(2), [7d](SO(3)CF(3))(2), [8b](Cl)(2), [8f](NO(3))(2), 9b, and 10b were also characterized by single-crystal X-ray diffraction.