Bicyclic Guanidine Research Articles

Chiral Guanidines-catalyzed Nucleophilic Addition Reactions Using 5<i>H</i>-Oxazol-4-ones: Development of Catalytic Asymmetric Syntheses of Chiral α-Tetrasubstituted Hydroxycarboxylic Acid Derivatives

Chiral α-hydroxycarboxylic acids and their derivatives bearing a tetrasubstituted α-carbon atom are well-recognized chiral synthons and components of various pharmaceuticals and biologically active natural products. Despite their high demand, preparative methods of these compounds are quite limited due to the difficulty in achieving stereoselective construction of the tetrasubstituted carbon stereogenic center at the α-position. To address this issue, we developed asymmetric nucleophilic addition reactions of 5H-oxazol-4-ones to carbon electrophiles using chiral bicyclic guanidine catalysts containing a hydroxyl group at the appropriate position as a preparative method of α-tetrasubstituted hydroxycarboxylic acid derivatives. In this article, we introduce the nucleophilic addition reactions with several types of electrophiles, such as aldehydes, alkynyl carbonyl compounds, allenyl carbonyl compounds, vinyl ketones, dienones, and 2-chloroacrylnitrile. Derivatizations of the products of these reactions into corresponding α-hydroxycarboxylic acid derivatives and their application to a natural product synthesis are also described.

Optoelectronic and structural studies of a Ni(II) complex including bicyclic guanidine ligands: DFT calculations

Structural and optoelectronic properties of NiCl2C14H26N6 complex (1) using DFT calculations by various exchange correlation potentials including PBE–Generalized Gradient Approximation (PBE–GGA) and hybrid density functional B3LYP are investigated. The complex belongs to monoclinic crystallographic system with P2(1)/n space group, Z = 4. Selected 1 as monomeric unit (1−mon) through CH···Cl hydrogen bonds (HBs) constructs a 2–D coordination polymeric plane. The dispersion corrected density functional theory (DFT−D) calculations determine the binding energies of the non−conventional CH···Cl HBs constructor of the crystalline network (1−net). The calculated−B3LYP/LANL2DZ/6–311+G(d, p) electronic spectrum in gas phase shows four major bands in the range of 200–240 nm that could assigned to MLCT and LMCT transitions with n → π∗ and π∗ → σ∗ characters. The calculated results of electronic transitions in water using PCM method include broad bands with blue shifts in comparison with the vacuo one. Density of states results by the PBE–GGA show that this complex has a wide band gap (3.0 eV). The optoelectronic properties predict that it can be efficiently used in the optoelectronic molecular devices.

Mononuclear Ruthenium and Osmium Complexes with a Bicyclic Guanidinate Ligand: Synthesis and Catalytic Behavior in Olefin Isomerization Processes

The preparation of the first mononuclear RuII, RuIV, and OsII complexes containing the anion of the bicyclic guanidine 1,3,4,6,7,8‐hexahydro‐2H‐pyrimido[1,2‐a]pyrimidine (Hhpp) as a chelating ligand, namely [RuX{κ2‐(N,N′)‐hpp}(η6‐arene)] [arene = p‐cymene, X = Cl (2a), Br (2b), I (2c); arene = C6Me6, X = Cl (7)], [RuCl{κ2‐(N,N′)‐hpp}(η3:η3‐C10H16)] (9; C10H16 = 2,7‐dimethylocta‐2,6‐diene‐1,8‐diyl), and [OsCl{κ2‐(N,N′)‐hpp}(η6‐p‐cymene)] (11), is described. Compounds 2a–c, 7, 9, and 11 have been fully characterized by elemental analysis, HRMS, IR and NMR spectroscopy. In addition, the structure of 2a has been unequivocally confirmed by single‐crystal X‐ray diffraction methods. The catalytic behavior of these metal guanidinate complexes in the base‐free redox isomerization of allylic alcohols is explored, with the ruthenium(IV) derivative 9 showing the best performance (TOF up to 5940 h–1). All of the synthesized complexes have also proven to be active in the isomerization of the allylbenzene estragole into the industrially relevant 1‐propenylbenzene anethole, with a trans selectivity of up to 95 %.

Urea derivatives from carbon dioxide and amines by guanidine catalysis: Easy access to imidazolidin-2-ones under solvent-free conditions

A novel methodology to easily access imidazolidin-2-ones from propargylamines, primary amine and CO2 with TBD (1,5,7-triazabicyclo[4.4.0]dec-5-ene) or MTBD (7-methyl-1,5,7-triazabicyclo[4.4.0]dec-5-ene) as catalysts under solvent-free conditions is here reported. Bicyclic guanidines, able to catalyze the formation of oxazolidinones from secondary propargylamines and CO2, are now presented for the first time as effective organocatalysts for the chemical fixation of CO2 into linear and cyclic ureas. Plausible reaction pathways are proposed on the basis of experimental findings.

Coupling Reactions of Alkynyl Indoles and CO2 by Bicyclic Guanidine: Origin of Catalytic Activity?

Density functional theory calculations were used to investigate the three possible modes of activation for the coupling of CO2 with alkynyl indoles in the presence of a guanidine base. The first of these mechanisms, involving electrophilic activation, was originally proposed by Skrydstrup et al. (Angew. Chem. Int. Ed. 2015, 54, 6682). The second mechanism involves the nucleophilic activation of CO2 . Both of these electrophilic and nucleophilic activation processes involve the formation of a guanidine-CO2 zwitterion adduct. We have proposed a third mechanism involving the bifunctional activation of the bicyclic guanidine catalyst, allowing for the simultaneous activation of the indole and CO2 by the catalyst. We demonstrated that a second molecule of catalyst is required to facilitate the final cyclization step. Based on the calculated turnover frequencies, our newly proposed bifunctional activation mechanism is the most plausible pathway for this reaction under these experimental conditions. Furthermore, we have shown that this bifunctional mode of activation is consistent with the experimental results. Thus, this guanidine-catalyzed reaction favors a specific-base catalyzed mechanism rather than the CO2 activation mechanism. We therefore believe that this bifunctional mechanism for the activation of bicyclic guanidine is typical of most CO2 coupling reactions.

Metal-Free Synthesis of N-Aryl Amides using Organocatalytic Ring-Opening Aminolysis of Lactones.

Catalytic ring-opening of bio-sourced non-strained lactones with aromatic amines can offer a straightforward, 100 % atom-economical, and sustainable pathway towards relevant N-aryl amide scaffolds. Herein, the first general, metal-free, and highly efficient N-aryl amide formation is reported from poorly reactive aromatic amines and non-strained lactones under mild operating conditions using an organic bicyclic guanidine catalyst. This protocol has high application potential as exemplified by the formal syntheses of drug-relevant molecules.

The guanylated bioamine agmatine – A theoretical investigation of its structure and exceptional high basicity in the gas phase

The guanylated bioamine agmatine, H2N(CH2)4NC(NH2)2⇌H2N(CH2)4NHC(NH2)NH, formed by decarboxylation of the proteinogenic amino acid arginine, plays an important role in the physiological processes of living organisms. Its structure is marked by the flexibility of the four-carbon chain connecting the two potentially basic amino and guanidino sites. Owing to these noteworthy properties and the questionable literature data on its protonated forms, its structure and intrinsic basicity were carefully scrutinized at the B3LYP/6–311+G(d,p) level. Three types of isomerism (prototropy and geometrical isomerism of the guanidino group, and rotational isomerism of the chain) were considered in this structural investigation. Various tautomers, as well as E/Z-isomers of the free guanidino group, various coiled and open conformations of the alkyl chain, and potential protonation sites (alkylamino and guanidine imino) were analyzed. Due to n-π conjugation in the guanidino group, its amino nitrogen (erroneously taken in the literature as the site of monoprotonation) is the least probable candidate as a protonation site. The H2N(CH2)4NC(NH2)2 tautomer slightly predominates in the tautomeric mixture. One of its coiled conformations, facilitating the formation of an internal hydrogen-bond, possesses the lowest enthalpy, whereas one of its open conformations has the lowest Gibbs energy. The guanidine imino-nitrogen is without any doubt the favored site of protonation in the gas phase and in solution. Monoprotonated agmatine prefers the coiled conformations, all stabilized by intramolecular H-bonding, in a scorpion-shaped arrangement. The DFT-estimated PA of agmatine is higher than the experimental PAs reported for other bidentate bioamines (cadaverine, putrescine, and histamine) and close to that of the superbasic bicyclic guanidine MTBD. A guanylation of one amino group of diamines (NH2 in putrescine or NMe2 in tetramethyldiamines) increases their PA by ca. 60kJmol−1.

Organocatalytic Coupling of CO2 with a Propargylic Alcohol: A Comprehensive Mechanistic Study.

The metal-free coupling of a propargylic alcohol with CO2 catalysed by guanidine derivatives was investigated in detail through the combination of online kinetic studies by in situ attenuated-total reflection IR (ATR-IR) spectroscopy and DFT calculations. Bicyclic guanidines, namely 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) and 7-methyl-1,5,7-triazabicyclo[4.4.0]dec-5-ene (MTBD) are effective catalysts for the conversion of 2-methyl-3-butyn-2-ol to α-methylene cyclic carbonate and oxoalkyl acyclic carbonate under mild reaction conditions. The lower selectivity of TBD in comparison with MTBD towards the formation of α-methylene cyclic carbonate was elucidated from DFT calculations and is related to the bifunctional activity (base/H-bond donor) of TBD decreasing the Gibbs free energy of the reaction path for the formation of the acyclic carbonate.

Synthesis of Bicyclic and Tricyclic Chiral Guanidinium Salts by an Intramolecular Alkylation Approach

Synthetic studies leading to three new types of chiral guanidinium scaffolds are described. Structures of interest include bicyclic guanidine derivatives with various substitution patterns around the guanidine core, as well as a highly rigid tricyclic scaffold. The challenging targets were accessed by application of mercury(II)‐promoted guanylation of N‐Boc‐substituted thioureas and Ishikawa's desulfurative cyclization. In addition to our synthetic logics, a scalable synthesis of the tricyclic guanidinium salt is presented.

A theoretical study of intramolecular H–bonding and metal–ligand interactions in some complexes with bicyclic guanidine ligands

The reported complexes formulated as [MnCl2(hppH)2] (1), [FeCl2(hppH)2] (2), and [NiCl2(hppH)2] (3) and a new theoretically designed example, [CuCl2(hppH)2] (4), have been used for calculations at the B3LYP/LANL2DZ/6-311G (d, p) level of density functional theory (DFT). The intramolecular hydrogen bonds (HB) NH⋯Cl could followed through their physicochemical properties such as, vibrational frequency, electronic transmission in UV–Visible spectroscopy, metal–ligand donor–acceptor interactions in NBO analysis, total energy and frontier molecular orbital energy. These properties influenced by the relationship of structure and metal–ligand electron density exchange will be discussed. The computations revealed that the stronger NH⋯Cl HB exists in complexes with longer MCl and MNimine bonds and shorter H⋯Cl bond, and vice versa that confirms the shortest and longest HBs in 4 and 3, respectively. These results agree with the second–order perturbation energies obtained by NBO analysis within charge transfer from the proton–acceptor chlorine to the p∗ orbital of the N atom. The calculated electronic absorption spectra by TD–DFT calculations show the larger Cl− to M2+ donation in 2 and 3 in comparison with 1 and 4 that confirms the stronger HB in 1 and 4 compared to 2 and 3. In order to find the basis set effect on the structure, vibrational frequencies, and electronic transitions, we use another basis set def2-TZVP includes polarization function for Mn element in complex 1. The obtained test results showed unremarkable differences between two basis sets LANL2DZ and def2-TZVP (cf. Figs. S2, S3 and Tables S1 and S2 in Supplementary Materials).

Absolute Configuration and Body Part Distribution of the Alkaloid 6-epi-Monanchorin from the Marine Polychaete Chaetopterus variopedatus

As a result of the first study on secondary metabolites from the cosmopolitan bioluminescent marine tube polychaete Chaetopterus variopedatus, a new bicyclic guanidine alkaloid, 6-epi-monanchorin (1), along with the previously known monanchorin (2) were isolated. The structure of 1 was elucidated by spectroscopic and chemical methods, including a cleavage of the C₁-0₇ bond to obtain a secondary alcohol (3), which was used to determine the absolute configurations by Mosher's method. It was found that 1 and 2 were mainly accumulated in a secreted mucus special organ of the worm (food net), where green and blue-green microalgae were detected. A biosynthetic pathway to 6-epi-monanchorin and monanchorin from dietary polyeic fatty acid precursors was proposed.

ChemInform Abstract: Synthesis of Cyclic Guanidines via Silver‐Catalyzed Intramolecular Alkene Hydroamination Reactions of N‐Allylguanidines.

The silver-catalyzed hydroamination of tosyl-protected N-allylguanidines is described. These reactions provide substituted cyclic guanidines in high yields. The reactions are amenable to the construction of quaternary stereocenters as well as both monocyclic and bicyclic guanidine products.

Exploring the Intricacies of Weak Interactions in Metal-Metal Bonds Using an Unsymmetrical Carbonyl Precursor and a Triple-Bonded W2(6+) Paddlewheel.

Stepwise reaction of W(CO)6 with tetramethylated bicyclic guanidinate ligands, characterized by a central C(N)3 unit joining two fused six-membered rings with CH2CMe2CH2 units spanning two of the nitrogen atoms, allowed isolation of W2(μ-CO)2(μ-TMhpp)2(η(2)-TMhpp)2, 1, a precursor of W2(TMhpp)4Cl2 ( J. Am. Chem. Soc. 2013 , 135 , 17889 ; TMhpp = [(CH2CMe2CH2)2(C(N)3)]). Subsequent heating of 1 followed by reaction with TlPF6 generates [W2(TMhpp)4](PF6)2, 2. Compound 1 has an edge-sharing bioctahedral (ESBO) arrangement with a W2(μ-CO)2(4+) core having semibridging carbonyl groups, while 2 has a paddlewheel structure with a W2(6+) core spanned by four tetramethyl-substituted bicyclic guanidinate ligands. This compound also has hexafluorophosphate anions along the metal-metal bond that are nestled within methylene groups with the aid of a network of weak C-H···F interactions that prevent a close approach of the fluorine atoms to the dimetal unit. Theoretical computations were carried out on ditungsten model complexes supported by three ligand sets: bicyclic guanidinate, guanidinate, and formamidinate. The computations show that the π-accepting ability of the carbonyl groups significantly lowers the energy of the σ* orbital, and thus, the energy falls below that of the δ orbital. This information along with the diamagnetism of both 1 and 2-as shown by the sharp signals in the (1)H NMR spectra that support a lack of unpaired electrons (S = 0)-is consistent with the electronic configuration of σ(2)π(2)σ*(2)δ(2) (π(2)δ(2)) and thus a formal bond order of 2 for 1 and σ(2)π(4) for the triple-bonded W2(6+) core in 2. A comparison of the W-W bond lengths in 2, its chloro precursor W2(TMhpp)4Cl2, and the corresponding analogue W2(hpp)4Cl2 shows a substantial effect from the axially coordinated ligand, distal lone pair in determining the length of the metal-metal bond for these paddlewheel species. The importance of the ligands in tuning the energy level of the metal-metal bonds that may lead to dramatic changes in physical properties is also discussed. It is noteworthy that bicyclic guanidinates with the strongest π-donating ability push upward the energy level of the δ orbital, thus allowing the compounds to be easily oxidized.

A chiral analog of the bicyclic guanidine TBD: synthesis, structure and Brønsted base catalysis

Starting from (S)-β-phenylalanine, easily accessible by lipase-catalyzed kinetic resolution, a chiral triamine was assembled by a reductive amination and finally cyclized to form the title compound 10. In the crystals of the guanidinium benzoate salt the six membered rings of 10 adopt conformations close to an envelope with the phenyl substituents in pseudo-axial positions. The unprotonated guanidine 10 catalyzes Diels–Alder reactions of anthrones and maleimides (25–30% ee). It also promotes as a strong Brønsted base the retro-aldol reaction of some cycloadducts with kinetic resolution of the enantiomers. In three cases, the retro-aldol products (48–83% ee) could be recrystallized to high enantiopurity (≥95% ee). The absolute configuration of several compounds is supported by anomalous X-ray diffraction and by chemical correlation.

15N NMR Spectroscopy, X-ray and Neutron Diffraction, Quantum-Chemical Calculations, and UV/vis-Spectrophotometric Titrations as Complementary Techniques for the Analysis of Pyridine-Supported Bicyclic Guanidine Superbases

Pyridine substituted with one and two bicyclic guanidine groups has been studied as a potential source of superbases. 2-{hpp}C5H4N (I) and 2,6-{hpp}2C5H3N (II) (hppH = 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine) were protonated using [HNEt3][BPh4] to afford [I-H][BPh4] (1a), [II-H][BPh4] (2), and [II-H2][BPh4]2 (3). Solution-state (1)H and (15)N NMR spectroscopy shows a symmetrical cation in 2, indicating a facile proton-exchange process in solution. Solid-state (15)N NMR data differentiates between the two groups, indicating a mixed guanidine/guanidinium. X-ray diffraction data are consistent with protonation at the imine nitrogen, confirmed for 1a by single-crystal neutron diffraction. The crystal structure of 1a shows association of two [I-H](+) cations within a cage of [BPh4](-) anions. Computational analysis performed in the gas phase and in MeCN solution shows that the free energy barrier to transfer a proton between imino centers in [II-H](+) is 1 order of magnitude lower in MeCN than in the gas phase. The results provide evidence that linking hpp groups with the pyridyl group stabilizes the protonation center, thereby increasing the intrinsic basicity in the gas phase, while the bulk prevents efficient cation solvation, resulting in diminished pKa(MeCN) values. Spectrophotometrically measured pKa values are in excellent agreement with calculated values and confirm that I and II are superbases in solution.

Formation of a Radical Tricationic Tetraborane(4) by Hydride Abstraction from sp3–sp3‐Hybridized Diboranes

The formation, electronic structure, and reactivity of the recently‐reported radical tricationic tetraborane [B4(hpp)4]·3+ (hpp = 1,3,4,6,7,8‐hexahydro‐2H‐pyrimido[1,2‐a]pyrimidinate), which features a planar rhomboid B4 unit and bridging bicyclic guanidinate (hpp) substituents, is reported. New experiments show that the tetraborane could be synthesized by hydride abstraction either in a very fast reaction from the electron‐rich sp3–sp3‐hybridized diborane [HB(hpp)]2, or in a very slow reaction from the cationic, less electron‐rich sp3–sp3‐hybridized diborane [HB(hpp)2BPCy3]+. The results argue for an electron‐transfer in the rate‐determining step of a complex reaction sequence. Quantum chemical calculations highlight the role of the hpp substituents for the stabilization of the radical tricationic tetraborane [B4(hpp)4]·3+. Further analysis shows that [B4(hpp)4]·3+ is highly reactive, and especially a very strong electrophile. This reactivity makes the compound very interesting because boron atoms in such a low oxidation state (<2) are expected to be strong nucleophiles.

Synthesis of Cyclic Guanidines via Silver-Catalyzed Intramolecular Alkene Hydroamination Reactions of N-Allylguanidines.

The silver-catalyzed hydroamination of tosyl-protected N-allylguanidines is described. These reactions provide substituted cyclic guanidines in high yields. The reactions are amenable to the construction of quaternary stereocenters as well as both monocyclic and bicyclic guanidine products.

Experimental Basicities of Phosphazene, Guanidinophosphazene, and Proton Sponge Superbases in the Gas Phase and Solution.

Experimental gas-phase superbasicity scale spanning 20 orders of magnitude and ranging from bicyclic guanidine 7-methyl-1,5,7-triazabicyclo[4.4.0]dec-5-ene to triguanidinophosphazenes and P3 phosphazenes is presented together with solution basicity data in acetonitrile and tetrahydrofuran. The most basic compound in the scale-triguanidinophosphazene Et-N═P[N═C(NMe2)2]3-has the highest experimental gas-phase basicity of an organic base ever reported: 273.9 kcal mol(-1). The scale includes besides the higher homologues of classical superbasic phosphazenes and several guanidino-substituted phosphazenes also a number of recently introduced bisphosphazene and bis-guanidino proton sponges. This advancement was made possible by a newly designed Fourier transform ion cyclotron resonance (ICR) mass spectrometry setup with the unique ability to generate and control in the ICR cell sufficient vapor pressures of two delicate compounds having low volatility, which enables determining their basicity difference. The obtained experimental gas-phase and solution basicity data are analyzed in terms of structural and solvent effects and compared with data from theoretical calculations.

Haliclonadiamine Derivatives and 6-epi-Monanchorin from the Marine Sponge Halichondria panicea Collected at Iriomote Island.

Four new haliclonadiamine analogues, (10Z,12E)-haliclonadiamine (1), (10E,12Z)-haliclonadiamine (2), and halichondriamines A (3) and B (4), were isolated from the Okinawan marine sponge Halichondria panicea together with haliclonadiamine (5) and papuamine (6). The structures of 1-4 were elucidated on the basis of their spectroscopic data by comparisons with those for 5 and 6. Further separation of the remaining fraction led to the isolation of a new bicyclic guanidine alkaloid, 6-epi-monanchorin (7), along with monanchorin (8). Compound 7 is the epimer of 8 at the 6 position. Compounds 1-6 inhibited the growth of Mycobacterium smegmatis with inhibition zones of 12, 7, 8, 7, 16, and 12 mm at 10 μg/disc, respectively. Compounds 2-4 exhibited weak cytotoxicities against the Huh-7 (hepatoma) human cancer cell line and were 2-fold less active than 5 and 6. Compounds 7 and 8 were not active against M. smegmatis at 20 μg/disc or the cancer cell line at 10 μM.

Inside Back Cover: Bicyclic Guanidine Catalyzed Asymmetric Tandem Isomerization Intramolecular‐Diels–Alder Reaction: The First Catalytic Enantioselective Total Synthesis of (+)‐alpha‐Yohimbine (Chem. Asian J. 3/2016)

Inside Back Cover: Bicyclic Guanidine Catalyzed Asymmetric Tandem Isomerization Intramolecular‐Diels–Alder Reaction: The First Catalytic Enantioselective Total Synthesis of (+)‐alpha‐Yohimbine (Chem. Asian J. 3/2016)