Trans Substituents Research Articles

Broad-spectrum cyclic boronate β-lactamase inhibitors featuring an intramolecular prodrug for oral bioavailability

Early efforts to broaden the spectrum and potency of cyclic boronic acid β-lactamase inhibitor vaborbactam included a series of 7-membered ring boronates. Exploration of stereoisomers and incorporation of heteroatoms allowed identification of the all-carbon cyclic boronate with substituents trans as the preferred core structure, showing inhibition of Class A and C enzymes. Crystal structures of one analog bound to important β-lactamase enzymes were obtained. When isolated under acidic conditions, these compounds spontaneously formed a neutral cyclic anhydride (intramolecular prodrug) which was shown to have much-improved oral bioavailability (52–69%) compared to the ring-opened carboxylate salt (9%).

Ambiphilic boryl groups in a neutral Ni(ii) complex: a new activation mode of H2.

The concept of metal–ligand cooperation opens new avenues for the design of catalytic systems that may offer alternative reactivity patterns to the existing ones. Investigations of this concept with ligands bearing a boron center in their skeleton established mechanistic pathways for the activation of small molecules in which the boron atom usually performs as an electrophile. Here, we show how this electrophilic behavior can be modified by the ligand trans to the boron center, evincing its ambiphilic nature. Treatment of diphosphinoboryl (PBP) nickel–methyl complex 1 with bis(catecholato)diboron (B2Cat2) allows for the synthesis of nickel(ii) bis-boryl complex 3 that promotes the clean and reversible heterolytic cleavage of dihydrogen leading to the formation of dihydroborate nickel complex 4. Density functional theory analysis of this reaction revealed that the heterolytic activation of H2 is facilitated by the cooperation of both boryl moieties and the metal atom in a concerted mechanism that involves a Ni(ii)/Ni(0)/Ni(ii) process. Contrary to 1, the boron atom from the PBP ligand in 3 behaves as a nucleophile, accepting a formally protic hydrogen, whereas the catecholboryl moiety acts as an electrophile, receiving the attack from the hydride-like fragment. This manifests the dramatic change in the electronic properties of a ligand by tuning the substituent trans to it and constitutes an unprecedented cooperative mechanism that involves two boryl ligands in the same molecule operating differently, one as a Lewis acid and the other one as a Lewis base, in cooperation with the metal. In addition, reactivity towards different nucleophiles such as amines or ammonia confirmed the electrophilic nature of the Bcat moiety, allowing the formation of aminoboranes.

Chiral 7-Oxabicyclo[2.2.1]heptane Building Blocks for Prostanoids

Methyl (Z)-3-[(2R,3R,4S,5S)-5-(2-methoxy-2-oxoethyl)-3,4-(isopropylidenedioxy)tetrahydrofuran-2-yl]-prop-2-enoate was synthesized, and its intramolecular carbocyclization was studied. This reaction proceeds smoothly and quickly under the action of t-BuOK in THF, leading to three cycles with the methoxy and methoxycarbonylmethyl side substituents trans to each other. A stepwise route for the formation of methyl (1R,2R,6S,7R,8R,9R)-9-(2-methoxy-2-oxoethyl)-4,4-dimethyl-3,5,10-trioxatricyclo[5,2,1,02,6]decane-8-carboxylate and methyl (1R,2R,6S,7R,8S,9S)-9-(2-methoxy-2-oxoethyl)-4,4-dimethyl-3.5,10-trioxatricyclo-[5,2,1,02,6]decane-8-carboxylate with the trans orientation of the side substituents is proposed. The observed stereochemical result of the reaction is interpreted in terms of epimerization in the C1 and C4 centers of methyl (Z)-3-[(2R,3R,4S,5S)-5-(2-methoxy-2-oxoethyl)-3,4-(isopropylidenedioxy)-tetrahydrofuran-2-yl]prop-2-enoate, which ultimately leads to the cyclized β,β’-cis diastereomer of methyl (Z)-3-[(2R,3R,4S,5S)-5-(2-methoxy-2-oxoethyl)-3,4-(isopropylidenedioxy)-tetrahydrofuran-2-yl]prop-2-enoate and the corresponding α,α’-cis diastereomer, a precursor of methyl (1R,2R,6S,7R,8S,9S)-9-(2-methoxy-2-oxoethyl)-4,4-dimethyl-3,5,10-trioxatricyclo[5,2,1,02,6]decane-8-carboxylate.

SN2 Reaction of Diarylmethyl Anions at Secondary Alkyl and Cycloalkyl Carbons

The substitution reaction of the diethyl allylic and propargylic phosphates with Ar2CH anions was applied to sec‐alkyl phosphates to compare reactivity and stereoselectivity. However, the substitution took place on the ethyl carbon of the diethyl phosphate group. We then found that the diphenyl phosphate leaving group ((PhO)2PO2) was suited for the substitution at the sec‐alkyl carbon. Enantioenriched diphenyl sec‐alkyl phosphates with different substituents (Me, Et, iPr) on the vicinal position underwent the substitution reaction with almost complete inversion (>99% enantiospecificity). The substitution reactions of cyclohexyl phosphates possessing cis or trans substituents (Me and/or tBu) at the C4, C3, and C2 positions of the cyclohexane ring were also studied to observe the difference in reactivity among the cis and trans isomers. A transition‐state model with the phosphate leaving group ((PhO)2PO2) in the axial position was proposed to explain the difference. This model was supported by computational calculation of the virtual substitution reaction of the structurally simpler “dimethyl” cyclohexyl phosphates (leaving group = (MeO)2PO2) with MeLi. Furthermore, the calculation unexpectedly indicated higher propensity of (PhO)2PO2 as a leaving reactivity than alkyl phosphate groups such as (MeO)2PO2 and (iPrO)2PO2.

Platinum(IV) complexes of primary amines via oxidative addition

Oxidative addition allows to synthesize the first cycloplatinated primary amines in which the platinum adopts oxidation state +IV: Square planar organoplatinum(II) complexes can be converted with elementary diiodine and dibromine to the corresponding pseudo-octahedral Pt(IV) derivatives. The reaction may be monitored with the help of NMR spectroscopy and five of the target products have been characterized by single crystal X-ray diffraction. Two of these Pt(IV) complexes provide structural evidence for cis addition of the halogen and for the pronounced trans effect of the Pt–C bond. The latter is also reflected in a substitution reaction promoted by AgBF4: The halide substituent trans to the C in the coordination sphere can be selectively replaced by the pseudohalide cyanate. In the absence of a suitable silver salt, the pseudo-octahedral Pt(IV) complexes are inert in substitution reactions.

ChemInform Abstract: Diastereoselective Synthesis of α,α,α′‐Trisubstituted Pyrrolidines and Piperidines by Directed Sequential Lithiation/Alkylation.

The stereocontrolled synthesis of α,α,α′-trisubstituted pyrrolidines and piperidines has been accomplished through α′-lithiation/trapping of the corresponding α,α-disubstituted Boc-protected azaheterocycle with various electrophiles. The relative configuration of the major diastereomer has the α′-substituent trans to the α-aryl group in the pyrrolidines but cis to the α-aryl group in the piperidines. The diastereoselectivity of the lithiation/alkylation of pyrrolidines is unaffected by TMEDA but decreases in the presence of (−)-sparteine in diethyl ether.

Diastereoselective synthesis of α,α,α′-trisubstituted pyrrolidines and piperidines by directed sequential lithiation/alkylation

The stereocontrolled synthesis of α,α,α′-trisubstituted pyrrolidines and piperidines has been accomplished through α′-lithiation/trapping of the corresponding α,α-disubstituted Boc-protected azaheterocycle with various electrophiles. The relative configuration of the major diastereomer has the α′-substituent trans to the α-aryl group in the pyrrolidines but cis to the α-aryl group in the piperidines. The diastereoselectivity of the lithiation/alkylation of pyrrolidines is unaffected by TMEDA but decreases in the presence of (−)-sparteine in diethyl ether.

Experimental and Theoretical Studies on the Platinum-Mediated Selective C(sp)–Si Bond Cleavage of Alkynylsilanes

A series of cis-alkynyl(silyl)platinum(II) complexes was prepared via the chemoselective C(sp)–Si bond cleavage of alkynylsilanes by a platinum(0) complex ligated with the P–N hemilabile bidentate ligand. The coordination of the triple bond to the platinum center triggers selective C(sp)–Si bond cleavage. Hammett plots of the 31P{1H} NMR spectroscopic properties (δ and J values) reflect an electronic effect on platinum(II) complexes; trans substituents of arylethynyl groups are influenced, but cis-positioned silyl groups are not affected, as evidenced by 29Si{1H} NMR. In comparison, Hammett plots show that C(sp)–Si bond cleavage rates are accelerated by electron-rich alkynylsilanes, which is opposite to the ordinary oxidative addition of aryl halides to transition metals often observed in catalytic cross-coupling reactions. A DFT calculation reveals that intermediates and transition states are stabilized by electron-rich alkynylsilanes and that the five-membered hemilabile P–N ligand is essential, in which a reactive electron-deficient 14-electron platinum(0) species is produced via the dissociation of nitrogen, giving rise to a monodentate phosphine coordination. Electron-rich alkynylsilanes allow decreased π back-donation from the platinum center to the ligand, accelerating the dissociation of the more labile nitrogen. Steric congestion between diisopropylphosphino and silyl groups thermodynamically disfavors C(sp)–Si bond cleavage.

Hyperaromatic Stabilization of Arenium Ions: Acid-Catalyzed Dehydration of 2-Substituted 1,2-Dihydro-1-naphthols

Rate constants for acid-catalyzed dehydration of cis-2-substituted 1,2-dihydro-naphthols are well correlated by the Taft relationship log k = -0.49 - 8.8σ(I), with minor negative deviations for OH and OMe. By contrast the trans substituents show a poor correlation with σ(I) and in most cases react more slowly than their cis isomers. The behavior is consistent with rate-determining formation of a 2-substituted carbocation (naphthalenium ion) intermediate that for cis reactants possesses a 2-C-H bond suitably oriented for hyperconjugation with the charge center. For the trans isomers the 2-substituent itself is oriented for hyperconjugation in the initially formed conformation of the cation. It is argued that k(cis)/k(trans) rate ratios for substituents (Me, 8.4; Bu(t), 12.7; Ph, 3.8; NH(3)(+), 160; OH, 440) reflect their hyperconjugating ability relative to hydrogen. Faster reactions of trans isomers are observed for substitutents known (RS, N(3)) or suspected (EtSO, EtSO(2)) of stabilizing the cation by a π or σ neighboring group effect. The good Taft correlation is taken to indicate that cis substuents are reacting normally, differentiated only by their inductive effects. The slower reactions of the trans isomers are the judged to be "abnormal". This is confirmed by comparing effects of cis and trans β-OH substituents on the reactivities of dihydro phenols, naphthols, and phenanthrols. Whereas k(H)/k(OH) for cis substituents varies by less than 8-fold and is consistent with the influence of an inductive effect of the OH group (k(H)/k(OH) ≈ 2000), k(H)/k(OH) for the trans substituents varies by 3 orders of magnitude, reflecting the additional influence of the lesser hyperconjugating ability of a C-OH bond compared to a C-H bond. The magnitude and variation of this difference is consistent with C-H hyperconjugation conferring aromatic character on the arenium ions.

Competitive formation of cis and trans derivatives in the nucleophilic substitution reactions of cyclophosphazenes having a mono-spiro P–NHR group

Nucleophilic substitution reactions of N(3)P(3)Cl(4)[NH(CH(2))(3)NMe] (1) and N(3)P(3)Cl(4)[NH(CH(2))(3)O] (2) with mono-functional alcohols (methanol, 2,2,2-trifluoroethanol, phenol) and a secondary amine (pyrrolidine) were used to investigate the relationship between the incoming nucleophile and the proportions of products with substituents that are cis or trans to the spiro NH moiety. The reaction products were characterized by elemental analysis, mass spectrometry, (1)H and (31)P NMR spectroscopy and the configurational isomers by X-ray crystallography. Six products have been characterised with the substituent cis to the spiro NH group for the alcohol (methanol, phenol) and pyrrolidine derivatives of both compounds 1 and 2, compared to just one derivative with the substituent trans to the spiro NH group, that for the pyrrolidine derivative of compound 2. For each reaction the relative proportions of cis and trans isomers were determined by (31)P NMR measurements of the reaction mixtures. It was found that the reactions of compound 1 with all three alcohols and of compound 2 with methanol lead to exclusive formation of isomers with the substituent cis to the NH moiety, whereas all other reactions lead to mixtures of cis and trans isomers in different ratios under standard reaction conditions. However, when crown ether is included in the reaction medium for the reactions of compound 2 with both 2,2,2-trifluoroethanol and phenol, it is found that only cis isomers are formed. All these results are rationalised in terms of the competition between at least two effects; the cis-directing effect by hydrogen bonding of the incoming nucleophile to the spiro N-H group already present on the cyclophophazene ring and the cis-directing effect of the sodium cation coordinating to the oxygen lone pairs of the P-O moiety of the spiro ring.

Flexibility of N-Heterocyclic Carbene Ligands in Ruthenium Complexes Relevant to Olefin Metathesis and Their Impact in the First Coordination Sphere of the Metal

We present a detailed static and dynamics characterization of 11 N-heterocyclic carbene (NHC) ligands in Ru complexes of the general formula (NHC)Cl(2)Ru horizontal lineCH(2). Analysis of the dynamic trajectories indicates that the nature of the N substituent can result in extremely different flexibilities of the Ru complexes. In almost all the cases the N substituent trans to the Ru-ylidene bond is severely folded so that it protects the vacant coordination position at the Ru center. Limited flexibility is instead associated with the N substituent on the side of the Ru-ylidene bond. NHCs with a single ortho substituent, either a simple Me or a bulkier i-Pr group, have a preferential folding that bends the unsubstituted side of the ring toward the halide-Ru-halide plane. Analysis of the dynamics trajectories in terms of buried volume indicates that the real bulkiness of these systems can be somewhat modulated, and this flexibility is a key feature that allows NHCs to modulate their encumbrance around the metal in order to make room for bulky substrates. Analysis of the buried volume in terms of steric maps showed that NHCs with mesityl or 2,6-diisopropylphenyl N substituents have quite different reactive pockets: rather flat with constant pressure on the halide-Ru-halide plane in the former and vault-shaped with higher pressure on the sides in the latter. Regarding the NHCs with an ortho tolyl or i-Pr group on the N substituent, the steric maps quantify the higher impact of the unsubstituted side of the ligand in the first coordination sphere of the metal and evidence the overall C(s)- and C(2)-symmetric reactive pockets of the corresponding complexes. We believe that a detailed characterization of the differently shaped reactive pockets is a further conceptual tool that can be used to rationalize the experimentally different performances of catalysts bearing these ligands or to devise new applications.

Nature of the Bonding in Metal-Silane σ-Complexes

The nature of metal silane sigma-bond interaction has been investigated in several key systems by a range of experimental and computational techniques. The structure of [Cp'Mn(CO)(2)(eta(2)-HSiHPh(2))] 1 has been determined by single crystal neutron diffraction, and the geometry at the Si atom is shown to approximate a trigonal bipyramid; salient bond distances and angles are Mn-H(1) 1.575(14), Si-H(1) 1.806(14), Si-H(2) 1.501(13) A, and H(1)-Si-H(2) 148.5(8) degrees. This complex is similar to [Cp'Mn(CO)(2)(eta(2)-HSiFPh(2))] 2, whose structure and bonding characteristics have recently been determined by charge density studies based on high-resolution X-ray and neutron diffraction data. The geometry at the Si atom in these sigma-bond complexes is compared with that in other systems containing hypercoordinate silicon. The Mn-H distances for 1 and 2 in solution have been estimated using NMR T(1) relaxation measurements, giving a value of 1.56(3) A in each case, in excellent agreement with the distances deduced from neutron diffraction. Density functional theory calculations have been employed to explore the bonding in the Mn-H-Si unit in 1 and 2 and in the related system [Cp'Mn(CO)(2)(eta(2)-HSiCl(3))] 3. These studies support the idea that the oxidative addition of a silane ligand to a transition metal center may be described as an asymmetric process in which the Mn-H bond is formed at an early stage, while both the establishment of the Mn-Si bond and also the activation of the eta(2)-coordinated Si-H moiety are controlled by the extent of Mn --> sigma*(X-Si-H) back-donation, which increases with increasing electron-withdrawing character of the X substituent trans to the metal-coordinated Si-H bond. This delocalized molecular orbital (MO) approach is complemented and supported by combined experimental and theoretical charge density studies: the source function S(r,Omega), which provides a measure of the relative importance of each atom's contribution to the density at a specific reference point r, clearly shows that all three atoms of the Mn(eta(2)-SiH) moiety contribute to a very similar extent to the density at the Mn-Si bond critical point, in pleasing agreement with the MO model. Hence, we advance a consistent and unifying concept which accounts for the degree of Si-H activation in these silane sigma-bond complexes.

C–H activation of diphenylphosphinoaryl-derivatives with dimethyltetrakis(trimethylphosphine)iron(II)

Fe(CH 3) 2(PMe 3) 4 reacts with 1-(diphenylphosphino)naphthalene or benzyldiphenylphosphine within 4 h at 20 °C to give the novel metallated methyl iron complexes Fe(CH 3){P(C 6H 5) 2(C 10H 6)}(PMe 3) 3 ( 1) and Fe(CH 3){(C 6H 4)CH 2P(C 6H 5) 2}(PMe 3) 3 ( 3), respectively, via selective activation of the C–H bond of the pre-chelating ligands. The complexes are thermally unstable releasing metal through a reductive elimination of the aromatic backbone and leading to a C,C-coupling product that is regiospecifically methylated, namely 8-methyl(diphenylphosphino)naphthalene ( 2). Carbonylation (1 bar, 20 °C, 1 h) of complex 1 effects monosubstitution of a trimethylphosphine ligand trans to the metallated 8-C atom to afford Fe(CH 3){P(C 6H 5) 2(C 10H 6)}(CO)(PMe 3) 2 ( 4). The remaining methyl group in the parent complex 1 reacts with trimethylsilylethyne and tert-butylethyne affording the new complexes 5 and 6 bearing an alkynyl substituent trans to the diphenylphosphino anchoring group. The complexes 1 and 3– 6 are diamagnetic and possess octahedral coordination geometry. All novel complexes were fully characterized by spectroscopic methods and by X-ray diffraction.

Hydroheteroarylation of Alkynes under Mild Nickel Catalysis.

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Hydroheteroarylation of Alkynes under Mild Nickel Catalysis

Nickel complexes having a bulky tri(sec-alkyl)phosphine ligand catalyze hydroheteroarylation of alkynes at 35 degrees C. Selective activation of an Ar-H bond over an Ar-CN bond of N-protected 3-cyanoindoles is achieved by a proper choice of ligand and/or an N-protecting group. The catalysis is applicable to a diverse range of heteroarenes to afford cis-hydroheteroarylation products in highly chemo- and stereoselective manners. Excellent regioselectivity is observed with unsymmetrical alkynes to give the corresponding heteroaryl-substituted ethenes having a larger substituent trans to an aryl group.

Synthesis and G-quadruplex binding studies of new 4-N-methylpyridinium porphyrins

A series of cationic porphyrins carrying 1-3 meso-N-pyridinium groups has been synthesised, and their binding to G-quadruplex DNA has been explored by surface plasmon resonance (SPR) and circular dichroism spectroscopy. Two trans substituents appear to be sufficient for tight binding; preferential binding to the anti-parallel intramolecular human telomeric DNA was observed for the A2trans and A3 porphyrins. The A2trans is able to induce the formation of an anti-parallel G-quadruplex in a K+ free solution, mimicking the effect of a molecular chaperone.

The molecular binding interactions of inhibitors and activators of phosphoenolpyruvate carboxylase

We have performed molecular modelling studies of the binding to maize phosphoenolpyruvate carboxylase (PEPC) of phosphoenolpyruvate (PEP) and a number of representative competitive inhibitors. We predict that all these compounds share a common binding mode and that the differences in inhibitory activity of the various inhibitors arise mainly from either increased hydrophobic interactions of cis substituents or small but significant changes in their binding mode arising from steric clashes of trans substituents with the active site. We have also performed molecular modelling studies of glucose-6-phosphate (G6P) and a number of other allosteric activators in their putative allosteric binding site in this enzyme. We predict that these molecules share the same binding mode for their phosphate moiety while some of them engage in a variety of hydrogen bonding interactions with residues from different subunits of the enzyme, and others establish hydrophobic and van der Waals interactions with other regions of the allosteric binding site.

Sub-picosecond time-resolved absorption spectroscopy of a push–pull type p, p′-substituted trans-azobenzene

Relaxation dynamics of an azobenzene derivative with push–pull type substituents trans-(4-methoxyphenylazo)-4 ′-nitrobenzene ( trans-MNAB) upon the S 2 ← S 0 excitation in acetonitrile was studied by sub-picosecond time-resolved transient absorption spectroscopy which exhibited a fast formation of the lowest excited singlet (S 1) state within 1 ps and a vibrational cooling of the ground state (S 0). All the events of the S 1 state completed within <5.0–6.0 ps, suggesting that isomerization of trans-MNAB proceeds mainly via the S 1 state. The absence of remarkable substituent effect on the dynamics of the excited states strongly suggested the absence of the rotational cis– trans isomerization mechanism via the S 2 state.

Spirodienone route for the stereoselective methylene functionalization of p-tert-butylcalix[4]arene.

A new method for the regio- and stereoselective functionalization of two distal methylene groups of p-tert-butylcalixarene (1a) is described. Reaction of the meso bis(spirodienone) calixarene derivative 2a with bromine afforded the tetrabrominated product 3a derived from exo 1,4-additions of bromine to the diene subunits. A phase-transfer-catalyzed reaction of 3a with aqueous NaOH/CH2Cl2 yielded the exo bis(epoxide) calixarene derivative 4. Heating 3a in a vacuum eliminated two molecules of HBr and afforded a product (5b) that retained the C(i) symmetry of the starting material. X-ray analysis indicated that the calixarene derivative 5b possesses two exocyclic double bonds of E configuration. Calixarene 5b undergoes reaction with nucleophiles at the exocyclic double bonds, with concomitant bond shifts and expulsion of the bromine atoms. Selective trans monodeuteration of two methylene groups of 1 was achieved by reaction of 5b with NaBD4 followed by aromatization of the labeled spirodienol derivative. Reaction of 5b with RONa/ROH (R = Me, Et) afforded the methylene-substituted bis(spirodienone) derivatives 9a and 9b possessing two trans alkoxy groups. X-ray crystallography of 9b indicated that the two trans substituents are located at pseudoequatorial positions of the methylene groups. LiAlH4 reduction of the substituted bis(spirodienone) derivatives afforded calix[4]arenes incorporating trans alkoxy groups at two distal methylene positions.

Enantioselective Dicarbonylation of Styrene to Isotactic Poly[1‐oxo‐2‐phenylpropane‐1,3‐diyl] with phosphinodihydrooxazole‐palladium(ii) complexes: Model studies for enantioface selection preliminary communication

AbstractThe first steps, believed to be involved in the highly enantioselective copolymerization of styrene and carbon monoxide to poly[1‐oxo‐2‐phenylpropane‐1,3‐diyl] with phosphinodihydrooxazole‐palladium(II) complexes, were investigated. The insertion of carbon monoxide into [Pd(Me)(PˆN)(solvent)] TfO (PˆN = (S)‐2‐[2‐(5H‐benzo[b]phosphindol‐5‐yl)phenyl]4‐benzyl‐4,5‐dihydrooxazole (1)) and of styrene into [Pd(Me)(PˆN)(solvent)] TfO were highly regioselective (alkyl and acyl substituents trans to N); moreover, the olefin insertion took place with essentially complete enantioface discrimination.