Stereospecific S-assisted cationic olefin cyclization a synthetic application to thiasteroid

A new annelation method involving cationic olefin cyclization reaction $\text{via}$ neighboring sulfur participation, which proceeds in an entirely stereospecific manner, is demonstrated on a thiasteroid synthesis.     

Correction: HCOOH disproportionation to MeOH promoted by molybdenum PNP complexes

Correction for ‘HCOOH disproportionation to MeOH promoted by molybdenum PNP complexes’ by Elisabetta Alberico et al., Chem. Sci., 2021, 12, 13101–13119, DOI: 10.1039/D1SC04181A.

The authors regret that in Scheme 2 of the original article, complexes 7 and 8 were drawn incorrectly. The solid-state structure of both complexes, as established by X-ray analysis, had been previously reported (7 (ref. 1) and 8 (ref. 2)). In both complexes, the PNP ligand adopts a facial tridentate coordination to molybdenum and not a meridional one, as erroneously shown in Scheme 2 of the original article. The correct ligand arrangements in the metal coordination sphere for complexes 7 and 8 are reported below in Scheme 1.Open in a separate windowScheme 1Mo–PNP complexes tested in the dehydrogenation of HCOOH.Open in a separate windowScheme 2Proposed mechanisms for HCOOH dehydrogenation (red), disproportionation (blue) and decarbonylation (green) promoted by 5. Evidence for the formation of a Mo(iv) species is based on the detection by NMR of H₂ and HD following addition of DCOOD to Mo(H)_n species (see Fig. SI-31).Please note that complex 8 is also shown in Scheme 4 in the proposed mechanism for HCOOH decarbonylation (green part), and in Fig. 2. In both cases, the correct structure for complex 8 is reported below in Scheme 2 and Fig. 1.Open in a separate windowFig. 1 ¹H and ³¹P{¹H} NMR spectra of a toluene-d₈ solution of {Mo(CH₃CN)(CO)₂(HN[(CH₂CH₂P)(CH(CH₃)₂)₂]₂} 4 in the presence of 100 equivalents of HCOOH ([Mo] 10⁻² M, [HCOOH] 1 M), before (a) and after heating at 90 °C for 1 hour (b). Spectra were recorded at room temperature. Signals related to complex 5 are marked by red dots.Open in a separate windowFig. 2Molecular structure of {Mo(CO)₂(CH₃CN)[CH₃N(CH₂CH₂P(CH(CH₃)₂)₂)₂]} 9. Displacement ellipsoids correspond to 30% probability. Hydrogen atoms are omitted for clarity.Furthermore, a mistake was made in the caption of Fig. 6, showing the solid-state structure of complex 9: the latter has been incorrectly described as a Mo(i)-hydride species {Mo(H)(CO)₂(CH₃CN)[CH₃N(CH₂CH₂P(CH(CH₃)₂)₂)₂]}. The correct formula, in agreement with the X-ray structure, is as follows and is shown above in Fig. 2: {Mo(CO)₂(CH₃CN)[CH₃N(CH₂CH₂P(CH(CH₃)₂)₂)₂]}.The Royal Society of Chemistry apologises for these errors and any consequent inconvenience to authors and readers.     

Zirconium complexes containing a diarylamido diphosphine ligand: Structural preferences controlled by ligand electronics and sterics

This work describes the preparation of [PNP]ZrX₃ ([PNP]⁻ = [N(o-C₆H₄PⁱPr₂)₂]⁻; X = Cl, Me, CH₂SiMe₃) whose structural preference is found to be a function of the electronic and steric nature of the monodentate ligand X⁻. The reaction of ZrCl₄(THF)₂ with [PNP]Li in toluene at room temperature generates [PNP]ZrCl₃ as a red solid in 60% yield. Alkylation of [PNP]ZrCl₃ with three equivalents of Grignard reagents in diethyl ether at −35 °C produces cleanly [PNP]ZrR₃ (R = Me, CH₂SiMe₃) as yellow crystalline materials. An X-ray diffraction study of [PNP]ZrCl₃ showed it to be a chloride-bridged binuclear species {[PNP]ZrCl₂(μ−Cl)}₂ in which both zirconium atoms are 7-coordinate whereas that of [PNP]ZrMe₃ revealed a mononuclear, 6-coordinate core structure. Interestingly, with the incorporation of more sterically demanding alkyls, [PNP]Zr(CH₂SiMe₃)₃ is a 5-coordinate compound wherein the amido phosphine ligand is κ²-N,P bound to zirconium. The solution structures of these molecules were also assessed by variable-temperature NMR spectroscopy.     

Reductive PET cycloreversion of oxetanes: singlet multiplicity, regioselectivity, and detection of olefin radical anion

Cycloreversion of 2-(p-cyanophenyl)-4-methyl-3-phenyloxetane (1) is achieved using 1-methoxynaphthalene (2) as electron-transfer photosensitizer. The experimental results are consistent with the reaction taking place from the singlet excited state of the sensitizer. Ring splitting of the radical anion 1*- occurs with cleavage of O-C2 and C3-C4 bonds, leading to products (acetaldehyde and p-cyanostilbene) different from the reagents used in the Paterno-Büchi synthesis of 1. The olefin radical anion involved in the electron-transfer process has been detected by means of laser flash photolysis.     

Ring-closing olefin metathesis of 2,2'-divinylbiphenyls: a novel and general approach to phenanthrenes

[reaction: see text] The ring-closing olefin metathesis (RCM) of 2,2'-divinylbiphenyls, using a second-generation RCM ruthenium-based catalyst, leads to differently substituted phenanthrenes in quantitative yield under very mild reaction conditions, independent of both nature and position of the groups present on the biphenyl moiety.     

Preparation and characterization of exfoliated polystyrene/clay nanocomposites using a cationic radical initiator-MMT hybrid

The exfoliated polystyrene (PS)/clay nanocomposites were prepared via in situ polymerization using a cationic radical initiator-intercalated montmorillonite hybrid. The exfoliated structure resulted from the predominant intra-gallery polymerization over the extra-gallery polymerization owing to the anchored radical initiator inside the clay galleries. Several critical properties of the nanocomposites such as the initial thermal degradation temperature, glass transition temperature, storage modulus, Young’s modulus, and tensile strength were estimated and compared with earlier reported literature values. The improvements in such properties were either comparable or much greater than the reported literature values, mainly due to the efficient exfoliation and dispersion of the clay in the PS matrix.     

Synthesis and incorporation of a cationic pyridinium nucleoside mimic in duplex DNA: effect on long-distance radical cation transport

[Structure: see text] A convenient method for the synthesis and solid-phase incorporation of a cationic nucleoside mimic and its role in DNA long-range charge transfer are described.     

Correction: Highly selective acid-catalyzed olefin isomerization of limonene to terpinolene by kinetic suppression of overreactions in a confined space of porous metal–macrocycle frameworks

Correction for ‘Highly selective acid-catalyzed olefin isomerization of limonene to terpinolene by kinetic suppression of overreactions in a confined space of porous metal–macrocycle frameworks’ by Wei He et al., Chem. Sci., 2022, 13, 8752–8758, https://doi.org/10.1039/d2sc01561g.

The authors regret that there were errors in Fig. 2, Fig. 5 and Fig. 6 in the original article and Fig. S18 of the ESI. The stereochemistry of the chemical structural formulas for (−)-α-pinene (6) and (−)-β-pinene (7) was incorrectly reversed. The correct versions of the figures are shown below, and in the updated version of the ESI.Open in a separate windowFig. 2Metal–macrocycle framework (MMF). (a) Self-assembly of asymmetrically twisted Pd^II-macrocycles into (b) a porous crystal MMF (sticks model) with five enantiomeric pairs of binding pockets (surface model). (c) Previously reported site-selective adsorption of (−)-α-pinene (6) (space-filling model) on the channel surface of the MMF.¹ Blue, yellow, red, or black dashed circles indicate the ceiling-, side-, bottom-, or tubular-pockets of the MMF, respectively. MMF: Pd, yellow; Cl, green; N, blue; C, grey. 6: C, pink; H, white. Hydrogen atoms attached to the MMF were omitted for clarity. Green or blue surface represents exposed Cl or N–H groups of the MMF, respectively.Open in a separate windowFig. 5Investigation of the inhibitory effects of additives on the isomerization reaction of 2 using 2-NBSA@MMF at 25 °C for 102 h.Open in a separate windowFig. 6Crystallographic study of MMFs soaked in (a) a CHCl₃ solution containing 1 (1.0 M), (b) a CHCl₃ solution containing 2 (1.0 M), and (c) a CH₃CN solution containing 7 (1.0 M). MMF: stick model or surface model; 1 and 7: space-filling model; water and CHCl₃: stick model. Red dashed circles indicate the bottom pocket of the MMF. MMF: Pd, yellow; Cl, green; N, blue; C, grey. 1: C, yellow; H, white. 7: C, pink; H, white. Water and CHCl₃: O, red; H, white; C, grey; Cl, green. Hydrogen atoms attached to the MMF were omitted for clarity. Green and blue surface represents exposed Cl and N–H groups of the MMF, respectively.The Royal Society of Chemistry apologises for these errors and any consequent inconvenience to authors and readers.     

Synthesis and characterization of aryl substituted bis(2-pyridyl)amines and their copper olefin complexes: investigation of remote steric control over olefin binding

The aryl-functionalized pyridylamine 2-(i)PrC(6)H(4)N(H)py (1) and bis(2-pyridyl)amines of the type ArN(py)(2) for Ar = Mes (2), 2,6-Et(2)C(6)H(3) (3), 2-(i)PrC(6)H(4) (4), 2,6-(i)Pr(2)C(6)H(3) (5), and 1-naph (6), have been prepared by the palladium-catalyzed cross-coupling of substituted anilines with 2-bromopyridine, and have been characterized by (1)H and (13)C NMR NMR, FTIR, MS, and TGA. Complexes of these new N-aryl bis(2-pyridyl)amines have been prepared for the acid salts [H{ArN(py)(2)}]BF(4) where Ar = Mes (7) and 2-(i)PrC(6)H(4) (8), and the dimeric bridged complexes [Cu{ArN(py)(2)}(μ-X)(Y)](2) where X/Y = Cl(-) and Ar = Ph (9), 2-(i)PrC(6)H(4) (10), and 1-naph (11), in addition to X = OH(-), Y = H(2)O and Ar = Mes (12). The olefin complexes [Cu(Ar-dpa)(styrene)]BF(4) for Ar = Ph (13), Mes (14), 2-(i)PrC(6)H(4) (15), and 1-naph (16), in addition to the norborylene complexes of Ar = Mes (17) and 2-(i)PrC(6)H(4) (18) have been prepared and characterized by (1)H and (13)C NMR, FTIR, and TGA. The crystal structures have been determined for compounds 1-17. Secondary amine 1 crystallizes in hydrogen-bonded head-to-tail dimers, while the N-aryl bis(2-pyridyl)amines 2-6 crystallize in a three-bladed propellar conformation, having nearly planar geometries about the amine nitrogen. The geometry about copper centers in the dimeric complexes 9-12 is distorted trigonal bypyramidal, with the axial positions occupied by one of the two pyridyl nitrogens and one of the bridging ligands (i.e., Cl or OH). The copper atoms in each of the olefin complexes 13-17 are coordinated to the two pyridine nitrogen atoms and the appropriate olefin; consistent with a pseudo three-coordinate Cu(I) cation. Distortion of pyridyl ring geometries about the copper centers, and concomitant bending of the aryl groups away from the CuN(amine) vectors were found to correlate with the steric bulk of the aryl group present in both dimeric and olefin complexes. Such distortion is also observed to a lesser extent in the acid salts as well. The (1)H and (13)C NMR spectra of [Cu(Ar-dpa)(olefin)]BF(4) exhibit an upfield shift in the olefin signal as compared to free olefin. A good correlation exists between the (1)H and (13)C NMR Δδ values and olefin dissociation temperatures, confirming that the shift of the olefin NMR resonances upon coordination is associated with the binding strength of the complex.     

Aromaticity of benzene in the facial coordination mode: a structural and theoretical study

The effects of facial coordination of benzene to a trinuclear transition-metal cluster have been studied by structure correlation and DFT calculational methods. Data taken from the X-ray crystal structures of twelve complexes [(eta-C(5)H(4)R")Co(3)(micro(3)-eta(2):eta(2):eta(2)-C(6)H(4)RR')] 1 b-1 m were analyzed by using standard statistical methods. The prototypal facial arene ligand is considerably expanded with respect to free benzene and shows a small but highly significant Kekulè distortion (d(CC)=1.42, 1.45 A). DFT MO calculations were carried out on the model complexes [(eta-C(5)H(5))M(3)(micro-eta(2):eta(2):eta(2)-C(6)H(6))] 1 a (M=Co), 2 (M=Rh), and 3 (M=Ir). Ring currents in the facial benzene and apical cyclopentadienyl ligands have been assessed by nucleus independent chemical shift (NICS) calculations. Compared to the free ligand (with the optimized D(6h) structure as well as with D(3h) and a C(3v) geometries similar to that in the prototypal facial arene), facial benzene exhibits somewhat reduced but still substantial cyclic electron delocalization (CED). The calculated order of CED is benzene approximately [(CO)(3)Cr(eta-C(6)H(6))] 4 > 1 > 2 > 3.     

New chemistry of olefin complexes of platinum(II) unravelled by basic conditions: synthesis and properties of elusive cationic species

The evolution in basic medium ([RO-] = 1 M in methanol, R = H or Me) of five-coordinate platinum(II) compounds, [PtCl2(eta2-C2H4)(N-N)], 2a-c, (N-N = N,N,N',N'-tetramethyl-1,2-ethanediamine, a; 2,2'-bipyridyl, b; 1,10-phenanthroline, c) leads to the formation of [PtCl(eta1-CH2CH2-OCH3)(N-N)], 5a-c. The analogous compound 5d (N-N = 2,9-dimethyl-1,10-phenanthroline, d) can also be prepared, but not via transformation of the five-coordinate species 2d in basic medium where it is quite stable. 5d can instead be prepared by reaction of d with a strongly basic methanol solution of Zeise's anion [PtCl3(eta2-C2H4)](-), 1. In such a medium the di-anionic trans-[PtCl2(OR)(eta1-CH2CH2-OCH3)](2-) species (1") reacts with to form exclusively 5d. Hydrolysis of with acids bearing weakly coordinating anions leads to [PtCl(eta2-C2H4)(N-N)]+, 3a-c, as stable cations; upon the same treatment 5d does not generate 3d, but it reacts with HCl to give 2d in almost quantitative yield. Cationic complexes 3b, 3c, here reported for the first time, were reacted with some nucleophiles and their behaviour compared with that of the already known 3a. In 3b, 3c the metal centre competes with the coordinated ethene for binding to nucleophiles; therefore the acetylacetonate anion can either add to the olefin (affording compounds 6b, 6c ) or to the metal ion replacing the ethene ligand (yielding compounds 7b, 7c). Under similar conditions, 3a gives exclusively 6a. Secondary amines readily add to ethene in 3b, 3c, affording the addition products 8b, 8c, which undergo a ready cyclization to an azaplatinacyclobutane ring (9b, 9c). The remarkable ease of the four-membered ring formation has been related to the high electrophilic character of the metal core in 3b, 3c.     

Sequential photodecomposition of bisacylgermane type photoinitiator: Synthesis of block copolymers by combination of free radical promoted cationic and free radical polymerization mechanisms

A block copolymer of cyclohexene oxide (CHO) and styrene (St) was prepared by using bifunctional visible light photoinitiator dibenzoyldiethylgermane (DBDEG) via a two‐step procedure. The bifunctionality of the photoinitiator pertains to the sequential photodecomposition of DBDEG through acyl germane bonds. In the first step, photoinitiated free radical promoted cationic polymerization of CHO using DBDEG in the presence of diphenyliodonium hexafluorophosphate (Ph₂I⁺PF) was carried out to yield polymers with photoactive monobenzoyl germane end groups. These poly(cyclohexene oxide) (PCHO) prepolymers were used to induce photoinitiated free radical polymerization of styrene (St) resulting in the formation of poly(cyclohexene oxide‐block‐styrene) (P(CHO‐b‐St)). Successful blocking has been confirmed by a strong change in the molecular weight of the prepolymer and the block copolymer as well as NMR, IR, and DSC spectral measurements. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 4793–4799, 2009     

Regiochemical control of monolignol radical coupling: a new paradigm for lignin and lignan biosynthesis

BACKGROUND: Although the lignins and lignans, both monolignol-derived coupling products, account for nearly 30% of the organic carbon circulating in the biosphere, the biosynthetic mechanism of their formation has been poorly understood. The prevailing view has been that lignins and lignans are produced by random free-radical polymerization and coupling, respectively. This view is challenged, mechanistically, by the recent discovery of dirigent proteins that precisely determine both the regiochemical and stereoselective outcome of monolignol radical coupling. RESULTS: To understand further the regulation and control of monolignol coupling, leading to both lignan and lignin formation, we sought to clone the first genes encoding dirigent proteins from several species. The encoding genes, described here, have no sequence homology with any other protein of known function. When expressed in a heterologous system, the recombinant protein was able to confer strict regiochemical and stereochemical control on monolignol free-radical coupling. The expression in plants of dirigent proteins and proposed dirigent protein arrays in developing xylem and in other lignified tissues indicates roles for these proteins in both lignan formation and lignification. CONCLUSIONS: The first understanding of regiochemical and stereochemical control of monolignol coupling in lignan biosynthesis has been established via the participation of a new class of dirigent proteins. Immunological studies have also implicated the involvement of potential corresponding arrays of dirigent protein sites in controlling lignin biopolymer assembly.     

ECE and disproportionation: Part VI. General resolution. Application to potential step chronoamperometry

The competition between homogeneous and heterogeneous electron transfers in ECE-DISP mechanisms: A+eB BC C+eD B+C→A+D is analyzed in the context of potential step chronoamperometry. Starting from the expressions of the apparent number of electrons for the limiting situation, ECE, DISP1, DISP2, the transition between two of these “pure” mechanisms is systematically investigated. The general system is then analyzed for large values of the kinetic parameters, i.e., in the particular case of a stationary state arising from mutual compensation of diffusion and chemical reaction (“pure kinetic” conditions). This provides an estimation of the range of parameters where numerical analysis of the general case is actually necessary together with tests of accuracy of the numerical computation procedures. In this framework the analysis of the most general case involving no particular assumption about the magnitude of the thermodynamic and kinetic parameters leads to a three-dimensional kinetic zone diagram which allows to predict the effects of the intrinsic (equilibrium and rate constants) and operational (time, concentration) parameters on the displacement of the system from one limiting situation to the other. On these bases, the practical effectiveness of single-step techniques in discriminating between the limiting mechanisms is discussed. It is shown that the discrimination between DISP2 and either ECE and DISP1 can be easily carried out in terms of either concentration or time dependence of the current response. The discrimination between ECE and DISP1 is much more difficult and requires a high experimental accuracy to be effective.     

Studies related to carba-pyranoses: a radical decarboxylation approach to monocarba-disaccharides

(1--> 1), (1--> 3) and (1--> 4) acetal-linked monocarba-disaccharides have been synthesised from a series of glucosylated gamma- and delta-lactonic acids prepared from common intermediate, obtained from the Diels-Alder reaction of maleic anhydride and (E)-1-(2',3',4',6'-tetra-O-acetyl-beta-D-glucopyranosyloxy)-3-(trimethylsiloxy)buta-1,3-diene 1. Thiohydroxamic ester 14, prepared from gamma-lactonic acid 9, gave, upon treatment with tert-butyl thiol and light, the lactone 15. Subsequent lithium aluminium hydride reduction and acetylation gave the (1--> 3) acetal-linked monocarbadisaccharides 1,6-di-O-acetyl-3-O-(2',3',4',6'-tetra-O-acetyl-beta-D-glucopyranosyl)-2,4-dideoxy-5a-carba-beta-L-threo-hexopyranose 16. In a similar manner, protected monocarba-disaccharides 13, 19, 30, and 35 possessing L-ido, L-xylo, D-arabino and L-ido configurations of the carba-pyranose ring have been prepared. Treatment of thiohydroxamic esters 14 and 17 with either tert-butyl thiol or trityl thiol, dimethyl sulfide, oxygen and light gave alcohols 20 and 22. Subsequent lithium aluminium hydride reduction and aceytlation gave the monocarbadisaccharides 1,4,6-tri-O-acetyl-3-O-[2',3',4',6'-tetra-O-acetyl-beta-D-glucopyranosyl]-2-deoxy-5a-carba-beta-L-arabino-hexopyranose 21 and 1,2,4,6-tetra-O-acetyl-3-O-(2',3',4',6'-tetra-O-acetyl-beta-D-glucopyranosyl)-5a-carba-beta-L-glucopyranose 23 respectively.     

Binding of cationic and neutral phenanthridine intercalators to a DNA oligomer is controlled by dispersion energy: quantum chemical calculations and molecular mechanics simulations

Correlated ab initio as well as semiempirical quantum chemical calculations and molecular dynamics simulations were used to study the intercalation of cationic ethidium, cationic 5-ethyl-6-phenylphenanthridinium and uncharged 3,8-diamino-6-phenylphenanthridine to DNA. The stabilization energy of the cationic intercalators is considerably larger than that of the uncharged one. The dominant energy contribution with all intercalators is represented by dispersion energy. In the case of the cationic intercalators, the electrostatic and charge-transfer terms are also important. The DeltaG of ethidium intercalation to DNA was estimated at -4.5 kcal mol(-1) and this value agrees well with the experimental result. Of six contributions to the final free energy, the interaction energy value is crucial. The intercalation process is governed by the non-covalent stacking (including charge-transfer) interaction while the hydrogen bonding between the ethidium amino groups and the DNA backbone is less important. This is confirmed by the evaluation of the interaction energy as well as by the calculation of the free energy change. The intercalation affects the macroscopic properties of DNA in terms of its flexibility. This explains the easier entry of another intercalator molecule in the vicinity of an existing intercalation site.     

Electrochemical radical-polar crossover:a radical approach to polar chemistry

Radical-polar crossover(RPC) reaction bridges the gap between one-and two-electron reactivities,thus providing an ideal solution to overcome the limitations of both radical and polar chemistry.In this manifold,organic electrochemistry provides a uniquely facile strategy to access a diverse array of radical intermediates,thus broadening the chemical space of the RPC concept.This review highlights the synthetic advances in the field of electrochemical RPC reactions since 2020,with an emphasis on t...     

Nitroxyl radical addition to pentafulvenones forming cyclopentadienyl radicals: a test for cyclopentadienyl radical destabilization.

Photochemical Wolff rearrangements in alkane solvents of the 6-diazo-2,4-cyclohexadienones 4 and 13-15 give pentafulvenone (1), 2,3-benzopentafulvenone (2), dibenzopentafulvenone (3), and 2,4-di-tert-butylpentafulvenone (16), as identified by conventional UV and IR spectroscopy. Reactions of these fulvenyl ketenes with tetramethylpiperidinyloxyl (TEMPO) proceed by addition of TEMPO to the carbonyl carbon forming delocalized radicals for 1 and 2 which add one or more further TEMPO molecules, while the initial radical products formed from 3 and 16 dimerize. The rate constants of these reactions compared to hydration rate constants for the same compounds show the benzannulated derivatives 2 and 3 fit a previous correlation k(2)(TEMPO) vs k((H(2)O), whereas for 1 and 16 there is evidence for inhibition of reactions with radicals. The deviations are consistent with an absence of aromatic stabilization of the cyclopentadienyl radicals from 1 and 16 that is compensated in the benzannulated derivatives.