Reduction In THF Research Articles

Electrosynthesis of π-Extended Porphyrins Via Reductive Decyanation

The development of new electrochemically driven synthetic routes to obtain π-extended porphyrins, or new synthetic porphyrins in general, is highly desirable since electrosynthesis is often able to afford a clean conversion of reactants to products without a great deal of contamination. At the same time, electrosynthesis, when combined with classical electrochemical measurements, can often provide unique insights into the mechanism and intermediates involved in formation of the desired product. This is explored in the current work where a novel electrosynthetic method for generation of π-extended porphyrins and the corresponding reductive decyanation mechanism is described. The heterogenous anion induced electrosynthetic method is also compared to a homogenous chemical synthesis method to obtain the same products, using anion-induced electron transfer (AIET). The starting porphyrins are β,meso-fused malononitrile derivatives are represented as MTPP(MN)2 and the π-extended, di-fused products as MTPP(VCN)2, where TPP = the dianion of tetraphenylporphyrin, MN = malononitrile, VCN = vinyl cyanide and M = H2, NiII, CuII and ZnII. Structures of these porphyrins are shown in Chart 1. The easily synthesized π-extended porphyrins with two fused cyanobenzene (benzonitrile) rings are characterized by three facile and reversible one-electron ring-centered reductions in THF containing 0.1 M TBAP. There is also a fourth irreversible reduction at the edge of the solvent. Figure 1

Facile Heterogeneous and Homogeneous Anion Induced Electrosynthesis: An Efficient Method for Obtaining π-Extended Porphyrins.

Two closely related electrosynthetic approaches are applied for the preparation of novel π-extended tetraphenylporphyrins from malononitrile-appended meso-β di-fused porphyrins, represented as MTPP(MN)2, where TPP = the dianion of tetraphenylporphyrin and MN = malononitrile. The first method involves application of a controlled reducing potential at a platinum electrode in CH2Cl2, while the second proceeds via cyanide anion induced electron transfer. Both methods produced the same decyanated, π-extended di-fused porphyrins represented as MTPP(VCN)2 where VCN = vinyl cyanide and M = H2, NiII, CuII, or ZnII in almost quantitative yields. The final isolated and purified porphyrin products are characterized by a split Soret band ranging from 411-497 nm and two broad intense Q bands. The new π-extended porphyrins are easier to reduce than the parent MTPP or MTPP(MN)2 compounds by 760-800 mV and 180-190 mV, respectively, and possess an electrochemical HOMO-LUMO gap ranging from 1.48 to 1.66 V. They are also characterized by two reversible one-electron ring-centered reductions in CH2Cl2 and three reversible one-electron ring-centered reductions in THF. A fourth irreversible reduction is seen in THF at more negative potentials and is assigned to one or two of the fused cyanobenzene rings of the macrocycle.

A Series of Diamagnetic Pyridine Monoimine Rhenium Complexes with Different Degrees of Metal-to-Ligand Charge Transfer: Correlating (13) C NMR Chemical Shifts with Bond Lengths in Redox-Active Ligands.

A set of pyridine monoimine (PMI) rhenium(I) tricarbonyl chlorido complexes with substituents of different steric and electronic properties was synthesized and fully characterized. Spectroscopic (NMR and IR) and single-crystal X-ray diffraction analyses of these complexes showed that the redox-active PMI ligands are neutral and that the overall electronic structure is little affected by the choices of the substituent at the ligand backbone. One- and two-electron reduction products were prepared from selected starting compounds and could also be characterized by multiple spectroscopic methods and X-ray diffraction. The final product of a one-electron reduction in THF is a diamagnetic metal-metal-bonded dimer after loss of the chlorido ligand. Bond lengths in and NMR chemical shifts of the PMI ligand backbone indicate partial electron transfer to the ligand. Two-electron reduction in THF also leads to the loss of the chlorido ligand and a pentacoordinate complex is obtained. The comparison with reported bond lengths and (13) C NMR chemical shifts of doubly reduced free pyridine monoaldimine ligands indicates that both redox equivalents in the doubly reduced rhenium complex investigated here are located in the PMI ligand. With diamagnetic complexes varying over three formal reduction stages at the PMI ligand we were, for the first time, able to establish correlations of the (13) C NMR chemical shifts with the relevant bond lengths in redox-active ligands over a full redox series.

Spectroelectrochemical study of complexes [Mo(CO)2(η3-allyl)(α-diimine)(NCS)] (α-diimine = bis(2,6-dimethylphenyl)-acenaphthenequinonediimine and 2,2′-bipyridine) exhibiting different molecular structure and redox reactivity

The redox properties and reactivity of [Mo(CO)2(η3-allyl)(α-diimine)(NCS)] (α-diimine = bis(2,6-dimethylphenyl)-acenaphthenequinonediimine (2,6-xylyl-BIAN) and 2,2′-bipyridine (bpy)) were studied using cyclic voltammetry and IR/UV–Vis spectroelectrochemistry. [Mo(CO)2(η3-allyl)(2,6-xylyl-BIAN)(NCS)] was shown by X-ray crystallography to have an asymmetric (B-type) conformation. The extended aromatic system of the strong π-acceptor 2,6-xylyl-BIAN ligand stabilises the primary 1e−-reduced radical anion, [Mo(CO)2(η3-allyl)(2,6-xylyl-BIAN•−)(NCS)]−, that can be reduced further to give the solvento anion [Mo(CO)2(η3-allyl)(2,6-xylyl-BIAN)(THF)]−. The initial reduction of [Mo(CO)2(η3-allyl)(bpy)(NCS)] in THF at ambient temperature results in the formation of [Mo(CO)2(η3-allyl)(bpy)]2 by reaction of the remaining parent complex with [Mo(CO)2(η3-allyl)(bpy)]− produced by dissociation of NCS− from [Mo(CO)2(η3-allyl)(bpy•−)(NCS)]−. Further reduction of the dimer [Mo(CO)2(η3-allyl)(bpy)]2 restores [Mo(CO)2(η3-allyl)(bpy)]−. In PrCN at 183 K, [Mo(CO)2(η3-allyl)(2,6-xylyl-BIAN•−)(NCS)]− converts slowly to 2e−-reduced [Mo(CO)2(η3-allyl)(2,6-xylyl-BIAN)(PrCN)]− and free NCS−. At room temperature, the reduction path in PrCN involves mainly the dimer [Mo(CO)2(η3-allyl)(bpy)]2; however, the detailed course of the reduction within the spectroelectrochemical cell is complicated and involves a mixture of several unassigned products. Finally, it has been shown that the five-coordinate anion [Mo(CO)2(η3-allyl)(bpy)]− promotes in THF reduction of CO2 to CO and formate via the formation of the intermediate [Mo(CO)2(η3-allyl)(bpy)(O2CH)] and its subsequent reduction.

Electron Transfer Behavior of Pincer-Type {RhNO}8Complexes: Spectroscopic Characterization and Reactivity of Paramagnetic {RhNO}9Complexes

Fil: Pellegrino, Juan. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Oficina de Coordinacion Administrativa Ciudad Universitaria. Instituto de Quimica, Fisica de los Materiales, Medioambiente y Energia; Argentina. Universidad de Buenos Aires; Argentina

Reduction and Protonation of Mo(IV) Imido Complexes with depe Coligands: Generation and Reactivity of a S = 1/2 Mo(III) Alkylnitrene Intermediate

Reduction and protonation of Mo(IV) imido complexes with diphosphine coligands constitutes the second part of the Chatt cycle for biomimetic reduction of N2 to ammonia. In order to obtain insights into the corresponding elementary reactions we synthesized the Mo(IV) ethylimido complex [Mo(CH3CN)(NEt)(depe)2](OTf)2 (2-MeCN) from the Mo(IV)-NNH2 precursor [Mo(NNH2)(OTf)(depe)2](OTf) (1). As shown by UV-vis and NMR spectroscopy, exchange of the acetonitrile ligand with one of the counterions in THF results in formation of the so far unknown complex [Mo(OTf)(NEt)(depe)2](OTf) (2-OTf). 2-MeCN and 2-OTf are studied by spectroscopy and X-ray crystallography in conjunction with DFT calculations. Furthermore, both complexes are investigated by cyclic voltammetry and spectroelectrochemistry. The complex 2-OTf undergoes a two-electron reduction in THF associated with loss of the trans ligand triflate. In contrast, 2-MeCN in acetonitrile is reduced to an unprecedented Mo(III) alkylnitrene complex [Mo(NEt)(CH3CN)(depe)2]OTf (5) which abstracts a proton from the parent Mo(IV) compound 2-MeCN, forming the Mo(III) ethylamido complex 5H and a Mo(II) azavinylidene complex 6. Compound 5 is also protonated to the Mo(III) ethylamido complex 5H in the presence of externally added acid and further reduced to the Mo(II) ethylamido complex 7. The results of this study provide further support to a central reaction paradigm of the Schrock and Chatt cycles: double reductions (and double protonations) lead to high-energy intermediates, and therefore, every single reduction has to be followed by a single protonation (and vice versa). Only in this way the biomimetic conversion of dinitrogen to ammonia proceeds on a minimum-energy pathway.

A Surfactant-Free Strategy for Synthesizing and Processing Intermetallic Platinum-Based Nanoparticle Catalysts

Using Pt(3)Fe nanoparticles as an example, a surfactant-free Np-KCl matrix method (Np stands for nanoparticle) is developed for the synthesis of nanoparticles with controlled size and structure. In this method, the Np-KCl assembly is formed in a one-pot reduction in THF at room temperature. KCl is an insoluble byproduct of the reaction and serves as a matrix that traps the nanoparticles to avoid particle agglomeration and to control the coalescence of nanoparticles during thermal annealing up to 600 °C. By varying the molar ratio of metal precursors and KCl, as well as the time and temperature of annealing, the final particle sizes and crystalline order can be independently controlled. After thermal processing, nanoparticles were released from the KCl matrix and transferred in an ethylene glycol-water solution to support materials forming a uniform Np-support assembly. A detailed study of the synthesis of ordered intermetallic Pt(3)Fe nanoparticles with an average diameter of 4 nm, using this Np-KCl method, is provided as an example of a generally applicable method. This surfactant-free strategy has been extended to the synthesis of other bi- and trimetallic nanoparticles of Pt-transition metals.

1,7-Dinitroperylene bisimides: facile synthesis and characterization as n-type organic semiconductors

1,7-Dinitroperylene bisimides ( 1a– 1b) and 1-nitroperylene bisimides ( 2a– 2b) were synthesized under mild condition in high yields, and were characterized by FT-IR, 1H NMR, UV–vis, HRMS spectra, cyclic voltammetry, and thermogravimetric analyses. These compounds are stable up to 260 °C according to thermogravimetric analyses. They undergo two quasi-reversible one-electron reductions in THF at modest potentials. The nitro functionalities provide stability of n-type charge carriers by lowering the LUMO to resist ambient oxidation.

Synthesis of Benzo[b]siloles via KH-Promoted Cyclization of (2-Alkynylphenyl)silanes

(2-Alkynylphenyl)silanes undergo intramolecular cyclization in the presence of an excess or a subequimolar amount of potassium hydride to give a variety of new 2-substituted benzosiloles in good to excellent yields. Some of these compounds showed a high fluorescence quantum yield both in solution and in the solid state, and they also showed reversible reduction in THF.

A Stereoselective Access to Dihydroxylated Pyrrolidines by Reductive Ring Contraction of 1,2-Oxazines

Protected dihydroxylated 1,2-oxazine derivatives such as RAC- 4 were converted into tetrahydro-1,2-oxazines RAC- 7 by employing BH 3 ·THF for the stereoselective reduction of the C=N bond. Compound 7A underwent a reductive ring contraction on hydrogenation in the presence of palladium on charcoal to provide RAC- 5 in good yield. Enantiopure 1,2-oxazine derivatives 13 and 14 were prepared using (-)-menthol as chiral auxiliary. Their diastereo-selective dihydroxylation and BH 3 ·THF reduction furnished enantiopure tetrahydro-2 H-1,2-oxazine derivatives 17 and 18 in good overall yield. Enantiomers (6 R)- 18 and (6 S)- 18 were transformed into the two enantiomeric hydroxylated pyrrolidine derivatives 5 in moderate yield.

Electrochemical investigations of platinum phenylethynyl complexes

We report the electrochemical characterization of a family of five platinum phenylethynyl complexes by cyclic voltammetry, bulk electrolysis, and spectroelectrochemistry. All have irreversible oxidations near +1.2 V vs. Ag∣AgCl. None has any reductive electrochemistry within the potential window of 50/50 benzene + acetonitrile. trans-Bis(tri- n-butylphosphine)bis(phenylethynyl)platinum(II) undergoes reduction in THF at −2.786 V vs. Ag∣AgCl. The number of electrons passed during the oxidation ranges from 1.15 to 2.29. Photophysical measurements show that as the phenylethynyl chain length increases, the absorbance wavelength increases and the emission wavelength shows a weak but similar trend. Spectroelectrochemistry illustrates the loss of the absorbance band over successive oxidative sweeps. Results indicate that the oxidation takes place at the Pt(II) center and is followed by chemical reactions that generate a new species.

Solvent-dependent diastereoselectivities in reductions of beta-hydroxyketones by SmI2.

The reductions of a series of beta-hydroxyketones by SmI(2) were examined in THF, DME, and CH(3)CN using methanol as a proton source. Reductions in THF and DME typically lead to the syn diastereomer with DME providing higher diastereoselectivities. Reductions in CH(3)CN provided the anti diastereomer predominantly. This study reveals that solvation plays an important role in substrate reduction by SmI(2). [reaction: see text]

3(1,3-Heterazolidin-3-yl-methyl)-1,3-oxazolidines and their reduction with borane–THF

Preparation of bis-heterazolidines bonded by a CH 2, CH 2–S–CH 2 or CH 2SCH 2SCH 2 groups through their nitrogen atoms is reported: 3-(1,3-oxazolidin-3-ylmethyl)-1,3-oxazolidine 1, 3-(4,4-dimethyl-1,3-oxazolidin-3-ylmethyl)-1,3-oxazolidine 2, 3-(1,3-diazolidin-3-ylmethyl)-1,3-diazolidine 3, 3-(1,3-thiazolidin-3-ylmethyl)-1,3-thiazolidine 4, 3-(1,3-thiazolidin-3-ylmethylsulfanylmethyl)-1,3-thiazolidine 5 and 3-(1,3-oxazolidin-3-ylmethylsulfanylmethyl-sulfanylmethyl)-1,3-oxazolidine 6. The solid state structures of 4 and 5 were determined by X-ray diffraction analyses. BH 3–THF reduction reactions of compounds 1– 6 were investigated. N→BH 3 mono- and di-adducts of 1– 6 were prepared and their structures calculated (ab initio 3-21G*).

Instantaneous SmI2/H2O/amine-mediated reductions in THF.

The SmI(2)-mediated reductions of ketones, imines, and alpha,beta-unsaturated esters have been shown to be instantaneous in the presence of H(2)O and an amine in THF. The SmI(2)-mediated reductions are not only shown to be fast and quantitative by the addition of H(2)O and an amine, but the workup procedures are also simplified. Competing experiments with SmI(2)/H(2)O/amine confirmed that alpha,beta-unsaturated esters could be selectively reduced in the presence of ketones or imines. Comparison of analogue ligands showed that nitrogen and phosphorus ligands are superior to oxygen and sulfur ligands in these reductions. The trialkylphosphine 1,2-bis(dimethylphosphino)ethane (DMPE) provided a primary kinetic isotope effect, yielding a k(H)/k(D) of 4.5.

Redox and magnetic switching in 1,3,5-acceptor-substituted benzenes: reversible formation of radical anions, dianions and trianions in doublet, triplet, and quartet spin states

1,3,5-Tris(dicyanovinyl)-substituted benzene 1 can accept one, two, and three electrons stepwise, as shown by (spectro)electrochemical methods. When the corresponding redox stages are attained by K-metal reduction in THF and 2-methyltetrahydrofuran, paramagnetic resonance and optical techniques can identify equilibria between adjacent redox states and different (para)magnetic stages. It can be shown that the dianion can adopt a triplet state whereas the trianion is present in a doublet and quartet spin multiplicity. Similar findings are established for the methoxy-substituted derivative 2. The formation of the different paramagnetic stages is closely connected to the association of the anions with alkali-metal countercations.

Disodium Tetrasupersilyltetragallanediide Na2Ga4R*4·2THF (R* = SitBu3) − Preparation of a Novel Gallium Cluster Compound via Dichlorodisupersilyldigallane R*2Ga2Cl2

Red tetragallanediide Na2Ga4R*4·2THF was prepared by reduction of the monogallane R*GaCl2·THF in heptane or the tetrahedro-tetragallane R*4Ga4 in benzene/tetrahydrofuran (THF) with sodium at 100 °C. Potassium worked analogously as a reduction agent. As an intermediate of the dehalogenation of R*GaCl2·THF, the orange digallane R*2Ga2Cl2 and the dark violet tetrahedrane R*4Ga4 were formed. The extreme air- and moisture-sensitive Na2Ga4R*4·2THF was stable against Me3SiCl, but reacted with oxygen under formation of R*4Ga4, and with GaBr under formation of several products (isolation and X-ray structure analysis of R*GaBr2·THF). According to X-ray structure analyses, (i) the tetragallanediide contains a nonplanar Ga4 ring with short Ga−Ga bonds, whereby three Ga atoms on opposite sides of the ring are each connected with one of the two Na(THF) groups, (ii) the digallane is dimeric, whereby it takes the structure of As4S4 (exchange of As/S against Ga/Cl). The possible ways of R*GaCl2·THF reduction are discussed, an explanation of the structure of Na2Ga4R*4·2THF is given, and halides RnEnHal2 (E = Al−Tl) are presented.

Synthesis of N,N-bis[2-(2-pyridyl)ethyl]amino steroids and related compounds intended as chiral ligands for copper ions

Compounds with the N,N-bis[2-(2-pyridyl)ethyl]amino structure (RPY2) are useful tridentate ligands for copper(I) ions, which can bind and activate oxygen from the atmosphere. For diastereoselective and enantioselective oxidation reactions, hitherto unknown chiral ligands possessing tripodal structures have been synthesized starting from homochiral steroids. The double Michael addition of primary steroidal amines and aminoalcohols to 2-vinyl pyridine was not very succesful. However, homochiral bidentate ligands with N-[2-(2-pyridyl)ethyl]amino steroid structure could be obtained by this procedure in most cases. New routes (acylation of the bidentate ligands with 2-pyridylacetic acid followed by BH3·THF reduction, or reductive amination of steroidal ketones, acylation and borane reduction) to the desired tridentate RPY2, also at sterically hindered positions, are described. In the last reaction sequence, ‘mixed’ tridentate ligands can also be obtained. Copper complexation and oxygen activation with these ligands are briefly discussed.

Reactions of Odd-Electron Cobaltacycles: Characterization of a Persistent 17-Electron Anionic Intermediate in Electron-Transfer-Catalyzed (ETC) Substitution Reactions

The cobaltafluorene complex Cp(PPh3)CoC12H8 (1) undergoes an electrochemically irreversible one-electron reduction in THF (Ep,c = ca. −2.56 V vs ferrocene) with release of PPh3 to give a persistent 17-electron Co(II) monoanion, 2-, which was characterized by electrochemistry and by ESR spectroscopy. The unpaired spin density in 2- is highly delocalized, with the majority being located in the cyclopentadienyl ring rather than the cobaltacyclic fragment. Reoxidation of 2- in the presence of L (L = phosphines, phosphites) forms Cp(L)CoC12H8 in high yield. Slow electron-transfer-catalyzed (ETC) substitution processes are found when 1 is electrolyzed in the presence of P(OMe)3, with the efficiency of the catalysis being dependent on both the relative and absolute concentrations of 1 and P(OMe)3. The substitution reaction is accounted for by a model in which the anion 2- is in equilibrium with the 19-electron adducts [Cp(L)CoC12H8]-, where L = THF, PPh3, P(OMe)3. Further reduction of 2- is possible in a reversible one-electron reduction (E1/2 = −2.80 V) to an 18-electron dianion that is stable in solution. The dianion 22- was characterized by 1H NMR spectroscopy and shown to have Cs or higher symmetry.

Organometallic Chemistry sans Organometallic Reagents: Modulated Electron‐Transfer Reactions of Subvalent Early Transition Metal Salts

AbstractThe potential of low‐valent, early transition‐metal reagents as selective reductants in organic chemistry has been foreshadowed by intensive research on the ill‐defined and heterogeneous subvalent titanium intermediates generated in the McMurry reaction and its numerous variants. As part of the long‐term research effort to develop soluble, well‐defined transition‐metal reductants of modulated and selective activity toward organic substrates, the THF‐soluble reductant, titanium dichloride, has been throughly examined, as well as the analogous ZrCl2 and HfCl2 reagents, all of which are readily obtainable by the alkylative reduction of the Group tetrachloride by butyllithium in THF. Noteworthy is that such interactions of MCl4 with butyllithium in hydrocarbon media lead, in contrast, to M(III) or M(IV) halide hydrides. Analogous alkylative reductions in THF applied to VCl4, CrCl3, and MoCl5 have yielded reducing agents similar to those obtained from Mcl4 but gradated in their reactivity. Such reductants have proved capable of coupling carbonyl derivatives, benzylic halides, acetylenes and certain olefins in a manner consistent with an oxidative addition involving a two‐electron transfer (TET). Such a reaction pathway is consistent with the observed stereochemistry of pinacol formation from ketones and for the reductive dimerization of alkynes. In contrast of the reaction of CrCl3 with two equivalents of butyllithium, which leads to a CrCl intermediate, the interaction of CrCl3 in THF with four equivalents of butyllithium at –78°C yields a reagent of the empirical formulation, LiCrH4.2LiCl.2 THF, as supported by elemental and gasometric analysis of its protolysis. This hydridic reductant cleaves a wide gamut of s̀ carbon–heteroatom bonds (C–X, C–O, C–S and C–N), towards which the CrCl reductant is unreactive. The type of cleavage and/or coupled products resulting from the action of “LiCrH4” on these substrates in best understood as arising from single‐electron transfer (SET). In light of the aforementioned findings, the gradated reducing action noted among TiCl2, HfCl2 and CrCl, as well as the contrasting reducing behavior between CrCl and LiCrH4, there is no doubt that future research with early transition metals will continue to yield novel reductants of modulated and site‐selective reactivity.

Alkali Metal Induced Rupture of a Phosphorus−Phosphorus Double Bond. Electrochemical and EPR Investigations of New Sterically Protected Diphosphenes and Radical Anions [ArPPAr]•-

The synthesis and characterization of the new diphosphenes DmtPPDmt (2a) (Dmt = 2,6-dimesityl-p-tolyl) and DxpPPDxp (3a) (Dxp = 2,6-di(m-xylyl)phenyl) are described. The electrochemical behavior of 2a, 3a, and DmpPPDmp (1a) (Dmp = 2,6-dimesitylphenyl) has been examined by cyclic voltammetry. Compounds 1a−3a display reversible reductions in THF (room temperature, 0.1 M nBu4NBF4) at −2.08, −2.15, and −2.08 V vs SCE, respectively. Chemical and electrochemical reduction of solutions of 1a−3a led to the stable radical anions [DmpPPDmp]•- (1b), [DmtPPDmt]•- (2b), and [DxpPPDxp]•- (3b) which were studied by EPR spectroscopy. The EPR data indicate the unpaired spins reside in PP π* molecular orbitals. Chemical reduction of 1a−3a with either Na metal or sodium naphthalenide yields sodium salts Na[ArPPAr], which show additional EPR signals that have been attributed to the presence of ion-paired species. No ion pairing was detected by EPR spectroscopy for the corresponding magnesium, potassium, or lithium salts of 1...