Effects of heteroatoms on 1,2-rearrangements of 3-membered ring carbenes

1,2-rearrangements of carbenes: CCH₂X(X = CH₂, NH and O) are studied by using ab initio gradient method. Heteroatoms N and O stabilize the carbene and decrease its reactivity, mainly by changing frontier molecular orbitals, but retain the way of the reaction. The reaction starts from the attack of the migrating hydrogen on the carbene p AO and ends with the entrance of the hydrogen into the carbene σ orbital. Reactivities are in the order of X = CH8>NH>0. The reaction is exothermic or endothermic according to whether the product is a 4n+2 or 4n π electron molecule.     

Chemiluminescence from arylcarbene oxidation: Phenylchlorocarbene and (2-chlorophenyl)carbene

Chemiluminescence is observed in the thermal reaction of phenylchlorocarbene or (2-chlorophenyl)carbene and O₂, matrix-isolated in Ar. The chemiluminescence spectra closely match the phosphorescence of the corresponding carbonyl compounds. The reactivity of both carbenes towards O₂ is very different. Singlet carbene phenylchlorocarbene reacts thermally only slowly with O₂ up to 60 K. The oxidation products phenylchloroformate, benzoyl chloride and O(³P) are mainly formed photochemically on irradiation of the diazirine precursor. Triplet carbene (2-chlorophenyl)carbene reacts readily with O₂ at cryogenic temperatures to give mostly 2-chlorobenzaldehyde-O-oxide. The carbonyl-O-oxide is photochemically easily cleaved to give 2-chlorobenzaldehyde and O(³P). The reaction step leading to carbonyl compounds in their excited states is in both carbene oxidations the recombination of the free carbene and O(³P).     

Formation of chiral ionic liquids and imidazol-2-ylidene metal complexes from the proteinogenic aminoacid l-histidine

Treatment of the amino acid derivative Bz-His-OMe with excess n-propyl bromide gave the corresponding histidinium salt [Bz-His(n-propyl)₂-OMe⁺Br⁻]. It features a melting point of 39 °C and may serve as a useful readily available optically active ionic liquid. Its subsequent treatment with silver oxide gave the corresponding l-histidine derived chiral N-heterocyclic carbene complex [“(carbene)₂Ag · AgBr₂”]. Transmetallation by treatment with Pd(CH₃CN)₂Cl₂ or [Rh(cod)Cl]₂ led to the formation of the respective chiral late metal imidazol-2-ylidene complexes [“(carbene)₂PdCl₂”] and [“(carbene)RhCl(cod)”], respectively. Four diastereomers of the square planar palladium system were observed. Due to the additional chirality center in the l-histidine-derived “Arduengo-carbene ligand” two diastereomers of the rhodium carbene complex were formed.     

Axial and equatorial carbene ligands of dimanganese and dirhenium monocarbene complexes: Synthesis, characterisation and structural studies

The binuclear alkoxycarbene complexes [M₂(CO)₉{C(OEt)C₄H₃Y}] (M = Mn, Y = S(1), O(2); Re, Y = S(3), O(4)) were synthesised and characterised, giving axial carbene ligands for the dimanganese complexes, and equatorial carbene ligands for the dirhenium complexes. Aminolysis of these complexes with ammonia and n-propylamine yielded complexes [M₂(CO)₉{C(NHR)C₄H₃Y}] (R = H, M = Mn, Y = S(5), O(6); Re, Y = S(7), O(8); R = propyl, M = Mn, Y = S(9), O(10); Re, Y = S(11), O(12)). For the smaller NH₂-substituted carbene ligands, the X-ray structures determined showed equatorial carbene ligands for both dimanganese and dirhenium complexes, while the NHPr-substituted carbene complexes retained the original configurations of the precursor alkoxy carbene complex, indicating that the steric effects of both the M(CO)₅-fragment and the carbene ligand substituent can affect the coordination site of the carbene ligands of Group VII transition metal complexes in the solid state.     

Strukturuntersuchungen an mono- und bis-alkoxy(triphenylsilyl)-carben-komplexen des rheniums

Steric and electronic influences on bond lengths and angles at the carbene carbon atoms of cis-Re₂(CO)₉C(OR)SiPh₃ (I: R = CH₃, II, R = C₂H₅) and cis,trans-Re₂(CO)₈[C(OEt)SiPh₃]₂ (III) are discussed based on their structural analyses. I (ReRe 305.2(1) pm; ReC(carbene) 209(2) pm) and II (two independent molecules; ReRe 305.0(3) and 305.2(4) pm; ReC (carbene) 208(5) and 210(5) pm) differ by the cis and trans positions of the alkyl groups at the partial C(carbene)O double bonds. The change in configuration affects the bond angles at the carbene carbon. In III the carbene ligands are bonded to different rhenium atoms; cis to one Re atom and trans to the other Re atom (ReRe bond 309.1(2) pm). The ReC(carbene length of the trans- carbene ligand is significantly shorter (185(3) pm) than that of the cis-carbene ligand (208(3) pm).     

Rapid Interception of CnF2n+1(O)SO. Radical with Copper-Based Carbene: A Novel Access to Perfluoroalkanesulfinate Ester

In this communication, an unprecedented interception of C_nF_2n+1(O)SO^. radical with a copper-based carbene has been established. Distinguished by wide substrate scopes and mild reaction conditions, this novel radical–carbene coupling reaction (RCC reaction) provides a fundamentally different and mechanistically interesting strategy for the synthesis of perfluoroalkanesulfinate esters.     

Heteronuclear (WFe) and homonuclear (FeFE) μ-alkylidene complexes of transition metals from mononuclear terminal carbene complexes. crystal structures of {WFe[μ-η1, η3-C(OCH2CH3)(CHCHCH3](CO)8} and [fe2{μ-η2,η3-C[OCH2CH3[CHC(OCH2CH3)(CH3]}(CO)6

The reactions of mononuclear carbene complexes of W and Fe of the type CO)_mMC(OR)(CH_2nCHCR′″ (M  = FE, W; m = 4 and 5; n = 0, 2, 3; R′, R″ = C, CH₃, OEt) with Fe(CO)₅ have been studied. In all cases the reaction leads to new hetero (WFe) or homo (FeFe) μ-alkylidene complexes, the position of the double bond depending strongly on n.     

Formal carbene insertion into CO double bond: A facile approach to the synthesis of 2H-chromenes

A formal carbene insertion into CO double bonds of 2-hydroxycinnamaldehydes catalyzed by Rh₂(esp)₂ with Na₂CO₃ as additive has been disclosed by using aryldiazoacetates as carbene precursors. 2H-chromenes with a quaternary carbon at C-2 position were readily obtained from simple starting materials in good yields with excellent diastereoselectivity under mild reaction conditions in one pot. The reaction mechanism was investigated and proposed as the formation of epoxide intermediate from Rh(II)-associated carbene and (E)-2-hydroxycinnamaldehyde and followed by a cascade epoxide ring-opening procedure in which the (E)-intermediate was converted into the (Z)-intermediate to give (Z)-products.     

Structural studies of carbene complexes: The crystal structure of the secondary carbene complex RuI2[CHN(CH3)(p-CH3C6H4)](CO)(CN-p-CH3C6H4)(P(C6H5)3)

Crystal and molecular structures of the title compound have been determined from a three-dimensional X-ray analysis using diffractometer data. The crystals are triclinic, space group P$\text{1}$, with Z = 2 in a unit cell of dimensions a = 11.640(1), b = 10.9139(8), c = 16.587(2) Å, α = 87.983(5), β = 99.670(6), λ = 62.250(5)°. Full matrix least squares refinement has given a final R-factor of 0.043 for 2726 reflections for which I > 2σ(I).The crystal structure consists of discrete molecules of neutral complex together with water molecules which are hydrogen bonded into pairs [O ? O separation 2.60 Å]. 0The (H₂0)₂ units do not hydrogen bond to any other atoms. The ruthenium coordination is octahedral with trans carbene and isocyanide, cis iodides, and cis phosphine and carbonyl ligands. The Ru-donor distances are 2.776(2) [I trans to -PPh₃], 2.782(1) [I trans to -CO], 2.342(4) [PPh₃], 1.855(15) [CO], 2.046(15) [C(carbene)], and 1.998(16) Å [C(isocyanide)]. The bond lengths are discussed in terms of the trans effects of the ligands. The C(carbene)-N distance is 1.26(2) Å and the Ru—C(carbene)—N angle is 141.5(5)°.     

Bidentate Donor-Functionalized N-Heterocyclic Carbenes: Valuable Ligands for Ruthenium-Catalyzed Transfer Hydrogenation

Ruthenium complexes are by far the most studied compounds that catalyze hydrogen transfer reactions. In this review, we describe the use in this field of ruthenium complexes bearing bidentate donor-functionalized N-heterocyclic carbene ligands. The review specifically covers the application in transfer hydrogenations of (κ²-C_NHC,Y)-ruthenacyclic compounds where the Y donor atom is a N, P, O, or S atom, and where the N-heterocyclic carbene ligand is a classical imidazol-2-ylidene, a benzimidazol-2-ylidene, a mesoionic 1,2,3-triazolylidene, or an imidazol-4-ylidene ligand. Tridentate donor-functionalized N-heterocyclic carbene complexes thus fall outside the scope of the review. Applications in (asymmetric) transfer hydrogenation of ketones, aldehydes, imines, alkenes, and nitrobenzene are discussed.     

Density functional investigations on the (H2O)n·CCH and (H2O)n·HCC complexes (n=1–3)

The electronic structures and energies of (H₂O)_n·CCH and (H₂O)_n·HCC complexes (n=1–3) between CCH and water have been theoretically investigated at the UB3LYP/6-311++G(2df,p)//UB3LYP/6-311G(d,p) level. The complexes with n=2–3 are cyclic structures with homodromic hydrogen-bond chain. The (H₂O)_n·CCH (n=1–3) complexes show increasing stabilities towards CCH- or H₂O-eliminations of 2.3, 5.8 and 7.6 kcal/mol and are energetically more stable than the corresponding (H₂O)_n·HCC complexes by 0.8, 2.7 and 3.4 kcal/mol, respectively, due to the charge-separation-enhanced hydrogen bonds within (H₂O)_n·CCH (n=2,3). Strong interactions between CCH and (H₂O)₂ and (H₂O)₃ clusters suggest special solvent effects of water on the chemical behavior of unsaturated radicals.     

A novel isotope analysis of oxygen in uranium oxides: comparison of secondary ion mass spectrometry,glow discharge mass spectrometry and thermal ionization mass spectrometry

The natural variation of the oxygen isotopic composition is used among geologists to determine paleotemperatures and the origin of minerals. In recent studies, oxygen isotopic composition has been recognized as a possible tool for identification of the origin of seized uranium oxides in nuclear forensic science. In the last 10 years, great effort has been made to develop new direct and accurate n(¹⁸O)/n(¹⁶O) measurements methods. Traditionally, n(¹⁸O)/n(¹⁶O) analyses are performed by gas mass spectrometry. In this work, a novel oxygen isotope analysis by thermal ionization mass spectrometry (TIMS), using metal oxide ion species (UO⁺), is compared to the direct methods: glow discharge mass spectrometry (GDMS) and secondary ion mass spectrometry (SIMS). Because of the possible application of the n(¹⁸O)/n(¹⁶O) ratio in nuclear forensics science, the samples were solid, pure UO₂ or U₃O₈ particles. The precision achieved using TIMS analysis was 0.04%, which is similar or even better than the one obtained using the SIMS technique (0.05%), and clearly better if compared to that of GDMS (0.5%). The samples used by TIMS are micrograms in size. The suitability of TIMS as a n(¹⁸O)/n(¹⁶O) measurement method is verified by SIMS measurements. In addition, TIMS results have been confirmed by characterizing the n(¹⁸O)/n(¹⁶O) ratio of UO₂ sample also by the traditional method of static vacuum mass spectrometry at the University of Chicago.     

Synthesen im system {carbonylmetallat/ketenimin/säure}: XXX. Neue ferra- und rhenaazetidine

The metal carbonyl anions [Fe(η-C₅H₅(CO)₂]^? and [Re(CO)₅] undergo regio- and site-specific [2 + 2]-cycloadditions with the ketenimines Ph₂CCNR (R = Me, Ph) to give the (isolable) anionic complexes [L_n$\text{M{C(CPh}{}_2\text{)N(R)C}$(O)}]^? (L_nM = Fe(η-C₅H₅)CO, Re(CO)₄) which have been alkylated and acylated at the exocyclic oxygen atom of the carbonyl function. The result is stable neutral complexes having a metallaazetidine structure which is composed of an α-metallated enamine and an N,O carbene part. IR, ¹H, and ¹³C NMR data are presented.     

Novel scorpionate-type triscarbene ligands and their silver and gold complexes

New silver(I) carbene complexes were obtained starting from the N-heterocyclic carbene ligand precursors {[HB(RImH)₃]Br₂} (R = Bn, Mes and t-Bu) and {[HC(MeBImH)₃](BF₄)₃}, by treatment of the imidazolium salt with Ag₂O. Use of the tris-imidazolylborate precursors resulted in stable, well-characterized trimetallic complexes of general formula {Ag₃[HB(RIm)₃]₂}Br, which were successfully employed as carbene transfer reagents in the synthesis of related gold(I) complexes by transmetallation. The silver complexes also proved to be active catalysts of the coupling of aryl iodides with terminal alkynes (the Sonogashira reaction), although related bimetallic silver complexes were found to exhibit enhanced reactivity.     

A direct and practical approach for the synthesis of Au(I)-NHC complexes from commercially available imidazolium salts and Au(III) salts

A direct and practical approach for the synthesis of Au(I)-NHC complexes from imidazolium salts and commercially available aurate salt (MAuCl₄·2H₂O) is described. The reaction proceeded without sacrificing carbene transfer agent (Ag₂O) or using highly sensitive free NHC.     

Synthesis,crystal structures and spectroscopic studies of praseodymium(III) malonate complexes

Reactions of malonic acid (H₂mal) with PrCl₃·6H₂O afforded the known complex [Pr₂(mal)₃(H₂O)₆]_n (1), and compounds [Pr₂(mal)₃(H₂O)₆]_n·2nH₂O (2·2nH₂O), [PrCl(mal)(H₂O)₃]_n·0.5nH₂O (3·0.5nH₂O) and [Pr(mal)(Hmal)(H₂O)₃]_n·nH₂O (4·nH₂O) using various reaction ratios, reaction media (H₂O, MeOH) and pH values. Analogous reactions with CeCl₃·7H₂O afforded compounds [Ce₂(mal)₃(H₂O)₆]_n (5), [CeCl(mal)(H₂O)₃]_n·nH₂O (6·nH₂O) and [Ce(mal)(Hmal)(H₂O)₃]_n·nH₂O (7·nH₂O). Compounds 2·2nH₂O and 3·0.5nH₂O were characterized by X-ray crystallography, and 4–7 by microanalytical and spectroscopic data. The malonate(-2) ligand adopts three different coordination modes in the structures of 1–3, i.e., the μ₂-κO:κO′:κO″ and the μ₄-κ²O:κO′:κ²O″:κO? in 1 and 2 leading to a 3D network structure, and the μ₃-κ²O:κO′:κ²O″:κO? in 3 promoting an 1D structure. The thermal decomposition of 1 and 3·0.5nH₂O was monitored by TG/DTA and TG/DTG measurements. The structural features of 1–3 are discussed in terms of known malonato(-2) Ln^III and Ca^II complexes. The bioinorganic chemistry relevance of our results is discussed.     

Reaktionen von carben-komplexen mit tetramethylcyanoguanidin

Tetramethylcyanoguanidine, NCNC(NMe₂)₂, reacts with pentacarbonyl-[methoxy(phenyl)carbene]chromium, (CO)₅Cr[C(Ph)OMe], via substitution of the carbene ligand to give pentacarbonyl(tetramethylcyanoguanidine)chromium. X-ray structure analysis shows that the CrNCN fragment is nearly linear in the crystal, and that the CNC angle is 122.9(1)°. In solution rapid syn-anti isomerization at the NC double bond occurs. The reaction of tetramethylcyanoguanidine with pentacarbonyl[acetoxy(phenyl)carbene]-chromium and -tungsten, (CO)₅M-[C(Ph)OCO(O)Me] (M = Cr, W), yields (CO)₅M[C(PHNC(NMe₂)₂], where the acetoxy group of the carbene ligand is exchanged for the NC(NMe₂)₂ group of the cyanoguanidine.     

(Aminocarbene)(Divinyltetramethyldisiloxane)Iron(0) Compounds: A Class of Low‐Coordinate Iron(0) Reagents

The combined use of aminocarbene and divinyltetramethyldisiloxane (dvtms) as supporting ligands enables the access of unprecedented low‐coordinate iron(0) alkene compounds [L_nFe(η²:η²‐dvtms)] (L=N‐heterocyclic carbene (NHC) or cyclic (alkyl)(amino)carbene (CAAC), n=1 or 2) from the reactions of FeCl₂ with alkali‐metal reducing agents, free aminocarbene ligands, and dvtms. The iron(0) species deliver their {L_nFe⁰} fragments to perform redox reactions with Ph₂SiH₂, S₈, Se, and DippN₃, furnishing novel aminocarbene‐supported iron(IV) silylene, all‐ferrous iron–sulfur/selenium cubanes, and bis(imido)iron(IV) compounds. These conversions demonstrate the potential synthetic utility of the carbene‐supported iron(0) complexes as a valuable class of low‐coordinate iron(0) reagents.     

Alkoxycarbene complexes of nickel(II)

Alkynylnickel complexes trans-C₆Cl₅Ni(PPhMe₂)₂CCR (IIIa, R  H; IIIb, R  Me; IIIc, R  Et; IIId, R  CH₂OH; IIIe, R  CH₂CH₂OH; IIIf, R  Ph; IIIg, R  C₆H₄OMe-p) have been prepared from trans-[C₆Cl₅Ni-(PPhMe₂)₂L]ClO₄ and monosubstituted acetylenes in the presence of triethylamine, and their reactions with alcohols in the presence of perchloric acid were studied. Complexes IIIa and IIIe afforded alkoxycarbene complexes trans-[C₆Cl₅Ni-(PPhMe₂)₂{C(OR′)Me}]ClO₄ (IVa, R′  Me; IVb, R′  Et; IVc, R′  n-Pr) or trans-C₆Cl₅Ni(PPhMe₂)₂{$\text{C(CH}{}_2\text{)}{}_3\text{O}$}]ClO₄(IVd), respectively, but IIIb either decomposed or afforded trans-C₆Cl₅Ni(PPhMe₂)₂CHC(OMe)Me, depending on the amount of acid used. Treatment of IVaIVd with amines resulted in deprotonation to give α-alkoxyvinyl complexes, trans-C₆Cl₅Ni(PPhMe₂)₂C(OR′)CH₂ (VIaVIc) or trans-C₆Cl₅Ni(PPhMe₂)₂$\text{CCHCH}{}_2\text{CH}{}_2\text{O}$ (VId), the reaction being reversible. A ¹H NMR study indicated: (i) that the carbene methyl and the vinyl protons IV or VI are D-exchangeable by MeOD without catalyst; (ii) that the basicity of VIa is comparable to those of amines; (iii) that the carbene complexes IVaIVc have two isomers due to hindered rotation about the C(carbene)O bond in solution, IVb existing in the Z-form in the solid state; (iv) that the rotationalbarriers (°G^≠) about the C(carbene)O bond in IVb and the NiC-(carbene) bond in IVd are 20 (or more) and 11.7 kcal/mol, respectively. These results are explained in term of double bond character of the carbene carbon and its surrounding atoms.     

Facile synthesis of [1]benzothieno[3,2-b]benzothiophene from o-dihalostilbenes

A convenient one-pot synthesis of [1]benzothieno[3,2-b]benzothiophene from readily available o-dihalostilbenes using combined reagents of sodium sulfide nonahydrate (Na₂S·9H₂O) or sodium hydrosulfide hydrate (NaSH·nH₂O) and sulfur is described along with plausible reaction paths in this intriguing reaction.