Thermodynamic Isomer Research Articles

Schleyer hyperconjugative aromaticity in indene scaffolds

In the present research, the Schleyer hyperconjugative aromaticity was used to enhance the aromaticity of indene scaffolds. The first section of the research focused on examining the thermodynamic stability and electronic characteristics of the four structural isomers of indene. Results indicate that 1H-indene scaffolds are the most stable thermodynamic isomers of indene derivatives exceeding 100 kJ/mol. The hierarchy of stability is as follows: 1H-indenes > 2H-indenes > 5H-indenes > 4H-indenes. The aromaticity of 5- and 6-membered rings in the isomeric structures was investigated using the B3LYP/6–311 + G(d,p) method in the second section of the study. The hyperconjugative aromaticity of 5MR and 6MR was assessed using the harmonic oscillator model of aromaticity index, Bird index, Shannon index, aromatic fluctuation index, and the nucleus independent chemical shift. The results reveal that introducing electron-donating groups on 6MR enhances the aromaticity of 5MR. Moreover, adding two fluorine atoms on the Csp3 site negatively affects the aromaticity and induces antiaromaticity in the indene scaffold. Based on aromaticity data, the order of increasing aromaticity in the presence of different groups is F < CH3 < H < SiH3 < GeH3 < Si(CH3)3 < Ge(CH3)3. The extent of the Schleyer hyperconjugation interaction was also quantified using the E(2) parameter obtained from the NBO calculations.

Diastereoselectivity Switch During Alkene Reductions: Diastereodivergent Syntheses of Molecular Fossils via MHAT or Homogeneous Catalytic Hydrogenation Reactions.

Sixteen geosterane derivatives were synthesized in up to 57 % overall yields in four steps harnessing the olefin cross-metathesis (OCM) and Metal hydride H atom transfer (MHAT) or homogeneous hydrogenation reactions as key steps. Drawing on this strategy, the diastereomeric ratio (d. r.) reached up to 24 : 1 for the thermodynamic isomer and 7 : 1 for the other isomer in the hydrogenation step. In a geological sample from northeast Brazil, we confirmed the putative structures previously assumed as methyl 2-(3α-5αH-cholestan) acetate, methyl 2-(3β-5αH-cholestan)acetate, and methyl 6-(3β-5αH-cholestan)hexanoate, as well three new molecular fossils of approximately 120 million years old. We also proved the migration marking ability of those carboxylic acids derived from forerunner geosteranes during an oil migration event, which suggests their aptitudes as molecular odometers. Our approach demonstrated swiftness and effectiveness in preparing a molecular library of geological biomarkers would also be appropriate to generate stereochemical diversity in molecular libraries for medicinal chemistry and natural product anticipation.

Design of hydroxyl- and thioether-functionalized iron-platinum dimetallacyclopentenone complexes. Crystal and electronic structures, Hirshfeld and docking analyses and anticancer activity evaluated by in silico simulation

A series of metal-metal bonded iron-platinum heterobimetallic complexes has been prepared by carbon-carbon coupling of [(OC)3Fe(µ-C=O)(µ-dppm)Pt(PPh3)] (1) (dppm = Ph2PCH2PPh2) with the terminal alkynols 4-pentyn-1-ol, 4-pentyn-2-ol and the internal thioether-functionalized alkyne 1,4-bis(p-tolylthio)but-2-yne. The resulting dimetallacyclopentenones [(OC)2Fe(µ-dppm){µ-C(=O)C((CH2)3OH)=CH}Pt(PPh3)] (2a) and [(OC)2Fe(µ-dppm){µ-C(=O)C(CH2CHOHCH3)=CH}Pt(PPh3)] (2b) featuring intramolecular hydrogen O-H···O=C bonding have been structurally characterized, as well as [(OC)2Fe(µ-dppm){µ-C(=O)C(CH2S(p-tolyl))=C(CH2S(p-tolyl))}Pt(PPh3)] (2c). NMR studies reveal that coupling of 1 with HCC-CH2CH2CH2OH yields initially the kinetic isomer [(OC)2Fe(µ-dppm){µ-C(=O)C(H)=C((CH2)3OH)}Pt(PPh3)] (2a’), which upon heating converts quantitatively to the thermodynamic isomer 2a. Furthermore, the molecular structures of series 2 were investigated by means of Hirshfeld surface analyses. As targets for a bioinformatic study, molecular docking studies were applied to the complexes 2 and revealed the occurrence of intermolecular interactions with four different protein residues associated with breast (1hk7 and 3eqm) and prostate cancer (2q7k and 2ax6). According to the docking results, this family of complexes displays better activity on prostate cancer proteins rather than breast cancer ones. The molecular structures of compounds 2 were also optimized using DFT quantum chemical calculations. The large band gap implies low reactivity and good stability of these complexes.

Problems of polycyclic aromatic hydrocarbons studying in the waters of North-Eastern Caspian Sea

The problems of studying the negative factors of oil and gas activity in the waters of the North-Eastern Caspian Sea are presented. Despite the active anthropogenic activity, the specificity of hydrocarbon pollution in this part of the water area is poorly understood. The approaches to the analysis of the qualitative composition and identification of sources of polycyclic aromatic hydrocarbons (PAHs) pollution based on the indicator ratios of kinetic and thermodynamic isomers are presented. These approaches make it possible to identify the source of pollution, but a full assessment requires taking into account the seasonal dynamics of the sta- tus of aquatic systems. Estimates of the migration features of PAHs and their definitions in the aqueous phase are given. The problems of transition and the study of polyarenes to bottom sediments are assessed. The issues of the degradation of PAH and oil products in the aquatic systems of the region are considered, recommendations are given to improve the development of the research base of the reviewed topics.

Mechanism of H2 Production by Models for the [NiFe]-Hydrogenases: Role of Reduced Hydrides.

The intermediacy of a reduced nickel-iron hydride in hydrogen evolution catalyzed by Ni-Fe complexes was verified experimentally and computationally. In addition to catalyzing hydrogen evolution, the highly basic and bulky (dppv)Ni(μ-pdt)Fe(CO)(dppv) ([1](0); dppv = cis-C2H2(PPh2)2) and its hydride derivatives have yielded to detailed characterization in terms of spectroscopy, bonding, and reactivity. The protonation of [1](0) initially produces unsym-[H1](+), which converts by a first-order pathway to sym-[H1](+). These species have C1 (unsym) and Cs (sym) symmetries, respectively, depending on the stereochemistry of the octahedral Fe site. Both experimental and computational studies show that [H1](+) protonates at sulfur. The S = 1/2 hydride [H1](0) was generated by reduction of [H1](+) with Cp*2Co. Density functional theory (DFT) calculations indicate that [H1](0) is best described as a Ni(I)-Fe(II) derivative with significant spin density on Ni and some delocalization on S and Fe. EPR spectroscopy reveals both kinetic and thermodynamic isomers of [H1](0). Whereas [H1](+) does not evolve H2 upon protonation, treatment of [H1](0) with acids gives H2. The redox state of the "remote" metal (Ni) modulates the hydridic character of the Fe(II)-H center. As supported by DFT calculations, H2 evolution proceeds either directly from [H1](0) and external acid or from protonation of the Fe-H bond in [H1](0) to give a labile dihydrogen complex. Stoichiometric tests indicate that protonation-induced hydrogen evolution from [H1](0) initially produces [1](+), which is reduced by [H1](0). Our results reconcile the required reductive activation of a metal hydride and the resistance of metal hydrides toward reduction. This dichotomy is resolved by reduction of the remote (non-hydride) metal of the bimetallic unit.

La3N@C92: An Endohedral Metallofullerene Governed by Kinetic Factors?

Different structures have been proposed so far for the C92 isomer that encapsulates M3N (M = La, Ce, Pr). We show here that the electrochemical properties of the predicted most abundant (thermodynamic) isomer for La3N@C92 does not agree with experiment. After a systematic search within the huge number of possible C92 isomers, we propose other candidates with larger electrochemical gaps for La3N@C92 before its structure could be finally determined by X-ray crystallography. We do not discard that the thermodynamic isomer could be detected in future experiments though.

Secondary kinetic deuterium isotope effect in oxidative addition reaction of cycloplatinated(II) complexes with MeI

Oxidative addition reaction of CH3I or CD3I with the cycloplatinated(II) complexes [PtMe(CˆN)L], 1a–1e, in which CˆN = 2-phenylpyridinate (ppy) or benzo[h]quinolinate (bhq), and L = PPh3, PMePh2 or P(OPh)3, in CH2Cl2 solvent gave the organoplatinum(IV) products [PtMe2I(CˆN)L]. NMR spectroscopy (1H and 31P), and X-ray crystallography (supported by DFT calculations) clearly indicated that in each case the thermodynamic isomer products 2a–2e, with I ligand trans to C of CˆN rather than the related kinetic isomer in which I ligand is trans to Me, is obtained. Kinetics of the reactions were studied by using UV–vis spectroscopy and the reactions are suggested to occur by a common SN2 mechanism, on the basis of their clean second-order kinetics, failure of radical traps to inhibit the reactions, and the observation of large negative values of ΔS‡. Rates of the reactions are significantly dependent on nature of the phosphine ligand L with the trend being PMePh2 > PPh3 > P(OPh)3. The kinetic isotope effect (KIE) values, kH/kD, were obtained in the present study and are found to be close to the normal value of 1 in the corresponding reactions, showing that nature of the ancillary ligands containing phosphorous donor atoms and the cycloplatinated ligands, CˆN, are not effective on the values of kH/kD. For example, KIE values, in reactions involving the ppy complexes [PtMe(ppy)L], 1a–1c, at 20 °C are 1.06 (L = PPh3, 1a), 0.97 (L = P(OPh)3, 1c) and 0.92 (L = PMePh2, 1b). Note that nature of the CˆN ligands are found to be almost non-effective on both rates of the reactions and the related KIE values.

Mechanism and selectivity of methyl and phenyl migrations in hypervalent silylated iminoquinones.

Chlorosilanes R(X)(Y)SiCl (R = Me, Ph; X, Y = Me, Ph, Cl) have been reported to react with Pb(ONO(Q))2 (ONO(Q) = 3,5-di-tert-butyl-1,2-quinone-(3,5-di-tert-butyl-2-oxy-1-phenyl)imine) to give five-coordinate (X)(Y)Si(ON[R]O), in which the R group has migrated from silicon to nitrogen. This migration is intramolecular, as confirmed by the lack of crossover between (CH3)3SiCl and (CD3)3SiCl in their reaction with Pb(ONO(Q))2. Reaction of PhSiMeCl2 takes place with high kinetic stereoselectivity to produce isomer Ph(Cl)Si(ON[Me]O) in which the phenyl is axial in the trigonal bipyramid, which subsequently isomerizes to the thermodynamic isomer with axial chlorine. This indicates that migration takes place preferentially from the stereoisomer of the octahedral intermediate, κ(3)-Ph(CH3)(Cl)Si(ONO(Q)), in which the phenyl and methyl groups are mutually trans, indicating that the observed complete selectivity for methyl over phenyl migration is due to intrinsic differences in migratory aptitude. DFT calculations suggest that migration takes place from this isomer not because it undergoes migration faster than other possible stereoisomers, but because it is formed most rapidly, and migration occurs faster than isomerization.

Crystal structures and DFT calculations of new chlorido-dimethylsulfoxide-MIII (M = Ir, Ru, Rh) complexes with the N-pyrazolyl pyrimidine donor ligand: kinetic vs. thermodynamic isomers

New chlorido-dimethylsulfoxide-iridium(III), ruthenium(III) and rhodium(III) complexes with the 2-(1H-pyrazol-1-yl)-pyrimidine (pyrapyr) ligand (OC-6-N1)-[Rh(III)Cl3(DMSO-κS)(pyrapyr)] (1a, N = 3 and 1b, N = 4); (OC-6-N1)-[Ru(III)Cl3(DMSO-κS)(pyrapyr)] (2a, N = 3 and 2b, N = 4) and (OC-6-N1)-[Ir(III)Cl3(DMSO-κS)(pyrapyr)] (3a, N = 3 and 3b, N = 4) have been synthesized and characterized by spectroscopic techniques and by single crystal X-ray diffraction studies (1a, 1b, 2a, 2b, a disordered crystal 3a/3b and a cocrystal 3a·3b). In all cases, the metal centers show octahedral geometry coordinated to three chloride ligands and one S coordinated dimethylsulfoxide (DMSO-κS). The coordination sphere of the metal is completed by the pyrapyr molecule. Two different coordination modes are observed: (i) the DMSO-κS is opposite to the pyrimidinic N atom (IUPAC nomenclature is OC-6-31 denoted herein as trans); (ii) DMSO-κS is opposite to the pyrazolic N atom (IUPAC nomenclature is OC-6-41 denoted as cis). For Rh(III) the kinetic product (cis) yields the thermodynamic (trans) upon heating a solution of the kinetic product and both isomers have been X-ray characterized. Conversely for Ru(III), both kinetic and thermodynamic complexes have been obtained by using different procedures. Both isomers have been characterized by X-ray crystallography and the kinetic product does not yield the thermodynamic upon heating a solution of the former. Furthermore, the Ir(III) behaves differently, since both isomers are energetically equivalent and both isomers co-crystallize in the solid state. The kinetic/thermodynamic mechanism that yields the different isomers has been studied by using theoretical DFT calculations for each metal. Finally, two Ru(II) complexes (OC-6-N1)-[Ru(II)Cl2(DMSO-κS)2(pyrapyr)] (1a, N = 3 and 4b, N = 4) are also described and X-ray characterized. They were obtained as minor products during the synthesis of 2a.

Reactivity of Silyl-Substituted Iron–Platinum Hydride Complexes toward Unsaturated Molecules: 4. Insertion of Fluorinated Aromatic Alkynes into the Platinum–Hydride Bond. Synthesis and Reactivity of Heterobimetallic Dimetallacylopentenone, Dimetallacyclobutene, μ-Vinylidene, and μ2-σ-Alkenyl Complexes

Insertion of p-FC6H4C≡CH, p-CF3C6H4C≡CH, and m-CF3C6H4C≡CH into the Pt–H bond of [(OC)3Fe{Si(OMe)3}(μ-dppm)Pt(H)(PPh3)] (1a) yields first the σ-alkenyl complexes [(OC)3Fe{μ-Si(OMe)2(OMe)}(μ-dppm)Pt(ArC═CH2)] (2a, Ar = C6H4F-p; 2d, C6H4CF3-p; 2e, C6H4CF3-m), which react in a second step with the liberated PPh3 ligand to afford the structurally characterized μ-vinylidene complexes [(OC)3Fe(μ-dppm){μ-C═C(H)C6H4F-p}Pt(PPh3)] (3a) and [(OC)3Fe(μ-dppm){μ-C═C(H)C6H4R}Pt(PPh3)] (3d, R = p-CF3; 3e, R = m-CF3). In contrast, treatment of 1a with o-FC6H4C≡CH produces first [(OC)3Fe{μ-Si(OMe)2(OMe)}(μ-dppm)Pt(o-FC6H4C═CH2)] (2b), which evolves to the dimetallacyclopentenone complex [(OC)2Fe(μ-dppm){μ-C(═O)C(H)═C(C6H4F-o)}Pt(PPh3)] (4b′). The latter slowly rearranges to the structurally characterized thermodynamic isomer [(OC)2Fe(μ-dppm){μ-C(═O)C(C6H4F-o)═C(H)}Pt(PPh3)] (4b). Treatment of 1a with 2,4-F2C6H3C≡CH produces via transient alkenyl complex 2c an isomeric mixture of [(OC)3Fe(μ-dppm){μ-C═C(H)C6H3F2-2,4}Pt(PPh3)...

Re2(CO)6(μ-thpymS)2 (thpymSH = pyrimidine-2-thiol) as a versatile precursor to mono- and polynuclear complexes: X-ray crystal structures of fac-Re(CO)3(PPh3)(κ2-thpymS) and two isomers of ReRu3(CO)13(μ3-thpymS)

The dirhenium complexes Re2(CO)6(μ-thpymS)2 (1) and eq-Re2(CO)9{κ1-(S)-SN2C4H8} (2) are obtained from the reaction of tetrahydropyrimidine-2-thiol (thpymSH) with Re2(CO)8(NCMe)2 or Re2(CO)10. Complex 1 proves to be an excellent precursor to a range of monometallic and cluster complexes acting as a source of “Re(CO)3(thpymS)”. Thus, reactions with triphenylphosphine (PPh3) and bis(diphenylphosphino)methane (dppm) afford mononuclear fac-Re(CO)3(PPh3)(κ2-thpymS) (3) and fac-Re(CO)3(κ1-dppm)(κ2-thpymS) (4), respectively. A crystal structure of 3 reveals that the pyrimidine-2-thiolate binds in a chelating fashion. Reactions with M3(CO)12 (M=Ru, Os) at moderate temperatures afford mixed-metal clusters. With Ru3(CO)12 in boiling thf isomeric tetranuclear butterfly clusters ReRu3(CO)13(μ3-thpymS) (5–6) result which differ in the position of the capping pyrimidine-2-thiolate ligand on the cluster surface. Thus in the kinetic isomer 5, the pyrimidine-2-thiolate caps the ReRu2 face while in the thermodynamic isomer 6 it caps the Ru3 face. A similar reaction with Os3(CO)10(NCMe)2 in boiling benzene affords predominantly the ReOs2-capped tetranuclear cluster ReOs3(CO)13(μ3-thpymS) (7), together with small amounts of (μ-H)Os3(CO)9(μ3-thpymS) (8) resulting from loss of rhenium. Cluster 7 is stable in refluxing toluene but slowly converts to isomeric 9 in xylene at 140°C but only in low yields. The crystal structures of isomeric 5–6 have been solved and show only small variations in the cluster core geometry. Density functional calculations show that clusters 6 and 9 are slightly more stable than 5 (ΔG 2.0kcalmol−1) and 7 (ΔG 1.5kcalmol−1), respectively, and a mechanism is proposed for these conversions in which the pyrimidine-2-thiolate becomes bidentate after release of the pyrimidine group.

Mechanistic Features of Isomerizing Alkoxycarbonylation of Methyl Oleate

The weakly coordinated triflate complex [(P^P)Pd(OTf)](+)(OTf)(-) (1) (P^P = 1,3-bis(di-tert-butylphosphino)propane) is a suitable reactive precursor for mechanistic studies of the isomerizing alkoxcarbonylation of methyl oleate. Addition of CH(3)OH or CD(3)OD to 1 forms the hydride species [(P^P)PdH(CH(3)OH)](+)(OTf)(-) (2-CH(3)OH) or the deuteride [(P^P)PdD(CD(3)OD)](+)(OTf)(-) (2(D)-CD(3)OD), respectively. Further reaction with pyridine cleanly affords the stable and isolable hydride [(P^P)PdH(pyridine)](+)(OTf)(-) (2-pyr). This complex yields the hydride fragment free of methanol by abstraction of pyridine with BF(3)·OEt(2), and thus provides an entry to mechanistic observations including intermediates reactive toward methanol. Exposure of methyl oleate (100 equiv) to 2(D)-CD(3)OD resulted in rapid isomerization to the thermodynamic isomer distribution, 94.3% of internal olefins, 5.5% of α,β-unsaturated ester and <0.2% of terminal olefin. Reaction of 2-pyr/BF(3)·OEt(2) with a stoichiometric amount of 1-(13)C-labeled 1-octene at -80 °C yields a 50:50 mixture of the linear alkyls [(P^P)Pd(13)CH(2)(CH(2))(6)CH(3)](+) and [(P^P)PdCH(2)(CH(2))(6)(13)CH(3)](+) (4a and 4b). Further reaction with (13)CO yields the linear acyls [(P^P)Pd(13)C(═O)(12/13)CH(2)(CH(2))(6)(12/13)CH(3)(L)](+) (5-L; L = solvent or (13)CO). Reaction of 2-pyr/BF(3)·OEt(2) with a stoichiometric amount of methyl oleate at -80 °C also resulted in fast isomerization to form a linear alkyl species [(P^P)PdCH(2)(CH(2))(16)C(═O)OCH(3)](+) (6) and a branched alkyl stabilized by coordination of the ester carbonyl group as a four membered chelate [(P^P)PdCH{(CH(2))(15)CH(3)}C(═O)OCH(3)](+) (7). Addition of carbon monoxide (2.5 equiv) at -80 °C resulted in insertion to form the linear acyl carbonyl [(P^P)PdC(═O)(CH(2))(17)C(═O)OCH(3)(CO)](+) (8-CO) and the five-membered chelate [(P^P)PdC(═O)CH{(CH(2))(15)CH(3)}C(═O)OCH(3)](+) (9). Exposure of 8-CO and 9 to (13)CO at -50 °C results in gradual incorporation of the (13)C label. Reversibility of 7 + CO ⇄ 9 is also evidenced by ΔG = -2.9 kcal mol(-1) and ΔG(‡) = 12.5 kcal mol(-1) from DFT studies. Addition of methanol at -80 °C results in methanolysis of 8-L (L = solvent) to form the linear diester, 1,19-dimethylnonadecandioate, whereas 9 does not react and no branched diester is observed. DFT yields a barrier for methanolysis of ΔG(‡) = 29.7 kcal mol(-1) for the linear (8) vs ΔG(‡) = 37.7 kcal mol(-1) for the branched species (9).

Kinetic versus thermodynamic isomers of the deltahedral dicobaltadicarbaboranes having nine to 12 vertices

The lowest energy structures for the dicobaltadicarbaboranes Cp2Co2C2Bn−4Hn−2 (n=9, 10, and 11) are found by density functional theory to be the most spherical borane deltahedra with the carbon atoms at degree 4 vertices and the cobalt atoms at degree 5 or 6 vertices. For the icosahedral 12-vertex dicobaltadicarbaboranes Cp2Co2C2B8H10 with only degree 5 vertices, the lowest energy structures are those without Co–Co or C–C edges. These theoretical predictions agree well with experimental data on the numerous known Cp2Co2C2Bn−4Hn−2 (n=9, 10, 11, and 12) derivatives. Thus for the nine-vertex Cp2Co2C2B5H7 system only the two lowest energy isomers are found experimentally. For the 10-vertex Cp2Co2C2B6H8 system three of the six lowest energy isomers have been synthesized. The 11- and 12-vertex Cp2Co2C2Bn−4Hn−2 systems provide examples of stable high energy isomers with direct Co–Co or C–C bonds arising from the synthetic methods used. Thus one of the experimentally known 11-vertex Cp2Co2C2B7H9 isomers is a high-energy structure with adjacent carbon atoms lying ∼26kcal/mol above the global minimum. In addition to this high-energy isomer seven of the 12 predicted Cp2Co2C2B7H9 isomers within 6kcal/mol of the global minimum have been synthesized. Similarly, for the icosahedral Cp2Co2C2B8H10 derivatives, high-energy isomers with a Co–Co bond lying 21.8kcal/mol above the global minimum and with a C–C bond lying 19.4kcal/mol above the global minimum have both been synthesized as stable compounds. Pyrolysis of these high energy Cp2Co2C2B8H10 isomers at temperatures up to 340°C gives a lower energy isomer with neither a Co–Co nor a C–C bond. Further pyrolysis of this Cp2Co2C2B8H10 isomer at the incredibly high temperature of 650°C for an organometallic reaction gives a complicated mixture of seven of the 12 possible lowest energy isomers, namely those with neither Co–Co nor C–C bonds.

Thermal Isomerization of N-Bridged Cobalt Corrole Complexes through a Transiently Formed Axial Carbenoid

The reaction of cobalt(III) octaethylcorrole (CoOEC) with ethyl diazoacetate (EDA) under a nitrogen atmosphere led to the formation of bridged complexes with the carbene moiety inserted into one of the cobalt−nitrogen bonds. The bridged complexes underwent unique thermal isomerization, presumably via the transiently formed axial carbene complex. One of the thermodynamic isomers was characterized by 1H NMR and mass spectroscopic methods.

Bulky Guanidinato Nickel(I) Complexes: Synthesis, Characterization, Isomerization, and Reactivity Studies

Reactions of lithium complexes of the bulky guanidinates [{(Dip)N}(2)CNR(2)](-) (Dip=C(6)H(3)iPr(2)-2,6; R=C(6)H(11) (Giso(-)) or iPr (Priso(-)), with NiBr(2) have afforded the nickel(II) complexes [{Ni(L)(μ-Br)}(2)] (L=Giso(-) or Priso(-)), the latter of which was crystallographically characterized. Reduction of [{Ni(Priso)(μ-Br)}(2)] with elemental potassium in benzene or toluene afforded the diamagnetic species [{Ni(Priso)}(2)(μ-C(6)H(5)R)] (R=H or Me), which were shown, by X-ray crystallographic studies, to possess nonplanar bridging arene ligands that are partially reduced. A similar reduction of [{Ni(Priso)(μ-Br)}(2)] in cyclohexane yielded a mixture of the isomeric complexes [{Ni(μ-κ(1)-N-,η(2)-Dip-Priso)}(2)] and [{Ni(μ-κ(2)-N,N'-Priso)}(2)], both of which were structurally characterized. These complexes were also formed through arene elimination processes if [{Ni(Priso)}(2)(μ-C(6)H(5)R)] (R=H or Me) were dissolved in hexane. In that solvent, diamagnetic [{Ni(μ-κ(1)-N-,η(2)-Dip-Priso)}(2)] was found to slowly convert to paramagnetic [{Ni(μ-κ(2)-N,N'-Priso)}(2)], suggesting that the latter is the thermodynamic isomer. Computational analysis of a model of [{Ni(μ-κ(2)-N,N'-Priso)}(2)] showed it to have a Ni-Ni bond that has a multiconfigurational electronic structure. An analogous copper(I) complex [{Cu(μ-κ(2)-N,N'-Giso)}(2)] was prepared, structurally authenticated, and found, by a theoretical study, to have a negligible Cu···Cu bonding interaction. The reactivity of [{Ni(Priso)}(2)(μ-C(6)H(5)Me)] and [{Ni(μ-κ(2)-N,N'-Priso)}(2)] towards a range of small molecules was examined and this gave rise to diamagnetic complexes [{Ni(Priso)(μ-CO)}(2)] and [{Ni(Priso)(μ-N(3))}(2)]. Taken as a whole, this study highlights similarities between bulky guanidinate ligands and the β-diketiminate ligand class, but shows the former to have greater coordinative flexibility.

Fluorine for Hydrogen Exchange in the Hydrofluorobenzene Derivatives C6HxF(6-x), where x = 2, 3, 4 and 5 by Monomeric [1,2,4-(Me3C)3C5H2]2CeH: The Solid State Isomerization of [1,2,4-(Me3C)3C5H2]2Ce(2,3,4,5-C6HF4) to [1,2,4-(Me3C)3C5H2]2Ce(2,3,4,6-C6HF4)

The reaction between monomeric bis(1,2,4-tri-tert-butylcyclopentadienyl)cerium hydride, Cp'2CeH, and several hydrofluorobenzene derivatives is described. The aryl derivatives that are the primary products, Cp'2Ce(C6H(5-x)F(x)) where x = 1,2,3,4, are thermally stable enough to be isolated in only two cases, since all of them decompose at different rates to Cp'2CeF and a fluorobenzyne; the latter is trapped by either solvent when C6D6 is used or by a Cp'H ring when C6D12 is the solvent. The trapped products are identified by GC/MS analysis after hydrolysis. The aryl derivatives are generated cleanly by reaction of the metallacycle, Cp'((Me3C)2C5H2C(Me2)CH2)Ce, with a hydrofluorobenzene, and the resulting arylcerium products, in each case, are identified by their (1)H and (19)F NMR spectra at 20 degrees C. The stereochemical principle that evolves from these studies is that the thermodynamic isomer is the one in which the CeC bond is flanked by two ortho-CF bonds. This orientation is suggested to arise from the negative charge that is localized on the ipso-carbon atom due to C(o)(delta+)F(o)(delta-) polarization. The preferred regioisomer is determined by thermodynamic rather than kinetic effects; this is illustrated by the quantitative, irreversible solid-state conversion at 25 degrees C over two months of Cp'2Ce(2,3,4,5-C6HF4) to Cp'2Ce(2,3,4,6-C6HF4), an isomerization that involves a CeC(ipso) for C(ortho)F site exchange.

Water Lends a Hand

CHEMISTRY Although the global chirality of molecular aggregates is strongly influenced by the individual chirality of the building blocks, it is not generally a simple matter to predict one from the other. In a series of careful experiments, Johnson et al. uncover the subtle environmental factors that determine the helical handedness of rosette nanotubes assembled in solution from small organic heterocycles bearing a chiral side chain. The heterocycles (which are self-complementary toward hydrogen bonding) first form hexameric supermacrocycles, which in turn stack into a helical arrangement. Dissolution of a single enantiomer of this building block in methanol gives rise to one helical isomer, but addition of as little as 1% water to the solvent instead induces opposite helicity in the stacks. The authors show that the water-induced product is thermodynamically favored, but faces a larger kinetic barrier than its counterpart to formation in pure methanol (though aggregation in both senses appears to be accelerated in the absence of water). They further find that chiral inversion of the kinetic isomer in methanol can be catalyzed by the thermodynamic isomer. Inversion is also possible at an early stage by heating, though after 3 days the kinetic isomer becomes stereochemically locked, a result attributed in part to extensive solvation. — MSL J. Am. Chem. Soc. 129 , 5735 (2007).

Prediction of pKaValues ofnido-Carboranes by Density Functional Theory Methods

A great parallel exists between metal complexes of cyclopentadienyl and arene ligands on one side and metal complexes of the nido derivatives of the icosahedral o-carborane clusters. With few exceptions, the metal complexation in the cluster can be viewed as the substitution of one or more bridging hydrogen atoms by the metal. Therefore, a necessary requirement for the complexation is the deprotonation of the nido cluster to generate a coordination site for that metal. The reaction to remove these protons, which most probably is one of the most commonly done processes in boron and metallaborane chemistry, is barely known, and no quantitative data are available on the magnitude of their pKa values. With the purpose of determining the acidity of nido-carboranes, a procedure to calculate the pKa values of nido boron clusters is presented in this paper for the first time. To this objective, some nido clusters have been selected and their geometry and NMR-spectroscopic properties have been studied, giving a good correlation between the theoretical and experimental data in both geometry distances and 11B NMR spectroscopy. Of notice is the result that proves that the singular carbon atom in the thermodynamic isomer of [C2B10H13]- is definitely part of the cluster and that its connection with the C2B3 face would be better defined by adding additional interactions with the two boron atoms nearest to the second cluster carbon. The pKa values of the nido species have been calculated by correlating experimental pK(a) values and calculated reaction Gibbs energies DeltaG(s). Some pKa values of importance are -4.6 and +13.5 for 7,8-[C2B9H13] (1) and 7,8-[C2B9H12]- (2), respectively.

Thiapentadienyl−Rhodium−Phosphine Chemistry1

The reactions of (Cl)Rh(PR3)3 (R = Me, Et) with the anionic thiapentadienide reagent lithium 2,3-dimethyl-5-thiapentadienide have been investigated. Treatment of (Cl)Rh(PMe3)3 with lithium 2,3-dimethyl-5-thiapentadienide produces ((1,2,5-η)-2,3-dimethyl-5-thiapentadienyl)Rh(PMe3)3 (1), in which the thiapentadienyl ligand is σ-bonded to rhodium through the sulfur end of the chain and π-bonded through the carbon end (C1−C2 double bond). In solution at room temperature, 1 undergoes a rapid dynamic process involving release and recoordination of the thiapentadienyl double bond C1−C2, causing two of the phosphine ligands to become equivalent. The intermediate in which the double bond C1−C2 is released from the rhodium center can be “trapped” as a dioxygen adduct, ((5-η)-2,3-dimethyl-5-thiapentadienyl)Rh(PMe3)3(η2-O2) (2). When it is stirred at room temperature in tetrahydrofuran solution, 1 slowly rearranges to ((1,4,5-η)-2,3-dimethyl-5-thiapentadienyl)Rh(PMe3)3 (3), the thermodynamic isomer in which the thiap...

Exocyclic–Endocyclic N‐Acyliminium Ion Equilibration via an Intramolecular α‐Thioamidoalkylation in the Synthesis of Fused N,S‐Heterocyclic Systems: Some New Parameters

AbstractThe reactivity of N‐[aryl(or alkyl)thio(oxo or seleno)alkylamidals 6 bearing the N‐(CH2)n‐X‐(CH2)m‐Ar functionality towards neat TFA has been examined. Substrates of type 6 give, together with the products 11, 17, 19, and 25 of the “expected” π‐cyclization process, the “unexpected” products 12, 18, and 26 resulting from a new tandem heteroamidoalkylation/isomerization/π‐cyclization. The composition of the final isomeric mixture depends on the temperature of the reaction, with a high selectivity in favor of the “expected” π‐cyclization product 11 as a thermodynamic isomer. Furthermore, the results demonstrate again the relevance of TFA catalysis and support two reaction pathways involving the formation of an aza‐sulfonium cation A as a valuable intermediate which undergoes the π‐cyclization process alone or in tandem with the thiocyclization/isomerization to give the cyclized products cited above. For the latter tandem reaction, we have established clearly that the n value should be equal to 1, the process can be performed independently for both the succinimide and phthalimide series, the reaction depends on the nucleophilicity of the heteroatom, and can be generalized to other heterocycles. Only the role of the benzene substitution has still to be totally elucidated. (© Wiley‐VCH Verlag GmbH &amp; Co. KGaA, 69451 Weinheim, Germany, 2005)