Showing papers on "Double bond published in 2010"

C−H Activation for the Construction of C−B Bonds

Abstract: A number of studies were conducted to demonstrate C-H activation for the construction of C-B bonds. Investigations revealed that the conversion of C-H bonds to C-B bonds was both thermodynamically and kinetically favorable. The reaction at a primary C-H bond of methane or a higher alkene B 2(OR)4 formed an alkylboronate ester R' -B(OR)2 and the accompanying borane H-B(OR2. The ester and the borane were formed on the basis of calculated bond energies for methylboronates and dioaborolanes. The rates of key steps along the reaction pathway for the conversion of a C-H bond in an alkane or arene to the C-B bond in an alkyl or arylboronate ester were favorable. These studies also highlighted the accessible barriers for C-H bond cleavage and B-C bond formation during the borylation of alkanes and arenes.

Crystalline 1H‐1,2,3‐Triazol‐5‐ylidenes: New Stable Mesoionic Carbenes (MICs)

Abstract: In 2001, Crabtree and co-workers first reported complex A, which features an imidazole ring bound at the C5 position (III), and not at C2 as commonly observed.[7] More recently, Huynh and co-workers[8] and Albrecht and co-workers[9a] showed that pyrazolium and 1,2,3-triazolium salts can serve as precursors to metal complexes of type B and C, which feature pyrazolin-4-ylidenes IV and 1,2,3-triazol-5-ylidenes V as the ligand, respectively. As a consequence of their lineage, these have also been referred to as N-heterocyclic carbenes (NHCs). However, as no reasonable canonical resonance forms containing a carbene can be drawn for free ligands III–V without additional charges (see V′), these ligands have been described as abnormal or remote carbenes (aNHCs or rNHCs, respectively).[10] As they are, in fact, mesoionic compounds,[11] we suggest naming this family of compounds mesoionic carbenes (MICs). There have been no reported dimerizations of MICs III and IV, which suggests that the Wanzlick equilibrium pathway for classical carbenes is disfavored;[12] this observation should lead to relaxed steric requirements for their isolation. Moreover, experimental and theoretical data suggest that MICs III–V are even stronger electron-donating species than NHCs I and II, which opens up interesting perspectives for their applications.[10] Our recent success in the isolation of a free imidazol-5-ylidene III[13] and pyrazolin-4-ylidenes IV (cyclic bent allenes),[14,15] prompted us to investigate the possibility of preparing new types of stable neutral compounds that feature a lone pair of electrons on the carbon atom.[16] Preliminary calculations (B3LYP, 6–311G(d,p); for details, see the Supporting Information) predicted that the parent MIC V is located at an energy minimum, about 32 kcalmol−1 above the regioisomeric parent 1,2,4-triazol-5-ylidene II. Furthermore, parent V is predicted to exhibit an appreciably large singlet–triplet band gap (56 kcalmol−1), which is a good predictor of carbene stability and thus of possible isolation. Herein, we report the preparation, isolation, and characterization of two free 1,2,3-triazol-5-ylidenes of type V. By analogy with the synthetic route used for preparing NHCs and the related species III and IV, 1,2,3-triazolium salts (2a,b) were targeted as precursors for the desired 1,2,3-triazol-5-ylidenes (Va,b). A sterically hindered flanking aryl substituent (2,6-diisopropylphenyl, Dipp) was selected to provide kinetic stabilization to the ensuing free ligand. 1,2,3-Triazole 1 was obtained in 83% yield from the copper-catalyzed azide–alkyne cycloaddition (CuAAC, click chemistry) of 2,6-diisopropylphenyl azide and phenylacetylene.[17] The one-pot conversion of aniline into the desired aryl azide, followed in situ by CuAAC as reported by Moses and co-workers[18] was found to be especially convenient for the synthesis of 1. Alkylation of 1 with methyl or isopropyl trifluoromethanesulfonate afforded the corresponding tri-azolium salts in moderate to excellent yields (2a and 2b, respectively; Scheme 2). Scheme 2 Synthesis of the free 1,2,3-triazol-5-ylidenes Va,b. Potassium bases have been identified as the reagents of choice for the depronation of carbene precursors, as they avoid the formation of stable carbene–alkali-metal adducts that are commonly encountered when lithium bases are used.[12,13,14a,19] Gratifyingly, triazolium salts 2a,b were cleanly deprotonated with either potassium bis(trimethylsilyl)amide or potassium tert-butoxide in ethereal solvents to afford the corresponding MICs Va and Vb in 55 and 39% yield, respectively. Deprotonation was evidenced by the disappearance of the triazolium CH signal in their 1H NMR spectra (2a: δ =8.62 ppm; 2b: δ =8.85 ppm) and the appearance of a signal at low field in the 13C NMR spectrum (Va: δ = 202.1 ppm; Vb: δ =198.3 ppm). The structure of Va was unambiguously confirmed by X-ray crystallography (Figure 1).[20] In the solid state, Va contains a planar heterocycle, characterized by bond lengths that are intermediate between those of single and double bonds; both of these features are indicative of electronic delocalization. Upon deprotonation, the C5 carbon bond angle becomes more acute (2a: 106°; Va: 100°), which is consistent with an increased s character in the σ lone pair orbital of Va compared to the C–H bonding orbital of the precursor 2a. This is in agreement with the generally observed trend for carbenes and their conjugate acids.[5] Figure 1 Molecular views (thermal ellipsoids set at 50 % probability) of 2a (top) and Va (bottom) in the solid state. For clarity, counter ions, solvent molecules, and H atoms are omitted, except for the ring hydrogen of 2 a. Selected bond lengths [A] ... In the solid state, with the exclusion of oxygen and moisture, free 1,2,3-triazol-5-ylidene Va (m.p. 50–52°C decomp.) remained stable for several days at −30°C and for a few hours at room temperature. By contrast, Vb (m.p. 110–112°C) was significantly more stable, showing no sign of decomposition after three days at room temperature in the solid state. Upon heating in a benzene solution for 12 hours at 50°C, Va decomposed to give, among other products, triazole 3 (Scheme 3; for details, see the Supporting Information). We surmise that the latter product results from a nucleophilic attack of the carbon lone pair of Va on the methyl group of a second molecule of Va, giving rise to heterocycles 4 and 5, which react together to afford the observed product 3. This apparent rearrangement is reminiscent of that recently observed in the formation of imidazol-2-ylidenes of type I from imidazol-5-ylidenes of type III that contain an electro-philic Y group.[21] In agreement with this hypothesis, MIC Vb, which contains the less-electrophilic isopropyl group at the N3 position, appears much more robust with respect to this decomposition pathway. Scheme 3 Degradation of free 1,2,3-triazol-5-ylidene Va, and analogy with the rearrangement of III into I. To evaluate the donor properties of 1,2,3-triazol-5-yli-denes, the [(Va)Ir(CO)2Cl] complex was prepared by addition of Va to [{Ir(cod)Cl}2] (cod = 1,5-cyclooctadiene), followed by treatment with an excess of carbon monoxide. The CO vibration frequencies (ν =2061 and 1977 cm−1; νavg = 2019 cm−1) are in line with those of the analogous iridium complex, previously reported by Albrecht and co-workers (νavg = 2021 cm−1),[9a] and are indicative of donor properties that are superior to those of NHCs I and II (νavg = 2022–2031 cm−1),[22] but inferior to those of MICs III (νavg = 2003–2006 cm−1)[23] and IV (νavg = 2002 cm−1).[14b] Free 1H-1,2,3-triazol-5-ylidenes, as exemplified by compounds Va,b, possess an ensemble of properties that portend to their utility. The synthesis of their precursors is short and efficient, from readily available starting materials, yet is modular and thus amenable to a wide variety of potential analogues. As with other mesoionic carbenes III and IV, the dimerization of MICs of type V has not been observed; therefore, the preparation of comparatively unhindered MICs is predicted to be viable. Their donor properties are greater than those of NHCs of type I and II, but they are nonetheless available by deprotonation using mild bases (e.g. alkoxides), thus signaling their potential for applications, such as nucleophilic organocatalysis. Free triazolylidenes V complement the rapidly growing numbers of neutral carbon-based κ1C ligands that are now available. We predict that many other classes of MICs, that are derived from a variety of heteroaromatic scaffolds, can be isolated. This endeavor is currently the object of ongoing efforts in our laboratory.

Water-soluble atmospheric organic matter in fog: exact masses and chemical formula identification by ultrahigh-resolution fourier transform ion cyclotron resonance mass spectrometry.

Abstract: The detailed molecular composition of water-soluble atmospheric organic matter (AOM) contained in fog water was studied by use of electrospray ionization ultrahigh-resolution Fourier transform ion cyclotron resonance mass spectrometry. We identified 1368 individual molecular masses in the range of 100−400 Da from negative-ion spectra obtained after reversed-phase extraction with a hydrophilic solid phase sorbent. The detected organic anions are multifunctional with a variety of oxygenated functional groups. We observe organic nitrogen, organic sulfur, and organic nitrogen−sulfur compounds as well as many species with only C, H, and O elemental composition. Analysis of the double bond equivalents (DBE = the number of rings plus the number of double bonds to carbon) suggests that these compound structures range from highly aliphatic to aromatic with DBE values of 1−11. The compounds range in their extent of oxidation with oxygen to carbon ratios from 0.2 to 2 with an average value of 0.43. Several homologou...

Steering S−H and N−H Bond Activation by a Stable N-Heterocyclic Silylene: Different Addition of H2S, NH3, and Organoamines on a Silicon(II) Ligand versus Its Si(II)→Ni(CO)3 Complex

Abstract: The strikingly different behavior of the ylide-like, N-heterocyclic silylene LSi: (5: L = CH[(C═CH2)CMe(NAr)2]; Ar = 2,6-iPrC6H3) versus its LSi→Ni(CO)3 complex 13 to activate E−H bonds (E = S, N) of small molecules is reported. Remarkably, conversion of 5 with hydrogen sulfide leads exclusively to the first isolable silathioformamide, L′Si(═S)H (16: L′ = CH[C(Me)NAr]2; Ar = 2,6-iPrC6H3) with a donor-supported Si═S double bond and four-coordinate silicon. The latter result demonstrates the unusual ambivalent reactivity of 5 by combining two modes of reactivity involving S−H bond activation and subsequent 1,4- and 1,1-addition, respectively. In addition, 5 can serve as a ligand with well-balanced σ-donor and π-acceptor capabilities toward transition metals. This has been demonstrated by the isolable [Ni0(arene)] complexes 12a−e (arene = MenC6H6−n, n = 0−3), which are ideal precursors for the formation of the corresponding Ni(CO)3 complex 13. The latter activates a S−H bond in hydrogen sulfide, too, but the...

What Factors Influence the Rate Constant of Substrate Epoxidation by Compound I of Cytochrome P450 and Analogous Iron(IV)-Oxo Oxidants?

Abstract: The cytochromes P450 are a versatile range of mono-oxygenase enzymes that catalyze a variety of different chemical reactions, of which the key reactions include aliphatic hydroxylation and C═C double bond epoxidation. To establish the fundamental factors that govern substrate epoxidation by these enzymes we have done a systematic density functional theory study on substrate epoxidation by the active species of P450 enzymes, namely the iron(IV)-oxo porphyrin cation radical oxidant or Compound I. We show here, for the first time, that the rate constant of substrate epoxidation, and hence the activation energy, correlates with the ionization potential of the substrate as well as with intrinsic electronic properties of the active oxidant such as the polarizability volume. To explain these findings we present an electron-transfer model for the reaction mechanism that explains the factors that determine the barrier heights and developed a valence bond (VB) curve crossing mechanism to rationalize the observed tr...

Oxoboryl Complexes: Boron−Oxygen Triple Bonds Stabilized in the Coordination Sphere of Platinum

Abstract: Monomeric oxoboranes have hitherto been detected only as short-lived species in gas-phase or low-temperature matrix experiments. Here, we report formation of the oxoboryl complex trans-[(Cy3P)2BrPt(B≡O)] (Cy being cyclohexyl) by means of reversible liberation of trimethylsilylbromide from the boron–bromine oxidative addition product of dibromo(trimethylsiloxy)borane and [Pt(PCy3)2] in room-temperature toluene solution. The platinum complex is inert toward oligomerization, even under photolytic conditions and at elevated temperatures. The bromide was substituted by thiophenolate, and spectral parameters of both products as well as results of computational and x-ray diffraction studies are in agreement with the formulation of a triple bond between boron and oxygen. The boron–oxygen distance of 120.5(7) picometers shows a bond shortening of 7.2% as compared with a double bond, which is similar to the shortening observed in carbon–carbon analogs.

Palladium-Catalyzed Cross-Coupling of Internal Alkenes with Terminal Alkenes to Functionalized 1,3-Butadienes Using C-H Bond Activation: Efficient Synthesis of Bicyclic Pyridones

Abstract: Transition-metal-catalyzed cross-coupling through C H bond activation is emerging as one of the most important tools for carbon–carbon bond formation. In general, vinylogous compounds can be synthesized by Wittig, Heck, and Suzuki reactions, from the condensation of carbonyl compounds, C H addition to alkynes, or by means of organometallic alkenyl compounds, but direct alkenylation using C H bond activation remains particularly attractive for constructing carbon–carbon double bonds owing to their synthetic simplicity and use of readily available reagents. Vinylborates, vinyl halides, alkenyl acetates, and cyclic 1,3-dicarbonyls have been known for the direct alkenylation of arene and (hetero)arene C H bonds. In a more simple and synthetically useful alkenylation, terminal alkenes have been applied as the coupling partners. However, little attention has been paid to the direct alkenylation of alkenyl C H bonds with an alkene as the coupling partner using C H bond activation. 1,3-Butadienes, as a class of versatile organic synthetic reagents, have usually been prepared by indirect methods. To date, only two reports have been documented for their direct synthesis, involving coupling two simple terminal alkenes, owing to the difficulty in activating two alkene substrates at the same time (Scheme 1). Although two examples involving the reaction of 3-methyl-1H-indenes with tert-butyl acrylate were also reported, no work has been directed to the direct alkenylation of open-chain internal alkenes with another alkene as the coupling partner. In order to realize the direct cross-coupling of an internal alkene with a terminal alkene, the low reactivity of an internal alkenyl C H bond should be overcome. We envisioned the introduction of a structural element that could increase the reactivity of an internal alkenyl C H bond. Thus, we hypothesized that a 1,2-dithiane group at the terminal position of an alkene should satisfy the requirement on activating an internal alkenyl C H bond, and a-oxoketene dithioacetals were chosen as the internal alkenes. Herein, we report the palladium(II)-catalyzed direct cross-coupling of a-oxoketene dithioacetals with terminal alkenes as well as the synthesis of bicyclic pyridones [Eq. (1)].

A highly specific ferrocene-based fluorescent probe for hypochlorous acid and its application to cell imaging

Abstract: A highly specific ferrocene-based fluorescent probe, (9-anthryl)ethenylferrocene, has been designed, synthesized and characterized for fluorescence imaging of hypochlorous acid (HOCl) in live cells. The design strategy for the probe is based on the strong quenching effect of electron-donor ferrocene on anthracene fluorescence via an intramolecular charge transfer process, and is accomplished through constructing the conjugated molecule by using a cleavable double bond as a linker. The double bond in the probe reacts selectively with HOCl rather than the other reactive oxygen species (e.g., ˙OH, ˙O2−, 1O2, and H2O2) in pH 7.4, accompanied by more than 100-fold fluorescence enhancement. Moreover, the probe is cell membrane permeable, and its applicability has been successfully demonstrated for fluorescence imaging of HOCl in HeLa cells.

Influence of triple bonds as π-spacer units in metal-free organic dyes for dye-sensitized solar cells

Abstract: Four metal-free organic sensitizers (TC101-TC104) with triple bonds in pi-spacers and five reference dyes (TC, TC105, TPC1, D5, and TH208) without triple bonds were applied in dye-sensitized solar cells to study the influence of triple bonds as pi-spacer units on their photoelectrochemical properties and dye-sensitized solar cells (DSCs) performance. Results show that the introduction of triple bond could red-shift the dye's absorption spectrum due to the enhancement of the pi-spacer. However, the spectrum red-shift is much less than that of the introduction of double bond because of more electronegativity of triple bond. The incident photon-to-current conversion efficiency reveals that the electron transfer yield (Phi(nu)(ET)) of DSCs becomes larger with the introduction of triple bond. Electrochemical impedance spectroscopy analysis reveals that the introduction of triple bond almost does not change the electron lifetimes in TiO2 films but decreases the effective diffusion lengths.

Synthesis of 4- and 4,5-functionalized imidazol-2-ylidenes from a single 4,5-unsubstituted imidazol-2-ylidene.

Abstract: Using the nucleophilicity of NHCs and aNHCs, as well as the leaving group ability of the former, the carbon−carbon double bond of imidazol-2-ylidenes can be readily mono- and difunctionalized. These results provide also a new light on the formation of abnormal carbene adducts from classical unsaturated NHCs.

Synthetic and Mechanistic Studies on Pd(0)-Catalyzed Diamination of Conjugated Dienes

Abstract: Various dienes and a triene can be regioselectively diaminated at the internal double bond with good yields and high diastereoselectivity using di-tert-butyldiaziridinone (5) as the nitrogen source and Pd(PPh3)4 (1−10 mol %) as the catalyst. Kinetic studies with 1H NMR spectroscopy show that the diamination is first-order in total Pd catalyst and inverse first-order in PPh3. For reactive dienes, such as 1-methoxybutadiene (6g) and alkyl 1,3-butadienes (6a, 6j), the diamination is first-order in di-tert-butyldiaziridinone (5) and zero-order in the olefin. For olefins with relatively low reactivity, such as (E)-1-phenylbutadiene (6b) and (3E,5E)-1,3,5-decatriene (6i), similar diamination rates were observed when 3.5 equiv of olefins were used. Pd(PPh3)2 is likely to be the active species for the insertion of Pd(0) into the N−N bond of di-tert-butyldiaziridinone (5) to form a four-membered Pd(II) complex (A), which can be detected by NMR spectroscopy. The olefin complex (B), formed from intermediate A via li...

Chemoselective Conjugate Reduction of α,β-Unsaturated Ketones Catalyzed by Rhodium Amido Complexes in Aqueous Media

Abstract: Although a notable feature of Noyori’s Ru−TsDPEN complex is that the transfer hydrogenation reaction is highly chemoselective for the C═O functional group and tolerant of alkenes, our early report indicated that the chemoselectivity could be switched from C═O to C═C bonds in the transfer hydrogenation of activated α,β-unsaturated ketones. Now we have found that a variety of α,β-unsaturated ketones, even without other electron-withdrawing functional groups, could be reduced on the alkenic double bonds with high selectivities employing amido-rhodium hydride complex in aqueous media, and up to 100% chemoselectivity has been achieved. It is notable that the chemoselectivity was improved significantly on going from organic solvent to water. Moreover, a 1,4-addition mechanism has been proposed on the basis of the corresponding experimental details and computational analysis.

Solid-State [2+2] Photodimerization and Photopolymerization of α,ω-Diarylpolyene Monomers: Effective Utilization of Noncovalent Intermolecular Interactions in Crystals

Abstract: [2+2] Photocycloaddition of olefins is a very useful reaction in synthetic organic chemistry to obtain cyclobutane-containing molecules, which are almost inaccessible by other methods. The reaction, when performed in the crystalline state, occurs more efficiently and selectively than in homogeneous solution due to tight and regular molecular arrangement in the crystal state. Despite numerous examples for the solid-state [2+2] photodimerization of monoenes, however, it is still a challenge to prepare not only dimers but also higher oligomers and polymers from conjugated polyenes, which have multiple reactive double bonds in a molecule. In our recent studies of the solid-state photoreactions of α,ω-diarylpolyenes, noncovalent intermolecular interactions in crystals were effectively utilized to prealign molecules in stacking arrangements, suitable for the [2+2] reaction. With appropriate ring-substituents, [2+2] photodimerization and photopolymerization of the polyenes took place, although the degree of polymerization was relatively low. This review will describe the details of these reactions.

Theoretical and experimental studies on N-(6-methylpyridin-2-yl-carbamothioyl)biphenyl-4-carboxamide

Abstract: A novel thiourea derivative, N -(6-methylpyridin-2-yl-carbamothioyl)biphenyl-4-carbox amide, is synthesized and characterized by elemental analysis, FT-IR, NMR and single crystal X-ray diffraction study. There are two independent molecules A and B in the asymmetric unit. The short bond lengths of the C-N bonds in the central thiourea fragment indicate partial double bond character in this fragment of the title compound. These results can be explained by the existence of resonance in this part of the molecule. Each A and B molecule is stabilized with an intramolecular N-H•••O hydrogen bond which results in the formation of a pseudo six membered ring. In addition, the independent molecules are linked into a chain along the c axis by weak N-H•••S intermolecular hydrogen bonds. The conformational behavior and structural stability of the optimized geometry of the title compound were also investigated by utilizing ab- initio calculations with 6-31G* basis set at HF, BLYP, and B3LYP levels. The calculated parameters are in good agreement with the corresponding X-ray diffraction values.

Activation of ammonia by a Si=O double bond and formation of a unique pair of sila-hemiaminal and silanoic amide tautomers.

Abstract: The new silanone complex 3 is accessible in 82% yield and is capable of undergoing addition of ammonia under mild conditions, yielding the sila-hemiaminal 4 and, at the same time, its unique tautomer 5, the first silanoic amide. The unexpected formation of 3 is due to the presence of the basic exocyclic methylene group in the C3N2 ligand backbone. Strikingly, the tautomers 4 and 5 are in equilibrium in solution and can be cocrystallized in benzene or THF solutions having a SiOH···O═Si hydrogen bond as confirmed by single-crystal X-ray diffraction analysis.

Structure and Dynamic Features of an Intramolecular Frustrated Lewis Pair

Abstract: Di(mesityl)cyclohexenylphosphine undergoes hydroboration with Piers' borane [HB(C(6)F(5))(2)] to yield the cyclohexylene-anellated frustrated Lewis pair 5. This P/B pair splits H(2) with the formation of the product 4 and adds to the C=O double bond of phenyl isocyanate to yield 6. In the crystal, compound 5 features a puckered four-membered heterocyclic core structure with a long P-B bond (av. 2.197(5) A). The activation energy of the P-B cleavage of the frustrated Lewis pair 5 was determined by dynamic (19)F NMR spectroscopy at ΔG(≠)(298 K)=12.1±0.3 kcal mol(-1).

Si=X multiple bonding with four-coordinate silicon? Insights into the nature of the Si=O and Si=S double bonds in stable silanoic esters and related thioesters: a combined NMR spectroscopic and computational study.

Abstract: The electronic structures and nature of silicon−chalcogen double bonds Si═X (X = O, S) with four-coordinate silicon in the unique silanoic silylester 2 and silanoic thioester 3 have been investigated for the first time, by 29Si solid state NMR measurements and detailed DFT and ab initio calculations. 29Si solid state NMR spectroscopy of the precursor silylene 1 was also carried out. The experimental and computational study of 2 and 3, which was also supported by a detailed computational study of smaller model systems with Si═O and Si═S bonds, provides a deeper understanding of the isotropic and tensor components of their NMR chemical shifts. The general agreement between the experimental NMR spectra and the calculations strongly support our previous NMR assignment deduced from experiment. The calculations revealed that in 2 δ(29Si(═O))iso is shifted upfield relative to H2Si═O by as much as 175 ppm; the substituents are responsible for ca. 100 ppm of this shift, while the remaining upfield shift is caused ...

Crystal structures of proline-derived enamines

Abstract: The isolation and structural characterization of both aldehyde- and ketone-derived proline enaminones are reported and discussed. Crystal structures of 10 proline enamines provide information on stereochemical aspects, i.e., double bond configuration and syn- vs. anti-positioning of the carboxylate relative to the enamine double bond. Furthermore, the obtained crystal structures are compared with the density functional theory-calculated structures of the ground and transition state and the postulated Seebach–Eschenmoser transition state.

Organocatalyzed Conjugate Addition of Carbonyl Compounds to Nitrodienes/Nitroenynes and Synthetic Applications

Abstract: The purpose of this study is to point out the synthetic utility of a new class of Michael acceptors (nitrodienes and nitroenynes). The highly enantioselective organocatalytic Michael addition of carbonyl compounds to these functionalized nitroolefins has been carried out in the presence of (S)-diphenylprolinol silyl ether to achieve some interesting building blocks in high selectivities. The adducts thus obtained can be easily converted by taking advantage of the corresponding unsaturated carbon-carbon bond. In presence of the double bond, metathesis or electrophilic activation could be carried out whereas in the presence of the triple bond electrophilic activation could be conducted. We thus focused on a gold-catalyzed cyclization of the bis-homopropargylic alcohol to afford the corresponding substituted tetrahydrofuran. Then, we also demonstrated that organic and gold catalysts were compatible in a one-pot process. Indeed, we developed a one-pot enantioselective organocatalytic Michael addition to a nitroenyne followed by a gold-catalyzed acetalization/cyclization to achieve tetrahydrofuranyl ethers in high diastereo- and enantioselectivities with excellent yields.

Recent Developments in Asymmetric Organocatalysis

Abstract: Introduction Nucleophilic additions to electron-deficient C=C double bonds Nucleophilic additions to C=O double bonds Nucleophilic additions to C=N double bonds Nucleophilic additions to unsaturated nitrogen Nucleophilic substitutions at aliphatic carbon Cycloaddition reactions Oxidations Reductions Kinetic resolutions and desymmetrisations Miscellaneous reactions General Conclusion

On the TD-DFT accuracy in determining single and double bonds in excited-state structures of organic molecules.

Abstract: We present an analysis on the behavior of the TD-DFT approach in the determination of excited-state structures with particular attention to single and double bonds. The analysis is based on a direct comparison with the highly correlated CASPT2 ab initio approach. Six DFT exchange-correlation functionals differing in the Hartree−Fock exchange percentage and the type of correlation functional are considered and applied to the study of seven prototype organic molecules characterized by two families of excitations (acrolein, acetone, diazomethane, and propanoic acid anion for n−π* and cis-1,3-butadiene, trans-1,3-butadiene, and pyrrole for π−π*), and three protonated Schiff bases, used as model chromophores for 11-cis retinal. Our analysis allows pinpointing specific correlations between accuracy of the various functionals and category of excitation and/or type of chemical bond involved in the corresponding geometry relaxation. We confirm the role of the long-range correction of the potential to obtain a bala...

Synthesis and structure of [U{C(PPh2NMes)2}2] (Mes = 2,4,6-Me3C6H2): A homoleptic uranium bis(carbene) complex with two formal UC double bonds

Abstract: Treatment of H2C(PPh2NMes)2 (1, Mes = 2,4,6-Me3C6H2) with two equivalents of ButLi afforded the methandiide complex [Li2{C(PPh2NMes)2}2]2 (2); reaction of 2 with [UI3(THF)4] gave [U{C(PPh2NMes)2}2] (3), which is the first homoleptic uranium bis(carbene) complex with two formal UC double bonds.