The formation of metal—carbon bonds via insertion reactions in gaseous negative ion/molecule reactions

The negative ion mass spectra under chemical ionization conditions for a series of copper(II), nickel(II), and cobalt(II) Schiff base complexes have been measured. Reagent gases employed include methane, isobutane, and methane-d₄. For all complexes, molecular negative ions were produced with all three reagent gases via secondary electron capture processes. Ion/molecule reactions between the molecular negative ion and the neutral reactant gas appear to be the dominant processes for the formation of secondary ions. The secondary reactions lead to the incorporation of the CH₂ moiety into the nickel(II) complexes and the CH₃ group into the cobalt(II) compounds.     

有机合成中的C=C键形成新方法: 原子经济的金属复 分解闭环反应

综述了金属复分解闭环反应最新进展。金属复分解闭环反应已成为有机合成中的非常重要的方法,广泛应用于碳环、杂环化合物及天然产物的合成。原子经济的、绿色化学的优点展示出更加诱人的前景。     

Borrowing hydrogen: iridium-catalysed reactions for the formation of C-C bonds from alcohols

Alcohols have been employed as substrates for C-C bond-forming reactions which involve initial activation by the temporary removal of hydrogen to form an aldehyde. The intermediate aldehyde is converted into an alkene via a Horner-Wadsworth-Emmons reaction, nitroaldol and aldol reactions. The 'borrowed hydrogen' is then returned to the alkene to form a C-C bond.     

Synthesis of N-heterocyclic compounds via ene-yne metathesis reactions

Propargylamino and allylamino derivatives of cyclohexene and norbornene were subjected to tandem metathesis reactions with first and second generation Grubbs' catalysts 1 and 2. Results show that the method is compatible with suitably protected nitrogen-containing compounds. Cyclohexenes gave intriguing results in terms of the possibility to perform ring rearrangement metathesis (RRM) reactions, showing a difference with the analogous allyl and propargyl ether substrates.     

Tandem catalysis: generating multiple contiguous carbon-carbon bonds through a ruthenium-catalyzed ring-closing metathesis/Kharasch addition

Tandem catalysis can offer unique and powerful strategies for converting simple starting materials into more complex products in a single reaction vessel while generating less waste and minimizing handling. In this regard, Grubbs' ruthenium alkylidene (Cy3P)2Cl2Ru=CHPh is shown to catalyze two mechanistically distinct transformations to offer a unique protocol that effects multiple bond changes in a single operation. A tandem ruthenium-catalyzed olefin ring-closing metathesis (RCM)/Kharasch addition allows for the facile preparation of bicyclic [3.3.0], [4.3.0], and [5.3.0] ring systems in one step from the appropriately functionalized acyclic precursors. For substrates where the intramolecular Kharasch addition fails, an intermolecular Kharasch addition is possible. By combining the intra- and intermolecular Kharasch additions with RCM, three new contiguous carbon-carbon bonds with multiple stereocenters can be generated by the ruthenium catalyst in a controlled fashion in one operation through two mechanistically distinct pathways.     

Inner and outer phases of antibonding orbitals of transition metal bonds: olefin metathesis

The frontier orbital theory is applied to ruthenium olefin metathesis. The formal [2+2] cycloaddition step, that is, the key step involved in the catalytic cycle of the reaction, is found to be favored by the phases of the HOMO and LUMO, in sharp contrast to [2+2] cycloaddition reactions between olefins. In the LUMO of transition metal part, a d-orbital overlaps out of phase with the vacant p-orbital of the carbene in the inner space of the metal–carbon π bond as is expected, but the remote lobe of the d orbital overlaps in phase in the outer space of the bond. This is a characteristic feature of the antibonding orbitals of transition metal bonds. The outer orbital phase plays more important role in the interaction.     

Olefin metathesis as an inorganic synthetic tool: cross and ring closing metathesis reactions of diruthenium-bound omega-alkene-alpha-carboxylates

Diruthenium compounds containing one omega-alkene-alpha-carboxylate ligand, Ru2Cl(D(3,5-Cl2Ph)F)3(O2C(CH2)nCH=CH2) (n=1 (1a) and 2 (1b)), were prepared from the reaction between Ru2Cl(D(3,5-Cl2Ph)F)3(O2CCH3) (D(3,5-Cl2Ph)F=N,N'-bis(3,5-dicholorophenyl)formamidinate) and the corresponding omega-alkene-alpha-carboxylic acid. Compounds 1a and 1b both underwent olefin cross metathesis reactions catalyzed by (Cy3P)2Cl2Ru(=CHPh) to afford the dimerized compounds [Ru2Cl(D(3,5-Cl2Ph)F)3]2(mu-O2C(CH2)nCH=CH(CH2)nCO2) (n=1 (2a) and 2 (2b)). Similarly, diruthenium compounds containing two omega-alkene-alpha-carboxylate ligands, cis-Ru2Cl(D(3,5-Cl2Ph)F)2(O2C(CH2)nCH=CH2)2 (n=1 (3a), 2 (3b), and 3 (3c)), were prepared by substituting the acetate ligands in cis-Ru2Cl(D(3,5-Cl2Ph)F)2(O2CCH3)2 with the corresponding omega-alkene-alpha-carboxylate ligands. Compounds 3 exhibited different reactivity under olefin metathesis conditions: both 3b and 3c underwent the intramolecular ring closing reaction quantitatively to afford compounds cis-Ru2(D(3,5-Cl2Ph)F)2(mu-O2C(CH2)nCH=CH2(CH2)nCO2)Cl with n=2 (4b) and 3 (4c), respectively, but 3a displayed no metathesis reactivity. Molecular structures of compounds 1a/1b, 2a/2b, 3a/3b, and 4b were established via X-ray diffraction studies, confirming the formation of cross and ring closing metathesis products. Voltammograms of compounds 2 are nearly identical to those of compounds 1, indicating the absence of electronic interactions mediated by the tether derived from olefin metathesis.     

N-tosyloxycarbamates as a source of metal nitrenes: rhodium-catalyzed C-H insertion and aziridination reactions

The rhodium-catalyzed decomposition of N-tosyloxycarbamates to generate metal nitrenes which undergo intramolecular C-H insertion or aziridination reaction is described. Aliphatic N-tosyloxycarbamates produce oxazolidinones with high yields and stereospecificity through insertion in benzylic, tertiary, and secondary C-H bonds. Intramolecular aziridination occurs with allylic N-tosyloxycarbamates to produce aziridines as single diastereomers. The reaction proceeds at room temperature using a rhodium catalyst and an excess of potassium carbonate and does not require the use of strong oxidant, such as hypervalent iodine reagents. A rhodium nitrene species is presumably involved, as both reactions are stereospecific.     

Catalytic asymmetric ring-opening metathesis/cross metathesis (AROM/CM) reactions. Mechanism and application to enantioselective synthesis of functionalized cyclopentanes

Studies regarding the first examples of catalytic asymmetric ring-opening metathesis (AROM) reactions are detailed. This enantioselective cleavage of norbornyl alkenes is followed by an intermolecular cross metathesis with a terminal olefin partner; judicious selection of olefin is required so that oligomerization and dimerization side products are avoided. Results outlined herein suggest that the presence of suitably positioned heteroatom substituents may be critical to reaction efficiency. Mo-catalyzed tandem AROM/CM affords functionalized cyclopentyl dienes in >98% ee and >98% trans olefin selectivity; both secondary and tertiary ether products can be obtained. The examples provided include the catalytic synthesis of an optically pure cyclopentyl epoxide and dimethyl acetal. Mechanistic studies suggest that it is the more substituted benzylidene or silylated alkylidenes that are involved in the catalytic process (vs the corresponding Mo-methylidenes). Although electron rich benzylidenes react more efficiently, the derived electron poor Mo complexes promote AROM/CM transformations as well; alkylidenes that bear a boron substituent are unreactive.     

Osmium-carbon multiple bonds: Reduction and C-C coupling reactions

This paper shows that the redox equilibria hydride-alkenylcarbyne/alkenylcarbene and alkenyl-alkenylcarbyne/dienylcarbene are readily governed by the electronic properties of the ligands of the complexes. Because we have learned to control the position of these equilibria, we are able to build, step by step, osmium derivatives with cyclic alkenylcarbene ligands and osmacyclopentapyrrole complexes. The dihydride OsH₂Cl₂(PⁱPr₃)₂ reacts with alkynols, allenes, enynes, and dienes to give hydride-alkenylcarbyne derivatives, OsHCl₂(CCR′CR₂)(PⁱPr₃)₂, which can be transformed into dicationic species by replacement of chloride ligands by acetonitrile molecules. The selective deprotonation of the alkenylcarbyne ligand of [OsH(CCHCPh₂)(CH₃CN)₂(PⁱPr₃)₂]²⁺ affords the hydride-allenylidene [OsH(CCCPh₂)(CH₃CN)₂(PⁱPr₃)₂]⁺, which undergoes the reduction of the C_α-C_β double bond of the cumulene in the presence of alcohols. The insertion of monosubstituted alkynes into the Os-H bond of the hydride-allenylidene complex leads to alkenyl-allenylidene derivatives, which are transformed into dienylcarbene compounds. The coordination of carbon monoxide to the osmium atom of the latter promotes the 4π-conrotation of the dienylcarbene ligand, to afford a cyclic alkenylcarbene complex via an η¹-cyclopentadienyl intermediate. Through a similar cyclization, in acetonitrile under reflux, the alkenyl-allenylidene complexes are converted into osmapyrrole derivatives by means of the formation of three C-C bonds involving the three carbon atoms of the cumulene, the alkenyl ligand, and an acetonitrile molecule.     

Two types of Kramers rate equations for reactions in condensed media

Two cases of bilinear coupling of a particle to a medium are compared. They differ in that one of the mediums does not modify the particle potential, whereas the other does. Two corresponding kramers-type rate equations for adiabatic reactions are derived directly from a dynamic rate theory and interpreted from the view of the stochastic theory. Both equations become identical in the weak coupling limit. Their relations to transition-state theory are also discussed. Corresponding results for nonadiabatic reactions are considered. © 1994 John Wiley & Sons, Inc.     

Comparative studies of geometric and quasielastic characteristics of PP and SS bonds

It is demonstrated that density functional theory in B3PW91/aug-cc-pVTZ formulation provides a reasonable estimate of the geometrical and vibrational parameters of various PP and SS bonds. Calculated quasielastic characteristics of SS bonds, such as relaxed force constants f(SS), are approximately twice as large as f(PP). This means much higher elasticity of the PP bond, compared to the SS bond. From the dependence of the relaxed force constants on the formal bond order, the various PP bonds studied represent single bonds. By contrast, the more than twice as high rigidity of the SS bonds implies multiple SS bonding. Pronounced shortening of the SS bond on passing from less electronegative to more electonegative substituents at the sulfur atoms can be ascribed to the corresponding increase of the SS pi interactions. Essentially single PP bonds are not affected by electronic effects of this kind, which is the reason for the small variability of PP bond lengths in the gas phase. At the same time, intermolecular forces acting in crystals may strongly influence the length of highly elastic PP bonds.     

Hydrogenation of conjugated CC and CO bonds: Quantum chemical preview before metal catalysis

Hydrogenation of acrolein, ethylene and formaldehyde by molecular hydrogen in gas phases, as a preview before metal catalysis, is investigated using density functional theory (PW91 and B3LYP), ab initio (MP2), and composite theoretical methods (G2, CBS-QB3, and CBS-APNO). Compared to the most accurate CBS-APNO method used in this paper, PW91 functional underestimates the barrier while B3LYP reproduces close results. All the reaction barriers are predicted to be no less than 70 kcal/mol and the CC hydrogenation is thermodynamically favored. Kinetically, however, the hydrogenation of CO bond is more favorable than that of the CC bond, especially for the isolated CO. The conjugation effect in acrolein greatly reduces the kinetic preference for the isolated CO hydrogenation. It is revealed that there is a good relationship between the energy barrier and the increase of the molecular H₂ bond length from the reactant to transition state.     

Regioselective metathesis reactions of various polyunsaturated ketones and alcohols

The reactivity for metathesis reaction of some polyunsaturated cyclopentanols, and of a polyunsaturated ketone bearing vinyl groups or vinyl and propenyl groups has been examined. In the presence of two of these reactants, depending on the ruthenium catalyst employed, either the ring closing metathesis or the cross‐coupling metathesis was observed. Copyright © 2010 John Wiley & Sons, Ltd.     

Cross-metathesis and ring-closing metathesis reactions of amino acid-based substrates

Olefin tethers of variable length, introduced into a natural amino acid (side-chain of Ser, Cys; N-terminus of Arg; C-terminus of Phe and Tic; and in both the side-chain and either the N- or C-terminus of Ser, Cys and Tyr), undergo metathesis on treatment with Grubbs' second generation catalyst. Side-chain linked dimers of Ser, Cys and Tyr were obtained by cross-metathesis, while olefin installation at the N- and C-terminus led to dimers of Arg and Phe (or Tic), respectively. Ring-closing metathesis of the doubly alkenylated derivatives of Ser, Cys and Tyr gave 12-, 20- and 24-membered macrocycles.     

Structural diversity in copper-sulfur chemistry: synthesis of novel Cu/S clusters through metathesis reactions

With the ultimate goal of understanding the Cu(4)S cluster in nitrous oxide reductase, studies of the fundamental chemistry of nitrogen-donor ligand-supported copper-sulfur species have been pursued. Reactions of Cu(II)X(2) (X = Cl(-) or CF(3)SO(3)(-)), N,N,N',N'-tetramethyl-trans-(1R,2R)-diaminocyclohexane, and Li(2)S or Na(2)S(2) yielded clusters that contain [Cu(2)(micro-S(2))(2)](2+), [Cu(3)(micro-S)(2)](3+), [Cu(4)(micro-S(2))(2)](4+), and/or [Cu(6)(micro-S(2))(4)](4+) cores, depending on the specific reaction conditions, notably the nature of X and the sulfur source used. Copper(II) and/or Copper(III) and variable sulfur oxidation levels, including S(2-), S(2)(2-), and S(2)(-*), were identified by X-ray crystallography and spectroscopy.     

Synthesis of sugar-derived spiroaminals via lactamization and metathesis reactions

A series of sugar-derived spiroaminals has been synthesized by utilizing cross metathesis, ring closing metathesis and lactamization reactions as key steps from 1-C-alkylated glycosyl azides and important correlations in the spectral data between spiroaminals and their respective anomers are reported.