Carbon-Carbon Bond Forming Reactions via Photogenerated Intermediates.

The Introduction of the Radical Cascade Reaction into Polymer Chemistry: A One-Step Strategy for Synchronized Polymerization and Modification.

Review: 485 refs.

Graphynes are a family of two-dimensional carbon materials combining atoms in the sp²- and sp-hybridized state with an ordered arrangement in their structure. One of the promising methods for their synthesis is the Sonogashira cross-coupling reaction, which provides the formation of carbon-carbon bonds between an aryl halide and a terminal alkyne. Traditionally, this process is catalyzed by palladium compounds, but in this work, the possibility of synthesizing γ-graphyne using a nickel catalyst is demonstrated for the first time. The synthesis was carried out at a temperature of 60°C for 24 hours, using hexaiodobenzene as an aryl halide component and calcium carbide as a source of acetylene fragments. The use of IR and Raman spectroscopy confirmed the formation of triple carbon-carbon bonds in the obtained material. According to elemental analysis, the carbon content in the sample was 87.51 at.%. The morphology study by scanning electron microscopy revealed a characteristic flaky structure formed by γ-graphyne layers. X-ray phase analysis recorded a shift of the main diffraction maximum to 24.5° 2θ, which differs significantly from the position of the graphite reflection (26.5° 2θ) and indicates a modification of the crystal structure due to the introduction of acetylene bridges.

Organoboron chemistry continues to make an ever-increasing impact on general methodology in synthetic organic chemistry. Over the recent years advances in the reduction, hydroboration, and coupling protocols have served to broaden the synthetic chemists arsenal with methods illustrating the utility of boron. If one were to include boron enolate chemistry in conjunction with the previously mentioned areas this would encompass most of the literature related to organoboron chemistry. What remains paramount to the synthetic organic chemist are those synthetic methods that result in the formation of carbon-carbon bonds. With respect to organoboron chemistry, the most widely used methods (reduction and hydroboration) do not result in carbon-carbon bond formation. Although boron enolate chemistry and Suzuki based coupling reactions result in the formation of carbon-carbon bonds, it could be argued that boron is not directly involved in the formation of these bonds. However, there exists an area of organoboron chemistry that has been sporadically visited but has yet to be categorized. What is presented here is a review of the carbon-carbon bond forming reactions that are mediated through direct transfer of an appendage from boron to result in a new carbon-carbon bond. This review of boron mediated transfer reactions is not meant to be exhaustive, but it does span ∼40 years in an attempt to assemble a number of examples that best illustrate this category, and to provide a template for substantial expansion of this area. Keywords: boron mediated transfer, organoboron chemistry, hydroboration, coupling protocols, boron enolate chemistry, boron mediated transfer reactions

This review examines the use of baker's yeast as a reagent in organic synthesis, with an emphasis on the developments of the last 15 years. The transformations of various classes of compounds are examined, for example, reduction or formation of double bonds, acyloin condensation. The equivalent microbial transformations are compared in relevant cases. 1. Introduction 2. Reduction of β-Keto Esters 3. Reduction of other Keto Acid Derivatives 4. Reduction of α-Hydroxy Aldehydes and Ketones to Vicinal Diols 5. Reduction of Ketones with a Sulfur Containing Functional Group on the α-Carbon 6. Reduction of β-Diketones 7. Reduction of Carbon-Carbon Double Bonds 7.1. Preparation of Bifunctional Chiral Synthetic Building Blocks 7.2. Reduction of Trisubstituted Double Bonds of Larger Synthetic Intermediates 7.3. Reduction of other Trisubstituted Double Bonds 8. Formation of Carbon-Carbon Bonds 8.1. Acyloin Condensation 8.2. Enzymatic Cyclization of Squalenoids 8.3. Carbon-Carbon Bond Formation through Michael-Type Addition 9. Reduction of Carbonyl Groups for the Preparation of Optically Active Fluorinated Compounds 10. Reduction of Heterocyclic Ketones 11. Reduction of Aromatic and Unsaturated Ketones 12. Kinetic Resolution of Cyclic Racemic Ketones 13. Reduction of Nitro Containing Compounds 14. Ester Hydrolysis 15. Water Addition Across the Double Bond of α,β-Unsaturated Aldehydes 16. Dehydrogenation of Unnatural Fatty Acids

The formation of carbon-carbon bonds by radical cyclisation has become an important tool for the synthesis of natural products containing heterocyclic rings [1]. The majority of radical cyclisations in heterocyclic chemistry are still carried out using tributyltin hydride as the reducing agent. This has disadvantages, as the tributylin hydride is toxic and it is difficult to completely remove the tin species from the reaction mixture. The electrochemical version of similar intramolecular processes - in reactions involving aryl or vinyl bromides [2], bromoacetals [2], or α-bromoamides [2] with double or triple bonds in the presence of nickel(II) complexes as catalysts - has been shown to be highly efficient. The reaction can be performed under mild conditions using relatively simple equipment. It also avoids the hazardous or toxic reagents used in the tributyltin hydride method. Although most of the reactions have been reported in aprotic solvents, their toxicity, hazards and cost make them unattractive. This makes the search for non-toxic fluid alternatives highly desirable. Ionic liquids (ILs) represent a class of alternative solvents currently receiving serious consideration, as they have environmental and technological benefits [3]. Their characteristics render them promising replacements for volatile organic solvents (VOCs). As an extension of our previous research, we undertake further studies on analogs/derivatives of bromopropargyl esters as substrates. In the present work, we investigate the reductive intramolecular cyclization of bromopropargyl ethers derivatives, catalysed by electrogenerated (1,4,8,11-tetramethyl-1,4,8,11-tetraaza-cyclotetradecane)nickel(I), [Ni(tmc)]+ as the catalysts in N,N,N-trimethyl-N-(2-hydroxyethyl)ammonium bis(trifluoromethylsulfonyl)imide,[N1 1 1 2(OH)][NTf2] and 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, [C2mim][NTf2] by cyclic voltammetry and controlled-potential electrolysis. The results show that the reaction leads to the formation of the expected cyclic compounds, which are important intermediates in the synthesis of natural products with possible biological activities [4]. ACKNOWLEDGEMENTS We are grateful to the Fundação para a Ciência e Tecnologia (PestC/QUI/UI0686/2013 and FCOMP-01-0124-FEDER-037302) for financial support of this work. REFERENCES 1. B. Giese, “Radicals in Organic Synthesis: Formation of Carbon-Carbon Bonds”, Pergamon Press, Oxford (1986). 2. E. Dunach, M.J. Medeiros, S. Olivero, New J. Chem., 30, 1534 (2006). 3. Ionic liquids IIIB: Fundamentals, Process, Challenges, and Opportunities, ACS Symp. Ser. (Eds.: R. D. Rogers and K. R. Seddon), ACS, Washington DC, 2005. 4. P.K. Mandal, G. Maiti, S.C. Roy, J. Org. Chem., 63, 2829 (1998).

In the presence of a silyl triflate and an amine base, a nickel–phosphine complex catalyzes the direct conjugate addition of ethylene, α-olefins, and aryl alkenes to unsaturated aldehydes and ketones. The enolsilane products are isolated in good to very high yield, and in very high stereoselectivity for some cases. The alkene is a functional equivalent of an alkenylmetal reagent in the transformation. Supporting information for this article is available on the WWW under http://www.wiley-vch.de/contents/jc_2002/2008/z705163_s.pdf or from the author. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

Review: 229 refs.

Abstract

Reliable thermochemistry is computed for infinite stretches of pure-carbon materials including acetylenic and cumulenic carbon chains, graphene sheet, and single-walled carbon nanotubes (SWCNTs) by connection to the properties of finite size molecules that grow into the infinitely long systems. Using ab initio G3 theory, the infinite cumulenic chain (:C[double bond]C[double bond]C[double bond]C:) is found to be 1.9+/-0.4 kcal/mol per carbon less stable in free energy at room temperature than the acetylenic chain (.C[triple bond]C-C[triple bond]C.) which is 24.0 kcal/mol less stable than graphite. The difference between carbon-carbon triple, double, and single bond lengths (1.257, 1.279, and 1.333 A, respectively) in infinite chains is evident but much less than with small hydrocarbon molecules. These results are used to evaluate the efficacy of similar calculations with the less rigorous PM3 semiempirical method on the (5,5) SWCNT, which is too large to be studied with high-level ab initio methods. The equilibrium electronic energy change for C(g)-->C[infinite (5,5) SWCNT] is -166.7 kcal/mol, while the corresponding free energy change at room temperature is -153.3 kcal/mol (6.7 kcal/mol less stable than graphite). A threefold alternation (6.866, 6.866, and 6.823 A) in the ring diameter of the equilibrium structure of infinitely long (5,5) SWCNT is apparent, although the stability of this structure over the constant diameter structure is small compared to the zero point energy of the nanotube. In general, different (n,m) SWCNTs have different infinite tube energetics, as well as very different energetic trends that vary significantly with length, diameter, and capping.

Enzymatic carbon-carbon (C-C) bond formation reactions have become an effective and invaluable tool for designing new biological and medicinal molecules, often with asymmetric features. This review provides a systematic overview of key C-C bond formation reactions and enzymes, with the focus of reaction mechanisms and recent advances. These reactions include the aldol reaction, Henry reaction, Knoevenagel condensation, Michael addition, Friedel-Crafts alkylation and acylation, Mannich reaction, Morita-Baylis-Hillman (MBH) reaction, Diels-Alder reaction, acyloin condensations via Thiamine Diphosphate (ThDP)-dependent enzymes, oxidative and reductive C-C bond formation, C-C bond formation through C1 resource utilization, radical enzymes for C-C bond formation, and other C-C bond formation reactions.

Chapter 17 - Formation of Carbon-Carbon Bonds Via Radicals and Carbenes

The formation of polyynes with an odd number of conjugated triple bonds is synthetically demanding, particularly for complex architectures, since most established methods rely on irreversible C-C bond-forming reactions and operate under kinetic control. To address this problem, we have developed the synthesis of triynes via molybdenum-catalyzed alkyne metathesis of diynes, which allows thermodynamic control through reversible cleavage and formation of carbon-carbon triple bonds. We demonstrate the potential of this method through the synthesis of challenging, more complex products, including triyne precursors to [7]cumulenes, a pentayne, a heptayne, and dehydrobenzannulene macrocycles containing triynes. The key to the success of the proposed methodology is achieving a balance between the selectivity and reactivity of the molybdenum catalyst with the diyne precursors.

The synthesis of substituted tertrahydrofuran (THF) has been important because they are ubiquitous in many natural products such as annonecious acetogenins, polyether antibiotics and C-nucleosides. The efficient and stereoselective manner of the preparation of the substituted THF has been a significant challenge for synthetic chemists. There are numerous synthetic methodologies that involve the preparation of the multisubstituted THFs. Among those, the formation of carbon-oxygen or carboncarbon bonds via intramolecular SN1, SN2 or SN2' addition reactions is worthy to mention as the effective approaches to these hetero cyclic compounds. An approach that has been envisioned by us is to employ a methodology involving a stereoselective preparation of allenes by the SN2' hydride addition to an alkynyloxirane.

The equilibrium molecular structure of the octatetranyl anion, C8H(-), which has been recently detected in two astronomical environments, is investigated with the aid of both ab initio post-Hartree-Fock and density functional theory (DFT) calculations. The model chemistry adopted in this study was selected after a series of benchmark calculations performed on molecular acetylene for which accurate gas-phase structural data are available. Geometry optimizations performed at the CCSD/6-311+G(2d,p), QCISD/6-311+G(2d,p), and MP4(SDQ)/6-311+G(2d,p) levels of theory yield for C8H(-) an interesting polyyne-type structure that defies the chemical formula displaying a simple alternation of triple and single carbon-carbon bonds, [:C[triple bond]C-C[triple bond]C-C[triple bond]C-C[triple bond]CH](1-). In the optimized geometry of C8H(-), as one proceeds from the naked carbon atom on one side of the chain to the CH unit on the opposite side of the chain, the short (formally triple) carbon-carbon bonds decrease in length from 1.255 to 1.213 A whereas the long (formally single) carbon-carbon bonds increase (albeit only slightly) in length from 1.362 to 1.378 A (CCSD results). In striking contrast, both MP2 and DFT (B3LYP and PBE0) calculations fail in reproducing the pattern of the carbon-carbon bond lengths obtained with the CCSD, QCISD, and MP4 methods. The structures of three shorter n-even chains, C(n)H(-) (n = 2, 4, and 6), along with those of four n-odd compounds (n = 3, 5, 7, and 9) are also investigated at the CCSD/6-311+G(2d,p) level of theory.