Olefin Metathesis in Organic Chemistry

Transition metal catalyzed C? C bond formations belong to the most important reactions in organic synthesis. One particularly interesting reaction is olefin metathesis, a metal-catalyzed exchange of alkylidene moieties between alkenes. Olefin metathesis can induce both cleavage and formation of C? C double bonds. Special functional groups are not necessary. Although this reaction—which can be catalyzed by numerous transition metals—is used in industry, its potential in organic synthesis was not recognized for many years. The recent abrupt end to this Sleeping-Beauty slumber has several reasons. Novel catalysts can effect the conversion of highly fictionalized and sterically demanding olefins under mild reaction conditions and in high yields. Improved understanding of substrate–catalyst interaction has greatly contributed to the recent establishment of olefin metathesis as a synthetic method. In addition to the preparation of polymers with fine-tuned characteristics, the metathesis today also provides new routes to compounds of low molecular weight. The highly developed ring-closing metathesis has been proven to be key step in the synthesis of a growing number of natural products. At the same time interesting applications can be envisioned for newly developed variants of bimolecular metathesis. Improvements in the selective cross-metathesis of acyclic olefins as well as promising attempts to include alkynes as viable substrates provide for a vivid development of the metathesis chemistry.     

Synthesis and structural characterization of mixed carbene-carboxylate complexes of palladium(II)

Mixed carbene-carboxylate complexes of Palladium(II) have been prepared by reacting {1,1^′-dimethyl-3,3^′-methylenediimidazoline-2,2^′-diylidene} palladium(II) diiodide (1) [Angew. Chem. 107 (1995) 2602; Angew. Chem. Int. Ed. Engl. 34 (1995) 2371; J. Organomet. Chem. 557 (1998) 93] with AgO₂CR, where R=CF₃, CF₂CF₃ and CF₂CF₂CF₃. In this manner, {1,1^′-dimethyl-3,3^′-methylenediimidazoline-2,2^′-diylidene} palladium(II) bis(trifluo-roacetate) (2), {1,1^′-dimethyl-3,3^′-methylenediimidazoline-2,2^′-diylidene} palladium(II) bis(pentafluoropropionate) (3) and {1,1^′-dimethyl-3,3^′-methylenediimidazoline-2,2^′-diylidene} palladium(II) bis(heptafluorobutyrate) (4) were obtained. All three complexes were fully characterized by ¹H-, ¹³C- and ¹⁹F NMR spectroscopy as well as ESI mass spectrometry. X-ray crystal structure analyses of complexes 3 and 4 reveal mononuclear species with a square planar metal center coordinated by a cis-chelating dicarbene and two monodentate carboxylate ligands. The results show that the introduction of a cis-chelating N,N-heterocyclic carbene ligand stabilizes the palladium-carboxylate moiety effectively.     

Theoretical study on the mechanism of the 1CHCl + NO reaction

The complex doublet potential energy surface of the CHClNO system, including 31 minimum isomers and 84 transition states, is investigated at the QCISD(T)/6-311G(d, p)//B3LYP/6-31G(d, p) level in order to explore the possible reaction mechanism of the singlet CHCl with NO. Various possible isomerization and dissociation channels are probed. The initial association between 1CHCl and NO at the terminal N-site can almost barrierlessly lead to the chainlike adducts HClCNO a (a1, a2) followed by the direct Cl-extrusion to product P9 Cl + HCNO, which is the most feasible channel. Much less competitively, a (a1, a2) undergoes a ring-closure leading to the cyclic isomer c-C(HCl)NO d followed by a concerted Cl-shift and N-O cleavage of d to form the branched isomers ClNC(H)O f (f1, f2). Eventually, f (f1, f2) may take a direct H-extrusion to produce P7 H + ClNCO or a concerted 1,2-H-shift and Cl-extrusion to form P1 Cl + HNCO. The low-lying products P2 HCl + NCO, P3 Cl + HOCN, P14 HCO + 3NCl, P6 ClO + HCN, and P13 ClNC + OH may have the lowest yields observed. Our calculations show that the product distributions of the title reaction are quite different from those of the analogous 1CHF + NO reaction, yet are similar to those of another analogous 3CH2 + NO reaction. The similarities and discrepancies among the three reactions are discussed in terms of the substitution effect. The present article may assist in future experimental identification of the product distributions for the title reaction and may be helpful for understanding the halogenated carbene chemistry.     

含2，4－二硫代乙内酰脲分子片卡宾铁羰基簇合物的合成和结构

Ｆｅ_３（ＣＯ）_（１２）与５个２，４－二硫代乙内酰脲ＳＣＮＨＣ（Ｒ_１）（Ｒ_２）Ｃ（Ｓ）ＮＨ反应制得通式为Ｆｅ_３（ＣＯ）_８（ｕ３－Ｓ）_２（Ｌ）含卡宾配体的５个新铁羰基联合物（１～５），对其进行了元素分析、ＩＲ、~１ＨＮＭＲ和ＭＳ表征，并用Ｘ射线衍射法测定了簇合物３的晶体和分子结构，表明２，４－二硫代乙内酰脲分子片配位基：ＣＮＨＣ（ＣＨ_３）_２Ｃ（Ｓ）ＮＨ的卡宾碳具有ｓｐ~２成键特征，其Ｃ卡宾－Ｆｅ键长０．１８９８ｎｍ，３的分子几何构型维持母体物Ｆｅ_３（ＣＯ）_９（ｕ_３－Ｓ）_２的形状，其中卡宾取代了四方锥分子骨架基底平面Ｆｅ（１）Ｓ（１）Ｆｅ（３）Ｓ（２）的Ｆｅ（１）原子上轴向位置的一个端羰ＣＯ。     

烯烃的交叉复分解反应(CM)及其合成应用

概述了近年来烯烃交叉复分解反应的研究进展及其在有机中间体制备、碳水化合物的合成、高分子化学以及工业生产上的应用.     

Experimental and quantum-chemical study of complexation of carbene analogs with dinitrogen. Direct IR-spectroscopic observation of Cl2Si·N2 complexes in low-temperature argon-nitrogen matrices

Interaction of dichlorosilylene with dinitrogen in mixed Ar—N₂ matrices at 9 - 10 K was studied by IR spectroscopy. A donor-acceptor complex Cl₂Si·N₂ was found and characterized by six bands of symmetric (at 511.2, 508.9, and 506.5 cm^–1) and antisymmetric (at 500.1, 496.9, and 495.1 cm^–1) stretching vibrations of Si—Cl bonds in the most abundant isotopomers. Two bands at 498.7 and 493.5 cm^–1 observed in mixed matrices were tentatively assigned to Cl₂Si·(N₂)₂ complex. Several stretching vibration bands of minor isotopomers of SiCl₂ were detected for the first time in argon matrices. Assignment has been done for the isotopic structure of SiCl₂ associates with dinitrogen observed in N₂ matrices. Dimerization of SiCl₂ and its complexation with one and two N₂ molecules were studied by quantum-chemical DFT calculations (PBE and B3LYP functionals). The structures, energies, and vibrational frequencies of the Cl₂Si·N₂ and Cl₂Si·(N₂)₂ complexes and the Si₂Cl₄ dimer were determined. The energies of SiCl₂ complexation with one and two N₂ molecules obtained from PBE and B3LYP calculations are 0.3 and 0.6 kcal mol^–1, respectively. More accurate G2(MP2,SVP) calculations using the B3LYP geometries have predicted a higher stability of the Cl₂Si·N₂ complex (1.2 kcal mol^–1). The calculated and experimental vibrational frequencies of reagents and complexes are in good agreement. A correlation has been established between the PBE calculated energies of complexation of EHal₂ (E = Si, Ge, Sn, Pb) with N₂ and the experimentally observed shifts of E—Hal stretching vibrations in EHal₂ upon complexation. The strength of the complexes with N₂ increases on going from dihalosilylenes to dihaloplumbylenes.     

Co(III), Rh(III) & Ir(III)-Catalyzed Direct C−H Alkylation/Alkenylation/Arylation with Carbene Precursors

Metal carbenes play a pivotal role in transition-metal-catalyzed synthetic transfer reactions. The metal carbene is generated either from a diazo compound through facile extrusion of N₂ with a metal catalyst or in situ generated from other sources like triazoles, pyriodotriazoles, sulfoxonium ylides and iodonium-ylide. On the other hand, Co(III), Rh(III) & Ir(III)-catalyzed C−H functionalizations have been well established as a key synthetic step to enable the construction of various synthetic transformations. Interestingly, in recent years, merging of these two concepts C−H activation and carbene migratory insertion gained much attention, in particular group 9 metal-catalyzed arene C−H functionalizations with carbene precursors via carbene migratory insertion. In this review, we summarize recent advances in Co(III), Rh(III) & Ir(III)-catalyzed direct C−H alkylation/alkenylation/arylation with carbene precursors and also discuss key synthetic intermediates within the catalytic cycles.