C-H bond activation and subsequent C-C bond formation promoted by osmium: 2-vinylpyridine-acetylene couplings

Complex OsH2Cl2(PiPr3)2 promotes the C-H activation of 2-vinylpyridine and subsequently couples the activated substrate with a second 2-vinylpyridine and two acetylene molecules. In the absence of 2-vinylpyridine, the activated substrate is coupled with an acetylene unit to afford a 2-butadienylpyridine derivative.     

Selective,catalytic carbon-carbon bond activation and functionalization promoted by late transition metal catalysts

The selective catalytic activation and functionalization of carbon-carbon bonds in a series of substituted cyclopropane substrates has been developed using commercially available transition metal catalysts. Catalytic hydrogenation and olefination procedures, tolerant of a range of functional groups, have been discovered. Introduction of a chelate-assisting substituent such as [PPh2] is effective in altering the kinetic selectivity and lowering the activation barrier for the catalytic processes.     

Heterolytic activation of hydrogen as a trigger for iridium complex promoted activation of carbon-fluorine bonds

The cationic iridium(III) complex [IrCF(3)(CO)(dppe)(DIB)][BARF](2) where DIB = o-diiodobenzene, dppe = 1,2-bis(diphenylphosphino)ethane, and BARF = B(3,5-(CF(3))(2)C(6)H(3))(4)(-) undergoes reaction in the presence of dihydrogen to form [IrH(2)(CO)(2)(dppe)](+) as the major product. Through labeling studies and (1)H and (31)P[(1)H] NMR spectroscopies including parahydrogen measurements, it is shown that the reaction involves conversion of the coordinated CF(3) ligand into carbonyl. In this reaction sequence, the initial step is the heterolytic activation of dihydrogen, leading to proton generation which promotes alpha-C-F bond cleavage. Polarization occurs in the final [IrH(2)(CO)(2)(dppe)](+) product by the reaction of H(2) with the Ir(I) species [Ir(CO)(2)(dppe)](+) that is generated in the course of the CF(3) --> CO conversion.     

Intermolecular C-H bond activation promoted by a titanium alkylidyne

The transient titanium alkylidyne complex (PNP)TiCtBu (PNP = N-[2-P(CHMe2)2-4-methylphenyl]2-), prepared from alpha-hydrogen abstraction of the corresponding alkylidene-alkyl species (PNP)Ti=CHtBu(CH2tBu), can readily undergo intermolecular 1,2-addition of C-H bonds of benzene and SiMe4. Synthesis and reactivity, isotopic labeling, kinetics, and theoretical studies strongly favor an alkylidyne pathway and the alpha-H abstraction step to be the rate-determining step.     

Facile functionalization of a metal carbon bond by O-atom transfer

The facile conversion of M-R to M-OR that could be useful for the functionalization of electron-rich metal alkyl intermediates is shown to proceed via a Baeyer-Villiger-type pathway involving a nonredox, electrophilic, O-atom insertion in reactions with non-peroxo O-donors.     

过渡金属催化C-H键活化的硅氢化反应

过渡金属催化C-H键活化的硅氢化反应在材料科学和合成化学领域里具有重要意义。有机硅化合物在纺织、橡胶、机械、日化等材料领域有广泛应用,此外,它还是重要的有机合成中间体,作为亲核试剂应用到Hiyama偶联反应,反应具有经济、高效、环境友好等特点。近些年来,很多课题组在该领域进行研究,并取得了一定的成果_^[1]。我们将从过渡金属催化芳基/烷基C-H键活化的硅氢化反应出发,介绍近些年在此领域的研究进展。     

Ab initio coupled cluster determination of the equilibrium structures of cis- and trans-1,2-difluoroethylene and 1,1-difluoroethylene

The equilibrium structures of cis- and trans-1,2-difluoroethylene and 1,1-difluoroethylene, C(2)H(2)F(2), have been determined with high-level coupled cluster techniques combined with large basis sets, explicit consideration of core/valence, and scalar relativistic and higher order correlation effects. Excellent agreement was found with new semiexperimental structures, increasing the level of confidence in both approaches. Differences in bond lengths among ethylene and the fluoroethylenes are discussed.     

Chelation-assisted carbon-hydrogen and carbon-carbon bond activation by transition metal catalysts

Herein we describe the chelation-assisted C-H and C-C bond activation of carbonyl compounds by Rh1 catalysts. Hydroacylation of olefins was accomplished by utilizing 2-amino-3-picoline as a chelation auxiliary. The same strategy was employed for the C-C bond activation of unstrained ketones. Allylamine 24 was devised as a synthon of formaldehyde. Hydroiminoacylation of alkynes with allylamine 24 was applied to the alkyne cleavage by the aid of cyclohexylamine.     

Transition metal catalyzed carbon-silicon bond forming reactions using chlorosilanes promoted by Grignard reagents

New catalytic C--Si bond-forming reactions using chlorosilanes are described. These reactions proceed efficiently under mild conditions by the combined use of Grignard reagents and transition metal catalysts, such as Ti, Zr, Ni, and Pd. It is proposed that ate complex intermediates formed by the reaction of transition metals with Grignard reagents play important roles as the active catalytic species. The present study demonstrates the practical use of chlorosilanes in transition metal catalyzed silylation reactions providing convenient methods for allyl- or vinylsilane synthesis. The reaction pathways of these transformations as well as the scope and limitations are discussed.     

N-h and N-C bond activation of pyrimidinic nucleobases and nucleosides promoted by an osmium polyhydride

Complex OsH(6)(P(i)Pr(3))(2) (1) reacts with 1-methylthymine and 1-methyluracil to give OsH(3)(P(i)Pr(3))(2)(nucleobase') (2, 3) containing the deprotonated nucleobases (nucleobase') κ(2)-N,O coordinated by the nitrogen atom at position 3 and the oxygen bonded to the carbon atom of the ring at position 4. Similarly, the reactions of 1 with thymidine, 5-methyluridine, deoxyuridine, and uridine lead to OsH(3)(P(i)Pr(3))(2)(nucleoside') (4-7) with the deprotonated nucleoside (nucleoside') κ(2)-N,O coordinated by the nitrogen atom at position 3 and the oxygen bonded to the carbon atom at position 4 of the nucleobases. Treatment of complexes 5 and 7, containing nucleosides derived from ribose, with OsH(2)Cl(2)(P(i)Pr(3))(2) (8) in the presence of Et(3)N affords dinuclear species OsH(3)(P(i)Pr(3))(2)(nucleobase')-(ribose)(P(i)Pr(3))(2)H(2)Os (9, 10) formed by two different metal fragments. Complex 1 also promotes the cleavage of the N-C bond of 2-7 to give the dinuclear species {OsH(3)(P(i)Pr(3))(2)}(2)(nucleobase') (11, 12) with the nucleobase skeleton (nucleobase') κ(2)-N,O coordinated to both metal fragments. These compounds can be also prepared by reaction of 1 with 0.5 equiv of thymine and uracil. The use of 1:1 hexahydride:nucleobase molar ratios gives rise to the preferred formation of the mononuclear complexes OsH(3)(P(i)Pr(3))(2)(nucleobase') (13, 14; nucleobase' = monodeprotonated thymine or uracil). The X-ray structures of complexes 6, 11, and 14 are also reported.     

Molecular structure of 1,1-difluoroethylene as determined by gas phase electron diffraction and microwave data

The structure of 1,1-difluoroethylene was determined, from gas phase electron diffraction data obtained independently in Leiden and Tokyo and the rotational constants of F₂CCH₂, F₂CCHD and F₂CCD₂ derived from the microwave study by Chauffoureaux. The two electron diffraction data agreed without significant discrepancy. From a joint least squares analysis of the diffraction and microwave data, the following r_g bond distances and r_z bond angles were derived: CC = 1.340 ± 0.006 Å, C-F = 1.315 ± 0.003 Å, C-H = 1.091 ± 0.010 Å, ∠C-C-F = 124.7 ± 0.3°, ∠C-C-H = 119.0 ± 0.4°, where the uncertainties represent estimated limits of error.     

Mechanism of vinylic and allylic carbon-fluorine bond activation of non-perfluorinated olefins using Cp*(2)ZrH(2)

Cp(2)ZrH(2) (1) (Cp = pentamethylcyclopentadienyl) reacts with vinylic carbon-fluorine bonds of CF(2)=CH(2) and 1,1-difluoromethylenecyclohexane (CF(2)=C(6)H(10)) to afford Cp(2)ZrHF (2) and hydrodefluorinated products. Experimental evidence suggests that an insertion/beta-fluoride elimination mechanism is occurring. Complex 1 reacts with allylic C-F bonds of the olefins, CH(2)=CHCF(3), CH(2)=CHCF(2)CF(2)CF(2)CF(3), and CH(2)=C(CF(3))(2) to give preferentially 2 and CH(3)-CH=CF(2), CH(3)-CH=CF-CF(2)CF(2)CF(3), and CF(2)=C(CF(3))(CH(3)), respectively, by insertion/beta-fluoride elimination. In the reactions of 1 with CH(2)=CHCF(3) and CH(2)=CHCF(2)CF(2)CF(2)CF(3), both primary and secondary alkylzirconium olefin insertion intermediates were observed in the (1)H and (19)F NMR spectra at low temperature. A deuterium labeling study revealed that more than one olefin-dihydride complex is likely to exist prior to olefin insertion. In the presence of excess 1 and H(2), CH(2)=CHCF(3) and CH(2)=CHCF(2)CF(2)CF(2)CF(3) are reduced to propane and (E)-CH(3)CH(2)CF=CFCF(2)CF(3), respectively.     

The synthesis of brominated-boron-doped PAHs by alkyne 1,1-bromoboration: mechanistic and functionalisation studies

The synthesis of a range of brominated-B_n-containing (n = 1, 2) polycyclic aromatic hydrocarbons (PAHs) is achieved simply by reacting BBr₃ with appropriately substituted alkynes via a bromoboration/electrophilic C–H borylation sequence. The brominated-B_n-PAHs were isolated as either the borinic acids or B-mesityl-protected derivatives, with the latter having extremely deep LUMOs for the B₂-doped PAHs (with one example having a reduction potential of E_1/2 = −0.96 V versus Fc⁺/Fc, Fc = ferrocene). Mechanistic studies revealed the reaction sequence proceeds by initial alkyne 1,1-bromoboration. 1,1-Bromoboration also was applied to access a number of unprecedented 1-bromo-2,2-diaryl substituted vinylboronate esters directly from internal alkynes. Bromoboration/C–H borylation installs useful C–Br units onto the B_n-PAHs, which were utilised in Negishi coupling reactions, including for the installation of two triarylamine donor (D) groups onto a B₂-PAH. The resultant D–A–D molecule has a low optical gap with an absorption onset at 750 nm and emission centered at 810 nm in the solid state.

The synthesis of a range of brominated-B_n-containing (n = 1, 2) polycyclic aromatic hydrocarbons (PAHs) is achieved simply by reacting BBr₃ with appropriately substituted alkynes via a bromoboration/electrophilic C–H borylation sequence.     

Metal free oxidative CC bond cleavage: Facile and one-pot tandem synthesis of benzothiadiazine-1,1-dioxides

An interesting protocol for the synthesis of benzothiadiazine-1,1-dioxides through iodine-catalyzed one-pot dehomologative oxidation of styrenes and readily available 2-aminobenzenesulfonamide has been developed. Diverse benzothiadiazine-1,1-dioxides were prepared using I₂ as a catalyst, TBHP as an oxidant and Na₂CO₃ as a base. This reliable, metal and ligand free conversion involves dehomologation of styrene to aromatic aldehyde which on subsequent cyclisation affords benzothiadiazine-1,1-dioxides in good yield.     

Aliphatic and aromatic carbon-fluorine bond activation with Cp*(2)ZrH(2): mechanisms of hydrodefluorination.

Cp*(2)ZrH(2) (1) (Cp* = pentamethylcyclopentadienyl) reacts with primary, secondary, and tertiary monofluorinated aliphatic hydrocarbons to give Cp*(2)ZrHF (2) and/or Cp*(2)ZrF(2) and alkane quantitatively through a radical chain mechanism. The reactivity of monofluorinated aliphatic C-F bonds decreases in the order 1 degrees > 2 degrees > 3 degrees. The rate of hydrodefluorination was also greatly reduced with -CF(2)H and -CF(3) groups attached to the hydrocarbon. An atmosphere of H(2) is required to stabilize 1 against C-H activation of the Cp*-methyl groups and subsequent dimerization under the thermal conditions employed in these reactions. Reaction of 1 with fluorobenzene cleanly forms a mixture of Cp*(2)ZrHF, benzene, and Cp*(2)Zr(C(6)H(5))F. Detailed studies indicate that radicals are not involved in this aromatic C-F activation reaction and that dual hydrodefluorination pathways are operative. In one mechanism, hydridic attack by Cp*(2)ZrH(2) on the aromatic ring and fluoride abstraction is involved. In the second mechanism, an initial ortho C-H activation occurs, followed by beta-fluoride elimination to generate a benzyne complex, which then inserts into the zirconium-hydride bond.     

Preferential conformation of the carbon-fluorine bond and electronic interactions in substituted benzyl fluorides

CNDO/2 and PCILO calculations have been used to study the preferential conformations and relative stabilities of six para-substituted, three ortho-substituted and two diortho-substituted benzyl fluorides. In non-ortho-substituted compounds, the C-F bond of the CH₂F group lies preferentially in the plane of the phenyl ring. The HOMO energies show that conjugation exists between the substituent and the fluorine atom. Some experimental evidence has been obtained through the use of Raman spectroscopy on the crystalline state. For ortho-substituted compounds the calculations are in agreement with several previous studies which show that steric hindrance can lead to a change in the preferential conformation in the phenyl ring plane as occurs in 2,6-dichlorobenzyl fluoride.     

Facile and efficient synthesis of xanthenedione derivatives promoted by niobium pentachloride

Xanthenedione derivatives were synthesised in one-pot reactions between arylaldehyde derivatives and 1,3-cyclohexanedione promoted by niobium pentachloride. This new method is simple, costeffective, high-yielding with a good variety of substrates generality, and can be conducted within reasonable reaction times.