Alkyne Adducts Research Articles

Electrochemical behavior of the Cp*(dithiolato)Co(III) complex [(η 5-Cp*)Co(1,2-S 2C 2B 10H 10- S, S′)] and its derivatives: the effect of the ligand on half-sandwich cobalt(III) complexes

Reaction of the 16-electron dithiolatocobalt, [(η 5-Cp*)Co(Cab S, S′ )] ( 1) (Cab S, S′ =1,2-S 2C 2B 10H 10- S, S′), with a ligand L (L=CN t Bu, PMe 3, PEt 3, PHPh 2) affords the 18-electron complexes [(η 5-Cp*)Co(Cab S, S′ )(L)] ( 2) in high yield. The structures of 2 in solution were probed by NMR spectroscopy, and the solid-state structure of the diphenylphosphine derivative was established by single-crystal X-ray diffraction. Cyclic voltammetry of 1 shows that the complex undergoes a quasi-reversible one-electron oxidation and reversible reduction. The voltammetry of the corresponding 18-electron complex 2 was investigated rather extensively in terms of coordination and redox chemistry, in which the influence of the enhanced steric interaction between the bulky Cp* and incoming ligand could be observed. Cyclic voltammograms of alkylidiene or acetylene adducts of dithiolatocobalt complex 1, [(η 5-Cp*)Co{η 3-( S, S′, C′)-SC 2B 10H 10S(C′R 2)}] [C′R 2=CHSiMe 3 ( 3a), HCCPh ( 3b), (COOMe)CC(COOMe) ( 3c)], are also included.

Alkyne Adducts of [W2(OCH2tBu)8]. Cases of Perpendicular and Skewed Bridges in Equilibrium with Terminal Bonded Isomers

Alkynes react with a suspension of [W 2 (OCH 2 tBU) 8 ](M=M) in hydrocarbon solvents to give alkyne adducts W 2 (μ-RCCR')(OCH 2 tBu) 8 , where R = H and R' = Ph, Me, Me 3 Si and R = Me and R' = Me and Ph. For PhC≡CH, there is a μ-perpendicular mode of alkyne bonding, θ = 84°, but for MeC≡CH and MeC≡CMe, the bridging alkyne is distinctly skewed with respect to the W-W axis, 0 = 64° and 67°, respectively. In contrast, the PhC≡CMe adduct has the alkyne bonded to only one tungsten, η 2 -PhC≡CMe, with the alkyne C≡C axis being 30° from the W-W axis. In all of these structures, there are alkoxide bridges, and the dinuclear unit may be viewed as a confacial bioctahedron where either one alkyne and two alkoxides or three alkoxides occupy the bridging face. In toluene-d 8 , NMR spectroscopic studies indicate that certain of these alkyne adducts (MeC≡CH, MeC≡CMe, PhC≡CMe, and Me 3 SiC≡CH) exist as a mixture of alkyne-bridged and η 2 -alkyne-bonded isomers. The isomers are fluxional on the NMR time scale by rapid exchange of terminal and bridging alkoxide groups. However, μ-η 2 alkyne exchange is slow on the NMR time scale. The structural data are compared with electronic structure calculations employing density functional theory on model compounds of formula W 2 (alkyne)(OMe) 8 . The calculations suggest that a μ-parallel mode of bonding is energetically favorable relative to u-perpendicular and, further, that the energy difference between bridge and terminal alkyne adducts is, in all cases, less than 5 kcal mol - 1 .

Coordination of alkenes and alkynes to a cationic d(0) zirconocene alkoxide complex.

This paper describes the synthesis of base-free (C5R5)2Zr(OtBu)+ cations, the direct observation of nonchelated alkene and alkyne adducts of these cations, and studies of the thermodynamic and dynamic properties of these novel species. Reaction of %@mt;sys@%Cp%@/xs;55;%lnwidth@%'%@/xs;63;(%lnwidth-x55)@%%@mh;-x63@%%@sb@%2%@sbx@%%@/hd@%ZrMe%@sb@%2%@sbx@%%@mx@% (Cp' = C5H4Me) with tert-butyl alcohol followed by [Ph3C][B(C6F5)4] in benzene yields [%@mt;sys@%Cp%@/xs;55;%lnwidth@%'%@/xs;63;(%lnwidth-x55)@%%@mh;-x63@%%@sb@%2%@sbx@%%@/hd@%Zr%@/hd@%%@fn;(;vis;full;auto@%O%@ital@%%@ex@%t%@rsf@%%@exx@%%@/hd@%Bu%@fnx;);vis;full@%%@/hd@%%@mx@% ][B(C6F5)4] (1), which exists as %@mt;sys@%Cp%@/xs;55;%lnwidth@%'%@/xs;63;(%lnwidth-x55)@%%@mh;-x63@%%@sb@%2%@sbx@%%@/hd@%Zr%@/hd@%%@fn;(;vis;full;auto@%O%@ital@%%@ex@%t%@rsf@%%@exx@%%@/hd@%Bu%@fnx;);vis;full@%%@/hd@%%@fn;(;vis;full;auto@%ClR%@fnx;);vis;full@%%@ex@%+%@exx@%%@mx@% solvent adducts in C6D5Cl and CD2Cl2 solutions. Addition of ligands L (L = ethylene, propylene, propyne, 2-butyne, CO, phenylacetylene, allene, 1-hexene, cis-2-butene) to 1 in CD2Cl2 at -89 degrees C results in reversible formation of %@mt;sys@%Cp%@/xs;55;%lnwidth@%'%@/xs;63;(%lnwidth-x55)@%%@mh;-x63@%%@sb@%2%@sbx@%%@/hd@%Zr%@/hd@%%@fn;(;vis;full;auto@%O%@ital@%%@ex@%t%@rsf@%%@exx@%%@/hd@%Bu%@fnx;);vis;full@%%@/hd@%%@fn;(;vis;full;auto@%L%@fnx;);vis;full@%%@ex@%+%@exx@%%@mx@% adducts. NMR data for %@mt;sys@%Cp%@/xs;55;%lnwidth@%'%@/xs;63;(%lnwidth-x55)@%%@mh;-x63@%%@sb@%2%@sbx@%%@/hd@%Zr%@/hd@%%@fn;(;vis;full;auto@%O%@ital@%%@ex@%t%@rsf@%%@exx@%%@/hd@%Bu%@fnx;);vis;full@%%@/hd@%%@fn;(;vis;full;auto@%H%@sb@%2%@sbx@%C=%@/bd@%CHMe%@fnx;);vis;full@%%@ex@%+%@exx@%%@mx@% (4) indicate that the propylene coordinates unsymmetrically and is polarized with positive charge buildup at Cint. Equilibrium constants, defined by Keq = [Zr-L][1]-1[L]-1, vary in the order CO > propyne > 2-butyne > phenylacetylene > ethylene > allene > propylene > 1-hexene > cis-2-butene > vinyl chloride. Loss of L from %@mt;sys@%Cp%@/xs;55;%lnwidth@%'%@/xs;63;(%lnwidth-x55)@%%@mh;-x63@%%@sb@%2%@sbx@%%@/hd@%Zr%@/hd@%%@fn;(;vis;full;auto@%O%@ital@%%@ex@%t%@rsf@%%@exx@%%@/hd@%Bu%@fnx;);vis;full@%%@/hd@%%@fn;(;vis;full;auto@%L%@fnx;);vis;full@%%@ex@%+%@exx@%%@mx@% to give 1 appears to proceed via associative displacement by CD2Cl2 in most cases.

Alkyne adducts of [W2(OCH2tBu)8]n (M = M): comparisons of bridging and terminal addition products.

Alkynes are found to react with [W2(OCH2tBu)8] (M = M) in hydrocarbon solvents at room temperature or 45 degrees C to give 1:1 adducts. These are shown to be either bridged (mu-PhCCH and mu-MeCCMe) or terminal-bound (eta2-PhCCMe) in the solid state by single-crystal X-ray crystallography. In solution NMR spectroscopy reveals that bridged and terminal species exist in equilibrium for MeCCH, MeCCMe, and PhCCMe. By NMR spectroscopy the PhCCH and Me3SiCCH adducts are present in solution in bridging and terminally bonded species, respectively. The interconversion of bridged and terminal-bound adducts is chemically rapid but slow on the NMR time scale even though each type of adduct shows fluxional behavior. Calculations employing density functional theory have been carried out on alkyne adducts of the model template W2(OCH3)8 and reveal very small differences in energy between a mu-skewed structure and one having a terminal eta2-alkyne.

Insights into the Schrock 'chop-chop' reaction gained from density functional theory and preparation and structure of W2(mu-PhCCPh)(SC6H4-2-Me)6.

Computations employing density functional theory on the reactions between ethyne and the model compounds (HE)3M identical to M(EH)3, where M = Mo and W and E = O and S, predict that the alkyne adducts M2(mu-C2H2)(EH)6 are thermodynamically favored with respect to the metathesis products HC identical to M(EH)3 except when M = W and E = O; the reaction between (tBuO)3 W identical to CPh and 2-MeC6H4SH (> 3 equiv.) yields W2(mu-PhCCPh)(SC6H4-2-Me)6 consistent with expectations based on the calculations.

A comparative study of olefin or acetylene insertion into Ru–H or Os–H of MHCl(CO)(phosphine)2

The adduct OsHCl(C2D4)(CO)L2 (L = PiPr3) shows only very slow H/D exchange at 25 °C, but this is easily detectable at 65 °C; no ethyl species is formed in detectable concentration. RuHCl(CO)L2 shows no detectable C2D4 adduct, but Ru–H/C–D exchange at 60 °C is actually faster than for Os. DFT (B3PW91) calculations have been carried out to analyze the relative energies of the isomeric forms that would result from the addition of an alkene or an alkyne to MH(Cl)(CO)(PH3)2 (M = Os, Ru). Thus, 18-electron alkyne adducts are compared to the 18-electron vinylidene isomer and to the 16-electron vinyl complex. Similarly, the 18-electron alkene adduct is compared to the 18-electron carbene isomer and to the 16-electron ethyl complex. Two factors are found to control the products formed: (i) the Os complex favors unsaturated π-bonded ligands and an 18-electron count while Ru favors saturated ligands and an unsaturated metal center; (ii) the weaker π bond in the alkyne than in the alkene makes insertion or isomerization of an alkyne thermodynamically more favored than that of an alkene. This results in ethyl complexes being less favored than vinyl complexes in similar experimental conditions. For RuHCl(CO)L2′, where L′ is PiPr2[3,5-(CF3)2C6H3], 1 atm ethylene gives a detectable, colorless ethylene adduct, then also a detectable ethyl complex, all in facile equilibrium.

Activation of Terminal Alkynes at the Sulfur-Rich Bimetallic Site [MoIII2Cp2(μ-SMe)3]+: Alkyne−Vinylidene Conversion and C−S and C−C Couplings Promoted by Addition of Unsaturated Substrates (RC⋮CH, RN⋮C, SCS). Crystal Structures of μ-η1:η2-Vinylidene, μ-η1:η2-Acetylide, and μ-η1:η3-Vinyl−Thioether Compounds

Reactions of the bis(nitrile) compound [Mo2Cp2(MeCN)2(μ-SMe)3](BF4) (1) with terminal alkynes in a 1:1 ratio in dichloromethane at room temperature led to the alkyne adduct [Mo2Cp2(μ-SMe)3(RCCH)](BF4) (2: R = Tol (2a), Ph (2b), CH3CCH2 (2c), nPr (2d), CO2Me (2e), CF3 (2f)). Compounds 2a−d were readily deprotonated with Et3N to give neutral acetylide derivatives [Mo2Cp2(μ-η1:η2-C⋮CR)(μ-SMe)3] (3). Protonation of 3 afforded exclusively the vinylidene complexes [Mo2Cp2(μ-η1:η2-CCHR)(μ-SMe)3](BF3) (4). Reaction of 1 with an excess of terminal alkyne RCCH in dichloromethane gave either the six-membered metallacycle compounds [Mo2Cp2(μ-η2:η4-CRCHCRCHSMe)(μ-SMe)2](BF4) (5) or the S-methylthiophenium derivatives [Mo2Cp2(μ-η2:η4-C4H2R2SMe)(μ-SMe)2](BF4) (6), depending on the nature of the R groups. Further reactions of the alkyne adducts 2 with isocyanide and carbon disulfide led to the vinyl−thioether complexes [Mo2Cp2(μ-η1:η3-CRCR‘-SMe)(μ-SMe)2(RNC)](BF4) (7, 8) and to their CS2 adducts [Mo2Cp2(S2C−CR‘CRSMe)(μ-...

Ferrocenylalkynes as ligands in transition metal complexes

Several ferrocenylalkynes have been prepared and their reaction with dicobalt octacarbonyl led to the formation of [Co2(CO)6(μ-η2 ∶ η2-ethynylferrocene)] 1, [Co2(CO)6(μ-η2 ∶ η2-1,4-bis(ferrocenyl)butadiyne)] 2, [(Co2(CO)6)2(μ-η2 ∶ η2,μ-η2 ∶ η2-1,4-bis(ferrocenyl)butadiyne)] 3, [Co2(CO)6(μ-η2 ∶ η2-1-ferrocenylethynyl-2-hydro-1,2-dihydro-[60]fullerene)] 4, [Co2(CO)6(μ-η2 ∶ η2-1,1’-bis(phenylethynyl)ferrocene)] 5 and [(Co2(CO)6)2(μ-η2 ∶ η2,μ-η2 ∶ η2-1,1′-bis(phenylethynyl)ferrocene)] 6. The [Mo–Co]-alkyne adducts [MoCo(CO)5(η5-C5H5)(μ-η2 ∶ η2-ethynylferrocene)] 7 and [MoCo(CO)5(η5-C5H5)(μ-η2 ∶ η2-1,4-bis(ferrocenyl)butadiyne)] 8 were obtained from complexes 1, 2 and 3. The molecular structures of compounds 2, 3, 6 and 7 were determined by X-ray diffraction, only 6 has the cyclopentadienyl rings of the ferrocene unit eclipsed. The reaction of 1 with bis(diphenylphosphino)methane (dppm) and 1,1′-bis(diphenylphosphino)ferrocene (dppf) resulted in the formation of [Co2(CO)4(μ-η1 ∶ η1-dppm)(μ-η2 ∶ η2-ethynylferrocene)] 9 and [(Co2(CO)4)2(μ-η1 ∶ η1-dppf)2(η1-μ-η1-dppf)(μ-η2 ∶ η2-ethynylferrocene)2] 10, respectively. The reaction of HPPh2 with 7 gave the monosubstituted product [MoCo(CO)4(η5-C5H5)(PHPh2)(μ-η2,η2-ethynylferrocene)] 11. The bis(phosphido)-bridged complex [Co2(CO)6(μ-PPh2)2] undergoes regiospecific insertion with ethynylferrocene to give [Co2(CO)4{(μ-η4-PPh2CHCRC(O)}(μ-PPh2)] (R = ferrocene) 12, which was characterized by X-Ray diffraction and contains a five-membered metallacyclic ring.

Reduction of Metal-Stabilized α-CF3-Carbenium Ion Complexes under Mild Conditions: Synthesis, Structures, and Reactivity

Complexed α-CF3 propargyl alcohols of the general formula [(M2L6){μ-η2,η2-RC≡CCH(CF3)(OH)}] were prepared with M2L6 = Co2(CO)6, R = CH3(CH2)4– (1), R = C6H5– (2); M2L6 = Co2(CO)5P(C6H5)3, R = CH3(CH2)4– (3a,b), R = C6H5– (4a,b); M2L6 = Co2(CO)4dppm, R = C6H5– (5); M2L6 = Co(CO)3MoCp(CO)2, R = CH3(CH2)4– (6a,b), R = C6H5– (7a,b). An X-ray molecular structure of the propargyl-alcohol complex [{Co2(CO)4dppm}{μ-η2,η2-C6H5C≡CCH(CF3)(OH)}] (5) was also determined. The related carbenium ions [(M2L6){μ-η2,η3-RC≡CCH(CF3)}][BF4] (8–12) were obtained from the parent propargyl alcohol complexes by direct protonation with HBF4· Et2O in diethyl ether. These carbenium ions were reduced further by Zn in CH2Cl2 to give the alkyne adducts [(M2L6){μ-η2,η2-RC≡CCH2(CF3)}] (13–17), as confirmed by the X-ray molecular structure of [(Co2(CO)4dppm){μ-η2,η2-C6H5C≡CCH2(CF3)}] (17). Treatment of the carbenium ion complex [{Co(CO)3MoCp(CO)2}{μ-η2,η3-CH3(CH2)4C≡CCH(CF3)}][BF4] (8) with NaSMe unexpectedly afforded the reduced alkyne adduct [{Co(CO)3MoCp(CO)2}{μ-η2,η2-CH3(CH2)4C≡CCH2(CF3)}] (13), along with the alkyne-thioether diastereomers {Co(CO)3MoCp(CO)2}{μ-η2,η2-CH3(CH2)4C≡CCH(CF3)[(SMe)}] (18a,b). Presumably, all the reduction reactions proceed primarily by the formation of the transient radical species, which are subsequently transformed into the reduced alkyne complexes by hydrogen abstraction from the solvent medium. Interestingly, in the case of the complexed alcohols [{Co2(CO)5P(C6H5)3}{μ-η2,η2-RC≡CCH(CF3)(OH)}] (3a,b) and (4a,b), the reduction process occurs in acidic medium in THF/CH2Cl2. An extensive study of the electronic and steric factors that influence the stability and reactivity of the carbenium ions were performed, which allowed us to explain the behavior of the related radical species in solution during the reduction process.

Sulfanyl radical mediated cyclization of aminyl radicals

1-(2-Aminopheny)pent-1-yne 1 reacted with benzenethiol at 150 °C under radical conditions to give the thiol alkyne adduct 2, the benzothiophene 4 and the indole 5. Reaction of 1 with benzenesulfanyl radicals produced from diphenyl disulfine in the absence of hydrogen donors gave only the indole 5 in high yields. Formation of indole 5 was explained in terms of sulfanyl radical mediated aminyl radical cyclization onto the alkyne triple bond.

Reactivity of the Heterobinuclear Phenylacetylide-Bridged A-Frame [RhIr(CO)2(μ-CCPh)(Ph2PCH2PPh2)2][O3SCF3] with Small Molecules

The compound [RhIr(CO)2(μ-CCPh)(dppm)2][O3SCF3] (2, dppm = Ph2PCH2PPh2) is an “A-frame” species in which the bridging phenylacetylide group is σ-bound to iridium and functioning as a π donor to rhodium. Compound 2 reacts with CO and SO2 to give the carbonyl- and sulfur dioxide-bridged products [RhIr(CO)2(μ-CCPh)(μ-L)(dppm)2][O3SCF3] (L = CO (1), SO2 (5)), respectively. Reaction of 2 with a hydride source yields [RhIr(H)(CO)2(μ-CCPh)(dppm)2] (6), having the hydride terminally bound to iridium, and this product rearranges at room temperature to give the phenylvinylidene complex [RhIr(CO)2(μ-CC(H)Ph)(dppm)2] (7). Phosphines, olefins, and alkynes also bind terminally to iridium, yielding [RhIr(L)(CO)2(μ-CCPh)(dppm)2][O3SCF3] (L = PR3, olefin, alkyne). In the case of dimethyl acetylenedicarboxylate, rearrangement of the initial alkyne adduct occurs to give the alkyne-bridged product [RhIr(CO)2(μ-CCPh)(μ-CH3O2CC⋮CCO2CH3)(dppm)2][O3SCF3] (18). In addition, the initial adducts of terminal alkynes also rearrange b...

A General Route to Prochiral and Chiral Dicationic Molybdenum−Cobalt Acetylenic Complexes: Synthesis, Structures, and Reactivity

Several [Co−Co]−alkyne adducts [Co2(CO)6)(μ-η2,η2-HOCR1R2-C⋮C−CR1R2OH)] were prepared (R1 = R2 = H (1); R1 = H, R2 = CH3 (2); R1 = R2 = CH3 (3)) and their X-ray molecular structures determined, sho...

Reaction of ruthenium and iron alkynyl complexes with [Mo2(CO)4(η-C5H5)2] ‡

The reaction of the molybdenum dimer [Mo2(CO)4(η-C5H5)2] with [M(CCR)(CO)2(η-C5H5)] (M = Ru or Fe, R = Me or Ph) has given different products dependent on the alkynylmetal. The ruthenium-containing starting materials gave the expected dimolybdenum alkynyl adducts as the only isolable materials in moderate yield. In contrast the iron alkynyls underwent Fe–C bond cleavage to give the simple known alkyne adducts [Mo2(µ-η2-HC2R)(CO)4(η-C5H5)2] by a yet to be determined mechanism. The fluxional nature of the complex [Mo2{Ru(µ-CCMe)(CO)2(η-C5H5)}(CO)4(η-C5H5)2] was observed by solution NMR studies; the mechanism for equilibration of the molybdenum carbonyl groups at room temperature must involve disruption of the central Mo2C2 core.

Alternative Mechanisms of the Alkyne to Vinylidene Isomerization Promoted by Half-Sandwich Ruthenium Complexes. X-ray Crystal Structures of [Cp*RuCCHCOOMe(dippe)][BPh4] and [Cp*RuH(C⋮CCOOMe)(dippe)][BPh4] (dippe = 1,2-bis(diisopropylphosphino)ethane; Cp* = C5Me5)

The complex [Cp*RuCl(dippe)] reacts with 1-alkynes in MeOH in the presence of NaBPh4, yielding the metastable hydrido−alkynyl derivatives [Cp*Ru(H)(C⋮CR)(dippe)][BPh4] (R = COOMe, Ph or SiMe3), intermediates in the formation of the corresponding vinylidene complexes, to which these compounds rearrange both in solution and in the solid state. The X-ray crystal structures of the isomers [Cp*RuCCHCOOMe(dippe)][BPh4] and [Cp*RuH(C⋮CCOOMe)(dippe)][BPh4] have been determined. Kinetic studies show that the mechanism for this isomerization process seems to be dissociative and that it is inhibited in solution by strong acids. In contrast with this, no hydrido−alkynyl complex has been observed in the course of the reaction of 1-alkynes with [CpRuCl(dippe)]. Instead, the alkyne adducts have been detected, and isolated in some cases. Such species have only been observed in the Cp*Ru system for acetylene, and [Cp*Ru(η2-HC⋮CH)(dippe)]+ seems to be in equilibrium with the corresponding hydrido−alkynyl complex [Cp*RuH(C⋮CH)(dippe)]+, which isomerizes to [Cp*RuCCH2(dippe)]+ via the formation of the π-alkyne complex. The effects on these tautomerization processes of the phosphine, Cp*, and Cp ligands, as well as the R group of the alkyne, are discussed.

Reversible Complexation of Acetylene with (Perchlorato)cobalt(III) Porphyrins To Form a Novel Dicobalt(II) Bis(porphyrin) with a Vinylene-N,N‘ Linkage

(Perchlorato)cobalt(III) porphyrins bind acetylene reversibly in anhydrous CH2Cl2. The 1H NMR and UV−vis spectra of the acetylene adduct at room temperature are characteristic of a CoII N-substituted porphyrin. The ESR spectra of the acetylene adduct in CH2Cl2 at 4.2 and 77 K showed signals due to a π-cation radical and low-spin CoII. These spectroscopic data show that one acetylene combines with two cobalt porphyrins to generate a vinylene-N,N‘-linked bis(porphyrin) dicobalt(II) structure reversibly via an acetylene π-complex of CoII porphyrin π-cation radical. When coordinating anionic axial ligands (Cl-, SCN-) were attached, the vinylene-N,N‘ bis(tetraarylporphyrin) dicobalt(II) complexes were stabilized enough to be isolated and fully characterized.

Palladium-Catalyzed Additions of Terminal Alkynes to Acceptor Alkynes

The development of addition reactions wherein the product is the simple sum of the reactants plus anything else (only needed catalytically) constitutes an important goal for enhanced synthetic efficiency. The C−H bond of terminal alkynes (the donor alkynes) can be added to either terminal alkynes (self-coupling) or activated internal alkynes (cross-coupling) (the acceptor alkynes) in the presence of a catalytic amount of palladium acetate and an electron rich sterically encumbered ligand, tris(2,6-dimethoxyphenyl)phosphine. The activated internal alkynes for cross-coupling (the acceptor alkyne) include alkynes bearing an ester, sulfone, and ketone. Self-coupling is completely overwhelmed by cross-coupling, even at 1:1 ratios of donor and acceptor alkynes. The reaction exhibits extraordinary chemoselectivity with free carboxaldehydes, alcohols, ketones (saturated and conjugated), esters (saturated and conjugated), sulfones (saturated and conjugated), malonates, and silyl ethers all proving to be compatible. A 1:2 donor/acceptor alkyne adduct can also be optimized. Ethyl propiolate fails as an acceptor but its C-silylated analogue serves with the proper choice of silyl substituent. The products of the latter serve as useful precursors to β-keto esters. An iterative sequence is readily performed and led to a novel conformationally rigid retinoid analogue. The mechanism of this mild method for construction of conjugated enynes, versatile building blocks, is discussed.

Synthesis and Spectroscopic Characterization of (CO)6Fe2{μ-EC(H)C(H)E‘} (E ≠ E‘; E, E‘ = S, Se, Te) and (CO)6Fe2{μ-TeC(H)C(H)Te}. Structural Characterization of (CO)6Fe2{μ-SC(Ph)C(H)Se} and (CO)6Fe2{μ-SC(H)C(Ph)Te}

When acetylene gas was bubbled through methanol solutions containing the mixed-chalcogenide compounds (CO)6Fe2(μ-EE‘) (E ≠ E‘; E, E‘ = S, Se, Te), the acetylene adducts (CO)6Fe2{μ-EC(H)C(H)E‘} (1, 52%, E, E‘ = S, Se; 2, 46%, E, E‘ = S, Te; 3, 38%, E, E‘ = Se, Te) were obtained. In addition, trace amounts of the homochalcogenide derivatives (CO)6Fe2{μ-EC(H)C(H)E} (E = S, Se, Te) were also obtained. The Te2 compound (CO)6Fe2{μ-TeC(H)C(H)Te} (4) was obtained in 32% yield from the reaction of (CO)6Fe2(μ-Te2) with acetylene. Compounds 1−4 were characterized by IR and 1H, 13C, 77Se, and 125Te NMR spectroscopy. Crystallographic analysis of the phenylacetylene adducts (CO)6Fe2{μ-SC(Ph)C(H)Se} (5) and (CO)6Fe2{μ-SC(H)C(Ph)Te} (6) were carried out. The structures of both 5 and 6 can be described as Fe2SE (E = Se, Te) tetrahedral butterfly cores containing the phenylacetylene as a bridge between the two wingtip chalcogen atoms, with three terminally bonded carbonyl groups on each Fe atom.

Kinetics of the OH + C2H2 reaction in the presence of O2

Room-temperature rate constants for the removal of OH radicals in the gas-phase reaction with acetylene have been determined using pulsed H2O2 photolysis production of OH followed by cw UV-laser long-path absorption detection. The pressure dependences of the rate constants in N2, O2 and synthetic air as buffer gases have been investigated in the fall-off range between 1.5 and 100 k Pa. In N2 the pressure dependence can be described by the empirical expression of Gardiner: k={(k∞)a+(k0[M])a}1/a, using the parameter a=–⅓. This results in the rate constants k∞=(1.07 ± 0.07)× 10–12 cm3 s–1, and k0=(3.9 ± 1.6)× 10–29 cm6 s–1 for the given pressure range. Smaller effective rate constants were obtained in the presence of O2, owing to a fast regeneration channel for OH. In terms of the above expression the rate constants k∞=(4.6 ± 0.2) and (3.1 ± 0.1)× 10–13 cm3 s–1 and k0=(1.8 ± 0.2) and (1.9 ± 0.5)× 10–29 cm6 s–1 were obtained for O2 and synthetic air as buffer gases, respectively. Compared with N2 there is virtually no difference in the fall-off behaviour in O2 and synthetic air, indicating that the OH regeneration yield is independent of pressure. On the other hand, this yield was found to decrease with the O2 mixing ratio. A kinetic model is proposed that qualitatively explains this effect with different chemical properties of non-thermalized OH–acetylene adducts with regard to O2. This model is confirmed by measurements showing a significantly stronger dependence of the regeneration yield on the O2 mixing ratio in O2–He mixtures.

Lewis acid catalysis of the rearrangement of a dipalladium acetylene adduct to a vinylidene-bridged complex

Lewis acid catalysis of the rearrangement of a dipalladium acetylene adduct to a vinylidene-bridged complex

Alkyne Adducts of [Cp*Ru(SR)]2 and Intermediates of the Ruthenium Catalyzed Formation of Vinyl Thioethers (Z/E)-RSCR':CHR'' from RSH and R'C.tplbond.CR''

Alkyne Adducts of [Cp*Ru(SR)]2 and Intermediates of the Ruthenium Catalyzed Formation of Vinyl Thioethers (Z/E)-RSCR':CHR'' from RSH and R'C.tplbond.CR''