1071-81-4

The compound [(Ot-Bu)3WCt-Bu] (1) (t-Bu = C(CH3) 3) decomposes upon contact with water and several organic products are formed, including di-tert-butylacetylene, t-BuCCt-Bu. This process is reminiscent of the degradation of trinuclear metal-alkylidyne complexes in which free carbynes are ejected into solution, couple and form alkynes along with many other products. The reactivity pattern of the resulting t-BuC carbynes that includes extensive hydrogen abstraction, cleavage of alkynes and lack of reactivity towards alkenes is indicative of a quartet (S = 3/2) spin state. A similar spin state was assigned to other RC (R = alkyl) species that were produced by degrading M3-alkylidyne (M = transition metal) complexes in water. t-BuCCt-Bu is also produced during thermal decomposition of solid 1. In 1977 Fischer and co-workers reported a very similar process in which solids of Br(CO)4CrCR1 and Br(CO)4CrCR2 were co-thermolyzed to produce R1CCR2, R 1CCR1, and R2CCR2. Fischer had considered the involvement of free carbynes in the making of the alkynes but later resorted to other explanations. The current results suggest that his original proposal is indeed valid.

Using well-defined tungsten-based olefin metathesis catalysts, a family of partially substituted polyacetylenes have been synthesized via the ring-opening metathesis polymerization (ROMP) of monosubstituted cyclooctatetraenes (RCOT). These polymers are highly conjugated, as evidenced by their visible absorption maxima. They are of high molecular weight, as evidenced by gel permeation chromatography, and most members of the family are soluble in the as-synthesized, predominantly cis form. The polymers can be isomerized to the predominantly trans form using heat or light. The rate of thermal isomerization was monitored by visible absorption spectroscopy. Polymers containing, in general, secondary or tertiary groups immediately adjacent to the main chain remain soluble in the trans form and are, in most cases, still highly conjugated. Overall, there is a connection between the steric bulk of the side group in polymers of monosubstituted COTs, their effective conjugation length, and their solubility. The side group twists the main chain of the polymer and also induces a preference for cis units in the chain. The tradeoff between conjugation and solubility has been explored, and highly conjugated polyacetylenes that are still soluble have been discovered. In the solid state, these polymers are observed to be amorphous by wide-angle X-ray scattering and near-infrared scattering. The amorphous nature of these samples correlates with the relatively low temperature cis-trans isomerization in the solid state. Upon iodine doping, these polymers become electrically conductive, although their conductivities are smaller than those of unsubstituted polyacetylene. Both empirical and semiempirical computational methods indicate an increased preference for cis linkages in partially substituted polyacetylene chains and show twists around the single bonds adjacent to the side groups in the polymer chain. The relative magnitude of these twists can be used to rationalize the differences in solubilities of the various polyacetylene derivatives, and these models provide a means of visualizing the conformation of the polymer, at least on its smallest size regime. The computations have also been useful in the rational design of new soluble polyacetylene derivatives with high effective conjugation lengths. By modeling and then synthesizing chains containing sec-butyl and other secondary groups, these properties have been realized.

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Sonochemically enhanced Wurtz coupling using lithium metal has been investigated for a number of isomeric alkyl bromides under a variety conditions. The products result from direct coupling of short lived radicals formed at the metal surface rather than the secondary radicals which can be formed during coupling of aromatic halides and thus give rise to a single major product. Coupling has been extended to dibrominated aryl and alkyl compounds as well as showing that aryl-alkyl coupling is possible. Dibrominated alkyls were found to give low molecular weight oligomers although no reaction occurred for 1,2-isomers. The growth of oligomers in THF may be solubility limited. A simple model is proposed to explain these findings.

The effect of triphenylphosphine and metal loading and/or dispersion on the product distributions from di-tert-butylacetylene indicates that the surface structure of the metal particles also may affect stereospecificities by promoting different catalytic mechanisms at different sites.

We found that in situ generated cerium(IV) carboxylate generated by mixing the precursor Ce(OtBu)4 with the corresponding carboxylic acids served as efficient photocatalysts for the direct formation of carboxyl radicals from carboxylic acids under blue light-emitting diodes (blue LEDs) irradiation and air, resulting in catalytic decarboxylative oxygenation of aliphatic carboxylic acids to give C-O bond-forming products such as aldehydes and ketones. Control experiments revealed that hexanuclear Ce(IV) carboxylate clusters initially formed in the reaction mixture and the ligand-to-metal charge transfer nature of the Ce(IV) carboxylate clusters was responsible for the high catalytic performance to transform the carboxylate ligands to the carboxyl radical. In addition, the Ce(IV) carboxylate cluster catalyzed direct lactonization of 2-isopropylbenzoic acid to produce the corresponding peroxy lactone and ?3-lactone via intramolecular 1,5-hydrogen atom transfer (1,5-HAT).

Ni(I)-mediated single-electron oxidative activation of alkyl halides has been extensively proposed as a key step in Ni-catalyzed cross-coupling reactions to generate radical intermediates. There are four mechanisms through which this step could take place: oxidative addition, outer-sphere electron transfer, inner-sphere electron transfer, and concerted halogen-atom abstraction. Despite considerable computational studies, there is no experimental study to evaluate all four pathways for Ni(I)-mediated alkyl radical formation. Herein, we report the isolation of a series of (Xantphos)Ni(I)-Ar complexes that selectively activate alkyl halides over aryl halides to eject radicals and form Ni(II) complexes. This observation allows the application of kinetic studies on the steric, electronic, and solvent effects, in combination with DFT calculations, to systematically assess the four possible pathways. Our data reveal that (Xantphos)Ni(I)-mediated alkyl halide activation proceeds via a concerted halogen-atom abstraction mechanism. This result corroborates previous DFT studies on (terpy)Ni(I)- and (py)Ni(I)-mediated alkyl radical formation, and contrasts with the outer-sphere electron transfer pathway observed for (PPh3)4Ni(0)-mediated aryl halide activation. This study of a model system provides insight into the overall mechanism of Ni-catalyzed cross-coupling reactions and offers a basis for differentiating electrophiles in cross-electrophile coupling reactions.

A quaternary catalytic system consisting of a cobalt salt, a triarylphosphine ligand, a Grignard reagent, and pyridine has been developed for chelation-assisted C-H bond activation of an aromatic imine, followed by insertion of an unactivated internal alkyne that occurs at ambient temperature. The reaction not only tolerates potentially senstitive functional groups (e.g., Cl, Br, CN, and tertiary amide), but also displays a unique regioselectivity. Thus, the presence of substituents such as methoxy, halogen, and cyano groups at the meta-position of the imino group led to selective C-C bond formation at the more sterically hindered ortho positions. Under acidic conditions, the hydroarylation products of dialkyl- and alkylarylacetylenes underwent cyclization to afford benzofulvene derivatives, while those of diarylacetylenes afforded the corresponding ketones in moderate to good yields. A mechanistic investigation into the reaction with the aid of deuterium-labeling experiments and kinetic analysis has indicated that oxidative addition of the ortho C-H bond is the rate-limiting step of the reaction. The kinetic analysis has also shed light on the complexity of the quaternary catalytic system.

For the alkynyl-substituted olefines 1 - 14 activation parameters for the geometrical isomerisation have been determined in the gasphase by the single-pulse shoke-tube technique.By comparison of these barriers with the corresponding one of the isolated double bonds, each corrected by the steric energy contribution of the ground and transition state, a value of 7.8 +/- 1.3 kcal * mol-1 for the propargyl stabilisation energy (PrSE) has been derived. - Key Words: Resonance energy / Stabilisation energy / Propargyl resonance / Force field calculation / Intrinsic rotational barrier / Single pulse shock tube / Gasphase kinetics / Heats of hydrogenation

For the olefins 1-8 heats of formation have been derived from heats of hydrogenation and force-field calculations, respectively.From the kinetics of their geometrical isomerisation the corresponding values for the transition states were obtained.The rotational barriers, which vary by nearly 30 kcal/mol, can be described by a unique torsional potential (65.9 +/- 0.9 kcal/mol), which is independent of the degree of substitution, if a correction is made for the steric energy contribution in the ground- and transition-states. - Key Words: Rotational barriers / Olefins, strained / Heat of Hydrogenation / Force-field calculation

A series of new palladacyclopentane derivatives of formula has been prepared.The first X-ray crystal structure determination of a palladacyclopentane derivative is reported: the compound gives crystals belonging to the C2/c space group: a=16.643(9), b=11.174(4), c=7.451(3) Angstroem, β=116.05(9) deg, and Z=4; R=0.0340 for 826 observed reflexions.The metal co-ordination is square planar and the molecules lie on a two-fold axis.The palladacyclopentane ring shows a half-chair conformation with the two-fold axis running through Pd and the middle of the C(β)-C(β') bond.A study of the thermal decomposition of the palladacyclopentanes has been carried out: (L=PPh3; L2=dppe, tmen, bipy, dcpe, or dppb) gives butenes as the major products; cyclobutane (L=PPh3) and ethylene (L2=dppe or dcpe) are also formed as minor products.By comparing these results with those for the decomposition of some methyl-substituted palladacyclopentanes, it is shown that the presence of ethylene is not attributable to fragmentation of the metallacyclic skeleton, but rather to the rupture of the P-C bonds of the diphosphine ligands.The decomposition of palladacyclopentanes is also induced by Bun2O*BF3: linear C4 hydrocarbons are formed.

In the radiolysis of n-pentane-neopentane solid systems, the esr spectrum is assigned to pentyl radicals.These results are inconsistent with the analysis of final products by capillary gas chromatography (GC).The yields of the main products as a function of the composition of the mixture could be explained by the H atom abstraction theory and molecular structure effect.The physical properties of the solid exert a subtle effect upon the processes iniated by γ-radiation.

New dineopentyl complexes of palladium, with the formula (L2 = dppe, bipy; L = PMe2Ph) have been prepared by the alkylation of the corresponding dichlorides with Mg(CH2CMe3)Br or LiCH2CMe3.Thermal decomposition of these complexes in solution yields 2,2,5,5-tetramethylhexane.The reactions of these compounds with CO and with some electrophiles (Ph3C+, PhCH2Br, HCl) have been investigated.The reaction with CO produces dineopentyl ketone (when L2 = dppe; L = PMe2Ph) or promotes rapid decomposition to 2,2,5,5-tetramethylhexane (when L2 = bipy).The electrophiles (E+) attack the neopentyl ligand to eliminate ECH2CMe3.In the reaction of with PhCH2Br, has been isolated.A palladacyclic analogue of these systems, , has been prepared by the alkylation of with Li(CH2CMe2CMe2CH2)Li.

The dimer yields in the radiolysis of the neo-C5H12-cyclo-C5H11 (1 molpercent) mixtures at 77 and 4 K have been measured by capillary gas chromatography.Analysis of the results indicates that cyclopentyl radicals are produced selectively at 77K, while their selective formation is suppressed at 4K.This result coincides well with the data obtained by ESR spectroscopy.

The thermal decompositions of several platinacyclobutanes-bis(triethylphosphine)-(1), bis(triisopropylphosphine)-(2), and bis(tricyclohexylphosphine)-3,3-dimethylplatinacyclobutane (3) -have been examined in cyclohexane solution.Decomposition of 1 proceeds heterogeneously: it products platinum metal, neopentane, 1,1-dimethylcycloprorane, and ethilene and seems to be autocatalytic.Decompositions conducted using reaction mixtures containing mercury(0) proceed homogeneously and produce predominately 1,1-dimethylcyclopropane; any platinum metal produced is apparently amalgamated by Hg(0) and rendered catalytically inactive.Thermal decompositions of 2 and 3 proceed homogeneously, even in the absence of Hg(0), and yield 1,1-dimethylcyclopropane, bis(trialkylphosphine)platinum(0), and small quantity of neopentane.The rates of the decomposition decreases by addition of trialkylphosphine, and a study of these dependence of the rate on concentration of added phosphine for 2 indicates that one phosphine dissociates to produce a three-coordinate platinum(II) intermediate prior to reductive elimination of 1,1-dimethylcyclopropane.The Arrhenius activation parameters for decomposition of 2 and 3 are respectively Ea=46+/-3 kcal mol-1, log A=23+/-2 and Ea=42+/-2 kcal mol-1, log A=21+/-1.Substitution of deuterium for hydrogen in the triisopropylphosphine groups of 2 produces no change in the rate of the reaction.We cannot distinguish between mechanisms in which reductive elimination of 1,1-dimethylcyclopropane occurs directly from the three-coordinate intermediate and those in which oxidative addition of a C-H bond to platinum bycyclometalation of the phosphine produces a Pt(IV) intermediate in a step preceding rate-limiting reductive elimination of 1,1-dimethylcyclopropane.

The coupling of the organic halides bromobenzene, 1-iodobutane, 1-iodo-2,2-dimethylpropane, and benzyl chloride has been carried out by electrochemically generating and continuously recycling the nickel(0) complex promoter .This species undergoes oxidative addition by organic halides leading to ?-bonded organometallic nickel(II) derivatives.In these complexes the metal-carbon bond can be cleaved either by a straightforward thermal decomposition or by a cathodic reduction, depending upon whether the co-ordinated organic group is an alkyl or an aryl one.In the former case a high yield of the coupling product is obtained only by employing organic halides not bearing hydrogen atoms in the β position; when a β-elimination reaction can occur, the yield of the coupling product is, on the contrary, very low and can be significantly improved only by carrying out the reductive process at more negative potentials, at which a proposed nickel hydride intermediate can be reduced.When the co-ordinated organic group is an aryl one, reductive elimination of the organic group does not occur by thermal decomposition and the coupling product can be formed only by means of a cathodic reduction of the relevant organometallic nickel(II) derivative.An overall mechanism is proposed which is consistent with the data.

Ketones are readily converted into the corresponding geminal dimethyl derivatives on reaction with dimethyltitanium dichloride under mild conditions; tertiary alcohols react similarly.

A study of photobehavior of the sterically congested tri- and di-tert-butylethenes 9, 21, and 25 has afforded additional insights into the excited singlet state behavior of alkenes.In pentane solution the carbene-derived products 11, 12, 24, and 32 were formed, respectively.In methanol tri-tert-butylethene (9) afforded additionally ether 14 and the rearranged alkenes 15 and 16 derived from protonation of carbene intermediate 10. 1,1-Di-tert-butyl analogue 21 afforded no detectable ether or rearranged alkene products.However, irradiation in methanol-O-d resulted in incorporation of deuterium at the vinyl positions of 21 recovered after partial conversion, apparently from deuteration-deprotonation of carbene intermediate 22.By contrast, 1,2-di-tert-butyl isomer 25 afforded no products attributable to protonation of the corresponding carbene intermediate 23 but gave ether 35b via 1,2 addition of methanol across the double bond of 25, perhaps involving protonation of the highly polarizable orthogonal 1(?,?*) intermediate.

The thermal decomposition of dineopentylbis(triethylphosphine)platinum(II) (1) in cyclohexane solution at 157 deg C yields bis(triethylphosphine)-3,3-dimethylplatinacyclobutane (4) by a reaction which involves dissociation of 1 equiv of triethylphosphine, intramolecular oxidative addition of the C-H bond of a neopentyl methyl group to platinum (3), and reductive elimination of neopentane.Carbon-carbon bond formation resulting in production of dineopentyl is a detectable side-reaction.The overal reaction has Arrenius activation parameters: Ea ca. 49 kcal mol-1, log A ca. 20.The activation energy for phosphine dissociation is 27 - 35 kcal mol-1.Transfer of a hydrogen atom from the triethylphosphine group to aneopentyl moiety occurs at a rate approximately 3percent that of transfer of hydrogen from the methyl of one neopentyl group to the methylene of the other.Any processes which abstract α-methylene hydrogens from the neopentyl group occur at less than 1percent the rate of processes which abstract hydrogens from the neopentyl methyl groups.Substitution of deuterium for hydrogen in either the neopentyl methyl groups or the triethylphosphine groups slows the decomposition reactions (kH/kD ca. 3.0).The mechanism proposed for generation of 4 is based in part on deuterium-labeling experiments: comparison of results by using different labeling patterns for 1 demonstrates the special utility of "inverted" experiments in which hydrogen transfer from a specific site is examined in a system which is otherwise perdeuterated.The driving force for the conversion of 1 to 4 is not obvious: it may be relief of steric strain in 1 , changes in electronic energy due to reorganization of ligands around platinum, or changes in entropy.