Accessing Valuable Ligand Supports for Transition Metals: A Modified, Intermediate Scale Preparation of 1,2,3,4,5-Pentamethylcyclopentadiene.

A reliable, intermediate scale preparation of 1,2,3,4,5-pentamethylcyclopentadiene (Cp*H) is presented, based on modifications of existing protocols that derive from initial 2-bromo-2-butene lithiation followed by acid mediated dienol cyclization. The revised synthesis and purification of the ligand avoids the use of mechanical stirring while still permitting access to significant quantities (39 g) of Cp*H in good yield (58%). The procedure offers other additional benefits, including a more controlled quench of excess lithium during the production of the intermediate heptadienols and a simplified isolation of Cp*H of sufficient purity for metallation with transition metals. The ligand was subsequently used to synthesize [Cp*MCl2]2 complexes of both iridium and ruthenium to demonstrate the utility of the Cp*H prepared and purified by our method. The procedure outlined herein affords substantial quantities of a ubiquitous ancillary ligand support used in organometallic chemistry while minimizing the need for specialized laboratory equipment, thus providing a simpler and more accessible entry point into the chemistry of 1,2,3,4,5-pentamethylcyclopentadiene.

Cp*Co(IPr): synthesis and reactivity of an unsaturated Co(i) complex.

Synthesis of coordinatively unsaturated Cp*Co(IPr) (2), is accomplished by addition of free N-heterocyclic carbene IPr to [(Cp*Co)2-µ-(η(4):η(4)-toluene)] (1). Stoichiometric reactivity is consistent with a 16 electron species, as 2 undergoes ligand addition/NHC displacement and reversible reaction with dihydrogen. Cp*Co(IPr) represents an elusive example of a stable Cp*CoL fragment.

Synthesis of a bis(indenyl) Co(I) anion: a reactive source of a 14 electron indenyl Co(I) equivalent.

Alkali metal reduction of (η(5)-C9H5-1,3-(SiMe3)2)2Co (1) in tetrahydrofuran (THF) permits isolation of the unusual and reactive 20 electron Co(I) anion [Na(THF)6][(η(5)-C9H5-1,3-(SiMe3)2)2Co] (2). Crystallographic characterization of both 1 and 2 provide support for the one electron reduction from Co(II) to Co(I). Reactivity studies of 2 are further consistent with a Co(I) equivalent, based on both one electron chemical oxidation to reform 1 and reaction with a variety of σ and π donors. Upon addition of pyridines or vinyltrimethylsilane to 2, known dimer [(C9H5-1,3-(SiMe3)2)2Co2] (3) is formed, likely through 16 electron (η(5)-C9H5-1,3-(SiMe3)2)Co(L) intermediates. Ethylene addition to 2 establishes an equilibrium between (η(5)-C9H5-1,3-(SiMe3)2)Co(η(2)-H2C═CH2)2 (8) and 2, suggestive of reversible ligand ejection from 2. Crossover experiments between a related metal indenide salt and 2 confirm ligand extrusion from the anion, even in the absence of strong supporting donors. Reaction of 2 with PMe3 results in formation of 3, (η(5)-C9H5-1,3-(SiMe3)2)Co(PMe3)2 (13), and a paramagnetic species, with the product ratios being highly dependent on the conditions of synthesis. Collectively, 2 demonstrates an alternative entry point into the chemistry of 14 electron Co(I) equivalents when compared to typical ligand loss from neutral 18 electron cyclopentadienyl cobalt bis(ligand) complexes, perhaps permitting generation of low electron count species more effective for small molecule activation.

sp2 C-H activation of dimethyl fumarate by a [(Cp*Co)2-µ-(η4 : η4-toluene)] complex.

The well-defined oxidative addition of the vinylic sp(2) C-H bond of dimethyl fumarate is mediated by the cobalt triple decker complex [(Cp*Co)(2)-µ-(η(4) : η(4)-toluene)] (1) at ambient temperature, affording the dinuclear, bridging cobalt hydride, fumaryl compound (2). The C-H activation product has been characterized by mass spectrometry, NMR spectroscopy, and X-ray crystallography. Computational studies of 2 support asymmetric bonding interactions between the two metal centres and the bridging hydride/fumaryl fragments. Monitoring the reaction of dimethyl fumarate with 1 by (1)H NMR spectroscopy allows observation of intermediate [Cp*Co(MeO(2)CCH=CHCO(2)Me)](n) (n = 1 or 2) (3). Addition of 4 equivalents of dimethyl fumarate to 1 results in rapid formation of the bis(ligand) adduct Cp*Co(η(2)-MeO(2)CCH=CHCO(2)Me)(2) (5). Reversibility of the C-H activation was probed by reaction of additional dimethyl fumarate with 2, suggesting ligand induced reductive elimination is possible under ambient conditions. Reaction between 2 and strong σ or π ligands, such as PMe(3) or CO, affords the corresponding Cp*Co(η(2)-MeO(2)CCH=CHCO(2)Me)(L) (L = PMe(3) (7); L = CO (8)) complexes when heated, demonstrating the ability of 2 to undergo two electron redox processes. Further evidence for reversible C-H activation is provided by the isomerization of dimethyl maleate to the corresponding fumarate using 2, suggesting the complex can serve as a source of Co(I) under the appropriate catalytic conditions.

Development of more labile low electron count Co(I) sources: mild, catalytic functionalization of activated alkanes using a [(Cp*Co)2-µ-(η4:η4-arene)] complex.

Catalytic transfer dehydrogenation of silyl protected amines, requiring sp(3) C-H bond activation, is mediated by a bridging arene complex of the type [(Cp*Co)(2)-µ-(η(4):η(4)-arene)] under mild conditions. Mechanistic and qualitative rate studies establish the compound as a more reactive Co(I) source when compared to other known Cp*Co(I) complexes.

N2 hydrogenation from activated end-on bis(indenyl) zirconium dinitrogen complexes.

Sodium amalgam reduction of the bis(indenyl)zirconium dihalide complexes, (eta5-C9H5-1-iPr-3-Me)2ZrX2 (X = Cl, Br, I), yielded the corresponding end-on dinitrogen complexes, [(eta5-C9H5-1-iPr-3-Me)2Zr(NaX)]2(mu2, eta1, eta1-N2), with inclusion of 1 equiv of salt per zirconocene. The solid state structures of the chloro and iodo congeners establish short Zr N and elongated N N bonds, consistent with modest to strong activation of the coordinated dinitrogen molecule. Exposure of the N2 compounds to 1 atm of dihydrogen resulted in rapid N H bond formation to yield a hydrido zirconocene hydrazido compound concomitant with salt elimination. These studies establish a new structural type of zirconocene dinitrogen complex and demonstrate that side-on coordination of the N2 ligand in the ground state is not a prerequisite for dinitrogen hydrogenation.

Treatment with 1-alpha,25-dihydroxyvitamin D3 (vitamin D3) to inhibit carcinogenesis in the hamster buccal pouch model.

OBJECTIVE: To investigate whether systemic therapy with 1-alpha,25-dihydroxyvitamin D(3) (vitamin D(3) [hereinafter, VD(3)]) prevents tumor formation in a hamster buccal pouch model of carcinogenesis. DESIGN: Randomized trial in which a known carcinogen, 7,12-dimethylbenz[a]anthracene (DMBA), was applied to the buccal pouch of 40 hamsters. Animals were randomized to receive systemic VD(3) or no treatment and killed at 2, 6, and 14 weeks after the initiation of DMBA exposure. SETTING: Academic medical center. SUBJECTS: Forty male golden Syrian hamsters, aged 5 to 6 weeks, were used. INTERVENTIONS: A dose of 0.25 mug/kg of VD(3) via intraperitoneal injection was given to 20 animals 3 times per week. Of the remaining 20 control animals, 5 received placebo vehicle injection, and 15 received no further treatment. MAIN OUTCOME MEASURES: Timing, size, and number of tumors that developed in the 2 groups. RESULTS: Only 1 hamster treated with VD(3) developed a confirmed neoplasm compared with 7 of the control animals (P < .01). The mean +/- SD total diameter of gross lesions per animal in the VD(3)-treated group was 1.2 +/- 1.9 mm compared with 6.8 +/- 6.6 mm in the control group (P = .03). The time to onset of lesion formation was significantly delayed in those animals treated with VD(3), with a mean +/- SD time to development of 13.4 +/- 0.9 weeks, while the control animals developed lesions at 11.2 +/- 1.7 weeks (P = .02). CONCLUSIONS: Systemic VD(3) therapy delays carcinogenesis in the hamster buccal pouch model. Further investigation into the mechanisms through which VD(3) inhibits carcinogenesis may lead to development of effective chemopreventive agents to combat head and neck cancer.

Carbon-oxygen bond cleavage with eta9,eta5-bis(indenyl)zirconium sandwich complexes.

Treatment of the eta9,eta5-bis(indenyl)zirconium sandwich complex, (eta9-C9H5-1,3-(SiMe3)2)(eta5-C9H5-1,3-(SiMe3)2)Zr, with dialkyl ethers such as diethyl ether, CH3OR (R=Et, nBu, tBu), nBu2O, or iPr2O resulted in facile C-O bond scission furnishing an eta5,eta5-bis(indenyl)zirconium alkoxy hydride complex and free olefin. In cases where ethylene is formed, trapping by the zirconocene sandwich yields a rare example of a crystallographically characterized, base-free eta5,eta5-bis(indenyl)zirconium ethylene complex. Observation of normal, primary kinetic isotope effects in combination with rate studies and the stability of various model compounds support a mechanism involving rate-determining C-H activation to yield an eta5,eta5-bis(indenyl)zirconium alkyl hydride intermediate followed by rapid beta-alkoxide elimination. For isolable eta6,eta5-bis(indenyl)zirconium THF compounds, thermolysis at 85 degrees C also resulted in C-O bond cleavage to yield the corresponding zirconacycle. Both mechanistic and computational studies again support a pathway involving haptotropic rearrangement to eta5,eta5-bis(indenyl)zirconium intermediates that promote rate-determining C-H activation and ultimately C-O bond scission.

Translation initiation factor eIF4G-1 binds to eIF3 through the eIF3e subunit.

eIF3 in mammals is the largest translation initiation factor ( approximately 800 kDa) and is composed of 13 nonidentical subunits designated eIF3a-m. The role of mammalian eIF3 in assembly of the 48 S complex occurs through high affinity binding to eIF4G. Interactions of eIF4G with eIF4E, eIF4A, eIF3, poly(A)-binding protein, and Mnk1/2 have been mapped to discrete domains on eIF4G, and conversely, the eIF4G-binding sites on all but one of these ligands have been determined. The only eIF4G ligand for which this has not been determined is eIF3. In this study, we have sought to identify the mammalian eIF3 subunit(s) that directly interact(s) with eIF4G. Established procedures for detecting protein-protein interactions gave ambiguous results. However, binding of partially proteolyzed HeLa eIF3 to the eIF3-binding domain of human eIF4G-1, followed by high throughput analysis of mass spectrometric data with a novel peptide matching algorithm, identified a single subunit, eIF3e (p48/Int-6). In addition, recombinant FLAG-eIF3e specifically competed with HeLa eIF3 for binding to eIF4G in vitro. Adding FLAG-eIF3e to a cell-free translation system (i) inhibited protein synthesis, (ii) caused a shift of mRNA from heavy to light polysomes, (iii) inhibited cap-dependent translation more severely than translation dependent on the HCV or CSFV internal ribosome entry sites, which do not require eIF4G, and (iv) caused a dramatic loss of eIF4G and eIF2alpha from complexes sedimenting at approximately 40 S. These data suggest a specific, direct, and functional interaction of eIF3e with eIF4G during the process of cap-dependent translation initiation, although they do not rule out participation of other eIF3 subunits.

Synthesis of bis(indenyl)zirconium dihydrides and subsequent rearrangement to eta5,eta3-4,5-dihydroindenediyl ligands: evidence for intermediates during the hydrogenation to tetrahydroindenyl derivatives.

Exposure of eta9,eta5-bis(indenyl)zirconium sandwich complexes to 4 atm of H2 resulted in facile oxidative addition to furnish the corresponding zirconocene dihydrides, (eta5-C9H5-1,3-R2)2ZrH2 (R = SiMe3, SiMe2Ph, CHMe2). Continued hydrogenation completed conversion to the tetrahydroindenyl derivatives, (eta5-C9H9-1,3-R2)2ZrH2. Deuterium labeling studies established that dihydrogen (dideuterium) addition to the benzo rings is intramolecular and stereospecific, occurring solely from the endo face of the ligand, proximal to the zirconium. In the absence of dihydrogen, the bis(indenyl)zirconium dihydrides rearranged to new zirconium monohydride complexes containing an unusual eta5,eta3-4,5-dihydroindenediyl ligand, arising from metal-to-benzo ring hydrogen transfer. Mechanistic studies, including a normal, primary kinetic isotope effect measured at 23 degrees C, are consistent with a pathway involving regio- and stereoselective insertion of a benzo C=C bond into a zirconium hydride. The stereochemistry of the insertion reaction, and hence the eta5,eta3-4,5-dihydroindenediyl product, is influenced by the presence of donor ligands and controlled by the preferred conformation of the indenyl rings. Exposure of the zirconium hydrides containing the eta5,eta3-4,5-dihydroindenediyl rings to 1 atm of dihydrogen afforded the tetrahydroindenyl zirconium dihydride complexes, establishing the intermediacy of this unusual coordination environment during benzo ring hydrogenation.

Ligand-induced haptotropic rearrangements in bis(indenyl)zirconium sandwich complexes.

Addition of principally sigma-donating ligands such as THF, chelating diethers, or 1,2-bis(dimethyl)phosphinoethane to eta(9),eta(5)-bis(indenyl)zirconium sandwich complexes, (eta(9)-C(9)H(5)-1,3-R(2))(eta(5)-C(9)H(5)-1,3-R(2))Zr (R = alkyl or silyl), induces haptotropic rearrangement to afford (eta(6)-C(9)H(5)-1,3-R(2))(eta(5)-C(9)H(5)-1,3-R(2))ZrL adducts. Examples where L = THF and DME have been characterized by X-ray diffraction and revealed significant buckling of the eta(6) benzo ring, consistent with reduction of the arene, and highlight the importance of the zirconium(IV) canonical form. For the THF-induced haptotropic rearrangements, the thermodynamic driving force for ring migration has been measured as a function of indenyl substituent and demonstrates silylated sandwiches favor THF coordination and the eta(6),eta(5) bonding motif over their alkylated counterparts. In the case of chelating diethers, measurement of the corresponding equilibrium constants establish more stable eta(6),eta(5) adducts with five- over four-membered chelates and with smaller oxygen and carbon backbone substituents. Kinetic studies on both THF and DME addition to (eta(9)-C(9)H(5)-1,3-(SiMe(3))(2))(eta(5)-C(9)H(5)-1,3-(SiMe(3))(2))Zr established a first-order dependence on the incoming ligand, consistent with a mechanism involving direct attack of the incoming nucleophile on the eta(9),eta(5) sandwich. These results further highlight the ability of the indenyl ligand to smoothly adjust hapticity to meet the electronic requirements of the metal center.

Zirconium sandwich complexes with eta(9) indenyl ligands: well-defined precursors for zirconocene-mediated coupling reactions.

A family of isolable, well-defined bis-indenyl zirconium sandwich complexes, (eta(5)-C(9)H(5)-1,3-R(2))(eta(9)-C(9)H(5)-1,3-R(2))Zr (R = silyl, alkyl), have been prepared by either alkane reductive elimination or alkali metal reduction of a suitable zirconium(IV) dihalide precursor. Crystallographic characterization of two of these derivatives, R = SiMe(2)CMe(3) and CHMe(2), reveals unprecedented eta(9) coordination of one of the indenyl ligands. Variable-temperature and EXSY NMR studies establish that the eta(5) and eta(9) rings are rapidly interconverting in solution. The sandwich complexes serve as effective sources of low-valent zirconium reacting rapidly with both olefins and alkynes at ambient temperature. In contrast to bis-cyclopentadienyl chemistry, the olefin adducts of the bis-indenyl zirconium sandwiches undergo preferential C-H activation to yield the corresponding allyl hydride compounds, although reaction with excess olefin proceeds through the eta(2)-olefin adduct, forming the corresponding zirconacyclopentane.

Synthesis of a zirconium sandwich complex and crystallographic characterization of its adduct with tetrahydrofuran.

Reductive elimination of alkane from a silylated bis-indenyl zirconium isobutyl hydride affords a zirconium sandwich complex with an unusual eta6 indenyl ligand. Crystallographic characterization of its adduct with tetrahydrofuran has been achieved and reveals significant localization in the six-membered ring. Complexation of more potent ligands such as carbon monoxide and diphenylacetylene are effective in promoting haptotropic rearrangement of the eta6 indenyl ligand and provides familiar bent zirconocene derivatives. The zirconium sandwich compound also undergoes oxidative addition of carbon-hydrogen bonds of pyridyl ligands, resulting in a crystallographically characterized dimethylamino pyridyl hydride complex of zirconium.

Mass spectrometric analysis of the N terminus of translational initiation factor eIF4G-1 reveals novel isoforms.

In eukaryotes, translation initiation factor 4G (eIF4G) acts as the central binding protein for an unusually large number of proteins involved in mRNA metabolism. Several gene products homologous to eIF4G have been described, the most studied being eIF4G-1. By its association with other initiation factors, eIF4G-1 effects mRNA cap and poly(A) recognition, unwinding of secondary structure, and binding to the 43S initiation complex. Multiple electrophoretic isoforms of eIF4G-1 are observed, and multiple cDNAs have been reported, yet the relationship between the two is not known. We report here a new cDNA for eIF4G-1, present as a previously unidentified human expressed sequence tag, that extends the long open reading frame, provides a new in-frame initiation codon, and predicts a longer form of eIF4G-1 than reported previously. eIF4G isoforms from human K562 cells were cleaved with recombinant Coxsackievirus 2A protease and the N- terminal domains purified by m(7)GTP-Sepharose chromatography and polyacrylamide gel electrophoresis. Proteins were digested with proteolytic enzymes and peptides masses determined by matrix-assisted laser desorption ionization-time of flight mass spectrometry. In selected cases, peptides were sequenced by electrospray-mass spectrometry fragmentation. This identified the N termini of the three most abundant eIF4G-1 isoforms, two of which had not previously been proposed. These proteins appear to have been initiated from three different AUG codons.