Hydroamination of alkenes with dinitrogen and titanium polyhydrides.

An ideal synthesis of alkyl amines would involve the direct use of abundant and easily accessible molecules such as dinitrogen (N2) and feedstock alkenes1-4. However, this ambition remains a great challenge as it is usually difficult to simultaneously activate both N2 and a simple alkene and combine them together through carbon-nitrogen (C-N) bond formation. Currently, the synthesis of alkyl amines relies on the use of ammonia produced through the Haber-Bosch process and prefunctionalized electrophilic carbon sources. Here we report the hydroamination of simple alkenes with N2 in a trititanium hydride framework, which activates both alkenes and N2, leading to selective C-N bond formation and providing the corresponding alkyl amines on further hydrogenation and protonation. Computational studies reveal key mechanistic details of N2 activation and selective C-N bond formation. This work demonstrates a strategy for the transformation of N2 and simple hydrocarbons into nitrogen-containing organic compounds mediated by a multinuclear hydride framework.

Dinitrogen Cleavage and Multicoupling with Isocyanides in a Dititanium Dihydride Framework.

Dinitrogen (N2) activation and functionalization through N-N bond cleavage and N-C bond formation are of great interest and importance but remain highly challenging. We report here for the first time N2 cleavage and selective multicoupling with isocyanides in a dititanium dihydride framework. The reaction of a dinitrogen dititanium dihydride complex [{(acriPNP)Ti}2(µ-η1:η2-N2)(µ-H)2] (1) with an excess (four or more equivalents) of p-methoxyphenyl isocyanide at room temperature gave a novel amidoamidinatoguanidinate complex [(acriPNP)Ti{NC(═NR)NC(═NR)CH2NR}Ti(acriPNP)(CNR)] (2, acriPNP = 4,5-bis(diisopropylphosphino)-2,7,9,9-tetramethyl-9H-acridin-10-ide; R = p-MeOC6H4) through N2 splitting and coupling with three isocyanide molecules. When 1 equiv of p-methoxyphenyl isocyanide was used to react with 1 at -30 °C, the hydrogenation of the isocyanide unit by the two hydride ligands in 1 took place, affording an amidomethylene-bridged dititanium dinitrogen complex [{(acriPNP)Ti}2(µ-η1:η2-N2){µ-η1:η2-CH2N(p-MeOC6H4)}] (3), which upon reaction with another equivalent of p-methoxyphenyl isocyanide at room temperature gave an amidomethylene/nitrido/carbodiimido complex [(acriPNP)Ti(N═C═NR)(µ-N)(µ-η1:η2-CH2NR)Ti(acriPNP)] (4) through N2 cleavage and N═C bond formation. Further reaction of 4 with 1 equiv of p-methoxyphenyl isocyanide led to an unprecedented four-component (carbodiimido, nitrido, isocyanide, and amidomethylene) coupling, yielding an amidoamidinatoguanidinate complex [{(acriPNP)Ti}2{NC(═NR)NC(═NR)CH2NR}] (5), which on reaction with another equivalent of p-methoxyphenyl isocyanide afforded the isocyanide-coordinated analogue 2. The reaction of 1 with 2-naphthyl isocyanide also took place in a similar multicoupling fashion. Moreover, the cross-coupling reactions of the p-methoxyphenyl isocyanide-derived amidomethylene/nitrido/carbodiimido complex 4 with 2-naphthyl isocyanide, cyclohexyl isocyanide, and tert-butyl isocyanide were also achieved, which afforded the corresponding amidoamidinatoguanidinate products consisting of two different isocyanides. Density functional theory (DFT) calculations further elucidated the mechanistic details.

Hydrodeoxygenative Coupling and Transformation of Aldehydes at a N2-Derived Tetranuclear Titanium Imide/Hydride Framework.

Carbon-carbon bond formation via coupling of two organic components is among the most important chemical transformations in organic synthesis. Herein, we report an unprecedented hydrodeoxygenative coupling of aromatic aldehydes to form bibenzyls by a N2-derived tetranuclear titanium imide/hydride complex [(Cp'Ti)4(µ3-NH)2(µ-H)4] (1; Cp' = C5Me4SiMe3). Further reactions with the corresponding aldehydes under air afford hydrobenzamides together with a titanium oxo complex. Both hydride and imide ligands play an important role for the reductive coupling, hydrogenation processes, as well as the functionalization of the N2-derived imide units without the need of sacrificial reagents. These results demonstrate that the tetranuclear titanium imide/hydride framework is not only applicable for N2 activation and functionalization but also providing a new platform for the C-C bond formation using carbonyl compounds.

Aza-Michael Addition of Dinitrogen to α,ß-Unsaturated Carbonyl Compounds in a Dititanium Framework.

The direct use of dinitrogen (N2) as a building block for the synthesis of NN-containing organic compounds is of fundamental interest and practical importance but has remained a formidable challenge to date. Here, we report an unprecedented 1,4-conjugate (aza-Michael) addition of N2 to α,ß-unsaturated carbonyl compounds in a dititanium framework. The resulting hydrazinopropenolate products could be easily converted to diverse NN-containing organic compounds such as ß-hydrazine-functionalized esters and amides, pyrazolidinones, and pyrazolines depending on the types of Michael acceptors through protonation with MeOH. Further transformations of a hydrazinopropenolate titanium complex through C-C and N-C bond formations with electrophiles such as CO2 and benzaldehyde have also been achieved. The mechanistic details of the N2 addition reaction have been elucidated by computational studies, revealing the importance of redox-active metal centers in this event. This work showcases the potential of using N2 as a building block for the synthesis of NN-containing organic compounds through activation and functionalization in a molecular metal framework.

Dinitrogen Activation and Addition to Unsaturated C-E (E=C, N, O, S) Bonds Mediated by Transition Metal Complexes.

Dinitrogen (N2 ) activation and functionalization is of fundamental interest and practical importance. This review focuses on N2 activation and addition to unsaturated substrates, including carbon monoxide, carbon dioxide, heteroallenes, aldehydes, ketones, acid halides, nitriles, alkynes, and allenes, mediated by transition metal complexes, which afforded a variety of N-C bond formation products. Emphases are placed on the reaction modes and mechanisms. We hope that this work would stimulate further explorations in this challenging field.

Dinitrogen Cleavage and Functionalization with Carbon Dioxide in a Dititanium Dihydride Framework.

The activation and functionalization of dinitrogen (N2) with carbon dioxide (CO2) are of great interest and importance but highly challenging. We report here for the first time the reaction of N2 with CO2 in a dititanium dihydride framework, which leads to N-C bond formation and N-N and C-O bond cleavage. Exposure of a dinitrogen dititanium hydride complex {[(acriPNP)Ti]2(µ2-η1:η2-N2)(µ2-H)2} (1) (acriPNP = 4,5-bis(diisopropylphosphino)-2,7,9,9-tetramethyl-9H-acridin-10-ide) to a CO2 atmosphere at room temperature rapidly yielded a nitrido/N,N-dicarboxylamido complex {[(acriPNP)Ti]2(µ2-N)[µ2-N(CO2)2]} (2, 28%) and a diisocyanato/dioxo complex {[(acriPNP)Ti]2(NCO)2(µ2-O)2} (3, 52%) with release of H2. When the reaction of 1 with CO2 (1 atm) was carried out at -50 °C, complex 2 was selectively formed in 82% yield within 5 min. Heating 2 at 80 °C under 1 atm CO2 for 30 min afforded 3 in 67% yield. When 1 was allowed to react with 1.5 equiv of CO2 at room temperature, an isocyanato/nitrido/oxo complex {[(acriPNP)Ti]2(NCO)(µ2-N)(µ2-O)} (4) was exclusively formed in 89% yield within 5 min. The reaction of 4 with CO2 at room temperature almost quantitatively yielded the dioxo/diisocyanato complex 3 within 5 min. The mechanistic details were clarified by the 15N- and 13C-labeled experiments and density functional theory (DFT) calculations, providing unprecedented insights into the reaction of N2 with CO2. A titanium-mediated cycle for the synthesis of trimethylsilyl isocyanate Me3SiNCO from N2, CO2, and Me3SiCl using H2 as a reducing agent was also established.

Carboxylesterases for the hydrolysis of acetoacetate esters and their applications in terpenoid production using Escherichia coli.

Pathway engineering is a useful technology for producing desired compounds on a large scale by modifying the biosynthetic pathways of host organisms using genetic engineering. We focused on acetoacetate esters as novel low-cost substrates and established an efficient terpenoid production system using pathway-engineered recombinant Escherichia coli. Functional analysis using recombinant E. coli proteins of 18 carboxylesterases identified from the microbial esterases and lipases database showed that the p-nitrobenzyl esterase (PnbA) from Bacillus subtilis specifically hydrolyzed two acetoacetate esters: methyl acetoacetate (MAA) and ethyl acetoacetate (EAA). We generated a plasmid (pAC-Mev/Scidi/Aacl/PnbA) co-expressing PnbA and six enzymes of the mevalonate pathway gene cluster from Streptomyces, isopentenyl diphosphate isomerase type I from Saccharomyces cerevisiae, and acetoacetyl-coenzyme A ligase from Rattus norvegicus. The plasmid pAC-Mev/Scidi/Aacl/PnbA was introduced into E. coli along with plasmid expressing carotenoid (lycopene) or sesquiterpene (ß-bisabolene) biosynthesis genes, and the terpenoid production was evaluated following the addition of acetoacetate esters as substrates. These recombinant E. coli strains used MAA and EAA as substrates for the biosynthesis of terpenoids and produced almost equivalent concentrations of target compounds compared with the previous production system that used mevalonolactone and lithium acetoacetate. The findings of this study will enable the production of useful terpenoids from low-cost substrates, which may facilitate their commercial production on an industrial scale in the future. KEY POINTS: • PnbA from Bacillus subtilis exhibits acetoacetate hydrolysis activity. • A plasmid enabling terpenoid synthesis from acetoacetate esters was constructed. • Acetoacetate esters as substrates enable a low-cost production of terpenoids.

Hydrodeoxygenative Cyclotetramerization of Carbon Monoxide by a Trinuclear Titanium Polyhydride Complex.

The reductive coupling of carbon monoxide (CO) by metal hydrides is of fundamental interest and practical importance. Herein we report an unprecedented hydrodeoxygenative cyclotetramerization of CO by a trinuclear titanium polyhydride complex [(C5Me4SiMe3)Ti]3(µ3-H)(µ2-H)6 (1). The reaction of CO with 1 at -78 °C gave an ethen-1,2-diyl species [CH═CH]2- through the hydrodeoxygenative dimerization of two molecules of CO, which upon cycloaddition to another two molecules of CO afforded a cyclobuten-3,4-diyl-1,2-diolate unit [C4H2O2]4-. The hydrogenolysis of the [C4H2O2]4- species with H2 yielded a tetrahydrocyclobuten-1,2-diolate species [C4H4O2]2-, which on heating at 100 °C gave a cyclobuten-2-yl-1-olate product [C4H4O]2-. The acidolysis of the [C4H2O2]4- and [C4H4O]2- species with HCl afforded γ-butyrolactone and cyclobutanone, respectively.

Dinitrogen Activation and Hydrogenation by C5Me4SiMe3-Ligated Di- and Trinuclear Chromium Hydride Complexes.

Activation of dinitrogen (N2) by well-defined metal hydrides is of much interest and importance, but studies in this area have remained limited to date. We report here N2 activation and hydrogenation by C5Me4SiMe3-ligated di- and trinuclear chromium polyhydride complexes. Hydrogenolysis of [Cp'Cr(µ-Me)2CrCp'] (Cp' = C5Me4SiMe3) (1) with H2 in a dilute hexane solution under N2-free conditions affords the dichromium dihydride complex [Cp'Cr(µ-H)2CrCp'] (2), while hydrogenolysis of 1 in a concentrated solution or without solvent provides the trinuclear chromium tetrahydride complex [(Cp'Cr)3(µ3-H)(µ-H)3] (3). When the reaction is carried out in the presence of N2 in a dilute hexane solution, the tetranuclear diimide/dihydride complex [(Cp'Cr)4(µ3-NH)2(µ3-H)2] (4) is formed via N-N bond cleavage and N-H bond formation. The reaction of 2 with N2 at room temperature gives the tetranuclear imide/nitride/dihydride complex [(Cp'Cr)3(C5Me3(CH2)SiMe3)Cr(µ3-NH)(µ3-N)(µ-H)2] (5) via N2 cleavage and hydrogenation and C-H bond activation of a Cp methyl group. At -30 °C, the reaction of 2 with N2 affords the dinitride intermediate [(Cp'Cr)4(µ3-N)2(µ3-H)2] (6), which is quantitatively transformed to 5 at room temperature. Complex 5 reversibly converts to the stereoisomer 5'. The hydrogenation of a mixture of 5 and 5' with H2 affords 4. The reaction of 3 with N2 proceeds at 100 °C to afford [(Cp'Cr)3(µ3-NH)2] (7). This transformation has also been investigated by DFT calculations. Both experimental and computational studies suggest that N2 incorporation into the chromium hydride cluster is involved in the rate-determining step. This work represents the first example of N2 cleavage and hydrogenation by well-defined chromium hydride complexes.

Carbon-carbon bond cleavage and rearrangement of benzene by a trinuclear titanium hydride.

The cleavage of carbon-carbon (C-C) bonds by transition metals is of great interest, especially as this transformation can be used to produce fuels and other industrially important chemicals from natural resources such as petroleum and biomass. Carbon-carbon bonds are quite stable and are consequently unreactive under many reaction conditions. In the industrial naphtha hydrocracking process, the aromatic carbon skeleton of benzene can be transformed to methylcyclopentane and acyclic saturated hydrocarbons through C-C bond cleavage and rearrangement on the surfaces of solid catalysts. However, these chemical transformations usually require high temperatures and are fairly non-selective. Microorganisms can degrade aromatic compounds under ambient conditions, but the mechanistic details are not known and are difficult to mimic. Several transition metal complexes have been reported to cleave C-C bonds in a selective fashion in special circumstances, such as relief of ring strain, formation of an aromatic system, chelation-assisted cyclometallation and ß-carbon elimination. However, the cleavage of benzene by a transition metal complex has not been reported. Here we report the C-C bond cleavage and rearrangement of benzene by a trinuclear titanium polyhydride complex. The benzene ring is transformed sequentially to a methylcyclopentenyl and a 2-methylpentenyl species through the cleavage of the aromatic carbon skeleton at the multi-titanium sites. Our results suggest that multinuclear titanium hydrides could serve as a unique platform for the activation of aromatic molecules, and may facilitate the design of new catalysts for the transformation of inactive aromatics.

Synthesis and Diverse Transformations of a Dinitrogen Dititanium Hydride Complex Bearing Rigid Acridane-Based PNP-Pincer Ligands.

Studies on N2 activation and transformation by transition metal hydride complexes are of particular interest and importance. The synthesis and diverse transformations of a dinitrogen dititanium hydride complex bearing the rigid acridane-based acri PNP-pincer ligands {[(acri PNP)Ti]2 (µ2 -η1 :η2 -N2 )(µ2 -H)2 } are presented. This complex enabled N2 cleavage and hydrogenation even without additional H2 or other reducing agents. Furthermore, diverse transformations of the N2 unit with a variety of organometallic compounds such as ZnMe2 , MgMe2 , AlMe3 , B(C6 F5 )3 , PinBH, and PhSiH3 have been well established at the rigid acri PNP-ligated dititanium framework, such as reversible bonding-mode change between the end-on and side-on/end-on fashions, diborylative N=N bond cleavage, the formal insertion of two dimethylaluminum species into the N=N bond, and the formal insertion of two silylene units into the N=N bond. This work has revealed many unprecedented aspects of dinitrogen reaction chemistry.

Experimental and Computational Studies of Dinitrogen Activation and Hydrogenation at a Tetranuclear Titanium Imide/Hydride Framework.

The activation of N2 by a tetranuclear titanium(III) diimide/tetrahydride complex, [(Cp'Ti)4(µ3-NH)2(µ-H)4] (1) (Cp' = C5Me4SiMe3), which was obtained by the reaction of the Cp'-ligated titanium trialkyl complex Cp'Ti(CH2SiMe3)3 with H2 and N2, was investigated in detail by experimental and density functional theory studies. The reaction of 1 in the solid state with N2 (1 atm) at 180 °C gave the dinitride/diimide complex [(Cp'Ti)4(µ3-N)2(µ3-NH)2] (2) through the incorporation, cleavage, and partial hydrogenation of one molecule of N2 and release of two molecules of H2. At 130 °C, the formation of 2 was not observed, but instead, dehydrogenation of 1 took place through cleavage of the N-H bond in an imide ligand, followed by deprotonation of the other imide ligand with a hydride ligand, affording the dinitride/tetrahydride complex [(Cp'Ti)4(µ3-N)2(µ-H)4] (3). Upon heating under N2 (1 atm) at 180 °C, 3 was quantitatively converted to the dinitride/diimide complex 2. This transformation was initiated by migration of a hydride ligand to a nitride ligand to give one imide unit, followed by N2 coordination to a Ti atom and H2 release through the reductive elimination of two hydride ligands. The other imide ligand in 2 was formed by hydride migration to one of the two nitride ligands generated through the cleavage of the newly incorporated N2 unit. The hydrogenation of 2 with H2 (100 atm) at 180 °C afforded the tetraimide complex [(Cp'Ti)4(µ3-NH)4] (4). This reaction was initiated by σ-bond metathesis between H2 and a titanium-nitride bond, followed by migration of the resulting hydride ligand to the remaining nitride ligand. In all of these transformations, the interplay among the hydride, imide, and nitride ligands, including the reversible dehydrogenation/hydrogenation of imide and nitride species, at the multimetallic titanium framework has a critically important role.

Dinitrogen Activation by Dihydrogen and a PNP-Ligated Titanium Complex.

The hydrogenolysis of the PNP-ligated titanium dialkyl complex {(PNP)Ti(CH2SiMe3)2} (1, PNP = N(C6H3-2-PiPr2-4-CH3)2) with H2 (1 atm) in the presence of N2 (1 atm) afforded a binuclear titanium side-on/end-on dinitrogen complex {[(PNP)Ti]2(µ2,η1,η2-N2)(µ2-H)2} (2) at room temperature, which upon heating at 60 °C with H2 gave a µ2-imido/µ2-nitrido/hydrido complex {[(PNP)Ti]2(µ2-NH)(µ2-N)H} (3) through the cleavage and partial hydrogenation of the N2 unit. The mechanistic aspects of the hydrogenation of the N2 unit in 2 with H2 have been elucidated by the density functional theory calculations.

Gyroscope-Like Complexes Based on Dibridgehead Diphosphine Cages That Are Accessed by Three-Fold Intramolecular Ring Closing Metatheses and Encase Fe(CO)3, Fe(CO)2(NO)(+), and Fe(CO)3(H)(+) Rotators.

Reactions of trans-Fe(CO)3(P((CH2)mCH═CH2)3)2 (m = a/4; b/5, c/6, e/8) and Grubbs' catalyst (12-24 mol %, CH2Cl2, reflux) give the cage-like trienes trans- Fe(CO)3(P((CH2)mCH═CH(CH2)m)3 P) (3a-c,e, 60-81%). Hydrogenations (ClRh(PPh3)3, 60-80 °C) yield the title compounds trans- Fe(CO)3(P((CH2)n)3 P) (4a-c,e, 74-86%; n = 2m + 2), which have idealized D3h symmetry. A crystal structure of 4c suggests enough van der Waals clearance for the Fe(CO)3 moiety to rotate within the three P(CH2)14P linkages; structures of E,E,E-3a show rotation to be blocked by the shorter P(CH2)4CH═CH(CH2)4P linkages. Additions of NO(+)BF4(-) give the isoelectronic and isosteric cations [ Fe(CO)2(NO)(P((CH2)n)3 P)](+)BF4(-) (5a-c(+)BF4(-); 81-98%). Additions of [H(OEt2)2](+)BArf(-) (BArf = B(3,5-C6H3(CF3)2)4) afford the metal hydride complexes mer,trans-[ Fe(CO)3(H)(P((CH2)n)3 P)](+)BArf(-) (6a-c,e(+)BArf(-); 98-99%). The behavior of the rotators in the preceding complexes is probed by VT NMR. At ambient temperature in solution, 5a,b(+)BF4(-) and 6a(+)BArf(-) show two sets of P(CH2)n/2 (13)C NMR signals (2:1), whereas 5c(+)BF4(-) and 6b,c(+)BArf(-) show only one. At higher temperatures, the signals of 5b(+)BF4(-) coalesce; at lower temperatures, those of 5c(+)BF4(-) and 6b(+)BArf(-) decoalesce. These data give ΔH(⧧)/ΔS(⧧) values (kcal/mol and eu) of 8.3/-28.4 and 9.5/-6.5 for Fe(CO)2(NO)(+) rotation (5b,c(+)) and 6.1/-23.5 for Fe(CO)3(H)(+) rotation (6b(+)). (13)C CP/MAS NMR spectra show that the Fe(CO)3 moiety in polycrystalline 4c (but not 4a) undergoes rapid rotation between -60 and 95 °C. Approaches to minimizing these barriers and developing molecular gyroscopes are discussed.

Conversion of Dinitrogen to Nitriles at a Multinuclear Titanium Framework.

Activation and functionalization of N2 : A mixed diimide/dinitride tetranuclear titanium complex formed by activation of dinitrogen served as a unique platform for the synthesis of nitriles. Functional groups such as aromatic C-X (X=Cl, Br, I) bonds, nitro groups, and ammonia-sensitive aldehyde and chloromethyl moieties were compatible with the synthetic method.

Heterometallic polyhydride complexes containing yttrium hydrides with different Cp ligands: synthesis, structure, and hydrogen-uptake/release properties.

A new family of Y(4)/M(2) and Y(5)/M heterobimetallic rare-earth-metal/d-block-transition-metal-polyhydride complexes has been synthesized. The reactions of the tetranuclear yttrium-octahydride complex [{Cp''Y(µ-H)(2)}(4)(thf)(4)] (Cp'' = C(5)Me(4)H, 1-C(5)Me(4)H) with one equivalent of Group-6-metal-pentahydride complexes [Cp*M(PMe(3))H(5)] (M = Mo, W; Cp* = C(5)Me(5)) afforded pentanuclear heterobimetallic Y(4)/M-polyhydride complexes [{(Cp''Y)(4)(µ-H)(7)}(µ-H)(4)MCp*(PMe(3))] (M = Mo (2 a), W (2 b)). UV irradiation of compounds 2 a,b in THF gave PMe(3)-free complexes [{(Cp''Y)(4)(µ-H)(6)(thf)(2)}(µ-H)(5)MCp*] (M = Mo (3 a), W (3 b)). Compounds 3 a,b reacted with one equivalent of [Cp*M(PMe(3))H(5)] to afford hexanuclear Y(4)/M(2) complexes [{Cp*M(µ-H)(5)}{(Cp''Y)(4)(µ-H)(5)}{(µ-H)(4)MCp*(PMe(3))}] (M = Mo (4 a), W (4 b)). UV irradiation of compounds 4 a,b provided the PMe(3)-free complexes [(Cp''Y)(4)(µ-H)(4){(µ-H)(5)MCp*}(2)] (M = Mo (5 a), W (5 b)). C(5)Me(4)Et-ligated analogue [(Cp''Y)(4)(µ-H)(4){(µ-H)(5)Mo(C(5)Me(4)Et)}(2)] (5 a') was obtained from the reaction of 1-C(5)Me(4)H with [(C(5)Me(4)Et)Mo(PMe(3))H(5)]. On the other hand, the reaction of pentanuclear yttrium-decahydride complex [{(C(5)Me(4)R)Y(µ-H)(2)}(5)(thf)(2)] (1-C(5)Me(5): R = Me; 1-C(5)Me(4)Et: R = Et) with [Cp*M(PMe(3))H(5)] gave the hexanuclear heterobimetallic Y(5)/M-polyhydride complexes [({(C(5)Me(4)R)Y}(5)(µ-H)(8))(µ-H)(5)MCp*] (6 a: M = Mo, R = Me; 6 a': M = Mo, R = Et; 6 b: M = W, R = Me). Compound 5 a released two molecules of H(2) under vacuum to give [(Cp''Y)(4)(µ-H)(2){(µ-H)(4)MoCp*}(2)] (7). In contrast, compound 6 a lost one molecule of H(2) under vacuum to yield [{(Cp*Y)(5)(µ-H)(7)}(µ-H)(4)MoCp*] (8). Both compounds 7 and 8 readily reacted with H(2) to regenerate compounds 5 a and 6 a, respectively. The structures of compounds 4 a, 5 a', 6 a', 7, and 8 were determined by single-crystal X-ray diffraction.

Synthesis, Odour Characteristics, and Antimicrobial Activity of Optically Active (Z)-7-Decen-4-olide and Its Derivatives.

Six optically active (Z)-7-decen-4-olide derivatives (1a-1f) were synthesized in 99% enantiomeric excess using diastereomeric resolution. The odour properties of the racemic and optically active 1a-1f were evaluated in terms of their orthonasal aromas. All of the stereoisomers had different odour characteristics and thresholds. Decen-4-olides (1a-1c) had a strong fruity note, whereas undecen-4-olide (1d and 1e) and dodecen-4-olide (1f) had a strong green note. For 7-alken-4-olides (1a, 1d, and 1f), the (R)-enantiomer had a lower odour threshold than the (S)-enantiomer. In contrast, no difference in the odour threshold was observed for the enantiomers of the 8-alken-4-olides (1b and 1e). Furthermore, the antimicrobial activity against Escherichia coli (E. coli; ATCC 25922) and Staphylococcus aureus (S. aureus; ATCC 29213) were investigated. Although the no differences in the antimicrobial activity of the stereoisomers was observed, 1d and 1e showed slight antimicrobial activity against E. coli, whereas only 1f showed antimicrobial activity against S. aureus. No antimicrobial activity was exhibited by (R)-1f, whereas (S)-1f exhibited strong antimicrobial activity.

Synthesis and structures of the C5Me4SiMe3-supported polyhydride complexes over the full size range of the rare earth series.

The acid-base reaction of [Ln(CH(2)SiMe(3))(3)(thf)(2)] with Cp'H gave the corresponding half-sandwich rare earth dialkyl complexes [(Cp')Ln(CH(2)SiMe(3))(2)(thf)] (1-Ln: Ln=Sc, Y, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu; Cp'=C(5)Me(4)SiMe(3)) in 62-90% isolated yields. X-ray crystallographic studies revealed that all of these complexes adopt a similar overall structure, in spite of large difference in metal-ion size. In most cases, the hydrogenolysis of the dialkyl complexes in toluene gave the tetranuclear octahydride complexes [{(Cp')Ln(µ-H)(2)}(4)(thf)(x)] (2-Ln: Ln=Sc, x=0; Y, x=1; Er, x=1; Tm, x=1; Gd, x=1; Dy, x=1; Ho, x=1) as the only isolable product. However, in the case of Lu, a trinuclear pentahydride [(Cp')(2)Lu(3)(µ-H)(5)(µ-CH(2)SiMe(2)C(5)Me(4))(thf)(2)] (3), in which the C-H activation of a methyl group of the Me(3)Si unit on a Cp' ligand took place, was obtained as a major product (66% yield), in addition to the tetranuclear octahydride [{(Cp')Lu(µ-H)(2)}(4)(thf)] (2-Lu, 34%). The use of hexane instead of toluene as a solvent for the hydrogenolysis of 1-Lu led to formation of 2-Lu as a major product (85%), while a similar reaction in THF yielded 3 predominantly (90%). The tetranuclear octahydride complexes of early (larger) lanthanide metals [{Cp'Ln(µ-H)(2)}(4)(thf)(2)] (2, Ln=La, Ce, Pr, Nd, Sm) were obtained in 38-57% isolated yields by hydrogenolysis of the bis(aminobenzyl) species [Cp'Ln(CH(2)C(6)H(4)NMe(2)-o)(2)], which were generated in-situ by reaction of [Ln(CH(2)C(6)H(4)NMe(2)-o)(3)] with one equivalent of Cp'H. X-ray crystallographic studies showed that the fine structures of these hydride clusters are dependent on the size of the metal ions.

Hydrodenitrogenation of pyridines and quinolines at a multinuclear titanium hydride framework.

Investigation of the hydrodenitrogenation (HDN) of aromatic N-heterocycles such as pyridines and quinolines at the molecular level is of fundamental interest and practical importance, as this transformation is essential in the industrial petroleum refining on solid catalysts. Here, we report the HDN of pyridines and quinolines by a molecular trinuclear titanium polyhydride complex. Experimental and computational studies reveal that the denitrogenation of a pyridine or quinoline ring is easier than the ring-opening reaction at the trinuclear titanium hydride framework, which is in sharp contrast with what has been reported previously. Hydrolysis of the pyridine-derived nitrogen-free hydrocarbon skeleton at the titanium framework with H2O leads to recyclization to afford cyclopentadiene with the generation of ammonia, while treatment with HCl gives the corresponding linear hydrocarbon products and ammonium chloride. This work has provides insights into the mechanistic aspects of the hydrodenitrogenation of an aromatic N-heterocycle at the molecular level.

Gyroscope-like molecules consisting of three-spoke stators that enclose "switchable" neutral dipolar rhodium rotators; reversible cycling between faster and slower rotating Rh(CO)I and Rh(CO)2I species.

trans-Rh(CO)(Cl)(P((CH(2))(14))(3)P) is prepared from trans-Rh(CO)(Cl)(P((CH(2))(6)CH[double bond, length as m-dash]CH(2))(3))(2) by a metathesis/hydrogenation sequence, and converted by substitution or addition reactions to Rh(CO)(I), Rh(CO)(2)(I), Rh(CO)(NCS), and Rh(CO)(Cl)(Br)(CCl(3)) species; the Rh(CO)(Cl) and Rh(CO)(I) moieties rapidly rotate within the cage-like diphosphine, but the other rhodium moieties do not.