Formation of specific configurations at stereogenic nitrogen centers upon their coordination to zinc and mercury

The coordination of (R,R)-tetramethylcyclohexane-1,2-diamine derivatives with stereogenic nitrogen centers to zinc and mercury halides is investigated. It is shown that the resulting complexes display one specific configuration at the stereogenic nitrogen centers. This fact is unusual due to the fast inversion of nitrogen centers but highly desirable as the stereoinformation of the ligands is brought closer to the metal centers of the potential catalysts. A combination of NMR studies and quantum chemical calculations gives insight into the selective formation of one specific configuration at the stereogenic nitrogen centers of the zinc complexes.

Diphenyl[2-(phenyl-sulfon-yl)propan-2-yl]-λ(5)-phosphane-thione

The title compound, C(21)H(21)O(2)PS(2), was obtained from the corresponding dilithio methandiide by treatment with iodo-methane. The bond lengths and angles deviate considerably from those in the dimetallated compound. These differences are most pronounced in the PCS backbone. While the title compound features C-P and C-S distances of 1.9082 (17) and 1.8348 (17) Å, respectively, the dianion showed C-P(av) distances shortened by 11% [1.710 (4) Å] and C-S distances shortened by 12% [1.614 (3) Å]. Additionally, the P-C-S angle experiences a contraction by methyl-ation of the dianion from 121.4 (2) to 111.96 (9)° in the title compound.

Preparation of aminomethyl functionalised silanes via an α-lithiated amine: from their synthesis, stability and crystal structures to stereochemical issues

The preparation of aminomethyl functionalised silanes based on the α-lithiated amine, (1R,2R)-N,N,N',N'-tetramethylcyclohexane-1,2-diamine [(R,R)-TMCDA] is reported. This methodology can be applied for the synthesis of mono-aminomethyl substituted systems, but most remarkably also for di- and trifunctionalised compounds. The trapping of the lithiated amine is accompanied by transmetallation reactions resulting in the formation of (silylmethyl)silanes depending on the reaction temperature. The zinc(II) halide complexes of the mono-functionalised systems show the formation of exclusively one configuration of the stereogenic nitrogen atom, in which the spatially more demanding substituent exhibits the pseudo-equatorial position. The di- and trifunctionalised systems feature high sensitivity towards Si-C bond cleavage under re-formation of the (R,R)-TMCDA fragment.

Assembly of macrocycles by zirconocene-mediated, reversible carbon-carbon bond formation

Macrocyclic compounds have attracted considerable attention in numerous applications, including host-guest chemistry, chemical sensing, catalysis, and materials science. A major obstacle, however, is the limited number of convenient, versatile, and high-yielding synthetic routes to functionalized macrocycles. Macrocyclic compounds have been typically synthesized by ring-closing or condensation reactions, but many of these procedures produce mixtures of oligomers and cyclic compounds. As a result, macrocycle syntheses are often associated with difficult separations and low yields. Some successful approaches that circumvent these problems are based on "self-assembly" processes utilizing reversible bond-forming reactions, but for many applications, it is essential that the resulting macrocycle be built with a strong covalent bond network. In this Account, we describe how zirconocene-mediated reductive couplings of alkynes can provide reversible carbon-carbon bond-forming reactions well-suited for this purpose. Zirconocene coupling of alkenes and alkynes has been used extensively as a source of novel, versatile pathways to functionalized organic compounds. Here, we describe the development of zirconocene-mediated reductive couplings as a highly efficient method for the preparation of macrocycles and cages with diverse compositions, sizes, and shapes. This methodology is based on the reversible, regioselective coupling of alkynes with bulky substituents. In particular, silyl substituents provide regioselective, reversible couplings that place them into the α-positions of the resulting zirconacyclopentadiene rings. According to density functional theory (DFT) calculations and kinetic studies, the mechanism of this coupling involves a stepwise process, whereby an insertion of the second alkyne influences regiochemistry through both steric and electronic factors. Zirconocene coupling of diynes that incorporate silyl substituents generates predictable macrocyclic products in very high yields. In the absence of significant steric repulsion, the macrocyclization appears to be entropically driven, thereby providing the smallest strain-free macrocyclic structure. The scope of the reaction has been explored by variation of the spacer group between the alkynyl substituents and by incorporation of functional and chiral groups into the macrocycle. The size and shape of the resulting macrocycles are largely determined by the length and geometry of the dialkyne spacer, especially in the case of terminal trimethylsilyl-substituted diynes. For example, linear, rigid diynes with four or fewer phenylene rings lead to trimeric macrocycles, whereas bent or flexible diynes produce dimers. Depending on the reaction conditions, functional groups (such as N-heterocycles and imines) are tolerated in zirconocene coupling reactions, and in selected cases, they can be used to influence the shape of the final macrocyclic product. More recently, Cp(2)Zr(pyr)(Me(3)SiC≡CSiMe(3)) has been employed as a more general zirconocene synthon; it affords higher yields and increased functional group tolerance. Functional groups can also be incorporated through transformation of the zirconacyclopentadiene products, with acid hydrolysis to the corresponding butadiene being the most efficient derivatization. Furthermore, construction of chiral macrocycles has been accomplished by stereoselective macrocyclizations, and triynes have been coupled into three-dimensional cage compounds. We also discuss various design factors, providing a perspective on the utility of zirconocene-mediated couplings in the assembly of macrocyclic and cage compounds.

Mechanistic insight into stereoselective carbolithiation

This article addresses the mechanistic features of asymmetric carbolithiation of β-methylstyrenes. While often the presence of functional groups is required to obtain high enantioselectivities in carbolithiation reactions, simple β-methylstyrene also gives high selectivities in (-)-sparteine-mediated addition of alkyl lithium compounds. Computational studies on the carbolithiation of β-methylstyrene with (-)-sparteine show that the observed selectivities are the result of repulsion effects in the diastereomeric transition states between the (-)-sparteine⋅alkyl lithium adduct and the β-methylstyrene, upon approximation of the two reactants. In contrast, for the ortho-amino β-methylstyrene (E)-benzyl(2-propenylphenyl)amine (4) X-ray structure analyses of intermediate lithium amides indicate a carbolithiation mechanism in which one side of the double bond is shielded by the amide moiety, leaving only one side free for approach of the chiral alkyl lithium adduct.

(1R,2R)-N,N'-Diisobutyl-N,N'-dimethyl-cyclo-hexane-1,2-diamine

The title compound, C₁₆H₃₄N₂, is a chiral diamine with fixed R configuration at both stereogenic carbon centres of the cyclo­hexane backbone. Due to their different substituents, the two N atoms also become stereogenic. In the crystal structure, the configuration at one of the two nitro­gen centres is fixed, with the free electron pair pointing inward and the isobutyl group in a trans position towards the cyclo­hexane backbone resulting in an R configuration. The isobutyl group at the second N atom, however, is disordered with 75% S configuration and 25% R configuration. In both cases, the isobutyl group is arranged in a trans position towards the cyclo­hexane backbone.

(2,2-Dichlorovinyl)ferrocene

The title compound, [Fe(C₅H₅)(C₇H₅Cl₂)], represents a versatile building block for the preparation of π-conjugated redox-active compounds or polymetallic organometallic systems due to the presence of the electrochemically active ferrocenyl unit. It is therefore a potential starting material for the preperation of the corresponding alkyne. In the crystal, the alkenyl unit and the cyclo­penta­dienide ring are almost parallel, with an angle between the best planes of only 10.6 (4)°.

2-[(2-Hydr-oxy-2,2-diphenyl-ethyl)(meth-yl)amino]-N,N-dimethyl-ethanaminium bromide

The title compound, C₁₉H₂₇N₂O⁺·Br⁻, is the hydro­bromide of the trapping product of lithia­ted N,N,N′,N′-tetramethylethylenediamine (TMEDA) with benzophenone. Thereby, the N atom of the NMe₂ group is selectively protonated and the respective trapping product represents a potential tridentate ligand with one O and two N donor atoms. The H atoms at N (H2N) and O (H1O) are involved in hydrogen bonds with the Br⁻. The mol­ecular structure shows all donor atoms to be arranged on one side of the mol­ecule, thus indicating a potential threefold coordination of a Lewis acid.

(S)-1,2-Dimethyl-1,1,2-triphenyl-2-(4-piperidiniometh-yl)disilane chloride

The title compound, C(26)H(34)NSi(2) (+)·Cl(-), shows chirality at silicon. Because of its highly selective synthesis with an e.r. of >99:1 by means of a racemic resolution with mandelic acid, the free disilane is of great importance to the chemistry of highly enanti-omerically enriched lithio-silanes and their trapping products. N-H⋯Cl hydrogen bonding is present between the protonated nitro-gen atom of the piperidino group and the chloride counter-anion. The silicon-silicon distance as well as silicon-carbon and carbon-nitro-gen bond lengths are in the same ranges as in other quaternary, functionalized di- and tetra-silanes.

Crystal structures of n-BuLi adducts with (R,R)-TMCDA and the consequences for the deprotonation of benzene

Combinations of organolithium compounds and diamine bases have become a powerful tool in synthetic chemistry. Because of the structure-reactivity relationship, the elucidation of reaction mechanisms of these reagents is strongly connected with the structural determination of intermediate species. In mixtures of the diamine TMCDA (N,N,N',N'-tetramethylcyclohexane-1,2-diamine) and n-butyllithium, two different structures, the dimeric [n-BuLi x (R,R)-TMCDA]2 and the aggregate [(n-BuLi)2 x (R,R)-TMCDA]2, can be isolated, depending on the n-BuLi/TMCDA ratio. Thereby, [(n-BuLi)2 x (R,R)-TMCDA]2 is a rare example of an organolithium compound with a ladder arrangement of the central four-membered Li-C-Li-C rings. Two isomers of the ladder structure are formed in the crystal by changing from the enantiomerically pure to racemic TMCDA. As n-BuLi/TMCDA mixtures are also able to deprotonate benzene, these structures give hint to possible mechanisms. Supported by theoretical studies, transition states based on the dimer, the ladder structure, and a hypothetical monomer are discussed.

(1R,2R)-N,N'-Dimethyl-cyclo-hexane-1,2-diamine

The molecule of the title compound, C₈H₁₈N₂, possesses C₂ symmetry. Owing to its stereochemistry, it is used in the synthesis of chiral ligands and metal complexes for asymmetric synthesis. The cyclo­hexane ring shows a chair conformation with the amino groups in equatorial positions. Contrary to the literature, the title compound is not a liquid, but a crystalline solid at room temperature (293 K). The absolute configuration is assigned from the synthesis.

Remote sensing of in-use heavy-duty diesel trucks

On-road measurements in 2005 of carbon monoxide (CO), hydrocarbons, nitric oxide, nitrogen dioxide, and sulfur dioxide from 1641 individually identified heavy-duty diesel trucks at two locations in Colorado are reported. Carbon monoxide and nitric oxide show increasing emissions with increased altitude. Oxides of nitrogen (NOx) emissions have decreased with more recent model years over the last 10 years but are the same as vehicles that are 20 years old. At the Golden, CO site, there was a statistically significant decrease in fleet emissions of CO and NOx since a similar study in 1999. There was no emission trend for CO or NOx with gross vehicle weight or odometer in units of grams of pollutant per kilogram of fuel consumed. Data from this study suggest that on-road remote sensing can detect illegal, high sulfur fuel use from individual heavy-duty diesel trucks. Ammonia emissions from this study were below the detection limit of the instrument but will be useful as a baseline value for future comparison.