Angewandte Chemie International Edition

Nonracemic or racemic asymmetric catalysts can be further evolved into highly activated catalysts by association of chiral activators (see scheme). This process is particularly useful in racemic catalysis, where one enantiomer of the racemic catalyst is selectively activated. The activation works with racemic or chirally flexible ligands without the need for enantiomeric resolution.

Still in its infancy is the development of catalytic enantioselective hetero-Diels–Alder reactions of imines. In contrast, there are a series of chiral catalysts for the hetero-Diels–Alder reactions of carbonyl compounds, and great strides have been made especially in the development of enatioselective variants. An overview of what has been achieved to date and the investigation of the reaction mechanisms reveal similarites and differences between the reactions with carbonyl compounds and those with imines.

The prebiotic synthesis of small organic molecules is thought to be the first important step for the origin of life. New investigations indicate that relevant compounds can also be formed in the crust or in volcanic and hydrothermal vents under extreme conditions. Minerals such as magnetite Fe₃O₄ [Eq. (1)] or pyrite play an important role in these reactions.

In the past decade transition metal catalyzed reactions of allenes have become increasingly important. A new principle common in many of these reactions is the presence of functional groups on the carbon atom next to the allene unit which allows a degree of selectivity (regio-, stereo-, chemo-, and positional selectivity, see picture) difficult to achieve by other methods.

Microwave heating allows regio- and enantioselective allylic alkylation to be carried out in air in a few minutes in a convenient one-pot procedure [Eq. (1)]. The catalytically active molybdenum(0) species was formed in situ from inexpensive [Mo(CO)₆] and a chiral ligand.

Equal numbers of boron and silicon atoms are present in a new type of cage compound whose structure (see picture) is based on an incomplete B₄Si₄ cube in which the boron sites are three-coordinate. This compound, which was synthesized from tert-butyltrichlorosilane, 4-vinylphenylboronic acid, and aqueous aniline, is the first cube-based structure with pairs of the same atoms in adjacent positions.

Rapid intermolecular acid–alcohol hydrogen-bonded proton exchange and geometrical disorder, well below the melting temperature, form the basis of the observed amorphous behavior of salicyl salicylate 1. Compound 1 was observed in a stable, undercooled amorphous phase by ¹H and ¹³C NMR spectroscopy, and its crystal structure was determined by single-crystal X-ray diffraction. The amorphous state of 1 was confirmed by small-angle X-ray scattering (SAXS) studies.

Dendrimeric catalysts can present catalytic units in close proximity, thereby inducing useful cooperative effects (see picture). Commercial dendrimers were derivatized with chiral [Co(salen)] complexes and found to display enhanced reactivity relative to monomeric or even dimeric analogues in the hydrolytic kinetic resolution of epoxides.

New hexameric cyclophenyleneethynylenes (CPEs, 1) were synthesized by alkyne metathesis of meta-dipropynylated benzenes. Treatment of one of these CPE derivatives (R¹=tert-butyl, hexyl, dodecyl; R²=H, methyl) with [Os₃(CO)₁₀(MeCN)₂] led to a decacarbonyltriosmium–CPE complex in which the planarity of the ring was significantly disrupted, as evidenced by the results of the single-crystal X-ray diffraction analysis.

A new solution to an old problem: Remarkably regioselective de-O-benzylation of perbenzylated cyclodextrins (CDs) such as 1 (or α- and γ-CDs) occurs with diisobutylaluminum hydride, thus providing an innovative approach to the preparation of selectively modified cyclodextrins such as 2 in excellent yield.

That guest atoms donate electrons to framework atoms is a generally accepted concept. Nevertheless, evidence for the presence of neutral guest atoms in thermoelectric clathrate structures is presented in the form of experimental X-ray charge density analysis and X-ray absorption near-edge structure (XANES) data. The picture shows the deformation density (DD) determined by the maximum entropy method for Ba₈Ga₁₆Ge₃₀; the DD shows that both the Ba guest atoms are disordered.

Three molecular components self-assemble through zinc–pyridine coordination interactions to form a stable porphyrin pentamer (see picture). In this supramolecular complex energy absorbed by the outer zinc porphyrins is transferred to a central free-base porphyrin in 500 picoseconds.

A “merry-go-round” of iron valences is seen in an [Fe₃S₄]⁰ cluster with cysteine ligands. This phenomena is revealed by the observation of hyperfine-shifted ¹H NMR signals from the coordinated cysteine units, and the disappearance of these signals upon protonation of the cluster at low pH values. The proton binds to each of the three μ-bridging sulfides for a fraction of time (see scheme). The protonation of one μ-S causes the two iron atoms bridged by that μ-S to form a mixed-valence pair and the exchange of the proton from one μ-S to another causes a change in the iron valences in the cluster.

A highly diastereoselective hetero-Diels–Alder reaction, an O–C rearrangement, and a spatial/chemical desymmetrization are three new reactions that have been developed and exploited in the total synthesis of muricatetrocin C, an annonaceous acetogenin.

Combinatorial methods have recently been used to aid in the discovery of new pharmaceuticals and catalysts for organic synthesis. A simple and inexpensive strategy for using combinatorial methods to discover polymerization catalysts is reported, and using it enabled a new catalyst (1, see scheme) for the synthesis of syndiotactic polypropylene to be identified.

The novel charge-transfer salt Na⁺[CAl₄²⁻] was produced and characterized in the gas phase. The CAl₄²⁻ dianion contains a tetracoordinate planar carbon center (see picture), and the Na⁺ ion is coordinated to an edge of the dianion. The four-center peripheral ligand–ligand bond that is responsible for the planarity of CAl₄²⁻ is retained in the presence of the countercation; this provides the opportunity to design new bulk ionic materials with CAl₄²⁻ as a novel structural unit.

Long-range interactions are found in a neutral sheetlike copper compound in which each Cu^II ion is bridged by alternating end-on and end-to-end azide groups (see picture). Magnetic studies show unusual ferromagnetic interactions between Cu^II ions through both kinds of azide bridges in this complex (order at T_c=7 K), which is the first two-dimensional metal-azide-bridged compound with long-range ferromagnetic ordering properties.

Simple tripeptides (A) are asymmetric catalysts for the enantioselective conjugate addition of azide ion to enoate derivatives such as 1. Thus, chiral, nonracemic azides 2 (up to 85 % ee; 97 % yield) may be obtained under mild reaction conditions (toluene, 25°C; TMSN₃=trimethylsilyl azide).

Enhanced fluorescence as well as more sensitive fluorescence responses towards chiral quenchers are observed for dendrimer 1 (R=tBu) than its chiral core binaphthol unit. The fluorescence of the chiral dendrimer 1 is quenched efficiently and enantioselectively by chiral amino alcohols such as 2.

Modification of the DNA backbone by replacement of a single phosphate with an alkylamine is shown to generate a selective ligation catalyst. Related backbone modifications can be chemically read from a complementary DNA sequence. With this demonstration, it now appears possible to generate a two-stage replication cycle (see scheme), a simplified version of the biological three-stage DNA–RNA–protein cycle necessary for biological replication.

An extended network, {[Dy₄(OH)₄(asp)₃(H₂O)₈](ClO₄)₂⋅10 H₂O}_n, having three-dimensional channels with a spacing of 11.78 Å (see picture) has been prepared by mixing Dy(ClO₄)₃ and L-aspartic acid in aqueous solution, and subsequently adjusting the pH values to about 6.5 using aqueous NaOH solution. This open-framework compound is representative for the assembly of discrete lanthanide cubanes into supercubanes.

A marked influence on the catalytic epoxidation and hydroxylation of hydrocarbons by aqueous 30 % H₂O₂ in an aprotic solvent and on the formation of oxoiron(IV) porphyrin intermediates in m-chloroperoxybenzoic acid (m-CPBA) reactions in polar solvent is shown by the anionic axial ligands of an electron-deficient iron(III) porphyrin complex [Fe(tpfpp)X] (tpfpp=meso-tetrakis(pentafluorophenyl)porphinato dianion; see scheme).

Hole transfer from an excited, DNA-intercalated injector (1), to either G or to the “traps” GG or GGG occurs with similar rates, and always results in oxidation of the proximate guanine residue. These results indicate that charge trapping in GG or GGG must occur by a relaxation mechanism (structural and/or energetic) which is inherent in these sequences in DNA duplexes.

A concise and totally synthetic route involving the merger of a fully deprotected glycosylamine and a minimally protected aspartic acid containing peptide has enabled the homogeneous N-linked glycopeptide 1 to be synthesized. The other key conversions in the synthesis are the progression from glycal to free oligosaccharide hemiacetal and subsequent amination with high efficiency.

Site-specific DNA cleavage is achieved in the presence of 1 upon irradiation with only long wavelength UV light; no supplementary metallic or reducing agents are required. Compound 1 possesses analogues of the intercalator and carbohydrate moieties found in neocarzinostatin, an antitumor enediene antibiotic.

Coadsorbed water determines the reaction products when isobutane undergoes carbonylation by CO on a sulfated zirconia catalyst at 70°C. This is shown by the results of in situ solid-state ¹³C NMR and ex situ ¹³C high-resolution NMR spectroscopy and by GC-MS analyses. Methyl isopropyl ketone selectively forms in the absence of water, while pivalic acid represents the primary reaction product the presence of coadsorbed water (see scheme).

A solid-phase strategy for the synthesis of a new class of DNA–carbohydrate conjugates, termed nucleoglycoconjugates (see example shown), is presented. This phosphoramidite-based approach provides a general method to link carbohydrates within or at the termini of DNA oligonucleotides through stable phosphodiester bonds. The resulting glycoconjugates serve as substrates for enzymes used in the manipulation of DNA, and are effective as primers in extension reactions mediated by DNA polymerases, including the polymerase chain reaction.

Carbon nanotubes aligned perpendicular to a surface are used as electrodes to form novel, conducting polymer–carbon nanotube (CP–NT) composites. The SEM image (see picture) shows the CP–NT coaxial nanowires, in which an appropriate conducting polymer film is electrochemically deposited onto the carbon nanotube arrays partially immersed into an electrolyte solution.

Well-defined poly(alcohol)s were prepared directly by the living coordination polymerization of allene derivatives bearing hydroxy groups with [{(π-allyl)NiOCOCF₃}₂]/PPh₃ as an initiator (see scheme). R=CH₂OH, CHCH₃OH, C(CH₃)₂OH.

A new thermal coupling reaction forms silyloxy imidoyl tellurides 2 from silyl tellurides 1, carbonyl compounds, and phenyl isocyanide [Eq. (1)]. A mechanism that involves the generation of the silyl radical from the silyl telluride and subsequent chemoselective coupling with carbonyl compounds and the isocyanide is proposed.

A 29° twist of the central olefinic bond is observed in the sesquihomoadamantene cation radical (SH^+.). A careful comparison of the X-ray structures of neutral SH (right) and its cation radical (left) reveal the twist is not a result of relieving steric strain. Moreover, the changes in the bond lengths in SH^+.—the central C−C bond lengthens by 5 pm and the four single C−C bonds directly attached to the olefinic carbonatoms shorten by 3 pm—provide the first experimental evidence for the (classical) hyperconjugative stabilization of cationic charges.

Incorporation of boron into templated germanate frameworks has been achieved for the first time. Clusters of GeO₄ three-rings are connected by pairs of BO₄ tetrahedra into a two-dimensional framework structure with nine-ring channels (see picture; GeO₄: light gray; BO₄: dark gray). The structure topology is very similar to that of (NH₄)₄[(GeO₂)₃(GeO_1.5F₃)₂]⋅0.67 H₂O, although very different types and sizes of polyhedra are present in the structures.

Contrary to common belief [Co₄(CO)₁₂] does catalyze the Pauson–Khand reaction under practical laboratory conditions (70°C, 1 atm of CO) [Eq. (1)]. Its catalytic efficiency was further enhanced by an additive (cyclohexylamine, pyridine), which possibly facilitated its disproportionation into and promoted preservation of the catalytically active cobalt species. This is the first practical procedure using the commercially available and more stable [Co₄(CO)₁₂] cluster as a catalyst precursor.

Stability instead of explosiveness characterizes a novel diazonium ion T2* on a solid support, which opens a facile route to the versatile chemistry of diazo compounds and triazenes. This diazonium ion can be used as a linker for the immobilization and modification of amines (see scheme). The high stability also allows its application as a scavenger for amines, anilines, and phenols in combinatorial synthesis in the liquid phase.

Weakly coordinating anions and nonpolar solvents facilitate the isolation of the first NH₄⁺ and PH₄⁺ derivatives completely substituted with main group organometallic units. The triflate and BPh₄ salts of the [E(SnMe₃)₄]⁺ ions (E=N, P) undergo a rapid Me₃Sn⁺ ion exchange in THF and thus represent base-stabilized stannylium ions. The structure of the cation in [N(SnMe₃)₄]⁺[OSO₂CF₃]⁻ is depicted.

Highly reactive nitrilium phosphane ylide complexes 1 are formed as intermediates in the thermal decomposition of 7-phosphanorbornadiene complexes in the presence of 1-piperidinonitrile. They react with dimethylacetylenedicarboxylate in [3+2] cycloadditions to give 2H-1,2-azaphosphole complexes. pip=1-piperidino; R=Me, Ph.

A butterfly structure and competing magnetic exchange interactions between the manganese(II) ions and organic, ruthenocene-substituted α-nitronyl nitroxide (NIT) radicals characterize the spin frustration system of a new dimeric Mn^II complex (see scheme, IMH=reduced form of the imino nitroxide).

Multiple insertion of AlBr in bonds of AlBr₃ leads to Al₅Br₇⋅5 THF—a new type of low-valent aluminum halide. The actual, ionic structure of the subhalide [Al₅Br₆⋅6 THF]⁺[Al₅Br₈⋅4 THF]⁻ (the anion is shown schematically) appears plausible on the basis of density functional theory calculations. This and similar subhalides can play the role of potential intermediates in the electrolytic deposition of metals.

Resolution of racemic water-soluble [Ru(diimine)₃]Cl₂ complexes 2 can be efficiently performed by asymmetric extraction with lipophilic TRISPHAT ions Δ-1. Phase separation allows the isolation of the Δ and Λ enantiomers of the cationic complexes with selectivities (d.r.) of up to 49:1 (see scheme).

Pb₂²⁻forms the axle of the cartwheel-shaped compound [{(CO)₅W}₄Pb₂]²⁻ (see picture for structure). The Pb−Pb distance of 281 pm is the shortest of this kind observed so far, and is in agreement with the idea of a coordinatively stabilized diplumbaacetylide ion.

For at least a day, stability is imparted to the light-induced, metastable, high-spin state of an iron(II) complex with a strong ligand field (see scheme) at ≥20 K. The lifetime of this state is more than eight orders of magnitude longer than that predicted by the theory of the inverse energy-gap law.

A new in situ NMR method for the structural characterization of hydrogenation intermediates based on the PHIP effect (PHIP=parahydrogen-induced polarization): by means of a nuclear Overhauser effect (NOE) the intermediate bonding of hydrogenated [D₈]styrene to cationic Rh^I–diphosphane catalyst complexes could be detected (see picture).

The longest wavelength absorption of the dark blue solution of the tetragermabutadiene 1 (Tip=2,4,6-iPr₃C₆H₂) in n-hexane at 560 nm indicates a pronounced conjugation of the two Ge−Ge double bonds. In the crystal, 1 adopts the s-cis conformation with a dihedral angle for the Ge₄ framework of 22.5°.

Photochemically labile Te(CH₂CH₂OH)₂ is the first tellurium(II) thiolate with functionalized side chains. Its H₂O complex, Te(CH₂CH₂OH)₂⋅H₂O (see structure), is stabilized in the solid state by short O−H⋅⋅⋅S hydrogen bonds.

By maximizing the difference in the catalytic activity between enantiomers of racemic catalysts, high enantioselectivity can be achieved in the hydrogenation of ketones. This is accomplished through a combination of asymmetric activation and deactivation (see scheme).