Unusual Transformations Research Articles

Reactivity of Scorpionate-Anchored Yttrium Alkyl Complex toward Organic Nitriles

The mixed TpMe2/Cp-supported yttrium monoalkyl (TpMe2)CpYCH2Ph(THF) (1) reacted with 1 equiv of PhCN in THF at room temperature to afford the imine–enamine tautomer (TpMe2)CpY(N(H)C(Ph)═CHPh)(THF) (2) and the insertion product (TpMe2)CpY(N═C(CH2Ph)Ph)(THF) (3), in 61% and 12% isolated yields, respectively. 2 further reacted with PhCN in toluene at 120 °C to give the N–H bond addition product (TpMe2)CpY(N(H)C(Ph)NC(Ph)═CHPh) (4). Treatment of 1 with 1 equiv of anthranilonitrile produced the dimer [(TpMe2)CpY(μ-NHC6H4CN)]2 (5). The monomer product (TpMe2)CpY(NHC6H4CN)(HMPA) (6) can be obtained through the coordination of HMPA (hexamethylphosphoric triamide). The reaction of 5 with 1 in THF at room temperature gave the cyano group insertion product [(TpMe2)CpY(THF)]2(μ-NHC6H4C(CH2Ph)═N) (7). However, this reaction under the heating conditions gave an unexpected rearrangement product, (TpMe2)CpY(THF)(η2-NHC6H4C(CH2Ph)═NH) (8). 5 further reacted with o-aminobenzonitrile at 120 °C to afford the nucleophilic addition/cyclization product TpMe2Y[κ3-(4-NH═(C8N2H4)(2-NHC6H4)](HMPA) (9), accompanied with the elimination of the Cp ring. These results indicated that the yttrium alkyl complex exhibits high activity toward organic nitriles and reveals some unusual transformations during the insertion process. All these new complexes were characterized by elemental analysis and spectroscopic methods, and their solid-state structures were also confirmed by single-crystal X-ray diffraction analysis.

Carbaporphyrins, porphyrin isomers and the legacy of Emanuel Vogel

Vogel discovered a large number of bridged annulene structures, including constitutional porphyrin isomers, heteroporphyrinoid species such as the tetraoxaporphyrin dication, corrole isomers and novel expanded porphyrinoid systems. In this review, the impact of Vogel's work on the author's research program is discussed. Carbaporphyrinoid systems with one or two carbocyclic rings replacing the usual pyrrole subunits have been synthesized and their diatropic characteristics can be related to the presence of 18π electron delocalization pathways within these macrocycles. The preparation of a dideazaporphyrin also provides strong support for the concept, espoused by Vogel, that porphyrins are examples of bridged annulenes. Ring contractions of azuliporphyrins to benzocarbaporphyrins with tert-butyl hydroperoxide in the presence of base have been proposed to involve a Cope rearrangement where a cycloheptatriene unit rearranges to give a norcaradiene-fused carbaporphyrinoid, followed elimination of tert-butyl alcohol, and Vogel's investigations into these types of valence tautomerization processes provides insights into these unusual transformations. In addition, a new class of porphyrin isomers has been synthesized and these porphyrin analogs extend the observations previously made for N-confused porphyrins and Vogel's constitutional isomers.

Novel Fluorinated Indanone, Tetralone and Naphthone Derivatives: Synthesis and Unique Structural Features

Several fluorinated and trifluoromethylated indanone, tetralone and naphthone derivatives have been prepared via Claisen condensations and selective fluorinations in yields ranging from 22–60%. In addition, we report the synthesis of new, selectively fluorinated bindones in yields ranging from 72–92%. Of particular interest is the fluorination and trifluoroacetylation regiochemistry observed in these fluorinated products. We also note unusual transformations including a novel one pot, dual trifluoroacetylation, trifluoroacetylnaphthone synthesis via a deacetylation as well as an acetyl-trifluoroacetyl group exchange. Solid-state structural features exhibited by these compounds were investigated using crystallographic methods. Crystallographic results, supported by spectroscopic data, show that trifluoroacetylated ketones prefer a chelated cis-enol form whereas fluorinated bindone products exist primarily as the cross-conjugated triketo form.

Measuring the force of sound

The action of ultrasound on mechanically responsive functional groups — so-called mechanophores — embedded in a polymer chain often permits unusual chemical transformations. There is now a systematic effort to quantify the reactivity of mechanophores in relation to their structure.

Unusual phase transformations in synthetic NaF-bearing fluid inclusions in quartz

Unusual phase transformations in synthetic NaF-bearing fluid inclusions in quartz

The Two Faces of SAM

An unusual radical mechanism enables ribosome methylation in bacteria.

Unusual Ligand Transformations Initiated by dppm Deprotonation in Methylene-Bridged Rh/Os Complexes

The reaction of [RhOs(CO)(3)(μ-CH(2))(dppm)(2)][CF(3)SO(3)] (dppm = μ-Ph(2)PCH(2)PPh(2)) with 1,3,4,5-tetramethylimidazol-2-ylidene (IMe(4)) results in competing substitution of the Rh-bound carbonyl by IMe(4) and dppm deprotonation by IMe(4) to give the two products [RhOs(IMe(4))(CO)(2)(μ-CH(2))(dppm)(2)][CF(3)SO(3)] and [RhOs(CO)(3)(μ-CH(2))(μ-κ(1):η(2)-dppm-H)(dppm)] [3; dppm-H = bis(diphenylphosphino)methanide], respectively. In the latter product, the dppm-H group is P-bound to Os while bound to Rh by the other PPh(2) group and the adjacent methanide C. The reaction of the tetracarbonyl species [RhOs(CO)(4)(μ-CH(2))(dppm)(2)][CF(3)SO(3)] with IMe(4) results in the exclusive deprotonation of a dppm ligand to give [RhOs(CO)(4)(μ-CH(2))(μ-κ(1):κ(1)-dppm-H)(dppm)] (4) in which dppm-H is P-bound to both metals. Both deprotonated products are cleanly prepared by the reaction of their respective precursors with potassium bis(trimethylsilyl)amide. Reversible conversion of the μ-κ(1):η(2)-dppm-H complex to the μ-κ(1):κ(1)-dppm-H complex is achieved by the addition or removal of CO, respectively. In the absence of CO, compound 3 slowly converts in solution to [RhOs(CO)(3)(μ-κ(1):κ(1):κ(1)-Ph(2)PCHPPh(2)CH(2))(dppm)] (5) as a result of dissociation of the Rh-bound PPh(2) moiety of the dppm-H group and its attack at the bridging CH(2) group. Compound 4 is also unstable, yielding the ketenyl- and ketenylidene/hydride tautomers [RhOs(CO)(3)(μ-κ(1):η(2)-CHCO)(dppm)(2)] (6a) and [RhOs(H)(CO)(3)(μ-κ(1):κ(1)-CCO)(dppm)(2)] (6b), initiated by proton transfer from μ-CH(2) to dppm-H. Slow conversion of these tautomers to a pair of isomers of [RhOs(H)(CO)(3)(μ-κ(1):κ(1):κ(1)-Ph(2)PCH(COCH)PPh(2))(dppm)] (7a and 7b) subsequently occurs in which proton transfer from a dppm group to the ketenylidene fragment gives rise to coupling of the resulting dppm-H methanide C and the ketenyl unit. Attempts to couple the ketenyl- or ketenylidene-bridged fragments in 6a/6b with dimethyl acetylenedicarboxylate (DMAD) yield [RhOs(κ(1)-CHCO)(CO)(3)(μ-DMAD)(dppm)(2)], in which the ketenyl group is terminally bound to Os.

Dougall’s bilateral ₂𝐻₂-series and Ramanujan-like 𝜋-formulae

The modified Abel lemma on summation by parts is employed to investigate the partial sum of Dougall’s bilateral 2 H 2 _2H_2 -series. Several unusual transformations into fast convergent series are established. They lead surprisingly to numerous infinite series expressions for π \pi , including several formulae discovered by Ramanujan (1914) and recently by Guillera (2008).

Reaction of (±)-7,7-dichloro-4-(1-methylethylidene)-bicyclo[3.2.0]hept-2-en-6-one with ozone

The reaction of (±)-7,7-dichloro-4-(1-methylethylidene)bicyclo[3.2.0]hept-2-en-6-one with ozone involves mainly the exocyclic double bond, and subsequent unusual transformations of ozonides thus formed lead to anomalous products.

Reactions of bridging C3 ligands in diiron complexes: Unconventional routes to new functionalized organic frames

Diiron complexes containing C3 ligands, such as vinyliminium and vinylalkylidene bridging units, display unusual reaction patterns, not observed when the same organic fragments are bound to a single metal center, or not coordinated. Bridging vinyliminium complexes [Fe2{μ-η1:η3-C(NMe2)CH=CR}(μ-CO)(CO)(Cp)2][SO3CF3] undergo nucleophilic addition at the iminium C or at α-C position, which is uncommon since non-coordinated vinyliminium species generally undergo conjugated (Michael type) nucleophilic attack. Likewise, bridging vinyliminium ligands undergo new and unusual transformations consisting of the deprotonation and replacement of the α-CH by a variety of functional groups. These reactions, resulting in the formation of C–C and C–heteroatom single and double bonds, produce new bridging ligands of the type [μ-C(NMe2)C(X)CR) (X = S, O, Se, SPh, CNMe, NNCHCO2Me]. Removal of the vinylalkylidene ligands from the bridging coordination is achieved by a [3 + 2] cycloaddition with alkynes. The reaction leads to the formation of ferrocenes containing one polysubstituted Cp ring, which results from the cycloaddition of the bridging C3 ligand with alkynes. This result suggests a new possible route for the synthesis of polyfunctionalized ferrocenes.

C−H Bond Activation Reactions of Ethers That Generate Iridium Carbenes

Two important objectives in organometallic chemistry are to understand C-H bond activation reactions mediated by transition metal compounds and then to develop efficient ways of functionalizing the resulting products. A particularly ambitious goal is the generation of metal carbenes from simple organic molecules; the synthetic chemist can then take advantage of the almost unlimited reactivity of this metal-organic functionality. This goal remains very difficult indeed with saturated hydrocarbons, but it is considerably more facile for molecules that possess a heteroatom (such as ethers), because coordination of the heteroatom to the metal renders the ensuing C-H activation an intramolecular reaction. In this Account, we focus on the activation reaction of different types of unstrained ethers, both aliphatic and hemiaromatic, by (mostly) iridium compounds. We emphasize our recent results with the Tp(Me2)Ir(C(6)H(5))(2)(N(2)) (1.N(2)) complex (where Tp(Me2) denotes hydrotris(3,5-dimethylpyrazolyl)borate). Most of the reactivity observed with this system, and with related electronically unsaturated iridium species, starts with a C-H activation reaction, which is then followed by reversible alpha-hydrogen elimination. An alpha-C-H bond is, in every instance, broken first; when there is a choice, cleavage of the stronger terminal C(sp(3))-H bonds is always preferred over the weaker internal C(sp(3))-H (methylene) bonds of the ether. Nevertheless, competitive reactions of the unsaturated [Tp(Me2)Ir(C(6)H(5))(2)] iridium intermediate with ethers that contain C(sp(3))-H and C(sp(2))-H bonds are also discussed. We present theoretical evidence for a sigma-complex-assisted metathesis mechanism (sigma-CAM), although for other systems oxidative addition and reductive elimination events can be effective reaction pathways. We also show that additional unusual chemical transformations may occur, depending on the nature of the ether, and can result in C-O and C-C bond-breaking and bond-forming reactions, leading to the formation of more elaborate molecules. Although the possibility of extending these results to saturated hydrocarbons appears to be limited for this iridium system, the findings described in this Account are of fundamental importance for various facets of C-H bond activation chemistry, and with suitable modifications of the ancillary ligands, they could be even broader in scope. We further discuss experimental and theoretical studies on unusual alkene-to-alkylidene equilibria for some of the products obtained in the reactions of iridium complex 1.N(2) with alkyl aryl ethers. The rearrangement involves reversible alpha- and beta-hydrogen eliminations, with a rate-determining metal inversion step (supported by theoretical calculations); the alkylidene is always favored thermodynamically over the alkene. This startling result contrasts with the energetically unfavorable isomerization of free ethene to ethylidene (by about 80 kcal mol(-1)), showing that the tautomerism equilibrium can be directed toward one product or the other by a judicious choice of the transition metal complex.

Unraveling the Mechanisms of Isoprenoid Biosynthetic Enzymes: Mechanistic Studies of the Early Stage Enzymes

Isoprenoids (or terpenoids) are a large and structurally diverse class of biomolecules that are essential for the survival of all forms of life. Despite the vast differences in their final structures and functions, the early steps of isoprenoid biosynthesis in all organisms follow one of only two known biosynthetic pathways: the mevalonate pathway or the methyl erythritol phosphate (MEP) pathway. Interestingly, while humans utilize the mevalonate pathway, many human pathogens rely exclusively on the MEP pathway for the biosynthesis of their isoprenoid compounds. This has led to a number of mechanistic studies of the MEP-specific pathway enzymes, with the ultimate goal of developing small molecule inhibitors as potential drugs. In addition to their therapeutic value, many of the MEP pathway enzymes also catalyze unusual chemical transformations that are not well understood. In this review, we will highlight the recent work by us and others towards the elucidation of the catalytic mechanisms of several key enzymes involved in the early stages of isoprenoid biosynthesis. These include 1-deoxy-D-xylulose-5-phosphate reductoisomerase (DXR) and 1-hydroxy-2-methyl-2-(E)-butenyl-4-diphosphate reductase (IspH) of the MEP pathway, and the type II isopentenyl diphosphate:dimethylallyl diphosphate isomerase (IDI-2) from Staphylococcus aureus. The functions of these enzymes are validated or identified as potential drug targets.

Metal-Free Oxidative Cross-Coupling of Unfunctionalized Aromatic Compounds

A new strategy for mixed biaryl synthesis has been developed using the hypervalent iodine(III) reagents. The unique reactivities of the sigma-heteroaryl iodine(III) intermediates generated in situ are the key element for the unusual metal catalyst-free transformations and strict control of the product selectivities.

Influence of steric pressure on the activation of carbon dioxide and related small molecules by uranium coordination complexes

Recent reports on new types of reactions and bonding using uranium coordination complexes have marked uranium as an effective candidate for small molecule activation and potentially as a key participant in catalytic processes. This review discusses the advantages of employing uranium in coordination chemistry, with emphasis on the importance of ligand design and the promotion of unusual chemical transformations by steric pressure. The activation of industrially relevant C1 feedstocks such as CO and CO(2) by uranium complexes with their exemplary abilities to stabilize highly reactive charge-separated complexes are highlighted in this article. Spectroscopic and DFT studies are also presented, demonstrating the important methods that are utilized for investigating the electronic properties of these uranium complexes.

Metamaterials for light rays: ray optics without wave-optical analog in the ray-optics limit

Volumes of sub-wavelength electromagnetic elements can act like homogeneous materials: metamaterials. In analogy, sheets of optical elements such as prisms can act ray-optically like homogeneous sheet materials. In this sense, such sheets can be considered to be metamaterials for light rays (METATOYs). METATOYs realize new and unusual transformations of the directions of transmitted light rays. We study here, in the ray-optics and scalar-wave limits, the wave-optical analog of such transformations, and we show that such an analog does not always exist. Perhaps, this is the reason why many of the ray-optical possibilities offered by METATOYs have never before been considered.

Reaction of lupane and oleanane triterpenoids with Lawesson's reagent

The reactions of selected triterpenic oxo compounds with Lawesson's reagent were investigated. We examined sulfurization of some oxygen compounds and for these reactions several hindered ketones, one aldehyde, α-hydroxyketones, esters, or anhydrides were chosen. We synthesized 15 new sulfur derivatives, including thioketone 16, dimeric sulfides 17– 19, and thiaderivatives 20– 22. We also observed unusual transformations, which afforded oxathiaphosphinines 23a, 23b, and dithiaphospholanes 24. The prepared compounds failed to demonstrate any significant cytotoxic activity.

Intermolecular reactions of electron-rich heterocycles with copper and rhodium carbenoids

This tutorial review describes the reactions of the electron-rich heterocycles pyrrole, furan, indole and benzofuran with copper and rhodium carbenoids. Two main reaction pathways are possible, involving either a concerted non-synchronous cyclopropanation or zwitterionic intermediates. A diverse range of products are possible and the outcome is very dependent on the structure of the heterocycle and the carbenoid. To emphasize this point the carbenoids are considered in terms of three classes: acceptor, acceptor-acceptor and donor-acceptor carbenoids. Unusual catalytic asymmetric transformations can be achieved with this chemistry while the asymmetric induction is strongly influenced by how the carbenoid can approach the heterocycles. This tutorial review gives an overview of the general features that govern the chemistry of metal carbenoids with heterocycles and presents a mechanistic rationale for the range of products that can be formed.

Unusual chemical transformations of natural flavonoids

Unusual chemical transformations of natural flavonoids

Phase Transformations in Pearlitic Steels Induced by Severe Plastic Deformation

The paper presents an overview of a number of unusual phase transformations which take place in pearlitic steels in conditions of the severe deformation, i.e. combination of high pressure and strong shear strain. Strain-induced cementite dissolution is a well-documented phenomenon, which occurs during cold plastic deformation of pearlitic steels. Recently new results which can shed additional light on the mechanisms of this process were obtained thanks to 3DAP and HRTEM investigations of pearlitic steel deformed by high pressure torsion (HPT). It was shown that the process of cementite decomposition starts by carbon depletion from the carbides, which indicates that the deviation of cementite’s chemical composition from the stoichiometric is the main reason for thermodynamic destabilisation of cementite during plastic deformation. Important results were obtained regarding the distribution of released carbon atoms in ferrite. It was experimentally confirmed that carbon segregates to the dislocations and grain boundaries of nanocrystalline ferrite. Another unusual phase transformation taking place in nanocrystalline pearlitic steel during room temperature HPT is a stress induced α→γ transformation, which never occurs during conventional deformation of coarse grained iron and carbon steels. It was concluded that this occurred due to a reverse martensitic transformation. The atomistic mechanism and the thermodynamics of the transformation, as well as issues related to the stability of the reverted austenite will be discussed.

Structure-dependent transformations of the tetrachlorocyclopentadienone dimer and the product of its substitutive rearrangement in reactions with NaBH4, CrCl2, LiAlH4, and Zn

Dechlorination of the tetrachlorocyclopentadienone dimer and perchloro-7-oxotricyclo[6.1.0.04,8]nona-2,5-diene-9-carboxylic acid N,N-diethylamide with hydride ion donors (NaBH4, LiAlH4, etc.) and electron donors (CrCl2 or Zn) were studied. The possible pathways of unusual transformations of both sterically hindered molecules are considered.