3+2] versus [2+2] addition of metal oxides across C=C bonds. Reconciliation of experiment and theory

The reaction of a metal oxide with a double bond is the initial step in the osmium-catalyzed cis-dihydroxylation of olefins. The mechanism of the addition of osmium tetraoxide and rhenium(VII) oxides across C=C bonds was controversial. Early work indicated a [3+2] addition, and later kinetic studies suggested an initial [2+2] addition, whereas recent quantum-chemical calculations showed the [3+2] addition to be favored. Experiment and theory have now become reconciled. In this Account, we discuss recent contributions to the mechanistic debate as well as future challenges.     

Cytotoxic Activity of Silyl- and Germyl-Substituted 4,4-Dioxo-3a,6a-Dihydrothieno[2,3-d]isoxazolines-2

The [2+3] dipolar cycloaddition of nitrile oxides to the double C = C bonds of thiophene-1, 1-dioxides leads to formation of the fused isoxazolines-2 (1, 2). Tumor growth inhibition of these compounds strongly depends on the nature of group IV A element increasing from slightly active tert-butyl derivatives to silicon and germanium containing analogues. The products of benzonitrile oxide cycloaddition have greater cytotoxic effect than the compounds obtained from the cycloaddition reaction of 2, 5-disubstituted thiophene-1, 1-dioxides with acetonitrile oxide. Fused silyl substituted isoxazolines-2 are stronger NO-inducers than their germyl and tert-butyl analogues.     

Synthesis of Group 9 Metal‐Olefin Complexes with Identical Ligand Frameworks and Comparison of their Catalytic Activity in [2+2+2] Cycloaddition and other Addition Reactions

The preparation and catalytic evaluation of the three group 9 (cyclopentadienyl)metal(trimethylvinylsilane) complexes of the type C₅H₅M(H₂CCHSiMe₃)₂ (M=cobalt, rhodium and iridium) are reported. The complexes were investigated in [2+2+2] cycloaddition as well as in hydrogenation, hydroformylation and hydroboration reactions. Despite the identical organic frameworks and structural parameters, the complexes display remarkable reactivity and stability differences.     

[60]富勒烯[2+1]环加成反应研究进展

对[60]富勒烯的[2 1]环加成反应、主要试剂及反应机理进行了综述。     

Improved Synthesis of 5-Substituted 1H-Tetrazoles via the [3+2] Cycloaddition of Nitriles and Sodium Azide Catalyzed by Silica Sulfuric Acid

A silica supported sulfuric acid catalyzed [3+2] cycloaddition of nitriles and sodium azide to form 5-substituted 1H-tetrazoles is described. The protocol can provide a series of 5-substituted 1H-tetrazoles using silica sulfuric acid from nitriles and sodium azide in DMF in 72%-95% yield.     

Synthesis of Multisubstituted Silyloxy-based Donor-Acceptor Cyclobutanes by an Acid-Catalyzed [2+2] Cycloaddition

This account summarizes our recent efforts on the acid-catalyzed [2+2] cycloaddition of silyl enol ethers with α,β-unsaturated esters, giving donor-acceptor (D -A) cyclobutanes and their related reactions. The cycloaddition generally produces multisubstituted cyclobutanes in good yields with high diastereoselectivity. We found that aluminum Lewis acids and triflic imide (Tf₂NH) had good catalytic activity. The retro [2+2] cycloaddition proceeded at higher reaction temperatures, and in some cases, diastereoselectivity switching of cycloadducts was observed. A microflow protocol was established for Tf₂NH-catalyzed [2+2] cycloaddition. Although the cycloaddition usually requires cryogenic conditions in a batch reactor, the microreactor system enabled the production of D -A cyclobutanes, even at ambient temperature. [2+2] Cycloaddition of allylsilanes and alkyl enol ethers, instead of silyl enol ethers, afforded the corresponding D -A cyclobutanes.     

[3+2] Cycloaddition of Propargylamines and α‐Acylketene Dithioacetals: A Synthetic Strategy for Highly Substituted Pyrroles

A new [3+2] cycloaddition strategy for the direct synthesis of highly substituted pyrroles from the readily available α‐acylketene dithioacetals (or related substrates) and commercially available propargylamines under mild metal‐free conditions has been developed. In this reaction, the acyl group plays a critical role in driving the conjugate addition of propargylamine and further cyclization to give pyrroles. Furthermore, the wide scope was confirmed by the preparation of 1,2,3,4‐tetrasubstituted pyrroles (60–70% yields) via a formal 1,2‐acyl migrating [3+2] cycloaddition pathway with N‐methylprop‐2‐yn‐1‐amine as the secondary amine component.     

超分子化合物（C12N2H9）2（C12N2H8）[SW12O40]的合成与结构表征

利用水热法合成了超分子化合物（C12N2H9）2（C12N2H8）[SW12O40],通过元素分析和X射线单晶衍射确定了其晶体结构,该晶体为正交晶系,P2（1）2（1）2（1）空间群,晶胞参数为a=12．0168（3）A^。,b=21．0534（6）A^。,c=22．5820（6）A^。,V=5713．1（3）A^。^3,α=β=γ=90°,Z=4,Dc=3．977mg·m^-3,并通过IR和XRD进行了结构表征,采用热重分析研究了晶体的热稳定性。该化合物由邻菲罗林和Keggin结构[SW12O40]^2-阴离子组成,以氢键自组装成超分子结构化合物。     

Efficient Preparation of the Isoindoline Framework via a Six Component,Tandem Double A3‐Coupling and [2+2+2] Cycloaddition Reaction

The preparation of tetrasubstituted isoindolines from three alkyne units, two aldehyde units and a primary amine via three consecutive reactions, two aldehyde‐amine‐alkyne couplings (A³‐couplings) and a final [2+2+2] cycloaddition, in a single synthetic operation, is described. The A³‐couplings are catalyzed by copper bromide and the cycloaddition is catalyzed by Wilkinson’s catalyst. It was found that many catalysts known to be efficient at the cycloaddition step were not suitable when this step was part of the tandem reaction sequence. Wilkinson’s catalyst was found to be unique in its suitability for the overall domino reaction sequence.     

Superbase‐Catalyzed [4+2] Cycloaddition of Acetylenes to 3,6‐Di(pyrrol‐2‐yl)‐1,2,4,5‐tetrazine: A Facile Synthesis of 3,6‐Di(pyrrol‐2‐yl)pyridazines

Acetylenes undergo the [4+2] cycloaddition to 3,6‐di(pyrrol‐2‐yl)‐1,2,4,5‐tetrazine in the potassium hydroxide/dimethyl sulfoxide or potassium tert‐butoxide/dimethyl sulfoxide systems (80 °C, 2.5–4 h) to afford (after extrusion of the nitrogen molecule from the intermediate) 3,6‐di(pyrrol‐2‐yl)pyridazines in up to 73% yield, while under non‐catalytic conditions this reaction does not take place. This unusual result substantially extends the scope of synthetic application and mechanistic diversity of the Diels–Alder reaction. The step‐wise mechanisms involving the formation of [OH/tetrazine]⁻ or [t‐BuO/tetrazine]⁻ anionic intermediate complexes or cycloaddition of tetrazine to the acetylide anion are considered.     

Nitrones as Trapping Reagents of α,β‐Unsaturated Carbene Intermediates – [1,2]Oxazino[5,4‐b]indoles by a Platinum‐ Catalyzed Intermolecular [3+3] Cycloaddition

Some readily available Boc‐protected 2‐(3‐methoxy‐1‐propynyl)anilines and nitrones in platinum‐catalyzed reactions deliver [1,2]oxazino[5,4‐b]indoles. Twelve examples with yields of 41–95% are reported. Different substituents like nitro, trifluoromethyl, fluoro, bromo, and ester groups are tolerated. With regard to the mechanism, this reaction probably combines an initial intramolecular cyclization/elimination to vinylcarbenoid species and a subsequent stepwise intermolecular [3+3] cycloaddition with the nitrones.     

Interrogation of an Enzyme Library Reveals the Catalytic Plasticity of Naturally Evolved [4+2] Cyclases

Stereoselective carbon-carbon bond forming reactions are quintessential transformations in organic synthesis. One example is the Diels-Alder reaction, a [4+2] cycloaddition between a conjugated diene and a dienophile to form cyclohexenes. The development of biocatalysts for this reaction is paramount for unlocking sustainable routes to a plethora of important molecules. To obtain a comprehensive understanding of naturally evolved [4+2] cyclases, and to identify hitherto uncharacterised biocatalysts for this reaction, we constructed a library comprising forty-five enzymes with reported or predicted [4+2] cycloaddition activity. Thirty-one library members were successfully produced in recombinant form. In vitro assays employing a synthetic substrate incorporating a diene and a dienophile revealed broad-ranging cycloaddition activity amongst these polypeptides. The hypothetical protein Cyc15 was found to catalyse an intramolecular cycloaddition to generate a novel spirotetronate. The crystal structure of this enzyme, along with docking studies, establishes the basis for stereoselectivity in Cyc15, as compared to other spirotetronate cyclases.     

Cinchona Alkaloid‐Derived Thiourea‐Catalyzed Diastereo‐ and Enantioselective [3+2] Cycloaddition Reaction of Isocyanoacetates to Isatins: A Facile Access to Optically Active Spirooxindole Oxazolines

An efficient diastereo‐ and enantioselective [3+2] cycloaddition reaction of α‐aryl isocyanoacetates to isatins catalyzed by a quinine‐derived bifunctional amine‐thiourea‐bearing sulfonamide as multiple hydrogen‐bonding donor catalyst has been investigated. The corresponding adducts, which bear a spirocyclic quaternary stereocenter at the C‐3 position of the oxindole, were obtained in good yields (51–95%), high diastereoselectivities (up to >20:1 dr) and good to excellent enantioselectivities (up to 97% ee).     

Thermal Intramolecular [2+2] Cycloaddition: Synthesis of 3‐Azabicyclo[3.1.1]heptanes from Morita–Baylis–Hillman Adduct‐Derived 4,4‐Diaryl‐1,3‐dienes

Various 3‐azabicyclo[3.1.1]heptane derivatives were synthesized from Morita–Baylis–Hillman adduct‐derived 1,3‐dienes bearing a 4,4‐diaryl moiety through a thermal intramolecular [2+2] cycloaddition approach. By using the same approach, bicyclo[3.1.1]heptane, 3‐azabicyclo[3.2.0]heptane, and 3‐oxabicyclo[3.1.1]heptane derivatives could also be synthesized. A structurally similar dimethylallyl derivative underwent an intramolecular ene reaction to afford the pyrrolidine derivative.

     

Strukturuntersuchungen an einem Produkt der intramolekularen Diels-Alder-Reaktion Konfigurations- und Konformationsanalyse von 5-Methyl-2-phenyl-1H-2,3,3a,4,5,7a-hexahydroisoindol-1-on

Structure Elucidation of a Product of Intramolecular Diels-Alder-Reaction: Configurational and Conformational Analysis of 5-Methyl-2-phenyl-1H,2,3,3a,4,5,7a-hexahydroisoindol-1-one N-Allyl-N-phenylsorbamide 1 cyclizes at 160°C to give 5-methyl-2-phenyl-1H,2,3,3a,4,5,7a-hexahydroisoindol-1-one 2 . As a result of this intramolecular [4 + 2] cycloaddition four stereoisomeres 2a–2d can exist. The ¹H and ¹³C n.m.r. data are given. The ¹H n.m.r. studies demonstrate that the product of Diels-Alder cyclisation is the result of cis-fusion of the two rings. The nuclear Overhauser effect data provide further evidence for 2a in which the carbonyl group and methyl one are trans-arranged. The n.m.r. results are confirmed by X-ray studies.     

Palladium‐Catalyzed [2+1] Cycloadditions Affording Vinylidenecyclopropanes as Precursors of 7‐Membered Carbocycles

Palladium(II) acetate in association with secondary phosphine oxides provides an efficient catalytic system for [2+1] cycloadditions starting from oxanorbornene derivatives and tertiary propargyl esters giving rise to vinylidenecyclopropanes. This reaction is specific to bidentate phosphinito–phosphinous acid ligands generated from secondary phosphine oxides. The [2+1] cycloaddition was found broad in scope with a high tolerance to various functional groups. Moreover, vinylidenecyclopropanes were straightforwardly converted into oxabicyclo[3.2.1]oct‐2‐ene derivatives through a palladium‐catalyzed ring‐expansion. Finally, the oxa bridge cleavage of oxatricyclic compounds yields functionalized 7‐membered carbocycles.

     

Organic metal-complexes. XXI. Synthesis and structure of 3,5,15,17-tetraoxo-1,7,10,13,19,22-hexaoxa-cyclotetracosane and 2,12,14,24-tetraoxo-4,7,10,16,19,22-hexaoxa-25-mercurato-bicyclo[11.11.1]pentacosane

A new synthesis of the title crown via isoxazolo crown ether 7 and macrocyclic bis-β-eneaminoketone 10 is described. 7 can be synthesized in 14% yield by a non-template double-[3+2] cycloaddition of dinitrileoxide 5 prepared in situ from dinitropolyether 19 by dehydration with Ph–NCO and alkyne 6 . The compounds 16, 17 and 18 are synthesized by the same synthetic strategy. Comparable IR and ¹H NMR spectroscopic data of macrocyclic and non-cyclic compounds show, that macrocyclic conformation stabilizing effects can be ruled out. The structures of the macrocycles 1, 7, 10 and that of the Hg(II)-complex 25 , synthesized by reaction of 1 with Hg(OAc)₂ were established by single-crystal X-ray structure analyses. Both inter- and intramolecular hydrogen bonds are observed for the macrocyclic bis-β-eneaminoketone 10 , whereas only intramolecular hydrogen bonds are formed by 1 . In the Hg(II)-complex of 1 the mercury is bonded to two methylene groups. C Hg C is almost linear [177(1)°], the mean Hg C distance amounts to 215(1) pm. In addition to the Hg C bonds, each Hg makes a short contact to a carbonyl oxygen in a neighbouring molecule in the plane perpendicular to the C Hg C axis [Hg(1) O(1) = 279(1) pm, Hg(2) O(5) = 284(1) pm].     

Rhodium‐Catalyzed [5+2] Cycloaddition of 3‐Acyloxy‐1,4‐enyne with Alkene or Allene

We recently developed a completely new type of Rh‐catalyzed [5+2] cycloaddition by using 3‐acyloxy‐1,4‐enyne (ACE) as the 5‐carbon building block. In this update, we show that ACE can undergo intramolecular [5+2] cycloaddition with either an alkene or an allene in the presence of an appropriate rhodium catalyst and ligands to afford bicyclic compounds with multiple stereogenic centers. In most cases, cis‐fused bicyclo[5.3.0]decadienes are prepared highly diastereoselectively.

     

Single and Merged Beam Studies of the Reaction H2+ (v = 0,1; j = 0,4) + H2 → H3+ + H

Different approaches for studying low-energy (meV) reactive ion-molecule collisions in molecular beams are presented. In particular, differences in the photoionization (REsonant Multi-Photon Ionization / REMPI, Pulsed Field Ionization / PFI) experiments using single molecular beams vs. merged and crossed beams are discussed. We address the question of the proper determination of cross sections and rate constants in single beam experiments and the factors that influence the width of the collision energy distributions. We identify space charge effects as the dominant contribution under our experimental conditions. Problems pertinent to state-selected primary ion preparation with narrow energy distributions are highlighted. In light of the detailed data analysis, the derived relations are applied to the four-center reaction H₂⁺ + H₂ → H₃⁺ + H, which was studied in single and merged molecular beam experiments. We present merged beam data for the absolute reaction cross section for primary ions created in a storage ion source in the energy range from 5 meV up to 5 eV and relative state-selected cross sections for different vibrational and rotational ion states (v = 0,1; j = 0–4) generated by (3+1) REMPI for a mean collision energy of about 10 meV.     

Cycloaddition chemistry of organic molecules with semiconductor surfaces

Recent investigations have shown that cycloaddition reactions, widely used in organic chemistry to form ring compounds, can also be applied to link organic molecules to the (001) surfaces of crystalline silicon, germanium, and diamond. While these surfaces are comprised of Si=Si, Ge=Ge, and C=C structural units that resemble the C=C bonds of organic alkenes, the rates and mechanisms of the surface reactions show some distinct differences from those of their organic counterparts This article reviews recent studies of [2 + 2], [4 + 2] Diels-Alder, and other cycloaddition reactions of organic molecules with semiconductor surfaces and summarizes the current understanding of the reaction pathways.