A Cationic Imido Complex of Permethyltantalocene: H2 and Carbon−Hydrogen Bond Activation, [2 + 2] Cycloaddition Reactions, and an Unusual Reaction with Carbon Dioxide That Affords Coordinated Isocyanate

A theoretical study of the effect of the diprotonation on the nucleic acid bases (A : U, A : T and G : C) in Watson-Crick conformation has been carried out by means of DFT computational methods in vacuum. In addition, the corresponding neutral and monoprotonated binary complexes have been considered. Most of the diprotonated species studied are stable, even though the binding energy is positive due to the overall repulsive electrostatic term. Local electrostatic attractive forces in the regions of hydrogen bonds (HBs) are responsible for equilibrium geometries, as shown by the electric field lines connecting the electrophilic and nucleophilic sites involved in the HB interactions. Secondary electrostatic effects also affect the assembling of the nucleic acid complexes in either neutral or cationic form. In particular, the electric field lines flowing from electrophilic sites in one base to nucleophilic sites in the other reinforce the linking between them. Hence, when the nucleophilic site concerns the free lone pair of the heteroatom involved in the HB interaction as acceptor, the HB distance shortens. However, if the free lone pair of the HB acceptor interacts with an electrophilic site in the same molecule, the HB distance elongates, weakening the HB interaction. The topological analysis of the electron density distribution in HB regions indicates that neutral, monoprotonated and diprotonated complexes show no differences in the nature of their HB's.

2 + 2] cycloaddition reactions at terminal imido uranium(IV) complexes to yield isolable cycloadducts

Transition metal-catalyzed tandem reactions have attracted much attention due to their operational and economical efficiency in the synthesis of complex organic molecules. In particular, if an intermediate is highly reactive, its further transformation in a single operation will broaden the utility of the highly reactive intermediate molecule in organic synthesis. Synthesis of novel cyclic compounds through cobalt carbonyl-catalyzed tandem cyclization including the Pauson-Khand and the related reaction has been one of our current research themes. Recently, we reported the utilization of the highly reactive cyclopentadienone, which was produced by a Pauson-Khand reaction, for the synthesis of polycyclic compounds in a tandem fashion. Herein, we report cobalt-catalyzed intramolecular carbonylative cycloaddition of terminal diynes and a subsequent intermolecular cobalt-catalyzed [2+2+2] cycloaddition between the cyclopentadienone intermediate and the starting terminal diyne compound in the presence of cyclopentadiene. During the investigation of the cobalt carbonyl-catalyzed tandem cycloaddition reaction of α,ω-diynes with cyclopentadiene, it was found that two types of reaction products were obtained. One was a [4+2] cycloaddition product resulting from the carbonylative cyclization intermediate, cyclopentadienone, and cyclopentadiene. The other was a [2+2+2] cycloaddition product from the cyclopentadienone intermediate and the starting α,ω-diyne, or dimerization product of the diyne, as shown in Figure 1. Whereas internal diynes, produced only the [4+2] cycloaddition product in the Co-catalyzed tandem cycloaddition reaction with conjugated dienes, it was revealed in this study that terminal diynes, such as 1,6-heptadiene, resulted in a [2+2+2] and [4+2] cycloaddition product mixture when the reaction occurred in the presence of cyclopentadiene. Higher reactivity of terminal alkynes compared to internal alkynes may be attributed to the competition of diyne with cyclopentadiene in the quenching step of the tandem catalysis. With this in mind, we carefully examined the tandem reaction for α,ω-diynes and mono-substituted diynes and found that the steric and electronic properties of the diyne could control the reaction pathway between the [4+2] and [2+2+2] cycloadditions in the second step of the reaction scheme (Scheme 1). Adding diyne 1 (0.8 mmol) and cyclopentadiene (5 equiv.) to dichloromethane (30 mL) containing Co2(CO)8 (5 mol %) under 5 atm of CO pressure at 120 C for 18 h produced a mixture of [2+2+2] cycloaddition product 2 from the reaction of a cyclopentadienone intermediate with diyne 1 itself and [4+2] cycloaddition product 3 from the reaction of the cyclopentadienone intermediate with cyclopentadiene. The ratio of the products in the mixture varied according to the diyne used (Table 1). Competition between the diyne and cyclopentadiene at the second cycloaddition step after the production of a reactive cyclopentadienone intermediate by the carbonylative cycloaddition reaction in the presence of tandem catalysis was greatly influenced by the type of tether group in the terminal 1,6-diynes. In the reaction of the diynes containing a Ctether at the 4-position, carbonylative dimerization of the diyne occurred preferentially over carbonylative [4+2] cyclization of the diyne and cyclopentadiene (entries 1 and 2). Interestingly, cyclopentadiene did not take part as a diene in the cyclopentadienone quenching step in reaction of 1b at all owing to the higher reactivity of dimerization of the diyne compared to cyclization with cyclopentadiene. In the reaction of the diyne (1c) containing a N-tether at the 4-position, the chemoselectivity of the reaction was reversed to give the carbonylative [4+2] cycloadduct (3c) as a major product (entry 3). In order to investigate the chemoselectivity of the

The reaction of N‐methylisatoic anhydride 1 with a series of quincorine (QCI) and quincoridine (QCD) derivatives furnished the corresponding QCI‐ and QCD‐substituted anthranilic acid amides 4−9. In the synthesis of 4−9, we investigated the influence of the C5 substituent of 2‐aminomethyl derivatives of QCI and QCD on their basicity and polarity and, therefore, on their reactivity towards isatoic acid anhydride. By exposing a number of 2‐aminomethyl derivatives of QCI and QCD to (carbon dioxide from) air, the formation of carbon dioxide adducts 10−15 was observed. In an unusual reaction, we obtained the first set of chiral ammonium carbamates derived from primary amines, which provides a convenient method for purifying and stabilizing aminomethyl derivatives of QCI and QCD. All compounds were fully characterized by spectroscopic methods. In addition, the structure of 11 was proved by X‐ray crystal structure determination. The heterocyclic nitrogen atoms display trigonal‐pyramidal coordination geometries; the amido group is essentially planar. N‐2′ of the ammonium ion is a triple hydrogen bond donor to the carboxylato oxygen atoms of three different carbamate anions. The resulting net effect is a hydrophilic plane parallel to the ab plane. The crystal packing involves six classical hydrogen bonds. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)

Cycloaddition reactions of allenylphosphonates [(RO)(2)P(O)[(R(1))C═C═CR(2)(2)] with dialkyl acetylenedicarboxylates, 1,3-diphenylisobenzofuran, and anthracene have been investigated and compared with those of allenoates [(EtO(2)C)RC═C═CH(2)] and allenylphosphine oxides [Ph(2)P(O)(R(1))C═C═CR(2)(2)] in selected cases. Allenylphosphonates (RO)(2)P(O)(Ar)C═C═CH(2) with an α-aryl group preferentially undergo [4 + 2] cycloaddition with DMAD/DEAD under thermal activation, but in addition to the expected 1:1 (allene: DMAD) product, the reaction also leads to 1:2 as well as 2:1 products that were not reported before. When an extra vinyl group is present at the γ-carbon of allenylphosphonate [e.g., (OCH(2)CMe(2)CH(2)O)P(O)(Ph)C═C═CH(C═CHMe)], [4 + 2] cycloaddition takes place utilizing either the vinylic or the aryl end, but additionally a novel cyclization wherein complete opening of the [β,γ] carbon-carbon double bond of the allene is realized. In contrast to these, the reaction of allenylphosphonate (OCH(2)CMe(2)CH(2)O)P(O)(H)C═C═CMe(2) possessing a terminal ═CMe(2) group with DMAD occurs by both [2 + 2] cycloaddition and ene reaction. While the reaction of ═CH(2) terminal allenylphosphonates as well as allenylphosphine oxides with 1,3-diphenylisobenzofuran afforded preferentially endo-[4 + 2] cycloaddition products via [α,β] attack, the analogous allenoates [(EtO(2)C)RC═C═CH(2)] underwent exo-[4 + 2] cyclization. Under similar conditions, allenylphosphonates with a terminal ═CR(2) group gave only [β,γ]-cycloaddition products. An unusual ring-opening of a [4 + 2] cycloaddition product followed by ring-closing via [4 + 4] cycloaddition, as revealed by (31)P NMR spectroscopy, is reported. Anthracene reacted in a manner similar to 1,3-diphenylisobenzofuran, albeit with lower reactivity. Key products, including a set of exo- and endo- [4 + 2] cycloaddition products, have been characterized by single crystal X-ray crystallography.

Further chemistry of ruthenium butatrienylidene complexes

CO2 Capture via Crystalline Hydrogen-Bonded Bicarbonate Dimers

A series of 6-alkenyl-3-phenylcyclohex-2-enones has been synthesised and the structures of the products obtained from them on irradiation have been determined. The 6-propenyl compounds afforded a tricyclic 'parallel' [2 + 2] cycloaddition product and a bicyclic enone resulting from hydrogen abstraction in the biradical intermediate. The 6-butenyl and 6-pentenyl analogues gave 'crossed' cycloaddition products only. Although the regiochemistry of these cycloaddition reactions cannot be explained in terms of the 'rule of five', it is compatible with the concept of 'biradical conformation control' which is based on a consideration of the energy and structure of the possible 1,4-biradical intermediates.

Copper(I) is able to catalyze Huisgen 1,3-dipolar cycloaddition in a "click" fashion. This copper-catalyzed azide-alkyne cycloaddition (CuAAC) reaction presents excellent chemoselectivity and occurs over a wide-range of reaction conditions. It shows tolerance to variation in both pH and solvent polarity, thereby facilitating the ligation of peptides and proteins to produce peptidomimetics and synthetic proteins. In addition, the only product formed is a 1,4-disubstituted-1,2,3-triazole moiety, in many aspects resembling the natural peptide bond, including hydrogen-bonding capability, planarity, distance between the 1 and 4 substituents, and conformational restriction of the peptide backbone; thus the triazole-backbone-modified peptide, in which a triazole replaces the amide bond, may be anticipated to present a secondary structure similar to that of its natural counterpart. This Focus Review describes the scope and applications of copper(I)-catalyzed alkyne–azide cycloaddition in synthetic peptide/protein chemistry.

[reactions: see text] A rhodium complex of N-heterocyclic carbene (NHC) has been developed for intra- and intermolecular [4 + 2] and intramolecular [5 + 2] cycloaddition reactions. This is the first use of a transition-metal NHC complex in a Diels-Alder-type reaction. For the intramolecular [4 + 2] cycloaddition reactions, all the dienynes studied were converted to their corresponding cycloadducts in 91-99% yields within 10 min. Moreover, up to 1900 turnovers have been obtained for the intramolecular [4 + 2] cycloaddition at 15-20 degrees C. For the intermolecular [4 + 2] cycloadditions, high yields (71-99%) of the corresponding cycloaddition products were obtained. The reaction time and yield were highly dependent upon the diene and the dienophile. For the intramolecular [5 + 2] cycloaddition reactions, all the alkyne vinylcyclopropanes studied were converted to their corresponding cycloadducts in 91-98% yields within 10 min. However, the catalytic system was not effective for an intermolecular [5 + 2] cycloaddition reaction.

A catalytic, enantioselective spirocyclization of formanilides or formylindolines and enamides has been developed herein. The reaction proceeds through a sequential iridium-catalyzed hydrosilylatio...

The effects of a hydrogen bond acceptor and hydrogen bond donor on carbon dioxide absorption via natural deep eutectic solvents were studied in this work. Naturally occurring non-toxic deep eutectic solvent constituents were considered; choline chloride, b-alanine, and betaine were selected as hydrogen bond acceptors; lactic acid, malic acid, and fructose were selected as hydrogen bond donors. Experimental gas absorption data were collected via experimental methods that uses gravimetric principles. Carbon dioxide capture data for an isolated hydrogen bond donor and hydrogen bond acceptor, as well as natural deep eutectic solvents, were collected. In addition to experimental data, a theoretical study using Density Functional Theory was carried out to analyze the properties of these fluids from the nanoscopic viewpoint and their relationship with the macroscopic behavior of the system, and its ability for carbon dioxide absorption. The combined experimental and theoretical reported approach work leads to valuable discussions on what is the effect of each hydrogen bond donor or acceptor, as well as how they influence the strength and stability of the carbon dioxide absorption in deep eutectic solvents. Theoretical calculations explained the experimental findings, and combined results showed the superiority of the hydrogen bond acceptor role in the gas absorption process, with deep eutectic solvents. Specifically, the cases in which choline chloride was used as hydrogen bond acceptor showed the highest absorption performance. Furthermore, it was observed that when malic acid was used as a hydrogen bond donor, it led to low carbon dioxide solubility performance in comparison to other studied deep eutectic solvents. The cases in which lactic acid was used as a hydrogen bond donor showed great absorption performance. In light of this work, more targeted, specific, deep eutectic solvents can be designed for effective and alternative carbon dioxide capture and management.

Mild and environmentally benign Lewis acid promoted 1,3-dipolar cycloaddition reactions of α-hydrazonyl chlorides with alkenes in water are reported. These α-hydrazonyl chlorides, in the presence of Lewis acids, generate nitrile imines in situ which react with dipolarophiles to furnish the corresponding cycloaddition products. In many cases, the required times for the completion of the Lewis acid promoted 1,3-dipolar cycloaddition reactions in water were comparable to the equivalent reactions performed in an organic solvent. Analogous tetrahexylammonium chloride promoted 1,3-dipolar cycloaddition reactions were also performed. The comparison of reaction times and cycloadduct yields for the aforementioned 1,3-dipolar reactions in aqueous and organic media as well as the proposed role of the Lewis acid in the 1,3-dipolar cycloaddition reaction are described.

[ACCESS RESTRICTED TO THE UNIVERSITY OF MISSOURI AT REQUEST OF AUTHOR.] An allylic benzoate ester was prepared and used as a progenitor of vinylthionium ions for (4+3)-cycloaddition reactions. The chemistry was limited to 2-substituted and 2,5-disubstituted furans. In the case of 2-substituted furans, only Friedel-Crafts alkylation products were procured. In the case of 2,5-disubstituted furans, (4+3)-cycloaddition products were furnished in good yield. A catalytic, asymmetric variant using 2,5-dialkylfurans and chiral acids to procure cycloaddition products was unsuccessful. There was an opportunity to overcome the Friedel-Crafts alkylation problem that arose in a gold-catalyzed cycloaddition project. A sulfur-substituted unsaturated ester and alcohol were prepared, but silyl migration was a considerable problem upon activation by various agents. The problem of silyl migration was circumvented, but the target aldehyde was unreactive toward dienes in the presence of various Lewis acids. In further pursuit of an asymmetric (4+3)-cycloaddition reaction, 2-substituted furans were reacted with 2-tosyloxycyclopentanone in the presence of K2HPO4/H2O in perfluorobenzene as solvent and 30 mol% of an amino alcohol catalyst developed by David MacMillan. Enantioselectivities were modest for 2-alkylfurans and excellent for 2-arylchalcogenofurans (90% ee). 3-Substituted furans also work in this process.

ABSTRACTIn this article, we describe some of our results on the reactions of allenylphosphonates/allenyl phosphine oxides and allenyl diazaphosphole oxides, primarily cycloaddition and cyclization reactions. Depending on the substituents, both the α,β and β,γ -cycloaddition products have been observed. A novel cyclization reaction of a functionalized allenylphosphine oxide with diethylamine leading to 3-diethylamino-4-diphenylphosphinoyl-1-naphthol is reported. Representative compounds have been characterized by X-ray crystallography.