Enantioselective Reaction of 2H‐Azirines

2H‐Azirines are useful precursors for the synthesis of a variety of chiral aziridine and amine derivatives with a range of biological activities. Owing to the ring strain and the presence of a C=N double bond, 2H‐azirines are more reactive than other types of ketimine, and undergo a range of enantioselective reactions, including reduction and Diels–Alder reactions, as well as nucleophilic addition to the C=N double bond. Therefore, the enantioselective reactions of 2H‐azirines has become a hot topic, in particular within the last few years. In this Minireview, we focus on the enantioselective reactions of 2H‐azirines by using catalytic or stoichiometric amounts of chiral additives, the reaction mechanisms, and the applications of these reactions of 2H‐azirines and related compounds in organic synthesis.     

Criteria for the Efficiency,Stability, and Capacity of Abiotic Photochemical Solar Energy Storage Systems

The utilization of simple photochemical reactions for the storage of solar energy in the form of chemical energy in energy-rich products has often been considered in the further development and improvement of e. g. simple thermosolar techniques. The hitherto proposed criteria for the qualification of an abiotic photochemical system are, however, mostly of a qualitative nature, so a mutal comparison of the systems is not precise enough. In this article it is shown how a useful correlation on the basis of time-independent experimental data can be achieved and how, from the viewpoint of photochemistry, a comparative classification of known reactions is possible. The following reactions are compared: the [2 + 2]-photocycloadditions of norbornadiene, dimethyl 2,3-norbornadienedicarboxylate, and dicyclopentadienone, the photoisomerization of trans- to cis-diacetylindigo, the photodissociation of nitrosyl chloride as well as a photocatalytic redox reaction. The quantity of material required and storage efficiency are by far the most favorable in the case of trans-diacetylindigo. The main disadvantage of the latter however, is that the energy-rich cis-from rapidly reverts to the stable trans-form at elevated temperatures.     

Resonance revisited: A consideration of the calculation of cyclic conjugation energies

A homomolecular differential bond separation reaction may be defined as the difference between the conventional bond separation reactions involving the unsaturated system and its saturated counterpart. Such a reaction is homomolecular in that the basic molecular structures involved are the same on both sides of the reaction. The type of homodesmotic reaction that also conserves structure in this way may be termed a homomolecular homodesmotic reaction. Both types of homomolecular reactions are readily related to hydrogenation reactions and, more importantly, to each other. Δ B(n), the energy of the homomolecular differential bond separation reaction involving a system with n double bonds, and H(n), the corresponding homomolecular homodesmotic reaction, are related by: where h(1) and h(e) are the hydrogenation energies of the system's monoene and of ethylene, respectively. Both types of reactions yield measures of cyclic conjugation energies that for certain classifications of molecules are simply related to each other. Consideration of extra conjugation in the monoenes allows a ready interpretation of those cases in which a simple classification is not obtained. Ab initio calculations illustrating these effects have been carried out on a variety of molecules including many five- and six-membered ring systems using second order Møller-Plesset and density functional approaches. © 1997 by John Wiley & Sons, Inc.     

Proton-Coupled Electron Transfer (PCET) with 1,4-Bisguanidino-Benzene Derivatives: Comparative Study and Use in Acid-Initiated C-H Activation

Proton-coupled electron transfer (PCET) is of key importance in modern synthetic chemistry. Redox-active guanidines were established by our group as valuable alternatives to toxic high-potential benzoquinones in a variety of different PCET reactions. In this work, the PCET reactivity of a series of 1,4-bisguanidino-benzenes varying in their redox potentials and proton affinities is evaluated. The relevant redox and protonation states are fully characterized, and the compounds sorted with respect to their PCET reactivity by comparative PCET experiments supplemented by quantum-chemical calculations. Depending on the studied reactions, the driving force is either electron transfer or proton transfer; thereby the influence of both processes on the overall reactivity could be assessed. Then, two of the PCET reagents are applied in representative oxidative aryl-aryl coupling reactions, namely the intramolecular coupling of 3,3’’-4,4’’-tetramethoxy-o-terphenyl to give the corresponding triphenylene, the intermolecular coupling of N-ethylcarbazole to give N,N’-diethyl-3,3’-bicarbazole, and in the oxidative lactonization of 2-[(4-methoxyphenyl)methyl]-benzoic acid. Under mild conditions, the reactions proceed fast and efficient. Only small amounts of acid are needed, in clear contrast to the corresponding coupling reactions with traditional high-potential benzoquinones such as DDQ or chloranil requiring a large excess of a strong acid.     

Anilinolysis of reactive aryl 2,4‐dinitrophenyl carbonates: Kinetics and mechanism

The reactions of a series of anilines with phenyl 2,4‐dinitrophenyl ( 1 ), 4‐nitrophenyl 2,4‐dinitrophenyl ( 2 ), and bis(2,4‐dinitrophenyl) ( 3 ) carbonates are subjected to a kinetic investigation in 44 wt% ethanol–water, at 25.0 ± 0.1°C and an ionic strength of 0.2 M. Under amine excess pseudo‐first‐order rate coefficients (k_obs) are obtained. Plots of k_obs against free amine concentration at constant pH are linear, with slopes k_N. The Brønsted plots (log k_N vs. anilinium pK_a) for the anilinolysis of 1 – 3 are linear, with slope (β) values of 0.52, 0.61, and 0.63, respectively. The values of these slopes and other considerations suggest that these reactions are ruled by a concerted mechanism. For these reactions, the k_N values follow the reactivity sequence: 3 > 2 > 1 . Namely, the reactivity increases as the number of nitro groups attached to the nonleaving group increases. Comparison of the reactions of this work with the stepwise pyridinolysis of carbonates 1 – 3 indicates that the zwitterionic tetrahedral intermediate (T^±) formed in the pyridinolysis reactions is destabilized by the change of its pyridino moiety by an isobasic anilino group. This is attributed to the superior leaving ability from the T^± intermediate of anilines, relative to isobasic pyridines, which destabilize kinetically this intermediate. The k_N values for the anilinolysis of carbonates 1 – 3 are similar to those found in the reactions of these carbonates with secondary alicyclic amines. With the kinetic data for the anilinolysis of the title substrates and 4‐methylphenyl and 4‐chlorophenyl 2,4‐dinitrophenyl carbonates, a multiparametric equation is derived for log k_N as a function of the pK_a of the conjugate acids of anilines and nonleaving groups. © 2011 Wiley Periodicals, Inc. Int J Chem Kinet 43: 191–197, 2011     

Take a “Snapshot” of New Syntheses,Reactions, and Characterizations from Unusual Unsaturated Ring Strained Group 4 Metallacycles

Recently published syntheses, reactions and characterizations of unusual unsaturated ring strained Group 4 metallocene metallacycles like metalla-cyclocumulenes, -cycloallenes and -cycloalkynes with different ring size are updated for the last three years. There exist for some of these metallacycles, depending on the ring size, 7-, 5- and 4-membered compounds. The new results for these metallacycles are summarized here and considered in addition to the former published results. Additionally, several compounds of this type were now characterized by new reactions. For a better understanding of these compounds, some spectroscopical methods as well as theoretical calculations were published. Despite of these all-C-metallacycles, only in some cases the syntheses and reactions for the corresponding hetero-metallacycles were published too. Examples for these metallaheterocyclic compounds will not be considered in this article. All these unusual ring strained compounds have a great potential for a lot of interesting synthetic applications in the future. Additionally, they are very interesting from the theoretical point of view.     

Nickel: An Element with Wide Application in Industrial Homogeneous Catalysis

The efficiency and future development of the chemical industry are closely linked to catalysis. It has been estimated, for example, that 60 to 70% of all industrial chemicals have involved the use of a catalyst at some point during their manufacture. In the past two decades the share of the market credited to homogeneous transition metal catalysis increasead to 10–15%. Besides cobalt, which is used mainly in hydroformylation reactions, nickel is the most frequently used metal. Many carbon–carbon bond formation reactions can be carried out with high selectivity if catalyzed by organonickel complexes. Such reactions include, inter alia, carbonylation reactions, cyclic and linear oligomerization and polymerization reactions of monoenes and dienes, and hydrocyanation reactions. It was Reppe and Wilke who pioneered and shaped the field of homogeneous nickel catalysis. Great impetus was also given to the development of organonickel chemistry by Wilke and his students. Research in this area has contributed immensely towards an understanding of the reactions involved in catalysis.—This review is primarily concerned with nickel-catalyzed reactions which are of interest both preparatively and industrially; some mechanistic aspects are also dealt with.     

Reaction rates for O + HD → OH + D and O + HD → OD + H

We have calculated reaction rates for the reactions O + HD → OH + D and O + DH → OD + H using improved canonical variational transition state theory and least-action ground-state transmission coefficients with an ab initio potential energy surface. The kinetic isotope effects are in good agreement with experiment. The optimized tunneling paths and properties of the variational transition states and the rate enhancement for vibrationally excited reactants are also presented and compared with those for the isotopically unsubstituted reaction O + H₂ → OH + H. The thermal reactions at low and room temperature are predicted to occur by tunneling at extended configurations, i.e., to initiate early on the reaction path and to avoid the saddle point regions. Tunneling also dominates the low and room temperature reactions for excited vibrational states, but in these cases the results are not as sensitive to the nature of the tunneling path. Overbarrier mechanisms dominate for both thermal and excited-vibrational state reactions for T > 600 K. For the excited-state reaction (with initial vibrational quantum number n > 0) a transition state switch occurs for T > 1000 K for the O + HD(n = 1) → OD + H case and for T > 1500 K for the O + DH(n = 1) → OD + H reaction, and this may be a general phenomenon for excited-state reactions at higher temperature. In the present case the switch occurs from an early variational transition state where the vibrationally adiabatic approximation is expected to be valid to a tighter variational transition state where nonadiabatic effects are probably important and should be included.     

α,ω-Diisocyanatocarbodiimides, -Polycarbodiimides,and Their Derivatives

Research in the field of low-molecular weight, oligomeric and polymeric α,ω-diisocyanatocarbodiimides and -polycarbodiimides has been fruitful, not only in connection with these compounds themselves, but also—as so often happens in chemistry—with quite different problems. Novel synthetic methods, discoveries concerning the properties of low-molecular weight carbodiimides and phosphane imide derivatives, as well as results on the fragmentation reactions of four-membered heterocyclic compounds containing oxygen, phosphorus, and nitrogen, and a better understanding of the diisocyanate polyaddition process are among the many by-products of this research. The “high- and low-temperature formation” of polycarbodiimides and the homogeneous and heterogeneous catalysis of this process are described, and the fundamental importance of four-membered ring fragmentation mechanisms resulting in the formation of phosphane imide derivatives is outlined. Interesting building blocks for the diisocyanate polyaddition and polycondensation processes can be synthesized by many derivatization reactions of oligomeric and high-molecular weight polycarbodiimides and polyuretonimines. The in situ production of polycarbodiimides via matrix reactions in flexible polyurethane foams leads to a cellular arrangement of the material due to the pronounced symmetrical growth processes. Combination-foams with increased carbonation tendencies are formed in this way. Attention is drawn to several industrial applications of α,ω-diisocyanatopolycarbodiimides, of high-molecular weight cross-linked polyuretonimines, and of polycarbodiimide foams.     

Novel rate equations describing isochronous chemical reactions

A simple mathematical model involving two first-order Ordinary Differential Equations (ODEs) with fourth-degree polynomial nonlinearities is introduced. The initial-value problem for this system of two ODEs is solved in terms of elementary functions: for an open set of initial data, this solution is isochronous, i.e., completely periodic with a fixed period (independent of the initial data); in the complementary set of initial data, it blows up at a finite time. This system is likely to be of applicative interest: for instance it models the time evolution of two chemical substances in a spatially homogeneous situation, provided this evolution is characterized by six appropriate chemical reactions whose rates are simply expressed in terms of three a priori arbitrary parameters, or alternatively by five appropriate reactions whose rates are simply expressed in terms of two a priori arbitrary parameters.     

Kinetic Modeling of Hydrogen Abstractions Involving Sulfur Radicals

One of the requisites for the development of detailed reaction networks is the availability of accurate kinetic data. Group additivity based models linking the Arrhenius parameters to structural characteristics of the transition state have proven to be a valuable tool to obtain those data. In this work, group additivity values are presented to allow a broad range of C H and S H hydrogen abstraction reactions by S radicals to be modeled. Rate coefficients in the temperature range from 300 to 1500 K are obtained by using the CBS-QB3 method in the high-pressure limit and are corrected for tunneling and anharmonicity of rotation about the transitional bond. A total of 149 reactions are studied. From these reactions, a total of 52 group additivity values and 35 resonance corrections are derived. The general applicability of the group additivity method is demonstrated for a test set containing 25 reactions. At 300 K, rate coefficients are on average reproduced within a factor of 2.8. The mean absolute deviations on the Arrhenius parameters are 2 kJ mol⁻¹ for the activation energy and 0.38 for log A in which A is the pre-exponential factor.     

Photoinitiated Thiol-ene Reactions of Enoses: A Powerful Tool for Stereoselective Synthesis of Glycomimetics with Challenging Glycosidic Linkages

Thioglycosides and C-glycosides represent pharmacologically useful classes of glycomimetics that possess a high degree of biological stability. One emerging tool for the stereoselective synthesis of thioglycosides is the photoinitiated addition of thiols to unsaturated sugars. Moreover, thiyl radical-mediated reactions of exo-glycals and 1-substituted endo-glycals offer facile routes to β-C-glycosidic structures. This Concept article summarizes the thiol-ene coupling strategies developed recently by our group and Somsák's group for the synthesis of several kinds of glycomimetics which are difficult to synthesize by conventional methods. One unusual characteristic of the thiol-ene reactions of endo-glycals is that heating inhibits, whereas cooling promotes the reaction. This unique temperature dependence as well as the effects of the enose structures and thiol configurations on the efficacy and stereoselectivity of the reactions are also discussed.     

Carbon‐Centered Radical Addition and β‐Scission Reactions: Modeling of Activation Energies and Pre‐exponential Factors

A consistent set of group additive values ΔGAV° for 46 groups is derived, allowing the calculation of rate coefficients for hydrocarbon radical additions and β-scission reactions. A database of 51 rate coefficients based on CBS-QB3 calculations with corrections for hindered internal rotation was used as training set. The results of this computational method agree well with experimentally observed rate coefficients with a mean factor of deviation of 3, as benchmarked on a set of nine reactions. The temperature dependence on the resulting ΔGAV°s in the broad range of 300–1300 K is limited to ±4.5 kJ mol⁻¹ on activation energies and to ±0.4 on logA (A: pre-exponential factor) for 90 % of the groups. Validation of the ΔGAV°s was performed for a test set of 13 reactions. In the absence of severe steric hindrance and resonance effects in the transition state, the rate coefficients predicted by group additivity are within a factor of 3 of the CBS-QB3 ab initio rate coefficients for more than 90 % of the reactions in the test set. It can thus be expected that in most cases the GA method performs even better than standard DFT calculations for which a deviation factor of 10 is generally considered to be acceptable.     

Investigating the Active Oxidants Involved in Cytochrome P450 Catalyzed Sulfoxidation Reactions

Cytochrome P450 (CYP) heme-thiolate monooxygenases catalyze the hydroxylation of the C−H bonds of organic molecules. This reaction is initiated by a ferryl-oxo heme radical cation (Cpd I). These enzymes can also catalyze sulfoxidation reactions and the ferric-hydroperoxy complex (Cpd 0) and the Fe(III)-H₂O₂ complex have been proposed as alternative oxidants for this transformation. To investigate this, the oxidation of 4-alkylthiobenzoic acids and 4-methoxybenzoic acid by the CYP199A4 enzyme from Rhodopseudomonas palustris HaA2 was compared using both monooxygenase and peroxygenase pathways. By examining mutants at the mechanistically important, conserved acid alcohol-pair (D251N, T252A and T252E) the relative amounts of the reactive intermediates that would form in these reactions were disturbed. Substrate binding and X-ray crystal structures helped to understand changes in the activity and enabled an attempt to evaluate whether multiple oxidants can participate in these reactions. In peroxygenase reactions the T252E mutant had higher activity towards sulfoxidation than O-demethylation but in the monooxygenase reactions with the WT enzyme the activity of both reactions was similar. The peroxygenase activity of the T252A mutant was greater for sulfoxidation reactions than the WT enzyme, which is the reverse of the activity changes observed for O-demethylation. The monooxygenase activity and coupling efficiency of sulfoxidation and oxidative demethylation were reduced by similar degrees with the T252A mutant. These observations infer that while Cpd I is required for O-dealkylation, another oxidant may contribute to sulfoxidation. Based on the activity of the CYP199A4 mutants it is proposed that this is the Fe(III)-H₂O₂ complex which would be more abundant in the peroxide-driven reactions.     

A dimensionless reaction coordinate for quantifying the lateness of transition states

The Hammond‐Leffler postulate asserts that transition states of exothermic reactions are reactant‐like (early), whereas transition states of endothermic reactions are product‐like (late). Related postulates have been proposed to describe the sensitivity of activation barriers for reactions occurring on catalytic surfaces to the catalyst structure. To evaluate the validity of these postulates for different chemical reactions, a general method for classifying transition states as either early or late is needed. One can envision a dimensionless reaction coordinate that changes continuously and monotonically from 0 to 1 along a minimum energy reaction pathway. The value of the dimensionless reaction coordinate for the transition state (W_TS) classifies transition states as (a) early when W_TS < 0.5, (b) late when W_TS > 0.5, and (c) equidistant between reactants and products when W_TS = 0.5. In this article, we derive such a dimensionless reaction coordinate and illustrate its usefulness for several different chemical reactions. © 2009 Wiley Periodicals, Inc. J Comput Chem, 2010     

Recent Progress on Cyclic Nitrenoid Precursors in Transition-Metal-Catalyzed Nitrene-Transfer Reactions

Nitrene-transfer reactions are powerful synthetic tools for the direct incorporation of nitrogen atoms into organic molecules. The discovery of novel nitrene-transfer reactions has been dominantly supported not only by improvements in transition-metal catalysts but also by the employment of novel precursors of nitrenoids. Since pioneering work involving the use of organic azides and iminoiodinanes as practical synthetic tools for nitrogen-containing compounds was reported, a new approach using various N-heterocycles containing strain energy or a weak bond has emerged. In this review, we briefly summarize the history of nitrene-transfer chemistry from the viewpoint of its precursors. In particular, the use of N-heterocycles such as 2H-azirines, 1,4,2-dioxazol-5-ones, 1,2,4-oxadiazol-5-ones, isoxazol-5(4H)-ones, and isoxazoles is comprehensively described, showing the recent remarkable progress in this chemistry.     

Angular momentum conservation in unimolecular and recombination reactions

It is shown that the master equation describing fall-off effects in unimolecular and recombination reactions, with angular momentum (J) conservation taken into account, can be solved exactly if the assumption is made that the probability of collisional energy transfer in J is independent of initial state; this assumption is shown to be physically acceptable (from general conservation considerations and from trajectory calculations) for typical neutral radical recombination and decomposition reactions. This leads to a J-averaged master equation which can be readily solved by standard means. Illustrative computations using this treatment are presented.     

Preparation of Polymeric Analogs of Tetramethylurea and Their Use as Liquid-Liquid Phase-Transfer Catalysts

Polymeric analogs of tetramethylurea were prepared by free-radical polymerization of N,N,N'-trimethyl-N'-p-vinylbenzylurea and copolym-erization of this monomer with styrene. These polymers are soluble in toluene and have the ability to extract alkali metal ions such as lithium, sodium, and potassium. Moreover, these polymers act as phase-transfer catalysts for typical S _N 2 reactions, although the monomeric analogs tetramethylurea and N,N,N'-trimethyl-N'-benzylurea do not. A plausible mechanism for these catalytic reactions is proposed.     

New Method for the Synthesis of Lactones via Nickel-Catalyzed Isocyanides Insertion

A novel nickel catalyst for the reaction of tert-butyl isocyanide insertion was discovered. In this approach, 1,2-bis(diphenylphosphino)ethane (L3) serves as an efficient ligand, thereby allowing the preparation of lactones from (o-bromophenyl)phenylethanone derivatives. It is noteworthy that this is the first example of nickel acting as a metal catalyst in the reactions of tert-butyl isocyanide insertion. The significance of this methodology may draw many chemists’ attention in the field of isocyanide-incorporating reactions.

[Supplementary materials are available for this article. Go to the publisher's online edition of Synthetic Communications® for the following free supplemental resource(s): Full experimental and spectral details.]