Organocatalyzed Michael Addition of Aldehydes to Nitro Alkenes – Generally Accepted Mechanism Revisited and Revised

The amine‐catalyzed enantioselective Michael addition of aldehydes to nitro alkenes (Scheme 1) is known to be acid‐catalyzed (Fig. 1). A mechanistic investigation of this reaction, catalyzed by diphenylprolinol trimethylsilyl ether is described. Of the 13 acids tested, 4‐NO₂? C₆H₄OH turned out to be the most effective additive, with which the amount of catalyst could be reduced to 1 mol‐% (Tables 2–5). Fast formation of an amino‐nitro‐cyclobutane 12 was discovered by in situ NMR analysis of a reaction mixture. Enamines, preformed from the prolinol ether and aldehydes (benzene/molecular sieves), and nitroolefins underwent a stoichiometric reaction to give single all‐trans‐isomers of cyclobutanes (Fig. 3) in a [2+2] cycloaddition. This reaction was shown, in one case, to be acid‐catalyzed (Fig. 4) and, in another case, to be thermally reversible (Fig. 5). Treatment of benzene solutions of the isolated amino‐nitro‐cyclobutanes with H₂O led to mixtures of 4‐nitro aldehydes (the products 7 of overall Michael addition) and enamines 13 derived thereof (Figs. 6–9). From the results obtained with specific examples, the following tentative, general conclusions are drawn for the mechanism of the reaction (Schemes 2 and 3): enamine and cyclobutane formation are fast, as compared to product formation; the zwitterionic primary product 5 of C,C‐bond formation is in equilibrium with the product of its collapse (the cyclobutane) and with its precursors (enamine and nitro alkene); when protonated at its nitronate anion moiety the zwitterion gives rise to an iminium ion 6 , which is hydrolyzed to the desired nitro aldehyde 7 or deprotonated to an enamine 13 . While the enantioselectivity of the reaction is generally very high (>97% ee), the diastereoselectivity depends upon the conditions, under which the reaction is carried out (Fig. 10 and Tables 1–5). Various acid‐catalyzed steps have been identified. The cyclobutanes 12 may be considered an off‐cycle ‘reservoir’ of catalyst, and the zwitterions 5 the ‘key players’ of the process (bottom part of Scheme 2 and Scheme 3).     

Novel versatile imine—enamine chemosensor based on 6-nitro-4-oxo-4H-chromene for ion detection in solution, solid and gas-phase: synthesis, emission, computational and MALDI-TOF-MS studies

A new versatile emissive molecular probe (3) derived from 1,5-bis(2-aminophenoxy)-3-oxopenthane bearing two units of 6-nitro-4-oxo-4H-chromene- has been prepared by a Schiff-base condensation method using conventional and green, ultrasound-aided, methods. The dry yellow powder was characterized as the imine species (3). These imine species, however, where found to rapidly convert to their enamine form (4) in solution, under the presence of water traces. This reaction was computationally studied through Density Functional Theory (DFT) in order to investigate the relative stability of the molecular pair 3/4. The sensing properties of the enamine (4) towards various metal ions were investigated via absorption and fluorometric titrations in solution in dichloromethane, acetonitrile and DMSO. The compound shows a fluorescent turn-off response in the presence of Cu²⁺, Zn²⁺, Cd²⁺, Hg²⁺ and Ag⁺ over the other metal ions studied, such as Li⁺, Na⁺, K⁺, Ca²⁺, Co²⁺ and Ni²⁺, being stronger for Cu²⁺ and Hg²⁺. The gas phase chemosensing abilities of (3) were also explored suggesting (3) as new active MALDI-TOF-MS matrix by two dry methods showing a strong selectivity towards Cu²⁺ and Ag⁺. Our preliminary results show promising uses of (3) supported in PPMA films as metal ion solid chemosensor.     

Synthese und Deprotonierung eines Phosphinomethylimidazoliumsalzes. Zur Delokalisierung der π‐Elektronen in Aminovinylphosphanen [1]

Tetrafluorobo‐Synthesis and Deprotonation of a Phosphinomethylimidazolium Salt. On the π‐Electron Delocalization im Aminovinyl Phosphanes [1] The ylidic olefin 1, 3, 4, 5‐tetramethyl‐2‐methyleneimidazoline ( 1 , Im=CH₂) reacts with chlorodiphenylphosphane to give the phosphane salt [(Im‐CH₂)PPh₂]Cl ( 6 ) from which the vinylphosphane Im=CHPPh₂ ( 8 ) is obtained by deprotonation. 8 reacts with HBF₄ etherate to give the tetrafluoroborate salt [(Im‐CH₂)‐PPh₂] BF₄ ( 7 ). The crystal structures of 7 and 8 are determined and discussed. In 8 , π‐electron delocalisation is indicated both by structural data and temperature dependent n.m.r. spectroscopy     

Convenient synthesis of trifluoromethylated 2-pyrrolidone and 2-pyrrolone derivatives

The reaction of a set of enamines with methyl trifluoropyruvate and its imine was investigated. As a result, a simple procedure for synthesis of trifluoromethylated 2-pyrrolidone and 2-pyrrolone derivatives has been developed.     

A One‐Pot Biginelli Synthesis of 6‐Unsubstituted 5‐Aroylpyrimidin‐2(1H)‐ones and 6‐Acetyl‐1,2,4‐triazin‐3(2H)‐ones

The three‐component Biginelli‐like cyclocondensation reaction of enamines 1 , urea, and aldehydes in dioxane/acetic acid efficiently afforded the corresponding 6‐unsubstituted 3,4‐dihydropyrimidin‐2(1H)‐ones 2 in good yields (Scheme 1, Table). The corresponding reaction of azaenamine (=hydrazone) 7 with benzaldehyde and urea afforded 6‐acetyl‐1,2,4‐triazin‐3(2H)‐ones in good yields (Scheme 3).     

Action des Enamines Sur Les Allenes Phosphores: Synthese de 1-( N -morpholino ou N -pyrrolidino)-phosphomethylenbicycloalcanes

In this work we studied the reaction of enamines 1 with allenylphosphonates and -phosphine oxides 2 which lead to the 1-( N -morpholino or N -pyrrolidino)-phosphomethylen-bicyclolkanes 3 . The structure of compounds 3 was confirmed by NMR-( 1 H, 13 C, and 31 P) and IR spectroscopy.     

A mild procedure for the oxidative cleavage of substituted indoles catalyzed by plant cell cultures

We have developed a novel procedure for the oxidative cleavage of indole carbon double bonds in the presence of H₂O₂ using plant cell cultures as a catalytic system. The oxidative procedure has some advantages, such as mild reactions, good yields, easy work-up and safety.     

A dramatic effect of double bond configuration in N-oxy-3-aza Cope rearrangements—a simple synthesis of functionalised allenes

The first examples of low temperature N-oxy-3-aza Cope rearrangements, leading to functionalised allenes are described, where the Z-configuration of the enaminic double bond in the rearranging system proves critical.     

New access to fluorinated ketoglycolic acid derivatives from trifluoropyruvamides

Hydrates of trifluoropyruvamides are readily obtained from isocyanides and trifluoroacetic anhydride. Under catalysis with secondary amines, these hydrates were successfully reacted with various methylketones to form fluorinated aldol type compounds. The effect of the amine is discussed as well as Henry type additions with nitromethane.     

Enamines: efficient nucleophiles for the palladium-catalyzed asymmetric allylic alkylation

Enamines were tested to be efficient nucleophiles for palladium-catalyzed asymmetric allylic alkylation, avoiding the use of unstablilized ketone enolates formed by strong bases. The influence of the chiral metallocene-based ligands upon this reaction was studied in detail. It was shown that planar chirality played an important role in enantioselectivities. Meanwhile, different kinds of enamines and allylic acetate to the reactions were also investigated. High catalytic activity and excellent enantioselectivity (up to 99% ee) were obtained with pyrrolidine enamines of both aliphatic and aromatic ketone.