Additions of organomanganese reagents to conjugated nitroolefins

Diboron-promoted iron-catalyzed denitrative vinylation of β-nitrostyrenes with cycloketoximes

We have developed a new iron-catalyzed ring opening alkenylation of cycloketoximes with β-nitrostyrenes for the facile synthesis of flexible distal cyanoalkyl alkenes. Both non-active strained cycloketoximes and less-strained cycloketoximes could be coupled with β-nitrostyrenes with excellent regio- and stereoselectivity under base-free redox-neutral conditions. The diboron reagent played an irreplaceable role in this transformation. A reasonable radical-mediated process could be involved in this catalytic cycle.

Denitrative Cross-Couplings of Nitrostyrenes

Interestingly, β-nitrostyrenes, typically bench stable compounds, are highly promising cross-coupling partners, due to their excellent availability and well understood reactivity. In this review, we report on the discovery and advancements, in the field of stereoselective, denitrative cross-couplings of β-nitrostyrenes with miscellaneous organic reagents. The rapidly expanding field offers alternative access to a broad range of functionalized alkenes, including β-alkylated styrenes, chalcones, stilbenes, cinnamic acids, and conjugated sulfones and phosphonates. The most important mechanistic pathways are briefly discussed, to familiarize readers with the elementary reactions occurring during the coupling.

Cathodic hydrodimerization of nitroolefins

Nitroalkenes are easily accessible in high variety by condensation of aldehydes with aliphatic nitroalkanes. They belong to the group of activated alkenes that can be hydrodimerized by cathodic reduction. There are many olefins with different electron withdrawing groups used for cathodic hydrodimerization, but not much is known about the behaviour of the nitro group. Synthetic applications of this group could profit from the easy access to nitroolefins in large variety, the C–C bond formation with the introduction of two nitro groups in a 1,4-distance and the conversions of the nitro group by reduction to oximes and amines, the conversion into aldehydes and ketones via the Nef reaction and base catalyzed condensations at the acidic CH bond. Eight 1-aryl-2-nitro-1-propenes have been electrolyzed in an undivided electrolysis cell to afford 2,5-dinitro-3,4-diaryl hexanes in high yield. The 4-methoxy-, 4-trifluoromethyl-, 2-chloro- and 2,6-difluorophenyl group and furthermore the 2-furyl and 2-pyrrolyl group have been applied. The reaction is chemoselective as only the double bond but not the nitro group undergoes reaction, is regioselective as a ß,ß-coupling with regard to the nitro group and forms preferentially two out of six possible diastereomers as major products.

Copper(I) Thiolate Catalysts in Asymmetric Conjugate Addition Reactions

[structure: see text] Full conversion and enantioselectivities up to 83% have been obtained in the conjugate addition reactions of diethyl zinc to Michael acceptors catalyzed by well-defined (chiral) copper(I) aminoarenethiolates. Interesting differences between organozinc or Grignard reagents have been found: for cyclic enones R(2)Zn reagents afford better results, whereas earlier work showed that RMgX reagents react more selectively with acyclic enones.

Triethylaluminum- or triethylborane-induced free radical reaction of alkyl iodides and alpha,beta-unsaturated compounds

A series of alpha,beta-unsaturated compounds, 1a-c, 9, 13, and 17, were used as reactants in free radical conjugate addition reactions with different radicals generated from alkyl iodides such as 3, 4, or 5 in the presence of triethylborane-oxygen in air or via the use of triethylaluminum-benzoyl peroxide as a free radical initiator. When the reactions were carried out using triethylborane-air, the products, in most cases, were clean and were easily purified. However, higher yields of the 1,4-adducts and less side reactions occurred when less reactive substrates were used as Michael acceptors in reactions with triethylaluminum-benzoyl peroxide and alkyl iodide under similar conditions. A mechanism for this is proposed in Scheme 1.

Novel synthesis of alkenes via triethylborane-induced free-radical reactions of alkyl iodides and beta-nitrostyrenes

Reactions of (E)-beta-nitrostyrenes 1 and triethylborane 2 or tricyclohexylborane 4 in THF solution at room temperature in the presence of oxygen in the air as radical initiator generate high yields of trans-alkenes (E)-3 or (E)-5. Medium to high yields of different (E)-alkenes (E)-5, 7, 10, 12, and 14 also can be prepared when 1 reacts with different radicals, prepared from secondary alkyl iodides 6 and 8 or tertiary alkyl iodides 9, 11, and 13, in the presence of 2 and air as radical initiator. The generation of the only product (E)-alkenes can be explained by the generation of the benzylic radical A and/or B as the intermediate only and the mechanism is similar to Scheme 1. Both (E)- and (Z)-16a-c are generated when (E)- and (Z)-15a-c are used to react with adamantyl radical under similar conditions. Only (Z)-16d was observed when either (E)- or (Z)-15d was used to react with adamantyl radical. The generation of the (E)- and/or (Z)-alkenes can be explained by the free rotation of the A and/or B to generate A' and/or B' and vice versa, and the mechanism is proposed to be a free-radical reaction via NO2/alkyl substitution and is shown as Scheme 2.

Free-Radical Reactions of Trialkylboranes with beta-Nitrostyrenes To Generate Alkenes

beta-Nitrostyrenes 1 react with trialkylboranes under a nitrogen atmosphere to generate high yields of alkenes 2. The mechanism is proposed to be a free-radical reaction via NO(2)/alkyl substitution since the reaction is stimulated by the presence of a trace of oxygen in the nitrogen or tert-butyl peroxide or by photolysis and is retarded or inhibited by the addition of galvinoxyl to the solution.

A Highly Stereoselective One-Pot Tandem Consecutive 1,4-Addition-Intramolecular 1,3-Dipolar Cycloaddition Strategy for the Construction of Functionalized Five- and Six-Membered Carbocycles(,)(1)

Nitronates 3, possessing a suitably located olefinic moiety and arising from conjugate addition of Grignard reagent 2 to nitro olefins 1, are trapped as silyl nitronates 8 which undergo a facile intramolecular 1,3-dipolar cycloaddition. The resulting N-(silyloxy)isoxazolidines 9 are readily transformed into isoxazolines 6 and 7 upon treatment with acid, thus completing the entire sequence in one pot. While cycloaddition to the five-membered carbocycles, in general, proceeds smoothly at rt in a stereospecific manner, that to the six-membered ring is sluggish and less selective. The advantages of this strategy over the intramolecular nitrile oxide cycloaddition, namely greater stereoselectivity and adaptability to one-pot conditions, have been demonstrated. Extensive NMR investigations unravelled the stereochemistry unambiguously and underscored the inadequacy of coupling constants alone in determining the stereochemistry in cyclopentane systems. The explanation advanced to account for the selectivities, in terms of subtle differences among the putative transition state geometries, is in qualitative agreement with widely accepted assumptions pertaining to 1,3-dipolar cycloadditions.