1,721,070 research outputs found
Nitrones from Addition of Benzylic and Allylic Grignard Reagents to Alkyl Nitro Compounds: Chemo- Regio- and Stereoselectivity of the Reaction.”
No full textReaction of allyl and benzyl Grignard reagents with functionalized nitroalkanes affords nitrones in good yield.
This process shows considerable chemoselectivity; carbonyl groups and other highly reactive electrophilic functions
are unaffected by the reaction conditions (THF, -70 "C). A mixture of regioisomers 4 and 5 is usually obtained,
and the product distribution depends on the nature of the alkyl framework. An intermediate 3 is postulated,
and the isomeric pair of nitrones arises, very likely through a selective syn elimination. a-Substituted alkyl chaina
give mostly conjugated products 4 while unbranched chains afford predominantly the nonconjugated nitrones
5. The 4 f 5 ratio can be strongly modified by using a proton source of suitable strength; trichloroacetic acid produces
4 exclusively in the reaction of nitroethane with benzyl Grignard, while 2,6-dimethylphenol affords completely
the nonconjugated nitrone 5. The stereochemistry of the double bond is affected by the nature of the reagent
used. Benzyl Grignard gives only 2 nitrones 4 and 5; 2-butenylmagnesium chloride gives nonconjugated 2 nitrones
and a predominance of E isomer in the conjugated nitrone 5
Reaction of Aryl and Alkyl Nitro Compounds with 2-Butenyl Magnesium Chloride: Synthesis of a New Class of Nitrones
No full textAn easy method to synthesize in goodyields a new class of nitrones has been made ready
Mechanistic Studies of the Reaction of Nitro and Nitrosoarenes with Vinyl Grignard Reagents."
No full textThe reaction of ortho-substituted nitrobenzenes with 3 mol vinylmagnesium halides gives mainly 7-substituted indoles together with minor amounts of the aniline from complete reduction of the nitroarene. Under the same experimental conditions, para-substituted nitrobenzenes essentially lead to the corresponding anilines, with indoles being recovered in very low yield. Nitrosoarenes react with 2 mol Grignard reagent to give almost the same product distribution. An accurate analysis of the stoichiometry of the reaction established that in the first stage of the reaction nitroarenes are attacked at the oxygen atoms and are reduced to nitrosoarenes via enolate elimination. The nitroso derivative can undergo a 1,2-addition to give an N-aryl-N-vinylhydroxylamino magnesium salt. Hydrolysis of this intermediate affords hydroxylamine and the carbonyl derivative corresponding to the vinyl Grignard reagent, as proved by the reaction of nitroarenes with 2 mol Grignard reagent. In the presence of an excess of vinyl magnesium halide, a complete reduction to vinylaniline derivatives, which hydrolyse to aniline, occurs. The effect of the bulkiness of the substituent both in the nitroarene and in the Grignard reagent, the orientation of alkylation and the relative reaction rates of indole and aniline formation suggest that indoles arise via a completely different route: i.e. an inverse 1,2-addition to the NO double bond. The N-aryl-O-vinylhydroxylamino magnesium salt intermediate can undergo a [3,3]-sigmatropic rearrangement followed by a rapid ring closure. The third mole of Grignard reagent acts as a base on this bicyclic intermediate, re-aromatizing the six-membered ring. Elimination of water ultimately leads to the indol
Rearrangement of Nitrones: Acid Promoted Conversion of Vinyl Nitrones into N-(γ-Ketoalkyl)-N-Phenyl-Hydroxylamines
No full textTitle compounds can be obtained from α-vinyl-α-methyl-N-phenylnitrones in the presence of anhydrous trifluoroacetic acid via sigmatropic rearrangement of a protonated oxaziridine intermediate
A New Approach to the Synthesis of 2-Substituted Indoles: Reaction of Dimetallated ortho-Trimethylsilyl Methylanilides with Esters
No full textThe reaction of 2-trimethylsilylmethylanilides and esters in basic medium provides a new general method for the synthesis of indoles. The advantages of this method are the mild reaction conditions (-10-0°C), the ready availability of the starting materials and the use of a non-nucleophilic base (lithium 2,2,6,6-tetramethylpiperidide) to promote cyclisation
Direct α’ Alkylation of 2-Nitro Ketone Dianions
No full textDoubly deprotonated 2-nitro ketones (2-nitrocyclo-hexanones and α-nitroalkanones) are efficiently alkylated in the α′-position with alkenyl, alkyl or benzyl halides at -50 °C in tetrahydrofuran. Yields of the process are comparable with those reported for ketohydrazone or ketoimine derivatives and range from 65 to 85%
One-pot Chemoselective Reductive Alkylation of Nitroarenes: a New and General Method of Synthesis of Alkylanilines
No full textA New facile and general synthesis of alkylanilines by one-pot reductive alkylation of nitroarenes is reported. This method is based on the “in situ” reduction by hydrides (LiAlH4 or NaBH4) in the presence of catalytic amounts of PdC, of nitronate adducts arising from the conjugate addition of Grignard reagents to mononitroarenes. LiAlH4 showed to be a more efficient but less selective reducing agent than NaBH4. The reaction can be successfully applied to mono, homo and hetero bicyclic systems and allows to introduce a large variety of alkyl chains without isomerisation phenomen
Reaction of Nitro Compounds toward Grignard Reagents. A general method of synthesis of N-Alkyl or N-Aryl-Propargylhydroxylamines
No full textPropargyl bromide was converted to the resp. allenylmagnesium bromide and added to nitro compds. to give N-aryl-N propargylhydroxylamines and N-alkyl-N-propargylhydroxylamines RN(CH2C≡CH)OH I (R = Ph, 4-ClC6H4, 1-naphthyl, hexyl, cyclohexyl, 4-MeC6H4CH2CH2 etc.) in the presence of LiAlH4 and Pd/C. I are not further reduced to give, e.g., N-propargylanilines
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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