Para-substituted Products Research Articles

Liquid-phase regioselective benzylation of bromobenzene and other aromatics over microporous zeolites

Various solid acids were investigated for the liquid-phase benzylation of aromatics to industrially important substituted diphenylmethanes. Large-pore zeolites, particularly H-beta (BEA), were active and regioselective for the para-substituted product in the benzylation of arenes. With deactivated aromatics such as bromobenzene, the selectivity towards the para-substituted product decreased with reaction time. The decline in selectivity is attributed to the partial blockage of the zeolite channels by high molecular weight deposits, leaving only non-selective catalytic sites at the outside of the zeolite crystallites. These deposits result from self-condensation of benzyl chloride, and their formation can be greatly suppressed when the concentration of benzyl chloride in the reaction mixture is kept low. By adding benzyl chloride continuously to the reaction mixture, a selectivity towards the para-substituted product of more than 80% was achieved. Mesoporous materials such as montmorillonite K10 and H-Al-MCM-41 were active but not regioselective for the reaction. The pores of these materials are too large to impose restraints on the geometry of the products or the transition state. H-ZSM-5 with smaller pores was less active and showed no regioselectivity, because the active sites inside the channel system of this medium-pore zeolite are less accessible to the reactants, and most of the reaction is catalyzed at the outer surface.

Synthetic utilization of polynitroaromatic compounds. 1. S-derivatization of 1-substituted 2,4,6-trinitrobenzenes with thiols.

Reactions of 1-R-2,4,6-trinitrobenenes (R = alkyl, protected aldehyde, aminocarbonyl, cyano groups, or isoxazole ring) with thiol salts were investigated. In most cases, these reactions gave a mixture of minor para and major ortho substitution products. Reactions of N,N-disubstituted 2,4,6-trinitrobenzamides with S-,O-, and N-nucleophiles afforded products of substitution of the p-nitro group exclusively. 1-Cyano-2,4,6-trinitrobenzene was found to be the most reactive and the least selective: all three nitro groups can be substituted using an excess of thiol salts. 2-R-4, 6-dinitrobenzamides showed no regioselectivity under similar conditions to yield 1:1 mixtures of para and ortho isomers.

Substitution of Bridgehead Halogens by a Free-Radical Electron-Transfer Mechanism

The reactions of 1-bromo-7-nitro- and 1-bromo-6-nitro-1,4-methanonaphthalene (2) and (3), and 9-bromo-2-nitro, 10-bromo-2-nitro-, 9,10-dibromo-2-nitro- and 9,10-diiodo-2-nitro-9,10-ethano-9,10-dihydroanthracene (4)-(7). respectively, with the sodium salt (1) of p-toluenethiol gave substitution products that were shown to be formed by an SRN1 or a related radical chain mechanism. In the relatively slow substitution reactions of the salt (1) with compounds (2)-(5). That contain bromine at bridgehead positions that are either meta- or para-benzylic to an aromatic nitro group, the rates of substitution in the isomers where the leaving group was meta- benzylic to the aromatic nitro group were slightly greater than those for the corresponding para-benzylic isomer. In compounds (6)and (7) the halogens are at bridgehead positions that are either meta- or para-benzylic relative to an aromatic nitro group within the same molecule. In the case of the reaction of the dibromide (6) with the thiolate (1), substitution was slow and occurred more rapidly at the benzylic -bridgehead position meta to the nitro group than at the corresponding para-benzylic position. In contast , the reaction of the diiodide (7) with the thiolate (1) gave substitution products which formed more rapidly than in the corresponding reaction of the dibromide (6) and the regioselectivity was reversed, with substitution occurring more readily at the bridgehead position para-benzylic to the nitro group than at the corresponding meta- benzylic position. The ratio of meta to para substitution products, determined for the reactions of compounds (2)-(6) with the salt (1), were in the range 1.15-2.5:1, while the reaction of (7) with the same nucleophile afforded a meta-to-para ratio of 1:2:3. These ratios contrast not only with each other, but also with the differences in reactivities determined for other nitrobenzylic systems, which are known to undergo SRN1 substitution reactions with the same nucleophile. The differences in first, the regioselectivity of substitution between the bridgehead systems, and secondly, the differences in the observed rates of regioselective substitution are compared with other simple nitrobenzylic halides. These differences are rationalized in terms of the effect of fixing the C-X bond at a bridgehead position to be orthogonal with the plane of the nitroaromatic group; this results in a reduction of the rate constants of intramolecular electron transfer, with significant consequences on the detailed overall mechanism for these reactions.

Regioselective Alkylation of (η6-Triisopropylsilylbenzene)chromium(0) Complex with Organolithium Compounds

Abstract Reactions of tricarbonyl(η6-triisopropylsilyl(TIPS)benzene)chromium(0) complex 1b with various alkyllithiums proceed regioselectively, giving para-substituted TIPSbenzenes in high yield after oxidation with iodine. Vinyllithiums also react with the TIPSbenzenechromium complex to give para-TIPSstyrenes in good yield. In the reaction with phenyllithium, although the para-substituted product is obtained as a main product, deprotonation of the aryl group of the chromium complex also occurs competitively to afford bisTIPSbiphenyls as a by-product. Alkynyllithium, however, attacks the TIPS group to yield alkynylsilane and TIPSbiphenyl.

Selective synthesis using cyclodextrins as catalysts: Part 6. Cyclodextrin modification for para-selective hydroxymethylation and hydroxyethylation of phenol

4-(Hydroxymethyl)phenol is selectively synthesized from phenol and formaldehyde by use of β-cyclodextrins (β-CyDs) carrying quaternary ammonium, glucose and maltose residues as catalysts. The chemical modification of β-CyD largely promotes the catalytic activity, especially the rate-accelerating effect. Yield of the desired para-substituted product (16.8 mol%) for the β-CyD having the quaternary ammonium substituent (0.3 M) is 7.3 times as large as that (2.3 mol%) for unmodified β-CyD (the selectivities are 80 and 79%, respectively). Selective synthesis of 4-(hydroxyethyl)phenol from phenol and acetaldehyde is successfully achieved also by use of the β-CyD derivatives having sugar branches, which is in marked contrast with the marginal activity of unmodified β-CyD.

Friedel–Crafts Alkylation of Phenylsilanes with α-Chloro Sulfides

Abstract Treatment of trimethylphenylsilane with methyl chloro(methylthio)acetate in the presence of atannic chloride gave a mixture of the alkylated silylbenzenes, in which the meta- and para-substituted products predominated. The partial rate factors of this reaction suggest that the trimethylsilyl group slightly activates the meta- and para-positions of the silylbenzene.

Para-selective fries rearrangement of phenyl acetate in the presence of zeolite molecular sieves

The Fries rearrangement of phenyl acetate is catalysed by acidic zeolites such as H-Nu-2 and H-ZSM-5, with selectivities of 2-6:1 in favour of para-substituted products.

ChemInform Abstract: Nucleophilic Addition of Silyl Enol Ethers to Aromatic Nitro Compounds: Scope and Mechanism of Reaction.

AbstractDie Durchführung der Titelreaktion an Nitrobenzol (IV) führt bevorzugt zu den ortho‐substituierten Produkten (II); mit den sterisch anspruchsvolleren Silylenolethern (Ia) und (Id) hingegen wird durch Substitution in para‐Stellung überwiegend (III) gebildet [vgl.

Nucleophilic addition of silyl enol ethers to aromatic nitro compounds: scope and mechanism of reaction

In sharp contrast to alkali-metal enolates, silyl enol ethers and ketene silyl acetals add to aromatic nitro compounds in the presence of a fluoride ion source to give the intermediate dihydroaromatic nitronates, which can be observed by NMR. In situ oxidation of the intermediate with bormine or DDQ yields a-nitroaryl carbonyl compounds in moderate-to-high yields. The reaction is applicable to alkyl-, alkoxy-, and halogen-substituted nitrobenzenes as well as to heterocyclic and condensed nitroaromatic compounds. While substitution ortho to the nitro group predominates with sterically undemanding silyl reagents, para-substitution products are exclusively obtained with bulky reagents. However, by blocking the para position with an appropriate group such as chlorine, the addition can be directed to the ortho position. Halogen atoms of halogenated nitroaromatics and p-nitrocumenyl chloride are not displaced in the reaction, suggesting the absence of radical ion intermediates. Dihydroaromatic nitro derivatives can be isolated in some cases, such as anthracene and naphthalene systems which are less prone to rearomatize. The use of silicon reagents in organic synthesis has been ex- panding rapidly in the past few year^,^-^ and versatile methods for carbon-carbon bond formations have been developed based on silyl enol ethers activated by fluoride ion sources. Various groups have reported alkylation^,^^^^ arylati~ns,~~ aldol conden- sation~,~ a~ylations,~ and Michael additions.&* Our own interest in this area originated with a notion that the nucleophilic reactivity of such reagent combinations might be distinctively different from that of the classical metal enolates and, in particular, that the enhanced nucleophilicity might be attained without significantly increasing basicity to lead to novel carbon-carbon bond-forming reactions. For example, we have reported (Scheme I) that tri- alkylketene silyl acetals can be added to a,@-unsaturated ketones in the presence of tris(dimethylamino)sulfonium difluorotri- methylsili~onate ~ (TASF) to give products equivalent to 1,4-ad- ditions of ester en~lates.~ This same reagent combination also initiates group-transfer polymerization of methacrylate mono- mers via sequential Michael additions* (Scheme I). In this paper, we wish to record our results on the fluoride-assisted addition of silyl enol ethers to aromatic nitro compounds. Oxidation of the resulting intermediate nitronate gives highly versatile a-nitroaryl

Catalysis by 12-Heteropolymolybdic Acid. II. Friedel-Crafts-type Reaction of Aromatic Compounds

Abstract 12-Heteropolymolybdic acid, a heteropolyacid of Keggin-structure, behaves as an effective catalyst of the Friedel-Crafts-type reactions for aromatic compounds, such as alkylation by benzyl chloride and t-butyl chloride, acylation by acetyl chloride, and sulfonylation by tosyl chloride. The catalyst mainly gives para-substituted products for phenol and anisole. The reactions proceed via carbocation-generation mechanism brought about by the strong Brønsted acid.

ChemInform Abstract: ELECTROCHEMICAL REDUCTION OF ARENESULFONYL CYANIDES

AbstractMechanismus und Hammett‐Beziehung der elektrochemischen Reduktion von Benzolsulfonylcyanid und dessen p‐F‐, p‐Cl‐.

Einige neue Anwendungen nematischer Phasen in der Gaschromatographie

Abstract Nematic phases are in several special cases very useful for separation of benzene isomers and related compounds by gas-liquid-partition chromatography. In this paper methods for synthesis, melting and clearing points, and some typical cases of analytical application in the practical analysis of organic substances (separation and quantitative determination of ortho-, meta- and para-substituted products are given). The following substances have been used in their nematic state:

The transformations of aryloxysulfonium cations—II : Origin of para-substituted products and benzoxathians

2,6-Dimethylphenol is converted to its p-thiomethoxymethyl derivative on reaction with N,N′-dicyclohexylcarbodiimide and dimethyl sulfoxide in a moderately acidic medium. Similar treatment of ortho-unsubstituted phenols affords small amounts of products containing the 5,6-benzo-1,3-oxathian ring system in addition to the normal products described in the preceding paper. 1 Studies designed to elucidate the mechanisms of these transformations are described along with a second oxathian ring synthesis.