Aromatic and Heteroaromatic Series

The first report found in this context is a contribution by Hurd and Greengard [1]. In 1930, they attempted the pyrolysis of allyl aryl sulfides (at 207-240 °C), in analogy to reported examples with ethers. They claimed the formation of 2-allyl thio- 

The numerous papers by Kwart report on various aspects, including nucleophilic catalysis and mechanism [6, 8-10], evidence for the intermediate [11], and scope [7, 12-14]. 

At this stage, no specificity of the sulfur rearrangement had yet been disclosed, except the easy cyclization of allyl thiophenols, the postulated intermediates. 

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The reactions of 4-alkoxybut-3-en-2-ones with primary aromatic amines and diamines of the aromatic and heteroaromatic series follow analogous schemes. 

The oxidation potentials of TTF derivatives can be varied in a controlled way over a wide range by the introduction of appropriate substituents. The influence of substituents on oxidation potentials values of TTF has been discussed in many articles. For instance, the oxidation potentials of about 40 different substituted TTF derivatives have been measured under the same conditions [32]. In general, all substituents, with the exception of alkyl groups, increase oxidation potentials and the mesomeric effect of substituents have been shown to have an insignificant influence on the donor ability of TTF, in contrast to the trends observed for aromatic and heteroaromatic series. Some representative examples of oxidation potentials for substituted TTF derivatives are collected in Table 7. In the case of aryl-substituted TTFs, a usual Eox-cr+ correlation has been reported [63],... 

The Michaelis-Arbuzov reaction is currently used in aliphatic, aromatic, and heteroaromatic series. In aliphatic series it is used with success for the synthesis of dialkyl 3-(ethoxycarbonyl)propyl- and... 

Quite recently, the same research group compared the electrophilicity of 6-nitro-tetrazolo[l,5- ]pyridine and 6,8-dini-trotetrazolo[l,5- ]pyridine 11 with a series of electron-deficient aromatic and heteroaromatic compounds <2005JOC6242>. As reference nucleophiles, fV-methylpyrrole, indole, fV-methylindole, and some morpholino enamines were used. The reactivity of the electrophiles studied followed the linear-free energy relationship defined by Mayr et al. <2003ACR66>. 

In a series of important papers, MacMillan described the alkylation of electron rich aromatic and heteroaromatic nucleophiles with a,P-unsaturated aldehydes, using catalysts based upon the imidazoUdinone scaffold, further establishing the concept and utility of iminium ion activation. In line with the cycloaddition processes described above, the sense of asymmetric induction of these reactions can be rationalised through selective (F)-iminium ion formation between the catalyst and the a,P-unsaturated aldehyde substrate, with the benzyl arm of the catalyst blocking one diastereoface of the reactive Jt-system towards nucleophilic attack (Fig. 3). 

To characterize the relative gas-chromatographic retentions of condensed aromatics and heteroaromatics, inclu g thienothiophenes, benzo[b]thiophene, dibenzothiophene, naphthobenzothiophenes, and anthrabenzothiophenes, a system of indices. In, was proposed, In this system a series of similar linearly condensed hydrocarbons (such as benzene, naphthalene, anthracene, tetracene, pentacene,...) was used as a reference scale. The logarithm of the corrected retention volume (adjusted to 0°), log Ft, depends linearly upon the number of condensed benzene rings (z) in the molecule, both in the polar and nonpolar phases. In is expressed by Eq. (58) ... 

A series of complexes with the ir-bonded aromatic and heteroaromatic frameworks 6 and 9 was prepared by ligand exchange. An example is the preparation of the r 6-tricarbonylchromium complex of indole (30) (68JOM359). Ligand exchange may lead to complexes 9. Thus, interaction of 2,4,6-triphenylpyridine with L3Cr(CO)3 yields the trinuclear jr-complex... 

Microwave-assisted organic synthesis may also be used for carrying out the multicomponent reactions of ketones and 1,2-diamines [20, 21, 92, 100]. For example, the three-component reaction of o-PDA 1 with acetoacetic acid ethyl ester 83 and a series of aromatic and heteroaromatic aldehydes 84 proceeds under microwave irradiation with very high yields of diazepines 85 (up to 95%) [100]. Reaction of 2 equiv of cyclohexanone 86 with o-PDA 1 was also realized in a microwave field on a basic alumina surface in 4 min [92] (Scheme 4.27). 

There is current interest in the quantitative comparison of electrophilicities and nucleophilicities, particularly in carbon-carbon bond-forming reactions. The rates of a-adduct formation in acetonitrile of 10 electron-deficient aromatics and heteroaromatics with a series of reference carbon nucleophiles have been used to compare their electrophilicities, E. Values of E ranging from —13.2 for 1,3,5-trinitrobenzene, the least reactive studied, to -4.7 for 4,6-dinitrotetrazolo 1,5-a Ipyridinc, the most reactive, were determined.52 A reasonable correlation was found between electrophilicities and pA a values for water addition (eq. 1). These pA a values have also been found to... 

The A -methylthioimidazoyl moiety, among the various aromatic and heteroaromatic moieties used in the sulfur series, proved to be the one with the highest propensity to direct the alkylation of the corresponding organometallics (even those bearing an alkyl group in the a-position) to the a-carbon atom (Scheme 19, entry e). - ... 

A number of reviews and texts whose topics include photochemical reactions of aromatic compounds have been published during the year and will be mentioned here. A new volume of the Organic Photochemistry series has appeared and contains a chapter detailing the photochemical reactions of aromatic and heteroaromatic cations (e.g. cyclopropenlum ions, tropylium ions, pyrilium ions and... 

N-oxides can also be reduced. For example, the microbial deoxygenation of a series of aromatic and heteroaromatic N-oxide compounds, including quinoline N-oxides, isoquinoline N-oxides, 2-aryl-2H-benzotriazole 1-oxides, benzo[c]cinnoline N-oxide and azoxybenzene, has performed with bakers yeast-NaOH to afford quinolines and pyridines (291l... 

The concept of the closely related diamagnetic susceptibility exaltation was earlier discussed by Craig88 and Dauben et al,83 These groups suggested that the difference, the exaltation, in the observed molar magnetic susceptibility xm and that calculated from atomic and bond contribution Xm, provides a useful criterion for assessing aromaticity. Dauben et al.84 have recently provided an authorative survey of the subject and a comprehensive compilation of exaltation values for benzenoid, nonbenzenoid, and pseudo-aromatics, aromatic cations, keto aromatics and heteroaromatics. Examples of the last series are provided here in Table VI. Some uncertainties arise in the calculations of xm- for five-membered heteroaromatics. 

As with the other aromatic and heteroaromatic systems, functional group transformations can also be used to produce fluorinated 1,2,3-triazoles. A series of 1-aryl-1,2,3-triazole-4-carbaldehydes were converted to the corresponding difluoromethyl-substituted triazoles by the action of DAST (Fig. 3.96). The difluoromethylene derivatives as well as the precursor aldehydes were studied with respect to antitubercular activity. 

A series of aromatic and heteroaromatic fused pentathiepins has been prepared by reactions of S2CI2 with vicinal dithiols or their salts. The dithiols can be generated in situ (Scheme 31) <80JOC5122, 84JOC122l,85JA387l>. 

The nitro group in the ortho- or para-positions is one of the strongest electron-withdrawing substituent, activating an aromatic ring for a nucleophilic attack [5]. No wonder that a great deal of both oxidative and eliminative Sn reactions have been observed in the series of nitroaromatic compounds [11, 15-18, 25, 27, 28, 30-32, 36, 37, 97-100]. The VNS protocol has been particularly effective for the Sn reactions of a large number of nitro-substituted aromatic and heteroaromatic compounds [11-18, 25, 27, 28, 30-32, 36, 37, 45, 98-100], as illustrated by nucleophilic alkylation of 3-nitropyridine with vicarious methyl chloroacetate (Scheme 22) [100]. 

In 2009, Feng et al. reported the asymmetric hydrophosphonylation of a-keto esters catalysed for the first time by cinchona-derived thiourea organocatalysts. Thus, a series of aromatic and heteroaromatic a-keto esters reacted with dimethyl phosphite to afford the corresponding a-hydroxy phosphonates in high yields and enantioselectivities of up to 91% ee (Scheme 2.53). 

The major changes in the format and contents of this volume have been necessitated by economic considerations. Sulphur-containing heteroaromatic compounds which previously were included in these Reports and in the Reports on Aromatic and Heteroaromatic Chemistry are now covered in detail only in the new Reports, Heterocyclic Chemistry . The literature on these compounds has been reviewed up to March 1978 in Volume 5 of this series and thereafter in Volume 1 of the new series. Highlights of the literature of these compounds are given as Chapter 6 of this volume. The organization of the remainder of this volume is essentially the same as that of Volume 5 except that the section on the ylides and carbanionic compounds of selenium and tellurium has been considerably extended and the coverage of Chapter 3, Part III has been extended to include dithiocarbamates, xanthates, and trithiocarbonates. 

In the discussion that follows, various series of molecules are considered. First polyacetylene and polyene systems are treated, because they are the prototype models on which the interpretive scheme is based. Then the concepts are generalized to the more complex case of aromatic and heteroaromatic systems. 

A selection of biocatalytic deoxygenation reactions is shovm in Figure 1.8. The reducing power of baker s yeast in an ethanol-water mixture and sodium hydroxide at 60° C has been found effective for the rapid and selective reduction of a series of N-oxides like aromatic and heteroaromatic N-oxide compounds [118]. DMSO reductase from Rhodobacter sphaeroides f sp. denitrificans catalyzed the (S)-enantioselective reduction of various sulfoxides and enabled the resolution of racemic sulfoxides for the synthesis of (R)-sulfoxides with >97% ee [119,120]. Purified dimethyl sulfoxide reductase from Rhodobacter capsulatus resolved a racemic mixture of methyl p-tolyl sulfoxide by catalyzing the reduction of (S)-methyl p-tolyl sulfoxide and gave enantio-merically pure (J )-methyl p-tolyl sulfoxide in 88% yield, while whole cells of E. coli,... 

Ricci et al studied a series of thiourea catalysts for the Friedel-Crafts alkylation of aromatic and heteroaromatic compounds with nitroalkenes. They have succeeded in developing the Friedel-Crafts alkylation of indoles with nitroalkenes for the first time by means of a novel thiourea catalyst (13) (Scheme 2.48) [101]. The authors proposed the bifunctional nature of the thiourea catalyst, where thiourea activates the nitro group and at the same time the free alcoholic function interacts with the indole proton through a weak hydrogen bond, directing the attack of the incoming nucleophile on the Si face of the nitroalkene (Figure 2.18). 

Smith, G., U.D. Weimuth, P.C. Healy, and J.M. White. 2007. 3,5-Dinitrosalicylic acid in molecular assembly. El. Proton-transfer compounds of 3,5-dinitrosalicylic acid with polycychc aromatic and heteroaromatic amines, and overall series structural systemat-ics.Austr. J. Chem. 60 264—277. 

In the last ten years arylation has been tbe most studied homolytic aromatic substitution, also in the heteroaromatic series. Numerous data concerning a large variety of heterocycles have permitted the definition of many details for the individual substrates, without adding, however, anything particularly new as regards the general characteristics of the reaction, already outlined in the previous review of Norman and Radda. These characteristics are substantially the same as those observed in the homocyclic aromatic series, for which comprehensive reviews are available. There is therefore a sharp difference in behavior between arylation and other homolytic substitutions described in the previous sections. These latter have quite different characteristics, and sometimes they are not known, in the homocyclic series. 

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