Benzene ring fragmentation

H-NMR and C-NMR spectra of the parent benzimidazole are a simple combination of the imidazole fragment and the benzene ring fragment. In the H-NMR spectrum, the imidazole-H2 shows up as a singlet at 8.20 ppm, whereas the benzene-related H4+H7 and H5+H6 have chemical shifts at 7.60 ppm and 7.20 ppm, respectively, both as multiplets. In the... 

In such tables, typical chemical shifts are assigned to standard structure fragments (e.g., protons in a benzene ring). Substituents in these blocks (e.g., substituents in ortho, meta, or para positions) are assumed to make independent additive contributions to the chemical shift. These additive contributions are listed in a second series of tables. Once the tables are defined, the method is easy to implement, does not require databases, and is extremely fast. Predictions for a molecule with 50 atoms can be made in less than a second. On the other hand, it requires that the parent structure and the substituents are tabulated, and it considers no interaction... 

As in the case of thiazole and the alkylthiazoles, cleavage of the thiazole ring takes place at the 1,2 and 3,4 bonds, confirmed by a metastable peak. The other important peaks result from fragmentation of thiirenium ion, in the case of 4- and 5-phenylthiazole and of the phenyl ring. These latter are generally present in the spectra of all comptmnds with benzene-ring substituents, they occur at m/e 77, 76, 75, 51, 50, 39 (124). The ion m/e 45 (HCS" ) is always present. 

The mass spectra of arylthiazoles with funcmonal substituents cm the benzene ring have also been studied (125, 126). They possess the fragmentation pattern of the aromatic derivative corresponding to the substituent together with that of the thiazole ring described previously (126). 

The benzene rings A and B derived from the H NMR spectrum can be completed using Table 41.1. The way in which the enol ether is bonded is indicated by the correlation signal of the proton at Sh = 8.48. The structural fragment C results. Incorporating the C atom resonating at 5c = 123.3, which has not been accommodated in ring A or B and which is two bonds Jch) removed from the enol ether proton. 

Since fragment A was clearly assigned with the help of HH coupling constants, all of the C atoms not included in A, which, according to the CH COLOC plot, are two or three bonds apart from the equivalent protons at Sh = 6.72 (Table 43.1), belong to the benzene ring B. 

By carefully examining the fragmentation pattern of the metabolite and comparison with the mass spectra of the precursor molecule, it is often possible to determine not only the nature of the biotransformation, but also its position in the molecule. In the proceeding example, accurate mass measurement was used to determine that a hydroxyl group had been added to the benzene ring containing the fluorine substituent. 

The palladium-catalyzed carbonylation of aryl halides in the presence of various nucleophiles is a convenient method for synthesizing various aromatic carbonyl compounds (e.g., acids, esters, amides, thioesters, aldehydes, and ketones). Aromatic acids bearing different aromatic fragments and having various substituents on the benzene ring have been prepared from aryl iodides at room temperature under 1 atm... 

For example, the fact that ions of m/z [90 + R]+ and [104 + R]+ arise directly from the molecular ions of sulfones (Scheme 5.20) confirms a transformation with a new C-C bond formation between carbon atoms of the small ring and of the second benzene ring prior to the fragmentation of the M+. In this case the cyclopropyl moiety (maybe iso-merized) retains the charge and unpaired electron and attacks the second aromatic ring by a nucleophilic or radical mechanism. 

A series of four fluorene-phenylene vinylene copolymers 320-323 clearly demonstrates the effect of the exact position of CN groups in the vinylene fragment on the emission of the materials (Scheme 2.48) [408], Substitution of benzene rings in copolymers 320 and 321 by thiophene results in red-shifted PL and EL, where copolymers 322 and 323 exhibit pure red emission with chromaticity values very close to the standard red (CIE x = 0.66, y = 0.34), although no PLQY values were reported. The ITO/PEDOT/322/Ca/Al device showed a very... 

The molecule under subject is a push-pull system which contains a benzene ring with either a donor group D (i.e. an electron-rich fragment) and an acceptor group A (i.e. an electron-poor fragment) which are positioned at the opposite sides of the 7i-unsaturated ring.69... 

Neutral hexakis(methylsulfonyl)benzene (see Scheme 6.23) adopts a chair conformation. On the contrary, the tube conformer appears to be inherent in the corresponding anion-radical. The methylsulfonyl fragments at positions 1 and 4 of the bent benzene ring are nonequivalent. Moreover, one methylsulfonyl moiety is nonequivalent to all of the other five (Fabre et al. 2002). Scheme 6.23 depicts an intuitively constructed picture. Localization of spin-charge density within one methylsulfonyl group causes the attraction of the other from position 4. This makes the tube conformation the most stable in the case of hexakis(methylsulfonyl)benzene anion-radical. 

Besides hydroxylation of exogeneous phenol to the corresponding catechol, some imine and xylene-based Ugand systems have been shown to undergo (partial) hydroxylation of a central benzene-ring to yield a phenolate fragment in an intramolecular hydroxylation reaction [179,183-189]. Somewhat related reactivity was observed in the intramolecular hydroxylation at a benzyhc position of a sidearm on a specific aminopyridine ligand (compare Sect. 4.1). 

A benzofuran ring replaces one of the benzene rings of the biphenyl moiety present in many of the sartans in the rather more complex drug saprisartan (80-10). It is of note, further, that the acidic proton is provided in this case by a trifluorosulfo-namide instead of the more common tetrazole ring. Construction of the imidazole fragment begins by nitrosation of the (3-ketoester (79-1) by means of sodium nitrite in acid to afford the oxime (79-2). Reaction with acetyl chloride leads to the ester (79-3). Reaction of this last intermediate with the iminoether from propionitrile then affords the imidazole (79-4). 

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