Aromatic compounds, fused metalation

With regard to possible relations between homogeneous hydrogenation systems and heterogeneous HDS catalysts, polynuclear aromatic molecules are easier to reduce by metal complexes than isolated benzene rings, since the partial saturation of fused-ring aromatic compounds does not suffer from as dramatic a loss of resonance stabilization ... 

Various carbene complexes Cr(CO)5 =C(OMe)CR =CHR2 react with HC=C(CH2)4CN by benzannulation and subsequent intramolecular addition processes to form spirocyclic or fused aromatic compounds depending upon how the reaction is quenched. The reactions of various p-aminovinyl carbene complexes M(CO)5 =C(OEt)CH=CR(NHRi) (M = Cr, W) with alkynes R CaCH results in loss of ethanol and the formation of pyiidinylidene derivatives 53. These undergo protonation which results in replacement of the metal by a proton and the formation of the corresponding pyridinium salts. i ... 

As described in this chapter, transition-metal catalysts promote various types of cyclization reactions between C=N, C=0, N—H, O—H, and S—H bonds and alkynes in 5/6-endo/exo-dig manners. These reaction modes provide facile and atom-economical pathways to aromatic compounds such as pyrroles, indoles, isoquinolines, quinolines, furans, thiophenes, oxazoles, pyrones, and isoquinolones and their aza analogs and fused-ring congeners. Particularly notable is their utility in cascade reactions, which are step-economical approaches to target molecules, which increase rapidly in structural complexity. Therefore, these reactions can help provide solutions to meet the increasing demands of environmentally benign synthesis in modern organic chemistry. 

A number of synthetic procedures are available (Ai2). (2) For precisely defined stoichiometries, the isobaric, two-bulb method of Herold is preferred H5, H6, H2). (2) To generate compounds suitable for organic synthesis work, graphite and alkali metal may be directly combined, and heated under inert gas (Pl, lA). (5) Electrolysis of fused melts has been reported to be effective iN2). 4) Although alkali metal -amine solutions will react with graphite, solvent molecules co-inter-calate with the alkali metal. Utilization of alkali metal-aromatic radical anion solutions suffers the same problem. 

Chls and all tetrapyrroles are heteroaromatic compounds and the aromatic character of the underlying tetrapyrrole moiety and the reactivity of the functional groups in the side chains govern their chemistry. Three different classes of tetrapyrroles, differentiated by their oxidation level, occur in nature porphyrins (11, e.g. hemes), chlorins (12, e.g. chls) and bacteriochlorins (13, e.g. bchls). As a cyclic tetrapyrrole with a fused five-membered ring, the overall reactivity of chi is that of a standard phytochlorin 7. Such compounds are capable of coordinating almost any known metal with the core nitrogen atoms. Together with the conformational flexibility of the macrocycle and the variability of its side chains, this accounts for their unique role in photosynthesis and applications ... 

Of commercial interest are benzo- and other fused aromatic 1,2,3-diazaborine derivatives which have exhibited good antibacterial activity against a variety of microorganisms (155—157). The reaction of pyrazole or C-substituted pyrazoles with boranes yields the pyrazabole system, a class of exceptionally stable compounds. More than 70 species in this system have been reported and the subject comprehensively reviewed (158). These compounds have been used as ligands in transition-metal complexes (159). 

In each of the five sections of Chapter 3, the chemistry is reviewed in the following order (1) Reactivity of aromatic rings (thermal reactions not involving reagents, substitutions at carbon, additions to nitrogen, metallations) (2) Reactions of nonaromatic compounds (this enormous area, which overlaps extensively with nonheterocyclic chemistry, is reviewed with emphasis on the heterocyclic aspects) (3) Reactions of substituents (with emphasis on situations in which substituents behave somewhat differently when attached to a heterocycle note that for benzene-fused heterocycles, the benzene ring is treated as a substituent). 

Interpretation of interatomic vectors. Use of known atomic positions for an initial trial structure (a preliminary postulated model of the atomic structure) can be made, by application of Equations 6.21,4 and 6.21.5 (Chapter 6), to give calculated phase angles. Methods for obtaining such a trial structure include Patterson and heavy-atom methods. Such methods are particularly useful for determining the crystal structures of compounds that contain heavy atoms (e.g., metal complexes) or that have considerable symmetry (e.g., large aromatic molecules in which the molecular formula includes a series of fused hexagons). The Patterson map also contains information on the orientation of molecules, and this may also aid in the derivation of a trial structure. 

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