Aromatic examples

The preparations of over two hundred tetrahydro- and octahydro-pyrido[4,3-d]pyrimidines from piperidines or from purely aliphatic starting materials are described in the patent literature. Fully aromatic examples of the system have been prepared from pyridines and pyrimidines. 

Known 1,2,3-dithiazoles are represented by the structures in row (a) of Figure 1. Strictly, these are 1,2,3-dithiazolines, the first one being 1,2,3-dithiazoline-5-one. 1,2,5-Oxathiazoles are represented by l,2,5-oxathiazolidine-4-ones (b) and 3-methylene 1,2,5-oxathiazolines (c). 1,2,3-Oxathiazoles have been obtained only in the form of their S-oxides (d), which should rather be termed -azolidines and -azolidinones. Experimentally prepared S-oxides of 1,2,3-dithiazoles and 1,2,5-oxathiazoles are depicted in row (e) and should be termed dithiazolines and oxathiazolines. 1,2,3-Dithiazolyl radicals (f) are known, as well as 1,2,3-dithiazolium cations (g) which are the only formally aromatic examples of the ring system and which may be represented by a number of resonance structures (h). 

Multitubular reactors are mainly used in gas-phase partial oxidation processes, such as the air oxidation of light olefins, paraffins, and aromatics. Examples of chemistries where these reactors are used include the partial oxidation of methanol to formaldehyde, ethylene to ethylene oxide, ethylene and acetic acid to vinyl acetate, propylene to acrolein and acrylic acid, butane to maleic anhydride, isobutylene to methacrolein and methacrylic acid, and o-xylene to phthalic anhydride. An overview of the multitubular reactor process for the partial oxidation of n-butane to maleic anhydride is given here. 

The proline-catalysed aldol reaction of tetrahydro-4/f-thiopyran-4-one with aldehydes, which is accelerated by water <04SL1891>, gives the anti adducts with high diastereo- and enantioselectivity DMSO is the solvent of choice for aliphatic aldehydes and moist DMF for aromatic examples (Scheme 23). Desulfurisation of these thiopyrans with Raney-Ni gives products equivalent to aldol products derived from pentan-3-one <04TL8347>. 

Most of the work on such compounds has been directed towards their synthesis, with some emphasis on their conformations and reactions. Unlike the smaller rings, aromatic examples are absent. 

This review is divided into two main sections substitution reactions and addition reactions. Sometimes the classification of a particular reaction is somewhat arbitrary. Dealkylation reactions, some of which can superficially appear to be neither substitution nor addition reactions, are basically substitution reactions and a section is devoted to these reactions, including both aliphatic and aromatic examples. 

Any neutral monocyclic unsaturated hydrocarbon with an odd number of carbons in the ring must of necessity have at least one CH2 group in the ring and therefore cannot be aromatic. Examples of such hydrocarbons are cyclopropene, cyclopentadiene, and cycloheptatriene. 

The melting temperatures and thermal stabilities of several polybenzimidazoles are shown in TABLE I. For all aromatic polybenzimidazoies, melting points are over 600 C. Generally, the presence of other structural elements will reduce melting points and thermal stabilities. For example, TABLE I includes polybenzimidazoles condensed from either a tetraamine ether or a diacid ether. Strained polymer structures also reduce polybenzimidazole stability as evidenced by the low melting point of a polymer formed with a biphenyl-2,2 -dicarboxylate. Not surprisingly, the aliphatic, adipic acid forms a much less stable polybenzimidazole than the aromatic examples above it in the table. 

The most studied example of II was tetraphenylhydrazine cation radical, whose purple color led Hiinig to coin the term "violenes" ( ) for the series of vinologous species his group has studied extensively (3). The solution ESR spectrum of hydrazine radical cation had been recorded in a flow system (4). Aromatic examples of III had received extensive study (5), and diphenyl-picrylhydrazyl was the first example of a neutral radical stable both to dimerization and reaction with oxygen. The chemistry of 1,1-dimethyldiazenium cation had been studied (6), and interesting electrochemical work on arylhydrazine oxidations had been carried out by Cauquis and Genies (7). 

The delocalization of the electrons in the pi orbitals of benzene accounts for the properties of benzene and its derivatives, which differ from the properties of alkenes and other aliphatic compounds. The phenomenon is called aromaticity. A definition of aromaticity is that it occurs in compounds that obey the Hiickel rule-, i.e. that there should be a planar ring with a total of (An + 2) pi electrons (where n is any integer). Using this rule as a criterion certain nonbenzene rings show aromaticity. Such compounds are called nonbenzenoid aromatics. Examples are the cyclopentadi-enyl ion CjHj" and the tropyllium ion C7H7. Other compounds that have a ring... 

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