Oxygen as Heteroatom

Acidic Carboxonium Ions (Hydroxylated Cations). Acidic carboxonium ions are generally obtained by protonation of carbonyl compounds.541,542 Aldehydes and ketones protonate on the carbonyl oxygen atom in superacid media at low temperatures, and the corresponding carboxonium ions can be directly observed543-551 [Eqs. (3.61) and (3.62)]. 

Even protonated formaldehyde has been observed.548 Protonated acetaldehyde shows two isomeric forms, with the proton on oxygen being syn or anti to the methine proton (263).548 

The hydroxymethyl cation forms of protonated ketones (264) and aldehydes (265) contribute to the resonance hybrid. Based on 13C NMR studies,94 548 551 the degree of contribution of the hydroxymethyl cation forms can be quite accurately estimated. Similar studies have been carried out using 170 NMR spectroscopy.552 Recent theoretical studies (MP2/6-31G level)553 for protonated acetone have supported the 

The X-ray structure is of carboxonium ions 266560 and 267561 have been determined. The main features, a substantial lengthening of the C—O bonds and a shortening of the Cc=cr C bonds, were observed upon protonation of the starting ketones. 

The generation of the stable cyclopropenylcarbinyl cation 268 under superacid conditions is an exceptional example562 [Eq. (3.63)]. It appears that in this case the bulky terf-butyl group prevents the usually fast rearrangement of cyclopropenylcarbinyl cations to the corresponding homocyclopropenylium ions. 

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The oxygen as heteroatom in ethers or carbonyl compounds is weak to moderate Lewis base. Nevertheless, a highly reactive metal carbene complex can interact with the oxygen to generate oxygen ylide. The interaction between ether and metal carbene functional groups is believed to be rather weak as demonstrated by the facts that other metal carbene reactions, such as G-H insertion and cyclopropanation, can proceed in ethereal solvents." These experiments demonstrate that the formation of the metal ylide is much less favored in the equilibrium shown in Equation (1). ... 

Vogtle has suggested that synthetic ionophores be classified as follows (21) cor-onands are macromonocyclic compounds with any heteroatoms cryptands are bi- and polymacrocyclic ligands with any heteroatoms and podands are acyclic coronand and cryptand analogs. The term crown ether is reserved for coronands with only oxygens as heteroatoms. Such a classification will be used hereafter. 

Ion chromatography has been successfully applied to the quantitative analysis of ions in many diverse types of industrial and environmental samples. The technique has also been valuable for microelemental analysis, e.g. for the determination of sulphur, chlorine, bromine, phosphorus and iodine as heteroatoms in solid samples. Combustion in a Schoniger oxygen flask (Section 3.31 )is a widely used method of degrading such samples, the products of combustion being absorbed in solution as anionic or cationic forms, and the solution then directly injected into the ion chromatograph. 

Cyclic compounds in which one or more of the ring atoms is not carbon are termed heterocycles the noncarbon atoms are referred to as heteroatoms. We shall limit our discussions to compounds in which the heteroatoms are nitrogen, oxygen, or sulfur. For the purposes of studying and understanding their properties, heterocycles are conveniently grouped into two classes, i.e. non-aromatic and aromatic. 

Numbering always begins at the heteroatom in the case of morpholine, numbering starts at oxygen, the heteroatom of higher atomic number. Remember that an accepted alternative in nomenclature is to indicate a heteroatom by the prefix aza-, oxa, or thia-in the appropriate carbocycle (see Section 1.4). Thus, we could name piperidine as azacyclohexane, and tetrahydrofuran as oxacyclopentane. 

The species which are unknown and have not been identified as one of the major chemical lump such as alkanes, phenols and aromatics are lumped together as unidentified. However, the species in this lump include saturated and unsaturated cycloalkanes with or without side chains, which resembles the naphthenes, a petroleum refinery product group. A number of well known species in coal liquid are not mentioned in this lumping scheme. Such as heterocyclic compounds with sulfur, nitrogen or oxygen as the heteroatom, and other heteroatora containing species. Some of these compounds appear with aromatics (e.g. thiophenes, quinolines) and with phenols (e.g. aromatic amines), and most of them are lumped with the unidentified species lump. 

Only a few examples of ring syntheses by transformation of another ring have been published, as exemplified by the preparation of the sultines 168 by ring enlargement of five-membeted thiolane 1-oxides 166 (cf Section 8.10.9.2.3) and the 3,6-dihydro-l,2-dithiins 202 by catalytic transformation of vinylthiiranes 201 (cf Section 8.10.9.4.2). Because possibilities to synthesize six-membered rings with oxygen and/or sulfur as heteroatoms in 1,2-positions are rather limited, these reactions have been covered already in Section 8.10.9 together with alternative syntheses from alicyclic compounds. 

Table 4.66. l3C Chemical Shifts of Heterocycles with sp2 Ring Carbons (<5C in ppm). Part 1 Oxygen and Sulfur as Heteroatoms [416-424]. 

Nitrogen as Heteroatom. Amides are protonated on the carbonyl oxygen atom (cation 371) in superacid media at low temperatures, as shown first... 

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