Oxygen bases, compared with nitrogen base

NMR data for 4-methyloxazole have been compared with those of 4-methylthiazole the data clearly show that the ring protons in each are shielded. In a comprehensive study of a range of oxazoles. Brown and Ghosh also reported NMR data but based a discussion of resonance stabilization on pK and UV spectral data (69JCS(B)270). The weak basicity of oxazole (pX a 0.8) relative to 1-methylimidazole (pK 7.44) and thiazole (pK 2.44) demonstrates that delocalization of the oxygen lone pair, which would have a base-strengthening effect on the nitrogen atom, is not extensive. It must be concluded that not only the experimental measurement but also the very definition of aromaticity in the azole series is as yet poorly quantified. Nevertheless, its importance in the interpretation of reactivity is enormous. 

In the case of non-HBD solvents, such as DMSO, the measured pK values are absolute (that is, free from ion pairing) and can be directly compared with gas-phase acidities6 in addition, knowledge of the heats of ionization in DMSO7 allows the evaluation of a possible entropy effect when the two phases are compared. The mechanism of proton transfer between oxygen and nitrogen acids and bases in aqueous solution has been reviewed8. 

Alkylation of potentially tautomeric heteroaromatic systems under basic phase-transfer catalytic conditions normally occurs on the softer heteroatom [cf 57]. Thus, although 2- and 4-pyridones are alkylated on the annular nitrogen atom and the exocyclic oxygen atom, -alkylation of the 2-pyridones predominates to the extent of ca. 5 1 (or greater under soliddiquid reaction conditions [58]), whereas the relative predominance of A -alkylation of the 4-isomer is only ca. 3 1 [59] (Table 5.37 and 5.38). These ratios are comparable with those obtained for the base-catalysed alkylation of the pyridones by traditional methods and, not unexpectedly, S-alkyla-tion of the corresponding pyridthiones occurs to the total exclusion of A-alkylation [60]. Catalysed soliddiquid acylation has also been reported [58]. 

Pre-eminent amongst examples is the case of amides, which do not show the typical basicity of amines. Acetamide, for example, has pATa — 1.4, compared with a 10.7 in the case of ethylamine. This reluctance to protonate on nitrogen is caused by delocalization in the neutral amide, in which the nitrogen lone pair is able to overlap into the n system. This type of resonance stabilization would not be possible with nitrogen protonated, since the lone pair is already involved in the protonation process. Indeed, if amides do act as bases, then protonation occurs on oxygen, not on nitrogen. Resonance stabilization is still possible in the D-protonated amide, whereas it is not possible in the A-protonated amide. Note that resonance stabilization makes the D-protonated amide somewhat less acidic than the hydronium ion (pATa — 1.7) the amide oxygen is more basic than water. 

The hypothesis of a bifunctional mechanism involving allyl radical formation and oxygen incorporation on distinct sites is advocated by Haber et al. [147,152], This hypothesis is particularly based on experiments with Mo03, Bi203 and mechanical mixtures of these oxides, which are compared with bismuth molybdate catalysts. The reaction was carried out in cyclic operation (alternating feeds of oxygen and of propene diluted with nitrogen). The results are collected in Table 5. The authors con-... 

If these results are compared with calculated transannular distances shown in 236 for axial methyl groups based on X-ray data,235 then not only are the effects of bond-length changes on AG° evident (cf. 4-Me and 6-Me) but also interactions between axial methyl and oxygen is seen to be less than that between axial methyl and nitrogen.241... 

The oxygen and sulfur bases are weaker than the nitrogen bases, and accurate solution basicities are not available. Arnett s heat of protonation studies indicate that the order of decreasing basicity is ROR > ROH > HaO,98 a result that is in agreement with gas-phase measurements.99 Hydrogen sulfide in the gas phase has basicity comparable to that of water (Table 3.18), and substitution of H by alkyl produces stronger gas-phase bases just as does similar substitution on oxygen. 

Substitution by an aromatic group has a marked effect on solution base strength. One might be tempted to attribute the low basicity of aniline, di- and triphenylamines, and phenol compared with reference compounds (Table 3.17) to partial delocalization of the nonbonded electron pair on the nitrogen or oxygen into the n orbital system of the ring. But gas-phase results indicate the basicity... 

There are known many adducts of SiF with nitrogen-containing bases the majority of these being 1 2 adducts. Little is known about complexes of SiF with oxygen-containing bases, presumably owing to a much weaker interaction The heat of dissociation of the SiF 2 (CH3)30 complex at substantially below room temperature was reported to be 9.0 kcal rnole as compared to 40.5 kcal mole" ... 

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