With Exocyclic Oxygen Substituents

2 OXIDISED CHALCOGEN HOMOCYCLES 12.2.1 With Exocyclic Oxygen Substituents 

SioO from the corresponding cyclic sulfur allotropes [eqn (12.4)]. Interestingly, the use of an excess of CF3CO3H for the oxidation of cyclo-Sg results in ring contraction to give the dioxide 

The monoxide SgO forms the 0-bonded adduct SgO SbCl5 upon treatment with antimony pentachloride in CSy adduct formation results in pyramidal inversion at the three-coordinate sulfur atom. In contrast, the treatment of 850 with antimony pentachloride induces dimerisation to give the 2 1 adduct of SbCls with the cyclic dioxide SiyOy. The chair form of the two S O constituents of this dimerisation process are evident in the conformation of the 12-membered S12 ring in the adduct. 

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The protonation site of pyridazone [142] appears also to be the exocyclic oxygen, since a hypsochromic change occurs in the ultraviolet absorption upon protonation (Cookson and Cheeseman, 1972). The p/iTa-values of the 6-substituted derivatives follow a correlation with Op-constants of the substituents ... 

By far the most important systems under this heading are those with an exocyclic oxygen, i.e., phenols and aromatic ethers. They show an extremely wide variety of behaviour and many of the observations are still not fully understood. The question of their site of protonation was the subject of a controversy for a few years, but it is now clear that both the O- and the C-protonated forms [223], [224] and [225] occur, depending on the conditions (medium, temperature) and the substituents (R ). 

The reaction of aziridines and azetidine heterocycles with dinitrogen pentoxide in chlorinated solvents yields 1,2-nitramine nitrates and 1,3-nitramine nitrates respectively. In most cases yields are good to excellent, but, reactions are not as general as with the oxygen heterocycles the outcome of reactions is heavily dependent on the nature of the substituent on the exocyclic nitrogen. Some of the products from these reactions find use as melt-castable explosives i.e. Tris-X (46) and energetic plasticizers e.g. Bu-NENA (48) the latter is a component of some LOVA (low vulnerability ammunition) propellant formulations. 

Effects of oxygen substitutents in an aromatic ring upon an exocyclic rather than endocyclic carbocation charge center have also been measured. The possibility of comparing HO, MeO, and O substituent effects for the benzylic cations is provided by recent studies of quinone methides, including the unsubstituted / -quinone methide 23, which may be considered as a resonance-stabilized benzylic cation with a /xoxyanion substituent. 

Katritzky and coworkers utilized X-ray aystaUography to analyze the structure consequences of VAE in systems with two endocyclic double bonds and an exocyclic benzotriazole substituent (Figure 6.80). The two non-equivalent exocyclic bonds of the sp - hybridized carbon atom occupy pseudoaxial (flagpole) and pseudoequatorial (bowsprit) positions. Importantly, the benzotriazole residue generally assumes the pseudoaxial position. The exocychc C-N bond is 0.02-0.03A longer than the C-N bond of the model compound without oxygen. 

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