Stability adjacent heteroatoms

Substituted isoxazoles, pyrazoles and isothiazoles can exist in two tautomeric forms (139, 140 Z = 0, N or S Table 37). Amino compounds exist as such as expected, and so do the hydroxy compounds under most conditions. The stability of the OH forms of these 3-hydroxy-l,2-azoles is explained by the weakened basicity of the ring nitrogen atom in the 2-position due to the adjacent heteroatom at the 1-position and the oxygen substituent at the 3-position. This concentration of electron-withdrawing groups near the basic nitrogen atom causes these compounds to exist mainly in the OH form. 

The results so far suggest that stabilization of the intermediate carbocation by an adjacent heteroatom, aromatic ring or by steric factors stimulates fluorination of a CLH bond by sulfur tetrafluoride. 

Other stable carbocations are those with an adjacent heteroatom, e.g. oxygen, which can stabilize the cation through resonance (Scheme 5.8). 

The barrier to pyramidal inversion at arsenic in isopropylphenyltrimethylsilylarsine (165) is ca 74 kJ moP lower than in 11, whereas the less electronegative trimethylgermyl substituent reduces the barrier by ca 66 kJ mol Thus, the most important influence on pyramidal stability in these systems appears to be the atomic electronegativity of the adjacent heteroatom and not p -d bonding as had been suggested earlier as the source of barrier lowering in (R, R )/(R, S )-l,2-dimethyl-l,2-dipheny arsine (166) ". ... 

Azomethine ylide Sulfonium ylide Scheme 3.7 Stabilization of carbanion by adjacent heteroatoms. 

Stabilization of the singlet state by electron release from adjacent heteroatoms may be so pronounced as to cause the electrophilic properties of a carbene to vanish. 

It is known that 4 reacts faster than 2 to form dibromoacetone, and this is usually associated with a higher percentage of enol for 4 when compared to 2. In principle, the 0-H bond of the enol is polarized such that H is and will be attracted to the 6- bromine atom by intramolecular hydrogen bonding. The net result is stabilization of the enol form. The presence of heteroatoms at the a-carbon should stabilize the enol form, and this is generally correct. Titration of 2,4-pentanedione (7 known as acetylacetone) with bromine, as with the reaction of 2, gives 3-bromo-2,4-pentanedione. This experiment shows that there is 79.7% of the enol, 8, which is consistent with the concept of enol stabilization by an adjacent heteroatom substituent. 

The vast majority of platinum carbene complexes are stabilized by one or more adjacent heteroatoms, which allow TT-delocalization. However, there have been a few reports of compounds where there is no such stabilization, and we consider these first, before moving on to look at those with heteroatoms. 

Onium ions of small and large heterocyclics are usually produced by electrophilic attack on a heteroatom. In three- and four-membered rings nucleophilic attack on an adjacent carbon follows immediately, in most cases, and ring opening stabilizes the molecule. In large rings the onium ion behaves as would its acyclic analog, except where aromaticity or transannular reactions come into play (each with its electronic and steric pre-conditions). A wide diversity of reactions is observed. 

A comparative study on ylide stability as a function of the heteroatom type was carried out by Doering et al. [3,4]. They concluded that the phosphorus and sulfur ylides are the most stable ones. The participation of three-dimensional orbitals in the covalency determines the resonance stabilization of the phosphorus and sulfur ylides [5-8]. The nitrogen ylides are less stable from this point of view. The only stabilization factor involves electrostatic interactions between the two charges localized on adjacent nitrogen and carbon atoms [9]. 

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