Oxygen lone electron pairs, stabilizing

While the hydride shift illustrated in Scheme 5.12 cannot occur as a part of the pinacol rearrangement, the intermediate carbocation is subject to alkyl migrations. As shown in Scheme 5.13, a 1,2-alkyl shift results in transfer of the cation from a tertiary center to a center adjacent to a heteroatom. As the oxygen heteroatom possesses lone electron pairs, these lone pairs serve to stabilize the cation. Thus, the illustrated 1,2-alkyl shift transforms a carbocation into a more stable carbocation. 

On the basis of analysis of typical byproduct spectra W. Dmowski postulated a mechanism for the reaction [139]. In aHF as solvent the SF, species is generated in a solvolytic equilibrium. The strongly electrophilic SF, ion adds to the carbonyl oxygen, making the a-carbon atom highly electrophilic. A fluorine atom is then transferred intramolecularly to the carbon and sulfonyl fluoride is expelled. The resulting resonance-stabilized (by r-donation from fluorine lone electron pairs) a-fluoro carbenium ion adds fluoride from ambient (FHF) ions (Scheme 2.64). 

Oxygen lone pair cannot be used to stabilize positive charge in any of these stmctures all have six electrons around positively charged carbon... 

The interaction diagrams for the above conformations are identical with that of methyl vinyl ether (Fig. 30) except that the oxygen lone pair AO is replaced by an unoccupied carbon 2p AO. With this in mind we conclude that the transoid conformation of the cation, Ts, will be more stable than the cisoid conformation, Cs, since the <(>j—Pz two electron stabilizing interaction is greater for the Ts conformation. 

Analysis of the far IR-spectra of 3,4-dihydro-2//- pyran (13) (72JCP(57)2572> and 5,6-dihydro-2/f- pyran (14) (81JST(71)97> indicates that for both molecules the most stable conformation is a half-chair form. The barrier to planarity is greater for the former compound. These preferred structures are in accord with the half-chair conformation established for cyclohexene and its derivatives. The conformational mobility of cyclohexene is greater than that of the 3,4-dihydropyran. The increased stabilization of the pyran has been attributed to delocalization of the v- electrons of the alkenic carbon atoms and the oxygen lone-pairs (69TL4713). 

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