Heteroatom stabilization Subject

The intramolecular 4 + 3-, 3 + 3-, 4 + 2-, and 3 + 2-cycloaddition reactions of cyclic and acyclic allylic cations have been reviewed, together with methods for their generation by thermal and photochemical routes.109 The synthetic uses of cycloaddition reactions of oxyallyl cations, generated from polybromo and some other substrates, have also been summarized seven-membered rings result from 4 + 3-cycloadditions of these with dienes.110 The use of heteroatom-stabilized allylic cations in 4 + 3-cycloaddition reactions is also the subject of a new experimental study.111 The one-bond nucleophilicities (N values) of some monomethyl- and dimethyl-substituted buta-1,3-dienes have been estimated from the kinetics of their reactions with benzhydryl cations to form allylic species.112 Calculations on allyl cations have been used in a comparison of empirical force field and ab initio calculational methods.113... 

The regio- and diastereoselective rhodium-catalyzed sequential process, involving allylic alkylation of a stabilized carbon or heteroatom nucleophile 51, followed by a PK reaction, utilizing a single catalyst was also described (Scheme 11.14). Alkylation of an allylic carbonate 53 was accomplished in a regioselective manner at 30 °C using a j-acidic rhodium(I) catalyst under 1 atm CO. The resulting product 54 was then subjected in situ to an elevated reaction temperature to facilitate the PK transformation. 

The importance of carbanions a-substituted by heteroatoms in organic synthesis explains the vast amount of literature concerning the use of a-heterosubstituted organotins in transmetallation reactions. As the tin-lithium exchange is assumed to occur with a complete retention of configuration at the carbanion centre639, the enantioselective approach of such stabilized carbanions for synthesis has been the subject of recent developments. 

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. 

Some diols of heterocycles have been subjected to pinacol rearrangement. Mundy found that (37) gave only (38) when treated with cold concentrated sulfuric acid (equation 21). It was noted that (38) is not the product expected from initial cation stability arguments based on anticipated heteroatom dipole effects. The basis for this selectivity remains unclear. An attempt to detect alternative products at short reaction times failed to shed light on this question. 

As mentioned in previous sections, carbocation intermediates are subject to rearrangement to a more stable ion (sec. 2.7.B.iii). If the cation were stabilized in some manner, rearrangement would be much less likely, as when a heteroatom is attached to the electrophilic carbon. An example is the oxygen stabilized cation (3041 generated by reaction of a ketone with an acid catalyst. The electrons on the heteroatom are donated to the positive center leading to resonance stabilization (this is called back donation). Such a cation is usually... 

By analogy to the previous sections, we should consider Eq. 2.17 to evaluate the stabilities of carbanions. Because of the universal importance of acid-base chemistry. Chapter 5 is entirely devoted to this subject. There we will discuss both carbon acids (where the negative charge on A is primarily associated with a carbon) and heteroatom acids. Here we briefly mention trends associated with carbon acids with a goal of defining the essential nature of carbanions. Selected AH° values for the reaction of Eq. 2.17 in the gas phase for carbon acids are presented in Table 2.9. 

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