Heteroatomic anionic rearrangements

Both of these are examples of heteroatomic anionic rearrangements, that is, ones where X and Y are different. Homoatomic anionic rearrangements, where X = Y, are also possible. The one example known before the discovery of organometallic migrating groups was the C—C anionic rearrangement (//, S6), reviewed by Grovenstein elsewhere in this volume ... 

Acrylic acid derivatives with a heteroatom (N, O, S, Hal) at C-3 can be cleanly deprotonated at this position with BuLi or LDA at low temperatures [329, 333-338] (Scheme 5.37). Some of these anions rearrange to the a-metalated acrylates on warming [329], but can also decompose (see Section5.4.7). Non-heteroatom-substi-tuted lithium /3-lithioacrylate [329] or /3-magnesioacrylic acid derivatives [339, 340] have been prepared by bromine-lithium or halogen-magnesium exchange. 

Turning now to heteroatomic cases, the N—O, O—C, S—C, and N—C 1,2-anionic rearrangements have been described earlier in this chapter. Anionic rearrangements from doubly bonded nitrogen to carbon in imines, however, do not seem to have been studied. If carbanions can be generated from Al-silylimines, N=C rearrangement seems likely ... 

Similarly, the [3 + 4] annulation of the E- and Z-isomers of /1-heteroatom-substituted acryloylsilanes 156 with lithium enolates of ,/l-unsaturated methyl ketones 157 gave stereospecifically cis-5,6- and //r/w.v-5,6-disubstituted-3-cycloheptenones 160, respectively (equation 97). The stereospecificity in the annulation was explained by an anionic oxy-Cope rearrangement of the 1,2-divinylcyclopropanediol intermediate 159, which was generated through the Brook rearrangement of the initial 1,2-adduct 158219 - 223. 

The synthetic uses of hydrazones and oximes for carbon-bond formation differ little from those of imines as their anions represent enolate equivalents except for certain methods that have been developed for asymmetric induction. Conversely, the formal replacement of the carbon substituent of imines by a heteroatom (nitrogen for hydrazones and oxygen for imines) opens reaction pathways such as the Beckmann rearrangement of oximes and the Wolff-Kishner and Shapiro reductions of hydrazones that have no analogy in the chemistry of imines. 

On the other hand the inherent a-selectivity of allylsulfur carbanions can sometimes be transformed to a y-reactivity by a sigmatropic rearrangement, earning in addition the stereoselectivity typical for such reactions (see Section 4.S.2.3). Due to their significance in synthesis a lot of work has been done on reactions of heteroatom-substituted allyl anions with special emphasis on their use as homoenolate anion equivalents. The more recent developments, with the possibility of introducing diastereoselectivity, will be discussed later in Section 4.S.3.2.I. 

There are two main synthetic applications where the reaction of an allyl system with electrophiles is accompanied by an allylic rearrangement. One consists of the use of heteroatom-substituted allylic anions as homoenolate anion equivalents and the other represents a synthetically valuable alternative to the aldol reaction by addition of allyl metal compounds to aldehydes. 

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