Stannanes rearrangement

Upon treatment with a variety of mild Lewis acids, a-oxygenated allylic stannanes rearrange to the y isomers (Eq. 36) [56]. The process is stereospecific and highly regioselective. Thus (S)-a afford (5)-y-allylic stannanes, and vice versa, with essentially no loss of ee. 

In contrast to the intermediate hydroxystannanes, O-protected stannanes 7 are stable compounds which can be distilled or chromatographed and stored under nitrogen for months. Treatment of 7 with butyllithium in tetrahydrofuran at — 78,JC results in rapid tin/lithium exchange (< 1 min). No products resulting from Wittig rearrangement or formation of an ate complex 8 could be detected9. 

There are, however, serious problems that must be overcome in the application of this reaction to synthesis. The product is a new carbocation that can react further. Repetitive addition to alkene molecules leads to polymerization. Indeed, this is the mechanism of acid-catalyzed polymerization of alkenes. There is also the possibility of rearrangement. A key requirement for adapting the reaction of carbocations with alkenes to the synthesis of small molecules is control of the reactivity of the newly formed carbocation intermediate. Synthetically useful carbocation-alkene reactions require a suitable termination step. We have already encountered one successful strategy in the reaction of alkenyl and allylic silanes and stannanes with electrophilic carbon (see Chapter 9). In those reactions, the silyl or stannyl substituent is eliminated and a stable alkene is formed. The increased reactivity of the silyl- and stannyl-substituted alkenes is also favorable to the synthetic utility of carbocation-alkene reactions because the reactants are more nucleophilic than the product alkenes. 

Various nonracemic allenylstannanes have been prepared from nonracemic propargylic mesylates and (Bu3Sn)2CuLi. The stereochemistry of the displacement was shown to be anti by correlation with an allenic stannane prepared through Claisen orthoester rearrangement of a propargylic alcohol of known configuration (Scheme 33)80. 

The alkoxyalkynylstannanes show the familiar instability associated with alkoxyalkynes. The decomposition of a trialkyl(ethoxyalkynyl)stannane, 17, can be rationalized as follows. Elimination of ethene and rearrangement gives the stannylketene, which reacts with the alkoxyalkyne to give the ester 18 and the distannylketene 19 as the observed products (Equation (87)).241,250... 

For stannanes, there exists one example of a rearrangement (133 —> 134) which at first sight resembles a prototropic rearrangement, but is in fact a radical chain reaction [341] (Scheme 1.59). 

Imidates, rearrangement of, 14, 1 Imines, additions of allyl, allenyl, propargyl stannanes, 64, 1 additions of cyanide, 70, 1 as dienophiles, 65, 2 synthesis, 70, 1 Iminium ions, 39, 2 65, 2 Imino Diels-Alder reactions, 65, 2 Indoles, by Nenitzescu reaction, 20, 3 by reaction with TosMIC, 57, 3 Ionic hydrogenation, 71, 1 Isocyanides, in the Passerini reaction, 65, 1... 

Deoxygenation of allyttc alcohols.3 A method for conversion of allylic alcohols to l-alkenes is outlined in equation (I). The first step is an allylic rearrangement of an Oallyl xanthate to 2. The second step is an allyl transfer from sulfur to tin with tri-n-butyltin hydride to give the allylic stannane (3). The last step, destannylation, is a well-known mule to terminal alkcnes.4... 

In the presence of Lewis acids allyl silanes and stannanes react with epoxides generally at the sterically less demanding carbon atom. Other electron-rich alkenes, such as ketene acetals, can also be used as nucleophiles. The strong Lewis acids required might, however, also lead to rearrangement of the epoxide before addition of the nucleophile can occur (last reaction, Scheme 4.72). 

Formation of a pentacoordinate 1,2-oxastannetanide by a base-induced rearrangement of a bis(/3-hydroxyalkyl)-stannane has also been reported <1997PS513>. Synthesis of pentacoordinate anionic oxasiletanides, oxagermeta-nides, and oxastannetanides has been reviewed (Scheme 17) <19980M367, 2000JOM256, 2002CSR195>. 

There are numerous reports of stannane-mediated free-radical chemistry that does not fit neatly into the categories discussed above738-801. Some of these publications describe unusual outcomes and chemistry that is specific to a small subset of substrates, while others report radical rearrangements and fragmentations that take place after the initial, tin-mediated, radical-forming event. We have chosen a small set of examples that illustrate these concepts. 

Indeed, starting from the densely hydroxylated compound 122, formation of stannane 123 and [2,3]-sigmatropic rearrangement under Still s condition easily gave cyclohexene 124, whose hydroboration-oxidation led to protected carbapyranose 125. On the other hand, silylation of the hydroxymethyl moiety and hydration of double bond in... 

Wittig rearrangements proceed with predominant inversion, with radicals intervening in the mechanism.27 For example, stannane (/ )-30 of 88% ee rearranges, on transmetallation with alkyllithiums, to the alcohol (/ )-31 of 42% ee, a reaction demonstrating 74% invertive stereospecificity. 

---