Allyl stannates

In basic aqueous media, a kinetic study of the reaction between stannate(II) ions and alkyl halide shows that mono- and disubstituted organotin compounds are formed (Eq. 6.12a).27 The monosubstituted organotin compound is obtained after a nucleophilic substitution catalyzed by a complexation between the tin(II) and the halide atom. The disubstituted compound results from an electrophilic substitution coupled with a redox reaction on a complex between the monosubstituted organotin compound and the stannate(II) ion. Stannate(IV) ions prevent the synthesis of the disubstituted compound by complexation. Similarly, when allyl bromide and tin were stirred in D2O at 60° C, allyltin(II) bromide was formed first. This was followed by further reaction with another molecule of allyl bromide to give diallyltin(IV) dibromide (Eq. 6.12b).28... 

A second route to nonracemic /-oxygenated allylic stannanes utilizes an enantioselective deprotonation of allylic carbamates by BuLi in the presence of (—)-sparteine. The configurationally stable a-lithio carbamate intermediate undergoes enantioselective S/,-2 reaction with Bu3SnCl and Mc SnCI (Scheme 28)65. Once formed, the /-carbamoyloxy stannanes can be inverted by successive lithiation with. s-BuLi and stannation with R3SnCl (Scheme 29)65. The former reaction proceeds with S/.-2 retention and the latter by Sf2 inversion. Nonracemic allylic carbamates can also be used to prepare chiral stannanes. Deprotonation with. s-BuLi TMEDA proceeds stereospecifically with retention (Scheme 29)65. 

The hydrostannolysis of organic halides was discovered when an attempt at the hydro-stannation of allyl bromide resulted instead in hydrodebromination.37... 

Selective benzoylation (and allylation) of methyl (and benzyl 4-O-benzyl-a-L-rhamnopyranoside (4) has been studied by phase-transfer catalysis and by activation via the tributyl stannate. The former reaction gave the 2,4-dibenzyl ether (5) in high yield, whereas the latter process gave mainly the 3,4-diether (6). An alternative means of obtaining the 2,4-diether (5) involved selective tritylation to give (7) followed by benzylation and removal of the trityl group. These dibenzyl ethers have been employed in the synthesis of the tetrasaccharide repeating unit of the immunodeterminant of Klebsiella serotype K36 polysaccharide. Similar results have been recorded for the selective benzylation of other 4,6-blocked a-mannosides. The syntheses of 3-O-benzyl-D-ribose, 2-0-benzyl-D-ribose, and l,6-anhydro-2,3,2, 4-tetra-0-benzyl-jS-lactose have been recorded. The latter was employed in the synthesis of lacto-A -tetraose. ... 

Furlani, D. Marton, D. Tagliavini, G., Zordan, M. (1988) Hydrated a-bonded organometallic cations in organic synthesis. I. Allyl-, crotyl-, 1-methylallyl-, cyclohex-2-enyl-, and cinnamyl-stannation of carbonyl compounds in water, J. Organomet. Chem., 341,345-56. 

Acyclic a,a-disubstituted tin enolates 6 inevitably form as cis/trans-mixtures. Nevertheless, application of the chromium alkylation protocol with the modified salen complex 7 provides fair enantioselectivity with various alkylating agents R CH2X hke allyl bromide, benzyl bromide, allyl iodide, and ethyl iodoacetate, as outlined in Scheme 5.5. A plausible explanation is based on the assumption of a rapid cis/trans-isomerization of the tin enolates 6 through the C-bound tautomer and the postulate that one of the enolate diastereomers reacts distinctly faster than the other. The role of the additive BugSnOMe, which has a beneficial effect on the enantioselectivity, might be to catalyze the cis/trans-isomerization of the enolate. Several models have been proposed for the mechanisms of the enantioselective enolate alkylation like transmetallation of tin into a chromium enolate, formation of a stannate by iodine transfer from chromium to tin, as well as activation of the alkyl halide by chromium [5]. 

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