Stannanes reactions with aldehydes

Diallyldialkylstannanes with chiral alkyl substituents on the tin, show variable asymmetric induction in their Lewis acid catalyzed reactions with aldehydes. Using bis-(/f)-2-phenylbutyl-(di-2-propenyl)stannane, enantiomeric excesses of up to 54% were obtained via attack on the / e-face of the aldehyde96. 

Tin(IV) halide-catalysed reactions of 4-, 5-, and 6-alkoxy(alk-2-enyl)stannanes exhibit 1,5-, 1,6-, and 1,7-asymmetric induction, respectively. For example, 4-substituted (pent-2-enyl)stannanes (56) give e-hydroxy derivatives (57) with a syn anti ratio of >30 for hydroxy and benzyloxy substrates (i.e. R = OH, OCH2Ph). A key allyltin trichloride intermediate has now been identified, and the transition states for its reaction with aldehyde have been calculated as being over lOkcalmol" apart for the alternative product stereochemistries. ... 

This idea has been extended to pentadienylsilanes and -stannanes and provides a useful complement to the reagents discussed in Section III,A, for the preparation of homodienylic alcohols. Reaction with aldehydes in the presence of a Lewis acid at low temperatures affords the alcohols 7 in good yields (111,112) [Eq. (8)]. 

Achiral stannanes undergo reactions with aldehydes bearing a-asymmetry, and provide examples of matched diastereoselec-tivity with respect to 1 (eq 5), as well as cases of mismatched diastereoselection of these controlling factors (eq 6). ... 

In the same way, reaction of (8) with allenyl- or propargyl-stannanes affords intermediate borane derivatives which, upon reaction with aldehydes, produce the expected adducts with high selectivities (eqs 14 and 15). ... 

Alkoxyallylstannanes show excellent 1,5-asymmetric induction upon reaction with aldehydes. Consequently, reaction of stannanes 486 or 488 with 464 gives diols 487 or 489 with > 95% diastereoselectivity [155,156] (Scheme 71). In the case of 487, the stereochemical outcome of the reaction is sensitive to the chirality of the aldehyde. Upon reaction of ( S)-464 with (5)-486, a matched pair , one sees excellent stereoselection ( > 95%), whereas with the enantiomeric (R)-464 and (5)-486, a mismatched pair , the stereoselectivity falls to 70 30. 

Allylindium(III) and -bismuth(III) dihalides, generated by transmetallation of 5-benzyloxy-4-methylpent-2-enyl(tributyl)stannane, react with aldehydes with useful levels of 1,5-stereocontrol, a 93 7 ratio of 1,5-epimers in favor of the l,5-anti-(E)-stereoisomers typically being obtained using bismuth(III) iodide. These bismuth(III) and indium(III) halide-promoted reactions are of interest since they... 

Enol stannanes of cyclohexanone and propiophenone have been indicated to take part in r/treo-selective aldol reactions with benzaldehyde at low temperatures e.g. —78 °C), but to be erythro-seAsciiwe at higher temperatures ca 45 °C). Two complementary methods have been described for stereoselection in aldol-type reactions. Whilst a-mercurio-ketones show eryr/wo-selection in their reactions with aldehydes in the presence of boron trifluoride diethyl etherate, pre-formed lithium enolates and aldehydes, in the presence of simple trialkyl-boranes, lead to mixtures that are rich in the more stable threo-d do product. Aldol-type products arise from 1,3-alkyl migrations of alk-l-enyl alkyl acetals and ketals, in a reaction that is catalysed by boron trifluoride diethyl etherate (Scheme 52). Diastereoselection is possible, since (.E)-alkenyl acetals give the... 

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