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Reversible Reactions of Ethylene with Distannynes Under Ambient Conditions

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Science  25 Sep 2009:
Vol. 325, Issue 5948, pp. 1668-1670
DOI: 10.1126/science.1176443

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Tin Two-Step

Doubly and triply bonded carbon compounds have a well-studied tendency to link up with one another and form rings. The rates of these reactions and their relative susceptibilities to acceleration by heat versus light are encapsulated in the decades-old Woodward-Hoffmann rules. More recently, alkene and alkyne analogs have been prepared with heavier elements such as silicon and tin substituted for carbon. Peng et al. (p. 1668; see the Perspective by Sita) have now discovered that two distannynes (compounds with triply bonded tins) react readily with ethylene to form cycloadducts, with tin-carbon σ bonds taking the place of C-C and Sn-Sn π bonds. These products, characterized spectroscopically and crystallographically, are only loosely bound at room temperature, easily reverting to their multiply bonded precursors on gentle heating.

Abstract

Ethylene’s cycloadditions to unsaturated hydrocarbons occupy well-established ground in classical organic chemistry. In contrast, its reactivity toward alkene and alkyne analogs of carbon’s heavier-element congeners silicon, germanium, tin, or lead has been little explored. We show here that treatment of the distannynes AriPr4SnSnAriPr4 [AriPr4 = C6H3-2,6(C6H3-2,6-iPr2)2, 1] or AriPr8SnSnAriPr8 [AriPr8 = C6H-2,6(C6H2-2,4,6-iPr3)2-3,5-iPr2, 2] with ethylene under ambient conditions affords the cycloadducts Embedded Image (3) or Embedded Image (4) that were structurally and spectroscopically characterized. Ethylene incorporation in 3 and 4 involves tin-carbon σ bonding and is shown to be fully reversible under ambient conditions; hydrocarbon solutions of 3 or 4 revert to the distannynes 1 or 2 with ethylene elimination under reduced pressure or upon standing at ~25°C. Variable-temperature proton nuclear magnetic resonance studies showed that the enthalpies of reaction were near –48 (3) and –27 (4) kilojoules per mole.

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