The reactivities of some carbon-metal bonds.
thesisposted on 19.11.2015, 08:48 by David Ralph Miles. Walton
The chemistry of acetylenic compounds containing Group IVB metals is reviewed. The rates of acid-catalysed cleavage of the aryl--silicon bond in the 4-halogeno-2-trimethylsilylanisoles have been measured and found to agree with values predicted on the assumption that the effects of meta--halogen and ortho-methoxy substituents are additive. The order of inductive electron-release to a saturated carbon atom by trimethyl-silyl, -germyl, and -stannyl groups has been determined by rate measurements of the acid-catalysed cleavage of the aryl-tin bond in the compounds p-Me3MCH2C6H4SnMe3 (M = Si, Ge, Sn). In the light of the order obtained: Me3Sn > Me3Ge > Me3Si, comments are offered on the electronegativity order of the Group IVB elements and on the mechanisms proposed for the cleavage of aryl-metal (Group IVB) bonds by electrophilic reagents. A study has been made of the cleavage of aryl-ethynyltriethylgermanes and of the hydration of aryl-acetylenes; both reactions being catalysed by acid. The effects of nuclear substituents are analysed in terms of the Yukawa-Tsuno extended free-energy relationship, and are in accord with rate-determining protonations at the acetylenic carbon, B to the aryl group. The cleavage by acids of the ethynyl-germanium bond is pictured as a true electrophilic substitution at an acetylenic carbon, and a simple mechanism is proposed for this reaction. In view of the low reactivity of ortho relative to para substituents, field effects are thought to make an important contribution to the overall effects of para substituents. The preparation and physical properties of thirty-two new organometallic compounds are described.