This thesis is concerned with the preparation and reactions of some highly sterically hindered organosilicon compounds, mainly of the type TeiSiMe2X where Tsi denotes the (Me3Si)3C group. The first detailed study of the reactions of TsiS1Me20CN has shown that reactions with NaN3 in MeOH or KSCN or KOCN in MeCN give exclusively the corresponding TsiSiMe2X compounds (X = N3, NCS, or NCO), whereas those with other salts, viz. LiCl, CsF, KSCN, and KOCN, give the TsiSiMe2X species along· with other products, including in many cases TsiSiMe2NCO and TsiSiMe20H (from traces of water), and (in MeOH) TsiSiMe20Me. The reaction with MeOH alone was never found to give less than 10% ot TslSiMe20H, along with the expected TsiSiMe20Me, however carefully the MeOH was dried. The extreme sensitivity of the cyanate towards water was illustrated by the fact that the rate ot solvolysis in 'dry' MeOH was increased by~. 90% by addition of 0.05 vol-% of H2o, and the hydroxide was the sole product, whereas the rate for the triflate TsiSiMe2- OS02CF3 was increased by only 13% and the product was a 60:40 mixture of TsiSiMe20Me and TsiSiMe20H. The rate constant for the triflate increased linearly with the water concentration but that of the cyanate did not. The presence of NaOMe in MeOH led to rapid isomerization of the cyanate to the isocyanate, TsiSiMe2NCO, with the rate of isomerization being proportional to the base concentration: a possible explanation of this effect of base is suggested. Isomerization catalysed by ICI in CC14 was found to be of ca. second order with respect to both the cyanate and the IcI: A detailed kinetic study has been carried out of the the reactions of TsiSiMe2X compounds, with X c I, Br, Cl, or ON02' with various alkali metal salts, MY, viz. NaN3' CsF, KSCN, and KOCN in MeOH and with KSCN in MeCN. For X • I, Br, or Cl, the reactions have been shown to be much more complex than was previously thought, the compounds TsiSiMe20Me, TsiSiMe20H, and (Me3Si}2CHSiMe2oMe commonly being formed along with TsiSiMe2Y. The order of effectiveness of the salts in the formation of TsiSiMe2Y is CsF > NaN3 > KSCN > KOCN, except that for X - I the order for CsF and NaN3 is reversed. Approximate values of the activation parameters have been obtained: the activation entropies have very high negative values, consistent with formation of a very crowded transition state. The nitrate is markedly more reactive even than the iodide, and gives cleaner conversions into TsSiMe2Y compounds: the activation energies are much lower and the activation entropies much more negative than those tor the halides. Reactions of the compounds (Me3Si)2C(SiMe2H)(SiMe2Cl) with alkali metal salts have been shown to be much faster than those of TsiSiMe2Cl, casting doubt on an earlier suggestion that the ease of solvolysis of the former chloride might be due to anchimeric assistance by y-H.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:375169 |
| Date | January 1986 |
| Creators | El-Kaddar, Yousef Younis |
| Publisher | University of Sussex |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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