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Title: Heavier group 2 metals : application to intermolecular hydroamination, C-F activation and intramolecular hydroalkoxylation
Author: Brinkmann, Christine
ISNI:       0000 0004 2713 3903
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2011
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This thesis describes the reactivity of different heavier alkaline earth catalysts [M{X(SiMe3)2}2(THF)n]m (M = Ca, Sr, Ba; X = N, CH; n= 0, 2; m= 1, 2) in the intermolecular hydroamination of styrene derivatives. The scope of these reactions with respect to the substrate was determined and detailed kinetic studies to establish rate law and temperature dependence of the hydroamination reactions reported were conducted. Overall, it was found that [Ca{N(SiMe3)2}2]2 is favoured enthalpically (Ca: ΔH‡ = 51 kJ∙mol-1, Sr: ΔH‡ = 71 kJ∙mol-1) however the corresponding strontium bis(amide) proved a significantly better catalyst, likely due to a favourably high entropy of activation value (Ca: ΔS‡ = -168 J/mol-1 ·K-1, Sr: ΔS‡ = -92 J∙mol-1∙K-1). Large kinetic isotope effects of 4.1 and 7.9 at 55 °C for the intermolecular hydroamination of styrene with piperidine mediated by [Ca{N(SiMe3)2}2]2 and [Sr{N(SiMe3)2}2]2, respectively, suggest a rate-determining alkene insertion into the M-N bond with immediate or concerted protonolysis. The methodology used in these hydroamination reactions was extended to simple dienes, diphenylacetylene and an activated enyne. The catalyst initiation of the metal bis(amides) with piperidine was shown to be reversible and the equilibrium constant solvent dependent. Novel calcium and strontium dialkyl complexes [M{CH(SiMe3)2}2(THF)2] (M= Ca, Sr) were used to overcome the problem of catalyst initiation and showed a different solvent dependence. An enhanced reactivity was found for the dialkyl complexes compared to the metal bis(amides). This increased reactivity allowed the application in new reactions such as the C-F activation of fluorobenzenes. Furthermore, the use of these catalytic systems was successfully extended to intramolecular hydroalkoxylation reactions of alkynyl alcohols in the formation of five- and six-membered enol ethers. In this case, [Ba{N(SiMe3)2}2]2 displayed significant reactivity although the “catalyst of choice” for these reactions proved to be strongly dependent on substrate substitution pattern. Through detailed kinetic studies the catalyst, substrate and temperature dependence of the cyclisation reaction were established and an unusual rate law with inverse substrate dependence proposed.
Supervisor: Barrett, Anthony Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral