Synthesis, characterization and group modification of carbosilane based dendrimers




Hooper, Richard

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Carbosilane dendrimers have been prepared via iterative hydrosilylation/alkenylation reactions using a variety of chlorosilanes (HSiMe₃₋nC1n)and alkenyl nucleophiles (CH₂=CHCH₂MgBr and CH₂=CHMgBr). Characterization by multinuclear NMR spectroscopy of the stepwise trifurctate dendrimers RED₃ (E=Si; R=Ph; D=Dendritic carbosilane fragment) assists end-group counting (NMR, R vs D) and can be related to analogous spheroidal ED₄ and hexafunctional E₂D₆ systems (E=Si or Ge). Typical D units contain (prnSiMe₃₋n)N Nb linkers; where pr=(CH₂)₃, N=generation number, Nb=branch multiplicity at silicon. For higher symmetry ED₄ and E₂D₆ systems (and also trifurcate) the Nb=1 (1B series) or Nb=2 (2B series) also allows for end-group counting by proton NMR spectroscopy (Ph:D:Me). ²⁹Si NMR chemical shifts have been developed for topographical mapping of dendritic structure. Peripheral and core substitution reactions have been examined for a series of trifurcate and related Ge₂D₆ ‘masked trifurcate' dendrimers. Replacement of end groups with a range of substrates is facilitated by the reactivity of the intermediate silicon chlorine bonds formed by hydrosilylation reactions. Selective removal of the core phenyl group (via triflic acid), or alternatively oxidative Ge-Ge cleavage, allows for unprecedented ‘bifunctionalization’; offering routes into dendrimer periphery:core electron transfer (‘photon-harvesting’) processes. Methodology for a ‘rapid-synthesis’ of hyperbranched structures has been developed and examined on a range of initial core molecules. Use of multinuclear NMR spectroscopy via comparison with step-wise products, as well as GPC chromatography, gives some insight into the overall architectures produced. Alteration of Si branch point from Nb=2 to Nb=3, using presynthesised ‘hypercores’ facilitates an alternate method of end group counting for higher symmetry cores. This methodology allows for population counting at large N in dendrimer generation G(N) (D interior vs D periphery).



Hydrosilylation, Dendrimers, Chemistry, Organic