Synthesis, characterization and group modification of carbosilane based dendrimers
Date
2017-06-21
Authors
Hooper, Richard
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Abstract
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).
Description
Keywords
Hydrosilylation, Dendrimers, Chemistry, Organic