Title:
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Carbohydrate-based N-heterocyclic carbenes for enantioselective catalysis
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The synthesis of carbohydrate-based N-heterocyclic carbene ligands for the Rh-catalysed
asymmetric hydrosilylation of ketones has been explored.
Initial studies utilised an alkylation approach to globally modify the hydroxyl functionality of
a D-glucosamine scaffold. This strategy was amenable to a range of electrophiles; such as
BnBr and Mel, which gave access to a library of amino-,β-glycosides. The corresponding C2-
symmetric NRC.HCI salts could be synthesised from these amines, glyoxal and
paraformaldehyde. Furthermore, upon deprotonation, the carbohydrate-based NRCs could be
successfully ligated to [Rh(COD)Clh to give the Rh-NRC complexes. These catalysts were
evaluated in a typical benchmark reaction; the hydrosilylation of acetophenone. Two factors
were found to affect the er of the product alcohol (l-phenylethanol): (i) the use of apolar
soLvents such as hexane, and, (ii) the size of the group modifying the carbohydrate's
hydroxyls. For instance, the catalyst with the NRC ~ppendage derived from permethylated
glucosamine gave the highest er (80:20).
Next, the effect of varying the carbohydrate's stereochemistry at the C-1 and C-2 carbons on
the hydrosilyhltion reaction was investigated. To this end, four catalysts with NRC ligands
derived from α- and ,β-mannosamine and α- and ,β-glucosamine were synthesised.
Interestingly, a departure from the ,β-glucosamine scaffold was detrimental to the obsetved er
in the product alcohol. The all equatorial scaffold, at least across the C 1-3 positions of the
carbohydrate, was crucial for asymmetric induction.
Subsequently, the steric bias of the C-1 and C-3 positions was probed, given these carbons
flank the C-2 attachment of the carbohydrate moiety to the NRC ligand. While glycosylation
protocols gave access to C-1 ,β-thiophenyl-glucosides for NRC.HCI synthesis, derivatising
the C-3 hydroxyl was more challenging. This resulted in the development of a method based
on SNAr, which also led to several side investigations. The C-1 and C-3 sterically biased RhNRC
complexes turned out to be pseudo-enantiomeric catalysts for the hydrosilylation of
acetophenone, which gave the product alcohol in 30:70 and 86:14 er, respectively. Further
optimisation of this ligand system was undertaken, however, the stereoselectivity could not
be improved upon. Computational modelling was also performed in an attempt to probe the
catalysts' method of asymmetric induction but this ultimately provided no additional insight.
Finally, chelating carbohydrate-based NRC.HCI salts were synthesised. However, upon
ligation to Rh,- the chelating functionality, a hydroxyl group, was found not to be bonded to
the Rh-centre. While these complexes were active for the hydrosilylation of acetophenone, a
negligible er was detected in the product alcohol. Further work is still required to fully
evaluate this type of carbohydrate NRC ligand.
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