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ChemComm
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DOI: 10.1039/C7CC03076B
COMMUNICATION
Journal Name
Scheme 5 Alternative pathway for the catalytic Staudinger amidation involving silyl esters as amidating agents and reductants.
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Carboxylate attack of the silane initiates reduction of the
aminophosphonium species , generating the key silyl ester
intermediate along with the iminophosphorane . A further
reaction between these two species then regenerates the
phosphine catalyst and produces an aminosilylester species
that, ultimately, leads to the amide23,24 product
and
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silanol/siloxane by-products. We have demonstrated
experimentally that such silyl ester/silanamine species are
viable amidation precursors (Schemes 3 and 4). Amidation may
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be mediated intramolecularly from a mixed acyloxysilanamine 13
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species, or, in the presence of excess carboxylic acid, Si–N
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protonolysis may occur, releasing amine and entering a
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separate (and previously documented) amidation manifold
involving silane-mediated coupling of the free acid and
amine.21 We note that the in situ-generated silyl ester plays 17
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two key roles in the revised catalytic cycle. It, first, functions as
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For full details see the supporting information.
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an activated carboxylic acid and, second, as an enhanced
reductant, facilitating phosphine turnover; studies involving
both aspects are currently underway in our laboratory.25
In summary, we have provided an alternative pathway for
the catalytic Staudinger ligation that is congruous with existing
and new experimental results. Furthermore, we have shown
that silanes cannot simply be regarded as innocent terminal
reductants in reaction media that are Lewis basic/Brønsted
acidic and that silyl esters are likely to be involved as enhanced
reductants, or even more intimately in key bond forming steps
in many other catalytic reactions.26–29
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We are grateful to The School of Chemistry, Unversity of
Nottingham for funding (PhD studentship to K.G.A.).
Notes and references
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