Ambident reactivity of acetyl- and formyl-stabilized phosphonium ylides

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Date
2016-07-08
Authors
Byrne, Peter A.
Karaghiosoff, Konstantin
Mayr, Herbert
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Publisher
American Chemical Society, ACS
Published Version
10.1021/jacs.6b06264
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Abstract
The kinetics and mechanism of the reactions of formyl-stabilized ylide Ph3P═CHCHO (1) and acetyl-stabilized ylide Ph3P═CHCOMe (2) with benzhydrylium ions (Ar2CH+, 3) were investigated by UV–vis and NMR spectroscopy. As ambident nucleophiles, ylides 1 and 2 can react at oxygen as well as at the α-carbon. For some reactions, it was possible to determine the second-order rate constant for O-attack as well as for C-attack and to derive the nucleophile-specific parameters N and sN according to the correlation lg k (20 °C) = sN(E + N) for both nucleophilic sites. Generally, O-attack of benzhydrylium ions is faster than C-attack. However, the initially formed benzhydryloxyvinylphosphonium ions can only be observed by NMR spectroscopy when benzhydryl cations with high Lewis acidity are employed. In other cases, rearrangement to the thermodynamically more stable products arising from C-attack occurs. The results derived from our investigations are employed to rationalize the behavior of ambident nucleophiles 1 and 2 in reactions with carbon-centered electrophiles in general. It is shown that the principle of hard and soft acids and bases (HSAB) and the related Klopman–Salem concept of charge and orbital control lead to incorrect predictions of regioselectivity. We also show that the rate of the Wittig reaction of ylide 2 with aldehyde 14 is significantly faster than the rate of either C- or O-attack calculated using lg k (20 °C) = sN(E + N), thus indicating that the oxaphosphetane is formed by a concerted [2 + 2] cycloaddition.
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Keywords
Phosphonium ylide , Nucleophilicity , Reactivity , Ambident nucleophile , Marcus theory
Citation
Byrne, P. A., Karaghiosoff, K. and Mayr, H. (2016) 'Ambident Reactivity of Acetyl- and Formyl-Stabilized Phosphonium Ylides', Journal of the American Chemical Society, 138(35), pp. 11272-11281. doi: 10.1021/jacs.6b06264
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http://pubs.acs.org/doi/abs/10.1021/jacs.6b06264
URI
https://hdl.handle.net/10468/7703
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Chemistry - Journal Articles
Copyright
© 2016 American Chemical Society. This document is the Accepted Manuscript version of a Published Work that appeared in final form in Journal of the American Chemical Society, copyright © American Chemical Society after peer review and technical editing by the publisher. To access the final edited and published work see https://pubs.acs.org/doi/abs/10.1021/jacs.6b06264#