The modern interpretation of the Wittig reaction mechanism
The mechanism of the Wittig reaction has long been a contentious issue in organic chemistry. Even now, more than 50 years after its announcement, its presentation in many modern undergraduate textbooks is either overly simplified or entirely inaccurate. In this review, we gather together the huge bo...
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| Published in | Chemical Society reviews Vol. 42; no. 16; pp. 667 - 6696 |
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| Main Authors | , |
| Format | Journal Article |
| Language | English |
| Published |
England
22.07.2013
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| Subjects | |
| Online Access | Get full text |
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| Summary: | The mechanism of the Wittig reaction has long been a contentious issue in organic chemistry. Even now, more than 50 years after its announcement, its presentation in many modern undergraduate textbooks is either overly simplified or entirely inaccurate. In this review, we gather together the huge body of evidence that has been amassed to show that the Li salt-free Wittig reactions of non-stabilised, semi-stabilised and stabilised ylides all occur under kinetic control by a common mechanism in which oxaphosphetane (OPA) is the first-formed and only intermediate. The numerous recent significant additions to the subject including computational studies and experimental material pertinent to both steps of the reaction (OPA formation and its decomposition) are discussed in detail, and the currently accepted explanations for the source of the stereoselectivity in Wittig reactions are given. We also present the other mechanistic proposals that have been made during the history of the Wittig reaction, and show how they are unable to account for all of the experimental evidence that is now available. Details of certain experimental facts to do with Wittig reactions in the presence of Li cation are also included, although the precise mechanistic details of such reactions are yet to be established conclusively. We make the case that a clear distinction should henceforth be made between the unknown Li-present and the now well-established Li salt-free Wittig mechanisms.
Li salt-free Wittig reactions of all ylide types occur under kinetic control, and by a common [2+2] cycloaddition mechanism in which oxaphosphetane is the first-formed and only intermediate. |
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| Bibliography: | Declan G. Gilheany was born in Tyrone, N. Ireland. He studied chemistry at Queens University Belfast and obtained his PhD degree in 1983 in organophosphorus chemistry under the supervision of Dr B. J. Walker, having also been appointed temporary lecturer in organic chemistry during 19801981. From there he became a lecturer in synthetic organic chemistry at St. Patricks College, Maynooth, Ireland. During the period 19881990, he was a Fulbright Visiting Professor at Massachusetts Institute of Technology in the laboratory of Professor K. Barry Sharpless. He moved to his present position at University College Dublin in 1992. Dr Gilheany's main research interests lie in the area of catalytic asymmetric synthesis, especially asymmetric oxidation; structure and mechanism in organophosphorus chemistry, especially the Wittig reaction and the applications of P-chiral compounds in catalysis and the synthesis of small and medium heterocycles. Peter A. Byrne was born in Dublin, Ireland. He studied chemistry at University College Dublin, where he obtained his BSc degree in 2006. He carried out his doctoral studies from 20072011 under the supervision of Declan Gilheany and was awarded his PhD degree for his thesis entitled Investigation of Reactions Involving Pentacoordinate Intermediates: The Mechanism of the Wittig Reaction. Since then, he has worked as a postdoctoral fellow in the group of Prof. Gilheany carrying out research on the chemistry of small ring amines, hydrolysis of phosphonium ylides, and further work on the Wittig reaction. ObjectType-Article-1 SourceType-Scholarly Journals-1 ObjectType-Feature-2 ObjectType-Review-3 content type line 23 |
| ISSN: | 0306-0012 1460-4744 1460-4744 |
| DOI: | 10.1039/c3cs60105f |