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Prof. Kevin Shea (Smith College) 3.3: Rearrangements is shared under a CC BY 4.0 license and was authored, remixed, and/or curated by LibreTexts. This chapter focuses on synthetically useful rearrangements including the pinacol, Payne, benzilic acid, Favorskii, Tiffeneau-Demjanov, Wolff, Curtius, Baeyer-Villager, and Beckmann rearrangements.
- 3.1: Introduction to Neighboring Group Participation ...
The most common rearrangement being a hydride shift, but you...
- 3.1: Introduction to Neighboring Group Participation ...
- The Hofmann and Curtius Rearrangements
- The Key Rearrangement Step in The Hofmann and Curtius
- Formation of The Isocyanate
- Putting The Steps Together
- The Fate of The Isocyanate
- Summary: The Hofmann and Curtius Rearrangements
- Notes
- References and Further Reading
The Hofmann and Curtius rearrangements are two examples of a whole family of rearrangement reactions that share a common mechanistic step. [as do the Beckmann and Wolff rearrangements – see bonus topic 2] In the Hofmann rearrangement, anamide is treated with bromine and base (usually NaOH or KOH). Upon heating, an intermediate isocyanate is formed,...
Now we get down to business. The key step in the Hofmann and Curtius rearrangments is migration of a carbon atom to displace a leaving group on an adjacent nitrogen. This requires two curved arrows to draw, which are shown in the structure on the far left (below). 1. In the first curved arrow, the C-C bond breaks, and a new C-N bond forms. 2. In th...
The next step is going from our rearranged species to the isocyanate. For anyone accustomed to drawing resonance forms, this shouldn’t be too difficult. (As we’ll discuss below, studies have determined that isocyanate formation occurs at the same time as migration. But for our teaching purposes, I think it helps to treat this step in isolation. We ...
Studies into the mechanism of the Hofmann and Curtius rearrangements have determined that these two steps do not happen sequentially; they actually happen at the same time! (how do we know this? see Note 1) That is, the migration happens at the same time as isocyanate formation. We need to redraw the mechanisms of the Hofmann and Curtius to reflect...
Both the Hofmann and Curtius give rise to an isocyanate. You likely haven’t encountered isocyanates before. At first glance, they are pretty strange-looking species, but their chemistry is not so different from that of other carbonyl species such as esters, amides, and acyl halides. The isocyanate structure closely resembles that of carbon dioxide,...
The mechanism of these rearrangments is tricky at first glance, but is made considerably easier once you realize that you’ve seen a variant of the migration reaction before (i.e. in hydroboration-oxidation). I highly recommend drawing the ugly version first, because it will help you focus on seeing the bonds that form and break, before trying to re...
Note 1.Concerted or separate steps? If the Curtius was stepwise, the loss of nitrogen would lead to a nitrene. Nitrenes undergo a host of interesting reactions, such as insertion into C-H bonds (!). So one can design an experiment to test this: if one tries the following Curtius rearrangement, one could look for evidence of formation of a five-memb...
Ueber die Einwirkung des Broms in alkalischer Lösung auf Amide W. Hofmann Chem. Ber. 1881, 14, 2725-2736 DOI:10.1002/cber.188101402242 Original paper by A. W. Hofmann (who was a very productive che...A Mild and Efficient Modified Hofmann Rearrangement Xicai Huang, Mehran Seid, and Jeffrey W. Keillor The Journal of Organic Chemistry 1997, 62 (21), 7495-7496 DOI:10.1021/jo9708553 This is a modifi...METHYL CARBAMATE FORMATION VIA MODIFIED HOFMANN REARRANGEMENT REACTIONS: METHYL N-(p-METHOXYPHENYL)CARBAMATE Jeffrey W. Keillor and Xicai Huang Org. Synth. 2002, 78, 234 DOI: 10.15227/orgsyn.078.02...The Hofmann Reaction Wallis, Everett L.; Lane, John F. Org. React. 1946, 3, 267-306 DOI: 10.1002/0471264180.or003.07 An old but comprehensive (for its time) review on the Hofmann reaction (the amid...The most common rearrangement being a hydride shift, but you likely also saw alkyl shifts. Rearrangements definitely occur from carbocation intermediates (as pictured above); however, the most synthetically useful rearrangements proceed via mechanisms that don't involve true carbocation intermediates.
Oct 17, 2011 · Introduction to Rearrangement Reactions. Introduction to Rearrangement Reactions. Reactions that involve a carbocation intermediate may be accompanied by rearrangements where a pair of electrons from a C-H or C-C bond migrates toward the carbocation, resulting in breakage and formation of a C-H or C-C bond, and formation of a new carbocation.
NH. H. (100%) N. ". . . after ca. 10 seconds, a relatively violent reaction occurred which was accompanied by a dense cloud of white smoke and change in color from the characteristic yellow-green of the starting material to a dark brown."
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The most common migrations are 1,2 rearrangements in which migrating group moves to the adjacent atom. However, longer rearrangements are also possible. The rearrangement reactions are of three types, depending on the number of electrons the migrating atom or group carries along with it. a) Migration with electron-pair (nucleophilic)
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A 1,2-rearrangement is an organic reaction where a substituent moves from one atom to another atom in a chemical compound. In a 1,2 shift the movement involves two adjacent atoms but moves over larger distances are possible. Skeletal isomerization is not normally encountered in the laboratory, but is the basis of large applications in oil ...
