Stevens rearrangement
{{distinguish|Stephen aldehyde synthesis}}
{{Use dmy dates|date=April 2023}}
The Stevens rearrangement in organic chemistry is an organic reaction converting quaternary ammonium salts and sulfonium salts to the corresponding amines or sulfides in presence of a strong base in a 1,2-rearrangement.{{cite book | author = Pine SH | year = 2011 | title = The Base-Promoted Rearrangements of Quaternary Ammonium Salts. Organic Reactions | journal = Organic Reactions | pages = 403–464 | doi = 10.1002/0471264180.or018.04 | isbn = 978-0471264187 }}
Image:Stevens rearrangement overview.svg
The reactants can be obtained by alkylation of the corresponding amines and sulfides. The substituent R next the amine methylene bridge is an electron-withdrawing group.
The original 1928 publication by Thomas S. Stevens{{cite journal |vauthors=Stevens TS, Creighton EM, Gordon AB, MacNicol M | year = 1928 | title = CCCCXXIII.—Degradation of quaternary ammonium salts. Part I | journal = J. Chem. Soc. | pages = 3193–3197 | url =http://pubs.rsc.org/en/content/articlelanding/1928/jr/jr9280003193 | doi = 10.1039/JR9280003193 }} concerned the reaction of 1-phenyl-2-(N,N-dimethylamino)ethanone with benzyl bromide to the ammonium salt followed by the rearrangement reaction with sodium hydroxide in water to the rearranged amine.
Image:Stevens1928rearrangement.png
A 1932 publication{{cite journal | last1 = Stevens | first1 = T.S. | display-authors = etal | year = 1932 | title = 8. Degradation of quaternary ammonium salts. Part V. Molecular rearrangement in related sulphur compounds| doi = 10.1039/JR9320000069 | journal = J. Chem. Soc. | page = 69 }} described the corresponding sulfur reaction.
Reaction mechanism
The reaction mechanism of the Stevens rearrangement is one of the most controversial reaction mechanisms in organic chemistry.{{cite journal | last1 = Bhakat | first1 = S | year = 2011 | title = The controversial reaction mechanism of Stevens rearrangement: A review | journal = J. Chem. Pharm. Res. | volume = 3 | issue = 1| pages = 115–121 }} Key in the reaction mechanismM B Smith, J March. March's Advanced Organic Chemistry (Wiley, 2001) ({{ISBN|0-471-58589-0}})Strategic Applications of Named Reactions in Organic Synthesis Laszlo Kurti, Barbara Czako Academic Press (4 March, 2005) {{ISBN|0-12-429785-4}} for the Stevens rearrangement (explained for the nitrogen reaction) is the formation of an ylide after deprotonation of the ammonium salt by a strong base. Deprotonation is aided by electron-withdrawing properties of substituent R. Several reaction modes exist for the actual rearrangement reaction.
A concerted reaction requires an antarafacial reaction mode but since the migrating group displays retention of configuration this mechanism is unlikely.
In an alternative reaction mechanism the N–C bond of the leaving group is homolytically cleaved to form a di-radical pair (3a). In order to explain the observed retention of configuration, the presence of a solvent cage is invoked. Another possibility is the formation of a cation-anion pair (3b), also in a solvent cage.
Scope
Competing reactions are the Sommelet-Hauser rearrangement and the Hofmann elimination.
In one application a double-Stevens rearrangement expands a cyclophane ring.Macrocycle Ring Expansion by Double Stevens RearrangementKeisha K. Ellis-Holder, Brian P. Peppers, Andrei Yu. Kovalevsky, and Steven T. Diver Org. Lett.; 2006; 8(12) pp. 2511–2514; (Letter) {{doi|10.1021/ol060657a}} The ylide is prepared in situ by reaction of the diazo compound ethyl diazomalonate with a sulfide catalyzed by dirhodium tetraacetate in refluxing xylene.
Enzymatic reaction
Recently, γ-butyrobetaine hydroxylase,{{cite journal |vauthors=((Leung IKH)), Krojer TJ, Kochan GT, Henry L, von Delft F, ((Claridge TDW)), Oppermann U, McDonough MA, Schofield CJ | title = Structural and mechanistic studies on γ-butyrobetaine hydroxylase | journal = Chem. Biol. | volume = 17 | issue = 12 | pages = 1316–24 |date=December 2010 | pmid = 21168767 | doi = 10.1016/j.chembiol.2010.09.016 | doi-access = free }}{{cite journal |vauthors=Tars K, Rumnieks J, Zeltins A, Kazaks A, Kotelovica S, Leonciks A, Sharipo J, Viksna A, Kuka J, Liepinsh E, Dambrova M | title = Crystal structure of human gamma-butyrobetaine hydroxylase | journal = Biochem. Biophys. Res. Commun. | volume = 398 | issue = 4 | pages = 634–9 |date=August 2010 | pmid = 20599753 | doi = 10.1016/j.bbrc.2010.06.121 }} an enzyme that is involved in the human carnitine biosynthesis pathway, was found to catalyze a C-C bond formation reaction in a fashion analogous to a Stevens type rearrangement.{{cite journal |vauthors=Henry L, ((Leung IKH)), ((Claridge TDW)), Schofield CJ | title = γ-Butyrobetaine hydroxylase catalyses a Stevens type rearrangement | journal = Bioorg. Med. Chem. Lett. | volume = 22 | issue = 15 | pages = 4975–4978 |date=Aug 2012 | pmid = 22765904 | doi = 10.1016/j.bmcl.2012.06.024 }} The substrate for the reaction is meldonium.{{cite journal |vauthors=Simkhovich BZ, Shutenko ZV, Meirena DV, Khagi KB, Mezapuķe RJ, Molodchina TN, Kalviņs IJ, Lukevics E | title = 3-(2,2,2-Trimethylhydrazinium)propionate (THP)--a novel gamma-butyrobetaine hydroxylase inhibitor with cardioprotective properties | journal = Biochem. Pharmacol. | volume = 37 | issue = 2 | pages = 195–202 |date=January 1988 | pmid = 3342076 | doi =10.1016/0006-2952(88)90717-4 }}