Nitrenes, Carbenes, Diradicals, and Ylides. Interconversions of Reactive Intermediates

Wentrup, Curt (2011) Nitrenes, Carbenes, Diradicals, and Ylides. Interconversions of Reactive Intermediates. Accounts of Chemical Research, 44 6: 393-404. doi:10.1021/ar700198z

Author Wentrup, Curt
Title Nitrenes, Carbenes, Diradicals, and Ylides. Interconversions of Reactive Intermediates
Journal name Accounts of Chemical Research   Check publisher's open access policy
ISSN 0001-4842
Publication date 2011-06
Sub-type Critical review of research, literature review, critical commentary
DOI 10.1021/ar700198z
Volume 44
Issue 6
Start page 393
End page 404
Total pages 12
Place of publication Washington DC, United States
Publisher American Chemical Society
Collection year 2012
Language eng
Abstract Rearrangements of aromatic and heteroaromatic nitrenes and carbenes can be initiated with either heat or light. The thermal reaction is typically induced by flash vacuum thermolysis, with isolation of the products at low temperatures. Photochemical experiments are conducted either under matrix isolation conditions or in solution at ambient temperature. These rearrangements are usually initiated by ring expansion of the nitrene or carbene to a seven-membered ring ketenimine, carbodiimide, or allene (that is, a cycloheptatetraene or an azacycloheptatetraene when a nitrogen is involved). Over the last few years, we have found that two types of ring opening take place as well. Type I is an ylidic ring opening that yields nitrile ylides or diazo compounds as transient intermediates. Type II ring opening produces either dienylnitrenes (for example, from 2-pyridylnitrenes) or 1,7-(1,5)-diradicals (such as those formed from 2-quinoxalinylnitrenes), depending on which of these species is better stabilized by resonance. In this Account, we describe our achievements in elucidating the nature of the ring-opened species and unraveling the connections between the various reactive intermediates. Both of these ring-opening reactions are found, at least in some cases, to dominate the subsequent chemistry. Examples include the formation of ring-opened ketenimines and carbodiimides, as well as the ring contraction reactions that form five-membered ring nitriles (such as 2- and 3-cyanopyrroles from pyridylnitrenes, N-cyanoimidazoles from 2-pyrazinyl and 4-pyrimidinylnitrenes, N-cyanopyrazoles from 2-pyrimidinylnitrenes and 3-pyridazinylnitrenes, and so forth). The mechanisms of formation of the open-chain and ring-contraction products were unknown at the onset of this study. In the course of our investigation, several reactions with three or more consecutive reactive intermediates have been unraveled, such as nitrene, seven-membered cyclic carbodiimide, and open-chain nitrile ylide. It has been possible in some cases to observe them all and determine their interrelationships by means of a combination of matrix-isolation spectroscopy, photochemistry, flash vacuum thermolysis, and computational chemistry. These studies have led to a deeper understanding of the nature of reactive intermediates and chemical reactivity. Moreover, the results indicate new directions for further exploration: ring-opening reactions of carbenes, nitrenes, and cyclic cumulenes can be expected in many other systems.
Keyword Laser flash-photolysis
Nitrile ylides
Thermal interconversion
Q-Index Code C1
Q-Index Status Confirmed Code
Institutional Status UQ

Document type: Journal Article
Sub-type: Critical review of research, literature review, critical commentary
Collections: Official 2012 Collection
School of Chemistry and Molecular Biosciences
Version Filter Type
Citation counts: TR Web of Science Citation Count  Cited 48 times in Thomson Reuters Web of Science Article | Citations
Scopus Citation Count Cited 48 times in Scopus Article | Citations
Google Scholar Search Google Scholar
Created: Sun, 14 Aug 2011, 00:41:48 EST by System User on behalf of School of Chemistry & Molecular Biosciences