Carbenes overview

In 1999, Bob Atkinson wrote [1] that aziridination reactions were epoxida-tion s poor relation , and this was undoubtedly true at that time the scope of the synthetic methods available for preparation of aziridines was rather narrow when compared to the diversity of the procedures used for the preparation of the analogous oxygenated heterocycles. The preparation of aziridines has formed the basis of several reviews [2] and the reader is directed towards those works for a comprehensive analysis of the area this chapter presents a concise overview of classical methods and focuses on modern advances in the area of aziridine synthesis, with particular attention to stereoselective reactions between nitrenes and al-kenes on the one hand, and carbenes and imines on the other. 

It is obvious, that matrix isolation spectroscopy is the method of choice for the study of carbenes. The large number of carbenes identified by matrix isolation illustrates the strength of this analytical tool, as demonstrated by an excellent review on this topic by Sander et al. in 199314. In this comprehensive article all the work carried out until that date has been summarized. The goal of our review is to report on our own contributions to this field and to complete Sander s overview — taking over his classification of individual carbenes — by results achieved by us in this subject matter during the most recent epoch. 

The identification of unknown chemical compounds isolated in inert gas matrices is nowadays facilitated by comparison of the measured IR spectra with those computed at reliable levels of ab initio or density functional theory (DFT). Furthermore, the observed reactivity of matrix isolated species can in some instances be explained with the help of computed reaction energies and barriers for intramolecular rearrangements. Hence, electronic structure methods developed into a useful tool for the matrix isolation community. In this chapter, we will give an overview of the various theoretical methods and their limitations when employed in carbene chemistry. For a more detailed qualitative description of the merits and drawbacks of commonly used electronic structure methods, especially for open-shell systems, the reader is referred to the introductory guide of Bally and Borden.29... 

These observations indicate that there is no sharp borderline between cyclopro-panating and metathesis-catalyzing carbene complexes. Fortunately the number of carbene complexes which mediate both cyclopropanation and alkene metathesis is rather small, and in the detailed overview given in the following sections it will become apparent that most carbene complexes are highly selective and thus valuable reagents for organic synthesis. 

Metal complexes of stable carbenes are now known for almost all the metals of the periodic table. This chapter is divided into an overview of the NHC-main-group metal adducts that are generally synthesized by adding the free NHC to an appropriate metal precursor and a discussion of the various synthetic approaches toward transition metal complexes. 

This example, like those in Chapter 17, dealing with carbonyl hydration and preferential stabilization of singlet or triplet carbenes, places the calculations in the role of data gathering . Unlike these previous examples, the data here take the form of images while they can be used to extract numerical data , can also serve to furnish an overview. 

As a class, nitrenium ions are rather poorly characterized relative to similar reactive intermediates such as carbenes and carbenium ions. This simation alone is sufficient to motivate many fundamental studies into their structures and behavior. There are also several practical considerations that motivate their study. The following is intended as a brief overview of these latter areas. 

Cyclopropanation reactions are one set in an array of C-C bond-forming transformations attributable to metal carbenes (Scheme 5.1) and are often mistakenly referred to by the nonspecific term carbenoid. Both cyclopropanation and cyclopropenation reactions, as well as the related aromatic cycloaddition process, occur by addition. Ylide formation is an association transformation, and insertion requires no further definition. All of these reactions occur with diazo compounds, preferably those with at least one attached carbonyl group. Several general reviews of diazo compounds and their reactions have been published recently and serve as valuable references to this rapidly expanding field [7-10]. The book by Doyle, McKervey, and Ye [7] provides an intensive and thorough overview of the field through 19% and part of 1997. 

NHCs have become ligands almost as common as phosphines. This is widely exemplified in a book edited by Nolan.23 We will therefore limit this overview of carbenes as ligands to unusual applications and significant publications regarding their coordination chemistry. 

Fischer-type carbenes, 8, 218 iV-heterocyclic carbenes, 8, 219 non-heteroatom carbenes, 8, 217 overview, 8, 197—268 photochemistry, 1, 252 with Re(I), 5, 900 with rhodium, 7, 184 rotation, dynamic NMR studies, 1, 420 in Ru and Os half-sandwich preparations, 6, 575 silver-catalyzed insertions, 9, 568 with silver(I), 2, 203 as spectator ligands, 1, 66... 

Cope rearrangement, combined carbene C—H activation—Cope overview, 10, 177 as strategic reaction, 10, 178... 

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