Carbene classification

Carbene reactivity is strongly affected by substituents.117 Various singlet carbenes have been characterized as nucleophilic, ambiphilic, and electrophilic as shown in Table 10.2 This classification is based on relative reactivity toward a series of both nucleophilic alkenes, such as tetramethylethylene, and electrophilic ones, such as acrylonitrile. The principal structural feature that determines the reactivity of the carbene is the ability of the substituent to act as an electron donor. For example, dimethoxycarbene is devoid of electrophilicity toward alkenes because of electron donation by the methoxy groups.118... 

Table 10.2. Classification of Carbenes on the Basis of Reactivity toward Alkenesa... 

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. 

It is well known that metal carbenes can be classified as Fisher and Schrock carbenes. The classification is mainly based on the n electron density distribution on the M = C moiety (Scheme 4.2). On the basis of the n electron density distribution, carbene complexes of the Fisher-type (E) are normally electrophilic at the carbene carbon while carbene complexes of the Schrock-type (F) are nucleophilic at the carbene carbon. Similarly, metal vinylidenes could also be classified into the two types Fisher-type (G) and Schrock-type (H). The majority of isolated metal vinylidenes belong to the Fisher-type. On the basis of the 7t electron density distribution shown in... 

For a long time metal carbenes have been either classified as Fischer- or Schrock carbenes, depending on the oxidation state of the metal. Since the introduction of N-heterocycHc carbene complexes this classification needs to be extended because of the very different electronic character of these ligands. Carbenes—molecules with a neutral dicoordinate carbon atom—play an important role in all fields of chemistry today. The first examples in the field of organic chemistry were published by Doering and Hoffmann in the 1950s [1], while Fischer and Maasbol introduced them to organometallic chemists about ten years later [2,3]. But it took another 25 years until the first carbenes could be isolated [4-8]. 

Figure 14.9. (a) Orbital correlation diagram for the direct insertion of carbene into an olefin to form cyclopropane. Symmetry classification is with respect to the vertical bisecting mirror plane. (b) State correlation diagram showing the intended correlations and the avoided crossing of states So and S2. 

Classification of carbene complexes as Fischer or Schrock perhaps focuses too much on their differences and too little on their similarities. Both contain a metal-carbon bond of order greater than one. Whether the carbene carbon tends to seek or provide electrons will depend on the extent of it bonding involving the metal and the carbon substituents. Some carbene complexes lie between the Fischer/Schrock extremes, behaving in some reactions as nucleophiles and in others as electrophiles.69... 

In this section, the classification of carbenes and carbenoids bearing two different substituents is made by giving priority to the functional group which will appear in the later... 

The cycloaddition of phospholes with alkynes to synthesize phosphorins comes under this classification. For phosphorins to form, the transient 2H-phosphole has to react with alkynes followed by elimination of a carbene. This type of synthetic approach is less common and is discussed in the earlier chapter <1996CHEC-II(5)639>. 

With regard to a mechanistic classification of the known carbene-catalyzed processes, a further subdivison of these two major classes might be useful. Scheme 4 provides an attempt at a mechanism-based classification. 

Finally, there are also some special NHC-mediated transformations that do not completely fit into the classification, such as triazolylidene-catalyzed hydroacylations (Chan and Scheldt 2006). Aldehydes can serve as hydride donors for activated ketones partly following a standard 1,2-addition of the NHC to the aldehyde, but instead of the usual carbonyl umpolung a hydride ( H-umpolung ) transfer is initiated. A related Cannizzaro-type transformation has been described for indazole-derived carbene catalysts (Schmidt et al. 2007). 

The field of carbenes has been intensively investigated for the last two decades b. These studies have shown that carbenes as well as nitrenes and arynes play an important role in organic chemistry as reactive intermediates 2). Not only have these studies made it possible to understand many organic reactions in detail, they have also given use to a simple classification of organic reactions on the basis of these reactive intermediates. 

Since, for secrecy reasons, information on new processes and the state of their development is not always published, or only after long delays, the classification applied or recent developments may be misleading. For example, the potential of phase-transfer catalyzed processes may already be more important than the present literature indicates. The same statement could apply for areas such as amidocarbonylation, the synthesis of fine chemicals by means of metallocenes, the reductive/oxidative carbonylation of aromatic amines or nitro derivatives, Heck coupling using palladacycles and heterocyclic carbene complexes, catalytic McMurry coupling, or other proposed methods. Recent developments must therefore leave open the stage of development reached, perhaps signaling that at the time of publication no commercialized, licensable process is yet known to the scientific community. 

There are several parallels between the chemistries of the carbene and carbyne ligands. The classification of carbyne complexes into two major structural types—Fischer and Schrock—is perhaps the most obvious of these parallels. Complex 64 represents the prototypical Fischer carbyne complex Ccarbyne bonded to a low-valent metal with Ji-accepting CO ligands attached. Structure 65, on the other hand, is a classic example of a Schrock carbyne complex because a high-valent metal is present with electron-donating ligands attached. Atoms attached to Ccaibyne helpful in distinguishing between Fischer and Schrock carbene complexes (i.e., heteroatoms for the former and H and C for the latter), are less important in the case of carbyne complexes. It is convenient to classify carbyne complexes... 

Proposed mechanisms for the initial C-C bond formation may be broadly classified as carbenic [lb,10], carbocationic, involving hypervalent carbon intermediates [4,5], oxoniumylide [6,7], and free radical [8,9], while recognizing that the classification is somewhat arbitrary since many mechanisms embody several of these categories. 

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