Carbenoids, definition

The study of the Schlenk equilibrium for organozinc compounds represents a major chapter in the understanding of these reagents in general [26]. Before elaborating the studies on zinc carbenoids, it is appropriate to briefly review the definitive investigations on organozinc halides themselves. 

Before studying some examples more closely, let us consider some cases which are not listed in Table 13. There are numerous compounds SnX2 which are definitely monomeric but are nevertheless no carbene analogs since their valence electron number at the tin atom is at least eight. These compounds contain chelating ligands which can stabilize the carbenoid tin atom due to intramolecular Lewis acid-base interactions as shown by structure A and B (see also Chapter 3). 

It is known that the oxidation potentials of diazodiphenylmethane and Cu(I) in acetonitrile are very similar. With CuBr2 however, no radical-chain reaction takes place. Contrary to the copper perchlorates, CuBr2 and CuBr initiate identical reaction pathways involving copper carbenoids. No definite answer to this discrepancy is available 402). 

B uilding on the original proposal by Yates, the mechanism of this reaction is believed to involve the formation of copper carbenoids as intermediates, Scheme 1. Beyond the fact that copper, its ligands, the carbenoid fragment, and alkene are involved in the stereochemistry-determining event, as evidenced by Noyori et al. (2) and later by Moser (11, 12), little definitive mechanistic information has been acquired for this process. The basics of the mechanism will be discussed in this section. In subsequent sections detailing enantioselective variants, specific factors that have added to the understanding of this reaction will be addressed as will the models used to rationalize the observed stereochemistry. 

Whereas carbenoid character is definitely present in metalated alkyl vinyl ethers, lithiated alkyl and aryl vinyl sulfides and thioesters, which are easily available by hydrogen-lithium exchange, do not display carbenoid-typical reactions . They rather behave like nucleophilic reagents, so that their discussion is beyond the scope of this overview despite their utility in synthesis The same appiies to various derivatives of enamines, deprotonated in the vinyiic a-nitrogen position - . 

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. 

The photolysis of diazoalkanes both in the gas phase and in solution is a carbenoid reaction. Moreover, the results of EPR-spectroscopic investigations (Section IIB) demonstrate that triplet carbenes can be generated by irradiation of diazoalkanes. That the reactive intermediates in carbenoid reactions are free carbenes is usually taken as self-evident. While such an assumption is probably wholly justified in most cases, it is worth remembering that both in the gas phase and in solvents such as n-hexane, the electronic absorption spectra of simple diazoalkanes show definite fine structure (Bradley etal., 1964a). This implies that the photo-excited state is bonding (Hoffmann, 1966) and consequently may have a life-time long enough to enable it to react directly with another molecule... 

Unambiguous identification of r 2-vinyl ligands is often possible based on 13C-NMR spectra other less definitive spectroscopic probes may not reveal the presence of a metallocyclopropene. The carbenoid character of Ca is reflected in low-field chemical shifts of 230-290 ppm, whereas the four-coordinate Cp signal appears at much higher field, typically near 20-30 ppm (169). For comparison consider the chemical shifts of 151 and 128 ppm for the Ca and Cp carbons of the rj vinyl ligand in CpL3Mo-(V-CHCHBu ) [L = P(OMe)3] (170). 

The lack of reactivity of the stable carbenoids explains why no practically useful applications were found for these compounds, but they afford one of the most significant and fundamental contributions in the carbene field by definitively corroborating the existence and structure of these entities. 

The discovery of the participation of carbenoids in major industrial reactions has definitively established the importance and interest of these studies. 

One important class of cluster catalysts are dimetallic catalysts such as Rh2(carboxylate)4, which effect additions and insertions of carbenoids derived from diazoalkanes. This class is not included below because the strict definition of a cluster as containing three or more metal atoms has been adopted. This area of homogeneous catalysis has been reviewed recently " and is treated in the current edition concerning transition metals in organic synthesis. 

As mentioned by Doering and Knox (1956, footnote 9), the present meaning of the name carbene was collaboratively conceived by Doering, Winstein, and Woodward in a nocturnal Chicago taxi and later (1951) delivered diurnally in Boston . Carbenoids have been defined by Closs and Moss (1964) as intermediates that exhibit reactions qualitatively similar to those of carbenes without necessarily being free divalent carbon species . As mentioned already, this definition is applicable first of all to transition metal complexes of carbenes. 

Recently, some significant advances have been made that will likely have a major impact on the mechanistic understanding of these transformations. A stable rhodium-carbenoid complex has been characterized by X-ray crystallography [64]. As these systems are also capable of inducing catalytic carbenoid transformations, the X-ray crystallographic data lead to definitive information about the key metal carbenoid intermediate in catalytic reactions. 

In recent years dissociation of olefins other than tetraaminoethylenes into carbenoid halves has repeatedly been suggested or alleged to occur Steiic hindrance in the olefins and/or unusual ability of the substituents to stabilize divalent carbon have been offered as explanations for the proposed fragmentations. Though definitive contrary... 

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