Metal carbenoid complexes

The detailed mechanism of transition metal-catalyzed cyclopropanation using diazo compounds as a carbene source is still covered by clouds of controversy, but it is generally accepted that the reaction proceeds through metal-carbenoid complexes,17-21 and the valency of the metal ions (M) changes with carbenoid formation (Scheme 85). 

Beckhaus, R. C2 building blocks in the co-ordination sphere of electron-poor transition metals. Aspects of the chemistry of early-transition-metal carbenoid complexes. J. Chem. Soc., Dalton Trans. 1997, 1991-2001. 

It has been widely accepted that the carbene-transfer reaction using a diazo compound and a transition metal complex proceeds via the corresponding metal carbenoid species. Nishiyama et al. characterized spectroscopically the structure of the carbenoid intermediate that underwent the desired cyclopropanation with high enantio- and diastereoselectivity, derived from (91).254,255 They also isolated a stable dicarbonylcarbene complex and demonstrated by X-ray analysis that the carbene moiety of the complex was almost parallel in the Cl—Ru—Cl plane and perpendicular to the pybox plane (vide infra).255 These results suggest that the rate-determining step of metal-catalyzed cyclopropanation is not carbenoid formation, but the carbene-transfer reaction.254... 

For application in organic synthesis, the regiochemistry of insertion of carbenoids into un-symmetrical zirconacydes needs to be predictable. In the case of insertion into mono- and bicydic zirconacydopentenes where there is an wide variety of metal carbenoids insert selectively into the zirconium—alkyl bond [48,59,86], For more complex systems, the regiocon-trol has only been studied for the insertion of lithium chloroallylides (as in Section 3.3.2) [60]. Representative examples of regiocontrol relating to the insertion of lithium chloroal-lylide are shown in Fig. 3.2. 

There are no mechanistic details known from intermediates of copper, like we have seen in the studies on metathesis, where both metal alkylidene complexes and metallacyclobutanes that are active catalysts have been isolated and characterised. The copper catalyst must fulfil two roles, first it must decompose the diazo compound in the carbene and dinitrogen and secondly it must transfer the carbene fragment to an alkene. Copper carbene species, if involved, must be rather unstable, but yet in view of the enantioselective effect of the ligands on copper, clearly the carbene fragment must be coordinated to copper. It is generally believed that the copper carbene complex is rather a copper carbenoid complex, as the highly reactive species has reactivities very similar to free carbenes. It has not the character of a metal-alkylidene complex that we have encountered on the left-hand-side of the periodic table in metathesis (Chapter 16). Carbene-copper species have been observed in situ (in a neutral copper species containing an iminophosphanamide as the anion), but they are still very rare [9],... 

Functionalization of C-H bonds by metal carbenoid or nitrenoid insertions represents a promising alternative to the more traditional approach of direct activation by a metal center. Carbenoids and nitrenoids show unusual regio- and stereoselectivity in insertions into C-H bonds, and unlike insertions of metal centers, these are intrinsically functionalizations rather than activations, which must be followed by functionalization (although in either case, loss of the functionalized group, to regenerate the active metal complex, is still required for catalysis) [129]. The use of dimeric Rh(n) complexes in this area has been extensively developed [129]. 

Carbenes and carbenoids have long been recognized as a highly reactive species and are frequently used as intermediates in organic synthesis. From a synthetic perspective, however, most of the carbenes are relatively short-lived and are too reactive to be controlled. Recently, metal-carbene complexes (or metaUocarbenes) were found to be easier to control and are nowadays widely used in organic synthesis . 

The mechanism has not been fully clarified, but pure carbenes can be excluded, and a metal carbenoid is likely to be involved. The following results may be interpreted to indicate a possible complexation of the active species by hydroxy groups leading to reaction on the same face as this substituent. This would only be possible if an organozinc reagent is present. 

Related to metal complexes are metal carbenoids such as those formed when zinc reacts with di-iodomethane. In early examples, such as the efficient cyclopropanation of cyclohexenone 54, the zinc was activated by some copper.13 The active reagent is the zinc a-complex 56. One might suppose an a-elimination 57 would occur to give the carbene, but this is apparently not so. The active reagent 56, nearly but not quite a carbene, is known as a carbenoid. 

Further restrictions to the scope of the present article concern certain molecules which can in one or more of their canonical forms be represented as carbenes, e.g. carbon monoxide such stable molecules, which do not normally show carbenoid reactivity, will not be considered. Nor will there be any discussion of so-called transition metal-carbene complexes (see, for example, Fischer and Maasbol, 1964 Mills and Redhouse, 1968 Fischer and Riedel, 1968). Carbenes in these complexes appear to be analogous to carbon monoxide in transition-metal carbonyls. Carbenoid reactivity has been observed only in the case of certain iridium (Mango and Dvoretzky, 1966) and iron complexes (Jolly and Pettit, 1966), but detailed examination of the nature of the actual reactive intermediate, that is to say, whether the complexes react as such or first decompose to give free carbenes, has not yet been reported. A chromium-carbene complex has been suggested as a transient intermediate in the reduction of gfem-dihalides by chromium(II) sulphate because of structural effects on the reaction rate and because of the structure of the reaction products, particularly in the presence of unsaturated compounds (Castro and Kray, 1966). The subject of carbene-metal complexes reappears in Section IIIB. 

In addition to participating in [2 + l]-cycloaddition reactions, divalent reactive intermediates can form ylides in the presence of carbonyl or other Lewis basic functionalities.108 These ylides participate in cycloaddition or other pericyclic reactions to furnish products with dramatically increased complexity. While carbenes (or metal carbenoids) are well known to participate in these pericyclic reactions, silylenes, in contrast, were reported to react with aldehydes or ketones to form cyclic siloxanes109,110 or enoxysilanes.111,112 Reaction of silylene with an a,p-unsaturated ester was known to produce an oxasilacyclopentene.109,113,114 By forming a silver silylenoid reactive intermediate, Woerpel and coworkers enabled involvement of divalent silylenes in pericyclic reactions involving silacarbonyl ylides115 to afford synthetically useful products.82,116,117... 

As mentioned above, the electrophilic metal carbene complexes are stabilised by the presence of heteroatoms or phenyl rings at the divalent carbon atom, while hydrogen or alkyl groups stabilise the nucleophilic complexes. Therefore, there is a distinction between carbenoids and alkylidenes when designing carbene ligands corresponding to the former or the latter class. 

Optically active metal complexes have been recognized as excellent catalysts for the enantioselective cyclopropanation of carbenes with alkenes. Normally, diazo compounds react under metal catalysts in the dark to afford carbenoid complexes as key intermediates. Katsuki et al. have reported the ds-selective and enantioselective cyclopropanation of styrene with a-diazoacetate in the presence of optically active (R,R)-(NO + )(salen)ruthenium complex 80, supported under illumination (440 nm light or an incandescent bulb) [59]. The irradiation causes dissociation of the apical ligand ON + in 80, and thus avoids the splitting of nitrogen from the a-diazoacetate. 

Addition of diazo compounds to metallic complexes allows the formation of metal carbenoid species which can react with unsaturated molecules to form C-C or C=C bonds. 

One of the attractions of dirhodium paddelwheel complexes is their ability to catalyse a wide variety of organic transformations such as C-H insertions, cyclopropanations and ylide formation. A review on the application of high symmetry chiral Rh2(II,II) paddlewheel compounds highlights their application as catalysts for asymmetric metal carbenoid and nitrenoid reactions, and as Lewis acids.59 Their impressive performance as catalysts in C-H functionalisation reactions has been exploited in the synthesis of complex natural products and pharmaceutical agents. A recent review on catalytic C-H functionalisation by metal carbenoid and nitrenoid insertion demonstrates the important role of dirhodium species in this field.60... 

Carbenes are formally divalent carbon species and as such are very reactive. Certain transition metals stabilize the carbene. The metal arbene complexes which result, some of which are stable, are called "carbenoids . since an actual carbene is no longer present. 

Beckhaus R (1997) Carbenoid complexes of electron-deficient transition metals - Syntheses of and with short-lived building blocks. Angew Chem Int Ed 36 687... 

Of the three classes of divalent carbon species—free carbenes, reactive transition-metal carbene complexes (carbenoids), and stable metal carbenes—we restrict our consideration to the first two. Taking into account the fact that a fine reaction mechanism in planning the synthesis of specific molecules is of secondary importance, we discuss carbene and carbenoid reactions together. We concentrate solely on reactions and reaction sequences that result in a formation of a new heterocycle. Within subsections, the material is organized on the basis of reaction type. 

As already mentioned for rhodium carbene complexes, proof of the existence of electrophilic metal carbenoids relies on indirect evidence, and insight into the nature of intermediates is obtained mostly through reactivity-selectivity relationships and/or comparison with stable Fischer-type metal carbene complexes. A particularly puzzling point is the relevance of metallacyclobutanes as intermediates in cyclopropane formation. The subject is still a matter of debate in the literature. Even if some metallacyclobutanes have been shown to yield cyclopropanes by reductive elimination [15], the intermediacy of metallacyclobutanes in carbene transfer reactions is in most cases borne out neither by direct observation nor by clear-cut mechanistic studies and such a reaction pathway is probably not a general one. Formation of a metallacyclobu-tane requires coordination both of the olefin and of the carbene to the metal center. In many cases, all available evidence points to direct reaction of the metal carbenes with alkenes without prior olefin coordination. Further, it has been proposed that, at least in the context of rhodium carbenoid insertions into C-H bonds, partial release of free carbenes from metal carbene complexes occurs [16]. Of course this does not exclude the possibility that metallacyclobutanes play a pivotal role in some catalyst systems, especially in copper-and palladium-catalyzed reactions. 

---