Carbenoid complex

Carbenoid complexes with heterocyclic ligands as catalysts in enantioselective cyclopropanation of olefins 97S137. 

Chiral carbenoid complexes with 2,2 -bipyridine ligands for asymmetric synthesis 98YGK764. 

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). 

Ruthenium-carbenoid complex 1 catalyzed the isomerization of /3,7-unsaturated ethers to the corresponding vinyl ethers. This reaction is useful in the deprotection of allyl and homoallyl ethers (Scheme 39).65... 

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],... 

Another mechanism for benzene formation and parallel combustion is proposed by Germain and Laugier [129]. They suggest that toluene is yr-adsorbed on a surface cation via the nucleus, and then looses two ben-zylic H-atoms to form an o,a(a)-yr-adsorbed carbenoid complex, viz. 

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. 

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

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. 

Cotton, F. A., and C. M. Lukehart, Prog. Inorg. Chem., 1972, Vol. 16 (A comprehensive review of all phases of the chemistry of carbenoid complexes). 

The high dlastereoselectivity with a-hydroxy esters as auxiliaries indicates that a fairly rigid transition state must be involved. As a plausible explanation, the authors propose that the carbonyl group of the auxiliary interacts with the carbenoid complex 8.183 to generate a dipolar complex 8.184 prior to the cyclopropanation step (8-79). The complex is rigid, as only 5-ring but not 6-ring lactones are effective. 

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. 

The use of chiral iron-carbenoid complexes in the preparation of optically active cyclopropanes has been described. ... 

One could imagine generating carbene complexes from typical precursors to car-benes in organic chemistry, such as diazoalkanes. Fewer carbene complexes have been isolated by addition of diazoalkanes to transition metal complexes than by the more indirect methods presented previously, but many carbene or carbenoid complexes have been generated by this method as reactive intermediates. Two examples of carbene complexes that have been isolated from the reaction of a diazoalkane are shown in Equations 13.4a and 13.4b. 

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