Cationic carbene complex

If organic acids such as MeCOOH are used in place of HC1, cationic carbene complexes are formed (39), e.g.,... 

Lithiated thiazols react with (Ph3P)AuGl to a neutral gold thiazolate, which upon alkylation is transformed into the cationic carbene complex, while the reaction with (tht)AuC6Fs followed by alkylation leads to the neutral complex (Scheme 65).260... 

Oxidative decomplexation of the dialkylated organic ligand to provide a butyrolactone is accomplished by successive treatment of the cationic carbene complex with hydroxide followed by bromine96. 

Several carbene complexes prepared by the first route are sufficiently stable to be isolated and characterized. However, the cationic complexes prepared by the second and third methods are quite reactive species that usually cannot be isolated and for which only partial characterization has been possible through special spectroscopic techniques. Exceptions are those complexes bearing substituents R or R1 that are good cation-stabilizing groups. Consequently, these cationic carbene complexes are most commonly generated in situ from their precursors in the presence of the substrates desired for further reactions. 

Alkoxide anion abstraction by a Lewis acid from phosphorus ligand on a transition metal is one of the most useful methods to prepare cationic phosphenium complexes. It has been reported that OR anion abstraction from an alkoxyalkyl ligand, C(OR)R2, on a transition metal complex leads to the formation of a cationic carbene complex.39-42 It is similarly expected that OR anion abstraction from an alkoxysilyl ligand, Si(OR)R2, would cause the formation of a cationic silylene complex. Therefore, increased attention has been focused on the reaction with a Lewis acid of transition metal complexes bearing both alkoxyalkyl and phosphite ligands and of complexes bearing both alkoxysilyl and phosphite ligands. 

Stereoselectivity) is observed however, for ethylidene complexes of Fe(CO)(PR3)Cp (69) the products reflect trans selectivity. This difference in stereoselectivity has been suggested to be dependent upon which conformer is more reactive. The reaction of a chiral-at-iron cationic carbene complex (70) with styrene or vinyl acetate affords optically active cyclopropane products with high enantioselectivity (Scheme 24). h >3 intramolecular cyclopropanation, as in the case of (71), proceeds moderately well for the formation of norcarane-type ring systems however, intramolecular C-H insertion is a competing pathway when the alkene is highly... 

The reaction of polymeric beryllium chloride (BeCl2) with nucleophilic carbene (1) (R = Me, R = H) is reported (equation 2) to yield a cationic carbene complex (10). The structure of the cation in (10) consists of the beryllimn center adopting a distorted tetrahedral coordination. Although... 

Carbyne Complexes by Deprotonatlon of Cationic Carbene Complexes. 

Similar intermediate formation of cationic carbene complexes, involving phosphine-induced a-hydrogen abstraction to afford carbyne complexes, is also possible. However, evidence of such intermediates is lacking. 

Reaction of the bridging cyanocarbene complexes 428 [Y = SMe or N(Me)C(0)SMe] with MeS03Cp3 produces the cationic carbene complex 429 [with elimination of MeNCO when Y = N(Me)C(0)SMe] (260). An organosulfur compound (431) is formed in addition to the neutral alkylated complex 432 on alkylation of the trithiocarbonato complex 430 (261). 

The carbyne carbon atoms in both [W(CO)2 HB(pz)3)(CSMe)] (449) (266) and [W(CMe)Cp(CO)2] (450) (267) are nucleophilic and react with protons and the methylthio cation, SMe , to form cationic carbene complexes. Whereas the first type of carbene complex is typically electrophilic (257), the latter one, 451, is nucleophilic, and treatment with trifluoroace-tic acid produces a cationic metallathia cyclopropane complex (452) (268). 

In a general (and generalizing) view on this issue it can be stated that suitably selected ionic liquids are very likely to form catalytically active ionic catalyst solutions with a given transition metal catalyst if the latter is neither extremely electrophilic (acidic) nor extremely nucleophilic (basic). While extremely electrophilic catalyst complexes are likely to coordinate strongly even with those anions of the ionic liquid solvent which are generally regarded as weakly coordinating, extremely nucleophilic catalytic centers are likely to react with the ionic liquid s cation. Carbene complex formation by oxidative addition as well as dealkylation of the cation are possible deactivation pathways of the catalyst in such a case. 

Carbene complexes can also react by addition of alkane C-H bonds across the M=Cbond to generate a product containing two metal-carbon single bonds or materials derived from such products. This reaction is the reverse of the a-elimination, which is a common reaction that forms Schrock-t5q5e carbene complexes, as noted in Chapters 3,10, and 13. Such reactions of carbene complexes with the C-H bonds of alkyl groups were first observed to occur intramolecularly, particularly after generation of cationic carbene complexes. " However, intermolecular additions of C-H bonds of arenes ° and alkanes are also known. 

Many carbene complexes undergo nucleophilic attack at the carbene carbon. This chemistry is presented in more detail in Chapter 13 on metal-ligand multiple bonds. In brief, cationic carbene complexes tend to imdergo simple addition processes to generate neutral products (Equation 11.8), whereas neutral complexes can react by either addition (Equation 11.9) or by a sequence of addition and elimination reactions (Equation 11.10). Fischer has shown that the aminolysis of alkoxycarbene complexes occurs by initial attack at the carbene carbon by a mechanism similar to that for the aminolysis of organic esters. Although less studied than reactions of carbenes with nucleophiles, reactions of carbyne complexes with nucleophiles are also known, and these reactions generate carbene complexes. ... 

Alternative routes to the same or related carbene complexes also involve electrophilic abstractions at the a-carbon. As shown in Equations 12.38 and 12.39, trimethylsilyl triflate can be used to abstract alkoxo or hydroxo groups from the a-carbon to generate cationic carbene complexes. 

To further complicate the picture, the related isocyanide-substituted cation, [Os(=Cp-tolyl)(CNp-tolyl)(CO)(PPh3)2], undergoes a fast thermal reaction with HCl as depicted in Scheme 3. The cationic carbene complex produced is converted by treatment with LiEt3BH to the neutral substituted-benzyl complex labelled B in Scheme 3. The overall two-step transformation of A into B is a good example of the conversion of a metal-carbon triple bond to a metal-carbon single bond by sequential addition of "H+ followed by "H-" [11]. 

Weaker acids also add to Os(=CR)Cl(CO)(PPh3)2, for example, acetic acid with Os(sCp-tolyl)Cl(CO)(PPh3)2 gives the cationic carbene complex with a dihapto acetato-Ugand, [Os(Ti2-02CCH3)(=CHp-tolyl)(CO)(PPh3)2]+ [12]. 

The O-alkylation of acyl mercurials produces cationic carbene complexes (Gerhart and Schollkopf, 1967). Similar procedures have been used to prepare... 

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