Carbene-copper complexes

Chiral diaminocarbene complexes of copper were used in asymmetric conjugate addition of diethylzinc to Michael acceptors. Achiral copper carbene complexes derived from imidazolium salts were synthesized and characterized for the first time by Arduengo in 1993 [43]. In 2001, Woodward reported the use of such Arduengo-type carbene in copper-catalyzed conjugate addition and showed their strong accelerating effect [44]. The same year, Alex-... 

This group showed that isolable silver(I) diaminocarbene complexes can be use in situ instead of free carbenes, to generate the copper carbene complex. The silver salts that precipitates during the formation of the copper complex have not any negative effect on the conversion. This method is advantageous since most of the silver complexes are isolable, air-stable and easily obtained by treatment of the corresponding imidazohnium salt by 0.5 equiv of silver oxide (Scheme 53). The solid structure of 78 was analyzed by X-ray diffraction. 

Figure 23 X-ray structures of the copper carbene complexes 53 and 54 and geometry of the molecules of 55. 54 reproduced with permission from ACS publications. 

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],... 

Deprotonation of the imidazolium salt [HsTIME - KPEels with three equivalents of potassium fert-butoxide produced the free carbene TIME " " (2 ), which was isolated and fully characterized (28). Addition of one equivalent of 2 to [(CH3CN)4Cu](PF6) affords the copper carbene complex [(TIME ")2Cu2](PF6)2 (2 -Cu)... 

More recently, Naidu and West have utilized a ring expansion reaction of spiro azetidinium ylide 167 in the synthesis of pyrrolizidine alkaloids. Spiro azetidinium ylide 167 is generated through a Cu(acac)2-catalyzed intramolecular reaction of a copper carbene complex with a pendant amino moiety. Subsequent [l,2]-shift gives fused bicyclic products 168 and 169 as a diastereomeric mixture. Each diastereomer was further converted to naturally occurring pyrrolizidines ( )-turneforcidine and ( )-platynecine, respectively (Scheme 18). ... 

The Cu(I)-catalyzed decomposition of (alkynyloxysilyl)diazoacetates 119 furnishes the silaheterocycles 120 and/or 121 (equation 30) in modest yield63. In these cases, the photochemical extrusion of nitrogen from 119 does not lead to defined products and the thermal reaction is dominated by the 1,3-dipolar cycloaddition ability of these diazo compounds. In mechanistic terms, carbene 122 or more likely a derived copper carbene complex, is transformed into cyclopropene 123 by an intramolecular [1 + 2] cycloaddition to the triple bond. The strained cyclopropene rearranges to a vinylcarbene either with an exo-cyclic (124) or an endocyclic (125) carbene center, and typical carbene reactions then lead to the observed products. Analogous carbene-to-carbene rearrangements are involved in carbenoid transformations of other alkynylcarbenes64. 

Fig. 3.18. Mechanistic details on the transition-metal catalyzed (here Cu-catalyzed) cyclopropanation of styrene as a prototypical electron-rich alkene. The more bulky the substituent R of the ester group C02R, the stronger is the preference of transition state A over D and hence the larger the portion of the trans-cyclo-propane carboxylic acid ester in the product mixture.—The zwitterionic resonance form B turns out to be a better presentation of the electrophilic character of copper-carbene complexes than the (formally) charge-free resonance form C or the zwitterionic resonance form (not shown here) with the opposite charge distribution ( a to the C02R substituent, on Cu) copper-carbene complexes preferentially react with electron-rich alkenes.

The effect of ligands on the selectivity of carbenes generated by copper-catalyzed decomposition of diazo precursors has been recognized for a long time as proof of the participation of copper-carbene complexes (carbenoids) in these processes. 

Copper carbene complexes with iV-heterocyclic ligands as advanced catalysts 03AG(E)1088. 

Cyclocodimerization and cyclooligomerization reactions of cyclopropenes catalyzed by nickel complexes require alkenes with electron-withdrawing substituents as reaction partners. In this respect, these reactions are complementary to the copper-catalyzed additions discussed in the previous section which do not proceed with electron-poor alkenes due to the low nucleophilicity of the copper-carbene complex. 

The triacylcyclopropanes, which are, formally speaking, trimers of the ylidic C, unit are rather common in sulfonium ylide reactions (see Houben-Weyl, Vol. Ell, p 1414). Several mechanistic pathways leading to these products may apply,and depending on the decomposition mode for the sulfonium ylide, individual mechanistic steps may be different. The overall picture of the photochemical or copper-catalyzed decomposition, however, can be described as follows. The sulfonium ylide 4 is decomposed to give an oxocarbene 5 or a related copper carbene complex 6. These short-lived intermediates react with another sulfonium ylide molecule to form an 1,2-diacylethene 7 which most likely combines with the sulfonium ylide and yields the triacylcyclopropane in a Michael addition/ring-closure sequence. 

TOWARDS THE SYNTHESIS OF CARBINE COMPLEXES OF GOLD AND COPPER CARBENE COMPLEXES... 

Transition metal salts or complexes are known to catalyze effectively the cyclopropanation of olefins with diazoalkanes. Asymmetric synthesis with chiral copper catalysts (Nozaki et ai, 1966, 1968 Noyori et al., 1969 Moser, 1969), as well as a detailed kinetic study (Salomon and Kochi, 1973), has suggested the intervention of copper-carbene complexes as reactive intermediates. Recently synthesis of crysanthemic acid (CCXXXIV) (R = H) with high optical yield (60-70 %) has been achieved by applying this asymmetric catalysis (Aratani et al., 1975). The camphorglyoxime-cobalt(I) complex is also effective for the enantioselective reaction (Tatsuno et al., 1974). 

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