Carbene transfer reagents

The employment of silver(i) complexes having a chiral carbene ligand L (Figure 19) and the empirical formula [AgBrL] has been reported for use as both carbene-transfer reagents to the corresponding dichloropalladium(n)... 

In spite of the fact that silver(i) X-heterocyclic carbene complexes were widely employed as carbene-transfer reagents for the synthesis of other transition metal carbene complexes, their synthesis could also be achieved by the reaction of silver salts with relatively more labile carbene metal complexes, albeit rare. Complexes 71a-71c were reported to be synthesized from the reaction of the corresponding pentacarbonyl(carbene)chromium(i) complexes with silver(i) hexafluorophosphate in CDC13 under inert atmosphere (Scheme 17).117... 

Closely related with the synthesis of ylides from carbenes is the use of ylides as carbene transfer reagents (CTR), that is processes in which the ylide is cleaved homolytically, liberating the nucleophile and the carbene, which could remain both coordinated to the metal or not (Scheme 10). Diphosphirane (34) can be obtained from the diphosphene by reaction with sulfur ylide Me2S(0)=CH2, which behave as a carrier of the CH2 unit [95]. Recent work of Milstein et al. shows that sulfur ylides decompose in the presence of Rh derivatives with vacant coordination sites affording Rh(l)-carbene complexes [96, 97]. Complexes (35-37) can be obtained from... 

A very interesting amido functionahsed carbene was prepared by Legault et al. [116] from A-mesitylimidazole and 0-(2,4-dinitrophenyl)hydroxylamine, an electrophilic ami-nation reagent [117]. The exo-amino group is subsequently acylated to afford a zwitterionic amido functionalised carbene (see Figure 4.38). Reaction with silver(l) acetate and sodium carbonate [a rare variant of the silver(I) oxide method] yields the silver(l) carbene complex as a dimer with a Ag-Ag bond. The silver(l) carbene complex can be used as a carbene transfer reagent to synthesise the homoleptic monomeric copper(Il) carbene complex. 

The trihalogenomethyltin compounds act as carbene-transfer reagents towards al-kenes, often giving good yields of the dihalogenocyclopropanes (e.g. equation 6-7),25 but rather little use has been made of these reactions in organic synthesis. 

Phenylmercury derivatives are the best carbene-transfer reagents in this category. 1-Bromo-1,2,2,2-tetrafluoroethyl(phenyl)mercury (1) reacted with alkenes at elevated temperature to give l-fluoro-1-trifiuoromethylcyclopropanes 3 and 4 (X = F). Similarly, thermolysis of 1-bromo-l-chloro-2,2,2-trifluoromethyl(phenyl)mercury (2) in the presence of excess alkene yielded 1-chloro-l-trifluoromethylcyclopropanes 3 and 4 (X = Cl) and occasionally a small amount of 1 -bromo-1 -trifluoromethylcyclopropane 5. 1,1 -Dichloro-2,2,2-trifluoroethyl(phenyl)mer-... 

The ylide is prepared from 2,5-dichlorothiophene and dimethyl diazomalonate under catalysis by tetraacetatodirhodium and may be stored for a long time without special precautions at room temperature. As a carbene-transfer reagent, it is said to compare favorably with dimethyl diazomalonate itself because of usually higher yields and shorter reaction times. 

Electrophilic transition-metal-carbene complexes (Fischer carbene complexes) serve as formal carbene transfer reagents in reactions with alkenes to give functionalized cyclopropanes. This reaction behavior is well documented for alkoxycarbene complexes of elements of group In contrast, aminocarbene complexes exhibit a different reactivity over a wide range of conditions and [2 + 1] cycloadditions to alkenes represent exception. 

Fischer-type carbenes are known as potential carbene transfer reagents to electron-rich and electron-deficient alkenes. Little is known about the chemistry of carbene complexes with silicon substituents at the carbene C-atom, whereas complexes with germanium, tin, or lead have not yet been prepared. The tungsten-carbene complexes 6 react with an excess of ethyl vinyl ether to give l,2-diethoxy-l-(trialkylsilyl)cyclopropanes 7." Only the f-isomers were formed and similar results can be achieved by using the corresponding molybdenum or chromium complexes. On the other hand, no reaction takes place with 2,3-dihydrofuran or ethyl ( )-but-2-enoate. ... 

Williams and coworkers have synthesized a small library of new chiral imi-noalkyl imidazolium salts from amino acids using a modular design approach (Figure 4.18) [58]. Deprotonation with silver oxide yields the corresponding silver carbene transfer reagents, which have been tested as ligand sources in the palladium-catalyzed allylic alkylation of l,3-diphenylprop-3-enyl acetate with dimethyl malonate. 

Organozinc complexes have been used for a long time as carbene transfer reagents on double bonds (Simmons-Smith eyelopropanation) and arene rings which leads to their extension to eyelohexatrienes. 

Enantioselective Cyclopropanation. Chiral Cu(OAc)2 based complexes of Schiff bases have been used as a carbene transfer reagents in an asymmetric cyclopropane forming reaction. ... 

Use of this carbene transfer reagent allows the reaction to be carried out at 25°... 

Complexes of the form [(NHC)CuX] are also useful carbene transfer reagents for the synthesis of a range of transition metal catalysts. The use of these reagents is attractive owing to the very low cost of copper. It has been shown... 

Scheme 2.7 Silver(I) complexes as carbene transfer reagents.

Barret and Hill have extensively studied the reactivity and catalytic activity of NHC-supported Group 2 amido complexes. Similar to several reports with the alkali metals, tricoordinate NHC-alkaline earth metal complexes 58 could be formed directly from the corresponding conjugate acid by addition of a metal amide salt (Scheme 5.8). Whereas carbene-Li complexes were found to be excellent carbene transfer reagents, 58 could function as a stable carbene equivalent. Indeed, in the presence of Lewis base donors such as triphenylphosphine oxide or protic substrates such as 2-methoxyethylamine, liberation of the free carbene was observed by and NMR. While the authors did not attempt to isolate this free carbene or investigate any additional reactivity, they claimed that the carbene was dissociated under catalyt-ically relevant conditions. 

This complex can serve as a carbene-transfer reagent as described for (CsHioN2)Si. 

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