Carbenes transfer agent

Roland et al. obtained 23% ee in the addition of Et2Zn to cyclohexenone using the silver(I) complex 78 having a chiral backbone and methyl groups on the nitrogen atoms. This complex acts as an efficient carbene transfer agent towards Cu(OTf)2. The conjugate addition proceeds rapidly in toluene at 0 °C (Scheme 52). 

The air and moisture stable zwitterionic fV,N -dimethylimidazolium-2-carboxy-late 74 has been shown to be an efficient carbene transfer agent for various metals (Rh, Ir, Ru, Pd, Pt) (Fig. 24b) [192]. Since the access to derivatives of type 74 is of limited generality, the imidazolium-2-isobutylester salt 75 has been developed and used in the preparation of NHC complexes [193]. 

The reactivity of the carbene transfer agent ImMes2.AgCl towards a variety of osmium clusters (Os3(p-H)2(CO)i0, Os3(CO)i0(NCMe)2, Os4(p-H)4(CO)i2) and Ru4(p-H)4(CO)i2 has been examined, and a number of new NHC complexes resulting from C-H and C-N bond activation reported (29 -34), including examples containing coordinated silver and chloride atoms.31... 

Advantage The choice of transition metal complexes as starting materials isfar greater than with any other method, save that using carbene transfer agents. 

Of course, oxide is a base and thus it falls under the heading of reactions with basic transition metal compounds, but the silver carbene complexes are usually only synthesised because the silver atom coordinates only weakly and can easily be replaced by another metal of choice. It is therefore known as a carbene transfer agent. 

Silver carbene complexes are the most commonly used carbene transfer complexes [83], Other carbene transfer agents include lithium adducts [56], potassium complexes [53], molybdenum carbene complexes [83,84] or chromium carbene complexes [85],... 

Although the main emphasis with imino functionalised carbene hgands is on their silver complexes, as carbene transfer agents, and on late transition metal complexes - palladium. 

The obtained amino functionalised imidazolium salts could be used to generate the corresponding palladium(ll) carbene complexes using the silver(l) complexes as carbene transfer agents. Application of these palladium(ll) complexes (predominantly in situ) in asymmetric allylic alkylation reactions between ( )-l,3-diphenylprop-3-enyl acetate and dimethyl malonate (a standard reaction for this catalytic process [145]) gave up to 80% ee,... 

Note The difference is actually not in the use of the carbene transfer agent (the silver complex) versus the free NHC, but in the number of copper(I) ions added. 

The potassium complex of the hydroxyethyl functionalised carbene, once formed, can then be used as a carbene transfer agent. Patel et al. employed this compound in the synthesis of titanium(lV) complexes used as catalysts in the polymerisation of lactides [38]. Synthesis of the catalyst is facile and involves the reaction of the potassium complex of the hydroxyethyl functionaUsed carbene with [TiCOFV) ] (see Figure 4.7). Although the activity of the titanium(lV) carbene complex is considerably less then that of the potassium complex, the titanium complex acts as a masked NHC and thus a highly moisture and air stable NHC source [38]. 

Fig. 1 Grubbs second-generation catalyst (1), examples of carbenes (2-5), and a carbene transfer agent (6). Compounds 2, 3 are examples of NHCs...

Fig- 2 Full synthetic sequence for the production of the carbene transfer agent, 6... 

This experimental procedure involves a three-step synthesis of a carbene transfer agent, imidazo-2-yildene-silver(I) chloride (6) (Fig. 2). In the first step glyoxal-fcz.v (2,4,6-trimethylphenyl)unine (7) is produced, which is converted into l,3-d/5(2,4,... 

A straightforward preparation for an air-stable silver carbene complex using a standard wet chemistry kit was presented. This experiment removed the tedious requirements of a carbene synthesis and allows for the inclusion of carbenes and carbene transfer agents in the undergraduate laboratory curriculum. 

Canal JP, Ramnial T, Clybumc JAC (2008) A carbene transfer agent. In Roesky HW, Kennepohl D (eds) Experiments in sustainable chemistry, Wiley-VCH Verlag GmbH Co. KGaA, Weinheim (in press). 

Silver(i) oxide and copper(i) oxides can deprotonate two equivalents of azolium salt, yielding two equivalents of [(NHC)MX] and water. " These complexes are also useful carbene transfer agents, and therefore are discussed separately (see Section 2.2.4). 

Finally, Zr carbene complex [(7)ZrQ2(THF0]2 was tested as a carbene transfer agent, in a reaction similar to this observed with the Sc carbene complex [(7)ScQ (THF)2]. Not surprisingly and because of the strrMiger Zr=C braid, its reactivity was lower. Nevertheless, transmetalation to Ru(El), Co(II), and Pd(II) was successful [70],... 

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