Nucleophilic reactions Metal carbene complexes

The reaction of isocyanide complexes with nucleophiles gives metal-carbene complexes [49], which constitute an important branch of organometallic chemistry and are effective catalyst systems for a variety of processes [50, 51]. 

Other interesting reactions of nucleophiles with metal-carbene complexes include the reaction of 1-aminoethanol which leads to imino-substituted carbene complexes (Fischer and Knauss, 1971a) and the reaction of isonitriles which leads to the unusual complex XV (Aumann and Fischer, 1968a). 

The surprising stability of N-heterocyclic carbenes was of interest to organometallic chemists who started to explore the metal complexes of these new ligands. The first examples of this class had been synthesized as early as 1968 by Wanzlick [9] and Ofele [10], only 4 years after the first Fischer-type carbene complex was synthesized [2,3] and 6 years before the first report of a Schrock-type carbene complex [11]. Once the N-heterocyclic ligands are attached to a metal they show a completely different reaction pattern compared to the electrophilic Fischer- and nucleophilic Schrock-type carbene complexes. 

A decade after Fischer s synthesis of [(CO)5W=C(CH3)(OCH3)] the first example of another class of transition metal carbene complexes was introduced by Schrock, which subsequently have been named after him. His synthesis of [((CH3)3CCH2)3Ta=CHC(CH3)3] [11] was described above and unlike the Fischer-type carbenes it did not have a stabilizing substituent at the carbene ligand, which leads to a completely different behaviour of these complexes compared to the Fischer-type complexes. While the reactions of Fischer-type carbenes can be described as electrophilic, Schrock-type carbene complexes (or transition metal alkylidenes) show nucleophilicity. Also the oxidation state of the metal is generally different, as Schrock-type carbene complexes usually consist of a transition metal in a high oxidation state. 

Abstract The photoinduced reactions of metal carbene complexes, particularly Group 6 Fischer carbenes, are comprehensively presented in this chapter with a complete listing of published examples. A majority of these processes involve CO insertion to produce species that have ketene-like reactivity. Cyclo addition reactions presented include reaction with imines to form /1-lactams, with alkenes to form cyclobutanones, with aldehydes to form /1-lactones, and with azoarenes to form diazetidinones. Photoinduced benzannulation processes are included. Reactions involving nucleophilic attack to form esters, amino acids, peptides, allenes, acylated arenes, and aza-Cope rearrangement products are detailed. A number of photoinduced reactions of carbenes do not involve CO insertion. These include reactions with sulfur ylides and sulfilimines, cyclopropanation, 1,3-dipolar cycloadditions, and acyl migrations. 

Self-consistent field molecular orbital calculations by Fenske and coworkers have confirmed that nucleophilic additions to Fischer and related complexes [e.g., (CO)sCr=CXY, (T)5-C5H5)(CO)2Mn=CXY], are frontier orbital-controlled rather than charge-controlled reactions (7-9). Interaction of the HOMO of the nucleophile with the carbene complex LUMO (localized on Ca) destroys the metal-carbon w-interaction and converts the bond to a single one. 

Transition metal isocyanide complexes can undergo reactions with nucleophiles to generate carbene complexes. Pt(II) and Pd(II) complexes have been most extensively investigated, and the range of nucleophilic reagents employed in these reactions has included alcohols, amines, and thiols (56) ... 

Some carbyne complexes, in particular cationic ones with good Ji-accepting ligands, can react with nucleophiles to give carbene complexes [187,521]. Several reductions of carbyne complexes to carbene complexes by treatment with metal hydrides have been reported. Similarly, organolithium or other carbanionic reagents can react with electrophilic carbyne complexes to yield carbene complexes. Illustrative examples of both reactions are sketched in Figure 3.23. 

Irradiation of Fischer carbene complexes generates, by insertion of carbon monoxide, a metal-bound ketene intermediate. Photolytic reactions of carbene complexes are synthetically attractive, in that the reaction conditions are mild and the reactions of ketene intermediates with a variety of reagents is of significant scope. A low concentration of metal-bound ketene is probably obtained and in the absence of a nucleophile, the starting material can usually be recovered even after prolonged irradiation. The ketene intermediates are readily trapped with nucleophiles for example, dipeptides are formed in excellent yield and with very high diastereoselectivity upon irradiation of optically active carbenes in the presence of natural or urmatural a-amino acids (Scheme 28). Dipeptides and PEG-supported amino acids and dipeptides can also be used as nucleophiles. 

The reaction occurs well below the temperature at which most of the parent metal carbonyls exchange with free CO and so is a direct nucleophilic attack on coordinated CO, although it may alternatively proceed via a prior electron path. The resulting acyl anions can be isolated as their [R4N] " or [ (C6H5)3P 2N] salts but are reactive and are used directly in subsequent alkylations with organic halides, acetylenes, a-/i-unsaturated carbonyls and alkyloxonium salts to form organic condensation products or metal-carbene complexes. 

The reaction of these transition metal carbene complexes with some nucleophiles such as isocyanante, thiophenol, hydrazine, orhydroxylamine,have been studied. For example, the carbene part of the complex is converted into vinyl ether by pyridine 100>. 

Various elementary processes such as oxidative addition, reductive elimination, olefin and CO insertion into the metal-to-carbon bond have found extensive applications in organic synthesis. Other processes such as attack of nucleophiles on metal-bound CO and olefins, unique reactions of metal carbene complexes, and a-bond metatheses are among the topics of special interest to organometalhc chemists as well as to synthetic organic chemists. Our aim is to provide the reader with detailed accounts of elementary processes with the hope that the information provided here is used for further development of molecular catalysis. 

Actually, terminal metal carbene and alkylidene complexes are ubiquitous throughout the transition elements. The nomenclatural distinction between "carbene" and "alkylidene" represents a fundamental difference in reactivity. Metal carbene complexes usually behave as electrophiles, with typical reactions including cycloadditions to un-saturabed bonds (e.g. cyclopropanation of olefins). On the other hand, metal alkylidene complexes are nucleophilic, undergoing Wittig-type alkylations and olefin metathesis. 

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. ... 

Similar to the preceding class are reactions during which attack on CO takes place by an uncoordinated external reagent. Attack of external nucleophile occurs on the carbon atom. Alkylation of resulting acyl complexes leads to the formation of metal carbene complexes (see Chapter 5). 

XPS spectra " show that coordination of the RNC ligand causes a decrease of the electron density on the isocyanide carbon atom. Therefore, addition of nucleophiles to the carbon atom and addition of electrophiles to the nitrogen atom represent characteristic reactions of the isocyanide ligands. Reactions of metal isocyanide complexes with compounds of the type RXH (RX = RO, RNH, RS, etc.) lead to the formation of metal carbene complexes [Section 5.8.b, reactions (5.13)-(5.19)]. 

Complexes of nucleophilic carbenes are expected to react, like ylids, with electrophiles whereas complexes of electrophilic carbenes are expected to react, like carbocations, with nucleophiles and bases. All the complexes of terminal carbenes have in common the reactions with olefins, although their nature also varies. The principles of these reactions are detailed here, and application in catalysis and organic synthesis, are exposed in Parts IV and V respectively. Reactions of metal-carbene complexes leading to metal-carbyne complexes are mentioned in section 2. 

Metal-carbyne complexes M CR are less known than metal carbenes. The carbyne can also be nucleophilic (Schrock type) or electrophilic (Fischer type). Fischer-type metal-carbynes are obtained by reaction of BF3 on a neutral Fischer-type metal-carbene complex, whereas Sehroek earbynes are often obtained by deshydrohalogenation of a Schrock-type metal-carbene eomplex. They catalyze alkyne metathesis and, in particular, give heterocycles with unsaturated substrates. 

The Wittig-type olefination of carbonyl compounds is one of the characteristic reactions of carbene complexes. High-valent carbene complexes of early transition metals show ylide-like reactivity to vards carbonyl compounds. In 1976, Schrock first demonstrated that niobium and tantalum neopentylidene complexes 1 and 2, the typical nucleophilic Schrock-type carbene complexes, olefinate various carbonyl compounds including carboxylic acid derivatives [4]. 

The ease of formation of the carbene depends on the nucleophilicity of the anion associated with the imidazolium. For example, when Pd(OAc)2 is heated in the presence of [BMIM][Br], the formation of a mixture of Pd imidazolylidene complexes occurs. Palladium complexes have been shown to be active and stable catalysts for Heck and other C-C coupling reactions [34]. The highest activity and stability of palladium is observed in the ionic liquid [BMIM][Brj. Carbene complexes can be formed not only by deprotonation of the imidazolium cation but also by direct oxidative addition to metal(O) (Scheme 5.3-3). These heterocyclic carbene ligands can be functionalized with polar groups in order to increase their affinity for ionic liquids. While their donor properties can be compared to those of donor phosphines, they have the advantage over phosphines of being stable toward oxidation. 

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