Carbenes, generation metal complexes

The following chapter concerns another kind of low-valent organophosphorus compounds, namely phosphinidenes. Little is known about free phos-phinidenes in contrast to the corresponding transition metal complexes. Many new reagents have been generated exhibiting either electrophilic or nucleophilic properties. The reactivity of these carbene-like reagents is evaluated (K. hammer tsma). 

Enynes 79 can also undergo cycloisomerisation reactions in presence of NHC/ transition metal complexes (Scheme 5.32). The cycloadduct 124 can be prepared either in presence of complex 110 [36] or in presence of a NHC-Ni complex (prepared in situ from a mixture of [ (COD) ] and IDTB 123 [37]). In the latter case, the active catalytic species is believed to be a Ni-H species that is generated via C-H activation of the carbene ligand. 

Certain transition metal complexes catalyze the decomposition of diazo compounds. The metal-bonded carbene intermediates behave differently from the free species generated via photolysis or thermolysis of the corresponding carbene precursor. The first catalytic asymmetric cyclopropanation reaction was reported in 1966 when Nozaki et al.93 showed that the cyclopropane compound trans- 182 was obtained as the major product from the cyclopropanation of styrene with diazoacetate with an ee value of 6% (Scheme 5-56). This reaction was effected by a copper(II) complex 181 that bears a salicyladimine ligand. 

The most practical approach is the direct treatment of azolium salts with metal complexes under neutral or basic conditions [39,154-159]. Alternatively, the free carbenes can be generated in the presence of a suitable metal complex by reduction of a carbene precursor, e.g. a thiourea [160]. Stable, uncomplexed imidazoline-2-ylidenes, isolated for the first time in 1991 by Arduengo [161] (for further examples see [162-166]), are also convenient starting materials for the preparation of carbene complexes [167,168]. The corresponding diaminocarbene complexes can be obtained by treatment of the stable diaminocarbenes with transition metal complexes. Finally, at high temperatures many transition metal complexes catalyze the carbon-carbon bond scission of tetraaminoethylenes, forming carbene complexes [169-171]. Examples of such preparations are given in Table 2.8. 

Non-heteroatom-substituted carbene complexes can also be generated by treatment of electrophilic transition metal complexes with ylides (e.g. diazoalkanes, phosphorus ylides, nucleophilic carbene complexes, etc. Section 3.1.3). Alkyl complexes with a leaving group in the a-position are formed as intermediates. These alkyl complexes can undergo spontaneous release of the leaving group to yield a carbene complex (Figure 3.2). 

Electrophilic transition metal complexes can react with organic ylides to yield alkylidene complexes. A possible mechanism would be the initial formation of alkyl complexes, which are converted into the final carbene complexes by electrophilic a-abstraction (Figure 3.18). This process is particularly important for the generation of acceptor-substituted carbene complexes (Section 4.1). 

The most frequently used ylides for carbene-complex generation are acceptor-substituted diazomethanes. As already mentioned in Section 3.1.3.1, non-acceptor-substituted diazoalkanes are strong C-nucleophiles, easy to convert into carbene complexes with a broad variety of transition metal complexes. Acceptor-substituted diazomethanes are, however, less nucleophilic (and more stable) than non-acceptor-substituted diazoalkanes, and require catalysts of higher electrophilicity to be efficiently decomposed. Not surprisingly, the very stable bis-acceptor-substituted diazomethanes can be converted into carbene complexes only with strongly electrophilic catalysts. This order of reactivity towards electrophilic transition metal complexes correlates with the reactivity of diazoalkanes towards other electrophiles, such as Brpnsted acids or acyl halides. 

Acceptor-substituted carbene complexes are highly reactive intermediates, capable of transforming organic compounds in many different ways. Typical reactions include insertion into o-bonds, cyclopropanation, and ylide formation. Generally, acceptor-substituted carbene complexes are not isolated and used in stoichiometric amounts, but generated in situ from a carbene precursor and transition metal derivative. Usually only catalytic quantities of a transition metal complex are required for complete conversion of a carbene precursor via an intermediate carbene complex into the final product. 

Carbenes and transition metal carbene complexes are among the few reagents available for the direct derivatization of simple, unactivated alkanes. Free carbenes, generated, e.g., by photolysis of diazoalkanes, are poorly selective in inter- or intramolecular C-H insertion reactions. Unlike free carbenes, acceptor-substituted carbene complexes often undergo highly regio- and stereoselective intramolecular C-H insertions into aliphatic and aromatic C-H bonds [995,1072-1074,1076,1085,1086],... 

The use of functionalized isocyanides containing both the isocyanide function and the nucleophile in the same molecule leads to complexes with heterocyclic carbene ligands via a 1,2-addition across the C=N triple bond. Complexes with functionalized isocyanide ligands can be generated in template reactions or a nucleophile functionalized isocyanide can be reacted directly with a suitable metal complex. 

One of typical reactions of the Fisher-type metal carbene is interaction of the electron-deficient carbenic carbon with a pair of non-bonding electrons contributed by a Lewis base (B ) to generate a metal complex-associated ylide or a free ylide. The ylide intermediate thus generated is usually highly reactive and undergoes further reactions to give stable products (Figure 1). 

Ethers, sulfides, amines, carbonyl compounds, and imines are among the frequently encountered Lewis bases in the ylide formation from such metal carbene complex. The metal carbene in the ylide formation can be divided into stable Fisher carbene complex and unstable reactive metal carbene intermediates. The reaction of the former is thus stoichiometric and the latter is usually a transition metal complex-catalyzed reaction of a-diazocarbonyl compounds. The decomposition of a-diazocarbonyl compounds with catalytic transition metal complex has been the most widely used approach to generate reactive metal carbenes. For compressive reviews, see Refs 1,1a. 

Similar to those of oxygen and sulfur ylide, ammonium ylide or azomethine ylide can be generated by the interaction of metal carbene and amine or imine, respectively. As is the case of sulfur, nitrogen also has a strong coordinating ability to a metal complex. Consequently, metal complex-catalyzed diazo decomposition in the presence of an amine or imine usually requires high reaction temperatures (Figure 6). 

In view of the chemistry of a metal carbene complex, it is reasonable to expect that nitrene or metal-complexed nitrene will similarly interact with Lewis base to generate ylide-like species, which then proceeds to undergo further transformations (Figure 7). 

Certain transition metal complexes catalyze the decomposition of diazo compounds, where the metal-bound carbene intermediates behave differently from the free species generated by their photolysis or thermolysis. 

The elimination of a-hydrogen is not general and observed only with limited numbers of metal complexes. The elimination of a-hydrogen from the methyl group in the dimethylmetal complex 68 generates the metal hydride 69 and a carbene that coordinates to the metal. Liberation of methane by the reductive elimination generates the carbene complex 70. Formation of carbene complexes of Mo and Wis a key step in alkene metathesis. The a-elimination is similar to the 1,2-hydride shift observed in organic reactions. 

Carbenes, generated by several methods, are reactive intermediates and used for further reactions without isolation. Carbenes can also be stabilized by coordination to some transition metals and can be isolated as carbene complexes which have formal metal-to-carbon double bonds. They are classified, based on the reactivity of the carbene, as electrophilic heteroatom-stabilized carbenes (Fischer type), and nucleophilic methylene or alkylidene carbenes (Schrock type). 

At present, Mo, W, Re and Ru complexes are known to catalyse alkene metathesis [7]. This unique reaction, catalysed by transition metal complexes, is impossible to achieve by other means. Later, based on studies of the reactivities of Fischer-type carbene complexes, it was discovered that carbene complexes are the intermediates in alkene metatheses. WClg reacts with EtAlCl2 to afford the diethyltungsten complex 3 by transmetallation, and subsequent elimination of a-hydrogen generates ethane and the carbene complex 4 which is the active catalyst. 

The transmetallation of lithiated heterocycles has been described as a method to provide NHC complexes, and other Fischer-type carbene complexes [161]. The method proceeds via the alkylation of an alkylimidazole with BuLi to generate the lithiated azole, which can transmetallate to the metal complex. Further reaction with an acid or an alkylating agent would provide the desired NHC - M complex (Scheme 47). 

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