Carbenes transition metal complexes, catalytic

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 transition metal-catalyzed cyclopropanation of alkenes is one of the most efficient methods for the preparation of cyclopropanes. In 1959 Dull and Abend reported [617] their finding that treatment of ketene diethylacetal with diazomethane in the presence of catalytic amounts of copper(I) bromide leads to the formation of cyclopropanone diethylacetal. The same year Wittig described the cyclopropanation of cyclohexene with diazomethane and zinc(II) iodide [494]. Since then many variations and improvements of this reaction have been reported. Today a large number of transition metal complexes are known which react with diazoalkanes or other carbene precursors to yield intermediates capable of cyclopropanating olefins (Figure 3.32). However, from the commonly used catalysts of this type (rhodium(II) or palladium(II) carboxylates, copper salts) no carbene complexes have yet been identified spectroscopically. 

In addition to catalytically active transition metal complexes, several stable, electrophilic carbene complexes have been prepared, which can be used to cyclopropanate alkenes (Figure 3.32). These complexes have to be used in stoichiometric quantities to achieve complete conversion of the substrate. Not surprisingly, this type of carbene complex has not attained such broad acceptance by organic chemists as have catalytic cyclopropanations. However, for certain applications the use of stoichiometric amounts of a transition metal carbene complex offers practical advantages such as mild reaction conditions or safer handling. 

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. 

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. 

Murai and Chatani speculated that the two acetylene carbons should be converted into two carbene equivalents to give XVIII during the reaction." To trap this intermediate, the reaction of 6,11-dien-l-yne 69c, which has an olefin moiety in a tether, is carried out in the presence of [RuCl2(CO)3]2 in toluene at 80 °C for 4 h to give tetracyclic compound 71 in 84% yield. It is interesting to note that other transition metal complexes, such as PtCl2, [Rh(OOCCF3)2]2, [IrCl(CO)3] , arid ReCl(CO)s also show catalytic activity for this very complex transformation (Scheme 27). 

The vast majority of N-heterocyclic carbenes are based on 5-membered ring systems. It was found that sterically demanding substituents on the NHC are not only beneficial for the stability of the NHC, but also for its catalytic properties. Arguably, the most important and most often employed N-heterocyclic carbenes are imidazol-2-ylidenes IMes and IPr and the imidazolidin-2-ylidenes SIMes and SIPr (Fig. 3). The reactivity of the corresponding transition metal complexes is described in detail in the following sections. 

Lappert developed the thermolysis of an electron-rich olefin in the presence of a transition metal complex as another way to synthesise these compounds [4], When, in 1975, Clarke and Taube published their findings on carbon coordinated purine transition metal complexes [5], transition metal NHC complexes with functionalised NHC made their debut in biochemistry. The chemistry of carbenes from natural products became firmly established following the discovery that the catalytic activity of thiamine (vitamin Bl) is based on the intermediate formation of a carbene derived from thiazole [6-9] (see Figure 1.2). 

The development of CTOwn ether functionalised imidazolinm salts starts from the consideration that it is possible to link one polyether chain with two imidazolinm nnits at the end points. Since a transition metal can coordinate two imidazoUnm salts in trans fashion [131,162,208], two of these (poly)ether fnnctionaUsed bis-carbenes can form a macrocychc crown ether type hgand system with two transition metal carbene linkages. In favonrable cases, a pincer type C,0,C ms-coordination to the same transition metal is conceivable, bnt may not be very likely when the great affinity of late transition metals to NHC ligands and the aversion of these same late transition metals to ether donor Ugands is taken into account. However, hanilabile stabilisation of transition metal complexes in catalytic processes can certainly be hoped for. 

A related transformation to the previous carbene transfer reaction involves a nitrene ligand bonded to the metal center, in a metallonitrene intermediate in situ generated upon the appropriate selection of the catalyst and the nitrene precursor. As shown in Scheme 17, some transition metal complexes react with such a precursor to generate an unsaturated intermediate, generally electrophilic in nature, which might react with olefins or C—H bonds affording aziridines or amines in a catalytic manner. The most employed nitrene sources are hypervalent I(III) compounds such as PhI=NTs, chloramine-T or organic azides. 

With respect to the ionic hquid s cation the situation is quite different, since catalytic reactions with anionic transition metal complexes are not yet very common in ionic liquids. However, the 1,3-dialkyhmidazolium cation can act as a hgand precursor for the dissolved transition metal. Its transformation under the reaction conditions into a ligand has been observed in three different ways (i) formation of metal carbene complexes by oxidative addition of the imidazolium cation (ii) formation of metal-carbene complexes by deprotonation followed by coordination of the imidazolylidene on the metal center (iii) dealkylation of the imidazolium cation and formation of a metal imidazole complex. These different ways are displayed in a general form in Scheme 5.3-2. 

Heterocyclic imidazolylidene carbenes proved to be very interesting ligands of transition metal complexes. Their electronic properties have very often been compared to those of basic phosphines. They are indeed good a-donors but weak n-acceptors. A number of carbene complexes have been involved in catalytic reactions such as metathesis (Ru) or C-C coupling (Pd, Ni). They can be generated easily by deprotonation of an imidazoUum salt in the presence of an organic base. 

Synthesis and catalytic applications of transition metal complexes of multidentate N-heterocychc carbenes 13CJ0715. 

Reviews.—Recent reviews involving olefin chemistry include olefin reactions catalysed by transition-metal compounds, transition-metal complexes of olefins and acetylenes, transition-metal-catalysed homogeneous olefin disproportionation, rhodium(i)-catalysed isomerization of linear butenes, catalytic olefin disproportionation, the syn and anti steric course in bi-molecular olefin-forming eliminations, isotope-elfect studies of elimination reactions, chloro-olefinannelation, Friedel-Crafts acylation of alkenes, diene synthesis by boronate fragmentation, reaction of electron-rich olefins with proton-active compounds, stereoselectivity of carbene intermediates in cycloaddition to olefins, hydrocarbon separations using silver(i) systems, oxidation of olefins with mercuric salts, olefin oxidation and related reactions with Group VIII noble-metal compounds, epoxidation of olefins... 

In contrast to these classical types of NHCs, isomers with less extensive heteroatom stabilisation, termed here non-classical carbenes , initially received much less attention/ Non-classical carbenes can contain just one, or even no heteroatom in positions a to the carbene carbon atom. Due to the lower number of heteroatoms, n stabilising effects are substantially diminished when compared to classical carbenes, which in turn reduces the stability of the free carbene. On the other hand, the electron-withdrawing inductive influence of the heteroatoms is significantly decreased. As a consequence, these non-classical carbenes are surmised to be stronger donors than their classical analogues. Lately, they have gained interest especially as ligands in transition metal complexes because of their distinct electronic properties, which open up new synthetic and catalytic opportunities. 

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