Transition carbene transfer

It has been demonstrated that group 6 Fischer-type metal carbene complexes can in principle undergo carbene transfer reactions in the presence of suitable transition metals [122]. It was therefore interesting to test the compatibility of ruthenium-based metathesis catalysts and electrophilic metal carbene functionalities. A series of examples of the formation of oxacyclic carbene complexes by metathesis (e.g., 128, 129, Scheme 26) was published by Dotz et al. [123]. These include substrates where double bonds conjugated to the pentacarbonyl metal moiety participate in the metathesis reaction. Evidence is... 

Another remarkable property of iodorhodium(III) porphyrins is their ability to decompose excess diazo compound, thereby initiating carbene transfer reactions 398). This observation led to the use of iodorhodium(III) me.vo-tetraarylporphyrins as cyclopropanation catalysts with enhanced syn anti selectivity (see Sect. 2.2.3) s7, i°o) as wep as catalysts for carbenoid insertion into aliphatic C—H bonds, whereby an unusually high affinity for primary C—H bonds was achieved (see Sect. 6.1)287). These selectivities, unapproached by any other transition metal catalyst,... 

In order to rationalize the catalyst-dependent selectivity of cyclopropanation reaction with respect to the alkene, the ability of a transition metal for olefin coordination has been considered to be a key factor (see Sect. 2.2.1 and 2.2.2). It was proposed that palladium and certain copper catalysts promote cyclopropanation through intramolecular carbene transfer from a metal carbene to an alkene molecule coordinated to the same metal atom25,64. The preferential cyclopropanation of terminal olefins and the less hindered double bond in dienes spoke in favor of metal-olefin coordination. Furthermore, stable and metastable metal-carbene-olefin complexes are known, some of which undergo intramolecular cyclopropane formation, e.g. 426 - 427 415). 

It has been widely accepted that the carbene-transfer reaction using a diazo compound and a transition metal complex proceeds via the corresponding metal carbenoid species. Nishiyama et al. characterized spectroscopically the structure of the carbenoid intermediate that underwent the desired cyclopropanation with high enantio- and diastereoselectivity, derived from (91).254,255 They also isolated a stable dicarbonylcarbene complex and demonstrated by X-ray analysis that the carbene moiety of the complex was almost parallel in the Cl—Ru—Cl plane and perpendicular to the pybox plane (vide infra).255 These results suggest that the rate-determining step of metal-catalyzed cyclopropanation is not carbenoid formation, but the carbene-transfer reaction.254... 

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... 

Transition-metal catalysis, especially by copper, rhodium, palladium and ruthenium compounds, is another approved method for the decomposition of diazo compounds. It is now generally accepted that short-lived metal-carbene intermediates are or may be involved in many of the associated transformations28. Nevertheless, these catalytic carbene transfer reactions will be fully covered in this chapter because of the close similarity in reaction modes of electrophilic carbenes and the presumed electrophilic metal-carbene complexes. 

Transition-metal mediated carbene transfer from 205 to benzaldehyde generates carbonyl ylides 211 which are transformed into oxiranes 216 by 1,3-cyclization, into tetrahydrofurans 212, 213 or dihydrofurans 214 by [3 + 2] cycloaddition with electron-deficient alkenes or alkynes, and 1,3-dioxolanes 215 by [3 + 2] cycloaddition with excess carbonyl compound120 (equation 67). Related carbonyl ylide reactions have been performed with crotonaldehyde, acetone and cyclohexanone (equation 68). However, the ylide generated from cyclohexanone could not be trapped with dimethyl fumarate. Rather, the enol ether 217, probably formed by 1,4-proton shift in the ylide intermediate, was isolated in low yield120. In this respect, the carbene transfer reaction with 205 is not different from that with ethyl diazoacetate121, whereas a close analogy to diazomalonates is observed for the other carbonyl ylide reactions. 

Apart from this feature there are many similarities between ylides and carbene complexes, primarily among the structural criteria. The carbene carbon may be, but not necessarily, in a planar configuration, and the M—C bonding indicates some multiple bonding character just as in most of the ylides. On the other hand, carbene transfer reactions have been observed with ylides [e.g., Eq. (36)3, indicating that the carbene complex formalism can, indeed, be successfully applied with ylides. There is hope, therefore, for a fruitful symbiosis of ylide and carbene complex chemistry, which may soon become complementary as more data become available from this new area of transition metal chemistry. 

A select number of transition metal compounds are effective as catalysts for carbenoid reactions of diazo compounds (1-3). Their catalytic activity depends on coordination unsaturation at their metal center which allows them to react as electrophiles with diazo compounds. Electrophilic addition to diazo compounds, which is the rate limiting step, causes the loss of dinitrogen and production of a metal stabilized carbene. Transfer of the electrophilic carbene to an electron rich substrate (S ) in a subsequent fast step completes the catalytic cycle (Scheme I). Lewis bases (B ) such as nitriles compete with the diazo compound for the coordinatively unsaturated metal center and are effective inhibitors of catalytic activity. Although carbene complexes with catalytically active transition metal compounds have not been observed as yet, sufficient indirect evidence from reactivity and selectivity correlations with stable metal carbenes (4,5) exist to justify their involvement in catalytic transformations. 

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. 

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. 

Yang et al. used a similar protocol (an ether functionality supported on a primary alkyl halide carrier) to introduce an acetal on either side of the imidazole ring generating an ether functionalised ionic liquid (IL) imidazolium salt [183] (see Rguie 3.58). The anion could be varied without loss of the IL property (melting point below 1(X) °C) [184]. Synthesis of the transition metal carbene complexes (palladium) was done by carbene transfer ftom the corresponding silver(I) complexes or by reaction with the metal acetate (nickel) [162] (see Figure 3.64). 

The ligand was then used to form a variety of transition metal carbene complexes [207] (see Figure 3.72). Interestingly, more than one method for the formation of transition metal carbene complexes was successfully employed presence of an inorganic base (IC COj) to deprotonate the imidazolium salt and the silver(I) oxide method with subsequent carbene transfer to rhodium(I), iridium(I) and copperfi), respectively. The silver(I) and copper(I) carbene complexes were used for the cyclopropanation of styrene and indene with 1,1-ethanediol diacetate (EDA) giving very poor conversion with silver (< 5%) and qnantitative yields with copper. The diastereomeric ratio (endolexo) was more favonrable with silver than with copper giving almost a pnre diastereomer for the silver catalysed reaction of indene. 

Nielsen et al. have introduced a monoether linked bis-carbene [209] modelled on an amino linked bis-carbene ligand that acts as a C,N,C pincer ligand in a corresponding palladium(II) complex [156]. Synthesis of the ether linked bis-carbene is facile and involves the reaction of the l-co-dichloro-diethylether with 2 equiv. of methylimidazole. Subsequent reaction with silver oxide and carbene transfer to suitable transition metal precursor complexes affords the corresponding complexes (see Figure 3.73). 

Having seen that structural predictions are very difficult, we will now turn to the choice of transition metal. We have already seen the dependence of the coordination mode in square planar complexes on various factors and noticed the preference for polymeric chains with the silver(I) complexes owing to the linearly coordinated silver centre. Chiu et al. [325] reported on a series of arylmethylene and methyl wingtipped bis-carbene complexes of silver(I) (polymeric bridging) and palladium(II) (monomeric chelating). Carbene transfer to palladium was achieved in DMSO since solubility in CHjCfj was very poor. 

Given that thiazole is the active centre in vitamin Bl, thiamine has been the centre of intense research of its organocatalytic potential for decades it might be surprising that only very few examples of transition metal NHC complexes are known that use thiazol-2-ylidene or its benzo-annulated analogue benzothiazol-2-ylidene. As we have seen above, one major reason is the instability of the free carbene leading to dimerisation instead. Another major contribution is the apparent inability of thiazol-2-ylidene to coordinate to silver(I) [39] making carbene transfer from silver salts to other transition metals impossible. 

Transition metal carbyne complexes are described by the general formula L M=CR where the carbyne ligand (=CR) is bonded to the metal by a metal-carbon triple bond. Transition metal carbene complexes have found numerous applications in synthetic organic chemistry through a variety of carbene transfer and cycloaddition reactions [17]. In contrast, carbyne (L M=CR) and vinylidene (L M=C=CRR ) complexes have far fewer applications, in part because their overall chemistry is significantly less developed [18]. Addition reactions to transition metal vinylidene complexes will be discussed in Chapter 21. The first successful synthesis of a carbyne complex was reported by Fischer and co-workers in 1973 [Eq. (8) 19]. Subsequently, many other carbyne complexes have been synthesized by the classic route of Fischer or by new synthetic methods [20]. 

All of these carbenes are reactive intermediates that must be generated from the appropriate precursors in the presence of the alkene (or arene) which is to be cyclopropanated. The following methods of carbene-transfer reactions to C-C double bonds will be discussed path a. from a-halo-a-metal (or alkylmetal) compounds by a-elimination path b. from iodine or sulfur ylides by thermal, photochemical or transition metal catalyzed decomposition ... 

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. 

Photochemical decomposition of diazo(trimethylsilyl)methane (1) in the presence of alkenes has not been thoroughly investigated (see Houben-Weyl Vol. E19b, p 1415). The available experimental data [trimethylsilylcyclopropane (17% yield) and la,2a,3j8-2,3-dimethyl-l-trimethylsilylcyclopropane (23% yield)] indicate that cyclopropanation occurs only in low yield with ethene and ( )-but-2-ene. In both cases the formal carbene dimer is the main product. In reactions with other alkenes, such as 2,3-dimethylbut-2-ene, tetrafluoroethene or hexafluoro-propene, no cyclopropanes could be detected.The transition-metal-catalyzed decomposition of diazo(trimethylsilyl)methane (1) has been applied to the synthesis of many different silicon-substituted cyclopropanes (see Table 3 and Houben-Weyl Vol.E19b, p 1415) 3.20a,b,2i.25 ( iQp. per(I) chloride has been most commonly used for carbene transfer to ethyl-substituted alkenes, cycloalkenes, styrene, and related arylalkenes. For the cyclopropanation of acyl-substituted alkenes, palladium(II) chloride is the catalyst of choice, while palladium(II) acetate was less efficient, and copper(I) chloride, copper(II) sulfate and rhodium(II) acetate dimer were totally unproductive. The cyclopropanation of ( )-but-2-ene represents a unique... 

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. 

The carbene transfer occurs with retention of the aminocarbene configuration, indicating that an sp3 transition state is not formed. With H[AuBr4] and [W(CO)5 C(NMe2)Ph ], only the reductive substitution reaction occurs to give the gold(I) compound [ Au(Br)C(NMe2)Ph]. 

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