Carbene complexes reactions with nucleophiles

The importance of both frontier orbital-controlled and electronic charge-controlled factors in determining chemical reactivity has been recognized (16). These concepts are the key to interpreting two types of reactivity expected for carbene complexes, i.e., reactions with nucleophilic... 

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) ... 

In situation (a) a strong carbon-metal bond results. To this group belong the typical Schrock-type carbenes [e.g. Np3Ta=CH(7Bu)], many of which are nucleophilic at carbon. Situation (b) should also lead to nucleophilic carbene complexes, albeit with a weaker carbon-metal bond. Typical reactions of nucleophilic carbene complexes include carbonyl olefination (Section 3.2.4) and olefin metathesis (Section 3.2.5). 

The most common reaction of platinum(II) isocyanides is their reaction with nucleophiles. This reaction is a useful one for the formation of platinum carbene complexes, and this will be discussed in the next section. 

Wittig-type alkenation of the carbonyl group is possible with Ti carbene compounds [56], The reaction is explained by the formation of nucleophilic carbene complexes of Ti, although they are not isolated. In the carbonyl alkenation, the oxametallacyclo-butane intermediate 182 is formed by [2+2] cycloaddition of the carbene complex 181 with the carbonyl group. This intermediate is converted to the new alkene 183 and the Ti(IV) oxo species 184, which is a stable compound, and hence the carbonyl alkenation requires a stoichiometric amount of the Ti complex. Also, ester 185 is converted to the enol ether 187 via 186. 

Several heteroatom nucleophiles, for example, amines, alcohols, thiols, carboxylates, and dialkylphosphines, undergo Michael addition reactions with alkene- and alkyne-substituted carbene complexes. Reaction of alkyne-substituted chromium carbenes with urea affords products derived from Michael... 

On the other hand, neutral Os alkylidyne 82 undergoes reaction with methanol to give carbene complex 83 (equation 10.56).96 It would appear that 82 undergoes reaction with nucleophilic methanol at 0 first, which is followed by proton transfer to Os. Such reactivity would be consistent with that associated with Fischer carbyne complexes, yet the metal center is more electron-rich than the group 6 metal complex reactant in equation 10.55. 

Although many more examples of attack on coordinated CO have been published than examples of attack on isonitriles, the attack on isonitrile ligands is well established. The reaction in Equation 11.7 illustrates one example of such a nucleophilic attack on a coordinated isonitrile. The kinetics of this reaction are consistent with the direct nucleophilic attack of the amine on the coordinated isonitrile to form a stable intermediate, which undergoes a subsequent proton transfer to form the observed carbene complex. The resulting electron-rich carbene complex resists further nucleophilic attack. 

In carbyne osmium compounds such as [Os3(/i-COMe)(/z-H)(CO)io] nucleophilic attack on the carbyne carbon atom also takes place. By carrying out sequential reactions with nucleophiles and electrophiles, it is possible to break the C—O bond (Table 3.14, footnote reference A). The reaction furnishes an isolable carbene complex, [Os3(M-CHOMe)(/i-H)(CO)io]... 

In contrast to reactions with nucleophiles, only few reactions of Fischer-type carbene complexes with electrophiles are known. 

While the advent of NHC ligands brought much new activity to the field of nickel carbene chemistry, important progress was also made with more traditional Fischer-type carbene complexes. The typical route to methoxy(amino) or bis(amino) Fischer-type carbene complexes is the nucleophilic attack of alcohols or amines on coordinated isocyanides. " A new and efficient route to heteroatom-stabilized carbene nickel(ii) complexes was recently reported to occur by a protonation reaction of the nickel(O) complex Ni(GNXyl)(triphos)." Addition of 2 equiv. of HBF4 to solution of Ni(CNXyl)(triphos) in THF affords the stable dicationic nickel carbene complex [Ni C(H)N(H)Xyl (triphos)]2+(BF4-)2 (Equation (16)). 

Complexes with Fischer-type carbene ligands 954 have been fully characterized, and reactions with nucleophiles lead to reduetive elimination of an iminium salt. ... 

The [3+2] cycloaddition of Fischer alkynyl carbene complex 1 with diazomethane was first reported in 1973 (Scheme 5.1) [9]. The reaction initially generates a pyrazole carbene complex 2 as an intermediate, followed by the nucleophilic addition of the second diazomethane to the carbene center, and the denitrogenation occurs to give the A-metalated pyrazole 3. When trimethylsilyldiazo-methane (TMSCHN2) is used instead of diazomethane, the reaction of the initially formed [3+2] cycloadduct with TMSCHN2 was suppressed to give the pyrazole carbene... 

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. 

All around this chapter, we have seen that a,/J-unsaturated Fischer carbene complexes may act as efficient C3-synthons. As has been previously mentioned, these complexes contain two electrophilic positions, the carbene carbon and the /J-carbon (Fig. 3), so they can react via these two positions with molecules which include two nucleophilic positions in their structure. On the other hand, alkenyl- and alkynylcarbene complexes are capable of undergoing [1,2]-migration of the metalpentacarbonyl allowing an electrophilic-to-nucleophilic polarity change of the carbene ligand /J-carbon (Fig. 3). These two modes of reaction along with other processes initiated by [2+2] cycloaddition reactions have been applied to [3+3] cyclisation processes and will be briefly discussed in the next few sections. 

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. 

Photodriven reactions of Fischer carbenes with alcohols produces esters, the expected product from nucleophilic addition to ketenes. Hydroxycarbene complexes, generated in situ by protonation of the corresponding ate complex, produced a-hydroxyesters in modest yield (Table 15) [103]. Ketals,presumably formed by thermal decomposition of the carbenes, were major by-products. The discovery that amides were readily converted to aminocarbene complexes [104] resulted in an efficient approach to a-amino acids by photodriven reaction of these aminocarbenes with alcohols (Table 16) [105,106]. a-Alkylation of the (methyl)(dibenzylamino)carbene complex followed by photolysis produced a range of racemic alanine derivatives (Eq. 26). With chiral oxazolidine carbene complexes optically active amino acid derivatives were available (Eq. 27). Since both enantiomers of the optically active chromium aminocarbene are equally available, both the natural S and unnatural R amino acid derivatives are equally... 

Dihalocarbene complexes are useful precursors to new carbenes by nucleophilic displacement of the chlorine substituents. This has been nicely illustrated for Fe(TPP)(=CCl2) by its reaction with two equivalents of Re(CO)5J to give the unusual /t-carbido complex Fe(TPP)=C=Re(CO)4Re(CO)5 which also contains a rhenium-rhenium bond. " The carbido carbon resonance was observed at 211.7 ppm in the C NMR spectrum. An X-ray crystal structure showed a very short Fe=C bond (1.605(13) A, shorter than comparable carbyne complexes) and a relatively long Re=C bond (1.957( 12) A) (Fig. 4, Table III). " ... 

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