Carbene displacement

To complicate the issue further, there is now on record a stereospecific carbene displacement which looks very much like an SB2 reaction, with inversion (Landgrebe and Thurman, 1967) ... 

The first lanthanide-NHC complexes were isolated by Arduengo and coworkers in 1994.62 A stable carbene displaces THF in bis(pentamethylcy-clopentadienyl)-samarium-THF to form the samarium(II)-NHC complex 55 (Scheme 29). The addition of a second equivalent of NHC resulted in the isolation of the bis(NHC) adduct 56. Compound 56 was characterised in the solid state by single crystal X-ray diffraction and exhibits samarium-NHC bond distances of 2.837(7) A and 2.845(7) A, which are longer than the M-C bond in a-bonded alkyl lanthanide complexes. 

A synthesis of pyrrolizidine (109) from (108) by intramolecular carbene displacement has been described <88JCS(Pl)833>. 

Cobalt complexes are also prone to carbene displacement. In an attempted synthesis of [(IPr)Co(Cp)Me2] from [(Ph3P)Co(Cp)Me2], Baird and co-workers found that an equilibrium was set up between free IPr and the phosphine complex." Isolated [(IPr)Co(Cp)Me2] reacted with PMe3 resulting in complete displacement of the carbene ligand. The authors attributed this instability to steric crowding. 

An intermolecular carbenoid reaction followed by intramolecular displacement of acetate gives the clavulanic acid derivative (112) in one step from 4-acetoxyazetidin-2-one (91) (80CC1257). Carbene-induced reactions of penicillins and cephalosporins have been reviewed (75S547, 78T1731). 

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). " ... 

The ruthenium complexes were prepared in 50-80% yield by treatment of the imidazolium salts with potassium hexafluoro-t-butoxide, and then by (PCy3)2Cl2Ru = CHPh. A single phosphine is displaced by the carbene affording the desired complexes as air-stable solids that were purified by silica gel... 

The iron atom in [Fe(F2o-TPP)CPh2] is 0.294 A out of the mean porphyrin plane toward the carbene group, whereas [(MeIm)Fe(F2o-TPP)CPh2] exhibits a considerably smaller displacement of the iron atom from the mean plane of the porphyrin ring (0.122 A toward the CPh2 group). In contrast, the iron atom of... 

Gold carbene complexes preparation, oxidation, and ligand displacement. Journal of Organometallic Chemistry,... 

As heavier analogs of carbenes141) stannylenes can be used as ligands in transition-metal chemistry. The stability of carbene complexes is often explained by a synergetic c,7t-effect cr-donation from the lone electron pair of the carbon atom to the metal is compensated by a a-backdonation from filled orbitals of the metal to the empty p-orbital of the carbon atom. This concept cannot be transferred to stannylene complexes. Stannylenes are poor p-a-acceptors no base-stabilized stannylene (SnX2 B, B = electron donor) has ever been found to lose its base when coordinated with a transition metal (M - SnXj B). Up to now, stannylene complexes of transition metals were only synthesized starting from stable monomoleeular stannylenes. Divalent tin compounds are nevertheless efficient cr-donors as may be deduced from the displacement reactions (17)-(20) which open convenient routes to stannylene complexes. 

Halide displacement from the carbene ligands of Ru, Os, and Ir halocarbene complexes by N-, O-, and S-based nucleophiles frequently leads to the formation of new heteroatom-substituted carbene complexes. This important class of reactivity will be discussed in more detail in Section V,D, but it is appropriate here to illustrate the scope of this method with several examples ... 

The driving force for this transformation is the fact that the less electronegative nitrogen atom is a better Tr-donor than oxygen and can form a stronger bond with the carbene carbon atom. Hence displacement of alkoxide by amines and thiols is commonly observed, but the reverse reactions are seldom seen. 

It was noted in Section V,B that the chlorophenyl carbene complex 85 can be prepared by chlorine addition to carbyne complex 80. Treatment of 85 with one equivalent of PhLi does not afford 80, suggesting that the reaction sequence is reduction/substitution rather than substitution/reduc-tion. The recent report (127) of a nucleophilic displacement reaction of the molybdenum chlorocarbyne complex 87 with PhLi to generate phenylcar-byne complex 88 suggests that the intermediacy of the chlorocarbyne complex 86 in the above mechanism is not unreasonable. 

This hypothesis is supported by Chauvin s report (33) on a catalyst derived from (CO)5W=C(OEt)C4H9. This highly stable carbene-W(O) compound does not display catalytic activity for cyclopentene monomer. When mixed in the dark with TiCl4, a slow evolution of 1 equivalent of CO occurs. Subsequent thermal or photochemical activation produces ah extremely efficient catalyst system. Chauvin demonstrated that a high conversion to polypentenamer is obtainable at a W/cyclopentene ratio of 10 li at 5°C. The role of TiCI4 is not well understood nevertheless, it promotes carbonyl displacement which appears to be essential. 

It is also possible to isolate bis(carbene) complexes involving the heavier alkaline earth metals. Thus, the reaction of two equivalents of 4 (R = Me or (Bu, R = H) with calcium, strontium and barium bis(trimethylsilyl)amides [M N(SiMe3)2 2(thf)2] (M = Ca, Sr, Ba) resulted in the displacement of two thf molecules to afford the corresponding biscarbene species, 19 (19). The solubilities and stabilities of these complexes were found to decrease from calcium to barium. 

R = Pr) via a bromide displacement process. Halide displacements have been observed previously in the reactions of carbenes with Me3SiI (38). However, this represents the first such reaction with a haloborane. The X-ray crystal structure of 49 was determined and showed that both heterocyclic rings are planar and that the interpla-nar angle is 92.9°. The B-C(carbene) bond distance of 1.580(11) A is comparable to that found in 32 (1.603(3) A). 

The problem with carbene formation is that they can displace the phosphine ligands attached to the catalyst and deactivate the catalyst. In general, the active catalyst is a palladium(O) compound and this low oxidation state is best stabilized by very bulky phosphines such as P(lBu)3 mentioned above. 

From Chapter 7 it is apparent that the trichloromethyl anion is formed under basic conditions from chloroform, as a precursor of the carbene. The anion can also react with Jt-deficient alkenes (see Section 7.3) and participate in nucleophilic substitution reactions, e.g. 1,1-diacyloxy compounds are converted into 1,1,1-trichloroalkan-2-ols [58] (Scheme 6.35). Similarly, benzyl bromides are converted into (2-bromoethynyl)arenes via an initial nucleophilic displacement followed by elimination of hydrogen bromide [59] (Scheme 6.35). 

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