Complex, Imino carbene

The imino carbene complexes of tungsten and chromium (e.g., 81) also serve as nitrile ylide synthons (40). The mngsten complexes gave higher yields of the adducts and were strongly regioselective for product 82. For example, for 81 (M=W R = Me, Ph R = Pr, Ph R" = H), the pyrrole 82 was produced in yields of 65-75% with <1% of 83. This route to pyrroles thus has clear advantages over... 

Bishomoallyl alcohols, via allyindium reagents, 9, 703 Bis(hydrostannation), in tin-carbon bond formation, 3, 814 Bis(imidazolyl) ligands, chromium complexes, 5, 359 Bis(imido) systems, with chromium(VI), as models, 5, 377 Bis(imido)tungsten complexes, synthesis, 5, 749 Bis(imido)uranium(VI) complexes, synthesis, 4, 216-217 Bis(imino)carbenes, with Zr(IV), 4, 798 Bis(iminooxazolidine) complexes, biaryl-bridged, with Zr(IV) and Hf(IV), 4, 811-812... 

Pyrrole synthesis.3 Imino carbene complexes such as 1 react with alkynes in hexane at 70°, possibly by a [2 + 2]cycloaddition, to form pyrroles in 80-98% yield. Usually, only a single pyrrole is obtained. This heteroannelation is unusual because carbon monoxide is not incorporated to give a six-membered product. However, O-alkyl imidate carbene complexes such as 3 react with 1-alkynes to form 3-hydroxy-pyridines as the major product. 

A further approach to the synthesis of pyrroles other than that shown in Scheme 47 involves formation of l-metalla-5-aza-l,3,5-trienes 123 (M = W) by insertion of an alkyne into the M=C bond of an (imino)carbene complex 121, leading to chain extension by two carbon atoms ( alkyne route to pyrroles). Compounds 123 cyclize spontaneously to 2//-pyrrole complexes 124, from which ///-pyrroles 122 are obtained by disengagement of the metal unit (Scheme 48).l2d 2H-Pyrrole complexes of type 124 have been characterized by X-ray analysis, and 2/Z-pyrroles could be eliminated from such complexes if substituents other than hydrogen are attached to C2 of the compound.136125 137 l2al2b... 

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. 

A number of quantitative studies have appeared concerning addition to coordinated carbenes. Formation of the dimethylamino(imino)carbene complexes (1) from the reactions of [W(CO)5 C(4-RC6H4)Ph ] (2) (R = H, Me, CF3, or... 

Formation of five-membered heterocycles from imino carbene complexes... 

Formation of Five-Membered Heterocycles from Imino Carbene Complexes... 

Chromium (and tungsten) imino carbene complexes 113 are found to be synthetic equivalents for nitrile ylides on the basis of the resonance structure (Scheme 5.23) [34]. The reaction of the imino carbene complex 113 with alkyne 114 affords pyrrole 115. Imidazoles 116 and 120, pyrrolidines 118, and oxazoline 119 are also obtained by the [3+2] cycloaddition of the imino carbene complexes 113 with benzonitrile, alkenes 117, and benzaldehyde, respectively. Although the regiochemical outcome of these reactions is unpredictable, the regioselectivity can be correlated with the resonance structure in the case of the reaction with 3-hexyn-2-one. 

SCHEME 5.23 Synthesis of five-membered iV-heterocycles 115, 116, and 118-120 from imino carbene complexes 113. 

Gold(I) isonitrile complexes were first reported (131) in 1956, when it was found that they were generally less stable than the isonitrile complexes of other transition metals. This was attributed to the low tendency of gold to form bonds with double bond character. A number of isonitrile complexes have been made, however, and they have been used extensively in the preparation of carbene and imino complexes of gold(I). 

Note Isomerism in the ligand sphere ofpalladium(II) carbene complexes with imino functionalised carbene chelate ligands is observed with moderately donating solvents like THF and acetonitrile as the solvent has to be hemilabile itself... 

Seeing the dicationic complex in Figure 3.42 that is stabilised by two solvent molecules (weakly coordinating acetonitrile) immediately raises the expectation that two imino functionalised NHC ligands should in principle be able to coordinate in a chelate fashion to the same metal atom. This is realised in a trigonal bipyramidal rhodium(I) complex, where the two carbene moieties are in the axial position [130] (see Figure 3.43). 

Figure 3,43 Synthesis and structure of rhodium(l) carbene complexes with imino functionalised NffC ligands.

Figure 3.45 Bromination of a goldf I) imino functionalised carbene complex.

Figure 3.46 Proposed reaction mechanism for the triple bromination of a goldf I) carbene complex with an imino functionalised NHC ligand.

Note The silver (I) carbene complex features Ag-N amide bonds in a five-membered met-allacycle with Ag-Ag bonds, but the copper(II) complex has Cu-0 imino oxide bonds in a... 

Other catalytic systems were used recently, namely a Ddppi system (67, Scheme 8.14) for dicyclopentadiene and di(methylcyclopentadiene) [96] and imino-phosphine complexes (Scheme 8.19, 97 and 98) or hybrid terdentate carbene hgand complexes (99) [133] for norbornene. Furtlierrnore, the copolymerization of various substituted norbomadienes and norbomenes has been achieved with Rh6(CO)i6 under water gas shift reaction conditions (55 ""C, water and 100 atm CO) [134],... 

Other interesting reactions of nucleophiles with metal-carbene complexes include the reaction of 1-aminoethanol which leads to imino-substituted carbene complexes (Fischer and Knauss, 1971a) and the reaction of isonitriles which leads to the unusual complex XV (Aumann and Fischer, 1968a). 

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