Ruthenium isonitriles

Jones and co-workers reported the ruthenium-catalyzed synthesis of indol derivatives in a single step starting from di-o,o -substituted aromatic isonitriles a representative example is shown in Scheme 77 366>366a... 

This transformation proceeds through coordination of the isocyanide group to the ruthenium complex (structure 172), followed by insertion of the C-bound ruthenium into the benzylic C-H bond (intermediate 173). After ruthenium-mediated addition of the benzylic carbon to the isonitrile carbon and tautomerization, the desired product was obtained via elimination of the ruthenium complex. 

Cycloadditions on a ruthenium(n) complex between 2 equiv. of phenylacetylene and various types of isonitriles were described for the first time by Singleton.367 3673 These transformations were shown to proceed through coordinatively unsaturated ruthenacycle intermediates to furnish the corresponding imino-2,5-diphenylcyclopentadiene complexes. 

Heterometal alkoxide precursors, for ceramics, 12, 60-61 Heterometal chalcogenides, synthesis, 12, 62 Heterometal cubanes, as metal-organic precursor, 12, 39 Heterometallic alkenes, with platinum, 8, 639 Heterometallic alkynes, with platinum, models, 8, 650 Heterometallic clusters as heterogeneous catalyst precursors, 12, 767 in homogeneous catalysis, 12, 761 with Ni—M and Ni-C cr-bonded complexes, 8, 115 Heterometallic complexes with arene chromium carbonyls, 5, 259 bridged chromium isonitriles, 5, 274 with cyclopentadienyl hydride niobium moieties, 5, 72 with ruthenium—osmium, overview, 6, 1045—1116 with tungsten carbonyls, 5, 702 Heterometallic dimers, palladium complexes, 8, 210 Heterometallic iron-containing compounds cluster compounds, 6, 331 dinuclear compounds, 6, 319 overview, 6, 319-352... 

The product distribution depends on the isocyanide used. Only with aromatic (R = o-tolyl or 2,6-xylyl), and not with aliphatic isocyanides (R = Bu , Bu or benzyl) are isonitrile insertion products 24 formed. Aryl isocyanide insertion is obviously very fast since the competing formation of 23 from CO insertion is not observed complexes 22 are only minor products. With aliphatic isocyanides, the thpp complexes 22, from double cycloaddition of dmad cf. Section 3.1.1), are the major products (70 to >95% of the product mixture) and indicate a strongly increased 1,3-dipolar reactivity, i.e. the intermolecular second cycloaddition is preferred to the intramolecular CO insertion. Compared with the ruthenium compound 17, the thpp in 22 is strongly bound to the metal and can only be decomplexed oxidatively with cerium(iv), or under 80 bar of CO. 

A problem is that the Pauson-Khand reaction uses two equivalents of cobalt. More efficient versions, many of them catalytic, using other metals have been developed. These include carbonyl complexes of titanium, molybdenum, tungsten (Scheme 7.15), rhodium and ruthenium (Scheme 7.16). Rhodium, iridium and iron (Scheme 7.17) have also been used with two alkynes to give cyclopentadienones, often as complexes 7.59. A version of the Pauson-Khand reaction employing a nickel catalyst and an isonitrile in place of CO has been developed. The product is an imine, which can be hydrolysed to a cyclopentenone. 

Substitution is, of course, much slower in [Ru(pc)L2] than in [Fe(pc)L2]. A D mechanism operates in both series of complexes the five-coordinate intermediates show very little discrimination. Variable-temperature proton nmr studies of axial ligand exchange in 1-methylimidazole- and 4-r-butylpyridine-benzyl isonitrile-ruthenium-tetraphenylporphyrin complexes show that tetraphenylporphyrin has a much smaller cis effect here than in analogous iron(II) systems. Again tt-bonding effects are important in determining kinetic parameters. ... 

This researeh was extended to include non-symmetrical amidinate complexes of the same type with mixed Cp and Cp ligands, such as the cationic species [Cp Ru 2-PrN=C(Me)NPr )RuCp(L)] (138 L = none, NCMe, CNBu , PMc3 BF4 salts). For 138 (L = CNBu ), there is NMR evidence that the isonitrile ligand switches between the ruthenium centers. Coordinatively unsaturated intermediates play a key role in catalytic processes, and thus complexes 138 (L = none PFe salt) and 135 (PFe salt) were found to catalyze the atom-transfer radical cyclization of A7-allyl trichloroacetamides. It was discovered that the greater protection offered by the steric bulk of the Cp" ligand led to the superior performance of 135 (PF salt). ... 

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