Iridium complexes acetylene

The iridium complex [Ir(cod)(//2-,PrPCH2CH2OMe)]+BF4 (22) in dichloro-methane at 25 °C at 1 bar H2 is a particularly active catalyst for the hydrogenation of phenyl acetylene to styrene [29]. In a typical experiment, an average TOF of 50 mol mol-1 h-1 was obtained (calculated from a turnover number, TON, of 125) with a selectivity close to 100%. The mechanism of this reaction has been elucidated by a combination of kinetic, chemical and spectroscopic data (Scheme 14.10). 

Another unusual three-component coupling reaction involving an imine as intermediate has been developed by Ishii who has shown that a C-H bond adjacent to the nitrogen atom of an imine can be activated by an iridium complex. Carbo-metallation reactions of acetylenic compounds may then be achieved, which lead to unsaturated imines 155 (Scheme 8.67) [122]. 

Calculated (B3LYP) enthalpies of addition and the effects of phosphine methylation (substitution of PMca for PH3) in reactions of some iridium complexes with C-H compounds (as well as with molecular hydrogen) are summarized in Table VI. 3 [46], It can be seen that the addition of aryl and especially acetylene C-H bonds is thermodynamically more favorable than the addition of simple alkyl C-H bonds. Addition of an aryl C-H bond has been found to be at least 16 kcal mol less exothermic than H2 addition. However, on the basis of the Bryndza-Bercaw relationship [47]... 

C ]H,o, Fluorene, iridium complex 29 232 C]3H jNO, Formanide,lV-ll-(l-naphthal-enyl)ethyl]-, rhenium complex 29 217 CijHijP, Phosphine, methyldiphenyl-, iron complex 26 61, 28 171 molybdenum complex, 27 9 nickel complex, 28 101 tungsten complexes. 27 10, 28 328,28 331 CijHio, Acetylene, diphenyl-, molybdenum complex, 26 102-105, 28 11, 13 C14H12, Benzene, 1,2-ethenediylbis-, platinum complex, 26 140 C14H14, Benzene. l,l -(l,2-ethanediyl)-bis-, 26 192... 

The dimerization of terminal acetylenes by transition metal catalysts has provided a highly attractive route to unsaturated C-4 units. Recent developments in this field include head-to-tail coupling by Ti or Pd catalysts to give 2,4-disubstituted enynes and head-to-head dimerization of ethynylsilanes by Pd or Rh complexes or by iridium complexes to give 1,4-disubstituted enynes k... 

Acetylenes having a terminal hydrogen sometimes add the acetylenic carbon-hydrogen bond across the planar d complex to form a hexa-coordinate hydridoacetylide complex 31). The only well-studied example is the reaction of ethyl propiolate with the iridium complex (XI) to form (XXVII). 

Hasegawa, Y Gridnev, I. D. Ikariya, T. Enantioselective and Z/E-Selective Conjugate Addition of a-Substituted Cyanoacetates to Acetylenic Esters Catalyzed by Bifunctional Ruthenium and Iridium Complexes. Angew. Chem. Int Ed. 2010,49,8157-8160. 

Industrially, large quantities of vinyl ethers can be prepared following Reppe s ethynylation reaction that involved reacting acetylene gas with alcohols [101]. Due to the challenges and hazards associated with the experiment as well as the required handling of acetylene gas (under pressure) alternative approaches to the synthesis of vinyl ethers are typically used in small laboratory settings. One such approach used common iridium complexes to catalyze... 

The hydration of alkynes with water-soluble iridium complexes was studied in detail for the first time by Chin and coworkers. In particular, (TPPTS)2(CO) IrCl-H20 (TPPS = OT-trisulfonated triphenylphosphine) was found to catalyze the hydration of terminal alkynes to give ketones at room temperature and in MeOH as solvent (900 turnovers). The hydration did not occur in the presence of TPPTS only and occurred very slowly in the presence of other water soluble complexes. The hydration of acetylene was much faster than those of terminal alkynes probably due to steric hindrance, whereas internal alkynes did not react at all [40] (Scheme 20). 

Measurements of the enthalpy of thermal decomposition of five co-ordinate perfluoro-olefin and -acetylene-iridium(I) complexes according to the equation... 

Attempts to turn this acetylene dimerization reachon into a catalyhc polymeriza-hon process have failed thus far. In the presence of excess phenylacetylene, the iridium(I) complex 23 activates another Caikynyi—H bond and hansforms, after a hydrogen shift, to the stable (vinyl)(alkynyl) iridium(III) system 27 (Equahon 12.10). 

Since it is known that halo(phenyl)acetylenes add oxidatively to Vaska s complex to give (7-phenylethynyl iridium(III) halides, 32112, the intervention of phenyliodonium iridium(III) and rhodium(III) intermediates, 33, in the alkynyliodonium reactions seems plausible. In any case, the production of cr-alkynyl complexes with alkynyl(phenyl)-iodonium triflates appears to be both more general and efficient21. 

Not surprisingly, these rhodium and iridium carbene complexes were tested for their catalytic behaviour in the transfer hydrogenation of benzophenone and acetophenone (M +3), the hydrosilylation of alkynes (M +1) and also the catalytic cyclisation of acetylenic carboxylic acids (M +1). Hydrogenation works better for iridium than rhodium and for aromatic than for aliphatic ketones [40,43,44]. The iridium(I) complex is the first iridium catalyst showing activity for the cyclisation of acetylenic carboxylic acids [40]. The results for the hydrosilylation reactions were very moderate. 

The reactions of fluoroalkyl acetylenes with complexes of cobalt, rhodium, and iridium give a wide range of compounds, some of which have been indicated above. Other examples for which l9F NMR... 

A number of metal complexes catalyses specific alkyne polymerizations, giving rise to four-, six- or eight-membered carbocyclic rings. The first work in this area was the nickel-catalysed formation of cyclooctatetraene (40) from acetylene by the group of Reppe ", but since then formation of cyclic systems from acetylenes has been found to be also catalysed by molybdenum, cobalt, iridium and tantalum. ... 

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