[12112-67-3] · C16H24Cl2Ir2 · Di-m-chlorobis(1,5-cyclooctadiene)diiridium(I) · (MW 671.71)
Physical Data: mp 190 °C (dec); 1H NMR (CDCl3) d 4.22 (m, =CH2), 2.15 (m, CH2), 1.40 (CH2).6
Solubility: sparingly sol acetone; sol CH2Cl2 and toluene; insol diethyl ether.
Form Supplied in: orange-red crystalline solid.
Preparative Method: a solution of 6 mL of 1,5-cyclooctadiene in 35 mL of ethanol and 20 mL of water is added to 2.0 g of IrCl3.3H2O in a round-bottomed flask. The mixture is refluxed under nitrogen for 24 h, during which time an orange-red product precipitates from solution. The mixture is cooled to rt and di-m-chlorobis(1,5-cyclooctadiene)diiridium(I) is collected by filtration, washed with cold methanol, and dried in vacuo at 25 °C for 8 h.1
Handling, Storage, and Precautions: the dry solid is moderately air-stable.
Cope and Claisen [3,3]-sigmatropic thermal rearrangements are of considerable utility in synthetic organic chemistry. Although such reactions proceed at high temperatures (ca. 200 °C), transition metals can be employed to accelerate the rearrangements. Indeed, [Ir(m-Cl)(1,5-cod)]2 is a fairly active catalyst precursor for the rearrangement of allyl imidates to allyl amides,2a as well as allylic thionobenzoates to allylic thiolobenzoates2b under mild conditions. Reactions are not selective, as both [3,3]- and [1,3]-rearrangement products are generated in approximately equal amounts (eq 1). Use of palladium(II) complexes circumvents this complication, giving exclusive [3,3] regioselectivity. The mechanism of iridium(I) catalysis seems to involve carbonium ion intermediates formed by allyl-oxygen bond cleavage.
In another rearrangement reaction, [Ir(m-Cl)(1,5-cod)]2 can be used in conjunction with Iodomethane to catalyze the
isomerization of methyl formate to acetic acid (eq 2).7 Reactions are conducted in a carboxylic acid solvent. Water inhibits acetic acid formation and allows an alternative pathway to proceed that affords methane and carbon dioxide as principal products.
The dimerization of bicyclo[2.2.1]heptadiene is promoted by [Ir(m-Cl)(1,5-cod)]2, giving exclusive formation of a dimer with exo-trans-exo configuration (eq 3).3 Evidence has been presented that suggests this dimerization proceeds via a metal-carbon s-bonded intermediate.
Hydrosilation of allylic halides with Triethoxysilane carried out in the presence of a catalytic amount of [Ir(m-Cl)(1,5-cod)]2 affords predominantly halopropyltrialkoxysilanes.4 Disilane formation is observed, however, in analogous hydrosilations of 1-hexene using H2SiEt2.7 Finally, while [Ir(m-Cl)(1,5-cod)]2 can be used to catalyze the hydroboration of vinylarenes, selectivities are poor and product distributions are complicated by significant amounts of alkane formation derived from hydrogenation of the substrate.5 Regioselectivities for this reaction are dependent upon the nature of the catalyst precursor employed.8
Stephen A. Westcott
University of North Carolina, Chapel Hill, NC, USA