Iridium trans

The corresponding iridium enamide complexes and their alkyl hydride counterparts are much more stable, and a full NMR characterization of the alkyl hydride proved possible in the DIPAMP series. Here, as in the corresponding rhodium chemistry, the presumed dihydride precursor proved to be elusive [31]. By employing a different approach to enamide complexes in which an iridium bis-enamide complex was allowed to react with the diphosphine (Fig. 6) both major and minor enamide complexes could be prepared separately the path to one of them is shown in Fig. 6. The trick was to employ menthyl esters so that stereo-chemically homogeneous Ir complexes were formed. Some additional structural features of the intermediates were derived from detailed NMR analysis, and especially the role of the OMe group in coordinating to iridium trans to the hydride [32]. 

Conditions cited for Rh on alumina hydrogenation of MDA are much less severe, 117 °C and 760 kPA (110 psi) (26). With 550 kPa (80 psi) ammonia partial pressure present ia the hydrogenation of twice-distilled MDA employing 2-propanol solvent at 121°C and 1.3 MPa (190 psi) total pressure, the supported Rh catalyst could be extensively reused (27). Medium pressure (3.9 MPa = 566 psi) and temperature (80°C) hydrogenation usiag iridium yields low trans trans isomer MDCHA (28). Improved selectivity to aUcychc diamine from MDA has been claimed (29) for alumina-supported iridium and rhodium by iatroduciag the tertiary amines l,4-diazabicyclo[2.2.2]octane [280-57-9] and quiaucHdine [100-76-5]. 

The iridium(II) complexes are rarer that those of rhodium(II). Iridium does not seem to form carboxylates Ir2(02CR)4 with the lantern structure complexes analogous to trans-RhX2 (PR3 )2 are not formed with bulky tertiary phosphines, probably because the greater strength of Ir-H bonds leads to IrHX2(PR3)2. 

The ethanal was converted to its 2,4-DNP derivative, obtained in 87% yield.) It may also be noted that for this redox reaction, iridium retains the +3 state throughout. The more labile chloride trans to phosphine is replaced... 

A polymer-supported iridium catalyst 4 has been prepared and used in the isomerization of the double bonds in aryl allyl ethers and aryl allylic compounds with excellent trans-scIcctivity and without conventional workup procedures (Scheme 45).73... 

Quickly, it became clear that iridium and rhodium do not cleanly fit the Chalk-Harrod mechanism as does platinum. For electron-rich silanes and relatively unhindered terminal alkynes, the major product is the (Z)-vinylsi-lane (Scheme 3, B) from apparent unusual trans-addition to the alkyne.22 This observation was followed by important and confusing discoveries. First, rhodium, under appropriate conditions, will catalyze the isomerization of the (Z)-vinylsilane product B to the (ft)-vinylsilane product A.23 Second, rhodium can also catalyze the reverse, contra-thermodynamic reaction of the (ft)-vinylsilane A to the (Z)-vinylsilane B.24... 

The other well-characterized metal (i.e., iridium) for trans-addition processes to terminal alkynes suffers from similar limitations.55,5511 53f While iridium provides good selectivity for (Z)-/3-vinylsilanes (Table 4, G) with MePh2SiH (Table 4, entry 1), silanes with electron withdrawing groups (entry 6) and bulky alkynes (entry 4) exhibit significant deterioration in selectivity.25... 

Ir4(CO)ii(PPh3)149 The structures of the di- and tri-substituted iridium derivative are shown in Fig. 14. In all cases, the phosphines are around the basal plane. However, whereas the two phosphines are in relative trans-positions in the di-substituted compound, in the tris-derivative two of the phosphines are obliged to occupy relative c/s-position and, as a result, are involved in more steric interaction. 

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