COD ligand

The 3,5-bis(trifluoromethyl)pyrazolate analog [Ir(cod)(/x-3,5-(CF3)2pz)]2 does not enter into oxidative addition with iodine, methyl iodide, or acetylenes. The mixture of pyrazolate and 3,5-bis(trifluoromethyl)pyrazolate gives [(rj -codllrf/x-pz)(/L-3,5-(CF3)2pz)Ir(rj -cod)], which reacts with bis(trifluoromethyl)acetylene in a peculiar manner [83JCS(CC)580], producing 145, where 3,5-bis(trifluoromethyl) pyrazolate is replaced by the ethylene bridge and the rj -coordination mode of one of the cod ligands is converted into the rj -allylic mode. 

The iridium catalyst was activated by briefly purging the system with atmospheric pressure of hydrogen to remove the COD ligand followed by degassing the... 

The Rh complex 72 shown in Fig. 35 was obtained by simple addition of the N-heterocyclic carbon(O) ligand to [Rh(p-Cl)(CO)2l2 in THF solution [103,104]. The transition metal complexes 73 and 74 of the all carbon four-membered cyclic ligand shown in Fig. 15 were prepared by adding the rhodium or iridium compounds [M(p-Cl)(cod)]2 to the freshly generated allene at 20° C in THF solution [105, 106]. The cod ligand can easily be replaced by admitting CO at room temperature to THF solutions of the complexes to give 75 and 76, respectively, as shown in Fig. 35. 

Figure 7.12 shows the FI traces taken from the 2D plots at 7.3, 3.0 and Oppm. The aromatic resonances at 7.3 ppm (bottom trace in Figure 7.12) are very well resolved, while the aliphatic and vinyl protons of cod ligand (middle trace) are broad and hidden in the baseline. The upper trace, taken at Oppm, represents the pure signal of methyl proton bonded to silicon residue. Despite the fact that 2D CRAMPS experiment enabled us roughly to assign the position of cod protons, the exact chemical shifts of other hydrogens remain equivocal. 

Delivery of hydrogen occurs syn to the polar functional group. Presumably, the stereoselectivity is the result of coordination of iridium by the functional group. The crucial property required for a catalyst to be stereodirective is that it be able to coordinate with both the directive group and the double bond and still accommodate the metal hydride bond necessary for hydrogenation. In the iridium catalyst illustrated above, the cycloocta-diene (COD) ligand in the catalyst is released upon coordination of the reactant. 

Cl, N02) undergoes cod substitutions with isocyanides and phosphines.190 However, when R is the strongly electron withdrawing N02 group, reaction with PEt3 leads to nucleophilic attack on the coordinated cod ligand affording (63) instead of cod substitution. 

The formation of (63) is also a nice example of the influence of the nature of R in the Pt(l,5-cod)-(R2N4) complexes on the sensitivity of the coordinated cod ligand for nucleophilic attack. Only if R contains the strongly electron withdrawing N02 group is reaction of the alkenic bonds with nucleophiles observed.190... 

Rotation of PtL2 units has been observed in other systems. The inequivalent olefinic environments of the COD ligands in 8 (80) and Ru3Pt(/i-H)(/i4-f/2-C=CfBu)(CO)9(COD) (33) (113) undergo exchange, while the inequivalent phosphine environments in 34 also show a slow exchange at ambient... 

An alternative method for the synthesis of 2,5-disubstituted platinacyclopentanes is via the reaction of Pt(COD)2 with 1,3-butadiene which affords the vinyl-substituted metallacycle 87a as a white crystalline solid in 76% yield. The COD ligand is labile and may be replaced by either two equivalents of PMe3 or CNCMe3 (Scheme 29). Each of the compounds 87a, 87b and 87c were structurally characterised, revealing the trans arrangement of a-substituents.107 The a,a (head to head) re-giochemistry observed for 87 is in contrast to that observed from the reaction of... 

The platinacycloheptane 112 was synthesised in 3% yield by the reaction of (COD)PtCl2 with a appropriate di-Grignard, and subsequent reaction with PPh3 displaced the COD ligand (Scheme 35). The products of thermal decomposition of 112 at 60 °C were found to be 1-hexene (83%) and 2-hexene (17%). Interestingly, compound 112 is reported to be less thermally stable than the five- and six-membered ring analogues.80... 

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