Decarbonylation diphosphine complexes

Decarbonylation of Benzaldehyde Using Cationic Diphosphine Complexes of Rhodium (I)... 

The mono(diphosphine) complexes, [Rh(dppp)]BF4 and RhCl-(dppp), are less effective than [Rh(dppp)2] + but are still more active than RhCl(PPh3)3. The mono(diphosphine) catalysts also decompose slowly under the reaction conditions, which renders them less useful than the bis(diphosphine) catalysts. The slower rate of decarbonyla-tion observed with the mono(diphosphine) catalysts compared with the bis(diphosphine) catalysts presumably is due to the lower basicity of the former which retards the rate of oxidative addition (vide infra). Consistent with this is the observation that [Rh(COD)(dppp)]BF4 (COD = 1,5-cyclooctadiene) shows a higher rate for catalytic de-carbonylation of benzaldehyde than does [Rh(dppp)]BF4 (22). An additional observation is that the type of anion, Cl or BF4 , has no apparent effect on decarbonylation rates for the bis(diphosphine) catalysts however, for the mono(diphosphine) complexes the chloride salts show slightly lower rates than their tetrafluoroborate analogues. 

The above data which are presented in Tables I-IV show that the diphosphine complexes are effective catalysts for the aldehyde decarbonylation. Although these reactions tolerate various functional groups such as carboxylic acids, ethers, ketones, olefins, and aryl chlorides,... 

CATALYTIC DECARBONYLATION OF ALDEHYDES WITH CATIONIC DIPHOSPHINE COMPLEXES OF Rh(l)... 

The rates of catalytic decarbonylation of benzaldehyde using mono-diphosphine complexes of Rh and Ir provide an interesting comparison. First of all, the mono-diphosphine complexes of Rh and Ir are not robust under the conditions of the catalytic reaction and therefore are of little practical use. However, they do provide useful data for mechanistic arguments. With Rh, the bis-diphosphine catalysts [Rh(P-P)2] are always more active than their mono-diphosphine analog [Rh(P-P)] when neat aldehyde is used as solvent. Although Rh-P bond rupture is not necessary with the coordinatively unsaturated mono-diphosphine complexes, the rhodium may not be electron-rich enough to promote facile oxidative addition. In support of this argument, the presence of the diolefin cod in the coordination core, [Rh(cod)(dppp)]", increased the activity of decarbonylation by a factor of 6 compared with [Rh(dppp)]. With Ir... 

The research on catalytic decarbonylation using diphosphine complexes was supported by the National Science Foundation. The Johnson Matthey Company is acknowledged for generous loans of rhodium and iridium trichloride. 

The study of alkene insertions in complexes containing diphosphine ligands turned out to be more complicated than the study of the CO insertion reactions [13], When one attempts to carry out insertion reactions on acetylpalladium complexes decarbonylation takes place. When the reaction is carried out under a pressure of CO the observed rate of alkene insertion depends on the CO pressure due to the competition between CO and ethene coordination. Also, after insertion of the alkene into the acetyl species (3-elimination occurs, except for norbomene or norbomadiene as the alkene. In this instance, as was already reported by Sen [8,27] a syn addition takes place and in this strained skeleton no (3-elimination can take place. Therefore most studies on the alkene insertion and isolation of the intermediates concern the insertion of norbomenes [21,32], The main product observed for norbomene insertion into an acetyl palladium bond is the exo species (see Figure 12.8). 

Catalytic decarbonylation of benzaldehyde using several iridium complexes has also been examined. Results of these experiments are shown in Table 8. The main points to be made here are (i) [Ir(P-P)2] catalysts have activities that are ca. twenty times lower than their Rh analogs (ii) the iridium mono-diphosphine catalysts are better than the 6w-diphosphine Ir catalysts (opposite trend noted using rhodium, see Table 5) and (iii) IrCl(CO)(PPh3)2 is a much better catalyst than RhCl(CO)(PPh3)2 and is also better than most of the iridium diphosphine catalysts. The results for the [M(P-P)2] catalysts may be explained in terms of the proposed mechanistic scheme in Figure 11.3. Since Ir-P bonds should be stronger than Rh-P bonds, the value of ki will be smaller for the Ir catalysts, thus... 

Scheme 8.4 Rhodium-catalyzed decarbonylation of heptanal with rhodium complexes bearing chelating diphosphine ligands.

---