Rhodium, catalysis

Addition of Organometallic Reagents to Enones in Aqueous Media Rhodium-catalyzed 1,4-addition of organometallic reagents to a,p-unsaturated compounds was first developed by Miyaura in 1997. Thus, Rh(acac)(CO)2/dppb was found to catalyze the 1,4-addition of aryl- and alkenylboronic acids to several ot,(3-unsaturated ketones in water-containing solvents at 50°C. The reaction conditions were successfully modified for the development of an asymmetric variant of this process by Hayashi and Miyaura in 1998. The important points of modification are (1) the use of Rh(acac)(C2H4)2/(5)-binap as a catalyst and 

With regard to the origin of stereoselectivity in these reactions catalyzed by Rh/ (X)-binap, the stereo-determining step is the insertion of an enone to the carbon-rhodium species coordinated with (5)-binap. As shown in Eq. (3.10), for example, 2-cyclohexen-l-one should approach from its Isi face to avoid the steric hindrance of the phenyl groups of the ligand, leading to the 1,4-adduct in (5) configuration, which is consistent with the observed stereochemical outcome  

By tethering a,p-enones with another carbonyl-containing moiety, Rrische developed rhodium-catalyzed asymmetric 1,4-addition/aldol cyclization reactions. These reactions proceed with high diastereo- and enantioselectivity, furnishing structurally complex cyclic compounds in a single step [Eqs. (3.11-3.13)]. 

Not only organoboron reagents but also organosilicon reagents are suitable nucleophiles in the rhodium-catalyzed asymmetric 1,4-additions to a,p-enones in 

A rhodium-catalyzed asymmetric synthesis of chiral titanium enolates by 1,4-addition of aryltitanium reagents to a,p-enones under aprotic conditions has 

---

In contrast to triphenylphosphine-modified rhodium catalysis, a high aldehyde product isomer ratio via cobalt-catalyzed hydroformylation requires high CO partial pressures, eg, 9 MPa (1305 psi) and 110°C. Under such conditions alkyl isomerization is almost completely suppressed, and the 4.4 1 isomer ratio reflects the precursor mixture which contains principally the kinetically favored -butyryl to isobutyryl cobalt tetracarbonyl. At lower CO partial pressures, eg, 0.25 MPa (36.25 psi) and 110°C, the rate of isomerization of the -butyryl cobalt intermediate is competitive with butyryl reductive elimination to aldehyde. The product n/iso ratio of 1.6 1 obtained under these conditions reflects the equihbrium isomer ratio of the precursor butyryl cobalt tetracarbonyls (11). 

Rhodium catalysis have been used for formation of ylides by intramolecular reactions. 

The reduction of a structurally simple acyclic tetrasubstituted enamide has been achieved [15c], also using rhodium catalysis (Scheme 9.35). 

Rhodium catalysis in an aqueous-organic biphasic system was highly effective for intramolecular [2+2+2] cyclotrimerization. It has been shown that the use of a biphasic system could control the concentration of an organic hydrophobic substrate in the aqueous phase, thus increasing the reaction selectivity. The intramolecular cyclization for... 

Branching at an olefinic carbon atom inhibited the reaction markedly, the most dramatic case being that of 2,3-dimethyl-2-butene. It should be noted that the product in this case is nearly exclusively 3,4-dimethylpen-taldehyde for either cobalt or rhodium catalysis (7). Thus, a general rule that products containing a formyl group attached to a quaternary carbon atom are not formed (49) remains valid. Hydroformylation proceeds only after isomerization has occurred. 

Information published from several sources about 1970 presented details on both the halide-containing RhCl(CO)(PPh3)2- and the hydride-containing HRh(CO)(PPh3)3-catalyzed reactions. Brown and Wilkinson (25) reported the relative rates of gas uptake for a number of different olefinic substrates, including both a- and internal olefins. These relative rates are listed in Table XV. 1-Alkenes and nonconjugated dienes such as 1,5-hexadiene reacted rapidly, whereas internal olefins such as 2-pentene or 2-heptene reacted more slowly by a factor of about 25. It should also be noted that substitution on the 2 carbon of 1-alkene (2-methyl-l-pentene) drastically lowered the rate of reaction. Steric considerations are very important in phosphine-modified rhodium catalysis. 

With cobalt catalysts, hydroformylation of ethyl cinnamate gave 91% of the hydrogenation product ethyl hydrocinnamate (15) and only 8% of the expected lactone, 16 (72). However, rhodium catalysis was effective in directing the reaction in favor of hydroformylation (70). The comparative results obtained with cobalt and rhodium are outlined in Table XXV. 

Another noncatalytic step proposed by King et al. (18) in iron carbonyl/base catalysis of the WGSR involves the formation of formate ion however, we recently observed that formate formation appears to have little importance in the related rhodium catalysis of hydrohydroxymethylation. We plan to perform studies of the CO + KOH and C02 + KOH reactions independent of catalysis to more fully appreciate the relationship of these reactions to solution pH and thus the catalytic activity. 

Since Shell s report on the use of phosphines in the cobalt catalysed process, which included preliminary data for the use of rhodium as well [1], many industries started to apply phosphine ligands in rhodium catalysed processes [2], While alkylphosphines are the ligands of choice for cobalt, they lead to slow reactions when applied in rhodium catalysis. In the mid-sixties the work of Wilkinson showed that arylphosphines should be used for rhodium and that even at mild conditions active catalysts can be obtained [3], The publications were soon followed by those of Pruett, in which phosphites were introduced (Figure 8.1) [4],... 

Lactones in rhodium catalysis, 32 365-366 Lacunary Keggin anion, heteropolyanions, 41 ... 

M(CxC) matrix, 32 290-291, 311-313 Measurements, interpretation of, in experimental catalysis, 2 251 Mechanism see also specific types cobalt catalysis, 32 342-349 dehydrocyclization, 29 279-283 rhodium catalysis, 32 369-375 ruthenium catalysis, 32 381-387 space, 32 280... 

---