Rhodium -, trans

On catalytic hydrogenation over a rhodium catalyst the compound shown gave a mixture containing as 1 ten butyl 4 methylcyclohexane (88%) and trans 1 ten butyl 4 methylcyclo hexane (12%) With this stereochemical result in mind consider the reactions in (a) and (b)... 

Rhodium forms a-bonded alkyl, trans- Rhl(CNCR])fiU ] [45151-89-1] [Rhl2(P(C2H2)3)2CH3] [47829-28-7]- aryl, [Rh(P(CH3)3)3C2H2]... 

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]. 

Similarity with cobalt is also apparent in the affinity of Rh and iH for ammonia and amines. The kinetic inertness of the ammines of Rh has led to the use of several of them in studies of the trans effect (p. 1163) in octahedral complexes, while the ammines of Ir are so stable as to withstand boiling in aqueous alkali. Stable complexes such as [M(C204)3], [M(acac)3] and [M(CN)5] are formed by all three metals. Force constants obtained from the infrared spectra of the hexacyano complexes indicate that the M--C bond strength increases in the order Co < Rh < [r. Like cobalt, rhodium too forms bridged superoxides such as the blue, paramagnetic, fCl(py)4Rh-02-Rh(py)4Cll produced by aerial oxidation of aqueous ethanolic solutions of RhCL and pyridine.In fact it seems likely that many of the species produced by oxidation of aqueous solutions of Rh and presumed to contain the metal in higher oxidation states, are actually superoxides of Rh . ... 

Tbe discovery of the catalytic properties of [RhCl(PPh3)3] naturally brought about a widespread search for other rhodium phosphines with catalytic activity. One of those which was found, also in Wilkinson s laboratory, was trans-[Rh(CO)H(PPh3)3 which can conveniently be... 

The rhodium catalyst previously discussed is employed in the hydrogenation of / -hydroxybenzoic acid. The resulting mixture of cis and trans products is separated by virtue of the ready formation of the lactone of the cis product, which is then hydrolized to the hydroxy acid. 

Closely related to the use of rhodium catalysts for the hydrogenation of phenols is their use in the reduction of anilines. The procedure gives details for the preparation of the catalyst and its use to carry out the low-pressure reduction of /j-aminobenzoic acid. Then, as in the preceding experiment, advantage is taken of the formation of a cyclic product to carry out the separation of a mixture of cis and trans cyclohexyl isomers. 

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. 

P. M. Maitlis Trans. NY. Acad. Chemistry of some novel rhodium 10 Pentamethylcyclopentadienyl 0... 

Rhodium complexes with chelating bis(oxazoline) ligands have been described to a lesser extent for the cyclopropanation of olefins. For example, Bergman, Tilley et al. [32] have prepared a family of bis(oxazoline) complexes of coordinatively unsaturated monomeric rhodium(II) (see 20 in Scheme 13). Interestingly, the use of complex 20 in the cyclopropanation reaction of styrene afforded mainly the cis cyclopropane cis/trans = 63137), with 74% ee and not the thermodynamically favored trans isomer. No mechanistic suggestions are proposed by the authors to explain this unusual selectivity. 

Finally, Jessop and coworkers describe an organometalhc approach to prepare in situ rhodium nanoparticles [78]. The stabilizing agent is the surfactant tetrabutylammonium hydrogen sulfate. The hydrogenation of anisole, phenol, p-xylene and ethylbenzoate is performed under biphasic aqueous/supercritical ethane medium at 36 °C and 10 bar H2. The catalytic system is poorly characterized. The authors report the influence of the solubility of the substrates on the catalytic activity, p-xylene was selectively converted to czs-l,4-dimethylcyclohexane (53% versus 26% trans) and 100 TTO are obtained in 62 h for the complete hydrogenation of phenol, which is very soluble in water. 

The Rh and Ir complexes 85-88 (Fig. 2.14) have been tested for the intramolecular hydroamination/cyclisation of 4-pentyn-l-amine to 2-methyl-1-pyrroline (n = 1). The reactions were carried out at 60°C (1-1.5 mol%) in THF or CDCI3 The analogous rhodium systems were more active. Furthermore, the activity of 87 is higher than 85 under the same conditions, which was attributed to the hemilabihty of the P donor in the former complex, or to differences in the trans-eSects of the phosphine and NHC ligands, which may increase the lability of the coordinated CO in the pre-catalyst [75,76]. 

Study of the mechanism of the rhodium-catalyzed hydroamination of ethylene with secondary amines indicated that the piperidine complex trans-RhCl(C2H4)(piperidine)2 can serve as a catalyst precursor [109, 110]. 

As would be expected for a highly electrophilic species, rhodium-catalyzed carbenoid additions are accelerated by aryl substituents, as well as by other cation-stabilizing groups on the alkene reactant.205 When applied to 1,1-diarylethenes, ERG substituents favor the position trans to the ester group.206 This can be understood in terms of maximizing the interaction between this ring and the reacting double bond. 

Fig. 10.10. Steric interactions in rhodium-catalyzed addition of methyl 2-diazobut-3-enoate to styrene (a) and cis and trans butene (b). Reproduced from J. Am. Chem. Soc., 125, 15902 (2003), by permission of the American Chemical Society.

The hydrogenation of para-substituted anilines over rhodium catalysts has been investigated. An antipathetic metal crystallite size effect was observed for the hydrogenation of /Moluidinc suggesting that terrace sites favour the reaction. Limited evidence was found for catalyst deactivation by the product amines. Catalysts with pore diameters less than 13.2 nm showed evidence of diffusion control on the rate of reaction but not the cis trans ratio of the product. 

Li-tetrafluorobenzenedicarboxylatobis [trans-carbonylbis(triphenylphos-phine)rhodium(I)] complexes in pyridine resulted in hemidecarboxylation in each case [Eq. (98)] (79). Prolonged heating in an attempt to... 

Cyclopropanation of C=C bonds by carbenoids derived from diazoesters usually occurs stereospeciflcally with respect to the configuration of the olefin. This has been confirmed for cyclopropanation with copper 2S,S7,60 85), palladium 86), and rhodium catalysts S9,87>. However, cyclopropanation of c -D2-styrene with ethyl diazoacetate in the presence of a (l,2-dioximato)cobalt(II) complex occurs with considerable geometrical isomerization88). Furthermore, CuCl-catalyzed cyclopropanation of cis-2-butene with co-diazoacetophenone gives a mixture of the cis- and trans-1,2-dimethylcyclopropanes 89). 

The change in selectivity is not credited to the catalyst alone In general, the bulkier the alkyl residue of the diazoacetate is, the more of the m-permethric acid ester results 77). Alternatively, cyclopropanation of 2,5-dimethyl-2,4-hexadiene instead of l,l-dichloro-4-methyl-l,3-pentadiene leads to a preference for the thermodynamically favored trans-chrysanthemic add ester for most eatalyst/alkyl diazoacetate combinations77 . The reasons for these discrepandes are not yet clear, the interplay between steric, electronic and lipophilic factors is considered to determine the stereochemical outcome of an individual reaction77 . This seems to be true also for the cyclopropanation of isoprene with different combinations of alkyl diazoacetates and rhodium catalysts77 . 

Remarkably, Claver et al. showed that in a square planar rhodium carbonyl chloride complex, two bulky phosphite ligands (65) were able to coordinate in a trans orientation.214 Diphosphite ligands having a high selectivity for linear aldehyde were introduced by Bryant and co-workers. Typical examples are (67)-(70).215,216... 

Intermolecular cyclopropanation of olefins poses two stereochemical problems enantioface selection and diastereoselection (trans-cis selection). In general, for stereochemical reasons, the formation of /ra ,v-cyclopropane is kinetically more favored than that of cis-cyclopropane, and the asymmetric cyclopropanation so far developed is mostly /ram-selective, except for a few examples. Copper, rhodium, ruthenium, and cobalt complexes have mainly been used as the catalysts for asymmetric intermolecular cyclopropanation. 

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