Chlororhodium complexes

In contrast to heterogeneous metal catalysts, the chlororhodium complex is not sensitive to sulfur poisoning,thus allowing the saturation of double bonds in the presence of mercapto functions. 

Previous work has shown that the electronic characteristics of the benzene substituent in triarylphosphine chlororhodium complexes have a marked influence on the rate of olefin hydrogenation catalyzed by these compounds. Thus, in the hydrogenation of cyclohexene using L3RhCl the rate decreased as L = tri-p-methoxyphenylphosphine > triphenylphosphine > tri-p-fluorophenylphosphine (14). In the hydrogenation of 1-hexene with catalysts prepared by treating dicyclooctene rhodium chloride with 2.2-2.5 equivalents of substituted triarylphosphines, the substituent effect on the rate was p-methoxy > p-methyl >> p-chloro (15). No mention could be found of any product stereochemistry studies using this type of catalyst. 

In our initial studies on the [5+2] cycloaddition, several metal catalysts were screened. Rhodium(I) systems were found to provide the optimum yields and generality [26]. Since the introduction of this new reaction in 1995, our group and others have reported other catalyst systems that can effect the cycloaddition of tethered VCPs and systems. These new catalysts thus far include chlororhodium dicarbonyl dimer ( [RhCl(CO)2]2 ) [27], bidentate phosphine chlororhodium dimers such as [RhCl(dppb)]2 [28] and [RhCl(dppe)]2 [29], and arene-rhodium complexes [(arene)Rh(cod)] SbFs [30]. [Cp Ru(NCCH3)3] PFg has also been demonstrated to be effective in the case of tethered alkyne-VCPs [31], but has not yet been extended to intermolecular systems or other 2n -components. 

Besides solid transition metals, certain soluble transition-metal complexes are active hydrogenation catalysts.4. The most commonly used example is tris(triphenylphosphine)-chlororhodium, which is known as Wilkinson s catalyst.5 This and related homogeneous catalysts usually minimize exchange and isomerization processes. Hydrogenation by homogeneous catalysts is believed to take place by initial formation of a rc-complex, followed by transfer of hydrogen from rhodium to carbon. 

Many papers formulate the starting chlororhodium(III) porphyrin just as RhCl(P), as if a trans ligand L in MC1(P)L were easily lost (path c, X = Cl). However, the conditions of preparations point to a predominance of hexacoor-dinate aqua species, RhCI(P)H20. Only in one case the formation of a pentacoordinated rhodium(III) halide, the iodide RhI(TMP), seems well-documented [61], see Sect. 2.1.3. The formation of interesting heterobimetallic porphyrins, e.g. (TPP)RhMn(CO)s, [path c, X = Mn(CO)s] was formulated as starting from RhCl(TPP) [63], but the work referred to [264] clearly stated that hexacoordinate species, namely RhCl(TPP)H20, RhCl(TPP)(EtOH), or RhCl(TPP)CO were involved. On the other hand, the heterobimetallic species appear to be pentacoordinate about the rhodium, in accord with many metal-metal-bonded porphyrin complexes [222] (see also below). 

The dihydrido complexes (Table 62) can be obtained by the oxidative addition of molecular hydrogen to rhodium(I) complexes (equation 186).10,119,922""926 The tri(f-butyl)phosphine complexes can be prepared either from the chlororhodium(I) complex,923 or rhodium trichloride.927 The former method seems more reliable since the latter reports the complex as a matt green substance, a color uncharacteristic of tertiary phosphine rhodium(III) complexes. Indeed, Masters and Shaw report that the related tertiary phosphines PBu2R (R = Et, Pr) give green rhodium(II) complexes in this reaction (see Section 48.5.2.1 above).268,269... 

A different type of cyclizalion by means of the rhodium complex is observed in the case of a compound containing a I-al-6-ene system. Thus reaction of (-l-)-citTonellal (4) with I eq. of tris(triphenylphosphine)chlororhodium in freshly distilled chloroform at room temperature for 15 hr. results in the formation of (-t-)-neoisopulegol (5) and (—)-isopulegDl (6) in the ratio of 3 1. 

The second approach to the generation of trivalent rhodiumcorrole complexes involved reacting dideoxybiladiene-ac 2.106 in methanol with tetracarbonyldi-p-chlororhodium(I) (Rh2(CO)4Cl2). ° After addition of PPh3, a Rh(III) corrole 2.134... 

There arc two main classes of catalysts neutral chlororhodium(I) diphosphane complexes and cationic rhodium(I) complexes having the general structure [Rh(diphosphane)(olefin)2]+. The cationic complexes are often more active and more selective than the corresponding chloro complexes60. In contrast to heterogeneous hydrogenation catalysts rhodium phosphane complexes are not pyrophoric. However, they are sensitive to oxygen and should be stored and handled under an inert atmosphere. 

Chlororhodium phosphine complexes also react with simple alkenes such as ethylene to form isolable ethyl complexes similar to those proposed as intermediates in alkene hydrogenations catalyzed by Rh(I) . Ethylrhodium(III) is also formed on protonation of / -CpRh(C2H4)2... 

Asymmetric hydrogenation. Morrison et al. have reported on asymmetric hydrogenations catalyzed by rhodium(I) complexes of the Wilkinson type containing chiral ligands. This type of asymmetric synthesis had been carried out previously with relatively inaccessible phosphine ligands that are asymmetric at phosphorus. Phosphines that are asymmetric at carbon are more readily available and appear to be more efficient. Thus reduction of (E)- 3-methylcinnamic acid with prereduced tris(neomenthyldiphenylphosphine)chlororhodium in the presence of triethylamine leads to 3-phenylbutanoic acid, +34.5°, which contains 61% enantiomeric excess of the S-isomer. Hydrogenations of olefins exhibit a lower degree of asymmetric bias. 

Tris(triphenylphosphine)chlororhodium(l) (Wilkinson s Catalyst). This complex catalyzes the hydrogenation of alkenes. It is prepared by the reduction of rhodium trichloride in the presence of triphenylphosphine (1). 

In general, reactivity toward molecular oxygen is lower for complexes of the second row transition metals than for third row complexes [3-7]. It has been found, however, that a cobalt(I) phosphine complex is far more reactive toward O2 than either the rhodium(I) or iridium(I) compounds having the same ligand system [70]. This leads, in at least one instance, to the rather unexpected reactivity sequence Co > Ir > Rh (Table 6). As in the iridum case, rhodium(I) forms side-bonded peroxo complexes with dioxygen. Reaction products, however, can be quite different as in the oxygenation of /nj(triphenylphosphine)chlororhodium(I), equation (23) [31]. 

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