Chemoselectivity rhodium

Whitl997 Whitehouse, D.L., Nelson, K.H., Jr, Savinov, S.N. and Austin, D.J., A Chemoselective Rhodium(II) Mediated Solid Phase 1,3-Dipolar Cycloaddition and its Application to a Thermally Self-Cleaving Furan Scaffold, Tetrahedron Lett., 38 (1997) 7139-7142. 

Rhodium complexes of (R,i )-1-benzyl-3,4-dithioether-pyrrolidines were also prepared by these authors, who further investigated them as ligands of rhodium complexes in the hydroformylation of styrene but, in all experiments, the enantioselectivity was lower than 3% ee, whereas the chemoselectivity was of 97% (Scheme 10.5). ... 

In 1968 Wilkinson discovered that phosphine-modified rhodium complexes display a significantly higher activity and chemoselectivity compared to the first generation cobalt catalyst [29]. Since this time ligand modification of the rhodium catalyst system has been the method of choice in order to influence catalyst activity and selectivity [10]. 

It has been shown previously how water-soluble rhodium Rh-TPPTS catalysts allow for efficient aldehyde reduction, although chemoselectivity favors the olefmic bond in the case of unsaturated aldehydes [17]. The analogous ruthenium complex shows selectivity towards the unsaturated alcohol in the case of crotonaldehyde and cinnamaldehyde [31]. 

Aizenberg and Milstein [78] have found rhodium complex-catalyzed reactions between polyfluorobenzenes and hydrosilanes which resulted in the substitution of fluorine atoms by hydrogen and were both chemoselective and regioselective (Eq. (6) ... 

The asymmetric hydroformylation of functionalized aliphatic alkenes is generally more difficult than the hydroformylation of vinyl arenes. The rhodium-catalyzed hydroformylation of vinyl acetate (36) yields 2- and 3-acetoxypropanals, 37 and 38, with high chemoselectivity. Ethyl acetate and acetic acid can also be found as by-products. One of the potential applications of vinyl acetate hydroformylation is the production of enantiopure propane 1,2-diol (Scheme 6). 

The different synthetic applications of acceptor-substituted carbene complexes will be discussed in the following sections. The reactions have been ordered according to their mechanism. Because electrophilic carbene complexes can undergo several different types of reaction, elaborate substrates might be transformed with little chemoselectivity. For instance, the phenylalanine-derived diazoamide shown in Figure 4.5 undergoes simultaneous intramolecular C-H insertion into both benzylic positions, intramolecular cyclopropanation of one phenyl group, and hydride abstraction when treated with rhodium(II) acetate. 

A series of water-soluble polyether-substituted triphenyl phosphines (PETPPs) la-c has been successfully employed by Jin et al. [11] in the thermoregulated hydroformylation of 1-dodecene in the biphasic water/toluene system. The catalysts exhibit very good catalytic properties with conversions up to 93% and about 85% selectivity for aldehyde formation. The catalyst derived from rhodium(III) chloride and ligand Ic could be reused in four consecutive cycles without significant loss of activity or chemoselectivity. The n-selectivity of the product aldehydes was not determined. 

In summary, the chemistry of the donor/acceptor-substituted carbenoids represents a new avenue of research for metal-catalyzed decomposition of diazo compounds. The resulting carbenoids are more chemoselective than the conventional carbenoids, which allows reactions to be achieved that were previously inaccessible. The discovery of pan-tolactone as an effective chiral auxiliary, and rhodium prolinates as exceptional chiral catalysts for this class of rhodium-carbenoid intermediate, broadens the synthetic utility of this chemistry. The successful development of the asymmetric intermolecular C-H activation process underscores the potential of this class of carbenoids for organic synthesis. 

It was apparent from the beginning (Scheme 16.7) that there were four potentially independent aspects of reactivity 1) the rate of bimolecular transfer of the diazo ester to the rhodium-complex [10a, 22] 2) the ratio [21] of C-H insertion to /9-H elimination [(34-1-35 -h 36 -h 37)/33] 3) the chemoselectivity [(34-i-35)/(36-i-37)] [4] and 4) the diastereoselectivity [9] of the insertion (34/35 or 36/37). As a prelude to the development of an effectively chiral catalyst, we felt that it was important to experimentally explore these aspects of reactivity. 

In the carboxylate series, the TPA catalyst (entry 4) was the most selective for methine over methylene insertion. Should this remarkable chemoselectivity prove to be general, this complex may add a possibility for high chemoselectivity not previously observed with rhodium(ll) catalysts. The other carboxylate catalysts show less preference for CH over CH2 insertion. We expect that the CH/CH2 ratios would be more pronounced with a less carefully balanced substrate. In the carboxamidate class, MPPIM catalyst (entry 9) was more selective than the corresponding MeOX catalyst (entry 10), with the MEPY catalyst (entry 8) being the least discriminating for CH over CH2 insertion. 

As with any modern review of the chemical Hterature, the subject discussed in this chapter touches upon topics that are the focus of related books and articles. For example, there is a well recognized tome on the 1,3-dipolar cycloaddition reaction that is an excellent introduction to the many varieties of this transformation [1]. More specific reviews involving the use of rhodium(II) in carbonyl ylide cycloadditions [2] and intramolecular 1,3-dipolar cycloaddition reactions have also appeared [3, 4]. The use of rhodium for the creation and reaction of carbenes as electrophilic species [5, 6], their use in intramolecular carbenoid reactions [7], and the formation of ylides via the reaction with heteroatoms have also been described [8]. Reviews of rhodium(II) ligand-based chemoselectivity [9], rhodium(11)-mediated macrocyclizations [10], and asymmetric rho-dium(II)-carbene transformations [11, 12] detail the multiple aspects of control and applications that make this such a powerful chemical transformation. In addition to these reviews, several books have appeared since around 1998 describing the catalytic reactions of diazo compounds [13], cycloaddition reactions in organic synthesis [14], and synthetic applications of the 1,3-dipolar cycloaddition [15]. 

While the perfluorinated acetates do prefer insertion, they are still capable of forming 1,3-dipoles and have demonstrated interesting effects on the regioselectivity of intramolecular cycloaddition reactions, presumably through Lewis acid-mediated effects on the dipolarophile [83]. Other chemoselectivity effects have been noted in the intramolecular cycloaddition reactions and may or may not be partially induced by conformation and sterics [84]. It was further demonstrated thaL when possible, O-H insertion is the predominant outcome over other types of insertion for rhodium]II)-car-benes, independently of the catalyst. However, cycloaddition reactions have been demonstrated to be hgand-dependent [85]. 

The perfluoroacetamide catalysts, rhodium(II) trifluoroacetamidate [Rh2(tfm)4] and rhodium(II) perfluorobutyramidate [Rh2(pfbm)4], are interesting hybrid molecules that combine the features of the amidate and perfluorinated ligands. In early studies, these catalysts were shown to prefer insertion over cycloaddition [30]. They also demonstrated a preference for oxindole formation via aromatic C-H insertion [31], even over other potential reactions [86]. In still another example, rhodium(II) perfluorobutyramidate showed a preference for aromatic C-H insertion over pyridinium ylide formation, in the synthesis of an indole nucleus [32]. Despite this demonstrated propensity for aromatic insertion, the perfluorobutyramidate was shown to be an efficient catalyst for the generation of isomtinchnones [33]. The chemoselectivity of this catalyst was further demonstrated in the cycloaddition with ethyl vinyl ethers [87] and its application to diversity-oriented synthesis [88]. However, it was demonstrated that while diazo imides do form isomtinchnones under these conditions, the selectivity was completely reversed from that observed with rhodium(II) acetate [89, 90]. 

Other hydrogenation methods are less chemoselective. Use of Raney nickel provides hydroxylamines in low yield °. Hydrogenation of 1-acetonaphthone oxime over rhodium-chiral phosphine catalysts was found to proceed under harsh conditions and provided low... 

Chatani s proposed mechanism bears some similarity to that of Jun s reaction (Scheme 9.12). They both begin with hydroamination of the C=C 7t-bond of a rhodium vinylidene. The resultant aminocarbene complexes (71 and 62) are each in equilibrium with two tautomers. The conversion of 71 to imidoyl-alkyne complex 74 involves an intramolecular olefin hydroalkynylation. Intramolecular syn-carbome-tallation of intermediate 74 is thought to be responsible for ring closure and the apparent stereospecificity of the overall reaction. In the light of the complexity of Chatani and coworkers mechanism, the levels of chemoselectivity that they achieved should be considered remarkable. For example, 5 -endo-cyclization of intermediate 72 was not observed, though it has been for more stabilized rhodium aminocarbenes bearing pendant olefins [27]. 

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