Rhodium complexes transfer hydrogenation

Directed intramolecular transfer hydrogenations are catalyzed by rhodium complexes with the pendant alkene acting as an internal sacrificial olefin (Equation (39)). 

Water-soluble complexes constitute an important class of rhodium catalysts as they permit hydrogenation using either molecular hydrogen or transfer hydrogenation with formic acid or propan-2-ol. The advantages of these catalysts are that they combine high reactivity and selectivity with an ability to perform the reactions in a biphasic system. This allows the product to be kept separate from the catalyst and allows for an ease of work-up and cost-effective catalyst recycling. The water-soluble Rh-TPPTS catalysts can easily be prepared in situ from the reaction of [RhCl(COD)]2 with the sulfonated phosphine (Fig. 15.4) in water [17]. 

Whatever the route to a rhodium dihydride alkene complex, the hydrogen must be transferred sequentially to the double bond. It had always been assumed that the first C-H bond is formed / to the amido-group, so that the more stable Rh-substrate chelate is formed. This is the alkylhydride isomer observed in stoichiometric NMR studies at low temperatures, and is supported by studies under catalytic turnover conditions, assuming a normal isotope effect... 

Rhodium diphosphine catalysts can be easily prepared from [Rh(nbd)Cl]2 and a chiral diphosphine, and are suitable for the hydrogenation of imines and N-acyl hydrazones. However, with most imine substrates they exhibit lower activities than the analogous Ir catalysts. The most selective diphosphine ligand is bdppsuif, which is not easily available. Rh-duphos is very selective for the hydrogenation of N-acyl hydrazones and with TOFs up to 1000 h-1 would be active enough for a technical application. Rh-josiphos complexes are the catalysts of choice for the hydrogenation of phosphinyl imines. Recently developed (penta-methylcyclopentyl) Rh-tosylated diamine or amino alcohol complexes are active for the transfer hydrogenation for a variety of C = N functions, and can be an attractive alternative for specific applications. 

Brunner, Leitner and others have reported the enantioselective transfer hydrogenation of alpha-, beta-unsaturated alkenes of the acrylate type [50]. The catalysts are usually rhodium phosphine-based and the reductant is formic acid or salts. The rates of reduction of alkenes using rhodium and iridium diamine complexes is modest [87]. An example of this reaction is shown in Figure 35.8. Williams has shown the transfer hydrogenation of alkenes such as indene and styrene using IPA [88]. 

BINAP has been extensively used for the asymmetric hydrogenation, transfer hydrogenation and isomerisation of double bonds using both ruthenium and rhodium complexes. 

While this manuscript was under preparation, a considerable number of examples of sohd-phase-attached catalysts appeared in the literature which is a clear indication for the dynamic character of this field. These include catalysts based on palladium [131, 132], nickel [133] and rhodium [134] as well applications in hydrogenations including transfer hydrogenations [135, 136] and oxidations [137]. In addition various articles have appeared that are dedicated to immobilized chiral h-gands for asymmetric synthesis such as chiral binol [138], salen [139], and bisoxa-zoline [140] cinchona alkaloid derived [141] complexes. 

The treatment of [Cp MCl2]2 (M = Rh and Ir) with (S,S)-TsDPEN gave chiral Cp Rh and Cp Ir complexes (12a and 12b Scheme 5.9). An asymmetric transfer hydrogenation of aromatic ketones using complex 12 was carried out in 2-propanol in the presence of aqueous KOH (1 equiv.) the results obtained are summarized in Table 5.4. In all of the reactions, the (S)-alcohols were obtained with more than 80% enantiomeric excess (ee) and in moderate to excellent yields. The rhodium catalyst 12a was shown to be considerably more active than the iridium catalyst... 

The first use of a metal catalyst in the DKR of secondary alcohols was reported by Williams et al. [7]. In this work, various rhodium, iridium, ruthenium and aluminum complexes were tested. Among them, only Rh2(OAc)4 and [Rh(cod)Cl]2 showed reasonable activity as the racemization catalyst in the DKR of 1-phenylethanol. The racemization occurred through transfer-hydrogenation reactions and required stoichiometric amounts of ketone as hydrogen acceptor. The DKR of 1-phenylethanol performed with Rh2(OAc)4 and Pseudomonas Jluore-scens lipase gave (R)-l-phenylethyl acetate of 98%e.e. at 60% conversion after 72 h. 

The inefficiency of the platinum/hydrogen reduction system and the dangers involved with the combination of molecular oxygen and molecular hydrogen led to a search for alternatives for the reduction of the manganese porphyrin. It was, for example, found that a rhodium complex in combination with formate ions could be used as a reductant and, at the same time, as a phase-transfer catalyst in a biphasic system, with the formate ions dissolved in the aqueous layer and the manganese porphyrin and the alkene substrate in the organic layer [28]. 

Choukroun, R.. Iraqi, A.. Gervais, D., Daran, f.C. and Jeannin, Y. (1987) A semibridging hydrido zirconium-rhodium complex A possible way to catalytic hydrogen transfer on do-de systems. Organometallics. 6. 1197-1201. 

Anomalous concentration dependence observed in the asymmetric transfer hydrogenation of imines with formic acid, catalysed by chiral rhodium-diamine complexes, has been attributed to the participation of both reactant and product in the formation of formate salt. The probable resting state of the catalyst is a rhodium hydride species.373... 

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