Rh-monophos

An example of a 103Rh-31P correlation experiment of Rh(MonoPhos)2-(COD)BF4 is shown in Figure 11.5. Here also, two inequivalent 31P atoms are connected to the same Rh center [23]. 

Few data are available for rhodium complexes based on monodentate ligands. In a recent study, the rate of COD hydrogenation in Rh(MonoPhos)2(COD)BF4 was determined as 0.071 min-1, which is somewhat slower than the corresponding DuPhos complex [20]. 

Nitrile groups in the substrate may also cause problems. Minnaard, Feringa and de Vries reported the enantioselective hydrogenation of a range of substituted 2-acetamido-cinnamates at 5 bar pressure using Rh/MonoPhos. Whereas most substrates could be hydrogenated with turnover frequencies (TOFs) of between 200 IT1 and 600 h-1, the 4-cyano-substituted substrate was hydrogenated very slowly at this pressure with a TOF of only 4 h 1 [40]. 

Solvent Dependency of Enantioselectivity in Rh/MonoPhos -Catalyzed Asymmetric Hydrogenation of 5a... 

Excellent results have been obtained in the asymmetric hydrogenation of itaconic acid (97% ee) and dimethyl ester (94% ee) using Rh/MonoPhos.31 The latter substrate could be hydrogenated in 99% ee using the piperidine ligand 24d.40 These hydrogenations are relatively fast and have been carried out on a 100-g scale with an S/C of 10,000. Preliminary results with some alkylidene and benzylidene succinates were also very promising. 

Figure 5.24 Immobilization of [Rh(COD)(MonoPhos)2]BF4 on Al-TUD-1. (Redrawn from Simons et al. [106].)...

Chiral monodentate phosphites and phosphoramidites are also effective ligands for Rh-catalyzed asymmetric hydrogenation of enamide substrates. As seen in the structure of MonoPhos illustrated in Figure 1.2, combination of the mod-ihed BINOF backbone and the amine part gives a structural variety to this type of ligand. Combinatorial methods are effective for optimization of the chiral structures.Elucidation of the hydrogenation mechanism catalyzed by the MonoPhos-Rh complex is in progress." ... 

Scheme 9.9 Heterogeneous catalysis of enantioselective hydrogenations with self-supported MonoPhos/Rh catalysts.

In 2002, Zhou and coworkers designed and synthesized a new type of spiro phosphoamidites 1 based on spirobiindane backbone [20]. These spiro monopho sphoramidite ligands 1, especially SIPHOS (la), showed high enantioselectivities (up to 99.7% ee) in the rhodium catalyzed asymmetric hydrogenation of arylenamides (Scheme 8.4) [15]. The reaction was performed under typical conditions with rhodium catalyst, usually produced in situ from [Rh(COD)2 BP.i with 2 equiv of... 

Jankowski, R McMullin, C. L. Gridnev, 1. D. Orpen, A. G. Pringle, P. G. Is restricted M-P rotation a common feature of enantioselective monophos catalysts An example of restricted Rh-P rotation in a secondary phosphine complex. Tetrahedron Asymmetry 2010,21,1206-1209. 

Enantioselective reduction of ketopantolactone and methyl benzoylformate was also examined by the same authors. Quite recently, Bakos and coworkers reported an enantioselective hydrogenation of dehydroalanine using an in situ-produced [Rh (COD)(S)-MonoPhos)2]Bp4 complex on AI2O3 [83,84]. After the optimization of the flow reaction conditions, a highly enantioselective production of (R)-acetylalanine methyl ester (>99% conversion and 96-97% ee) was achieved. 

Carmona and Lamata described the synthesis of several metal complexes bearing BINOL-based phosphoramidites 57-58. The preparation of these complexes involved the reaction of dimethylamino phosphorus-BINOLate (MonoPhos) with the metal precursors [(ri -C5Me5)]-M (M = Rh, Ir or Ru), giving the respective complexes in yields ranging from 74 to 96%. The complexes were characterized by analytical and spectroscopic means, including the determination of the crystal structures of some of them. 

Ding and coworkers replaced the terpyridine moieties with a chiral Eeringa s MonoPhos ligand to obtain a UPy-based bifunctional monomer 18 [38]. With the addition of [Rh(cod)2]Bp4 to the solution of 18 in CH2CI2, it could form a linear-assembled polymer with alternative orthogonal P-Rh coordination and UPy dimerization in the main chain (Scheme 10.5). The supramolecular coordination polymer 19 was insoluble in the apolar solvent such as toluene, which assured facile recovery and reusability of this catalyst. More importantly, this kind of heterogeneous catalyst showed excellent reactivity and enantioselectivity in the hydrogenation reaction. 

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