Rhodium diphosphane

Figure 6 Impregnation of Al-MCM-41 with Rhodium diphosphane complex results in the supported catalyst...

Cationic rhodium diphosphane catalysts have also been prepared from (yt4-2,5-norbornadi-ene)rhodium acetylaeetonate by subsequent treatment with trimethylsilyl trifluoromethanesul-fonate and the corresponding diphosphane117. In some cases, this method proved to be superior because it gave purer products than other methods. 

The rhodium complex 293 of a calix[4]arene carrying diphenylphos-phinomethoxy groups on the lower rim catalyzes the hydroformylation of styrene to 2-phenylpropanal and 3-phenylpropanal in a 95 5 ratio. The rate, however, is lower than that usually observed with rhodium-diphosphane complexes and is postulated to possibly be the result of the semi-encapsulated character of the metal atom in the complex. 

The first example of a combination of a metal-catalyzed substrate synthesis with a biotransformation conducted in a one-pot maimer proceeding according to Scheme 19.23 was reported by Hanefeld, Maschmeyer, Sheldon, and coworkers in 2006 [58]. In this pioneering work, enantioselective hydrogenation of methyl N-acetyl amino acrylate (72) with a heterogenized rhodium-diphosphane complex as catalyst gave the N-acetyl alanine (S)-73 with 100% conversion and 95% ee. This intermediate was then directly converted in situ after separation of the immobilized metal catalyst by means of an L-amino acylase (Scheme 19.23). This enzymatic resolution then led to the formation of the desired amino acid L-alanine (l-74 (S)-74) with 98% conversion and with an excellent enantiomeric excess of >98%. 

Reetz, M.T. and WaJdvogeJ. S.R. (1997) p-CycJodextrin-modified diphosphanes as Jigands for supramolecular rhodium catalysts. Angew. Chem., Int. Ed. Engl., 36, 865-867. 

P. Knochel and co-workers used diphosphines as ligands in the rhodium-catalyzed asymmetric hydroboration of styrene derivatives." The best results were obtained with the very electron rich diphosphane, and (S)-1-phenylethanol was obtained in 92% ee at -35 °C, with a regioselectivity greater than 99 1 (Markovnikoff product). A lower reaction temperature resulted in no reaction, while a higher temperature resulted in lower enantioselectivity and regioselectivity. The regioselectivity was excellent in all cases. Irrespective of the electronic nature of the substituents, their position and size had a profound effect on the enantioselectivity. 

Demay, S., Volant, F., Knochel, P. New C2-symmetrical 1,2-diphosphanes for the efficient rhodium-catalyzed asymmetric hydroboration of styrene derivatives. Angew. Chem., tnt. Ed. Engl. 2001,40,1235-1238. 

A novel chiral ligand type for asymmetric hydroformylation with rhodium and platinum complexes11 125,1 54,156 is the hydroxyproline derived (2S,4>S)-l-(/err-butoxycarbonyl)-4-(diphenylphosphino)-2-[(diphenylphosphino)methyl]pyrrolidine [(-)-BPPM]2-6. As with other diphosphane systems, modifications of BPPM by exchange of either one or both of the different diphenylphosphane groups with dibenzophosphole (DBP) units [BPPM-2DBP, BPPM-4DBP and BPPM-(DBP)2] have been studied158. 

Stereoisomeric 1,4-diphosphanes, 3a- and 3/ -diphenylphosphino-2a-(2-diphenylphosphino-ethyl)-5a-cholestane and DIPHOL analogs 3a- and 3/ -(5//-benzo[6]phosphindol-5-yl)-2a- 2-(5//-benzo[ ]phosphindol-5-yl)ethyl -5a-cholestane, have been prepared from cholestanone155 and used in rhodium-catalyzed asymmetric hydroformylations. 

Several other examples of this reaction type have been reported96-98. The nortricyclene skeleton is also obtained in asymmetric rhodium(I)-catalyzed addition of phenylethvne to norbornadiene, if[Rh(nbd)(—)-Norphos]PF6 is used as catalyst51. This results in optically active 3-(phenylethynyl)tricyclo[2.2.1.02,6]heptane (21) in 45% yield and 60% optical purity. Replacement of (—)-Norphos by other diphosphanes leads to other products or product mixtures. 

In the hydrogenation of optically active azomethines (prepared from chiral a-phenylcthyl-amine and ketones), the effect of reaction parameters, i.e., catalyst dispersion, mass and solvent polarity, on the diastereoselectivity has been studied20. The diastereoselective hydrogenation of several related chiral imines using nonchiral or chiral diphosphane ligands/rhodium has been reported to yield the corresponding amines with diastereomeric ratios up to 99.7 0.3 141. 

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. 

Preparation of Cationic Rhodium(I) Diphosphane Complexes General Procedure64 ... 

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