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Chemistry (Weinheim an der Bergstrasse, Germany)

Development of a continuous-flow system for asymmetric hydrogenation using self-supported chiral catalysts.


PMID 19685536

Abstract

Well-designed, self-assembled, metal-organic frameworks were constructed by simple mixing of multitopic MonoPhos-based ligands (3; MonoPhos=chiral, monodentate phosphoramidites based on the 1,1'-bi-2-naphthol platform) and [Rh(cod)(2)]BF(4) (cod=cycloocta-1,5-diene). This self-supporting strategy allowed for simple and efficient catalyst immobilization without the use of extra added support, giving well-characterized, insoluble (in toluene) polymeric materials (4). The resulting self-supported catalysts (4) showed outstanding catalytic performance for the asymmetric hydrogenation of a number of alpha-dehydroamino acids (5) and 2-aryl enamides (7) with enantiomeric excess (ee) ranges of 94-98 % and 90-98 %, respectively. The linker moiety in 4 influenced the reactivity significantly, albeit with slight impact on the enantioselectivity. Acquisition of reaction profiles under steady-state conditions showed 4 h and 4 i to have the highest reactivity (turnover frequency (TOF)=95 and 97 h(-1) at 2 atm, respectively), whereas appropriate substrate/catalyst matching was needed for optimum chiral induction. The former was recycled 10 times without loss in ee (95-96 %), although a drop in TOF of approximately 20 % per cycle was observed. The estimation of effective catalytic sites in self-supported catalyst 4 e was also carried out by isolation and hydrogenation of catalyst-substrate complex, showing about 37 % of the Rh(I) centers in the self-supported catalyst 4 e are accessible to substrate 5 c in the catalysis. A continuous flow reaction system using an activated C/4 h mixture as stationary-phase catalyst for the asymmetric hydrogenation of 5 b was developed and run continuously for a total of 144 h with >99 % conversion and 96-97 % enantioselectivity. The total Rh leaching in the product solution is 1.7 % of that in original catalyst 4 h.

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