Oxazaborolidine, polymer-bound

The use of such an oxazaborolidine system in a continuously operated membrane reactor was demonstrated by Kragl et /. 58] Various oxazaborolidine catalysts were prepared with polystyrene-based soluble supports. The catalysts were tested in a deadend setup (paragraph 4.2.1) for the reduction of ketones. These experiments showed higher ee s than batch experiments in which the ketone was added in one portion. The ee s vary from 84% for the reduction of propiophenone to up to >99% for the reduction of L-tetralone. The catalyst showed only a slight deactivation under the reaction conditions. The TTON could be increased from 10 for the monomeric system to 560 for the polymer-bound catalyst. 

Cross-linked polymers bearing IV-sulfonyl amino acids as chiral ligands were converted to polymer bound oxazaborolidine catalysts by treatment with borane or bromoborane. In the cycloaddition of cyclopentadiene with methacrolein, these catalysts afforded the same enantioselectivities as their non-polymeric counterparts238. 

A polymer-bound oxazaborolidine catalyst enantioselective borane reductions of ketones, Tetrahedron Asymm. 1995, 6, 2755-2766. 

In reactions with polymer-bound catalysts, a mass-transfer limitation often results in slowing down the rate of the reaction. To avoid this disadvantage, homogenous organic-soluble polymers have been utilized as catalyst supports. Oxazaborolidine 5, supported on linear polystyrene, was used as a soluble immobilized catalyst for the hydroboration of aromatic ketones in THF to afford chiral alcohols with an ee of up to 99% [40]. The catalyst was separated from the products with a nanofiltration membrane and then was used repeatedly. The total turnover number of the catalyst reached as high as 560. An intramolecularly cross-linked polymer molecule (microgel) was also applicable as a soluble support [41]. 

A disulfide-linked bis(aminoethanol) 82 prepared from L-cystine also catalyzes the borane reduction of ketones. Other oxazaborolidine derivatives are obtained from 83, 84, " and 85, " and polymer-bound species. Those derived from the ephedra bases find use in the asymmetric reduction of imines. bicyclic oxazaborolidine generated... 

The influence of temperature on the selectivity of the reduction of acetophenone and cyclohexylmethylketone by various oxazaborolidines 3.71 (Ar = Ph, R = Me, Bu, Ph) has been examined [S5]. The use of polymer-bound 3.71 has also been proposed [FS3]. Some mechanistic [DTIJ and theoretical investigations of the reductions have also been carried out [DLl, LS6, N4, NU2, QBl]. 

The use of oxazaborolidine catalysts in asymmetric reductions of ketones has been extensively studied by Corey et al. Recently, a polymer-bound oxazaborolidine Scheme 3.6.5) was developed and successfully used in the asymmetric reduction of acetophenone. 1-Phenylethanol was obtained in 93% yield and 98% enantiomeric excess. 

Fracl995 Franot, C., Stone, G.B., Engeli, R, Spondlin, C. and Waldvogel, E., A Polymer-Bound Oxazaborolidine Catalyst Enantioselective Borane Reductions of Ketones, Tetrahedron Asymmetry, 6 (1995) 2755-2766. 

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