mL and DMF and saturated with methylamine gas at 0 OC. The vessel was sealed and agitated for 1 day. The polymer was washed successively in dioxane, ethanol, 2 N NaOH/i-PrOH (l:l), water (until eluate neutral), ethanol, and ether. After drying in vacuo, the polymer (3.7 g/3.8 mequiv of amino groups/l g of dry weight) was suspended in a mixture of water (1.5 mL), ethanol (0.5 mL), triethylamine (7 mL), and 4chloropyridine hydrochloride (4.7 g) in a glass pressure vessel, sealed and heated for 4 days at 140 OC. The polymer was washed as before, and unreacted amino groups were blocked by acetylation (acetic anhydride in CH2C12, then base wash). The washed DMAP polymer was dried at 150 OC in vacuo until constant weight. Incorporation of pyridine groups was determined by potentiometric chloride titration of the hydrochloride salt bound to the polymer: 2.53 mequiv/g compared to 3.15 mequiv/g prior to acetylation.Polymeric l-Acyl-4-(diaUtylamino)pyridinium Chlorides. In a typical experiment, the anhydrous 4-(dialkylamino)pyridine polymer was swelled in methylene chloride (freshly distilled from P205 under argon) and treated with excess benzoyl chloride at 0 OC. The polymer was filtered and washed with methylene chloride under anhydrous conditions until the washings contained negligible amounts of benzoyl chloride-by the silver nitrate test in alcohol (less than 0.1% of total pyridine groups as indicated by GC). The polymer was dried under vacuum at room temperature and was stable at -10 OC for several months. After treatment with a primary (e.g., benzyl) amine in methylene chloride, a pure amide was recovered by filtration and acid/base wash. The amount of amide corresponded to 0.8 mequiv/g of acyl substitution on the polymer. Anhydrous manipulation as above and those involving transfer between two polymers were most conveniently carried out by using a circulating system described in Figure 1, containing Teflon columns (1-4-(10) Warshawsky, A,; Deshe, A.; Rossey, G.; Patchornik, A. React. Polym. 1984, 2, 301.mL volume) joined to the solvent distillation apparatus, waste, and vacuum pump via Teflon tubing.In summary, we have shown for the first time the possibility to perform highly efficient condensation reactions, by transferring polymerbound electrophiles (Le., active esters) via a mediator (shadchan) to polymer-bound nucleophiles (i.e., amines). We have also shown the possibility of on-line monitoring which is relevant for automation.The mediator methodology developed here is believed not to be limited to acylation and related processes but to be expandable to other chemical processes that involve the creation of activated intermediates. These possibilities are currently under investigation. Acknowledgment. We thank the Etta P. Schiff Trust and the Bantrell Fund for financial support. This work is dedicated to Prof. Arieh Berger on t h e 10th anniversary of his death.
Chiral coordinatively saturated cobalt(III) complexes with Schiff bases of enantio pure amino acids are formed as Λ and ∆ isomers, which are not transformed into each other under normal conditions. These complexes catalyze the formation of enantiomerically enriched cyanohydrins from aldehydes and Me 3 SiCN under homo and heterogeneous ca talysis.Catalysts containing chiral elements in the cationic part are widely used in asymmetric catalysis. 1 However, the chemistry of polynuclear catalysts, which are salts of chiral anions with achiral cations, has started to develop actively in the recent time. 1-3 For instance, a family of bimetallic catalysts based on chiral binaphthol with an element of asymmetry in the anionic part has recently been developed. 1 Such catalysts are used, in particular, in the aldol condensation and Michael addition. Chiral octa hedral phosphates are used to shift the equilibrium in solutions of mutually transforming enantiomers toward one of them with enantiomeric excess up to 98%. 2 A new class of chiral borate anions, viz., derivatives of enan tiomerically pure amino acids, was created. 3 In the pres ence of copper salts, these compounds catalyze the enantioselective cyclopropanation of styrene. We have previously 4 synthesized potassium bis(N salicylidene aminoacidato)cobaltates, which are coordinatively satu rated cobalt complexes with two perpendicular tridentate ligands, the latter being the Schiff bases of salicylaldehyde and (S) amino acids. These complexes were used as chiral substrates for asymmetric alkylation in syntheses of enantiomerically enriched amino acids. 4 Results and DiscussionIn the present work, we report the use of the chiral anionic* complexes, viz., potassium Λ bis[N salicylidene (S) aminoacidato]cobaltates, 4 as catalysts in the synthe ses of enantiomerically enriched cyanohydrins. These complexes were synthesized according to the somewhat modified literature procedure 5 (Scheme 1).The isomers were separated and purified by column chromatography. The complexes exist as meridian Λ and ∆ stereoisomers, which are not transformed into each other under normal conditions (i.e., they are stereo chemically inert) and can be separated chromatographi cally. Thus, we prepared individual Λ and ∆ stereo isomeric cobalt complexes of the Schiff bases of salicyl aldehyde and the following amino acids: glycine**, (S) valine, (S) phenylalanine, (S) tryptophan, and * The electrophoresis data that confirm the ionic structure of the complexes are given in Experimental. ** In the case of glycine, two enantiomeric complexes are formed, which were separated by the crystallization of the diastereomeric salts with brucine. 6
Cyanohydrins Q 0521Anionic Chiral Cobalt(III) Complexes as Catalysts of Asymmetric Synthesis of Cyanohydrins. -Chiral coordinatively saturated Co(III) complexes with Schiff bases of enantiopure amino acids catalyze the formation of enantiomerically enriched cyanohydrins from aldehydes and Me3Si-CN. -(BELOKON*, Y. N.; MALEEV, V. I.; MAL'FANOV, I. L.; SAVEL'EVA, T. F.; IKONNIKOV, N. S.; BULYCHEV, A. G.; USANOV, D. L.; KATAEV, D. A.; NORTH, M.; Russ. Chem. Bull. 55 (2006) 5, 821-827; Nesmeyanov Inst. Organoelem. Compd., Russ. Acad. Sci., Moscow 119991, Russia; Eng.) -H. Toeppel 05-082
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.