Poly(ethylene glycol)-supported dialkylbo-rane1

 · (MW ~10 000)

(soluble polymer-supported reagent for capture of amino alcohols)

Alternate Name: PEG-dialkylborane.

Solubility: soluble in CHCl3, CH2Cl2, benzene, toluene, DMF, DMSO, CH3CN, CH3OH, H2O, and acetone; insoluble in diethyl ether, t-butyl methyl ether, and iso-propanol.

Form Supplied in: solution (1 M) in toluene.

Preparative Methods: by multistep synthesis from monometh-oxy poly(ethylene glycol) methane sulfonate MW ~5000.2

Purification: dissolve in a minimum of CH2Cl2 and precipitate in a cold anhydrous diethyl ether solution. Recover the precipitate by filtration and wash with anhydrous diethyl ether.

Handling, Storage, and Precautions: as a solid, the reagent is very moisture sensitive and must be stored under argon in a desicator. As a solution in toluene, storage in a sealed bottle under argon at room temperature will ensure stability for many months.

Introduction

High-throughput solution-phase organic synthesis is generating the need for a whole new genre of polymer-supported reagents and catalysts to assist in rapid reaction purification.3 This has led to strategies called resin-quench4 and resin-capture5 that are designed to remove excess starting materials or sequester products, respectively. While resin supports are, by far, the most commonly used for this purpose, the inherent heterogeneity and nonlinear kinetics associated with solid-phase approaches has inspired the development of soluble polymers, such as poly(ethylene glycol) (PEG) as adjuncts.1

Fishing Out b-Amino Alcohols6

The PEG-dialkylborane reagent (1) was developed as a generic soluble polymer approach to remove b-amino alcohol products from complex reaction mixtures via their respective 1,3,2-oxazaborolidines in a process termed ‘fishing out’(1).7

In a typical procedure, the reaction mixture at the end of the reaction sequence shown in 2 was added to a solution of the reagent (in anhydrous toluene). The solution was then stirred for 1 h at 70 °C and then added into diethyl ether. The precipitated PEG-supported oxazaborolidines were isolated by filtration and the parent b-amino alcohol was released by addition to a solution of HCl (1 M solution in methanol). This procedure allowed isolation of amino alcohols in very high purity (> 95%).


1. Wentworth, P., Jr; Janda, K. D., Chem. Commun. 1999, 1917.
2. Zhao, X.-Y.; Janda, K. D., Tetrahedron Lett. 1997, 38, 5437.
3. Shuttleworth, S. J.; Allin, S. M.; Sharma, P. K., Synthesis 1997, 1217.
4. Kaldor, S. W.; Siegel, M. G.; Fritz, J. E.; Dressman, B. A.; Hanh, P. J., Tetrahedron Lett. 1996, 37, 7193.
5. Keating, T. A.; Armstrong, R. W., J. Am. Chem. Soc. 1996, 118, 2574.
6. Hori, M.; Janda, K. D., J. Org. Chem. 1998, 63, 889.
7. Hodge, P.; Waterhouse, J., J. Chem. Soc. Perkin Trans I 1983, 2319.

Paul Wentworth, Jr. & Kim D. Janda

The Scripps Research Institute and the Skaggs Institute for Chemical Biology, La Jolla, CA, USA



Copyright 1995-2000 by John Wiley & Sons, Ltd. All rights reserved.