[51901-85-0]  · C6H4BBrO2  · B-Bromocatecholborane  · (MW 198.81)

(mild Lewis acid capable of selective cleavage of ether, ester and carbamate protecting groups;1,2 catalyst for Diels-Alder reactions3,4)

Alternate Name: 2-bromo-1,3,2-benzodioxaborole.

Physical Data: mp 47 °C; bp 76 °C/9 mmHg; fp 41 °C.

Solubility: sol dichloromethane.

Form Supplied in: colorless solid which fumes in air.

Preparative Method: can be conveniently prepared by treating Catechol with Boron Tribromide.1,2,5 The reagent is isolated by distillation as a moisture-sensitive, low-melting solid.

Handling, Storage, and Precautions: flammable solid, moisture sensitive; may be conveniently stored and dispensed as a 0.2 M solution in dichloromethane. If properly protected from moisture, these solutions are stable for a period of months.1 Use in a fume hood.

Cleavage of Ether, Ester, and Carbamate Protecting Groups.

B-Bromocatecholborane has been used to cleave a variety of ether, ester, and carbamate protecting groups.1,2,6-9 The reactions typically proceed at rt in dichloromethane using 1-2 equiv of the reagent. Boron Trifluoride Etherate has been used as a coreagent.2 After the cleavage of carbamate protecting groups the product amines should be freed of reactive bromide byproducts immediately upon workup in order to avoid problems of N-alkylation, especially when a reactive halide (e.g. benzyl bromide from cleavage of the Cbz protecting group) is involved.1

A series of competition experiments were performed in order to determine the relative reactivity of a number of common protecting groups towards B-bromocatecholborane. The results of these experiments suggest the following tentative order of reactivity: MOM and MEM ethers > t-Boc > Cbz and t-Bu ethers > benzyl ethers > allyl ethers > t-Bu esters and secondary alkyl ethers > benzyl esters and primary alkyl ethers >> alkyl esters.1 Selective cleavage of multifunctional substrates is possible when sufficient differences in reactivity exist; however, an excess of the reagent may be necessary when coordinating groups are present (eqs 1-4).1,7

B-Chlorocatecholborane has been used for similar transformations.1,9-11 This reagent is observed to be less reactive than the bromo analog and may be preferable when higher selectivity is required.1

Diels-Alder Reactions.

B-Bromocatecholborane also functions as a catalyst for the Diels-Alder reaction (eq 5).3,4 It is a very effective catalyst with a wide variety of dienophiles including a,b-unsaturated aldehydes, ketones, esters, thioesters, and nitriles. Turnover was seen with all dienophiles tested with the exception of acrylonitrile.3

In a comparison of the Diels-Alder reaction of cyclopentadiene with a variety of a,b-unsaturated esters and thioesters catalyzed by Aluminum Chloride or B-bromocatecholborane, the latter reagent afforded higher levels of endo selectivity.4

Related Reagents.

Boron Tribromide; Bromodimethylborane.

1. Boeckman Jr., R. K.; Potenza, J. C. TL 1985, 26, 1411.
2. King, P. F.; Stroud, S. G. TL 1985, 26, 1415.
3. Kelly, T. R.; Maity, S. K.; Meghani, P.; Chandrakumar, N. S. TL 1989, 30, 1357.
4. Wladislaw, B.; Marzorati, L.; Gruber, J. PS 1991, 59, 185.
5. Gerrard, W.; Lappert, M. F.; Mountfield, B. A. JCS 1959, 1529.
6. Lam, L. K.-P.; Jones, J. B. CJC 1988, 66, 1422.
7. Anthony, J.; Knobler, C. B.; Diederich, F. AG(E) 1993, 32, 406.
8. Kerrick, S. T.; Beak, P. JACS 1991, 113, 9708.
9. Everhart, E. T.; Craig, J. C. JCS(P1) 1991, 1701.
10. Golinski, M.; Heine, M.; Watt, D. S. TL 1991, 32, 1553.
11. Golinski, M.; Heine, M.; Orlicky, D. J.; Fitz, T. A.; Watt, D. S. Eicosanoids 1992, 5, 87.

Paul C. Anderson & Yvan Guindon

Bio-Méga/Boehringer Ingelheim Research, Laval, Québec, Canada

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