[51901-85-0] · C6H4BBrO2 · B-Bromocatecholborane · (MW 198.81)
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.
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.
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
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
Paul C. Anderson & Yvan Guindon
Bio-Méga/Boehringer Ingelheim Research, Laval, Québec, Canada