[89449-88-7]  · C6H14BBrS2  · Bromobis(isopropylthio)borane  · (MW 241.01)

(mild Lewis acid capable of converting methoxyethoxymethyl (MEM) ethers into isopropylthiomethyl ethers;1 deprotection of MEM ethers;1 and cleavage of mono methoxymethyl (MOM) (or MEM) ethers of 1,2- and 1,3-diols1)

Physical Data: bp 75-80 °C/2 mmHg; light-yellow oil.

Solubility: sol most organic solvents such as pentane, CH2Cl2, etc.

Analysis of Reagent Purity: 1H NMR (CDCl3) d 3.64 (hept, 2H, J = 6.8 Hz, CH), 1.385 (d, 12H, J = 6.8 Hz, Me).

Preparative Method: 1 to a stirred mixture of 3.57 g (0.010 mol) of bis(isopropylthio)lead2 in 20 mL of pentane under argon at 25 °C is added a solution of 0.95 mL (0.010 mol) of Boron Tribromide in 10 mL of pentane. After the mixture is stirred for 20 min, 10 mL of CH2Cl2 is added and the mixture is heated under reflux for 2.5 h. The reaction mixture is cooled to 25 °C and the white precipitate (PbBr2) is removed by filtration under argon. The filtrate is distilled under argon to remove solvents and then distilled under reduced pressure: 2.13 g (90% yield) of a yellow oil; bp 75-80 °C/2 mmHg.

Handling, Storage, and Precautions: should be stored under argon at 0 °C. The compound has an unpleasant odor and reacts rapidly with water and other protic solvents.

Bromobis(isopropylthio)borane converts various MEM ethers into the corresponding isopropylthiomethyl ethers. Among several examples shown below, treatment of the MEM ether of cholesterol in CH2Cl2 at -95 °C with 3-5 equiv of (i-PrS)2BBr for 5 min followed by the addition of 4-Dimethylaminopyridine (DMAP) (2 equiv/equiv of boron reagent) and stirring the mixture at -95 °C for 15 min, then at -50 °C for 10 min, before quenching with aqueous potassium carbonate gives an 89% yield of the isopropylthiomethyl ether of cholesterol along with a 9% yield of cholesterol (eq 1).1 Presumably, DMAP displaces isopropylthiolate anion from the boron complex, thereby providing a reactive S-nucleophile to attack the dioxymethylene carbon of the boron-coordinated MEM ether.

Under different conditions, (i-PrS)2BBr cleaves MEM ethers into the corresponding alcohols. For example, in the absence of DMAP, reaction of MEM ether (1c) with 1.5 equiv of (i-PrS)2BBr in CH2Cl2 at -95 °C for 5 min followed by treatment with aqueous sodium bicarbonate provides an 88% yield of the corresponding alcohol and only a 2% yield of the isopropylthiomethyl ether.

Bromobis(isopropylthio)borane also cleaves mono MOM (or MEM) ethers of 1,2- and 1,3-diols to the corresponding diols.1 Treatment of glycol mono MOM ethers with 1.2 equiv of (i-PrS)2BBr in CH2Cl2 at -78 °C followed by methanol at 23 °C for 1 h affords excellent yields of the corresponding diols (eqs 2-4). It should be noted that these glycol mono MOM ethers under conventional acid-catalyzed processes (using protic or Lewis acids) produce cyclic diol formals as the major product.

Deprotection of MEM ethers with other Lewis acidic boron reagents (such as 2-chloro-1,3,2-dithiaborolane3 and Bromodimethylborane4) to the corresponding alcohols has also been reported.

1. Corey, E. J.; Hua, D. H.; Seitz, S. P. TL 1984, 25, 3.
2. (a) Pelter, A.; Levitt, T. E.; Smith, K.; Jones, A. JCS(P1) 1977, 1672. For a review of thioboranes: (b) Mikhailov, B. M. RCR 1968, 935.
3. Williams, D. R.; Sakdarat, S. TL 1983, 24, 3965.
4. Guidon, Y.; Morton, H. E.; Yoakim, C. TL 1983, 24, 3969.

Duy H. Hua & Jinshan Chen

Kansas State University, Manhattan, KS, USA

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