Trimethylsulfoxonium bromide (TBM, CAS 25596-24-1) is a crystalline salt made directly from DMSO and methyl bromide. It is the precursor to the Corey–Chaykovsky ylide used to install epoxides and cyclopropanes in pharmaceutical and steroid synthesis, and it sits at the start of several steroid epoxide routes.
(CH3)2SO + CH3Br40–80 °C, neat or MeCN→[(CH3)3SO]+ Br−
Reaction 1: Synthesis of trimethylsulfoxonium bromide (TBM) from DMSO and methyl bromide
Where TBM Is Used
Deprotonated with sodium hydride or potassium tert-butoxide, TBM generates dimethylsulfoxonium methylide, which transfers a methylene onto aldehydes and ketones to form epoxides and glycidyl ethers. The same ylide adds across α,β-unsaturated carbonyls to build cyclopropanes through conjugate addition and ring closure.
In steroid manufacturing, the TBM-derived ylide converts a canrenone intermediate to the corresponding 9α,11α-epoxide on the eplerenone route, a key methylene-bridge step. Beyond steroids, TBM epoxidation feeds oxirane building blocks used across fine chemical and API production, where a stable, weighable methylene donor is preferred over diazo compounds. It is also the reagent of choice when a cyclopropane ring must be installed on an enone during scale-up, because the sulfoxonium ylide tolerates ester, nitrile, and amide functionality that other methylene donors would attack.
Production from DMSO and Key Properties
TBM is manufactured by reacting DMSO with methyl bromide, either neat or in acetonitrile at 40–80 °C. Because the sulfoxide oxygen is nucleophilic, methyl bromide attacks at oxygen to form the sulfoxonium cation. On cooling, TBM crystallizes as a white, hygroscopic solid that is recrystallized to pharma-grade specifications. Low water is required in the feedstock, since water consumes methyl bromide and lowers the yield of the quaternary salt.
| Property | Value |
|---|---|
| Molecular formula | C₃H₉BrOS |
| CAS number | 25596-24-1 |
| Appearance | White crystalline solid |
| Main use | Corey–Chaykovsky methylene transfer (epoxides, cyclopropanes) |
Table 1: TBM key properties and use
Why TBM and DMSO Feedstock Quality
TBM is produced by methylating DMSO with methyl bromide, so the DMSO feedstock defines the final salt quality. Pharma-grade DMSO with controlled water and low residue is required, because trace impurities carry through methylation and recrystallization into the sulfoxonium and can fail residual-solvent and heavy-metal testing.
The ylide generated from TBM is more stable and more selective than the related sulfonium ylide, which makes it the practical choice when a clean epoxide or cyclopropane must be installed in a multi-ton API campaign. Avoiding diazo reagents also removes a significant safety and handling burden from the plant. For buyers, a pharma-grade TBM specification is tied directly to pharma-grade DMSO supply, so consistent salt quality depends on a controlled, low-residue DMSO feedstock rather than on the downstream recrystallization alone.
References
- Corey, E. J.; Chaykovsky, M. Dimethylsulfoxonium methylide: a reagent for methylene transfer. J. Am. Chem. Soc., 1965, 87(6), 1353–1364.
- Corey, E. J.; Chaykovsky, M. Methylenecyclopropane synthesis. J. Am. Chem. Soc., 1965, 87(6), 1354–1356.