DMSO – Pyridine-SO3 (Parikh-Doering)
Mechanism + Description
This widely used Swern variant uses pyridine-sulfur trioxide complex as an activating agent in a mechanism analogous to the Swern and Pfitzner-Moffat mechanisms.
These reagents are common in scale-up examples.
HSO4 is a common impurity in a commercial pyridine-SO3 complex. This can catalyze undesired side reactions, but can be effectively neutralized by the addition of extra pyridine to give 2 C5H5NH SO4.
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Relevant scale-up example
- Atom efficiency (by-products)
Sulphur trioxide is oxidized to sulfate. Pyridine is recovered from the reaction unchanged, but never reused. The organic base is converted to the corresponding sulfate salt.
- Safety concerns
This oxidation can be very exothermic. Me2S may off-gas from the reaction mixture.
- Toxicity and environmental/aquatic impact
Pyridine is toxic to humans and the environment. SO3 is an acidic gas and contributes to acid rain. The DMS by-product occurs naturally in the environment, but it is an irritant, volatile, and has a disagreeable smell.
- Cost, availability & sustainable feedstocks
Relatively cheap and available on-scale. Pyridine can also be purchased in polymer bound form, though at a cost premium.
- Sustainable implications
Pyridine used to be made from coal tar and coal gasification, but greener routes from acetaldehyde and ammonia are now extensively used.