Flow chemistry let a University of Cambridge team generate highly reactive diazo compounds in small, continuous amounts and consume them directly in carbon–carbon bond-forming reactions. In a 2016 report, the researchers described a sequence that made up to four new carbon–carbon bonds without isolating intermediate products. “Forbidden” refers to the hazards and practical limits of producing diazo compounds in large batch quantities—not a legal ban.
What the researchers did
Steven Ley’s Cambridge team used a hydrazone precursor to make diazo reagents as they were needed. The precursor solution was pumped over activated manganese dioxide under high pressure in a cooled, contained system. The resulting diazo compound flowed directly into room-temperature flasks containing boronic acids, where it reacted without a precious-metal catalyst.
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The first coupling product remained a boronic acid, so it could react again with another diazo reagent. The team designed sequences using as many as three different diazo species with one boronic acid, reporting up to four newly formed carbon–carbon bonds without isolating the intermediates. That is the scope reported for this research demonstration, not a guarantee that every substrate or sequence will work in the same way.
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Diazo compounds can be highly reactive, making their production and handling challenging, particularly when large quantities are accumulated in a batch vessel. In the reported flow approach, small, dilute quantities were generated continuously and sent directly to the next reaction. The intermediate was consumed promptly rather than isolated and stored, and successive couplings could proceed without separating each product.
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| Process feature | Reported flow sequence | Batch-handling concern |
|---|---|---|
| Reactive diazo material | Generated continuously in small, dilute quantities | Large quantities may be produced or accumulated together |
| Intermediate handling | Sent directly to react with boronic acid | Isolation or accumulation adds handling of reactive material |
| Multiple bond-forming steps | Sequence extended without isolating each intermediate | Separate isolation between steps may be required |
This is a hazard-management strategy, not proof that flow chemistry makes the process risk-free. The report does not provide a numerical scale-up comparison or establish that the method removes the need for careful process controls.
What “forbidden chemistry” means
Ley used “forbidden” to describe the usual limits on making diazo compounds in large-scale batch operations. He said: “Producing diazo compounds in batch mode on [large] scale is normally forbidden,” explaining that continuous production and immediate use of small quantities can open opportunities to explore new reactions. The phrase describes a practical safety constraint, not a legal prohibition.
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Ley also characterized the coupling as “a room temperature reaction without using precious metals.” Mimi Hii of Imperial College London, whose team researches continuous-flow processes in organic synthesis, called the work “excellent” and said it demonstrated how reaction engineering and rates could achieve an outcome “in a controlled and safe way.” Her comment describes the value of the engineering approach; it should not be read as a claim that the chemistry has no hazards.
What the report does—and does not—show
The Chemistry World account, published on 10 February 2016, presents a research demonstration by Ley’s team. It suggests that avoiding expensive metal catalysts could be useful in drug manufacturing, where reducing the risk of metal contamination in active pharmaceutical ingredients may matter. But the report does not establish that this sequence has been used commercially or to make a drug.
- It reports up to four new carbon–carbon bonds in the demonstrated sequence.
- It does not give a yield, throughput benchmark, or cost analysis.
- It does not show commercial deployment or a drug manufactured by this method.
The underlying paper is cited as C. Battilocchio and colleagues, Nature Chemistry (2016), DOI 10.1038/nchem.2439. The article’s reported details and claims are available in Chemistry World’s 10 February 2016 report; the bibliographic record is also indexed at ResearchGate.
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