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By generating reactive diazo compounds a little at a time and sending them straight into a coupling reaction, a University of Cambridge team demonstrated a sequence that formed up to four new carbon–carbon bonds without isolating its intermediates. The 2016 report called this “forbidden chemistry” because producing diazo compounds in large batches is normally considered too hazardous—not because the reaction is legally prohibited.
What the researchers demonstrated
Steven Ley’s Cambridge team used a continuous-flow setup to make highly reactive diazo reagents from hydrazone precursors. A 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.
The initial coupling product was itself a boronic acid, allowing it to react again with another diazo reagent. The researchers extended the sequence to use as many as three different diazo species with one boronic acid, forming up to four new carbon–carbon bonds without isolating the intermediate products. This describes the demonstrated scope, not a guarantee that every starting material or sequence will work. Chemistry World’s 2016 report describes the study.
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Diazo compounds can be highly reactive, making their preparation and accumulation hazardous in batch processes. In the reported flow approach, small, dilute amounts were generated continuously and consumed promptly rather than collected or stored. The process links formation to use, limiting the need to handle a larger accumulated quantity.
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That is a hazard-management strategy, not proof that the chemistry is risk-free. Containment, pressure, cooling, reagent handling, and the specific reaction conditions still matter. Flow processing does not remove the need for appropriate engineering controls and chemical-safety procedures.
How the flow sequence compares with batch handling
| Process feature | Reported flow approach | Batch concern described in the report |
|---|---|---|
| Reactive diazo material | Generated continuously in small, dilute quantities | Production on large scale is normally considered hazardous |
| Handling after formation | Sent directly into the coupling reaction | Accumulating or isolating reactive material is the practical hazard the approach seeks to avoid |
| Building the carbon framework | Couplings can be repeated without isolating intermediate products | Repeated steps can require handling and isolating intermediates |
The comparison is qualitative; the report does not provide a numerical scale-up comparison.
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What “forbidden chemistry” means here
Ley used “forbidden” as shorthand for the practical limits of producing diazo compounds in large batches, not as a claim that the chemistry is illegal. He described the reaction as “a room temperature reaction without using precious metals.” He also said, “Producing diazo compounds in batch mode on [large] scale is normally forbidden,” adding that generating small quantities continuously and using them immediately creates opportunities to discover new reactions.
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Mimi Hii of Imperial College London, whose team researches continuous-flow processes in organic synthesis, called the study “excellent.” She said it demonstrated how understanding reaction engineering and reaction rates can help achieve the desired outcome in a controlled and safe way.
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What the result could mean for drug manufacturing
The authors suggested that metal-free coupling might be useful in drug manufacturing because avoiding expensive metal catalysts could reduce the risk of metal contamination in active pharmaceutical ingredients. That is a potential application, not an outcome established by this demonstration. The report does not describe a drug made by the sequence, commercial deployment, production throughput, yields, or a cost analysis.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Source and scope
Andy Extance’s Chemistry World report, published 10 February 2016, covers the Cambridge team’s demonstration and cites C. Battilocchio and colleagues’ paper in Nature Chemistry (2016), DOI 10.1038/nchem.2439. The specific experimental conditions and broader substrate scope should not be inferred beyond what the report describes.
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