Flash vacuum pyrolysis

In organic chemistry. flash vacuum pyrolysis (FVP) is a technique that entails heating a precursor molecule intensely and briefly. The method is used to induce transformations that cannot readily be achieved in solution methods such as in refluxing solvents. The technique is uncommon due in part to limitations on the scale on which reactions can be carried out.

In terms of implementation of FVP, two key parameters are the temperature and duration (or residence time).[1] Often the experiment entails volatilizing a precursor, which is drawn through a "hot zone" followed by rapid condensation. The apparatus typically is conducted under dynamic vacuum. The hot zone must impart heat to the gaseous molecules, so it is packed with solids to induce gas-solid collisions. The packing material is generally chemically inert, such as quartz.[2] The precursor (i) volatilizes with gentle heating and under vacuum, (ii) the precursor fragments or rearranges in the hot zone, and finally (iii) the products are collected by rapid cooling. Rapid post-reaction cooling and the dilution inherent in gases both suppress bimolecular degradation pathways.

The technique is applied to conversions that proceed via unimolecular pathways.[3][4]

## Examples

* 2-Acetoxydioxane, when heated at 425 °C converts to the highly reactive dioxene, via loss of acetic acid.[5]
* 2-Furonitrile has been prepared by flash-dehydration of 2-furoic acid amide or oxime over molecular sieves.[6]
* The strained ring benzocyclobutenone has been prepared by FVP from a simple benzoyl chloride precursor.[7]

Illustration of FVP process, the intermediate ketene is unstable and the product, like most organic compounds, tolerates brief, but not prolonged treatment at high temperatures.[7]

* FVP was used to effect a retro-[4+2] cycloaddition (extruding cyclopentadiene) in the earlier stages of the total synthesis of Galbulimima alkaloid GB-13.[8]

## References

1. ↑ McNab, Hamish "Chemistry without reagents: synthetic applications of flash vacuum pyrolysis" Aldrichimica Acta 2004, volume 37, pp. 19–26. http://www.sigmaaldrich.com/ifb/acta/v37/acta-vol37-2004.html#20
2. ↑ Wentrup, Curt (2014). "Flash (Vacuum) Pyrolysis Apparatus and Methods". Australian Journal of Chemistry 67 (9): 1150–1165. doi:10.1071/CH14096.
3. ↑ Wentrup, Curt (2017). "Flash Vacuum Pyrolysis of Azides, Triazoles, and Tetrazoles". Chemical Reviews 117 (5): 4562–4623. doi:10.1021/acs.chemrev.6b00738. PMID 28234461.
4. ↑ Tsefrikas, Vikki M.; Scott, Lawrence T. (2006). "Geodesic Polyarenes by Flash Vacuum Pyrolysis". Chemical Reviews 106 (12): 4868–4884. doi:10.1021/cr050553y. PMID 17165678.
5. ↑ Matthew M. Kreilein, James C. Eppich, Leo A. Paquette "1,4-Dioxene" Org. Synth. 2005, volume 82, pp. 99. doi:10.15227/orgsyn.082.0099
6. ↑ Jacqueline A. Campbell; McDougald, Graham; McNab, Hamish; Rees, Lovat; Tyas, Richard (2007). "Laboratory-scale synthesis of nitriles by catalyzed dehydration of amides and oximes under flash vacuum pyrolysis (FVP) conditions". Synthesis 2007 (20): 3179–3184. doi:10.1055/s-2007-990782.
7. ↑ 7.0 7.1 Peter Schiess; Ppatibha V. Barve; Franz E. Dussy; Andreas Pfiffner (1995). "Benzocyclobutenone By Flash Vacuum Pyrolysis". Org. Synth. 72: 116. doi:10.15227/orgsyn.072.0116.
8. ↑ Larson, Kimberly K.; Sarpong, Richmond (2009-09-23). "Total Synthesis of Alkaloid (±)-G. B. 13 Using a Rh(I)-Catalyzed Ketone Hydroarylation and Late-Stage Pyridine Reduction". Journal of the American Chemical Society 131 (37): 13244–13245. doi:10.1021/ja9063487. ISSN 0002-7863. PMID 19754185. https://doi.org/10.1021/ja9063487.

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