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27.24.2.2.2.2 Variation 2: Flash-Vacuum Pyrolysis of Acyl Ylides

DOI: 10.1055/sos-SD-027-00847

Schobert, R.; Gordon, G. J.Science of Synthesis, (2004271010.

Under conventional pyrolysis conditions (see Section 27.24.2.2.2.1) (2-oxoalkylidene)triphenylphosphoranes [Ph3P=C(R1)COR2; R1=H, alkyl] do not give the expected alkynes due to competing side reactions, including partial extrusion of triphenylphosphine and isomerization of the initially formed alkynes to allenes. However, terminal and simple aliphatic alkynes 80 are accessible by using flash-vacuum pyrolysis of stabilized phosphoranes 79 (Scheme 27).[‌421‌] The required apparatus typically consists of a quartz tube in a Büchi Kugelrohr oven where the sample is vaporized. The tube extends through a furnace that maintains temperatures between 250 and 1000°C, and the exit of the furnace is connected to a U-shaped cold trap. The entire pyrolysis line can be maintained at a pressure of 0.01Torr by a rotary oil pump.[‌421‌,‌422‌] Under these conditions the contact times in the hot zone are estimated to be of the order of 10ms and multigram quantities of alkynes can be obtained in a few hours. Vinylalkynes,[‌421‌,‌423‌] symmetrical and unsymmetrical 1,3-diynes,[‌424‌] conjugated bis(alkenynes),[‌425‌] 2-alkynoates,[‌426‌] enantiopure 4-aminoalk-2-ynoic acids,[‌427‌] and diacylalkynes (from ylides with R1=COCOR3)[‌428‌] have also been prepared by flash-vacuum pyrolysis of acylated ylides.

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