Formation of Cumulenes, Triple-Bonded, and Related Compounds by Flash Vacuum Thermolysis of Five-Membered Heterocycles

Abstract

Flash vacuum thermolysis of a large variety of heterocyclic compounds is a useful means of production of ketenes,

Flash vacuum thermolysis of 1,2,3-thiadiazoles

“…13 C-NMR δ 13. 6,13.9,21.1,21.2,34.0,34.2,112.4,113.4,126.7,126.8,126.9,127.3,128.5,128.5,128.7,128.8,137.5,141.4. MS (m/z) : C,74.95;H,6.86;S,18.19.…”

“…This type of ring contraction has been observed in the thermolysis of eight and nine-membered rings. 6,10 It should be noted that the zwitterion 18 was also obtained as the main product in the thermal reactions of thiadiazole 17 using DMF at 150-170ºC. 9b In that case the structure of this compound was determined by single crystal X-ray analysis and the yield of those reactions were about 39%.…”

“…3 Thioketenes (v) have been prepared and isolated by the application of the flash-vacuum thermolysis (FVT) technique. [4][5][6] Previously identified products of thermolysis or photolysis of thiadiazoles include alkynes (C), thiophenes (D), 1,3-dithiafulvenes (E), and 1,2-or 1,4-dithiins (F) (Scheme 1). 7 1,2,3-Thiadiazoles unsubstituted at position-5 or with a -Cl, -OAr or -SAr substituent undergo loss of N 2 in the presence of a base to give alkynethiolates and subsequent quenching with alkyl-or acyl halides provides easy access to alkynylthioethers.…”

Formation of (Aza)fulvenallene, Cyanocyclopentadiene, and (Aza)fluorenes in the Thermal Rearrangements of Indazoles, Azaindazoles, and Homoquinolinic Anhydride

“…3 When suitable substituents are present, cyclization of vinylcarbene intermediates 2 can result in the formation of three-or five-membered rings. 4 Analogous tautomerization in indazole 3 will in principle generate the isomers 4−7 (Scheme 1A) prior to elimination of N 2 from either the 3H-pyrazole 5 or the 7aH-pyrazole 7. These tautomerizations are all 1,5-H shifts, but the bottom row (3 → 4 → 5) is expected to be much faster than the top row (3 → 7 → 6), where all aromaticity is lost.…”

“…Pyrazoles and indazoles are of importance in synthetic, medicinal, and physical organic chemistry. , The pyrolysis of pyrazoles 1 is known to result in tautomerization and nitrogen extrusion with consequential formation of allenes and acetylenes (eq ). When suitable substituents are present, cyclization of vinylcarbene intermediates 2 can result in the formation of three- or five-membered rings …”