A synthetically useful source of propargyl radicals

Some aspects of radical chemistry in the assembly of complex molecular architectures

“…Alkynes or allenes can be obtained, depending on the disposition of the internal alkene with respect to the delocalised radical [40–41]. In the sequence displayed in Scheme 18, the addition–cyclisation of a malonyl radical to enyne 89 furnishes allenyl acetate 91 by cyclisation of propargyl radical 90 [41].…”

Propargyl Radicals in Organic Synthesis

“…In the reaction with acetylene, a lower activation energy is found for propargylic site reaction relative to allenic site reactivity, despite greater stability of the immediate allenic site product. [8] Propargyl radical -propargyl radical combination reactions are found, computationally and experimentally, to give comparable amounts of propargylpropargyl (2) and propargyl-allenyl (3) coupling at room temperature, with significantly lower amounts of allenyl-allenyl (4) coupling (Scheme 1). [9] There is a paucity of data on the kinetics of propargyl radical addition to synthetically relevant π systems, with the only data of some relevance being derived under conditions not common in organic synthesis.…”

Sulfur Betaines from S-Propargyl Xanthates. Unusual Chemistry from a Simple Functional Group

“…As shown in Scheme 31, heating TMS-substituted propargyl xanthate 164 in refluxing cyclohexane does not cause rearrangement into allene 165. 33 It is therefore now possible to initiate the radical chain process outlined in Scheme 1 without complications from the allene and betaine by simply operating at around 80 °C. Indeed, heating xanthate 164 in refluxing cyclohexane with N-benzylmaleimide in the presence of lauroyl peroxide as initiator furnished a high yield (85%) of the normal radical adduct 166.…”

“…Indeed, heating xanthate 164 in refluxing cyclohexane with N-benzylmaleimide in the presence of lauroyl peroxide as initiator furnished a high yield (85%) of the normal radical adduct 166. 33 Similarly, capture of the propargyl radical with phenyl vinyl sulfone afforded addition product 167 (the yield in parentheses is based on recovered starting material). Xanthate 142a, which in refluxing chlorobenzene is converted into diene 144 (R, R′ = H), produces the expected radical adduct 168 with N-benzylmaleimide when the radical reaction is conducted in refluxing cyclohexane.…”

“…Finally, while the intermolecular capture of propargyl radicals requires reactive alkenes, a simple alkene is sufficient in the intramolecular variant, as shown by the formation of pyrrolidine 170 from propargyl xanthate 169. 33 By controlling the reaction temperature, it is thus possible to proceed either through the chemistry of the betaine or through the radical chemistry of the S-propargyl xanthates. Moreover, xanthates can be used to produce complex allenes by generating propargyl radicals in an indirect manner, as illustrated by the sequence in Scheme 32.…”