Preparation and Reactivity of 1,3-Bis(alkylthio)allenes and Tetrathiacyclic Bisallenes

Abstract

Reactions of Ph(2)C(3) dianion, prepared from 1,3-diphenylpropyne and n-butyllithium, with alkyl thiocyanates or alkane dithiocyanates gave 1,3-bis(alkylthio)allenes 1 or tetrathiacyclic bisallenes 2, respectively. Thermal reactions of 1 gave thiophenes 4 and 7, benzothiepin 5, 1,2-bis(benzylidene)cyclobutane 6, thiete 8, and alpha,beta-unsaturated ketone 9, and the reactions of tetrathiacyclic bisallenes 2a gave a cyclic dimer, 1,2-bis(benzylidene)cyclobutane derivative 10, quantitatively. Irradiation of 1,3-bis(alkylthio)allenes 1 and tetrathiacyclic bisallenes 2a caused rearrangement to give alkynes 18, 20, and 21. In the irradiation of the cyclic bisallenes 2a, isomerizations from dl to meso and meso to dl isomers were also found. In the reactions of allenes 1 and cyclic bisallenes 2a with diphenyl diazomethane, the diazomethane reacted selectively with the double bond rather than with the sulfur atom.

Synthesis and Thermal Reaction of 1,3-Bis(alkylseleno)allenes

“…2:1 mixture of 4 and 5 was obtained from 5 in a refluxing p- xylene after 1.5 h. Thermal reaction of 1,3-bis(benzylseleno)allene 2 proceeded smoothly in a refluxing p- xylene, and enediynes 4 and 5 were formed in 36 and 8% yields, respectively, together with dibenzyl diselenide 8 (57%) and dibenzyl selenide 9 (25%) after 1.5 h. In the thermal reaction of unsymmetric allene 3 , the reaction was intricate, and only enediyne compounds 4 (28%) and 5 (7%) were isolated. The thermal reactivities of selenium-substituted allenes 1 and 2 were found to be different from those of the corresponding sulfur-substituted allenes, and the difference of the reactivities may be due to smaller bond dissociation energy of carbon−selenium bond than that of carbon−sulfur bond. The thermal reactions of sulfur-substituted allenes 1 , 2 , and 3 were found to proceed via radical mechanism since an addition of galvinoxyl in the reaction of 1 and 2 inhibited both reactions.…”

“…Allene chemistry has been widely studied, , and substituted allenes by heteroatoms have also received attention . Recently, we reported the synthesis and reaction of sulfur-substituted allenes, and the thermal reactions of 1,3-bis(alkylthio)allenes have been found to yield thiophene derivatives . Several selenium-substituted allenes are also known as isolable compounds or reactive intermediates .…”

Mechanistic Aspects and Synthetic Applications of Radical Additions to Allenes

“…Whereas there has been extensive work on thermal and photochemical [2 + 2] cycloadditions of allenes and enyne reactions involving diradical intermediates, there are other reactions of allenes proposed to proceed through similar species that are worth of mentioning. For example, Kamigata studied sulfur- and selenium-containing allenes under thermal and photochemical conditions . Acyclic 1,3-bis­(alkylthio)­allenes 721 (R = H or Ph) gave several products under thermal conditions (Scheme ).…”

“…For example, Kamigata studied sulfur-and selenium-containing allenes under thermal 230 and photochemical conditions. 231 Computational analysis indicates that the reaction of propadiene with benzene under thermal conditions likely occurs via a concerted pathway (Scheme 184). A stepwise pathway would entail the loss of aromaticity, which would counteract any favorable allylic radical stabilization in intermediate 785.…”

Thiophosphonium–Alkyne Cycloaddition Reactions: A Heavy Congener of the Carbonyl–Alkyne Metathesis

“…These reactions have been extensively utilized in synthetic chemistry . The carbonyl–alkyne metathesis proceeds via a four-membered oxete intermediate, which is usually directly transformed into the α,β-unsaturated ketone, − unless it is stabilized by strongly electron-withdrawing groups. − Due to the lower tendency of sulfur to form double bonds, thietes are more stable than oxetes, − and a dynamic equilibrium between the “closed” thiete and “open” α,β-unsaturated thioketone form was observed with thioether substituents. , Given these differences between oxetes and thietes, we became curious to explore how the introduction of another heavy main group element would affect the stability of the four-membered ring species. Although numerous examples for heavy main group carbonyls R 2 E=O and thiocarbonyls R 2 E=S have been synthesized, − the reactivity with alkynes is little developed.…”