Propiolic esters, cycloadducts

Surprisingly, the catalyzed reaction of alkenes with propiolate esters results in [2 + 2] cycloadducts, as well as ene adducts, in a manner that is related to the substitution on the alkene. No cycloadducts are formed from alkenes with two substituents on one end of the double bond. ... 

The 1,2-thiaphospholes (20) react thermally with norbornadiene, norbornene, or diethyl azo-dicarboxylate to produce double Diels Alder cycloadducts (24) and (25). With mixtures of norbornadiene and methyl acrylate or acrylonitrile, crossed double Diels-Alder cycloadducts are obtained. In the presence of AICI3 the 1,2-thiaphospholes (20) react with acrylic esters, acrylonitrile or methyl vinyl ketone to give tandem Diels-Alder/Michael adducts such as (26). The AICI3-promoted reaction of (20) with ethyl propiolate affords with the elimination of a sulfur atom the stable phosphinine (27) (Scheme 4) <94BCJ2785>. 

The combined action of ethyl propiolate and triethylamine on the salt (445) results in the ester (447), which arises from the intermediate cycloadduct (446). The triazolones (448) decompose thermally to nitrogen, carbon monoxide, and the imines ArN=CMe2. ° The stable betaine (450) is produced by the reaction of 4-phenyl-1,2,4-triazoline-3,5-dione (449) with diazodeoxybenzoin, PhCNjCOPh, whereas diazoacetophenone is reported to yield an oligomer (451). 

The kinetically stabilized cyclobutadiene 413 adds methyl propiolate in two senses to produce a mixture of the Dewar benzenes 414 and 415, but addition of a number of terminal alkynes RC=CH [R = C02Me, Tos or Ph2P(0)] to the r-butyl ester 416 gave only the former type of cycloadduct, i.e., 417. ... 

The fully intermolecular variant of this concept has also been realized. A variety of propiolates were coupled with allenes to afford a regioselective aromatic cycloadduct (Scheme 2.12) [9]. A symmetric ester bearing an alkyne was coupled successfully with butyl allene to afford arene 46 in high yields. Unsymmetrical internal alkynes (i.e., methyl- and phenyl-substituted propiolates) were also coupled with butyl-, cyclohexyl-, and phenyl-substituted allenes in high yields (47 to 50, Scheme 2.12). The current scope of the reaction is confined to the use of activated alkyne substrates in this cycloaddition. 

In 2006, Hanzawa and coworkers demonstrated that the cationic Rh(I) catalyst, prepared in situ from [RhCl(cod)]2 and AgSbFe, was efficient to promote the intramolecular [5+2] cycloaddition of a series of ester-tethered alkynyl VCPs [23]. The reactions were performed in fiuorinated alcohol media, such as hexafluoroisopropanol (HFIP), at room temperature. As shown in Scheme 20.8, the [5+2] cycloaddition of (3-cyclopropylacrylates 16 yielded the corresponding bicyclic lactones 17 in fair yields. The latter were actually generated by the isomerization of the initially formed [5+2] cycloadducts 18 during the work up. In addition, the scope of this methodology was successfully extended to propiolates 19, which led to the corresponding cycloadducts 20. In those cases, the best yields were obtained when a mixture of hexafluoroisopropanol and... 

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