NEGATIVELY SUBSTITUTED ACETYLENES .2. CYCLOADDITION REACTIONS WITH STYRENES

Activated acetylenes, such as dicyanoacetylene, acetylenedicarboxylic ester, and benzyne, react with styrenes to give 1:2 adducts of type 2 by a sequence of Diels-Alder addition and ene reaction. Unlike in previously reported additions of this nature, the styrene, and not the acetylene, acts as the enophile in the second step. From the reactions of dicyanoacetylene and benzyne with styrene, mixtures of threo and erythro adducts have been isolated, resulting from exoid and endoid attack by the styrene in the ene reaction. The fact that no β-phenylethyldihydronaphthalenes (e.g., 22) could be detected among the products indicates that in the transition state of the second-step ene reaction, hydrogen transfer has proceeded to a larger extent than carbon-carbon bond formation. The scope of these reactions is discussed. © 1969, American Chemical Society. All rights reserved.