Ruthenium-catalyzed cycloadditions between alkynyl phosphonates and bicyclic alkenes

Ruthenium-catalyzed cycloadditions of bicyclic alkenes with alkynyl phosphonates were investigated. In regard to the Ru-catalyzed [2+2] cycloadditions, the phosphonate moieties were found to be compatible, giving the corresponding cyclobutene cycloadducts in low to excellent yield (up to 96%). Alkynyl phosphonates showed lower reactivity than other heteroatom-substituted alkynes such as alkynyl halides, ynamides, alkynyl sulfides and alkynyl sulfones, and required a higher reaction temperature and much longer reaction time. To this end microwave heating was employed to expedite the reaction. While yields comparable to the conventionaly heated cycloadditions were not achieved, the reaction was much faster by microwave heating. In direct comparison over a 2 h period, yields were much greater in the microwave heated reactions. Computational studies determined that the electronic nature of the triple bond is sufficiently different in alkynyl phosphonates compared to other substituents. While investigating solvents of varying polarity in the microwave assisted [2+2] cycloaddition a new mode of reaction, previously unknown to this catalyst, was discovered. At elevated temperatures, in polar solvents, norbornadiene undergoes a Ru-catalyzed [2+2+2] homo Diels-Alder cycloaddition with alkynyl phophonates. This reaction was optimized to produce excellent yields with the model alkynyl phosphonate studied. Investigation of other alkynes revealed that the scope of this reaction may be limited to phosphonate substituted alkynes.