Abnormal effect of Gn2SO4 as compared to other guanidinium salts on rates and stereoselectivities of Diels–Alder reactions

An abnormal effect in that guanidinium sulphate increases the rates and endo product formation of the reaction of cyclopentadiene with methyl acrylate is recorded for the first time in Diels–Alder chemistry. Other guanidinium salts like chloride, bromide, acetate and perchlorate inhibit the reaction rates and give rise to more exo products. This contrasting effect of Gn2SO4 on the kinetics of the Diels–Alder reaction can be attributed to the dominant role of SO42− over the guanidinium cation.