an example of an electrocyclic reaction is the conrotatory thermal ring-opening of benzocyclobutane. the reaction product is a very unstable ortho-quinodimethane. this molecule can be trapped in an endo addition with a strong dienophile such as maleic anhydride to the diels-alder adduct. the chemical yield for the ring opening of the benzocyclobutane depicted in "scheme 2" is found to depend on the nature of the substituent r. with a reaction solvent such as toluene and a reaction temperature of 110 °c, the yield increases going from methyl to isobutylmethyl to trimethylsilylmethyl. the increased reaction rate for the trimethylsilyl compound can be explained by silicon hyperconjugation as the βc-si bond weakens the cyclobutane c-c bond by donating electrons.