Dramatic Effects of Boryl Substituents on Thermal Ring-Closing Reaction of Vinylallenes

The unidirectional thermal ring-closing reaction of <i>cis</i>-4-phenyl-5-borylpenta-1,2,4-triene giving 4-boryl-3-methylene-1-phenylcyclobutene proceeded significantly faster than that of the <i>trans</i>-isomer. The large rate difference between the <i>cis</i>- and <i>trans</i>-stereoisomers is ascribed to electronic participation of the vacant boron p orbital in the transition state SHOMO.