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The unidirectional thermal ring-closing reaction of cis-4-phenyl-5-borylpenta-1,2,4-triene giving 4-boryl-3-methylene-1-phenylcyclobutene proceeded significantly faster than that of the trans-isomer. The large rate difference between the cis- and trans-stereoisomers is ascribed to electronic participation of the vacant boron p orbital in the transition state SHOMO.
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Murakami et al. (2004) studied this question.
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