Cycloaddition reactions of alkenyl- and alkynylboron compounds

Due to their great versatility, boron-substituted dienophiles and dienes have emerged as attractive building blocks in cycloaddition reactions. The early works in the field have been reviewed.1 

The photocycloaddition of 1-alkenylboranes in cyclohexane provided m-fused head-to-head adducts for 2-cyclopen-tenone and a mixture of cis- and trans-isomer for 2-cyclohexenone (Equation (71)).397 

Cycloaddition reaction of m-chlorotributylstannane was catalyzed by BC13 or 9-Br-9-BBN at 25 °C (Equation (72)).404 A catalytic cycle involved [2+4]-reaction of 1-alkenylborane, generated in situ by Si-B transmetallation. 

The reactivity and selectivity of cycloaddition can be considerably increased in intramolecular versions. The protocol was first demonstrated in the Diels-Alder reaction between anthrone 253416,417 and 4-hydroxy-2-butenoate mediated by phenylboronic acid (Equation (73)).418,419 Another method developed for the intramolecular cycloaddition is the synthesis of trienylboranes 256 by hydroboration of terminal alkynes (Equation (74)).419-422 

2 The intramolecular version was studied to control the selectivity of the insertion 268.433 Cyclotrimerization of bis(boryl) acetylene provided a novel hexa(boryl)benzene 269 (cat represents catecholato group).434,435 

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