A new method for the in situ generation of chiral and achiral vinylboranes, and their use as reactive dienophiles in Diels-Alder reactions

Vita.Achiral vinyldihaloboranes, trivinylborane were generated in situ via a newly developed methodology which involves the reaction of vinyltributyltin with BBr$sb3$ or BCl$sb3.$ These compounds proved to be highly activated dienophiles in Diels-Alder reactions with simple dienes. Kinetic studies revealed that vinyldibromoborane is the most reactive dienophile among them. With a bimolecular rate constant of $rm 2times 10sp{-3} Msp{-1}ssp{-1}$ for its reaction with isoprene it is only second to tetracyanoethylene in its reactivity. This tremendous reactivity is apparently due to electronic effects of the bromine atoms bonded to boron. The dienophiles studied: vinyldibromoborane, vinyldichloroborane, divinylbromoborane, and trivinylborane gave the normal regiochemical products as predicted by Frontier Molecular Orbital Theory with 1- and 2-substituted-1,3-butadienes. Ab initio calculations indicate that vinyldihaloboranes adopt a planar structure, therefore the observed regioselectivities are electronically controlled. The involvement of a 4 + 3 transition state has been predicted for these reactions and this is proposed to be responsible for the endo-selectivities obtained for theses systems. Good to outstanding regioselectivities and endo-selectivities were observed comparable to Diels-Alder reactions catalyzed by Lewis acids. Thus vinyldibromoborane reacted with 2-tert-butyl-1,3-butadiene in less than 15 min at 25$spcirc$C to afford 4-tert-butyl-3-cyclohexen-1-ol and 3-tert-butyl-3-cyclohexen-1-ol in a 94:6, after oxidation with alkaline hydrogen peroxide. The other dienophiles gave regioselectivities $>$90%. The yields were good to excellent. The reaction of vinyldibromoborane with 1-phenyl-1,3-butadiene resulted in $>$99% regio- and endo-selectivity, and therefore a synthetically useful reaction. Chiral vinylboranes with auxilliaries derived from the hydroboration of chiral alkenes such as limonene, longifolene, $beta$-pinene were also studied. A modest chiral induction of 41% ee was obtained for the endo product of the reaction between dilongifolylvinylborane and cyclopentadiene. The vinylborane with camphosulfonyl groups as the chiral auxillaries was synthesized easily. However, this dienophile proved to be very stable towards Diels-Alder reactions with cyclopentadiene at up to 55$spcirc$C