Alkylhalovinylboranes: a new class of Diels–Alder dienophiles

Abstract

The Diels-Alder reactions of alkylhalovinylboranes have been investigated theoretically and experimentally. Alkylhalovinylboranes presented higher reactivity than the corresponding dialkylvinylboranes. Although endo/exo selectivities were high for the reactions with cyclopentadiene, facial selectivities for the chiral analogues were low. Our results demonstrate that the replacement of an alkyl group on the boron atom by a halogen increases the dienophilicity considerably.Scheme 1 Synthesis and Diels-Alder reaction of chiral dialkylvinylborane 1. † Electronic supplementary information (ESI) available: Experimental section, NMR spectra, computational methods, FMOs and global and local reactivity indices of reactants, optimized geometries of the transition structures (TSs) not included in the paper and charge transfer (GEDT) of TSs, cartesian coordinates, absolute energies including zero-point energy corrections, free energies and number of imaginary frequencies of all the stationary points reported in the paper and values of imaginary frequencies of all transition structures. See

Boron and Silicon-Substituted 1,3-Dienes and Dienophiles and Their Use in Diels-Alder Reactions

“…Oxidation of the boron-substituted cycloadduct (88) produced 5-norbornen-2-ol (89) in 48% yield and 4:1 endo:exo selectivity. Pellegrinet and co-workers reported preparation of some alkylhalovinylboranes and their Diels-Alder reactions (Scheme 24) [22]. They reasoned that these vinyl boranes would be less sterically hindered and more electron deficient than dialkylvinylboranes and hence should be more reactive in Diels-Alder chemistry.…”

Regioselectivity Switch Towards the Development of Innovative Diels‐Alder Cycloaddition and Productive Applications in Organic Synthesis

“…So far, several studies have been published on stereoselective cycloadditions of chiral vinylboranes. However, most of which use severe reaction conditions such as high temperature due to the usage of boronate ester [148,149] . Although alkyl boronate can perform cycloadditions at lower reaction temperatures, face selectivity was declined due to a lack of rigidity [150] .…”

“…However, most of which use severe reaction conditions such as high temperature due to the usage of boronate ester. [148,149] Although alkyl boronate can perform cycloadditions at lower reaction temperatures, face selectivity was declined due to a lack of rigidity. [150] To overcome these drawbacks, Brown and coworkers in 2020, devised an auxiliary that combines the characteristics of high reactivity and rigidity, where the stereoselective cycloaddition of alkenylborane to the arena of enantioselective synthesis was crafted in striking contrast to the reaction of unsaturated carbonyls.…”

Stereoselective [4+2]‐Cycloaddition with Chiral Alkenylboranes

“…Several reports have described the development of stereoselective [4+2]‐cycloadditions with chiral vinylboranes (Scheme B) . In 1997, Avery and co‐workers reported the reaction of vinylborane 1 with isoprene to provide the cycloadduct in 63:33 er after oxidation.…”

“…Due to the use of a boronate ester, high reaction temperature was necessary, which may be detrimental to selectivity . More recently, Pisanao and Pellegrinet described the cycloaddition of (+)‐2‐carene derived vinylborane 2 with cyclopentadiene to generate the product in 70:30 er after oxidation . Since the more reactive alkylhalovinylborane was used, the reaction temperatures were significantly lower compared to use of a boronate ester.…”