Palladium-catalyzed cross-coupling reaction of 1,1-diboryl-1-alkenes with aryl and alkenyl iodides was found to proceed stereoselectively, giving rise to the corresponding mono-coupled product as a single diastereomer with E-configuration. Second coupling of the initial product with another aryl iodide affords diverse triarylalkenes in their stereochemically pure form. This highly stereoselective approach for triarylalkenes allows one to synthesize both diastereomers in one pot from 1,1-diboryl-1-alkenes.
Double Horner-Wadsworth-Emmons reaction of (E)-2,3-diaryl-1,4-bis(diethylphosphonyl)but-2-ene with (p-substituted) benzaldehydes gave (1E,3E,5E)-1,3,4,6-tetraarylhexa-1,3,5-trienes in moderate to good yields. Substitution of electron-withdrawing or -donating groups at the para position of the 1,6-diphenyl groups induced a slight bathochromic shift of UV spectra measured in CHCl(3) compared with that of the parent 1,3,4,6-tetraphenylhexa-1,3,5-triene. Although fluorescence was not observed with all the trienes in CHCl(3), they markedly emitted visible light in powder forms with quantum yields of 0.15-0.44. Introduction of amino groups at the para position of the 3,4-diphenyl groups induced a bathochromic shift of emission maxima with good solid-state quantum yields. Thus, the tetraarylated triene framework is found to serve as a new class of fluorophores that exhibit aggregation-induced emission.
Cyclopropylidene lithium carbenoids reacted with bis(pinacolato)diboron in THF/Et2O at −110 °C to give various 1,1-diborylated cyclopropanes in good yields. Treatment of the diborylated cyclopropanes with 3-chloro-1-lithio-3-methyl-1-butyne produced the corresponding diborylated allenylcyclopropanes, which underwent ring-expansion in the presence of a Rh catalyst to give 1,2-diborylated methylenecyclopentenes conveniently.
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