Rh-Catalyzed Synthesis of Isobenzofurans via Donor/Donor-Type Metal Carbenoids and Their [4 + 2] Cycloaddition

A rhodium-catalyzed synthesis of isobenzofurans via donor- and donor-type metal carbenoids has been developed. Nosylhydrazones were used as carbene precursors, generating rhodium carbenoid species under basic conditions. These intermediates underwent intramolecular cyclization with ester groups to afford isobenzofurans, which subsequently participated in a highly endo-selective [4 + 2] cycloaddition with maleimides and other dienophiles. The reaction exhibited a broad substrate scope, accommodating various ester and aryl substituents while maintaining excellent regio- and stereoselectivity. Mechanistic studies, including control experiments, NMR analysis, and computational calculations, revealed that the reaction proceeds through a rhodium carbenoid intermediate, leading to the formation of isobenzofuran prior to cycloaddition. The endo-selectivity was found to originate from the difference in activation energies between the transition states, as supported by computational studies. Additionally, the isolation of the diazo intermediate and its direct conversion to isobenzofuran confirmed the stepwise nature of the transformation. This study expands the utility of donor- and donor-type carbenoids in organic synthesis, demonstrating their effectiveness in constructing highly reactive isobenzofurans under mild conditions.

Kinetically stabilized 1,3-diarylisobenzofurans and the possibility of preparing large, persistent isoacenofurans with unusually small HOMO-LUMO gaps

DFT calculations demonstrate that an isoacenofuran of any size possesses a smaller HOMO-LUMO gap than the corresponding acene bearing an isoelectronic π-system (i.e., the same total number of rings). Isoacenofurans show limited stability due in part to the highly reactive 1,3-carbons of the furan ring. Both 1,3-dimesitylisobenzofuran and 1,3-di(2',4',6'-triethylphenyl)isobenzofuran, each bearing sterically congesting ortho-alkyl groups on their phenyl substituents, have been synthesized and shown to adopt non-planar conformations with the ortho-alkyl groups located above and below the most reactive 1,3-carbons of the furan ring. These bulky substituents provide a strong measure of kinetic stabilization. Thus, 1,3-dimesitylisobenzofuran and 1,3-di(2',4',6'-triethylphenyl)isobenzofuran are significantly less reactive than 1,3-diphenylisobenzofuran toward the strong dienophiles DMAD and acrylonitrile. The insights gained here suggest that the synthesis of large, persistent, kinetically stabilized isoacenofurans with unusually small HOMO-LUMO gaps is achievable. As such, these molecules deserve increased attention as potential p-type organic semiconductors.