Reaction of Carboryne with Alkylbenzenes

Abstract

Carboryne (1,2-dehydro-o-carborane), in situ generated from the precursor 1-iodo-2-lithiocarborane, reacted with alkylbenzenes to give two regioisomers of the [4 + 2] cycloadducts as the major products in moderate to good yields, in which the steric factors play an important role in the regioselectivity. Minor products derived from benzylic C−H insertion reaction, annulation reaction, tandem [4 + 2] cycloaddition/homo Diels−Alder reaction, and tandem ene reaction/[2 + 2] cycloaddition were also isolated and characterized in the reaction of carboryne with toluene. The presence of AgF in the above reaction produced no notable changes in the product distributions and yields.

Recent advances in the chemistry of carborynes

“…As shown in Table 1, precursor B is a more efficient precursor for o-carboryne compared to A or C as much improved yields are obtained [3a,3b,17,18]. For the reaction of precursor B with anthracene, a minor 1,4-cycloadduct is also isolated in 4% yield in addition to the expected 9,10-addition product [18], which is not observed in the reactions of A, C or benzyne [19]. If the activated central aromatic ring is sterically inaccessible, such as phenanthrene, the [4 + 2] cycloaddition on the less reactive yet more accessible aromatic ring proceeds (Table 1, entry 4).…”

“…If the activated central aromatic ring is sterically inaccessible, such as phenanthrene, the [4 + 2] cycloaddition on the less reactive yet more accessible aromatic ring proceeds (Table 1, entry 4). the "fatty" cage in 1,4-addition fashion, and the other comes from the steric repulsions between the substituent R and the flanking cage B(3,6)-H hydrogen atoms in 2,5-addition fashion [18]. In general, the relative reactivities of 1,4-over 2,5-addition lie around 1, which indicates that the differences between the two types of steric interactions are small (Table 2, entries 1 and 3−6).…”

Functionalization of Carborane via Carboryne Intermediates

“…As suggested in Table 1.1, precursor B is a more efficient precursor for o-carboryne compared to A or C as much improved yields are obtained [53a, b, 59, 60]. For the reaction of precursor B with anthracene, a minor 1,4-cycloadduct is also isolated in 4 % yield in addition to the expected 9,10-addition product [60], which is not observed in the reactions of A, C, or benzyne [61]. If the activated central aromatic ring is sterically inaccessible, such as phenanthrene, the [4 + 2] cycloaddition on the less reactive yet more accessible aromatic ring proceeds (Table 1. 1, entry 4).…”

“…After careful analyses, they are identified as one tandem [4 + 2] cycloaddition/homo-Diels-Alder reaction product, one benzylic C-H insertion product, one tandem ene reaction/ [2 + 2] cycloaddition product, and one tandem ene reaction/dehydrogenation product (Scheme 1.11) [60].…”

“…The reactions of o-carboryne with various alkylbenzenes and trimethylsilylbenzene are also investigated to afford two [4 + 2] cycloaddition isomers as the major products (Scheme 1.12) [60].…”