Reactions of 1,2-Dehydro-o-carborane with Thiophenes. Cycloadditions and an Easy Synthesis of “Benzo-o-carboranes”¹

Abstract

Cycloaddition reactions of 1,2-dehydro-o-carborane with thiophene, 2,5-dimethylthiophene, 2,5-dimethylfuran, [8](2,5)thiophenophane, and [8](2,5)furanophane are investigated. In all cases, except the reaction with the thiophenophane, both 4 + 2 and 2 + 2 cycloadducts are formed, with the 4 + 2 product dominating. The 4 + 2 adducts of the thiophenes are not stable but extrude sulfur under the reaction conditions to give "benzo-o-carboranes".

Recent advances in the chemistry of carborynes

“…However, minor [2 + 2] cycloaddition products are also formed in the reactions of o-carboryne with 2,5-dimethylfuran and [8](2,5)-furanophane (Scheme 7) [22]. The dominance of [4 + 2] cycloaddition retains in the reaction of o-carboryne with thiophene and 2,5-dimethylthiophene, which is parallel to that observed in the reaction of benzyne with thiophenes.…”

“…The [4 + 2] cycloaddition products in the reaction of o-carboryne with thiophenes also undergo desulfurization reaction to afford benzocarboranes. However, reaction with [8](2,5)-thiophenophane surprisingly proceeds in [2 + 2] fashion (Scheme 7) [22].…”

Functionalization of Carborane via Carboryne Intermediates

“…The reaction of o-carboryne with furan results exclusively in [4 + 2] cycloaddition fashion, and it has been used as a trapping method of o-carboryne in the early 1990s [55a]. However, minor [2 + 2] cycloaddition products are also formed in the reactions of o-carboryne with 2,5-dimethylfuran and [8] (2,5)-furanophane (Scheme 1.14) [63]. The dominance of [4 + 2] cycloaddition retains in the reaction of o-carboryne with thiophene and 2,5-dimethylthiophene and paralleling with the benzyne reaction with thiophenes.…”

“…The [4 + 2] cycloaddition products in the reaction of o-carboryne with thiophenes also undergo desulfurization reaction to afford benzocarboranes. However, reaction with [8] (2,5)-thiophenophane surprisingly proceeds in [2 + 2] fashion (Scheme 1.14) [63].…”

“…And the different effect of the octamethylene bridge on the cycloadditions of [8] (2,5)-furanophane and [8] (2,5)-thiophenophane possibly lies in the strain energy changes that accompany the cycloadditions. This difference can be ultimately related to the geometrical differences in the products because of the longer C-S versus C-O bonds and the larger size of S versus O [63].…”