Synthesis of o-Carborane-Fused Pyrazoles through Sequential C–N Bond Formation

Abstract

Transition-metal-free synthetic method for o-carborane-fused pyrazoles as a new scaffold has been developed from the reaction of B(4)-acylmethyl or B­(3,5)-diacylmethyl o-carborane with 2-azido-1,3-dimethyl­imidazolinium hexafluorophosphate (ADMP) in the presence of DBU in acetonitrile through sequential diazotization and cyclization reaction in one pot, consequently allowing twofold C–N bond formation under extremely mild conditions and high functional group tolerance.

Synthesis of o‐carboranyl‐acyl‐substituted diazo compounds from B(4)‐acylmethyl carboranes and 2‐azido‐1,3‐dimethylimidazolinium hexafluorophosphate

“…and 1,8-diazabicyclo [5.4.0]-undec-7-ene (DBU) (2.0 equiv.) in tetrahydrofuran (THF) at 25 C for 3 h under a nitrogen atmosphere, the desired diazo o-carborane (2a) was obtained in 33% yield, and then o-carborane-fused pyrazole 6 was unexpectedly obtained in 8% yield (entry 1). Compound 2a was fully characterized by 1 H, 13 C, 11 B, and 11 B{ 1 H} NMR spectra as well as HRMS.…”

“…In this regard, we recently developed a transition-metal free synthetic method for o-carborane-fused pyrazoles from B(4)acylmethyl and B (3,5)-diacylmethyl o-carborane in one pot. 6 At this time, novel diazo compound which is substituted with an electron-withdrawing acyl and ocarboranyl group has been first isolated as an intermediate of the reaction. In continuation of our studies directed toward the development of synthetic approach using ocarborane 7 and diazo compounds 8 and on the basis of recently reported functionalization of o-carborane, 9 we described herein an efficient synthetic method for a wide range of o-carboranyl-acyl-substituted diazo compounds from the reaction of B(4)-acylmethyl o-carborane with 2-azido-1,3-dimethylimidazolinium hexafluorophosphate (ADMP) in good to excellent yields with good functional group tolerance under mild conditions.…”

Borirenes and Boriranes: Development and Perspectives

“…The corresponding carborane-embedded polycyclic molecules have exhibited distinctive properties in terms of Lewis acidity, photophysics and catalysis. [71][72][73][74][75][76][77][78][79][80][81][82][83] Correspondingly, the replacement of the benzene unit in benzoborirene with o-carborane leads to a class of carborane-based 3D analogues of benzoborirene (V in Figure 1). Since the electrons within the carborane cage do not engage in conjugation, [84][85] the borirene unit of 3D benzoborirene exhibits no local aromaticity.…”

“…In recent years, replacing the benzene units in fused polycyclic π‐systems with 3D aromatic carborane units has emerged as a valuable approach in molecular design. The corresponding carborane‐embedded polycyclic molecules have exhibited distinctive properties in terms of Lewis acidity, photophysics and catalysis [71–83] . Correspondingly, the replacement of the benzene unit in benzoborirene with o ‐carborane leads to a class of carborane‐based 3D analogues of benzoborirene ( V in Figure 1).…”

Regiodivergent metal-catalyzed B(4)- and C(1)-selenylation of o-carboranes

“…In our continuing efforts to develop synthetic methodology for the regioselective B–H functionalization of o -carboranes, 10 we envisioned site selective syntheses of o -carboranes bearing organoselenyl groups through transition metal-catalyzed, regioselective B–Se and C–Se bond formations. To date, a limited number of methods for the synthesis of such compounds have been reported (Scheme 1): (a) C(1)-selenylation through the reaction of o -carboranes with n -BuLi followed by the addition of Se and HCl, 11 (b) C(1,2)-diselenylation through the treatment of dilithio- o -carboranes obtained from o -carboranes and n -BuLi with diphenyl diselenide, 12 (c) B(9,12)-diselenylation through the reaction of o -carboranes with (SeCl) 2 in the presence of AlCl 3 (3.0 equiv.…”