Dearomative [2 + 2] Cycloaddition and Formal C–H Insertion Reaction of o-Carboryne with Indoles: Synthesis of Carborane-Functionalized Heterocycles

Abstract

o-Carboryne (1,2-dehydro-o-carborane) is a very useful synthon for the synthesis of a variety of carborane-functionalized heterocycles. Reaction of o-carboryne with N-protected indoles gave carborane-fused indolines if the protecting group was TMS via dearomative [2 + 2] cycloaddition or carboranyl indoles for N-alkyl ones through formal C-H insertion reaction. For N-aryl indoles, both reactions were observed, giving two products, in which the product ratio was dependent upon the nature of the substituents on the aryl rings. In general, electron-withdrawing substituents favor [2 + 2] cycloaddition, whereas electron-donating substituents promote a formal C-H insertion pathway. This reaction is also compatible with other heteroaromatics. Thus, a stepwise reaction mechanism was proposed to account for the experimental observations. These protocols offer general and efficient methods for the preparation of carborane-functionalized indoles and indolines as well as other heterocycles. The observed dearomative [2 + 2] cycloaddition represents the first example of indoles to undergo such reaction in the absence of transition metals or without UV irradiation. All new compounds were fully characterized by (1)H, (13)C, and (11)B NMR spectroscopy as well as HRMS spectrometry. Some were further confirmed by single-crystal X-ray analyses.

Broad Scope Extra‐Annular [4 + 2] Cycloaddition of o‐Carboryne with Styrenes: Efficient Route to Carborane‐Fused Polycyclics

“…13 C{ 1 H} NMR spectra were recorded on a Bruker DPX 400 spectrometer at 100 MHz or a Bruker DPX 500 spectrometer at 126 MHz. 13 C{ 1 H} NMR spectra were recorded on a Bruker DPX 400 spectrometer at 100 MHz or a Bruker DPX 500 spectrometer at 126 MHz.…”

“…13 C{ 1 H} NMR (101 MHz, CD 2 Cl 2 ) δ: 132.0, 131. After stirring at room temperature for 20 min, the reaction mixture was quenched with water and extracted with diethyl ether (5 mL × 3).…”

“…[10b] HRMS: m/z calcd for C 13 5,132.1,130.8,129.3,129.1,128.8,128.5,128.4,127.7 (aromatic C),76.5,73.9 (cage C),46.3,34.8 (CH and CH 2 13 C{ 1 H} NMR (126 MHz, CDCl 3 ) δ: 141.2, 134. 9, 131.3, 129.2, 129.1, 129.0, 128.9, 127.7, 127.6, 127.4 (aromatic C), 72.3, 70.5 (cage C), 43.5, 39.2 (CH and CH 2 ).…”

“…11 B{ 1 H} NMR (128 MHz, CDCl 3 ) δ: -3.9 (1B), -6.0 (2B), -9.3 (3B), -10.5 (1B), -11.8 (3B). [19] 133.9,133.6,133.0,132.9,130.4,130.3,129.5,129.3,127.6 (aromatic C),71.1,70.1 (cage C),42.5,38.9 (CH and CH 2 (d,J＝2.0 Hz,1H),7.53 (m,2H),7.30 (dd,J＝8.5,2.0 Hz,1H),7.03 (d,J＝8.4 Hz,2H),6.50 (dd,J＝8.4,0.9 Hz,1H) (aromatic CH),4.13 (dd,J＝12.9,6.5 Hz,1H),2.89 (dd,J＝14.7,6.5 Hz,1H),2.70 (dd,J＝14.6,13.1 Hz, 2H) (CH and CH 2 ). [19] 133.9,133.6,133.0,132.9,130.4,130.3,129.5,129.3,127.6 (aromatic C),71.1,70.1 (cage C),42.5,38.9 (CH and CH 2 (d,J＝2.0 Hz,…”

“…[4,5] In general, o-carboryne can be generated in situ from 1-Br-2-Li-1,2-o-C 2 B 10 H 10 , 1-I-2-Li-1,2-o-C 2 B 10 H 10 , 1-Me 3 Si-2-[IPh(OAc)]-1,2o-C 2 B 10 H 10 , or 1-OTf-2-Li-1,2-o-C 2 B 10 H 10 . [5][6][7][8] It reacts with a broad spectrum of substrates including alkenes, alkynes, (hetero)aromatics, ethers, amines, and ferrocenes in various types of reaction manners such as [4 + 2]/[3 + 2]/[2 + 2] cycloaddition, [4a,4c-4h,7-13] ene reaction/hydrogen abstraction, [4b,4i] and sp 2 /sp 3 C-H bond insertion reaction [13,14] to give a large series of o-carborane derivatives that have received growing interests. [15] Scheme 1 Reaction of o-carboryne with styrenes As a highly reactive dienophile, o-carboryne is expected to undergo extra-annular [4 + 2] cycloaddition reaction with styrenes.…”