Broad Scope [4 + 2] Cycloaddition of o-Carboryne with Pentafulvenes Using 1-Li-2-OTf-o-C₂B₁₀H₁₀ as Precursor

Abstract

o-Carboryne (1,2-dehydro-o-carborane) generated in situ from 1-Li-2-OTf-o-C 2 B 10 H 10 undergoes an efficient [4 + 2] cycloaddition with pentafulvenes at room temperature to give a series of carboranonorbornenes in good to high isolated yields. This reaction is compatible with many functional groups and has a very broad substrate scope from 6-mono-to 6,6′disubstituted pentafulvenes and from alkyl to aryl substituents. Further transformations of the resultant [4 + 2] cycloaddition products have been carried out, affording various multifunctionalized o-carboranes.

CB11H10– and Related Carborenes

“…Polyhedral boranes are often called boron analogues of hydrocarbons, and certain apects of the chemistry of closoboranes and their heterocyclic analogues such as closocarboranes indeed resemble the chemistry of aromatic hydrocarbons and heterocycles. 1 Since o-didehydroarenes (arynes) are important intermediates in organic chemistry, it is hardly surprising that their carborane analogues have attracted attention as well, 2 and 1,2-didehydro-closo-1,2-dicarbadodecaborane has been used in the synthesis of a wide range of carbonfunctionalized o-carboranes, 3 such as heterocycles, 4 amines, 5 cyclobutanes, 6 norbornenes, 7,8 and metal complexes. 9 The general term "carboryne" has been used for this class of intermediates, 3−13 in analogy to the term aryne.…”

“…The general term “carboryne” has been used for this class of intermediates, − in analogy to the term aryne. We wonder whether this is the best choice, because the ending “yne” suggests the presence of a triple bond between two adjacent vertices.…”

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

“…[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.…”

“…[9,10b] We spectulated that our newly developed o-carboryne precursor 1-OTf-2-Li-1,2-o-C 2 B 10 H 10 eliminating LiOTf at low temperatures may solve the chemoselectivity issue, [6][7][8] affording only extra-annular [4 + 2] cycloaddition products in the reaction with styrenes. [9,10b] We spectulated that our newly developed o-carboryne precursor 1-OTf-2-Li-1,2-o-C 2 B 10 H 10 eliminating LiOTf at low temperatures may solve the chemoselectivity issue, [6][7][8] affording only extra-annular [4 + 2] cycloaddition products in the reaction with styrenes.…”

“…Colorless crystals. C{ 1 H} NMR (101 MHz, CDCl 3 ) δ: 140 7,. 1 H NMR (400 MHz, CDCl 3 ) δ: 7.73(d, J＝1.6 Hz, 1H), 7.55 (m, 2H), 7.50 (dd, J＝7.9, 1.8 Hz, 1H), 7.47 (t, J＝7.5 Hz, 2H), 7.39 (t, J＝7.3 Hz, 1H), 7.18(d, J＝7.9 Hz, 1H) (aromatic CH), 2.99 (t, J＝7.0 Hz, 2H), 2.72 (t, J＝7.0 Hz, 2H) (CH 2 ) 13.…”