Intermolecular Cyclization between Carboranylphosphines and Electron-Deficient Alkynes

Abstract

An efficient and facile method to synthesize zwitterionic nido-carborane-fused five-membered P-heterocycles was developed. Intermolecular cyclization between carboranylphosphines and electron-deficient alkynes occurred at 50 °C in THF, affording the products with up to 82% yield. For unsymmetrical alkynes, the more electrical-deficient site is selectively connected to phosphorus in the products. Structures of three products have been confirmed by X-ray diffraction analysis of single crystals. The key step of the reaction is believed to be deprotonation of acidic C–H of carborane by the alkenyl anion derived from addition of carboranylphosphines to alkynes.

Redox-active carborane clusters in bond activation chemistry and ligand design

“…Importantly, the presence of the two phosphine groups is important for this outcome, as carboranyl monophosphines do not exhibit B-H bond activation reactivity. 73,74 To reiterate, the unusual reactivity of carboranyl diphosphine 1 described herein relies on the synergy between the electron-donating phosphine groups and the electron-accepting carborane cluster. In these novel transformations, the neutral closo-{C 2 B 10 } cluster accepts two electrons and becomes an open dianionic nido-{C 2 B 10 } cluster, while the two phosphine groups are converted into phosphonium cations.…”

Activation of Alkynes by a Redox-Active Carboranyl Diphosphine and Formation of Boron-Containing Phosphacycles

“…If only one phosphine group is connected to the cluster, as in the case reported by Duan and co-workers, the C−H bond of carborane is likewise deprotonated, and a putative cyclic ylide, which is unstable to deboronation, forms and eventually transforms into a saturated five-membered phosphorus-containing heterocycle. 24,25,36 In the case reported herein, the presence of two phosphine groups on the cluster unlocks the redox behavior of the carborane cage, resulting in the intramolecular oxidation of the B−H bond and formation of the phosphorus and boron-containing heterocycle.…”

“…Phosphines have been known to attack electron-deficient alkynes, leading to the formation of vinylphosphonium intermediates. , Recently, the reaction of carboranyl monophosphines and acetylenedicarboxylates was reported, wherein the remaining relatively acidic C–H bond of the cluster was deprotonated and a new C–C bond between the alkyne and carborane was formed . Although the outcome of that reaction was complicated by the loss of one boron vertex and the eventual formation of an 11-vertex nido -cluster, the overall transformation is conceptually analogous to the activation of alkynes by electron-rich phosphines for subsequent reactions with nucleophiles.…”

“…31,32 Recently, the reaction of carboranyl monophosphines and acetylenedicarboxylates was reported, wherein the remaining relatively acidic C−H bond of the cluster was deprotonated and a new C−C bond between the alkyne and carborane was formed. 25 Although the outcome of that reaction was complicated by the loss of one boron vertex and the eventual formation of an 11-vertex nido-cluster, the overall transformation is conceptually analogous to the activation of alkynes by electron-rich phosphines for subsequent reactions with nucleophiles. In the case presented herein, the synergy between the redox capability of carborane and the electronically conjugated phosphine groups at each cluster carbon atom was utilized in reactions with electrondeficient alkynes, ultimately leading to the activation of the B− H bond of the cluster.…”

“…A related, intramolecular cyclization of a carborane-supported phosphine with an alkyne fragment, resulting in a carborane-fused phosphole, has been reported by Duan and co-workers (Scheme b) . This group also reported the intermolecular activation of electron-deficient alkynes by carboranyl monophosphines, which resulted in the deprotonation and subsequent derivatization of the remaining carborane C-vertex to form a five-member phosphacycle (Scheme c) . In all literature examples, functionalization of the remaining unsubstituted cluster C-H vertex was reported, in some cases, accompanied by deboronation of the cluster and conversion from a closo -{C 2 B 10 } to a nido -{C 2 B 9 } cage.…”