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

Abstract

In this work, we report the reactivity of the carboranyl diphosphine, 1-PtBu2–2-PiPr2-C2B10H10, with terminal alkynes, resulting in the formation of boron-containing phosphacycles. The reported system combines the nucleophilic activation of electron-deficient terminal alkynes via electron-rich phosphine groups with the redox behavior of carborane clusters to promote a sequence of metal-free intramolecular B–H bond activation and cyclization, creating an alkenylphosphonium cycle fused with a reduced open nido-carborane cluster.

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

“…A closely related example of bond activation by redox-active 1 was discovered in its reactions with alkynes. 72 We found that electron-deficient terminal alkynes were activated by our system in a process that is driven by cluster reduction and formation of the nido -{C 2 B 10 } cage, yet the products 4a / 4b did not follow the same reactivity pattern as the case described above (Fig. 6).…”

Redox-Active Carboranyl Diphosphine as an Electron and Proton Transfer Agent

“…For the diamagnetic species present, the 31 P and 11 B NMR spectra of the product indicate that the cluster is still in the open nido - form. We have previously observed the isomerization of the C 2 -symmetric nido- cluster present in 2 into a C s -symmetric nido- cluster during oxidation attempts . The mass spectrum of the reaction mixture provided some information on the composition of the product.…”

“…We have previously observed the isomerization of the C 2 -symmetric nido-cluster present in 2 into a C s -symmetric nido-cluster during oxidation attempts. 52 The mass spectrum of the reaction mixture provided some information on the composition of the product. Two major species exhibiting isotope patterns typical for boron clusters were observed: one with smaller intensity, corresponding to the exact mass for 2, and another with larger intensity, corresponding to the loss of one BH unit from 2, thus suggesting a deboronation event.…”

“…We recently reported on the bond activation reactions of the carboranyl diphosphine 1-P t Bu 2 -2-P i Pr 2 –C 2 B 10 H 10 ( 1 ). , This redox-active carborane scaffold has the ability to accept two electrons in the cluster, while the two phosphine arms act as electron-donating sites. Carboranyl disphosphine 1 was found to activate main group hydrides yielding a C 2 -symmetric reduced open nido -{C 2 B 10 } 2– cluster, evidenced by the change in signal splitting from doublets to singlets in the 31 P NMR spectra, as well as crystallographic studies.…”