Palladium-Catalyzed Cross-Coupling Reaction of Lithiated Monocarba-closo-dodecaborate at the Carbon Vertex

Dedicated to Professor Manfred Schlosser for his outstanding contribution in organic and organometallic chemistry.

Abstract

A cross-coupling reaction of lithium species of monocarba-closo-dodecaborate {[closo-CHB₁₁H₁₁]^–} at the carbon vertex under palladium catalysis was developed. The properties of a metal–carborate complex in a transmetalation step were important to achieve the cross-coupling process with aryl halides, and the use of lithiated monocarba-closo-dodecaborate was found to promote the carbon–carbon bond formation to provide a series of C-arylated monocarba-closo-dodecaborate derivatives. Mechanistic investigations with DFT calculations suggested the efficient transmetalation with the lithium reagent.

Key words

monocarba-closo-dodecaborate - cross-coupling - lithium - transmetalation

• References and Notes

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• 9 A substantial amount of iodinated 1 was observed by ESI-MS analysis, while the quantification was not done due to the overlap of peaks with unreacted 1 in ¹¹B NMR or ¹H NMR spectra.
• 10 A ketone-type byproduct generated by the nucleophilic addition of 1a to the carbonyl group of 2g was obtained in ca. 30% yield.
• 11 Some aryl iodides having strong electron-withdrawing substituents, such as cyano group, caused the undesired halogen–lithium exchange reaction with the poor C–C bond formation. For these substrates, the previous conditions^5a should be utilized.
• 12 The calculations were performed at the B3LYP/SDD (for Pd, I) and 6-31+G* (for other atoms) level of the theory. All calculations were carried with the Frisch, M. J. et al. Gaussian 09, Revision C.01; Gaussian, Inc., Wallingford, CT, 2009. The full citation is given in the Supporting Information.
• 13 While the precise mechanism of trans- to cis-isomerization has not been clarified, we speculate that another arylpalladium complex should be involved via the second transmetalation. The low energy difference (>1 kcal/mol) between two isomers has been found by DFT calculations (see Supporting Information).
• 14 With the boron–carborate complex, the transmetalation rection to give cis-Pd(Ph)(carborate)(PMe₃) is found to need ca. 40 kcal/mol, in accordance with the experimental results. Detailed results will be separately reported in due course.