Efficient Synthesis of ortho-Carborane by Reaction of Propiolic Acid tert-Butyl Ester with Decaborane(14)

Abstract

Reaction of propiolic acid tert-butyl ester 1 and decaborane(14) at 90 °C did not give the expected carborane carboxylic acid tert-butyl ester 3 but directly unsubstituted ortho-carborane 2 which was isolated in 52% yield as pure material. An in situ de-tert-butoxycarbonylation of the first formed carborane carboxylic acid tert-butyl ester 3 via a six membered transition state is suggested for this domino process.

Key words

alkynes - carboranes - domino reactions - electrocyclic reactions - tert-butyl esters

References

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Experimental details: Decaborane(14) (1.18 g, 9.65 mmol) was heated in CH₃CN (15 mL) to reflux under exclusion of oxygen and moisture. After 30 min a solution of propiolic acid tert-butyl ester 1 (900 mg, 7.13 mmol) in toluene (15 mL) was added and the resulting mixture heated for 15 h to reflux upon which it turned dark. For work-up, MeOH (1 mL) was added and the mixture refluxed for 30 min, then concentrated in vacuo and filtered through a small plug of silica gel to remove base line impurities with an n-pentane-EtOAc 1:1 mixture as eluent. Purification by column chromatography (n-pentane-EtOAc = 10:1) furnished pure ortho-carborane 3 (533 mg, 52%) as a colorless wax-like solid. R_f (n-pentane-EtOAc = 5:1): 0.69; IR (KBr): 3070
cm^-1 (C-H), 2575 (B-H), 1015, 714; ¹H NMR (200 MHz, CDCl₃): δ = 0.70-3.80 (m_br, 10 H, BH), 3.56 (s_br, 2 H, CH); ¹³C NMR (50 MHz, CDCl₃): δ = 54.36 (2 C, CH); HRMS: calcd for C₂H₁₂B₁₀: 144.1943; found: 144.1943.

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Ortho-carborane was purchased from Acros.