In organoboron chemistry, the Brellochs reaction provides a way to generate the monocarboranes. The use of acetylenes to insert two carbons into boron hydrides is well established. The Brellochs method uses formaldehyde to insert single carbon atoms into boron hydrides.^[1]

Illustrative is the synthesis of CB₉H₁₄⁻ from commercially available decaborane.^[2]

B₁₀H₁₄ + CH₂O + 2 OH⁻ + H₂O → CB₉H₁₄⁻ + B(OH)₄⁻ + H₂

Oxidation of the arachno anion gives nido-6-CB₉H₁₂⁻. Base degradation of the latter gives arachno-4-CB₈H₁₄.