Alkyne Hydroboration

Alkyne Reactions: Hydroboration–Oxidation with R₂BH, H₂O₂/NaOH

Alkyne Reactions: Anti-Markovnikov Hydration with R₂BH then H₂O₂ / NaOH

Bulky dialkylboranes (R₂BH), such as 9-BBN, disiamylborane (Sia₂BH), and dicyclohexylborane, hydroborate alkynes in a syn, anti-Markovnikov fashion. The resulting vinylboranes survive long enough to undergo basic peroxide oxidation (H₂O₂, NaOH), which converts the C–B bond into C–O with retention at the migrating carbon. The reaction sequence passes through a vinyl enol that quickly tautomerizes to the carbonyl product: terminal alkynes → aldehydes; internal alkynes → ketones, typically at the less hindered position. Pair this with the HgSO₄ hydration (Kucherov) guide to plan regio-complementary strategies.

Below we follow a bulky dialkylborane across five mechanistic frames, use the same substrate set from our ozonolysis notes for examples, and highlight practical decisions (reagent choice, equivalents, quench). The reagent button below links to our R₂BH image (`r2bh.png`).

Quick Summary

Reagents/conditions: R₂BH (9-BBN, Sia₂BH, Cy₂BH) in THF at 0–25 °C, then H₂O₂, NaOH (aq) added cold (0–25 °C).
Outcome: Anti-Markovnikov hydration. Terminal alkynes → aldehydes; internal alkynes → ketones, usually at the less hindered carbon.
Mechanism: Syn hydroboration → peroxyborate formation → hydroxide-assisted B–O collapse → vinyl alkoxide protonation → enol release → enol–keto tautomerization.
Stereochemistry: Hydroboration is syn; migration retains configuration at the migrating vinyl carbon; tautomerization removes residual alkene stereochemistry.
Contrast: HgSO₄/H₂SO₄/H₂O hydrates Markovnikov (terminal → methyl ketone). Use the pair to access both regioisomers.

Mechanism (6 Frames) (Closed-shell anti-Markovnikov hydration)

Showcased substrate: bulky dialkylborane hydroboration of a cyclohexyl-containing internal alkyne.

Step 1: Syn hydroboration with bulky R2BH — Step 1 — Syn hydroboration: R₂BH adds across the alkyne, placing boron at the less hindered carbon and H syn to it.

Bulky dialkylboranes (9-BBN shown) avoid over-hydroboration, steering addition to the least hindered carbon and enforcing syn delivery.

Step 2: Vinylborane poised for oxidation — Step 2 — Vinylborane intermediate: boron (carrying two R groups) sits on the anti-Markovnikov carbon ready for HOO⁻ attack.

The syn addition leaves a cis vinylborane while the Markovnikov carbon retains the newly delivered hydrogen.

Step 3: Peroxyborate formation — Step 3 — Peroxyborate formation: HOO⁻ (from H₂O₂/NaOH) coordinates to boron, shifting electron density toward oxygen.

The hydroperoxide builds a peroxyborate complex. The migrating vinyl carbon retains its configuration and explicit hydrogen.

Step 4: Hydroxide expels boron — Step 4 — Hydroxide attacks boron, collapsing the B–O bond and releasing the vinyl alkoxide while BR₂ departs.

Hydroxide removes the boron fragment, leaving the vinyl alkoxide tethered to oxygen and the Markovnikov hydrogen still present.

Step 5: Vinyl alkoxide protonation begins tautomerization — Step 5 — Vinyl alkoxide carries the negative charge; water donates a proton as electrons flow toward the nascent carbonyl.

The alkoxide fires electrons into the C=O bond while water delivers a proton, priming the enol for tautomerization.

Step 6: Enol–keto tautomerization — Step 6 — Enol–keto tautomerization furnishes the aldehyde (terminal) or ketone (internal).

Proton transfers equilibrate the enol and carbonyl. Terminal alkynes become aldehydes; internal substrates yield ketones.

Mechanistic Checklist

Syn hydroboration installs boron at the less hindered carbon; the oxidation step preserves that regiochemistry.
Migration from B → O proceeds with retention at the migrating vinyl carbon.
Terminal alkynes reliably produce aldehydes; internal alkynes form ketones (sterics govern unsymmetrical cases).
Bulky R₂BH (9-BBN, Sia₂BH) prevents over-addition and enforces anti-Markovnikov orientation.
Contrast with HgSO₄/H₂SO₄/H₂O (Kucherov) for Markovnikov hydration to methyl ketones.

Worked Examples

Substrate: but-2-yne — Substrate — but-2-yne

Reagents: R₂BH then H₂O₂/NaOH — R₂BH / H₂O₂, NaOH

Product: butan-2-one — Product — butan-2-one

Terminal alkyne → aldehyde or (for internal positions) a ketone at the less hindered carbon.

Substrate: hex-3-yne — Substrate — hex-3-yne

Product: hexan-3-one — Product — hexan-3-one

Symmetric internal alkyne → single ketone product.

Substrate: phenylacetylene — Substrate — phenylacetylene

Product: 2-phenylethanal — Product — 2-phenylethanal

Terminal alkyne → aldehyde (no over-oxidation when workup is controlled).

Substrate: 4-methylpent-2-yne — Substrate — 4-methylpent-2-yne

Product: 3-methylhexan-2-one — Product — 3-methylhexan-2-one

Unsymmetrical internal alkyne → ketone at the less hindered carbon (retention during migration).

Multiple Unsaturations & Selectivity

Bulky R₂BH reagents hydroborate less hindered π bonds first (alkynes over isolated alkenes). Control equivalents and temperature to target a single C≡C.
Polyynes can hydroborate multiple times; deliver R₂BH slowly and cap equivalents to avoid over-addition.
Post-oxidation carbonyls (aldehydes/ketones) can participate in downstream reactions. Quench and work up promptly before aldol or Cannizzaro side reactions occur.
For Markovnikov hydration instead, choose HgSO₄ / H₂SO₄ / H₂O.

Practical Tips & Pitfalls

Choose the borane wisely: 9-BBN and Sia₂BH suppress second additions and enforce anti-Markovnikov regiochemistry.
Order of addition: Complete hydroboration before introducing peroxide/base. Add H₂O₂/NaOH slowly at 0–5 °C; the oxidation is exothermic.
Solvent/handling: THF is standard. Dialkylboranes are moisture-sensitive—keep solutions under inert atmosphere and exclude water until oxidation.
Quench & cleanup: After oxidation, neutralize carefully and extract. Borate residues can cling to glassware—rinse with aqueous base then water.
Regio mixture warning: Unsymmetrical internal alkynes may still give minor regioisomers. Sterics help but consider reporting mixtures when substituents are similar.

Exam-Style Summary

R₂BH followed by H₂O₂/NaOH hydrates alkynes anti-Markovnikov via syn hydroboration, peroxyborate formation, vinyl migration (retention), and enol–keto tautomerization. Terminal alkynes furnish aldehydes; internal alkynes give ketones, typically at the less hindered carbon. Contrast with HgSO₄ hydration, which delivers Markovnikov methyl ketones from terminal alkynes.

Interactive Toolbox

Mechanism Solver — play the R₂BH → peroxyborate → enol → carbonyl mechanism (alkyne_r2bh_naoh).
Reaction Solver — toggle between anti-Markovnikov (R₂BH) and Markovnikov (HgSO₄) hydration outcomes.
IUPAC Namer — practice naming products such as butan-2-one, hexan-3-one, 2-phenylethanal, and 3-methylhexan-2-one.

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