Alkyne Hydroboration–Oxidation

Introduction

Direct mercury(II)-catalysed hydration of a terminal alkyne gives a methyl ketone. Hydroboration–oxidation offers the complementary carbonyl position: the terminal carbon becomes an aldehyde. The route first makes a vinylborane, then an enol, which tautomerises to the aldehyde. A bulky borane is often selected for terminal alkynes to stop unwanted further addition to the newly formed double bond.

Core explanation

Consider R–C≡CH. In hydroboration, a B–H bond adds across one alkyne pi bond. Boron bonds mainly to the less substituted terminal carbon, while H bonds to the internal carbon. The first organoboron product retains a C=C bond and is therefore a vinylborane . Both fragments are delivered in a concerted syn process. There is no ordinary free vinylic carbocation available for hydride or alkyl shifts.

Oxidation with H₂O₂ under basic conditions replaces the carbon–boron connection with a carbon–oxygen connection. The immediate organic product is a terminal enol , R–CH=CH–OH. This enol tautomerises by proton movement and pi-bond relocation to R–CH₂–CHO, an aldehyde. The original terminal alkyne carbon is the aldehyde carbonyl carbon; the internal alkyne carbon becomes the adjacent CH₂. Drawing the enol explicitly prevents accidentally naming an alcohol rather than the final carbonyl compound.

For but-1-yne, CH₃CH₂–C≡CH, hydroboration–oxidation yields butanal, CH₃CH₂CH₂CHO. Direct HgSO₄/H₂SO₄/H₂O hydration of the same alkyne yields butan-2-one. The two methods put oxygen on opposite original triple-bond carbons before tautomerisation. This is an important synthetic comparison: one starting alkyne can provide either an aldehyde or a methyl ketone by reagent choice.

Why use a bulky borane rather than simple BH₃ for a terminal alkyne? The first hydroboration leaves an alkene-like vinylborane that can accept another borane addition, complicating product formation. A sterically hindered borane, such as disiamylborane or 9-BBN in common laboratory descriptions, helps limit that second addition while allowing addition to the original triple bond. The exact reagent and solvent vary by procedure, but the conceptual role is to control chemoselectivity and stop at the desired vinylborane stage.

An internal alkyne can also undergo hydroboration–oxidation. It yields a ketone after enol tautomerism because neither triple-bond carbon carried a terminal H in the same arrangement. If the internal alkyne is unsymmetric, regioisomeric ketones may arise unless substrate or reagent sterics create useful selectivity. The striking aldehyde outcome is specifically associated with terminal alkynes.

The word anti-Markovnikov describes the net position of oxygen, not a reaction of HBr radicals and not an isolated primary alcohol. Hydroboration–oxidation of a terminal alkene gives a primary alcohol; the analogous terminal alkyne gives an aldehyde because the first addition leaves C=C and its OH product is an enol. This distinction is a frequent examination trap and a useful test of whether the intermediate was actually drawn.

Step-by-step reasoning

Mark the terminal C≡CH carbon and the internal alkyne carbon. Add B to the terminal carbon and H to the internal carbon, leaving a C=C vinylborane. In the oxidation step, replace C–B with C–OH to draw R–CH=CH–OH. Tautomerise by shifting the O–H proton to the neighbouring carbon and C=C to C=O. Verify that the final aldehyde carbon is the original terminal alkyne carbon.

Visual explanation

Draw a three-stage strip: R–C≡CH → R–CH=CH–B(R′)₂ → R–CH=CH–OH → R–CH₂–CHO. Highlight the original terminal carbon in one colour throughout and circle it as the final aldehyde carbonyl carbon. Above the first arrow write bulky borane; above the second write H₂O₂/OH⁻; above the last write tautomerism.

Real-world analogy

Two neighbouring positions are linked by a double-strength connection after the first reagent acts. A temporary tag is placed on the outer position, then exchanged for oxygen. Only after a final rearrangement of a token and connecting bar does the stable endpoint appear. The temporary tag is boron, the outer position is terminal alkyne carbon, and the final endpoint is an aldehyde rather than an alcohol.

Real-world example

Butanal can be prepared conceptually from but-1-yne by bulky-borane hydroboration followed by basic peroxide oxidation. Butanal is a useful aldehyde building block for further nucleophilic additions and condensations. If butan-2-one were desired instead, direct mercury(II)-catalysed alkyne hydration would offer the complementary carbonyl location, subject to laboratory and process considerations.

Why?

Why is the terminal product an aldehyde rather than a primary alcohol? The alkyne still contains one pi bond after one B–H addition, so replacing B by OH makes an enol with OH on a C=C carbon. Tautomerisation converts that C–OH position to C=O while adding H to the neighbouring carbon. A terminal carbonyl carbon retains an H, meeting the definition of an aldehyde.

Common misconception

"Hydroboration–oxidation always gives an alcohol." That is the usual net result for an alkene, where the pi bond is fully consumed in the initial addition. An alkyne leaves a double bond after the first addition, so oxidation makes an enol that generally tautomerises to an aldehyde or ketone. The substrate's bond order changes the final functional group.

Worked example

Question: Predict the final organic product of pent-1-yne treated with a bulky borane followed by H₂O₂/OH⁻. Compare it with direct HgSO₄-catalysed hydration.

Reasoning: Boron attaches to terminal C1 and is replaced by OH, giving a terminal enol that tautomerises to an aldehyde at C1. Mercury-catalysed hydration places OH initially at C2 and yields a methyl ketone there.

Answer: Hydroboration–oxidation gives pentanal; direct mercury(II)-catalysed hydration gives pentan-2-one.

Quick check

1. Which original carbon of a terminal alkyne becomes the aldehyde carbonyl carbon after hydroboration–oxidation? Answer: The terminal carbon that initially carried H and received boron during hydroboration.

Exam focus

Specify a bulky borane for controlled terminal-alkyne hydroboration, then basic peroxide. Show the vinylborane and enol before naming the final aldehyde. Compare with mercury-catalysed hydration only after identifying which carbon gets oxygen in each route. Do not report a terminal alcohol as the usual alkyne product.

Advanced insight

The steric size of the borane influences both which alkyne carbon receives B and whether the first vinylborane undergoes unwanted second hydroboration. Internal alkyne regioselectivity can be less clean because both triple-bond carbons carry carbon substituents. A synthetic route may therefore prefer a terminal alkyne when an unambiguous aldehyde is the target.

Summary

Hydroboration–oxidation of a terminal alkyne places boron, then oxygen, on the less substituted terminal carbon. Oxidation forms a terminal enol, and tautomerism converts it to an aldehyde. Bulky borane helps avoid double addition to the initially formed vinylborane. The route complements mercury(II)-catalysed direct hydration, which usually makes a methyl ketone from the same terminal alkyne.

Practice questions

1. What does but-1-yne yield after controlled hydroboration–oxidation? Answer: Butanal, with the original terminal alkyne carbon becoming the aldehyde carbonyl carbon. 2. Why is a bulky borane useful for terminal alkynes? Answer: It discourages a second borane addition to the vinylborane produced by the first addition. 3. What intermediate appears after oxidation but before the final carbonyl product? Answer: An enol, with OH directly on a carbon of a C=C double bond. 4. Why does direct HgSO₄ hydration give a different carbonyl position? Answer: It has Markovnikov oxygen placement at the more substituted alkyne carbon, leading to a methyl ketone after tautomerism.