05 alkyne reagents

Alkyne to alkane

H2 (excess), Pd/C

Alkyne to cis (Z) alkene

H2, Lindlar's catalyst (or H2, Ni2B / P-2)

Alkyne to trans (E) alkene

Na (or Li), NH3(l)

Alkyne to vinyl halide (1 equiv HX)

HX (Markovnikov)

Alkyne to geminal dihalide

2 equiv HX

Alkyne to anti-Markovnikov vinyl bromide

HBr, ROOR (mixture of E/Z)

Alkyne to trans-dihaloalkene

1 equiv X2

Alkyne to tetrahaloalkane

2 equiv X2

Terminal alkyne to methyl ketone

H2O, H2SO4, HgSO4

Terminal alkyne to aldehyde

1) Sia2BH or 9-BBN 2) H2O2, NaOH

Internal unsymmetrical alkyne + HgSO4 hydration

Mixture of two ketones (symmetrical alkynes avoid this)

Alkyne to two carboxylic acids

1) O3 2) H2O (or hot KMnO4)

Terminal alkyne ozonolysis/KMnO4 products

A carboxylic acid plus CO2

Why use a bulky borane (Sia2BH, 9-BBN) with alkynes?

Prevents double hydroboration; stops at the vinyl borane

Lindlar catalyst composition

Pd on CaCO3, poisoned with lead (and quinoline)

Why does Lindlar stop at the alkene?

The poisoned catalyst is not active enough to reduce the alkene

Na/NH3 on a terminal alkyne

Does not work well: the terminal alkyne is deprotonated first

Product stereochemistry of H2/Lindlar

cis (Z), via syn addition

Product stereochemistry of Na/NH3

trans (E)

Enol formed from alkyne hydration becomes

Ketone (or aldehyde via anti-Markovnikov hydroboration)

Alkyne hydration regiochemistry (HgSO4)

Markovnikov: OH ends on the more substituted carbon, giving a ketone

Terminal alkyne to carboxylic acid with one fewer carbon

1) O3 2) H2O (loses the terminal carbon as CO2)

Terminal alkyne to longer alkyne

1) NaNH2 2) R-X (1° or methyl)