Alkynes hydroboration-oxidation

Hydroboration-Oxidation of Alkynes Hydroboration-oxidation of an alkyne gives anti-Markovnikov addition of water across the triple bond. Di(secondary isoamyl)borane, called disiamylborane, is used, since this bulky borane cannot add twice across the triple bond. On oxidation of the borane, the unstable enol quickly tautomerizes to an aldehyde. (See Section 9-9F.)... 

When you know what functional group you want to create, you can try to remember the various ways it can be synthesized. For example, a ketone can be synthesized by the acid-catalyzed addition of water to an alkyne, hydroboration-oxidation of an alkyne, oxidation of a secondary alcohol, and ozonolysis of an alkene. Notice that ozonolysis decreases the number of carbons in a molecule. 

Like alkenes (Sections 7.4 and 7.5), alkynes can be hydrated by either of two methods. Direct addition of water catalyzed by mercury(II) ion yields the Markovnikov product, and indirect addition of water by a hydroboration/ oxidation sequence yields the non-Markovnikov product. 

The hydroboration/oxidation sequence is complementary to the direct, mercury(ll)-catalyzed hydration reaction of a terminal alkyne because different products result. Direct hydration with aqueous acid and mercury(IJ) sulfate leads to a methyl ketone, whereas hydroboration/oxidation of the same terminal alkyne leads to an aldehyde. 

Problem 8.6 What alkyne would you start with to prepare each of the following compounds by a hydroboration/oxidation reaction ... 

The chemistry of alkynes is dominated by electrophilic addition reactions, similar to those of alkenes. Alkynes react with HBr and HC1 to yield vinylic halides and with Br2 and Cl2 to yield 1,2-dihalides (vicinal dihalides). Alkynes can be hydrated by reaction with aqueous sulfuric acid in the presence of mercury(ll) catalyst. The reaction leads to an intermediate enol that immediately isomerizes to yield a ketone tautomer. Since the addition reaction occurs with Markovnikov regiochemistry, a methyl ketone is produced from a terminal alkyne. Alternatively, hydroboration/oxidation of a terminal alkyne yields an aldehyde. 

Aldehydes are prepared by the hydroboration-oxidation of alkynes (see Section 5.3.1) or selective oxidation of primary alcohols (see Section 5.7.9), and partial reduction of acid chlorides (see Section 5.7.21) and esters (see Section 5.7.22) or nitriles (see Section 5.7.23) with lithium tri-terr-butox-yaluminium hydride [LiAlH(0- Bu)3] and diisobutylaluminium hydride (DIBAH), respectively. 

Hydroboration-oxidation of alkynes preparation of aldehydes and ketones Hydroboration-oxidation of terminal alkynes gives syn addition of water across the triple bond. The reaction is regioselective and follows anti-Markovnikov addition. Terminal alkynes are converted to aldehydes, and all other alkynes are converted to ketones. A sterically hindered dialkylborane must be used to prevent the addition of two borane molecules. A vinyl borane is produced with anU-Markovnikov orientation, which is oxidized by basic hydrogen peroxide to an enol. This enol tautomerizes readily to the more stable keto form. 

Laurel Schafer of the University of British Columbia reports (Organic Lett. 2003,5,4733-4736) that terminal alkynes undergo smooth hydroamination with a Ti catalyst. The intermediate imine 4 can be hydrolyzed to the aldehyde 5 or reduced directly to the amine 6. The alkyne to aldehyde conversion has previously been carried out by hydroboration/oxidation (J. Org. Chem. 1996, 61, 3224), hydrosilylation/oxidation (Tetrahedron Lett. 1984,25, 321), or Ru catalysis (J. Am. Chem. Soc. 2001, 123, 11917). There was no previous general procedure for the anti-Markownikov conversion of a terminal alkyne to the amine. 

Oxidation of alkenylboranes by alkaline hydrogen peroxide gives aldehydes or ketones depending on the substituent pattern of the alkenyl group thus, when alkaline hydrogen peroxide oxidation is combined with alkyne hydroboration, the resulting two-step process is a procedure for converting alkynes to carbonyl compounds (Equations B2.7 and B2.8). 

Hydroboration-Oxidation In Section 8-7 we saw that hydroboration-oxidation adds water across the double bonds of alkenes with anti-Markovnikov orientation. A similar reaction takes place with alkynes, except that a hindered dialkylborane must be used to prevent addition of two molecules of borane across the triple bond. Di(second-ary isoamyl)borane, called disiamylborane, adds to the triple bond only once to give a vinylborane. (Amyl is an older common name for pentyl.) In a terminal alkyne, the boron atom bonds to the terminal carbon atom. 

The hydroboration-oxidation of internal alkynes produces ketones. 

Hydroboration-oxidation is a two-step reaction sequence that converts an alkyne to a carbonyl compound. 

Hydroboration-oxidation of an internal alkyne forms a ketone. Hydroboration of a terminal alkyne adds BH2 to the less substituted, terminal carbon. After oxidation to the enol, tautomerization yields an aldehyde, a carbonyl compound having a hydrogen atom bonded to the carbonyl carbon. 

Hydration (H2O, H2SO4, and HgS04> and hydroboration-oxidation (BH3 followed by H2O2, HO") both add the elements of H2O across a triple bond. Sample Problem 11.4 shows that different constitutional isomers are formed from terminal alkynes in these two reactions despite their similarities. 

What alkyne gives each compound as the only product after hydroboration-oxidation CHgCHO... 

By hydration of an alkyne using hydroboration-oxidation R-C=C-H alkyne [1] BHg [2] H202,-OH 0 II RCH2 H (Section 11.10)... 

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