Internal alkynes hydroboration-oxidation

The hydroboration-oxidation of internal alkynes produces ketones. 

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. 

The hydroboration of disubstituted (internal) alkynes leads to a mixture of two ketones. When 2-pentyne reacts with borane, two vinylboranes are formed in virtually equal amounts 120 and 121. There is no significant difference in steric hindrance to make one transition state favored over the other, so both vinylboranes are formed. Subsequent oxidation leads to the isomeric ketone products (from their respective enols) 2-pentanone (122) and 3-pentanone H23). To conclude, hydroboration of a terminal alkyne leads to an aldehyde as the major product, whereas hydroboration of an internal alkyne gives a mixture of two isomeric ketones. 

We have now seen how to prepare both terminal and internal alkynes from acetylene and substituted acetylenes, and we have seen several common reactions of alkynes, including addition (HX, X, and H2O), hydroboration-oxidation, and reduction. Now let us move a step farther to consider what might be called the art of organic synthesis. 

Hydroboration-Oxidation (Section 7.7/ Hydroboration of an internal alkyne is syn... 

Excellent yields of the ketones are achieved from internal alkynes (Eq. 7.5 Table 7.8) [1] via hydroboration with 1 equiv of 9-BBN. The boron of 9-BBN adds to the less hindered carbon of carbon-carbon triple bond, and oxidation of the resulting B-alkenyl-9-BBN affords the corresponding ketones. 

A carbonyl compound will be the product of hydroboration-oxidation only if a second molecule of BH3 or R2BH does not add to the ir-bond of the boron-substituted alkene. In the case of internal alkynes, the substituents on the boron-substituted alkene prevent the approach of the second boron-containing molecule. In the case of terminal alkynes, however, there is an H instead of a bulky alkyl group on the carbon that the second molecule adds to, so there is less steric hindrance toward the second addition reaction. Therefore, either BH3 or R2BH can be used with internal alkenes, but the more sterically hindered R2BH should be used with terminal alkynes. 

TMS-alkynes are oxidized at the terminal carbon to carboxylic acids by hydroboration/oxidation (dicyclohexylborane/NaOH, H2O2). This does not work with TIPS-alkynes. Instead, TIPS-alkynes are cleanly monohydroborated at the internal carbon by 9-borabicyclo[3.3.1]nonane dimer to give (Z)- -borylvinyl-silanes. These can be oxidized in high yields to a-silyl ketones, or cross coupled with a bromide R Br (R = aryl, benzyl, dimethyl-vinyl) in the presence of NaOH and tetrakis(triphenylphos-phine)palladium(0) to give /3,/3-disubstituted vinylsilanes (Suzuki reaction eq 14). The same nucleophilic substituted vinylsilane can be added to an aromatic aldehyde to provide access to ( )-3-silyl allyl alcohols. ... 

Ketones can be prepared by oxidation of secondary aicohois, hydroboration of internal alkynes, addition of water to aikynes, and ozonoiysis of aikenes aromatic ketones can be prepared by the Friedei-Crafts reaction. 

Soderquist and coworkers have reported [19] that hydroboration with 9-BBN of 1-silylacetylenes, having bulkier groups on the Si, place the boron exclusively at the internal position of the alkyne as a kinetically less preferred process. Consequently, triisopropylsilyl-substituted alkynes afford the novel P-silylated vinyl-boranes, which on oxidation with alkaline hydrogen peroxide affords the corresponding ketones (Eq. 7.11) in excellent yields, a truly noteworthy example of the stability imparted to this functionality by the triisopropyl substitution on silicon [20]. It should be noted that earlier studies have shown failures to obtain (3-ketosilanes from suitable organoprecursors due to their known sensitivity to bases and nucleophiles [21]. 

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