Hydroboration of alkenes and it's mechanisms

It includes reaction and it's mechanism and also applications. It contains stereochemistry of hydroboration . It also contains many exambles about hydroboration.

1. TOPIC H YDROBORATION PRESENTED BY K.MUTHU KANNAN

2. HYDROBORATION of alkenes: Introduction: In organic chemistry, the hydroboration-oxidation reaction is a two step hydration reaction that converts alkene into an alcohol. The process results in the syn addition of a hydrogen and a hydroxyl group. Hydroboration-oxidation is an anti- Morkovnikov reaction , with the hydroxyl group attaching to the less substituted carbon. Reaction: Borane is an electron-deficient compound. It has only six valence electrons, so the boron atom in BH 3 cannot have an octet. Acquiring an octet is the driving force for the unusual bonding structures found in boron compounds. As an electron-deficient compound, BH 3 is a strong electrophile, capable of adding to a double bond.

3. MECHANISM : This hydroboration of the double bond is thought to occur in one step, with the boron atom adding to the less substituted end of the double bond. In the transition state, the electrophilic boron atom withdraws electrons from the pi bond, and the carbon at the other end of the double bond acquires a partial positive charge. This partial charge is more stable on the more substituted carbon atom. The product shows boron bonded to the less substituted end of the double bond and hydrogen bonded to the more substituted end.

4. Stereochemistry of Hydroboration The simultaneous addition of boron and hydrogen to the double bond leads to a syn addition . Boron and hydrogen add across the double bond on the same side of the molecule. (If they added to opposite sides of the molecule, the process would be an anti addition. ) The stereochemistry of the hydroboration–oxidation of 1-methylcyclopentene is shown next. Boron and hydrogen add to the same face of the double bond (syn) to form a trialkylborane . Oxidation of the trialkylborane replaces boron with a hydroxyl group in the same stereochemical position. The product is trans -2-methylcyclopentanol.

5. The second step (oxidation of the borane to the alcohol) takes place with retention of configuration. Hydroperoxide ion adds to the borane, causing the alkyl group to migrate from boron to oxygen. The alkyl group migrates with retention of configuration because it moves with its electron pair and does not alter the tetrahedral structure of the migrating carbon atom. Hydrolysis of the borate ester gives the alcohol. Exambles : (1)

6. (2) (3)

7. (4) (5) (6)

8. (7) Applications: Hydroboration is quite useful in the synthesis of some organic compounds. Hydroboration can also be used to produce organoborane compounds which are chemical compounds which have carbon boron bonds and are derivatives of BH3.These organoboranes can be reacted with some reagents to give alkali halides, alcohols, and amines which are quite useful.

9. Reference: Organic chemistry 8 th edition by L.G.WADE en.m.wikipedia.org/wiki/Wolff%E2%80%93Kishner_reduction https://images.app.goo.gl/upNNKjFCfnJC9Bop8 https://images.app.goo.gl/cLY4LVLUBHBzUTYL7 https://images.app.goo.gl/e4qioTixgvpKEMYJ9 https://images.app.goo.gl/tEx6RrEnvaB1VtzRA https://www.google.com/imgres?imgurl=https%3A%2F%2Fcdn.masterorganicchemistry.com%2Fwp-content%2Fuploads%2F2020%2F05%2F1-hydroboration-of-alkenes-with-bh3-e1588973703820.gif&imgrefurl=https%3A%2F%2Fwww.masterorganicchemistry.com%2Freaction-guide%2Fhydroboration-of-alkenes%2F&tbnid=rpFKpC0xx_YP8M&vet=1&docid=MWpW5nl8A0cctM&w=1164&h=222&itg=1&source=sh%2Fx%2Fim

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