Addition of BH3 to carbon carbon double bond

 The addition of BH3 to a carbon-carbon double bond is the first step in the hydroboration-oxidation reaction. This step is called hydroboration and it is an electrophilic addition reaction. The mechanism of this step involves the vacant 2p orbital of the boron electrophile pairing with the electron pair of the pi bond of the nucleophile. The hydrogen atom attached to boron is transferred to the carbon atom adjacent to the one that becomes bonded to boron, attaching to the less-substituted carbon. This results in the formation of an organoborane compound. The hydroboration step is repeated two additional times, successively reacting each B-H bond so that three alkenes add to each BH3. The reaction provides a more stereospecific and complementary regiochemical alternative to other hydration reactions such as acid-catalyzed addition and the oxymercuration-reduction process. 

Steps of Hydroboration reaction:

The hydroboration-oxidation reaction is a two-step process that involves the addition of borane to an alkene followed by oxidation. The first step is the hydroboration step, which begins with the addition of borane (BH3) to the double bond of the alkene. The reaction occurs rapidly. This leads to the transfer of a hydrogen atom to the carbon atom that is adjacent to the carbon bonded with the boron atom. The mechanism starts with a borane attacking the π bond at the less substituted and sterically less hindered site of an alkene forming a cyclic transition state. The overall result is a syn-addition of BH2 and hydrogen across the alkene double bond, producing an alkylborane. The reaction of a second borane molecule with the alkylborane produces a dialkylborane. The hydroboration reaction is an electrophilic addition reaction that results in the formation of an organoborane intermediate. The second step is the oxidation step, which involves the oxidation of the organoborane intermediate with an oxidizing agent such as hydrogen peroxide (H2O2) or sodium perborate (NaBO3). The oxidation step converts the organoborane intermediate into an alcohol by replacing the boron atom with a hydroxyl group.

Note: The reaction proceeds in an anti-Markovnikov manner, where the hydrogen from BH3 attaches to the more substituted carbon and the boron attaches to the less substituted carbon. This is because the hydroboration step occurs in a concerted syn addition of B and H across the double bond, with the boron adding to the less substituted carbon. Overall, the addition of BH3 to carbon carbon double bond is a Markovnikov's syn addition reaction.

Hydroboration-oxidation can be used for the synthesis of most types of alcohols, but there are some limitations.

Limitation:  The hydroboration-oxidation reaction has some limitations, which are listed below:

• The reaction is not suitable for the synthesis of primary alcohols as the reaction stops at the formation of the corresponding aldehyde.
• The reaction is not effective for the synthesis of tertiary alcohols as the reaction leads to the formation of a mixture of products.
• The reaction is not regioselective for internal alkenes, which can lead to the formation of a mixture of products.
• The reaction requires the use of borane, which is a toxic and pyrophoric reagent.
• The reaction requires careful handling and storage due to the instability of borane.

Despite these limitations, hydroboration-oxidation is still a useful and widely used method for the synthesis of alcohols from alkenes due to its high stereoselectivity and complementary regiochemistry.