A number of conversions can be carried out by using the hydroboration reaction. These are as follows:
(a) Conversion of a terminal alkyne to an aldehyde: A terminal alkyne can be converted to the corresponding aldehyde by carrying out its hydroboration with di(3-methyl-2-butyl) borane or disiamylborane (Sia2BH) followed by oxidation and hydrolysis of the resulting vinylborane with alkaline hydrogen peroxide (the name ‘disiamyl’ comes from ‘di-secondary-iso-amyl’). This crowded borane is a very selective hydroborating agent. When it reacts with the terminal alkyne, boron becomes attached to the less crowded terminal carbon predominantly.
Also, hydroboration, which cannot be stopped at the alkenylborane stage by using BH3:THF, can be stopped by using Sia2BH. 1-Heptyne, for example, can be converted into heptanol as follows:
(b) Conversion of an alkene to the corresponding alkane: When an alkene is subjected to hydroboration by treating with BH3: THF followed by treatment of the resulting alkylborane with anhydrous propanoic acid (CH3CH2COOH), an alkane is obtained. For example, 1-pentene can be converted into pentane as follows:
The trialkylborane undergoes protonolysis concertedly through a six-membered cyclic transition state to give pentane, when treated with propaonic acid.
(c) Conversion of an alkene to a primary amine: When an alkene is subjected to hydroboration and the resulting alkylborane is treated with hydroxylamine-O- sulphonic acid, H2NOSO3H, an intermediate ammonium boride is obtained. This undergoes rearrangement to give a compound which on acidic hydrolysis produces the primary amine. For example, 1-butene can be converted into 1-butanamine as follows:
(d) Conversion of an alkene into a primary alcohol with one more carbon atom: An alkene can be converted into a primary alcohol with one more carbon atom by carbonylation of the corresponding alkylborane in the presence of lithium trimethoxyaluminium hydride [LiAlH(OCH3)3] followed by alkaline hydrolysis of the resulting compound. For example, propene can be converted into 1-butanol (CH3CH2CH2CH2OH) as follows:
(e) Conversion of an alkene into a secondary alcohol: An alkene can be converted into a secondary alcohol containing (2 ¥ no. of carbon atoms in the alkene +1) carbon atoms by carbonylation of the corresponding alkylborane in the presence of water followed by hydrolysis of the resulting compound. For example, 1-butene can be converted into 5-nonanol [(CH2CH2CH2CH2)2CHOH] as follows:
— —
3 2
2
1. BH : THF
3 2 2 3 2 2 2 2 2 2 3 2
3 2 2 2 2
2. CO/H O
NaOH H O
3CH CH CH CH CH CH CH CH B C (CH CH CH CH )
| | 1-Butene
OH OH (CH CH CH CH ) COH
5-Nonanol
== ổổổổổặ
ổổổổặ
(f) Conversion of an alkene into a tertiary alcohol: An alkene can be converted into a tertiary alcohol containing (3 ¥ no. of carbon atoms of the alkene +1) carbon atoms by carbonylation of the corresponding alkylborane followed by oxidation and hydrolysis of the resulting compound. For example, propene can be converted into 4-propyl-4-heptanol [(CH3CH2CH2)3COH] as follows:
(g) Conversion of an alkene into an aldehyde: An alkene can be converted into an aldehyde with one more carbon atom by carbonylation of the corresponding alkylborane in the presence of lithium trimethoxyaluminium hydride followed by oxidation and hydrolysis of the resulting compound with H2O2 (pH 7). For example, propene can be converted into butanal (CH3CH2CH2CHO) as follows:
(h) Conversion of an alkene into a ketone: An alkene can be converted into a ketone containing (2 ¥ no. of carbon atoms of the alkene +1) carbon atoms by carbonylcation of the corresponding alkyl borane in the presence of water followed by oxidation and hydrolysis of the resulting compound with H2O2/OH①. For example, 1-butene can be converted into 5-nonanone [(CH3CH2CH2CH2)2C == O] as follows:
(i) 9-BBN and its uses: 9-Borabicyclo[3.3.1]nonane (commonly known as 9-BBN) can be prepared by carrying out hydroboration of 1,5-cyclooctadiene.
Mechanism: The mechanism of hydroboration of 1,5-cyclooctadiene to yield 9-BBN is as follows:
It is to be noted that hydroboration of the two double bonds of 1,5-cyclooctadiene takes place in two possible ways to give a mixture of 1,4- and 1,5-isomer. The 1,5-isomer, i.e., 9-BBN is thermodynamically more stable because it contains two stable and strain-free six-membered rings and so, it is formed predominantly.
The two important uses of 9-BBN are as follows:
(i) Only one of the three alkyl groups of a trialkylborane is converted into an aldehyde by carbonylation-reduction-oxidation sequence and the wastage of the alkene used cannot be avoided. This can, however, be avoided by carrying out hydroboration with 9-BBN. Also, a –CHO group can be much selectively introduced at the less hindered doubly-bonded carbon by using the sterically hindered reagent 9-BBN.
For example:
(ii) When 9-BBN is allowed to react with a compound containing two double bonds, hydroboration at the less crowded double bond takes place exclusively. For example:
1. Provide the structure of an alkene that would give each of the following alcohols as the major (or only) product on hydroboration-oxidation:
(a) 3 2
3 3
CH CH CH CHOH
| | CH CH
(b)
(c) (d)
Solution
(a)
2
3 2 3
CH|
CH CH C==CHCH (b)
(c) (d)
2 Predict the product (s) in each step of the following reaction sequences.
Give sterochemistry when it is appropriate.
(a)
(b)
(c)
(d)
(e)
(f)
(g) 3 3
3
3 2 2 2
:
BD THF Ag NO /KOH
CH|
CH CH CH —C==CH ổổổổổặ SổổổổổổặT Solution
(a) 1-Phenylethanol produces styrene (A) when heated with concentrated H2SO4. Hydroboration-oxidation of styrene produces 2-phenylethanol (C).
(b) The addition of BH3 to the double bond is stereospecifically syn and the conversion of C—B to the C—O bond by oxidation and hydrolysis proceeds with retention of configuration. Therefore, the products of this reaction sequence are as follows:
(c)
(d)
(e)
It is possible to convert alkylboranes to ketones if acidic dichromate is used as the oxidant.
(f)
(g)
Alkylboranes on treatment with basic silver nitrate (AgNO3) lead to coupling of the alkyl groups.
3. Write the structure, for the alcohol obtained by hydroboration-oxidation of each of the following alkenes:
(a) (b)
Solution
(a) If the two faces of the double bond are not equivalent in terms of steric hindrance, hydroboration takes place preferentially from the less crowded or less hindered face of the double bond. In this bicyclic alkene, the bottom face of the double bond is more hindered than the top face because of the two boat-axial or endohydrogens at C-5 and C-6.
(b)
The addition of Sia2BH occurs preferably from the less-hindered bottom face (b) of the double bond to yield mainly the alcohol (A).
4. C D Sia BH2 C H CH6 5 CH2 BH THF3: A B Styrene
+ ăổổổổ == ổổổổổặ + Identify A, B, C and D and compare the distribution of products is one reaction with that of the other reaction.
Solution
The regioselectivity in hydroboration-oxidation is caused mostly by steric factors. When BH3 adds to styrene, 80% event of addition of boron occurs at the less substituted and less sterically hindered carbon. However, when the sterically hindered borane Sia2BH is used for hydroboration, the reaction becomes more regioselective. Because of greater steric hindrance at the more substituted carbon, 98% event of addition of boron occurs at the less substituted carbon.
5. What is called thexylborane? Give its two important uses.
Solution 2,3-Dimethyl-2-butylborane — — —
3 3
3 2 2
3
CH CH
| |
(CH C H C BH or|—|— BH )
| CH
is called thexylborane.
Thexylborane is a valuable hydroborating agent and it is the most readily available of the monalkylboranes. Its two important uses are as follows:
(i) It is useful for the cyclic hydroboration of dienes. For example:
(ii) It is also used to make trialkylboranes containing three different alkyl groups by stepwise addition to two different alkenes. For example:
6. How would you accomplish the following transformation?
Solution This transformation can be carried out by using the sterically hindered disiamylborane which reacts more rapidly with terminal double bond than internal double bond.
1. How would you accomplish the following transformations:
(a) 3 2 2 3 2 — 3
OH| (CH ) C==CH ổổặ(CH ) C CH (b) (CH ) C3 2 ==CH2 ổổặ(CH ) CHCH OH3 2 2
2. How would you carry out the following transformations:
(a) CH (CH ) C3 2 2 ∫∫CH ổổặCH (CH ) CHO3 2 3 (b) CH (CH ) C3 2 2 ∫∫CH ổổặCH (CH ) COCH3 2 2 3
3. How can CH3(CH2)3CHDCHO can be prepared from CH3(CH2)3C ∫∫ CH?
[Hint: Sia2BD is to be used instead of Sia2BH]
4. Explain why a B—O bond is much stronger than a B—C bond.
5. Show how one may accomplish each of the following transformations in a practical manner:
(a)
(b)
(c) CH CH3 ==CH2 ổổặCH (CH ) COCH3 2 3 3
(d)
(e)
(f)
(g)
6. Show how hydroboration of an appropriate alkene can be used to prepare (S,S)- and (R,R)-PhCH(CH3)CH(OH)Ph.
7. How would you carry out the following transformations:
(a)
(b)
8. When BH3 is allowed to react with a large excess of 2-methylbut-2-ene and 2,3- dimethylbut-2-ene, “disiamylborane”. (Me2CHCHMe-)2BH and “thexylborane”, Me2CHCMe2BH2 are obtained instead of trialkylboranes—explain.
9. Indicate suitable reagents to carry out the following conversions (show the intermediate compounds):
10. Suggest a mechanism for each of the following reactions:
(a) (b)