Organoborane or Organoboron compounds

Synthesis and important reactions of organoboranes were discussed.

1. Prepared By Dr. Krishnaswamy . G Faculty DOS & R in Organic Chemistry Tumkur University Tumakuru Organoboranes or Organo boron Compounds

2. Organoborane Chemistry deals with the chemistry of organoboranes or organoboron compounds. The C-B bond has low polarity because of small electronegativity difference (C-2.55 and B-2.04). Because of low electronegativity boron often forms electron deficient compounds such as triorganoboranes . Organoboranes acts as strong electrophile in organic chemistry since boron is unable to gain a full octet of electrons. Organoboron compounds are important reagents in organic synthesis and the most important one is hydroboration . Introduction

3. Synthesis of Organoboranes From Grignard Reagents (2) From Borylation Simple organoboranes such as triethylborane can be prepared from trifluoroborane and ethyl Grignard reagent. Metal catalysed C-H borylation produce an organoboron compound.

4. (3) From alkenes Alkenes react with borane (BH 3 ) and its derivatives to give synthetically useful alkyl boranes .

5. The addition of borane to alkene is an electrophilic process and it takes place through a concerted process in which the alkene donates electron density to the borane and at the same time hydogen shifts to carbon. This reaction is known as hydroboration reaction. Hydroboration reaction

6. Hydroboration reaction is highly regioselective because of combination of steric as well as electronic factors. In hydroboration reaction boron bonds to less substituted carbon via syn addition resulting in anti- Morkovnikov product. This regiochemistry is just reverse of typical HX addition to the alkene.

7. Hydroboration reaction is very important reaction since boron can eventually be replaced by hydroxy , carbonyl, amine and halogen substituents with retention of configuration.

8. Important organo borane reagents

9. Reactions of Organoboranes Here the focus is on the following reaction of organoboranes

10. Isomerization reaction Some alkyl boranes rearrange at elevated temperature (160 o C and above) to form more stable isomer i.e. boron prefer to bond with least substituted terminal carbon to minimize unfavourable steric interaction.

11. Rearrangement is associated with the fact that hydroboration reaction is thermally reversible at 160 o C and above. This occurs intramolecularly through three membered cyclic bridged TS analogues to bridged carbocations .

12. Most widely used reaction of organo boranes is the oxidation to alcohols is known as hydroboration -oxidation. Alkaline hydrogen peroxide is the reagent usually employed to effect the oxidation. Oxidation reactions The trialkylborane is converted into trialkoxyborane by a series of Boron to Oxygen migrations. Finally hydrolysis of alkoxy boron bonds by aqueous alkaline solution generates alcohol.

13. Mechanism

14. Other oxidants includes molecular oxygen, sodium peroxycarbonate and amine oxides permit oxidation of organoborane to alcohols. The oxidation reaction proceeds with retention of configuration.

15. Alkyne hydroboration -oxidation strategy does not usually yield an alcohol as the resulting enol is tautomerized to form a ketone .

16. Protonolysis Organoboranes are stable to mineral acids (HCl, H 2 SO 4 , etc..) but are readily attacked by carboxylic acids. Boiling an alkyl borane with a liquid acid results in the formation of a hydrocarbons. The process is termed as protonolysis of organoboranes .

17. Mechanism This method of reduction of double bond is a good alternative to catalytic hydrogenation.

18. Protonolysis of the alkenylborane can lead to alkenes in good yield.

19. Carbonylation reaction Carbonylation of organoboranes with carbon monoxide leads to formation of Lewis acid-base complex which gives rise to range of products which depends on reaction conditions. Three possible migrations of groups may occur leading to formation aldehydes and ketones as well as primary, secondary and tertiary alcohols.

20. Single migration of group followed by reduction with mild reducing agent lithium trimethoxyaluminium hydride leads to formation of mono migrated product. This mono migrated product undergoes oxidation with alkaline hydrogen peroxide to give aldehyde. On the other hand treatment with LiAlH 4 followed by oxidative workup produces primary alcohols. Synthesis of aldehyde and alcohols

21. If the carbonylation reaction is done in the presence of a small amount of water at 100 °C, a second alkyl group migrates and after oxidative workup produce corresponding ketone . Synthesis of Ketones

22. The use of thexylborane (1,1,2-trimethylpropylborane) as the hydroboratingagent permits the synthesis of mixed trialkyboranes which upon carbonylation gives unsymmetrical ketones .

23. (b) cyclic hydroboration of dienes followed by carbonylation yields cyclic ketones . Due to low migratory aptitude of thexyl moiety in carbonylation and it serves as an anchor group

24. Synthesis of tertiary alcohols Carbonylation of trialkylboranes in the presence of ethylene glycol results in migration of all three alkyl group from Boron to carbonyl Carbon to give, after oxidation, the corresponding tert -alcohols.

25. Cyanidation reaction Since nitrile anion is isoelectronic with carbonyl and reacts with trialkyl boranes . Reaction of nitrile with trialkyl borane is known as cyanidation reaction. The intermediate cyanoborates are thermally stable hence to induce 1, 2-migration with electrophiles such as benzoyl chloride or trifluoroacetic anhydride. Cyanidation is useful alternative route to prepare ketones and trialkylmethanols . Formation of ketones and trialkylmethanols occurs under milder condition.

26. Formation of ketones and trialkylmethanols depends on the stoichiometry of the electrophile . Excess of TFAA results in the formation of trialkylmethanol after oxidation.