Interconverting Functional Groups

Question 1

When would BH3.THF not be suitable for use and why? What would be better to use alternatively?

Answer 1

It is not suitable for use when a carbonyl group is present as it could react with this, it also shows lower regioselectivity with a simple terminal alkene. 9-BBN would be better used here.

Question 2

What to reaction types can be used to add H2O across a C=C double bond?

Answer 2

• Hydroboration Oxidation
• Oxymercuration
• Direct methods are generally ineffective.

Question 3

What is the Upjohn modification? What does it need to work?

Answer 3

The Upjohn modification is a modification to Dihydroxylation that allows a safer alternative to using large quantities of dangerous osmium as it only requires it to be used catalytically. It also requires the use of NMO with BuOH/H2O.

Question 4

What is a syn addition?

Answer 4

A syn addition decreases the bond order while increasing the number of substituents. Things add to the same face of the alkene.

Question 5

Hydrogenation of alkenes - type of reaction, order of reactivity.

Answer 5

Hydrogenation reduces alkenes to alkenes.
It requires a homogeneous or heterogeneous catalysts (Pd/C is commonly used).
It is a syn addition so substituents added to same side of double bond.
Generally, least hindered alkenes react faster.

Question 6

Oxymercuration

Answer 6

• Regioselective when there is a difference in cation stability.
• Hydrogen is added to the least hindered side, however, unless the substrate us tetra substituted there are no diastereoisomers formed so the ‘anti’ selectivity in the oxymercuration step is not visible in the products. (this means the OH is added anti to the Hg)

Question 7

Hydroboration-Oxidation - type of reaction, what is used in the reaction.

Answer 7

• Syn addition.
• Regioselective if there is a difference in the ability to stabilise a positive charge in the transition state.
• There is no intermediate, only a transition state. This means both substituents add at the same face of the molecule.
• The addition occurs three times to form one boron surrounded by/attached to three of the alkanes (hydrogen adds each time from the BH2/BH)
• H2O2 and NaOH are used for the oxidation step and so replace the boron with a hydroxyl group on each molecule.
• Enantiomers are formed.

Question 8

Enantiomer

Answer 8

• Two enantiomers are mirror images of each other (left/right handed)

Question 9

Regioselectivity of Hydroboration-Oxidation

Answer 9

Considering the transition states (TS) it is evident why the BH2 adds to the least substituted carbon (the BH2 is slightly positive and the H is slightly negative charged due to the electonegativities of these atoms, therefore the carbons they attach to are the opposite of this and the carbon the hydrogen attaches to must be positive and a more substituted carbon is better at stabilising this positive charge so the BH2 must add to the least substituted carbon. - to remember this, think of steric hinderance also)

Question 10

9-BBN Key points

Answer 10

The same reasoning with regards to stabilisation of positive charge applies as with BH3.THF. However, 9-BBN is more bulky than BH3.THF and so steric hinderance helps to enhance the stereoselectivity. This means that in comparison to the 80-98% selectivity of BH3.THF, 9-BBN has a higher selectivity.
9-BBN does not react with carbonyls which is useful and means it can be more widely used.
It is air stable.

Question 11

Dihydroxylation of Alkenes

Answer 11

Dihydroxylation of alkenes produces a 1,2 -diol from an alkene by syn addition of two hydroxyl groups onto the molecule. It uses the dangerous substance osmium tetroxide and so the upjohn modification is often used instead.

Question 12

Ozonoylsis of Alkenes

Answer 12

• Breaks a C=C double bond and forms two C=O bonds.
• Is a 1,3 dipolar cycloaddition reaction.
• Forms a primary and then a secondary ozonide.

Question 13

Epoxidation of Alkenes

Answer 13

• All bond breaking/forming steps occur at the same time, however, the process is driven by the alkene attacking the electrophilic oxygen.

Question 14

Epoxide Ring Opening

Answer 14

• Ring opening can be done using amine nucleophiles or organolithiums.

Question 15

Ozonoylsis of Alkenes

Answer 15

• Breaks a C=C double bond and forms two C=O bonds.
• Is a 1,3 dipolar cycloaddition reaction.
• Forms a primary and then a secondary ozonide.

Question 16

Epoxidation of Alkenes

Answer 16

• All bond breaking/forming steps occur at the same time, however, the process is driven by the alkene attacking the electrophilic oxygen.

Question 17

Epoxide Ring Opening

Answer 17

• Ring opening can be done using amine nucleophiles or organolithiums.

Question 18

1,3- Dipolar Cycloaddition Reactions

Answer 18

The 1,3 Dipole means that a resonance form exists of the molecule that has two opposing charges in the 1,3 positions - ozone is an example of this.

Question 19

Reduction of Epoxides

Answer 19

Breaks an epoxide ring to form a hydroxyl group. Can use Lithium Aluminium Hydride or a with Aluminium Hydride .

Question 20

Oxymercuration of Alkynes

Answer 20

Oxymercuration of an alkyne yields a enol, which tautomerises to a ketone/aldehyde. Reaction requires the presence of water to add the hydroxyl group.

Question 21

Hydroboration of alkynes

Answer 21

Yields an enol which tautomerizes to an aldehyde/ketone.