Alkenes and Alkynes- Hydration and Oxidation of Alkenes Flashcards
Under what conditions can water add across alkenes
- Under acidic conditions
2. Reverse of E1 elimination of tertiary alcohol
What substrates can water be added to
- Only possible for substrates that can form a stabilised tertiary carbocation intermediate
What is uncertainty about addition of water under acidic conditions to an alkene
- Difficult to predict whether aqueous acid will hydrate alkene or dehydrate alcohol
- Alkenes can also polymerise under acidic conditions
What is a sequence that can be used to hydrate alkenes selectively
- Oxymercuration/demercuration sequence
- Add Hg(OAc)2 + H2O which breaks C=C
- Then add NaBH4 which breaks the HgOAc-C weak bond producing an alcohol
Why is oxymercuration/demercuration not used now
- Very toxic
Describe the mechanism for the oxymercuration/demercuration reaction
- Reaction proceeds via a mercurinium ion intermediate, which ring-opens at the most substituted position
- Weak C-Hg bond is cleaved under reductive conditions
What method can be used to form alcohols with anti-Markovnikov’s regioselectivity from alkenes
- Hydroboration of alkenes followed by oxidation
Describe borane derivatives
- Contain B-H bond
- They are electrophilic due to empty p-orbital on boron atom which can accept e-
- Boron has 3 valence electrons
- Less electronegative than hydrogen
Give 3 examples of common boranes
- B2H6- diborane
- H3B.THF
- H3B.SMe2
Describe the regioselectivity of hydroboration
- Hydroboration is concerted with a four-membered trnasition state
- Regioselectivity is determined by combination of steric and electronic factors
- Electronics- partial positive charge builds up on more substituted end of alkene
- Sterics- boron adds to the least hindered end of the alkene
- First borane formed is still electrophilic so undergoes two more hydroboration reductions to form trialkylborane
What happens once the trialkylborane has formed
- It can be oxidised into the alcohol using hydrogen peroxide and base
Describe the oxidation of trialkylborane using hydrogen peroxide and base
- Deprotonated hydrogen peroxide adds to empty p-orbital on boron atom
- Boronate undergoes bond migration
- Breaks weak O-O bond to form strong B-O bond
- Occurs 3 times to form trialkylborate
What happens once the trialkylborate is formed
- In the presence of the hydroxide, the trialkylborate undergoes hydrolysis to release the alcohol product and boric acid side product
Describe the stereochemistry of hydroboration
- It is a stereospecific process with hydrogen and boron adding to the same face of the alkene- syn-addition
- Oxidation does not affect stereochemistry ast eh migration step occurs with retention
How can you form epoxides from alkenes
- Treating alkenes with peroxycarboxylic acid (peracid)
2. Often meta-chloroperbenzoic acid (mCPBA) is a commercially available oxidant- as it contains a weak O-O sigma bond
Describe what happens in the formation of an epoxide from alkenes
- HOMO of alkene (piC-C) interacts with LUMO of mCPBA (sigma* O-O)
Describe the stereochemistry of epoxidation
- It is stereospecific
- Oxygen added to one face of the alkene- syn-addition
- Racemic but either top or bottom or meso compound
Describe epoxide ring-opening under basic conditions
- Under basic conditions, epoxides undergo regioselective ring-opening with good nucleophiles at the least hindered positions
- SN2 transition state- steric hindrance controlling factor
Describe epoxide ring-opening under acidic conditions
- The selectivity of ring-opening is lower
2. Both SN2 and loose SN2 transitions states are accessible leading to a mixture of regioisomers
How else can epoxides be ring-opened
- With hydroxides to form 1,2-diols
- Stereospecific SN2 ring-opening leading to inversion of configuration at one centre
- Racemic or meso epoxide could give racemic or meso diol
Describe example of epoxide ring-opening in nature
- Marine ladder polyethers are highly complex natural products
- While the biosynthesis is not clear, one suggestion is a dramatic epoxide ring-opening cascade
