Alkenes and Alkynes- Oxidation of alkenes

Question 1

How can 1,2-diols be synthesised directly from alkenes

Answer 1

1. Dihydroxylation
2. Uses osmium tetroxide (OsO4) to deliver 2 oxygen atoms
3. Both oxygen atoms are added from one face of the alkene- syn-addition

Question 2

Describe the stereochemistry of dihydroxylation

Answer 2

1. Stereospecific

2. Both oxygen atoms added to one face of the alkene- syn-addition

Question 3

Describe Upjohn dihydroxylation

Answer 3

1. Dihydroxylation is widely used in academia and in industry
2. But OSO4 is highly toxic + expensive
3. A catalytic variant was developed using N-methylmorpholine N-oxide (NMO) to oxidise OS(VI) back to OS(VIII)

Question 4

How can 1,2-diols be further oxidised

Answer 4

1. Using sodium periodate (NaIO4) to break the sigma C-C bond
2. Forms two carbonyls where OH was
3. Also same reaction using lead acetate Pb(OAc)4 which is known as a Criegee oxidation
4. Sodium periodate is beneficial as it avoids the use of stoichiometric lead

Question 5

Describe mechanism for addition of 1,2-diol to NaIO4

Answer 5

1. Addition of NaIO4 forms periodate ester
2. Concerted reaction with cyclic movement of electrons (pericyclic) to break the sigma C-C bond
3. I is reduced from I(VII) to I(V)

Question 6

What can ozone be used for

Answer 6

1. To oxidatively cleave alkenes directly
2. Produce two carbonyls
3. Ozonolysis

Question 7

Describe O3 structure

Answer 7

1. O3 is bent with a positively charged central oxygen atom and two terminal oxygen atoms that share a negative charge resonance
2. One double bond and one single
3. It is generated from oxygen and bubbled through a reaction

Question 8

Describe the mechanism for ozonolysis of an alkene

Answer 8

1. 1,3-dipolar cycloaddition forms a molozonide which is highly unstable and decomposes rapidly
2. 1,3-dipolar cycloaddition to decomposition products produces a ozonide- first stable product
3. This can then react Me2S to form two carbonyl compounds

Question 9

What else can ozonide react with

Answer 9

1. Oxidants (H2O2) to generate carboxylic acids

2. Reductants (NaBH4)to form two alcohols

Question 10

How can you form cyclopropanes from alkenes

Answer 10

1. Addition of carbenes

Question 11

What are carbenes

Answer 11

1. A neutral, divalent carbon atom containing 6 valence electrons
2. Filled sp2 hybridised orbital (lone pair)
3. empty p-orbital
4. Nucleophilic and electrophilic

Question 12

Describe the Simmons-Smith Cyclopropanation

Answer 12

1. Reaction generates a zinc carbenoid from the reaction of diiodomethane CH2I2 (carbene) with zinc metal
2. Forms ICH2ZnCH2I + ZnI2

Question 13

Describe the mechanism for the cyclopropanation of an alkene with ZNI-CH2-I

Answer 13

1. Direct carbene transfer without forming a free carbene species
2. Proceeds through a butterfly transition state
3. Carbon atom of the carbenoid can only add to one face of the alkene so always form the syn-product