other addition reactions

regioselectivity

refers to the preference of a reaction to take place at a specific position on a molecule when more than one position is possible

stereoselectivity

refers to the orientation in space of the reaction taking place at a specific position on a molecule

stereoselective reactions

have a preference to generate a specific stereoisomer as opposed to a racemic mixture

stepwise additions

• HX + ROOR

• X₂ addition

• X₂ in H₂O addition

• oxymercuration/demercuration

HX + ROOR

• regioselective (anti-Markovnikov)

• not stereoselective

• carbocation intermediate

X₂ addition

• not regioselective

• stereoselective (anti)

• cation not centered on carbon

X₂ in H₂O addition

• regioselective (Markovnikov)

• stereoselective (anti)

• cation not centered on carbon

oxymercuration/demercuration

• regioselective (Markovnikov)

• not stereoselective

• cation not centered on carbon

concerted additions

• hydroboration/oxidation

• H₂ addition

• epoxidation

• carbene addition

hydroboration/oxidation

• regioselective (anti-Markovnikov)

• stereoselective (syn)

H₂ addition

• not regioselective

• stereoselective (syn)

epoxidation

• not regioselective

• stereoselective (syn)

carbene addition

• not regioselective

• stereoselective (syn)

HX+ROOR addition mechanism

1. RO∙ attack on H-X, formation of RO-H σ bond and X∙

2. C=C attack on X∙, formation of C-X σ bond and C∙

3. C∙ attack on H-X, formation of C-H and X∙

anti-Markovnikov addition

H adds to the carbon with the least Hs already to make the most stable free radical

key features of HX+ROOR additions

• either side attack

• new sp³ carbons are racemic (if chiral)

• regioselective, most stable C∙ intermediate

• no carbocations or rearrangements

X₂ addition mechanism

1. C=C attack on X-X, formation of C-X σ bond and cyclic cation

2. X^- attack on cyclic cation, formation of C-X σ bond (anti addition)

anti addition

addition of X^- must be from opposite side to X⁺

key features of X₂ addition

1. anti addition

2. stereoselective: only a subset of possible stereoisomers are formed

3. not regioselective

4. no carbocations or rearrangements

5. can add two equivalents to alkyne to form tetrahaloalkane

X₂/H₂O addition mechanism

1. C=C attack on X-X, formation of C-X σ bond and cyclic cation

2. H₂O attack on cyclic cation, formation of C-OH₂⁺ σ bond

3. C-OH₂⁺ loses H⁺ to X^- to form neutral halohydrin

key features of X₂ additions in H₂O

• anti addition

• stereoselective (only a subset of possible stereoisomers are formed)

• Markovnikov addition, regioselective

• no carbocations or rearrangements

alcohol synthesis

• addition of H₂O/H⁺ with HX cation

• oxymercuration/demercuration (Hg(OAc)₂/H₂O, NaBH₄)

• hydroboration-oxidation (BH₃, H₂O₂/NaOH)

oxymercuration-demercuration mechanism

1. C=C attack on Hg(OAc)⁺, formation of C-Hg σ bond and cyclic cation

2. H₂O attack on cyclic cation, formation of C-OH₂⁺ σ bond

3. C-OH₂⁺ loses H⁺ to water/solvent

4. reduction of Hg(OAc) to H with NaBH₄ (stereoselectivity lost)

key features of oxymerc-demercuration

• oxymerc stereoselective, but demercuration is not, so both new sp³ carbons are racemic (if chiral)

• Markovnikov addition, regioselective

• no carbocations or rearrangements

hydroboration-oxidation mechanism

1. C=C attack on B-H, formation of C-B and C-H σ bond (every B-H can add to C=C, end result is a trialkylborane)

2. oxidation of BH₂ group (or BR₂ group) to OH

key features of hydroboration-oxidation

• syn-addition (B-H must approach C=C syn/add at the same time)

• stereoselective (only a subset of possible stereoisomers are formed)

• anti-Markovnikov addition, regioselective

H₂ addition (hydrogenation)

1. H₂ activated on catalyst, then adds across C=C bond

H₂ addition to alkyne with Lindlar’s catalyst (Pd/CaCO₃)

concerted syn addition, cis alkene product

H₂ addition to alkyne with Ni₂B

concerted syn addition, cis alkene product

Li or Na in liquid NH₃ addition to alkyne

radical addition, trans alkene product

key features of H₂ additions

• syn addition (H₂ approaches C=C from the same side)

• stereoselective (only a subset of possible stereoisomers are formed)

• not regioselective (symmetrical reagent)

• alkyne to alkene hydrogenation possible with specialty reagents