Alkyne Reactions

I.

R Reduction of Alkynes

A.

Relative Reactivity of 2 p -bonds Alkynes react like alkenes, but twice a) b) Hydrogenation of alkynes goes to alkanes Electrophilic Attack

R R A C C R AB A A or B R A R B R R A B R or R B A B B

c) Most of the Alkene reaction we learned will work for alkynes x 2

B.

cis

-Alkenes from Alkynes 1) Alkynes can be Hydrogenated under alkene conditions a) b) Pd/C or PtO 2 , 1 atm H 2 Gives saturated alkane product R C C R Pd/C H 2 2) H R R H H Lindlar Catalyst gives single H 2 addition a) b) Lindlar Catalyst = 5% Pd-CaCO 3 , Pb(OAc) 2 , quinoline = Less active catalyst surface, so only one p -bond is added to c) H 2 addition is syn (like for alkenes) and gives

cis

-alkene product N H

C C H

2

, Lindlar catalyst 25

o

C H C C H

C.

trans

-Alkenes 1) 2) 3) Na is a strong reducing agent: Na Na + + e Na dissolves in NH 3 (l) to give Na + + e- (solvated electron) Alkynes exposed to Na/NH 3 (l) are selectively reduced to

trans

-alkenes 1. Na, NH 3 (l) H R R C C R 2. H 2 O R H

4) Mechanism: (see book for orbital picture) R C C R Na C H R C R Alkyne Radical Anion NH 2 H C C R R trans-Alkenyl Radical Na H R C C R trans-Alkenyl Anion H C R C R H NH 2 H C R C R H

II.

Electrophilic Addition to Alkynes

A.

Alkynes are much like alkenes 1) p -bonds readily attacked by electrophiles 2) Terminal alkynes follow Markovnikov rule a) Electrophile ends up on less substituted Carbon b) Nucleophile ends up on more substituted Carbon N R C C H E N H R E

B.

HX Additions to Alkynes 1) Single addition usually gives anti product H 3 C C C CH 3 H Br Br H H 3 C 2) CH 3 Br The second addition gives the Geminal Dihaloalkane (Markovnikov Rule) H CH 3 H Br H Br Br H 3 C Br H 3 C H Br CH 3 3) H 3 C Terminal Alkynes give Marknovnikov Addition Products C C H HI -70 o C I CH 3 CI 2 CH 3 (65%) + H 3 C CH 2 (35%) 4) Stopping the reaction after only one addition is difficult for terminal alkynes

CH 3 CH 2 C C.

Halogenation of Alkynes 1) Anti addition of a single X 2 molecule can be done to get vicinal dihaloalkane anti addition product 2) The second addition gives a tetrasubstituted product C CH 2 CH 3 Br 2 , HOAc LiBr CH 3 CH Br 2 C C Br Br 2 CCl 4 CH 2 CH 3 E-3,4-dibromo-3-hexene CH 3 CH 2 CBr 2 CBr 2 CH 2 CH 3 R C D.

Ketones from Alkynes: Mercuric Ion Catalyzed Hydration 1) Like alkenes, Alkynes can be hydrated 2) The enol product undergoes

tautomerization

(interconversion of isomers by C=C, H shift) to give a ketone product H C 3) 4) R H 2 O, H + , HgSO 4 H C C O tautomerization H H C R Enol R R The Enol and Ketone are called

tautomers

The Hg 2+ Ketone cation catalyzes the reaction; mechanism not understood yet C O R

OH 5) The hydration step follows the Markovnikov Rule OH OH H 2 O, H + , HgSO 4 H C C 6) H Symmetric Alkynes give only 1 product H 2 O, H + , HgSO 4 HO C C H H C + C OH C C 7) Unsymmetric Alkynes give product mixtures HO H H 2 O, H + , HgSO 4 C C H C + C OH O H C C H C H + C H O OH O O C C H H

III. Anti-Markovnikov Additions to Alkynes

A.

Radical HBr additions 1) HBr adds to Alkynes by radical mechanism with radical initiator ROOR 2) Anti-Markovnikov Products due to need for most stable radical intermediate

ROOR + HBr ROH + Br H

3

C C

B.

C H Br H

3

C C C Br H Br H C H

Aldehydes from Hydroboration-Oxidation of Alkynes 1) 2) 3)

H

3

C

Like with alkenes, BHR 2 adds to less substituted side of an alkyne R groups prevent boration of both p -bonds Oxidation results in the enol  aldehyde tautomerization

C Br H

H 3 C C C H HB THF 2 H 3 C H C C H BR 2 H 2 O 2 OH H 3 C H C Enol C H OH H H C C H 3 C Aldehyde H O

IV. Alkenyl Halide Chemistry

A.

Alkenyl Halides don’t do S N 1 or S N 2 reactions 1) Alkenyl Halides preferentially eliminate to alkynes instead of substituting 2) 3) 4) Need strong base to get elimination, other nucleophiles give N.R.

Simple nucleophiles don’t give the substitution products Alkenyl cation that would form is very unstable H 2 C C H I S N 2 Br NO REACTION H 2 C C R H 2 O S N 1 Br R H 2 C C too high energy to exist

H

2

C

B.

C

Alkenyl Organometallics can form and act as nucleophiles

O H Mg H 1.

H

2

C C Br THF MgBr 2. H

2

O, H

+

H

2

C C H

3

C H C CH OH

3

V.

Organocuprate Chemistry

A.

Most organometallic reagents don’t react with alkyl halides 1) Alkyl and Alkenyl Metal reagents don’t attack haloalkanes fast enough

MgBr + Br NO REACTION

B.

2) CH Alkynyl Metal reagents can react with haloalkanes C nBuLi (base) Br THF, HMPA S N 2 Organocuprates are more reactive 1) Formation of Organocuprates 2 RLi + CuI R 2 CuLi + LiI C H 2 C CHLi + CuI (CH 2 =CH) 2 CuLi Lithium Diethenylcuprate 2) Organocuprates will couple their R groups with Haloalkanes

R

2

CuLi + R'X R R'

3) 4) Either alkyl or alkenyl organocuprates work for the coupling reaction Along with the Alkynyl Anion reactions, we now can form many different C—C bonds

CH

3

I 1. Li, Et

2

O 2. CuI (CH

3

)

2

CuLi CH

3

CH

3

I Et

2

O, 0

o

C CH

3

CH

2

CH

3 H 3 C H C C H Cl 1. Li, Et 2 O 2. CuI H 3 C H C C H 2 CuLi CH 3 CH 3 I HMPA H 3 C H C C H CH 2 CH 3 CH nBuLi (base) THF, HMPA C S N 2 Br C