Like alkenes, but ending Like alkenes, but ending -ene-ene turns intoturns into ––yne.yne.

HC CHHC CH

EthyneEthyne

5544

33

2211

1-1-PentynePentyne

BrBr11

2233

4455 66

5-Bromo-2-5-Bromo-2-hexynehexyne

-ol -yne-ol -yne>> OHOH

1122

33 2-Propyn-1-2-Propyn-1-olol

Alkynes: The C C Alkynes: The C C Triple Triple BondBond

NamesNames

Priority:Priority:

When the alkyne contains also double bonds, When the alkyne contains also double bonds, it is called an it is called an enyne.enyne. However, despite being However, despite being an “yne”, numbering begins closest to either an “yne”, numbering begins closest to either group: group:

11

2233 44

55

66

3-Hexen-1-3-Hexen-1-yneyne

1-Penten-4-1-Penten-4-yneyne

1122

33

44

55

When double and triple bond are equidistant When double and triple bond are equidistant from each terminus: from each terminus: Ene firstEne first (alphabetical) (alphabetical)

1122

33

44

5566

77

1-Hepten-4-yne1-Hepten-4-yne

SubstituentsSubstituents::

EthynylEthynyl 2-Propynyl 2-Propynyl (or (or

propargyl)propargyl)Rings:Rings: Naming follows hydrocarbon Naming follows hydrocarbon rule:rule:Smaller R is a substituent to larger R Smaller R is a substituent to larger R (ignore function)(ignore function)

3-Cyclobutyl-1-3-Cyclobutyl-1-hexynehexyne

EthynylcyclohexaEthynylcyclohexanene

1122

Two perpendicular Two perpendicular ππ bonds; bonds; sp sp hybridshybrids R R CC CC RR

EthyneEthyne

The Triple Bond is The Triple Bond is EnergeticEnergetic

Kcal molKcal mol-1-1

Heat of hydrogenationHeat of hydrogenation: : MoreMore than Two Alkene than Two Alkene Bonds Bonds (which would be ~ -60 kcal mol(which would be ~ -60 kcal mol-1-1))

DDH H ° HC CH H° HC CH H22C CHC CH2 2 HH33C C CHCH33229229 173173 99

00

Acetylene TorchAcetylene Torch

Hydrogens get more acidic Hydrogens get more acidic (blue)(blue)

Acidity:Acidity: RC C RC C HH + + BB R C R C C C + + HHB B

-- --:: ::

ppKKaa ~ ~ 25!25!

cf. CHcf. CH22 CH CH2 2 44, CH44, CH33 CH CH3 3

5050Why?Why? 50% s-50% s-

charactercharacter

KK

++ LiLi oror

LiLi LiLi++-- ++

++ CHCH33MgBMgBrr

MgBrMgBr++CHCH44

ppKKaa 25 25 ppKKaa 50 50

H H + Na NHH H + Na NH22++::

::

HH

NaNa C CC C:::: ----

NHNH33++

NaNHNaNH22

NaNa++ ++

ppKKaa 33 33

1 1 equiv.equiv.

::--

Synthetic Use of Synthetic Use of AcidityAcidity

--

11H NMR:H NMR: RC C RC C H H δδ = 1.7- = 1.7-3.1 ! 3.1 ! Recall:Recall: RCH CHRCH CH22 δδ = 4.6-6 = 4.6-6 ppmppm

AlkAlkene ene hydrogens: hydrogens: deshieldeddeshielded

Why?Why? Cylindrical electron current Cylindrical electron current shields alkynyl hydrogenshields alkynyl hydrogen

AlkAlkyneyne hydrogens: hydrogens: shieldedshielded

Recall alkene NMRRecall alkene NMR

Long Long range:range:

RCRCHH22 C C C C HH

J J = 2-4 Hz= 2-4 Hz

RCRCHH22 C C C C C CHH22 R’R’

JJ = 2-3 Hz = 2-3 Hz

CouplingCoupling::

1313C NMR:C NMR: δδ = 65-85 ppm: Also = 65-85 ppm: Also shielded. shielded.

Compare alkenes: Compare alkenes: 120-150ppm.120-150ppm.

HC CCHHC CCH22CHCH22CHCH22CHCH33

14-3114-316969 8484

IR spectra:IR spectra: diagnostic peaks for triple diagnostic peaks for triple bond and its attached H.bond and its attached H.

υυ (R(RC CC CR’) = R’) = 2120 cm2120 cm-1-1 ; ; υυ (RC (RC C HC H) = ) = 3300 3300 cmcm-1-1

~~

strongstrong

~~

Stability of Alkynes: Stability of Alkynes: Heats of Hydrogenation Heats of Hydrogenation

RevisitedRevisited

CHCHHCHC ++ HH22Special Special

cat.cat. CHCH22 CHCH22ΔΔH H ° = -44.9 ° = -44.9 kcal molkcal mol-1-1

CHCH22 CHCH22 ++ HH22Cat.Cat.

CHCH33 CHCH33ΔΔH H ° = -32.7 ° = -32.7 kcal molkcal mol-1-1

First First ΠΠ bond has more “heat content”, is bond has more “heat content”, is also more reactive. Allows for: also more reactive. Allows for:

RR11

CC CC RR22 ++ A BA BAA

CC CCRR22

RR11 BB

AACC CC

BB

RR11 RR22

++

Stereoselective alkene Stereoselective alkene synthesissynthesis

InternalInternal alkynes are alkynes are more stablemore stable than than terminal onesterminal ones

++ HH22

cat.cat.

++ HH22cat.cat.

ΔΔH H ° = -69.9 kcal mol° = -69.9 kcal mol-1-1

ΔΔH H ° = -65.1 kcal mol° = -65.1 kcal mol-1-1

Parallels the behavior of alkenes. Parallels the behavior of alkenes. Same reason: Same reason: hyperconjugationhyperconjugation. .

PreparationPreparation

1.1.Elimination E2 of Elimination E2 of DihaloalkanesDihaloalkanes

CC CC

HH HH

XX XX

CC CC CC CC

BrBrBrBr NaNa

B:B:-- HH

XX

B:B:--

NaNHNaNH22 excessexcess

NHNH33 liq. liq.

HH++, H, H22O O work work upup

75%75%

Application in Application in synthesis:synthesis:

RCH CHR R C C RRCH CHR R C C R

BrBrBrBrBrBr

BrBrBrBr22

NaNHNaNH22

NHNH33 liq liq

1,5-Hexadiyne1,5-Hexadiyne

2. Alkylation of Alkynyl Metals2. Alkylation of Alkynyl Metals

SSNN2 rules2 rules

LiLiTHFTHF

LiLiII

∆∆90%90%

BestBest: RI, THF, ∆. RBr or RCl need : RI, THF, ∆. RBr or RCl need “coordinating”“coordinating” additives: e.g. additives: e.g. ; or HMPA. Remember: Grignards ; or HMPA. Remember: Grignards don’t don’t workwork for RX, but O.K. for or for RX, but O.K. for or carbonyls.carbonyls.

HH22NN NHNH22

OO

+ CH+ CH33MgBrMgBrMgBrMgBr

CHCH22 O OOHOH

++ LiNHLiNH2 2 (l equiv)(l equiv) LiLiOO

OHOH

+ 2 CH+ 2 CH33MgBrMgBr MgBrMgBrBrMgBrMgHOHOOHOH

LiLi++LiLi

OO

CHCH33OHOH

CHCH33CHCHOO

ReactionReactionss

1. 1. ReductionsReductions

a. Complete a. Complete hydrogenationhydrogenationHH22, Pt, Pt

100100%%b. Partial hydrogenation:b. Partial hydrogenation: “Poisoned” “Poisoned”

Lindlar’s catalyst:Lindlar’s catalyst: Cis!Cis!

Pd-CaCOPd-CaCO33, Pb(OCCH, Pb(OCCH33))22, , quinolinequinoline

OO

NNHH22, , LindlarLindlar100%100%

HHHHCisCis-3-heptene-3-heptene

c. Na reduction:c. Na reduction: Trans!Trans! Via stepwise 2e Via stepwise 2e transfertransfer

+ Na°+ Na° NNHH33 liq. liq.

86%86%

HH

HHTransTrans-3--3-hepteneheptene

Equilibrates between cis and trans (more stable) Lipshutz

Mechanism:Mechanism:

Holiday

Na dissolves in liquid ammonia, makes “solvated” electronsNa dissolves in liquid ammonia, makes “solvated” electrons

2. Electrophilic additions.2. Electrophilic additions. Like Like alkenes.alkenes.a. HX:a. HX:

RRRR + + HH++ CC CC RRRR

HH

++ XX--

Anti to Anti to HH; ; pushes R pushes R transtrans

CC CCRR

HH RR

XX HH++

MarkovnikovMarkovnikovCC

RR

XXRCRCHH

22

XX--

RCRCHH22CCXX22RRGeminal Geminal dihalidedihalide

CHCHRCRCHXHX

CC CCXX

RR HH

HH HXHXRCXRCX22CHCH33

Markovnikov twiceMarkovnikov twice

spsp

++

sp sp 22

Internal alkynesInternal alkynes

Resonance with XResonance with X

δ 13C = 202.4 ppm

++

ν = 1987 cm -1~~

1.22 Å Linear!Linear!

Angew. Chem. Angew. Chem. 20042004, 43, 43, 1543., 1543.

Examples:Examples:HBrHBr BrBr BrBr

HIHIII II

II II++

HClHClClCl ClCl

Note:Note:

BrBr22

BrBr

BrBr NaNHNaNH22

NHNH33

HBrHBrBrBr BrBr

VicinalVicinal GeminaGeminall

b. Xb. X22: : Anti additionAnti addition, as for , as for alkenesalkenes

CHCH33

BrBr22

BrBr CHCH33

BrBr

BrBr22

BrBr BrBr

BrBr BrBr

c. c. Cat.Cat. HgSO HgSO44, H, H22O hydrationO hydration, Markovnikov, Markovnikov

CRCRRCRCCat. Cat. HgSOHgSO44HH22OO CC CC

HH

RR OHOH

RR TautomerizationTautomerizationOO

HH++ or or OH OH catalyzedcatalyzed

--

UnstableUnstable

RCHRCH22CCRRNo NaBH4

needed

OO

RR

Mechanism of tautomerizationMechanism of tautomerization

HH++ :: CCOHOH

RCHRCH

22

++ RRCC

OORCHRCH

22

HH++ RCHRCH22CCRR

OO--HH++

OHOH : : RCHRCH22CCRR

OO++HH++

CC CCHH

RR OO

RRCC CC

HH

RR

RR----

--

CC CCHH

RR OOHH

RRHH++ or or OH OH catalyzed catalyzed

--

CHCHRCRCHgSOHgSO44

HH22OO

cat.cat.

CCHOHO

RRCHCH22

RCCHRCCH33

OO

Methyl ketoneMethyl ketone

d. Radical HBr:d. Radical HBr: Anti-Markovnikov Anti-Markovnikov additionaddition

HBrHBr

ROORROOR

BrBr BrBr

HBrHBr

-Br-Br

BrBr BrBr

HH

HH

MixturesMixtures++

HaloalkenesHaloalkenes

No SNo SNN2:2:

No SNo SNN1:1:

But:But:

And, metal catalysts couple alkenyl halides to alkenes in the Heck reaction:

A variant with alkynes: Sonogashira reactionA variant with alkynes: Sonogashira reaction

CC CCRR

HH RR

XXR’C CHR’C CH++ CC CC

RR

HH RR

R’R’

BBRR22

e. Hydroboration-Oxidation e. Hydroboration-Oxidation

Use RUse R22BH (R = BH (R = bulkybulky group) to group) to protect protect alkenylborane: R = alkenylborane: R = cyclohexylcyclohexyl

CHCHRCRC ++ B-HB-H22

CC CCRR

HH

HH HH22OO22,, - -OHOH

OHOHCC CC

RR

HH

HH TautomerizationTautomerizationRCHRCH22CCHH

OO

Aldehyde Aldehyde !!

Steric controlSteric control

1.1. RR22BHBH

2.2. HH22OO22,,--OHOH

1.1. HBRHBR22

2.2. HH22OO22,,--OHOH

Therefore:Therefore: HH

OO

butbut HgSOHgSO44

HH22OO

cat.cat. OO

RRRR RCCHRCCH22

RR

OO

R = R : R = R : MixturesMixtures