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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