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Stereochemistry of eliminations from 2-butyl, system in acetic acid
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HO. 12, up 1031 - 1032, 1975.
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STEREOCHEMISTRYOF ELIMINATIONSFROM 2-RDTYI SYSTRR IN ACETIC ACID Marino Cavazza Iatitutodi Chimica Generale,Univeraita'diPisa (Reo.ived in UK 23 Jumrr~1975;aoooptdforyublioetiu 12 Fe-
1975)
An El mechanismvia ien-pairshas been poetulated'in the eliminationsfrom srythm-
and
threo-3-deutero-2-butyl toeylatein acetic acid in order to account
for observedprevalenceof egn-etereochemietry:
-
-
s.j&-2-butene-d
m-2-butene
Since the product step of this mechanisminvolvesthe leaving-greupanion, the nature of the latter would be expectedto affect the stereoapecificity of the elimination. This expectationappearanot to be fullfilledby the experiments.The data in Table 1 indeed show that the proportionof eyn-eliminationfrom 2-butyl system in acetic acid is remarkablyindependentof the nature of the leavinggroup for such differentleaving-groupsa8 chloride,bromide and tosylate. Anotherwidely quoted criterionfor the El ion-pairmechanismia the change of product dietribution(eliminationversus substitution)on changingthe leaving-group? Also this criterionfails for 2-butyl system. As shown in Table 2 the eliminationfraction is essentiallyunaffectedby leaving-group. Our results appear to rule out the interventionof either free ione or ionpairs in the acetolysisof 2-butyl system in agreementwith generalbehaviour 3 An E2 involving paenecarbonium4 transitionetate of secondaryeubstrates. appears to be a more likely mechanismfor these eubstratea.
The author ie indebtedto ProfeeeorAntoninoFava for
sthulating
end helpful commentsand to C.R.R.,Rome,for financialsupport. 1031
discussions
TABLE1 Percenta olefin formed by ~-elimination frem threo-3-deutero-2-butyl-X in glacial acetic acid X Cl
tie-2-butane -k 86
trana-2-butane -k
P-butene %
52
Br
86
50
OTeb
82
65
69 68
73 a) Measure&by ma68 epectrometryat low ionizingvoltage. It mu8t be noticed that percentsof g&-2-butene and tr8n8-2-butenefrem s-eliminatien reported ( X = Cl, Br ) representz values, because no allowanceha8 been made for the stereomutationat the a-carbonof the 2-butylhalide* which occur8 during elimination.However, the data are self-consist8nt because olefin analyseswere carried out at timee eelectedin such a way aa to have the same extent of stereomutationat the a-carbon inbothcaaea. That was measured by meana of the racemieation,in the reactionconditione, of optical active 2-butylhalides. b) From the data by Skell and Ball, ref. 1 TABLE2 Fractionof eliminationon eolvoly8ispreducteaof 2-butyl-Xb in glacial acetic acid' at 141° . X elimination/8olvoly&.a Cl
0.42
Br
0.46
I
0.42
OTE
0.44
a) l- and 2-butenes,P-butyl acetate b) 0.1 M RX c) 0.1 M NaOAc REFEREWCES
1) P.s.skelland w.L.Hall,J.Am.Chem.Soc.,85, 2851 (1963) 2) M.Cociveraand S.Winetein,J.Am.Chem.Soc.,85, 1702 (1963) 3) J.L.Fry,C.J.Lancelot,L.K.M.Lem,J.M.Harria,R.C.Bingham,D.J.Raber, R.E.Ralland P.v.R.Schleyer,J.Am.Chem.Soc.,92, 2538 (1970) 4) R.A.Bartechand J.F.Bunnett,J.Am.Chem.Sec.,91, 1376 (1969)
HO. 12, up 1031 - 1032, 1975.
Pwgww
-8.8.
Pdntod In Q@Jathikill.
STEREOCHEMISTRYOF ELIMINATIONSFROM 2-RDTYI SYSTRR IN ACETIC ACID Marino Cavazza Iatitutodi Chimica Generale,Univeraita'diPisa (Reo.ived in UK 23 Jumrr~1975;aoooptdforyublioetiu 12 Fe-
1975)
An El mechanismvia ien-pairshas been poetulated'in the eliminationsfrom srythm-
and
threo-3-deutero-2-butyl toeylatein acetic acid in order to account
for observedprevalenceof egn-etereochemietry:
-
-
s.j&-2-butene-d
m-2-butene
Since the product step of this mechanisminvolvesthe leaving-greupanion, the nature of the latter would be expectedto affect the stereoapecificity of the elimination. This expectationappearanot to be fullfilledby the experiments.The data in Table 1 indeed show that the proportionof eyn-eliminationfrom 2-butyl system in acetic acid is remarkablyindependentof the nature of the leavinggroup for such differentleaving-groupsa8 chloride,bromide and tosylate. Anotherwidely quoted criterionfor the El ion-pairmechanismia the change of product dietribution(eliminationversus substitution)on changingthe leaving-group? Also this criterionfails for 2-butyl system. As shown in Table 2 the eliminationfraction is essentiallyunaffectedby leaving-group. Our results appear to rule out the interventionof either free ione or ionpairs in the acetolysisof 2-butyl system in agreementwith generalbehaviour 3 An E2 involving paenecarbonium4 transitionetate of secondaryeubstrates. appears to be a more likely mechanismfor these eubstratea.
The author ie indebtedto ProfeeeorAntoninoFava for
sthulating
end helpful commentsand to C.R.R.,Rome,for financialsupport. 1031
discussions
TABLE1 Percenta olefin formed by ~-elimination frem threo-3-deutero-2-butyl-X in glacial acetic acid X Cl
tie-2-butane -k 86
trana-2-butane -k
P-butene %
52
Br
86
50
OTeb
82
65
69 68
73 a) Measure&by ma68 epectrometryat low ionizingvoltage. It mu8t be noticed that percentsof g&-2-butene and tr8n8-2-butenefrem s-eliminatien reported ( X = Cl, Br ) representz values, because no allowanceha8 been made for the stereomutationat the a-carbonof the 2-butylhalide* which occur8 during elimination.However, the data are self-consist8nt because olefin analyseswere carried out at timee eelectedin such a way aa to have the same extent of stereomutationat the a-carbon inbothcaaea. That was measured by meana of the racemieation,in the reactionconditione, of optical active 2-butylhalides. b) From the data by Skell and Ball, ref. 1 TABLE2 Fractionof eliminationon eolvoly8ispreducteaof 2-butyl-Xb in glacial acetic acid' at 141° . X elimination/8olvoly&.a Cl
0.42
Br
0.46
I
0.42
OTE
0.44
a) l- and 2-butenes,P-butyl acetate b) 0.1 M RX c) 0.1 M NaOAc REFEREWCES
1) P.s.skelland w.L.Hall,J.Am.Chem.Soc.,85, 2851 (1963) 2) M.Cociveraand S.Winetein,J.Am.Chem.Soc.,85, 1702 (1963) 3) J.L.Fry,C.J.Lancelot,L.K.M.Lem,J.M.Harria,R.C.Bingham,D.J.Raber, R.E.Ralland P.v.R.Schleyer,J.Am.Chem.Soc.,92, 2538 (1970) 4) R.A.Bartechand J.F.Bunnett,J.Am.Chem.Sec.,91, 1376 (1969)