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Trifluoroacetolysis of 2-phenylethyl p -toluenesulfonate. Evidence for phenonium ion
Tetrahedron
Letter8
No.45,
pp.
4409-4414,
1967.
Perp-m
Rena Ltd. Printed
in Gmat
Britfh.
TRIFLUOROACETOLYSIS OF 2-PRE3YLRTHYL @OLUENESULFONATR. EVIDENCE FOR PHENONIUM ION
J. Eric Nordlander and William C. Deadman (1) Department of Chemistry, Case-Western Reserve University Cleveland, Ohio 44106 (Beceived
in U.S.A.
19 June
1967)
The postulation of ethylenephenonlwn ion intermediates 2, first made by Cram (2) on stereochemical grounds, has proved to be a highly useful concept in correlating a wide variety of product, stereochemical, and kinetic data for reactions of&arylalkyl
theless, Drown and coworkers (5) of structures of type 1.
have
systems (3,4).
Never-
recently challenged the validity
In the case of solvolysis of symmetrical sys-
tems (with reepect to the possible hrldged Ions) unsubstituted in the phenyl ring, they have argued that persuasive kinetic evidence for phenyl participation is not found and without It phenonlum ions cannot be preferred over the corresponding simple open carbonlvm ions (5~).
Rapid
Wagner-Meerwein Interconversion of the latter species 2 has instead been
4409
No.43
4410 proposedto explain the atereochemlcalresults (5~). In Table I are listed eolvolyaiarates from the literaturefor ethyl toeylateand 2-phenylethyltosylate (6). From these data the TABLE1 PubXlahedTltrlmetrlcRate Data for Solvolysesof Ethyl Tosylate and 2-PhenylethylToaylateat 75'.'
106k1, sec.-1 ( 1 Ethyl 2-Phenylethyl k phenylethyl Solvent
tosylate
%R5OR CH3C02R
29.8
7.08
0.24
0.772
0.288
0.37
HCO$i
18.9
toaylate
39.4
k ethyl
2.1
aRef. 6.
phenylethyl:ethyl rate ratio 16 at most only 2.1 (uncorrectedfor phenyl Inductiveeffect (3b)),but lbs IncreaseIn proceedingfrom solvent ethanol (highlynucleophlllc(7),
poorly Ionizing (8)) to formic acid
(vice versa) suggestsfor the phenylethylsubstratea correapondlng -increase In phenyl aaalatancerelativeto solventaeslstanceIn ionlzation (6). It would be logicalto anticipatethat eubatantlalrate enhancements by the,&phenyl group might be found with solventaeven better Ionizing and/or more poorly nucleophlllcthan formic acid. Recent work, particularly that of Petersonand coworkers(9),
has made it apparentthet trl-
fluoroaceticacid aurpaaseaformic acid in these respects,and consequently
lo.45
4411
we have investigatedthe solvolyiesof ethyl and 2-phewlethyl toay'late In this
medium.
2-Phenylethyltoaylate2
(O.l&) reacted cleanly In boiling trlflu-
oroacetlcacid (72', 4 hr.) containingsodium trlfluoroacetate(Ql25M)and trlfluoroacetlc anhydride (1 wt.-$) to give exclusively2-phenylethyl trlfluoroacetate k
In 96s Isolatedyield (0.005-molescale). Rates were CF3C02H CF3C02Na
CH2CH20Ts #
/
cF3co,20+
0
/
CH,CH,O,CCF3
72'
zi%
&i
measured In unbufferedtrlfluoroacetlcacid (1s trlSluoroacetlcanhydrlde) (lo), using Peterson'sspectrophotometrlc procedure (gc);good first-order rate constantswere obtainedthroughout. The results are summarleedIn Table II. TABLE II
Spectrophotometrlc Rate Data for TrlSluoroacetolysls" of Ethyl Tosylateand 2-PhenylethylTosylate
Reactant (o.o5ora CH3CH20Ts
Temp.
125.0° 135.0° 145.00 75.0° b loo.o" b C6H5CH2CH20Ts 60.0" 70.0° 80.0' 75.0° b 1oo.oo b
106k,, -1 sec. 11.1 25.1 52.4 0.106 1.26 83.0 205 498 322 2434
LiH', kcal./mole 24.9
AS+, e.u.
relativekl 75'
100"
-19.4
1 1 20.2
-16.8
3040
1938
aTrlSluoroacetlcanhydrlde (1 wt.-$) added. %xtrapolated or Interpolated.
IO.45 At 75’ the rate of 2-phenylethyltosylateexceed8 that of ethyl tosylateby a factor of 3040, a strikingdeparturefrom the results In other solvents,Table I. This unprecedentedrate enhancementby the phenyl group where there is no net driving force for rearrangement can reasonably be attributed only to direct formation of the ethylenephonlum ion 2, as shonn in eq. 1.
.
31 CF3C0.9 H2CH20Ts
--*
CH2CH202CCF3
This simple mechanismrequiresthat the two methylenegroups become completely equivalent in the product, which was found to be the case.
Trifluoroacetolysls, In bufferedmedium, of 2-phenylethyl-l,l-d2 tosylate JJ was carried out for approximately one half-life at 72", and the trlfluoroacetate product and unreacted starting material were separated and purified. N.m.r. analysis showed, as expressed In eq. 2, that Isotopeposition equilibration was complete in the former, -i.e. 4b and -4c were present in equal amounts within experimental uncertainty (estimated +2$). Recovered reactant was only slightly rearranged, consisting of 955 s
and _& -3c (22%). A control experiment established that $J possessed complete isotope-position stability under the reaction conditions. The present results may be understood qualitatively in terms of the very low nucleophiliclty and high ionizing power associated with trifluoroacetic acid (9). In eolvolysls reactions lta poor nucleophilicity shocld depreciate not only the direct displacement mechanism but also specific solvation of electron-deficient centers, thereby accentcatlng intramolecL?larprocesses for carbonium-ion stabilization. As Its dielectric
4413
No.45
CH2CD2OTs
CD2CH202CCF3
CH2CD202CCP3+
CH2CD20Ts +
c
95%
constant is unexceptional(C= 8.32 at 25o) (11) its superiorionizing ability (gc) must derive largely from particularlyeffectiveanion Solvetion through hydrogenbonding. Thus, phenyl participationin ionizationof 2-phenylethyltosylate asserts itrrelfforcefullyIn trlfluoroacetlcacid. There is every reason to expect that additionalexpressionsof anchlmericassistancewhldh are Inconclusivein more nucleophilicmedia may bewme
decisive In trifluoro-
acetic acid. Such research la in progress. A detailedaccount of this and related work will be published shortly. REFERENCES 1.
Realpsentof NationalAeronauticsand Space AdministrationPredoctoral Traineeship,1964-67.
2.
(a) D.J. Cram, 2. &. m. u.,
3.
2,
-
sc)c.,71, -
3863 (19491, (b) D.J. Cram,
2129 (1952).
(a) The earlier literaturehas been criticallyrevlewedby A. Streitwleser, Jr., "SolvolyticDisplacementReactions,'McGraw-HillBook Co., Inc., New York, N.Y., 1962, pp. 144-152,159, 167-170, 181-182. (b) More recently,D.J. Cram, 2. 2. e.
&.,
86, 3767 (1964) has
provideda detaileddefense of the phenonium-ionconcept,in rebuttal to the challengeof Brown, ref. 3. As a leadingreferenceto very
No.45
recent work see (c) C.C. Lee and L. Noszko,
. 2. m.,
44,
-
2481 (1966). 4. Direct n.m.r. observationof several stable ethylenephenonium Ions from 2-arylethylprecursorsin very strong acid medium has now been report;'. a. A. Olah, M. B. Comlsarow,E. Namanworth,and B. Ramsey, . e.,
5.
5,
711 (1967).
(a) H.C.BrownIn IIT= TransitionState,w Special PublicationNo. 16, The ChemicalSociety,London, 1962, pp. 140 ff., (b) H.C. Brown, R. Bernhelmer,and K.J. Morgan,2. &. m. (c)
QOC., 8&
1280 (1965),
H.C. Brown, K.J. Morgan, and F.J. Chloupek,ibT;;.,a,
2137 (1965),
(d) H.C. Brown, R. Bsrnhelmer,C.J. Kim, and S.E. Scheppze, E., a, 6. a)
370 (1967). S. Winsteinand H. Marshall,2. &. m.
&.,
2, 1120 (1952), (b) S. Winstein,C.R. Lindegren,H. Marshall,and L.L. Ingraham,
*.,
a, 147 (1953). 7. Ref. 3a, pp. 63-69. 8. S.G. Smith, A.H. Falnberg,and S. Wlnsteln,2. &.
-
618 (1961). 9.
(a) P.E. Petersonand R.J. Bopp, w.,
&, 1283 (1967), (b) P.E. Petersonand J.E. Duddey, ., 88, 4990 (1966), (c) P.E. Peterson, R.E. Kelley, Jr., R. Belloli,and K.A. Sipp, ibid.,3, 5169 (1965), and
other papers in this series and referencesthereln.
10. Because of the Ngh temperaturesrequiredfor ethyl tosylate,kinetic complicationswere
encounteredwhen sodium trifluoroacetate was present.
11. (a) E.L. Mackor, P.J. Smit, and J.H.M~ der Waals, Trans. Faraday Sot., 3, 1309 (1957). (b) Comparewith those for acetic acld,g= 6.15 at -3 20 f and formic acid, c= 58.5 at 16', J.A. Rlddlck and E.E. Toops, Jr.,. in "Techniqueof Organic Chemistry," A. Weissberger,Ed., Vol. VII, 2n: Ed., IntersciencePublishers,Inc., New York, N.Y., p. 144,1k5.
Letter8
No.45,
pp.
4409-4414,
1967.
Perp-m
Rena Ltd. Printed
in Gmat
Britfh.
TRIFLUOROACETOLYSIS OF 2-PRE3YLRTHYL @OLUENESULFONATR. EVIDENCE FOR PHENONIUM ION
J. Eric Nordlander and William C. Deadman (1) Department of Chemistry, Case-Western Reserve University Cleveland, Ohio 44106 (Beceived
in U.S.A.
19 June
1967)
The postulation of ethylenephenonlwn ion intermediates 2, first made by Cram (2) on stereochemical grounds, has proved to be a highly useful concept in correlating a wide variety of product, stereochemical, and kinetic data for reactions of&arylalkyl
theless, Drown and coworkers (5) of structures of type 1.
have
systems (3,4).
Never-
recently challenged the validity
In the case of solvolysis of symmetrical sys-
tems (with reepect to the possible hrldged Ions) unsubstituted in the phenyl ring, they have argued that persuasive kinetic evidence for phenyl participation is not found and without It phenonlum ions cannot be preferred over the corresponding simple open carbonlvm ions (5~).
Rapid
Wagner-Meerwein Interconversion of the latter species 2 has instead been
4409
No.43
4410 proposedto explain the atereochemlcalresults (5~). In Table I are listed eolvolyaiarates from the literaturefor ethyl toeylateand 2-phenylethyltosylate (6). From these data the TABLE1 PubXlahedTltrlmetrlcRate Data for Solvolysesof Ethyl Tosylate and 2-PhenylethylToaylateat 75'.'
106k1, sec.-1 ( 1 Ethyl 2-Phenylethyl k phenylethyl Solvent
tosylate
%R5OR CH3C02R
29.8
7.08
0.24
0.772
0.288
0.37
HCO$i
18.9
toaylate
39.4
k ethyl
2.1
aRef. 6.
phenylethyl:ethyl rate ratio 16 at most only 2.1 (uncorrectedfor phenyl Inductiveeffect (3b)),but lbs IncreaseIn proceedingfrom solvent ethanol (highlynucleophlllc(7),
poorly Ionizing (8)) to formic acid
(vice versa) suggestsfor the phenylethylsubstratea correapondlng -increase In phenyl aaalatancerelativeto solventaeslstanceIn ionlzation (6). It would be logicalto anticipatethat eubatantlalrate enhancements by the,&phenyl group might be found with solventaeven better Ionizing and/or more poorly nucleophlllcthan formic acid. Recent work, particularly that of Petersonand coworkers(9),
has made it apparentthet trl-
fluoroaceticacid aurpaaseaformic acid in these respects,and consequently
lo.45
4411
we have investigatedthe solvolyiesof ethyl and 2-phewlethyl toay'late In this
medium.
2-Phenylethyltoaylate2
(O.l&) reacted cleanly In boiling trlflu-
oroacetlcacid (72', 4 hr.) containingsodium trlfluoroacetate(Ql25M)and trlfluoroacetlc anhydride (1 wt.-$) to give exclusively2-phenylethyl trlfluoroacetate k
In 96s Isolatedyield (0.005-molescale). Rates were CF3C02H CF3C02Na
CH2CH20Ts #
/
cF3co,20+
0
/
CH,CH,O,CCF3
72'
zi%
&i
measured In unbufferedtrlfluoroacetlcacid (1s trlSluoroacetlcanhydrlde) (lo), using Peterson'sspectrophotometrlc procedure (gc);good first-order rate constantswere obtainedthroughout. The results are summarleedIn Table II. TABLE II
Spectrophotometrlc Rate Data for TrlSluoroacetolysls" of Ethyl Tosylateand 2-PhenylethylTosylate
Reactant (o.o5ora CH3CH20Ts
Temp.
125.0° 135.0° 145.00 75.0° b loo.o" b C6H5CH2CH20Ts 60.0" 70.0° 80.0' 75.0° b 1oo.oo b
106k,, -1 sec. 11.1 25.1 52.4 0.106 1.26 83.0 205 498 322 2434
LiH', kcal./mole 24.9
AS+, e.u.
relativekl 75'
100"
-19.4
1 1 20.2
-16.8
3040
1938
aTrlSluoroacetlcanhydrlde (1 wt.-$) added. %xtrapolated or Interpolated.
IO.45 At 75’ the rate of 2-phenylethyltosylateexceed8 that of ethyl tosylateby a factor of 3040, a strikingdeparturefrom the results In other solvents,Table I. This unprecedentedrate enhancementby the phenyl group where there is no net driving force for rearrangement can reasonably be attributed only to direct formation of the ethylenephonlum ion 2, as shonn in eq. 1.
.
31 CF3C0.9 H2CH20Ts
--*
CH2CH202CCF3
This simple mechanismrequiresthat the two methylenegroups become completely equivalent in the product, which was found to be the case.
Trifluoroacetolysls, In bufferedmedium, of 2-phenylethyl-l,l-d2 tosylate JJ was carried out for approximately one half-life at 72", and the trlfluoroacetate product and unreacted starting material were separated and purified. N.m.r. analysis showed, as expressed In eq. 2, that Isotopeposition equilibration was complete in the former, -i.e. 4b and -4c were present in equal amounts within experimental uncertainty (estimated +2$). Recovered reactant was only slightly rearranged, consisting of 955 s
and _& -3c (22%). A control experiment established that $J possessed complete isotope-position stability under the reaction conditions. The present results may be understood qualitatively in terms of the very low nucleophiliclty and high ionizing power associated with trifluoroacetic acid (9). In eolvolysls reactions lta poor nucleophilicity shocld depreciate not only the direct displacement mechanism but also specific solvation of electron-deficient centers, thereby accentcatlng intramolecL?larprocesses for carbonium-ion stabilization. As Its dielectric
4413
No.45
CH2CD2OTs
CD2CH202CCF3
CH2CD202CCP3+
CH2CD20Ts +
c
95%
constant is unexceptional(C= 8.32 at 25o) (11) its superiorionizing ability (gc) must derive largely from particularlyeffectiveanion Solvetion through hydrogenbonding. Thus, phenyl participationin ionizationof 2-phenylethyltosylate asserts itrrelfforcefullyIn trlfluoroacetlcacid. There is every reason to expect that additionalexpressionsof anchlmericassistancewhldh are Inconclusivein more nucleophilicmedia may bewme
decisive In trifluoro-
acetic acid. Such research la in progress. A detailedaccount of this and related work will be published shortly. REFERENCES 1.
Realpsentof NationalAeronauticsand Space AdministrationPredoctoral Traineeship,1964-67.
2.
(a) D.J. Cram, 2. &. m. u.,
3.
2,
-
sc)c.,71, -
3863 (19491, (b) D.J. Cram,
2129 (1952).
(a) The earlier literaturehas been criticallyrevlewedby A. Streitwleser, Jr., "SolvolyticDisplacementReactions,'McGraw-HillBook Co., Inc., New York, N.Y., 1962, pp. 144-152,159, 167-170, 181-182. (b) More recently,D.J. Cram, 2. 2. e.
&.,
86, 3767 (1964) has
provideda detaileddefense of the phenonium-ionconcept,in rebuttal to the challengeof Brown, ref. 3. As a leadingreferenceto very
No.45
recent work see (c) C.C. Lee and L. Noszko,
. 2. m.,
44,
-
2481 (1966). 4. Direct n.m.r. observationof several stable ethylenephenonium Ions from 2-arylethylprecursorsin very strong acid medium has now been report;'. a. A. Olah, M. B. Comlsarow,E. Namanworth,and B. Ramsey, . e.,
5.
5,
711 (1967).
(a) H.C.BrownIn IIT= TransitionState,w Special PublicationNo. 16, The ChemicalSociety,London, 1962, pp. 140 ff., (b) H.C. Brown, R. Bernhelmer,and K.J. Morgan,2. &. m. (c)
QOC., 8&
1280 (1965),
H.C. Brown, K.J. Morgan, and F.J. Chloupek,ibT;;.,a,
2137 (1965),
(d) H.C. Brown, R. Bsrnhelmer,C.J. Kim, and S.E. Scheppze, E., a, 6. a)
370 (1967). S. Winsteinand H. Marshall,2. &. m.
&.,
2, 1120 (1952), (b) S. Winstein,C.R. Lindegren,H. Marshall,and L.L. Ingraham,
*.,
a, 147 (1953). 7. Ref. 3a, pp. 63-69. 8. S.G. Smith, A.H. Falnberg,and S. Wlnsteln,2. &.
-
618 (1961). 9.
(a) P.E. Petersonand R.J. Bopp, w.,
&, 1283 (1967), (b) P.E. Petersonand J.E. Duddey, ., 88, 4990 (1966), (c) P.E. Peterson, R.E. Kelley, Jr., R. Belloli,and K.A. Sipp, ibid.,3, 5169 (1965), and
other papers in this series and referencesthereln.
10. Because of the Ngh temperaturesrequiredfor ethyl tosylate,kinetic complicationswere
encounteredwhen sodium trifluoroacetate was present.
11. (a) E.L. Mackor, P.J. Smit, and J.H.M~ der Waals, Trans. Faraday Sot., 3, 1309 (1957). (b) Comparewith those for acetic acld,g= 6.15 at -3 20 f and formic acid, c= 58.5 at 16', J.A. Rlddlck and E.E. Toops, Jr.,. in "Techniqueof Organic Chemistry," A. Weissberger,Ed., Vol. VII, 2n: Ed., IntersciencePublishers,Inc., New York, N.Y., p. 144,1k5.