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Detection of the trans-Enol in Methyl Acetoacetate by 1H- and 13C-NMR Spectroscopy.

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CAS Registry numbers:
CH,=CH,, 74-85-1; Li, 7439-93-2; ( 2 ) , 13067-82-8; ( 3 ) , 917-57-7;
IS), 2123-72-0; (61, 2223-57-6; (?), 14660-39-0; (81, 109-72-8
Authentic ( 4 ) , which is certainly less Eiiiely divided and covered by
a hydroxide layer, does not react with ( 9 ) under these conditions.
121 Biphenyl ( 1 1 ) appears t o be the active “carrier”. The reactions described
proceed approximately as fast in the presence of ( 1 1 ) alone under
otherwise identical conditions: howevg, reaction of lithium with ( / I )
alone does not start easily in DMM. They also seem to occnr in the
presence of naphthalene alone, but are then much slower (small amounts
of stable (3) can be detected). In the absence of (//) and of naphthalene
no detectable reaction takes place. Action of Li on (11) in aprotic donor
solvents such as D M M atTords solutions containing equilibrium mixtures
of biphenyllithium f 12). and possibly also biphenyldilithium ( 1 3 ) with
( 1 1 ) and lithium. [Reviews: L! K a l w n a r a m a n and M . L! George. J.
Organometal. Chem. 4 7 , 225 (1973): N . L. Hal!., Chem. Rev. 74, 243
(1974): see also: D . F . Lindow. C. N . Corrrz, and R . G. Hury.!.. J. Amer.
Chem. SOC.94, 5406 (1972)]. At present we consider two mechanisms
(formulated for (11) as thecarrier): 1)(12) and/or (13) transfer electrons
and lithium cations to ethylene [see M. S x o r c : Carbanions, Living
Polymers. and Electron Transfer Processes. Wiley-Interscience. New York
19681. 2) Lithium which reacts directly with ethylene is formed (surface activation, transport into solution) in the reaction system
2 Li+ ( I l ) e L i + ( 1 2 ) $ ( / 3 ) . Transfer activity of the carriers [ ( / / ) is
active, naphthalene weakly active, anthracene inactive] and the energy
of their lowest antibonding molecular orbital [see A . Sfreitwieser, Jr.:
Molecular Orbital Theory for Organic Chemists. Wiley-lnterscience,
New York 1968, p. 4251 would seem t o be related.
[3] As usual in this field we use names and formulas which symbolize the true
structures, all of which are unknown. In the case of ( I ) the choice is particularly arbitrary. Possible structures are: (/) : ion pair, 2-lithioethyl radical and aggregated structures derived from both; (2) : monomeric covalent (21 and a cluster structure; (5) : cluster structure.
[4] K. Ziegler and H . 4 . Gellerr, Liebigs Ann. Chem. 567. 179 (19501: R .
A . Finnegan and H . W Kutta, J. Org. Chem. 30. 4138 (1965).
[ S ] P. D . Bartlerf, C . !I Goebel, and W P . Wrber, J. Amer. Chem. SOC.
91, 7425 (1969): A . Maercker and W Theysofin, Liebigs Ann. Chem.
747, 70 (1971).
[6] We have indications that higher homologs are also formed in decreasing
amounts, i. e. we observe the initial steps o f a “polymerization” of ethylene
[see K . Clusius and H . Moller, Helv. Chim. Acta 39, 363 (1956)l. The
first insertion ( 2 ) + ( 5 ) is faster than subsequent ones: the homologs
undergo partial decomposition by loss of lithium hydride and by reaction
with the solvent (scheme).
[7] H . Kuus, Uch. Zap. Tartu Gos. Univ. 1966, 133 [Chem. Abstr. 69,
67443 (196811: 1968, 245 [Chem. Abstr. 7 1 , 49 155 (I969)l.
[S] In the absence of solvent certain I-alkenes react at 20-120°C with
lithium suspension to form ( 4 ) and the corresponding I-lithio-I-alkynes
and I-lithio-1-alkenes can be detected as unstable intermediates: D.
L. Skinner, D. J . Peterson, and 7: J. Logan, J. Org. Chem. 32. 105
( I 967).
[I]
Detection of the rrans-Enol in Methyl Acetoacetate
by ‘H- and 13C-NMR Spectroscopy
Nevertheless, (3) can be enriched in yields of up to 15%
very simply by distillation of acetoacetic ester at normal pressure; the rest of the distillate consists of ( I ) and (2). If
distillation is performed at reduced pressure no trans-enol
(3) can be detected. Attempted photochemical cis-trans isomerization of ( 2 ) to (3) was unsuccessful.
Form (3) can be detected by neither UV nor IR spectroscopy[4! Even with the aid of ‘H-NMR spectroscopy (3) can
only be recognized on use of anisotropic solvents (Table 1).
Table I . ‘H-NMR data (G(TMS)=O ppm, coupling constants in Hz), [S]
and “C-NMR data (TMS=O ppm) 171 of acetoacetic ester ( I ) , the rwenol
form ( 2 ) . and the trans-enol form (3).
f1)
(2)
(3)
(1)
(2)
(31
CH,
CH2
1.93t
J=O.50
1.69 d
J=O.74
2.28 d
J=0.60
3.16m
c-I
c-2
167.89
173.19
168.11
49.72
89.73
91.03
’H-NMR
OCH3
3.46t
J=O.lR
3.49 s
3.26 d
J=O.46
”C-NMR
C-3
200.25
176.33
173.46
Vinyl-H
OH
4.91 q
J =0.74
4.99 qq
J=O.60.
J = 0.46
12.60 s
c-4
c-I
29.70
20.89
18.78
51.91
55.29
50.52
Support for the conformation proposed for (3) comes from
a comparison of the coupling constants in (2) and ( 3 ) . The
decrease in the ally1 coupling constant can be regarded as
indicative of isomerization about the double bond ;the increase
in the coupling constant between the ester methyl group and
the vinylic proton from an unmeasurable value to 0.46 Hz
can be explained by the W arrangement of the bonds involved
in the coupling in form (3).
Unequivocal confirmation of the existence of forms ( I ),
(2),and (3)16]isfurther provided by the I3C-NMR spectrum[’]
(Table 1).
Since changes in the charge density at the carbon atom
are known from experience to cause marked shifts of the
3C resonance frequencies, we have here a particularly impressive demonstration of the difference in the electron-withdrawing effect of the hydroxy group with [(Z)] and.without [(3)]
hydrogen bonding.
By Rudolf MatuschC*l
The keto-enol tautomerism of methyl acetoacetate ( I ) (2)
has been thoroughly studied and the position of the equilibrium determined in numerous ways“]. However, the transenol form (3), which should exist alongside the keto form
( 1 ) and the hydrogen-bonded cis-enol form (2), has not
hitherto been detected. Since energy values of up to 10
kcal/mol are reported‘’] for the hydrogen bond in (2), the
form (3) would appear to be at a considerable disadvantage.
Received: January 9, 1975 [Z 167 IE]
German version: Angew. Chem. 87, 283 f 1975)
CAS Registry numbers :
( I / , 105-45-3; ( 2 ) . 4525-25-1 ; (31, 4525-24-0
[I]
[2]
[3]
[4]
[S]
[6]
[*I Dr. R. Matusch
Fachbereich Pharmazie und Lebensmittelchemie der Universitit
355 Marburg, Marbacher Weg 6 (Germany)
260
171
a ) H . Grossmann, Z Phys. Chem. 109, 305 (1924): b) C . Giesstter-Prettr~,
C. R. Acad. Sci. 250, 2547 (1960): c) J . L. Burdett and M. 7: Rogers,
J. Amer. Chem. SOC.86, 2105 (1964).
M . A . Doflicier, 7: L. Gwshani, G. B. Kisriukowsk!., E. A. Smith, and
W E . Voughrrn, J. Amer. Chem. SOC.6 0 , 440 (1938).
“Spaltrohr” column H M S 500, Fischer Bonn-Bad Godesberg.
This applies lo the IR spectrum of a capillary film. A series of dilutions
has not yet been measured
60 MHz spectrum, Varian A-60A. 3O’Yi, in D d o l u e n e : G(TMS
internal)=0.00 ppm. measuring temp. 40°C.
The expression “tautomeric forms” has been purposely avoided here
since only ( 1 ) and ( 2 ) are tautomers in the usual sense: R . Marusch,
t o be published.
20 MHz spectrum, noisedecoupled; Varian CFT-20.30c!<,in D,-benzene.
Aiigew. Cham. inremar. Etiir.
I Vol. 14 f 1975) / N o .
4
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methyl, spectroscopy, acetoacetate, nmr, 13c, detection, enol, transp
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