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syn-Planar Conformation of Oxalic Acid Crystal Structure of the Oxalic Acid-Acetamide Complex.

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occur
but only above 300°C. We have now found an
ester migration that proceeds even at 100°C.
Pentamethyl 1,3-cyclopentadiene-1,2,3,4,5-pentacarboxylate
on alkylation with silver oxide and methyl or
ethyl iodoacetate in boiling benzene, gave 71% of (2a)
[m. p. 87-88 "C ; NMR (CDCI,): singlets at T = 6.15, 6.20,
6.28, 6.38, and 6.64ppm as 6:6:3:3:2; M + at m/e=428,
M+-OCH, at m/e 3971 and 90% of (2b)[41(m.p. 83°C).
A further ester migration, giving (4), occurred above
150°C; this process could be followed only in the case of
(Zc), it having the simpler NMR spectrum, where rearrangement to ( 4 c ) in o-dichlorobenzene resulted in two
further signals T = 6.03 and 6.45 ppm as 2 :3. The position
of the equilibria between the isomers ( 2 c ) : (3c) : (4c) in
o-dichlorobenzene at 160°C was estimated as 1: 1 :1.5.That
the second ester migration ( 3 ) + ( 4 ) i s slower than the
first migration ( 2 ) + (3) corresponds to analogous hydrogen rnigratiod6] on the cyclopentadiene skeleton.
Received: February 8,1972 [Z 611 b IE]
German version : Angew. Chem. 84,534 (1972)
syn-Planar Conformation of Oxalic Acid:
Crystal Structure of the Oxalic Acid-Acetamide
Complex
By Leslie Leiserowitz and Franz Nuderp1
Heating in an inert solvent such as toluene at 100-130°C
sets up an equilibrium between ( 2 ) and (3) from which
the vinylogous malonic ester (3) can be trapped in the
form of the anion ( 5 ) by addition of sodium hydride. The
esters ( 3 a ) [20%; m.p. 99-100°C; NMR (CDCI,): singlets at r=6.05, 6.07, 6.17, 6.24, 6.27ppm as 3:2:6:6:3]
and (3b) (m.p. 103°C) thus obtained show in their mass
spectra the (M' -CH,OH) signal characteristic of this
type of structure.
Equilibration of (2) and ( 3 ) is more than 99% intramolecular; when a 0.23 M solution of ( 2 a ) and (2c) is
heated for three half-lives of the rearrangement, no notable
amount of cross-product could be detected by mass
spectrometry. Thus the observed rearrangement probably
involves a sigmatropic [I,5]-ester migration"! It was
established by means of NMR spectroscopy that the reactions cleanly followed first order kinetics for more than
three half-lives, setting up a 1 :I equilibrium between ( 2 )
and (3) in all cases (Table 1).
Table 1. The reaction ( 2 ) + ( 3 )
Reaction
Temp.
("C)
Solvent
AH'
(kcal/mol)
A S
(cal mol
deg - ' I
(2a)
113-150
97-120
113-134
97-120
Chlorobenzene
Toluene
Chlorobenzene
Toluene
25.352.5
25.3 +2.3
25.1 k 3 . 7
26.952.5
- 9.7k4.4
- 10.9 53.5
+ (3a)
(2b) + (36)
13a)
+ (2a)
13b) + (26)
-10.4i4.2
- 6.6k3.5
[I]R. M . Acheson, Accounts Chem. Res. 4, 177 (1971).
[2] J . A. Berson and R . G. Salomon, J. Amer. Chem. SOC.93, 4620
(1971); R. A. Baylouny, ibid. 93, 4621 (1971).
[3] E. LeGoffand R. B. LaCount, J. Org. Chem. 29, 423 (1964); R. C.
Cookson, J . B. Henstock, J . Hudec, and B. R. D. Whitear, J. Chem. SOC.
C 1967,1986.
[4] ( 2 b ) is accessible also from ( I ) and ethyl diazoacetate, albeit
in poorer yield.
[5] A conceivable migration of the acetate side chain is degenerate in
this system and is thus not directly recognizable.
[6] W R. Rorh, Chimia 20,229 (1966).
514
We have studied, inter alia, the 1 :1 complex of acetamide
with oxalic acid as part of our X-ray investigation of molecular packing and its dependence on functional groups.
Although the molecular dimensions of the components in
carboxylic acid-amide complexes". compare well with
those of the individual species, as far as the latter are known,
we find striking deviations in this case. Here, oxalic acid
does not exist, as is usually found, in the anti-planar conformation ( I ) , but rather in the energetically unfavorable
syn-planar arrangement ( 2 ) .
Colorless crystals of the (hygroscopic) complex[31are obtained by slowly cooling a solution of equimolar parts of
acetamide and anhydrous oxalic acid in ethyl acetate or
nitromethane.
Crystal1og:aphic data : monoclinic, a =20.542, b = 9.709,
c=13.931A; ~=112"5';spacegroupA2/a;Z=8 (the unit
cell contains two molecules each of oxalic acid and acetamide); dcalc=1.537 g/cm3.
The structure was solved by the direct methodL4].Refinement with 2813 independent reflections gave an R factor
of 7.6%. The positions of all the hydrogen atoms could be
determined from the difference map (after anisotropic
refinement of the heavy atoms). This is shown in Fig. 1 for
the plane of one of the two independent oxalic acid molecules. Together with the experimental C-0 bond lengths
this proves the syn-planar conformation of the oxalic acid.
Table 1 permits a comparison of the geometrical parameters of (2) with those of the numerous published structural studies of ( I ) in the solid['*51and gaseous[61state.
No significant difference in the bond lengths between the
two conformers can be found, but there are such differences
[*I
Dr. L. Leiserowitz and Dr. F Nader
Department of Chemistry
[**I
Weizmann Institute of Science
Rehovot (Israel)
[**I Stipendiary of the Stiftung Volkswagenwerk
Angew. Chem. internat. Edit. Vol. 11 (1972) 1 N o . 6
This finding throws some doubt on the validity of the arguments used to explain the planarity of ( I ) . On the basis
of the C-C bond length of 1.54 A, which corresponds to
a normal sp3-sp3single bond, conjugation between the two
carbonyl groups was ruled out16771.
The coplanarity was
therefore explained as due to attractive forces between the
various hybridized 0 atoms[71.This situation does not
exist in (2); but nevertheless here too a strong tendency
to coplanarity is evident.
Received: November 19, 1971 [Z 587 IE]
German version: Angew. Chem. 84,536 (1972)
Fig. 1. Difference map for one of the two independent oxalic acid-molecules in the 1 :1 complex with acetamide; contour intervals 0.1t’A3.
Preparation of Non-noble Metals (Li, Ca, Sr, Ba,
Am, Cf) by Reduction of Their Oxides and
Fluorides with Hydrogen[**I
By Uwe Berndt, Bernkard Erdmann, and Cornelius Keller[*]
in bond angles. In (2) the angle C(l)-C(2)-0(4)
is widened by about 2 ” in comparison with ( I ) , and the angle
C( I)-C(2)=0(3)
is decreased by a corresponding amount.
In addition, (2) departs minimally from coplanarity, the
torsion angle between the two C O O H fragments being 2.1 ’.
Table 1. Structural parameters of oxalic acid (1) and (2).
-~
Bond lengths [A]
in the complex [a] Ref.[b] [l, 5, 61
12)
ill
C(I)-C(2)
C(2) = O(3)
c(2)-0(4)
0 ( 4 ) - . .O(5)
0 ( 3 ) - -O
- (6)
1S29 2 0.007
1.212 2 0.006
1.2982 0.006
2.590
2.728
1.53620.003
1.20220.006
1.291+0.010
119.6 i 0 . 5
114.1 i 0 . 4
126.220.5
2.1
121.7 0.4
112.0+0.3
126 3 k 0 . 4
180
[‘I
Bond angles
C(I)-C(2) = O(3)
C(l)-C(2)-0(4)
0(3)=C(l)-0(4)
0(3)=C(2)-C(1)=0(6)
+
[a] Mean values from two independent oxalic acld molecules.
[b] Mean values from six structure analyses.
Intramolecular interactions are clearly the cause of these
deviations. If the intramolecular 0 . ’ .0 distances are
calculated for the syn-planar conformation (2) on the
basis of the geometrical parameters of the anti-planar conformation ( I ) , the separation of the twoocarbonyl-oxygen
atoms [0(3) ... 0(6)] comes out at 2.80 A but that for the
two hydroxyl-oxygen atoms [0(4) ... 0 ( 5 0 at 2.48 A [compare the 0 ... 0 separations of 2.70-2.74 A in (I)]. At least
partial balance can be achieyed by altering the bond angles,
so that a separation of 2.59 A is found for 0(4)...0(5). The
deviation from coplanarity (2.1”) and its influence on the
O - . . O separation in question are so small that (2) can
well be designated as the syn-planar conformation.
[I] Chi-Min Huang, L. Leiserowitz,and G . M . J . Schmidt, to be published ; L. Leiserowitz, F. Nader, and G . M . J . Schmidt, to be published.
[2] I . Nuhringbauer and G . Larsson, Arkiv Kemi 30,91 (1968).
131 MacKenzie Rawles, Ind. Engl. Chem. Anal. 12, 737 (1940)
[4] J . Karle and I . L. Karle, Acta Crystallogr. 21, 849 (1966).
[5] R. G . Delaplane and J . A . Ibers, Acta Crystallogr. B 25,2432 (1969);
F . F . Iwasaki, H . Iwasaki, and Y. Saito, ibid. 23, 64, 56 (1967); F . R .
Ahmed and D. W J . Cruickshank, ibid. 6, 385 (1953).
[6] 2. Nahlouska, B. Nahloosky, and 7: G . Strand, Acta Chem. Scand.
24, 2617 (1970).
[7] E . G . C o x , H . W Dougzfl, and G . A . Jeflrey. J . Chem. SOC.1952,4855.
Angew. Chem. internaf. Edif. 1 Vol. 11 (1972) 1 No. 6
It is known (e.9. see Ref. [I]) that intermetallic compounds
are formed on reduction of mixtures of metal oxides and
noble metals with extremely pure hydrogen (po, I
pHIOl
torr), the process being termed “coupled
reduction”. With oxides of actinoids, lanthanoids, and other
non-noble elements (e. g. Zr, Hf, Nb, Ta) on the one hand
and Pt, Pd, Rh, or Ir on the other in the starting materials
at the requisite reaction temperatures (1100-1 55OoC),the
same ratio of metal to noble metal is found in the product
as in the starting mixture, but with Am/Pt, particularly at
higher temperatures, the product is poorer in Am[’”3 - ‘I.
A more detailed study of this and analogous systems has
led to surprising results : On preparation of intermetallic
phases of the elements Li, Ca, Sr, Ba, Am, and Cf ( =A) from
their oxides and/or fluorides with powdered noble metals
(=B) in flowing hydrogen at the lowest possible temperature, :he desired phase of composition AB,, AB,, AB,,
and/or AB, was obtained in high purity without change in
the introduced proportions. At higher temperatures, however, part of the more volatile component A evaporated
from these phases, so that a different A:B ratio was produced.
These observations make it possible to propose a method,
simple in principle, for preparing non-noble metals from
their oxides, fluorides, or other suitable compounds by
reduction with hydrogen : If a very finely powdered mixture
of metal oxide (Li,O, CaO, SrO, BaO, AmO,,,, or CfO, ,)
and noble metal is heated above 1100°C in a stream of pure
H,, one obtains the corresponding intermetallic phase in
very pure form (typical analytical values : 0,300-500 ppm,
N, < 100 ppm, H, < 100 torr-on cooling under He). If
these phases are further heated in a high vacuum
to
torr), the readily volatile components (Li, Ca, Sr, Ba,
Am, Cf) evaporate and can be isolated by condensation.
Because of its appreciably lower vapor pressure, the noble
metal-so far Pt, Pd, and Ir have been investigatedremains as solid phase and is available for a further reaction,
i.e. the noble metal functions wholly as catalyst for the
reduction of the metal oxide to metal by hydrogen (direct
reduction of the oxides in question is impossible because
of their low partial pressure of oxygen or their high bond
[*] Prof. Dr. C. Keller, U. Berndt, and Dr. B. Erdmann
Institut fur Radiochemie, Kernforschungszentrum
75 Karsruhe, Postfach 3640 (Germany)
[**I
This work was supported by the Deutsche Forschungsgemeinschaft
515
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