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Nature of the Bonding in Aminoquinones.

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results of Pople-Pariser-Parr calculations on the structural
isomers of ( I ) . Calculated values refer to systems with X =
NH, but it was shown[sl that for indigoids the spectroscopic
propertics of the isolated molecule with X = N H agree very
well with those of the corresponding molecule with X = S i n
the condensed phase.
Table 1. Comparison of the wavelengths Amax and intensities log E of
the longest-wavelength ultraviolet absorption maximum of compounds
of type ( 1 ) (measured in cyclohexane), and of their stretching fre(measured in KBr) with the wavelengths lmaX
and
quencies OC=O
oscillator strengths f of the longest-wavelength xi".-* transition a n d
the bond index p c = o of the C-0 bond, determined by the PoplePariser-Parr method.
Chromophore
X-NH
Q&
lcula
I
f
PCC
-
Compound
- -- -
0.42
0.74
( 5 )I
x-s
X
0
-2453
0.34
-Q
-x
0.55
-8
424
0.56
-
450
4.13
1674
315
3.95
1654
1709
-
- __
394
4.05
1692
1719
1520
318
3.94
1485
-
-
1668
1698
-
0.83
434
4.02
I642
374
3.72
I677
1488
38':
3.83
1640
1470
0.85
o = p
-x x-
0
501
Even the simplest Huckel molecular-orbital calculations
(carbon approximation [21) give the correct trend in spectroscopic properties in the series thioindigo, bisthiopheneindigo,
trans-42~~'-bi-(4,4-dimethylthiolan-3-one),as shown in
Table 2. Hence it is concluded that important properties of
the indigo dyes are determined by the topology of the
grouping (I).
0.81
x-
415
log
- -
___ - 0.27
rved
-
0
398
Ot
;.ma
(ni!*
-
-
~
1484
Table 2. Comparison of Huckel molecular-orbital result5 (carbon
approximation [2]) with spectroscopic data.
1 Calculated
~
Thioindiso ( f r u i i ~ )
Bisthionheneindigo
trans-~Z,?'-Bi-(4,4-dimethylthioian-3-one)
0.224
0.242
0.261
0.725
0.690
0.79X
I Observed (in CHCI,)
1x300
19800
21 800
1650
1641
I674
I693
-
__ -
0.82
If the calculated results for bispyrroleindigo (5) (X = NH)
in Table 1 are compared with those for group ( I ) in the
trans-s-cis-s-cis-orientation,
or if the experimental values for
the bisthiopheneindigo (5) (X = S) are compared with
those for compound (3u), then it is clear that (3u) - and
thus ( I ) - can be regarded as the basic chromophore of the
indigo dyes.
(a) Compound (3u) shows the unusually long-wavelength
absorption predicted by the theory (in comparison with
thioindigo the longest-wavelength absorption band is shifted
by only 88 m p to shorter wavelengths in spite of a reduction of the conjugated system from 22 to 10 ;r-electrons).
(b) Compound (3"
form.
hypsochromic shift in ethanol instead of the expected bathochromic shift [found for (2c): 41.= -4 m p ; (26): A). =
3 m v ; but (20) : A). = + 13 my]. Steric reasons are responsible
for these discrepancies between predicted and observed
results: The carbonyl groups on the six membered ring and
the SR groups cannot all be coplana; \riith the C = C double
which should have the same spectroscopic
bond. For ( & I ) ,
properties as (.?ti), the deviation from coplanarity due to
steric interaction of the CO-CH3 with the S-CH3 groups
is so great that the indigoid character is completely lost;
compounds (4.) and (46) have similar infrared and ultraviolet spectra to cr,p-unsaturated ketones having an alkylthio group at the $-position[8]. Since, owing to the
4
.
)
has n o symmetry centcr,
distortion, compound (
both the 3c=o and the ?c=cvibrations are infrared-active.
can be reduced reversit,ly to the leuco-
(c) Compound (3a) can, like thioindigo [6J, be reversibly
converted by irradiation to the cis-form; the hypsochromic
shift AA = --56 mp is in excellent agreement with the values
for thioindigo (AA = -53 mp) and the calculated values
(Ah = -55 mpj. Also the shift of the CO frequency from
3c=o = 1674 for (30) to 1719 cm-I for (36) is in agreement
with the change in the CO bond index from p c o = 0.81 to
0.85.
Thus the concepts developed by Dahne [71 to account for the
color of indigo are experimentally disproved.
[I] Part VI of Thcorctical and Spectroscopic Studies on Indigo
Dyes; Part V : M . Klessinger, Tetrahedron 1966, in press.
[2] M . Klessinger and W. Liirrke, Tetrahedron 19, Suppl. 2,
315 (1963).
[3] H. Hernrann, Dissertation, Universitdt Go ttingen , 1 966.
[4] Synthesis of (2n) to ( 4 b ) : W. Liirtke and H . Hertnnnn,
unpublished work.
[5] M . Klessinger and W. Liitrke, Chern. Ber. (1966), in press.
[6] G. M . Wy/nan and W. R . Brode, J. Amx. chern. SOC.73, 1487
(1951).
[7] D.Leitpold and S. Duhne, Theoret. chim. Acta 3, 1 (1965).
[ S ] K . Bowden, E. A . Brnrtde, and E. R . H . Jones, J. chern. SOC.
(London) 1946, 948; L. Bntetnnn and F. W. Shipley, ibid. 1955,
1997.
Nature of the Bonding in Aminoquinones
By Dr. S. Kulpe, Dr. D. Leupold, and Dr. S. DBhne
Institut fur Strukturforschung and Institut fur Optik und
Spektroskopie der Deutschen Akademie der Wissenschaften
zu Berlin (Germany)
The physicochemical behavior of aminoquinones, e.g. of
2,5-diamino-1,4-benzoquinones
( I ) , is determined, in our
opinion [1,*1, by coupling of dipolar structural elements of
merocyanine type. The compounds are, therefore, described
as quadrupolar merocyanines. Crystal-structure analysis of
2,5-diamino-3,6-dichloro-1,4-benzoquinone
(Fig. 1) con-
According to the calculations (Table 1) it is expected that
compounds (2b) and (2c) absorb at a wavelength 17 m y
higher, and more intensely, than ( 3 6 ) . However, only very
broad bands of rather low intensity at 386 and 374 my,
respectively, are observed; these undergo only a small
Angew. Chem. internat. Edit. Vof. 5 (1966) / No. 6
599
firms, in general, the bond lengths predicted121 by a simple
quantum-mechanical approximation o n the basis of this
concept. Interpretation of the properties of diaminoquinones
on the basis of a quinone structure 13341 is thus to be considered incorrect.
The coupling bonds CI-C2 and C4-C5 are much longer than
an aromatic bond (1.395 A). The polymethine chain shows
clearly an averaging of the bond lengths. In comparison with
the quinone structure [ 5 6 1 this amounts t o lengthening of the
C2-C3 and C5-C6 bonds and shortening of the C3-C4 and
C6-Cl bonds. The C-N separation is appreciably less than
the C-N single-bond distance (1.48 A).
I
I
IQ
Synthesis of 3,3,3-Trifluoroalanineand of
3,3,3-TrifluoroalanyI Peptides 1'1
By Prof. F. Weygand, Doz. W. Steglich, W. Oettmeier,
A. Maierhofer, and R. S. Loy
Organisch-Cheniisches Institut,
Technische Hochschule Miinchen (Germany)
The reaction of N-acyl-l-ethanesulfonyl-2,2,2-trifl~1oroethylamines ( I ) , X = SOzCzH5 r21, or of N-acyl-1-halogeno-2,2,2trifluoroethylamines ( I ) , X = C1 or Br "1, with triethylamine
(2) [I], which
gives rise to N-acyl-2,2,2-trifluoroiminoethanes
react with vinylmagnesium bromide to give N-acyl-l-trifluoromethylprop-2-enylamines (3). Oxidation of the latter
with potassium permanganate in acetone/water (1 :3) +
3 NH2S04 (4.5 vol- %) affords high yields of N-acyl-3,3,3-trifluoroalanines (4) [Ac = CsHsCO-, 92%, m.p. 152'C[*];
75 %, m.p. 11 2 "C].
Ac = CeHs-CHzO-CO-,
CH2=CH-MgBr
KMnO,
b
F S C - C H - C H ~ C H Z-*
NH-AC
(3)
C
b/2
Fig. 1. A molecule of 2,5-diamino-3,6-dichloro-l,4-benzoquinone
in
lid. Distances i n A.
(y,z)-projection about the center (x
0, y -- 0, z
Angles in degrees. Predicted values [Z]in brackets.
-
The X-ray structure analysis was carried out on reddishbrown needle-shaped crystals of 2,5-diamino-3,6-dichloro1,4-benzoquinone [X-ray density, 1.825 g/cm3; two molecules
per unit cell; space group, P21/c (P21/n was used in the calculations); lattice constants, a = 3.787 f 1 A, b = 10.771 i
3 A, c = 9.305 5 2 1\, = 91.2 i 1 "I.
About 730 intensities, determined photometrically or estis
mated, were used in the calculation. Patterson synthese[Po(u,w) and PO (v,w)] and M4-minimum functions obtained
therefrom gave first approximations for the x-, y-, and zcoordinates of the centers of gravity of the C1, 0, N, and Catoms. Fourier and Fourier difference syntheses in two and
three dimensions, as well as use of systems of linear structure
factor equarions [61,led to refinement of the coordinates. The
H atoms could be located approximately. The R factor for
the space data (including Fc = 0) is at present less than
14%. The mean error of the atomic distances given is estimated as 0.015 A. The positions of the coordinates are being
further refined.
The N-benzoyl compound ( 4 ) , Ac == C~HSCO-, is transformed by heating with concentrated hydrochloric acid into
3,3,3-trifluoroalanine hydrochloride, which can be dissolved
in chloroform by addition of triethylamine. Addition of
acetic acid liberates trifluoroalanine (5) in 75 % yield, which
sublimes above 21OoC. Compound (5) is markedly acidic
(pK; = 2.32, pK1 = 5.61 [ 3 9 . In contrast to unsubstituted
a-monoamino monocarboxylic acids it forms a crystalline
dicyclohexylammonium salt (m.p. 152 "C) and dissolves on
treatment with diazomethane in ether.
N-Benzyloxycarbonyl-3,3,3-trifluoroalaninecan be used in
the normal fashion to produce N-benzyloxycarbonyltrifluoroalanyl peptide esters, e . g . the methyl ester of benzyloxycarbonyl-3,3,3-trifluoroalanyl-~-phenylalanine,
m. p. 158 OC,
is obtained in 74 % yield using N-ethynylmethyldiethylamine
as condensing agent C41.
The method described can be adapted to prepare any aamino acid with a perfluoroalkyl side-chain. N-Benzoyl3,3,3-trichloroalanine, m. p. 163 OC, was made analogously.
Another synthesis of broad scope for the preparation of
derivatives of 3,3,3-trifluoroalanine involves treatment of (2)
with isonitriles. The adducts (6) initially formed [51 need not
be isolated but are transformed by treatment with dilute
acid into N-acyl-3,3,3-trifluoroalanineamides (7) ; e.g. the
cyclohexylamide of N-benzyloxycarbonyl-3,3,3-trifluoroalanine obtained in 8 0 % yield is identical with that prepared
from N-benzyloxycarbonyltrifluoroalanineitself.
Received: April 25th, 1966
[ Z 204 IEI
German version: Angew. Chem. 78, 639 (1966)
[ l ] S . Dahne and H . Paul, Chem. Ber. 97, 1625 (1964); S. Dalrne,
J . Ranft, and H . Paul, Tetrahedron Letters 1964, 3 3 5 5 .
[2] D. Leupold and S . DSihne, Theoret. chirn. Acta 3, 1 (1965).
[3] K . Wallenfels and W . Draber, Tetrahedron 20, 1889 (1964).
[4] M . Klessinger, Eighth European Congress on Molecular
I
Spectroscopy, Copenhagen, August 1965; Theor. chim. acta,
in press.
[ 5 ] J . Trotter, Acta crystallogr. 13, 86 (1960).
[6] L.Kutschabsky, Mber. dtsch. Akad. Wiss. Beriin 7,511 (1965).
600
F~C
-?H
- CO-
mR'
NH-Ac
(7)
Angew. Chem. internat. Edit.
i VOI.5 (1966) i NO.6
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