Configurational and conformational mobility in a cross-conjugated derivative of 1,3-diketo-2-iminopropane

Article abstract of DOI:10.1021/ol990636z

(Matrix presented) Mobility of configuration and conformation in the "push-pull" cross-conjugated derivative 1,3-diphenyl-2-(p-dimethylaminophenyl)iminopropane-1,3-dione has been investigated by NMR. The energetics of the thermal process that interconnects the syn-and anti-configurations of the imino group with the atropisomeric conformations of the skewed benzoyl groups are defined. It is proposed that a symmetric Y-delocalized transition state thermally interconnects the inversion of the imino configurations with the mobile interchange of the skewed benzoyl conformations. This connection is observable through a unique NMR window.

Full text of DOI:10.1021/ol990636z

ORGANIC
LETTERS
1999
Vol. 1, No. 4
561-564
Configurational and Conformational
Mobility in a Cross-Conjugated
Derivative of 1,3-Diketo-2-iminopropane
Yueyang Song, Lynne Spencer, William B. Euler, and William Rosen*
Department of Chemistry, UniVersity of Rhode Island, 51 Lower College Road,
Kingston, Rhode Island
wrosen@chm.uri.edu
Received May 5, 1999
ABSTRACT
Mobility of configuration and conformation in the “push−pull” cross-conjugated derivative 1,3-diphenyl-2-(p-dimethylaminophenyl)iminopropane-
1,3-dione has been investigated by NMR. The energetics of the thermal process that interconnects the syn- and anti-configurations of the
imino group with the atropisomeric conformations of the skewed benzoyl groups are defined. It is proposed that a symmetric Y-delocalized
transition state thermally interconnects the inversion of the imino configurations with the mobile interchange of the skewed benzoyl conformations.
This connection is observable through a unique NMR window.
The concepts of cross-conjugation1 and Y-delocalization2
have been part of the lexicon of chemistry for a number of
years.3 Whether the underlying σ-Y-shaped atomic frame-
works are capable of supporting fully conjugated π-electron
delocalized and stabilized systems has generated considerable
debate.4 Recently we have shown that unconstrained σ-Y-
frameworks, supporting cross-conjugated π-systems, have a
skewed geometry.5 In this Letter we report additional
examples of unconstrained, cross-conjugated systems that are
capable of sampling Y-delocalization. Instead of π-delocal-
ized planarity, the most obvious conformational hallmark in
our systems is mobility that equilibrates energetically
separable configurations.
1,3-diphenyl-1,2,3-propanetrione, I, that we laboriously
prepared in low yield,6 had begun to slowly decompose
hydrolytically when stored in “ultrapure” CDCl3 in a freezer
for several months. (The “ultrapure” solvent apparently
contained adventitious heavy water.) Examination of this
1
mixture by H-decoupled 15N NMR, after equilibrium was
reached, indicated that additional resonance bands had
appeared in relative amounts of 12% and 4% with respect
to the starting triimine resonance bands. The 12% resonances
appeared at δ ) 333.6 and 359.1 ppm (vs 15NH3) as singlets
while the 4% resonances appeared at δ ) 350.3 and 355.6
ppm as doublets having 3JN-N ) 2.7 Hz.5 We attributed the
12% resonances to hydrolytically derived IIa while the 4%
resonance doublets were attributed to IIb (see Scheme 1)
and we surmised that these 1-keto-2,3-diiminopropane
derivatives were an equilibrating 3:1 mixture of atropisomers.
We assumed further that the conformational and configura-
tional structures shown in Scheme 1 for IIa and IIb were
most reasonable on the basis of our previous experience with
Serendipity entered into our investigations when we noted
that the 15N-labeled NMR sample of the tris(phenylimino)-
(1) Phelan, N. F.; Orchin, M. J. Chem. Educ. 1968, 45, 633-637.
(2) Gund, P. J. Chem. Educ. 1972, 49, 100-103.
(3) Klein, J. Tetrahedron 1983, 39, 2733-2759.
(4) (a) Wiberg, K. J. Am. Chem. Soc. 1990, 112, 4177-4182. (b)
Shancke, A. J. Phys. Chem. 1994, 98, 5234-5239. (c) Traetteberg, M.;
Hopf, H. Acta Chem. Scand. 1994, 48, 989-993.
(5) Spencer, L.; Kim, M.; Euler, W. B.; Rosen, W. J. Am. Chem. Soc.
1997, 119, 8129-8130.
(6) Spencer, L.; Euler, W. B.; Traficante, D. D.; Kim, M.; Rosen, W.
Magn. Reson. Chem. 1998, 36, 398-402.
10.1021/ol990636z CCC: $18.00 © 1999 American Chemical Society
Published on Web 07/16/1999

Scheme 1
these types of cross-conjugated systems.6 Further investiga-
starting materials. The NMR spectral characteristics of IV
were analogous to those of III7 and are shown in Table 1.
tions as to the equilibrating nature of these atropisomers were
precluded, however, by the scarce availability of 15N-labeled
II. Consequently we examined a more easily prepared
1
derivative, unlabeled III. (The 13C and H NMR spectra of
Table 1. 1H and 13C NMRa Chemical Shift Information for
Compounds III and IV
pure, unlabeled II were complex but it was apparent that a
mixture of at least two isomers was present.)
The 1,3-diketo-2-iminopropane derivative, III, can have
four distinct atropisomeric forms. The phenylimino group
can either be syn or anti to the conformationally skewed
benzoyl group or the planar conjugated portion of the
molecule can occupy an s-cis (Z) or s-trans (E) configuration
(Scheme 1). Interconversion of the four atropisomers is easily
envisioned by rotating the benzoyl groups into and out of
the plane of the imino group. Our derived NMR spectral
assignments for III are displayed in Table 1 and are
consistent with III being a rapidly equilibrating mixture of
these four possible atropisomers, where anti-Z-III and syn-
E-III7 are the major isomers present. We can implicate the
syn-E-III structure as being more important than the syn-Z-
III structure since our previous observations,6,7 with several
model compounds, show that an s-trans-configuration for
conjugation is preferred in order to avoid lone pair repulsions.
The anti-Z-III and syn-Z-III configurations are implicated
as being part of the equilibrium based upon the observation
that the crystal structure of the parent III, 1,3-diphenyl-1,2,3-
propanetrione, exhibits a skewed conformation that is
between and closely aligned to these configurations.8 Un-
fortunately, we were unable to directly observe the proposed
equilibrium involving anti-Z-III with syn-Z-III or syn-E-
III. Consequently we turned our attention to the preparation
of 1,3-diphenyl-2-(p-dimethylaminophenyl)iminopropane-
1,3-dione, an example of a “push-pull” system that has
proven to be of value in similarly related cross-conjugated
derivatives,9 and investigation of its properties.
III (X ) H)
IV (X ) N(CH3)2)
atom no. 1H δ (ppm) 13C δ (ppm)a 1H δ (ppm) 13C δ (ppm)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
7.51
7.38
7.78
134.7
129.0
129.5
134.2
196.2
163.8
191.4
135.0
131.2
128.7
134.5
147.2
120.7
129.2
126.8
7.52
7.37
7.73
134.4b
129.3
129.0
134.8
190.3
140.0
189.0
136.1
128.7
129.5
134.4b
135.1
125.8
110.9
152.9
40.6
8.30
7.55
7.68
7.94
7.52
7.65
7.01
7.22
7.07
7.39
6.50c
3.00
a Assignments were established using appropriate 1D and 2D NMR
methods. b Separate resonances only at -80 °C. c Assignment of H14 based
upon an NOE to the -N(CH3)2 group of IV.
Compound IV was prepared in the usual manner5 using
p-nitroso-N,N-dimethylaniline and dibenzoylmethane as the
The similarities between III and IV are apparent, but the
differences proved most informative. (1) The carbonyl-C5
shifted upfield by 5.9 ppm and the carbonyl-C7 shifted
upfield by 2.4 ppm. The upfield shift of C5 implicates a more
highly conjugated s-trans system while the shift of C7
implies that the skewed benzoyl group is in a syn relationship
to a more charged ring than that found in III. These
(7) Spencer, L. Ph.D. Dissertation, University of Rhode Island, Kingston,
RI, 1998.
(8) Beddoes, R. L.; Cannon, J. R.; Heller, M.; Mills, O. S.; Patrick, V.
A.; Rubin, M.; White, A. N. Aust. J. Chem. 1982, 35, 543-556.
(9) Yavari, I.; Zonouzi, A.; Hekmat-Shoar, R. Tetrahedron Lett. 1998,
39, 3841-3842.
562
Org. Lett., Vol. 1, No. 4, 1999

observations are consistent with the syn-E-IV structure being
dominant. (2) Comparing position 13 between III and IV,
we note that the proton H13 of IV is shifted downfield by
0.38 ppm while the attendant carbon C13 is shifted downfield
by 5.1 ppm. The chemical shift changes associated with
position 13 can best be accommodated by an anti structure,
which must also be present as an equilibrium component.
Comparing the resonance structures of IV seems to cor-
roborate this conclusion (see below). (3) The imine C6 in
IV is shifted upfield by 23.8 ppm, in comparison to the same
position in III, which implies that there is considerable
negative charge associated with this position. (The position
6 of IV is observed as a singlet in the 1H-coupled 13C NMR
spectrum.) The large shift upfield at this position in IV
implicates considerable resonance delocalization of charge
as shown in Scheme 1. (4) The ipso C15 in IV is shifted
downfield by 26.1 ppm in comparison to III while the ipso
C12 is shifted upfield by 12.1 ppm. (The former resonance
appears as a complex multiplet in the 1H-coupled 13C NMR
spectrum while the latter resonance appears as a triplet of
at coalescence displayed 10 resonances rather than the 16
observed at lower temperatures. The most notable changes
were associated with the carbonyl carbons C5 and C7, a
broad resonance centered at 189.5 ppm, and the ipso carbons
C4 and C8, a broad resonance centered at 135.5 ppm.
Additionally, resonances for C2, C3, C9, and C10 collapsed
to two closely spaced signals at ∼129 ppm while the
resonances for C1 and C11 remained as a single peak at
134.4 ppm. The carbon resonances of ring 3 were unchanged
at coalescence in this experiment. Standard manipulation of
the variable temperature NMR data11 according to the
Arrhenius equation led to the following thermodynamic
parameters near and at coalescence: ∆Gq ) +67 kJ/mol;
∆Hq ) +63 kJ/mol; ∆Sq ) -12 J/mol‚K. These derived
thermodynamic parameters are consistent with an inversion
of configuration mechanism12 about an imine nitrogen and
thus syn-IV and anti-IV are interconverting at ∼+60 °C.
Closer inspection of the variable temperature CMR experi-
ment between 50 and 70 °C, in 2° steps, revealed the startling
observation that the carbon atoms of ring 3, e.g., those off
the symmetry axis only, separated into distinguishable
2
3
triplets having JC-H ) 8.5 Hz and JC-H ) 3.5 Hz.) These
large changes in chemical shift imply that there is consider-
able charge separation flowing from the push of the di-
methylamino group to the pull of the cross-conjugated
benzoyl groups as represented by the resonance structure
shown in Scheme 1 for IV.
1
topomeric resonances (see Figure 1). The H-decoupled
Our structural conclusions were supported by AM1
calculations.10 The syn-E-IV structure was found to be
slightly lower in energy than either the syn-Z-IV or anti-Z-
IV structures. Because of a ring-ring interaction, the anti-
E-IV structure was of much higher energy.
The above comparisons were informative, but in order to
make more accurate assessments we had to lower the NMR
probe temperature when examining IV because it exhibited
signs of coalescence signals near room temperature. Con-
sequently, we embarked upon an investigation of the
temperature-dependent dynamics displayed by IV.
For probing the variable temperature NMR effects dis-
played by IV we investigated the temperature range from
-90 to +90 °C in 10° steps (CD2Cl2 for -90 to 0 °C, CDCl3
for -30 to +60 °C, and Cl2CDCDCl2 for 0 to +90 °C).
Between -90 and -10 °C the 1-D 1H and 13C spectra were
observed to be well defined and all resonances were easily
identified (Table 1). The spectral resonances began to
broaden at ∼0 °C and subsequently coalesced at ∼60 °C.
At coalescence (between 60 and 65 °C, depending upon the
solvent) the proton resonances associated with the diben-
zoylmethano portion of the molecule appeared as two broad
manifolds centered at δ ) 7.5 and 7.8 ppm while the N,N-
dimethylanilino portion of the molecule appeared as sharp
signals that showed no apparent temperature-dependent
chemical shift changes. At +90 °C the protons of rings 1
and 2 appeared as three manifolds centered at δ ) 7.4 ppm
(4H), 7.6 ppm (2H), and 7.9 ppm (4H). The carbon spectrum
Figure 1. 1D-13C NMR spectrum for IV. The spectra were taken
on a JEOL Eclipse at 100 MHz. The solvent for the -60 °C
spectrum was CDCl2, for the 25 and 60 °C spectra the solvent was
CDCl3, and the solvent for the 80 °C spectrum was CDCl2CDCl2.
carbon resonances for positions 13 and 14 each separated
into two closely spaced lines, each having a separation of
(11) Gunther, H. NMR Spectroscopy - Basic Principles, Concepts and
Applications in Chemistry, 2nd ed.; John Wiley & Sons: New York, 1995;
pp 335-345.
(12) Eliel, E. L.; Wilen, S. H.; Mander, L. N. Stereochemistry of Organic
Compounds; Wiley-Interscience: New York, 1994; pp 550-555.
(10) The calculated heats of formation were syn-E-IV, 350.3 kJ/mol;
syn-Z-IV, 354.6 kJ/mol; anti-Z-IV, 369.9 kJ/mol; anti-E-IV, 1402.5 kJ/
mol. PC Spartan Plus v1.5, Wavefunction, Inc., 18401 Von Karman Ave.,
Ste 370, Irvine, CA 92612.
Org. Lett., Vol. 1, No. 4, 1999
563

∼0.06 ppm at coalescence. The methyl carbons at position
16 separated into a quartet-like manifold having unequal
spacings all near 0.1 ppm at coalescence. To eliminate the
possibility that the observed separations were a coupling
phenomenon, we examined the off-resonance 1H-coupled 13C
NMR of IV at +57 °C. Positions C13 and C14 were now a
through the window of an interconversion of conformational
and configurational isomers. The relative concentrations of
the four atropisomers at any given temperature are unknown
at present.
The hallmark of the cross-conjugated systems we have
studied is mobility of conformation and configuration. Until
we had prepared compound IV, however, we were unable
to directly observe the proposed structural isomers or skewed
conformers we believed were present in these equilibrium
systems. Now, through a unique nuclear magnetic resonance
window we are able to witness the individual configurational
and conformational isomers. Consequently, we can state with
some confidence that a Y-delocalized transition state must
connect the cross-conjugated atropisomer anti-Z-IV with the
syn-E-IV and syn-Z-IV conformers via an activation barrier
of about 65 kJ/mol. Experiments to further clarify the
thermodynamic parameters associated with this type of
fluxional atropisomerism are underway.
1
doublet of doublets (1JC-H(#13) ) 159 Hz; JC-H(#14) )
162 Hz) while the methyl groups at position 16 appeared as
four quartets having an integrated intensity ratio of ∼1:1.6:
1
1.6:1 and JC-H ) 136 Hz. More detailed inspection of the
separation of the 1H-decoupled carbon resonances at positions
13, 14, and 16 revealed that the separations for all three were
temperature dependent, increasing from 0 ppm at +45 °C
to as much as 0.13 ppm at +70 °C for position 16. We also
noticed two other features of importance in this experiment.
(1) The separation of the “doublets” for C13 and C14 were
equivalent, increasing with temperature to a value of ∼0.10
ppm at +70 °C. (2) The separation within the quartet-like
resonances associated with position 16 were unequal: the
more intense central resonances had a larger separation
(0.131 ppm at +70 °C) at all temperatures relative to their
separation from the wing resonances (downfield pair, 0.123
ppm; upfield pair 0.115 ppm at 70 °C). The observation of
these separations eliminated the possibility of some sort of
coupling phenomenon.13 Consequently, we associate the
separate topomeric resonances with an equilibrating mixture
of atropisomers, anti-Z-IV, syn-Z-IV, and syn-E-IV, viewed
Acknowledgment. We thank the Department of the Army
under the auspices of the AASERT program, Grant
DAAH0494G0150 (W.B.E. and W.R.), and the National
Science Foundation, Grant 9729819 (W.B.E.), for partial
support of this research.
OL990636Z
(13) Breitmaier, E.; Voelter, W. Carbon-13 NMR Spectroscopy, 3rd ed.;
VCH Publishers: New York, 1990; Chapter 3, pp 107-182.
564
Org. Lett., Vol. 1, No. 4, 1999