Diels-Alder reactivity of trialkyl 2-phosphonoacrylates with N-buta-1,3-dienylsuccinimide

Article abstract of DOI:10.1039/a702324c

N-Buta-1,3-dienylsuccinimide 1 reacts quantitatively with trimethyl 2-phosphonoacrylate 2a to furnish the ortho-[4 + 2]-cycloadduct 3a as a single stereoisomer. The cis axial/equatorial relationship between the succinimido and phosphonate groups respectively has been established by NMR and X-ray diffraction analyses. Triethyl 2-phosphonoacrylate 2b similarly undergoes cycloaddition with the diene 1 to give a 55:45 mixture of ortho stereoisomers 3b (cis axial/equatorial) and 4b (trans axial/axial). The activating and directing effect of the phosphonate group is discussed on the basis of a theoretical approach considering the van der Waals complexes.

Full text of DOI:10.1039/a702324c

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Diels–Alder reactivity of trialkyl 2-phosphonoacrylates with
N-buta-1,3-dienylsuccinimide
Nathalie Defacqz,a Roland Touillaux,b Bernard Tinant,b Jean-Paul Declercq,b
Daniel Peetersc and Jacqueline Marchand-Brynaert *,a
a Laboratoire de Chimie Organique de Synthèse, b Laboratoire de Chimie Physique et
Cristallographie and c Laboratoire de Chimie Quantique, Université Catholique de Louvain,
Département de Chimie, Place Louis Pasteur, 1, B-1348-Louvain-La-Neuve, Belgium
N-Buta-1,3-dienylsuccinimide 1 reacts quantitatively with trimethyl 2-phosphonoacrylate 2a to furnish the
ortho-[4 ϩ 2]-cycloadduct 3a as a single stereoisomer. The cis axial/equatorial relationship between the
succinimido and phosphonate groups respectively has been established by N M R and X-ray diffraction
analyses. Triethyl 2-phosphonoacrylate 2b similarly undergoes cycloaddition with the diene 1 to give a
55 :45 mixture of ortho stereoisomers 3b (cis axial/equatorial) and 4b (trans axial/axial). The activating and
directing effect of the phosphonate group is discussed on the basis of a theoretical approach considering
the van der Waals complexes.
N
N
C
5
5
Introduction
6
6
C
C
3
C
The synthetic applications of N-substituted 1-aminobuta-1,3-
dienes in [4 ϩ 2] cycloaddition processes1 have been well
illustrated.2–10 The usual protection of the terminal dienyl
amine involves amide, lactam and carbamate functions;11–13
such dienes react readily with various olefins bearing an
electron-withdrawing group (NO2, CHO, CO2R, CN) to furnish
exclusively the ortho-cycloadducts with a moderate to good
endo-selectivity.13–18 On the other hand, the use of N-1,3-
dienylimides in Diels–Alder reactions is poorly documented.19
In connection with studies of biologically active phosphonic
3
1
2
1
2
4
P
4
P
cis axial N/equat. P
3JP-C(3) = 5.4 Hz
4JP-C(6) = 12.6 Hz
trans axial N/axial P
3JP-C(3) = 0 Hz
4JP-C(6) = 5.4 Hz
Fig. 1 Characteristic heteronuclear C–P coupling constants
yellow oil that smoothly crystallized by slow evaporation from a
toluene solution (Scheme 1).
derivatives, we required
a flexible route towards ortho-
aminophosphonocyclohexane derivatives which could be fur-
ther functionalized on the six-membered ring. We selected the
Diels–Alder strategy using N-butadienylsuccinimide 120 and
vinylphosphonates as partners.
O
O
O
O
O
N
O
N
N
CO2R
CO2R
PO(OR)2
PO(OR)2
CO2R
Few reports are available on the use of vinylphosphonates as
dienophiles.21–22 They are generally less reactive than the cor-
responding α,β-unsaturated carbonyl compounds, and the
[4 ϩ 2] cycloadducts obtained are mixtures of regio- and
stereo-isomers (endo/exo). Ketovinylphosphonates are more
reactive.23–25 However, the selectivity is directed by the acetyl
group; accordingly, the final cycloadducts exhibit the phos-
phonate group in the meta position regarding the activating
substituent (OR) placed on the C(1) carbon atom of the diene.
We found that trialkyl 2-phosphonoacrylates 2 react rapidly
and quantitatively with N-butadienylsuccinimide 1 to furnish
ortho-cycloadducts as single or major stereoisomers 3, with a
cis axial/equatorial relationship between the succinimido and
phosphonate groups.
+
3
6
3
6
+
2
1
4
5
2
1
4
5
P(OR)2
O
1
2a R = Me
2b R = Et
3a (100%)
3b (55%)
4a (0%)
4b (45%)
Scheme 1 Cycloaddition of trialkyl 2-phosphonoacrylates
The H and 13C NMR spectra of 3a confirmed the presence
1
of a single regio- and stereo-isomer; they involve characteristic
features related to the half-chair conformation adopted by the
compound, and to the cis axial/equatorial relationship between
the succinimido and phosphonate groups respectively, as fur-
ther revealed by the X-ray diffraction analysis. The high value
(J 18.7)† for the coupling constant between the two geminal
protons H(6) is relevant. The carbon atoms of the imidyl car-
bonyls gave two broad signals (176.6 and 177.3 ppm), strongly
suggesting that the rotation of the succinimido group around
the N᎐C(3) bond is hindered by the cis bulky phosphonate
group. Two heteronuclear C–P coupling constants appeared to
be particularly influenced by the axial/equatorial position of
the phosphorus atom (Fig. 1): the JP–C(3) and JP–C(6) values were
5.4 and 12.6 respectively for compound 3a having the phospho-
nate group in the equatorial position.
Results and discussion
We first confirmed the poor reactivity of vinylphosphonates as
dienophiles: no cycloadduct could be detected (1H NMR and
TLC analyses) after refluxing N-butadienylsuccinimide 1 and
diethyl vinylphosphonate in acetonitrile for two weeks.
Interestingly, we observed a dramatic enhancement of the
alkene reactivity by the introduction of a carboxylate substitu-
ent in the geminal position. Thus, trimethyl 2-phosphono-
acrylate 2a quantitatively reacted with 1 in acetonitrile at 65 ЊC.
After 48 h, the crude mixture was purified by flash chroma-
tography on silica gel to furnish 3-succinimido-4-dimethyl-
phosphono-4-methoxycarbonylcyclohex-1-ene 3a as a pale-
Triethyl 2-phosphonoacrylate 2b26 was similarly reacted
with the diene 1 (Scheme 1): after 4 d at 65 ЊC in acetonitrile,
† J values in Hz.
J. Chem. Soc., Perkin Trans. 2, 1997
1965

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Fig. 2 Stereoscopic view of 3a (PLUTO)27
Table 1 Endocyclic torsion angles in the cyclohexene ring of 3a
PO3H2
C
PO3H2
C
H
X
(HCO)2N
X
C
C
Angle (Њ)
C
C
C
C
C
C
C(6)᎐C(1)᎐C(2)᎐C(3)
C(1)᎐C(2)᎐C(3)᎐C(4)
C(2)᎐C(3)᎐C(4)᎐C(5)
C(3)᎐C(4)᎐C(5)᎐C(6)
C(2)᎐C(1)᎐C(6)᎐C(5)
C(4)᎐C(5)᎐C(6)᎐C(1)
3.7(4)
8.4(3)
Ϫ38.5(2)
57.9(2)
15.6(4)
Ϫ45.5(3)
(HCO)2N
I (gauche configuration)
0.0 kcal mol –1
II (staggered configuration)
7.80 kcal mol –1
X = CO2H
X = CN
0.0 kcal mol –1
4.92 kcal mol –1
Fig. 3 Isomerization energies of model cyclohexenes
a mixture of two ortho-adducts 3b and 4b was quantitatively
recovered. According to the 1H and 13C NMR data, the
ratio of stereoisomers was 55:45, in favour of 3b. This
major compound showed the same spectral characteristics
as the reference compound 3a (JP–C(3) 5.4 and JP–C(6) 12.6); we
deduced a cis axial/equatorial relationship between the suc-
cinimido and phosphonate groups. We could reasonably
attribute the trans axial/axial stereochemistry to the minor
isomer 4b. For this isomer, free rotation of the succinimido
group around the N᎐C(3) bond could occur; accordingly, one
sharp line was observed at 176.3 ppm for the two equivalent
imidyl carbonyls. The axial position of the phosphorus atom
significantly influenced the relevant heteronuclear C–P coup-
ling constants: the P–C(3) coupling was suppressed and the
P–C(6) coupling appeared considerably diminished (J 5.4)
(Fig. 1).
The configuration of the cycloadduct 3a was unambiguously
confirmed by a single crystal X-ray diffraction analysis. As
shown in the stereoscopic view of the molecule (Fig. 2),27 the
succinimido and the phosphonate groups are cis to each other,
respectively in pseudoaxial and pseudoequatorial positions.
Selected geometrical parameters are presented in Table 1 and
are available as supplementary material. We have calculated
and O᎐P᎐O are also parallel with the same dihedral angle of 8Њ
as that observed in 3a.
᎐
The Diels–Alder cycloadditions involving 1,1-disubstituted
partners can proceed either by a concerted process or not.31–34
Whatever the mechanism involved, the selectivity leading to the
relative cis-configuration of the imido and phosphonate groups
in the final cyclic products 3 could be inspected theoretically.
From a theoretical point of view, a first approach would con-
sist of a comparison of the isomers related to the products in
order to determine their relative stability (the most stable com-
pound being the expected product). To facilitate such theor-
etical approaches ab initio, one must define a model for the
adduct molecules. To this end, methyl (or ethyl) groups were
replaced by hydrogen atoms correctly orientated to avoid
hydrogen bonding between substituents which would artificially
stabilize such structures. Further, the methylene groups of suc-
cinimide were also replaced by hydrogen atoms. This model
leads to two isomers (I and II) depending on the relative posi-
tions of the imido and phosphonic groups. Fig. 3 reports the
relative energies provided by the ab initio calculations at the
RHF/6-31G(d,p) level. These results show that in spite of the
steric effect resulting from the closeness of two bulky substitu-
ents, the gauche configuration is preferred to the staggered one.
To confirm this result and verify the independence of the effect
on the nature of the carboxylic substituent, the latter has been
replaced by a cyano group. Even though the stabilization is less
effective, the sterically favoured staggered configuration is less
stable.
This unexpected result suggests that some particular inter-
actions may exist between the imido and phosphonic groups. In
this hypothesis, the reactivity would be governed by such inter-
actions which must be present all along the reaction path, espe-
cially in the van der Waals pre-reactive complexes. In this pre-
liminary approach, we have searched for such complexes
between the substituted cis butadiene and ethylene moieties and
localized two of these on the energy hypersurface. The stabiliza-
tion of the complexes versus the reactant molecules ranges from
7.9 to 10.1 kcal molϪ1 (1 cal = 4.184 J) for the CO2H substitu-
ent, and from 4.2 to 5.9 kcal molϪ1 for the CN substituent. For
comparison, such a complex between cis-butadiene and ethyl-
ene is stabilized by 1.7 kcal molϪ1. The two complexes found
average values of P᎐O, P᎐O and P᎐C bond lengths over 62
᎐
structures of acyclic phosphonate derivatives bonded to a C
sp3 atom;28 they are respectively 1.460, 1.563 and 1.815 Å. The
observed values in 3a [1.445(2), 1.560(2) and 1.832(2) Å] com-
pare quite well with these means. As shown by the endocyclic
torsion angles (Table 1), the cyclohexene ring adopts a half-chair
conformation with the diad axis through the midpoint of the
double bond [∆C2(1–2) = 7.1Њ].29
In relation with the observed selectivity (endo directing effect
of the phosphonate group), it is worth noting the orientation of
the succinimido towards the phosphonate group. The best mean
planes through the succinimido ring and through O᎐P᎐O20 (A
᎐
or B) are nearly parallel to each other, the dihedral angle
between the two planes being ca. 8Њ. 8.6Њ for orientation A of
O20 and 8.4Њ for orientation B of O20. We have found in the
literature only one phosphonate derivative with a similar sub-
stituent in the γ-position with respect to the P atom: (E)-
diisopropyl
1-hydroxyimino-2-phthalimido-3-phenylpropyl-
phosphonate.30 In this molecule, the planes of the phthalimido
1966
J. Chem. Soc., Perkin Trans. 2, 1997

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Fig. 4 Phosphonoacrylate van der Waals complex
present a strong interaction between the imido part and the
calibrated with polystyrene. The 1H and 13C NMR spectra were
phosphonic group, the cyano or the carboxylic group. A stereo-
view of one of those complexes is shown in Fig. 4. In the
absence of hydrogen bonding, the most stable complexes are
obtained with the phosphonic group coming close to the imido
part. It is our opinion that the strength of the complexes keeps
the two reaction sites together long enough for a selective reac-
tion to take place.
Even though preliminary, these results lead to the conclusion
that favourable van der Waals interactions appear to occur
between the phosphonate and succinimido substituents, which
might govern the further reactivity of these species and produce
a Diels–Alder adduct despite the steric hindrance.
recorded on a Bruker AM-500 spectrometer, in CDCl3 solution;
chemical shifts are reported in ppm (δ) downfield from internal
Me4Si (J in Hz). The mass spectra (FAB or EI modes) were
obtained on a Finnigan MAT TSQ-70 instrument. The micro-
analyses were performed at University College London by Dr
Alan Stones.
N-Buta-1,3-dienylsuccinimide 1 was obtained by refluxing
succinimide and crotonaldehyde in toluene in the presence of
toluene-p-sulfonic acid as catalyst, followed by flash chroma-
tography on silica gel (230-400 mesh ASTM, supplied by
Merck) with ethyl acetate–hexane (25:75) as eluent.13 Trimethyl
2-phosphonoacrylate 2a was commercially available (Aldrich).
Triethyl 2-phosphonoacrylate 2b was obtained from triethyl-
phosphonoacetate by reaction with paraformaldehyde and
piperidine in refluxing methanol, followed by distillation over
phosphoric acid.26
Conclusions
The remarkable reactivity of phosphonoacrylates in [4 ϩ 2]
cycloaddition with butadienylsuccinimide opens
a route
towards functionalized cyclohexane derivatives equipped with
vicinal amino and phosphono groups (ortho products).
3-Succinimido-4-dimethylphosphono-4-methoxycarbonyl-
cyclohex-1-ene 3a
We have pointed out that the geminal substitution of the
alkenes strongly enhances their dienophilic reactivity, as com-
pared to the corresponding monosubstituted derivatives. In a
control experiment, we have reacted butadienylsuccinimide 1
with methyl acrylate; after 6 d at 65 ЊC in acetonitrile, the rate
of conversion into Diels–Alder product was ca. 80%. Under
similar conditions, vinylphosphonates were totally unreactive,
while the transformation of phosphonoacrylates was complete
within 2–4 d.
From a stereochemical point of view, we have revealed an
unexpected selectivity in favour of the cis axial/equatorial
cycloadducts 3; in terms of Alder’s rule, the phosphonate sub-
stituent exercises a more potent endo directing effect than the
carboxylate substituent. This observation constitutes the first
report of such a selectivity when PO3R and CO2R groups are in
direct competition facing a 1-substituted butadiene.
We could tentatively explain our results by the presence of
an imido N-protective group. This particular moiety creates
favourable interactions with the phosphonate substituent, as
found experimentally in the X-ray structure [CONCO/P(O)O
stacking], and confirmed theoretically: the most constrained cis
isomer 3 (R = H) was significantly stabilized regarding the less
stericallyhindered transisomer 4(R = H;relativeenergy:7.8kcal
molϪ1). Accordingly, particular interactions were found in the
pre-reactive complex leading to the cis stereoisomer 3 (R = H).
The theoretical predictions applied perfectly to the cyclo-
addition of trimethyl 2-phosphonoacrylate (R = Me). However,
using the more bulky triethyl 2-phosphonoacrylate (R = Et), a
mixture of cycloadducts 3 (cis, N axial/P equatorial) and 4
(trans, N axial/P axial) was recovered, most probably resulting
from the competition between favourable stacking interactions
and unfavourable steric interactions due to the P᎐O᎐R chains.
N-Buta-1-3-dienylsuccinimide 1 (300 mg, 1.98 mmol), trimethyl
2-phosphonoacrylate 2a (308 mg, 1.58 mmol) and hydro-
quinone (60 mg, 0.54 mmol) were dissolved in acetonitrile (2
ml) and heated under an argon atmosphere at 65 ЊC for 48 h.
After solvent evaporation, the crude mixture was purified by
flash chromatography on silica gel (230–400 mesh ASTM, sup-
plied by Merck) with dichloromethane–isopropyl alcohol (1:1)
as eluent. Yield: 638 mg (93%); mp 99–101 ЊC (from toluene);
νmax/cmϪ1 (CH2Cl2) 3458 (m), 2959 (w), 1709 (s), 1600 (w), 1434
(w), 1389 (w), 1358 (m), 1250 (m), 1177 (m), 1030 (m); δH (500
MHz) 2.19 (m, H-6), 2.30 (dt, H-6Ј, J(6,6Ј) 18.7), 2.39 (dt, H-5,
J(5,5Ј) 13.6), 2.71 (m, H-5Ј), 1.71 (br s, CH2 imide), 3.70 (d, OCH3
phosphonate, JH -P 11.0); 3.79 (d, OCH3 phosphonate, JH -P 11.0)
3.79 (s, OCH3 ester), 5.43 (m, H-2), 5.65 (m, H-3), 6.02 (m, H-
1); δC(125 Hz) 21.47 (d, C-6, JC-P 12.6), 21.88 (C-5), 27.80 (CH2
imide), 45.58 (d, C-3, JC-P 5.4), 52.35 (d, C-4, JC-P 135.0), 52.93
(OCH3 ester), 53.19 (d, OCH3 phosphonate, JC-P 7.2), 54.18 (d,
OCH3 phosphonate, JC-P 7.2), 120.45 (d, C-2, JC-P 90), 131.48
(C-1); 169.12 (d, CO ester, JC-P 5.4), 176.62 (br s, CO imide),
177.34 (br s, CO imide); MS m/z (FAB) 346 (M ϩ 1) (Found: C,
48.66; H, 5.80; N, 3.59. Calc. for C14H20NO7P: C, 48.69; H,
5.83; N, 4.05%).
3-Succinimido-4-diethylphosphono-4-ethoxycarbonylcyclohex-
1-ene 3b and 4b. The succinimide 1 (125 mg, 0.826 mmol), 2b
(190 mg, 0.804 mmol) and hydroquinone (14.2 mg, 0.128
mmol) were heated in acetonitrile (1 ml) at 65 ЊC for 4 d
(under Ar), as above. Yield: 312 mg (98%, yellow oil); νmax
/
cmϪ1 (CH2Cl2) 3475 (m), 2982 (w), 1711 (s), 1636 (w), 1445
(w), 1389 (w), 1357 (m), 1248 (m), 1177 (m), 1023 (m); cis
isomer 3b (major): δH (500 MHz) 1.17–1.36 (t, 9H, CH3
ester ϩ CH3 phosphonate), 2.14–2.35 (m, H-6 ϩ H-6Ј), 2.39
(dt, H-5, J(5,5Ј) 13.7), 2.68 (m, H-5Ј ϩ br s, CH2 imide), 3.94–
4.27 (q, 6H, OCH2 ester ϩ OCH2 phosphonate), 5.44 (m, H-
2), 5.69 (m, H-3), 6.02 (m, H-1); δC(125 MHz) 13.77 (CH3
ester), 16.00 (d, CH3 phosphonate, JC-P 7.2), 16.30 (d, CH3
phosphonate, JC-P 7.2), 21.65 (d, C-6, JC-P 12.6), 21.90 (C-5);
27.96 (CH2 imide), 45.76 (d, C-3, JC-P 5.4), 52.56 (d, C-4, JC-P
132.8), 61.79 (OCH2 ester), 62.78 (d, OCH2 phosphonate, JC-P
Experimental
General
Reagents and solvents were purchased from Aldrich. The IR
spectra were taken with a Perkin-Elmer 1710 instrument and
J. Chem. Soc., Perkin Trans. 2, 1997
1967

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7.2), 63.56 (d, OCH2 phosphonate, JC-P 7.2), 120.76 (d, C-2,
JC-P 9), 131.57 (C-1), 168.74 (d, CO ester, JC-P 5.4), 176.56 (br
s, CO imide), 177.43 (br s, CO imide); trans isomer 4b (minor):
δH (500 MHz) 1.17–1.36 (t, 9H, CH3 ester ϩ CH3 phosphon-
ate), 2.14–2.35 (m, H-6 ϩ H-6Ј), 2.56 (m, H-5 ϩ H-5Ј), 2.68
(br s, CH2 imide), 3.94–4.27 (q, 6H, OCH2 ester ϩ OCH2
phosphonate), 5.30–5.40 (m, H-2 ϩ H-3), 6.02 (m, H-1);
δC(125 MHz) 13.64 (CH3 ester), 16.13 (d, CH3 phosphonate,
JC-P 7.2), 16.23 (d, CH3 phosphonate, JC-P 7.2), 22.15 (d, C-6,
JC-P 5.4), 24.14 (d, C-5, JC-P 3.6), 27.86 (CH2 imide), 46.40
(C-3), 50.23 (d, C-4, JC-P 131.1), 61.48 (OCH2 ester), 62.66
(d, OCH2 phosphonate, JC-P 7.2), 63.33 (d, OCH2 phosphonate,
JC-P 7.2), 121.62 (d, C-2, JC-P 5.4), 130.58 (C-1), 168.84 (d, CO
ester, JC-P 5.4) 176.27 (CO imide). MS m/z(EI) 387 (M)
(Found: C, 52.69; H, 6.88; N, 3.35. Calc. for C17H26NO7P: C,
52.71; H, 6.76; N, 3.61%).
Acknowledgements
This work was generously supported by the Fonds National de
la Recherche Scientifique (FNRS, Belgium), the Fonds du
Développement Scientifique (FDS, UCL, Belgium), and the
Institut de Recherches Servier (Suresnes, France). N.D. and
J.M.-B. thank Dr A. Cordi for stimulating discussions.
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X-Ray analysis and structure determination
Crystallographic data for 3a: C14H20NO7P, Mr = 345.28, tri-
¯
clinic, P1, a = 8.688(2), b = 8.907(1), c = 12.424(1) Å,
α = 88.71(2), β = 75.73(2), γ= 61.97(2)Њ, V = 817.6(2) Å3, Z = 2,
Dc = 1.403 g cmϪ3. Parallelipiped crystal with approximate
dimensions 0.4 × 0.3 × 0.3 mm. Lattice parameters were refined
using 30 reflections in the range 5 р 2θ р 25Њ. Huber four-circle
diffractometer, graphite monochromatized Mo-Kα radiation
(λ = 0.71069 Å). 3210 independent reflections with sin θ/λ
р 0.62 ÅϪ1; 0 р h р 10, Ϫ9 р k р 10, Ϫ14 р l р 15, 2471
with I у 2(I). A standard reflection (0 Ϫ2 Ϫ1) was checked
every 50 reflections; no significant deviation was observed. The
structure was solved by direct methods using SHELXS86.35 All
H atoms, except those of one methyl group, were located from
difference Fourier synthesis; the H atoms of methyl C21 were
calculated with AFIX. Anisotropic least squares refinement
(SHELXL93)36 using F2 values; H isotropic with a common
refined thermal parameter (U = 0.079 Å2). 279 parameters.
w = 1/σ2(Fo)2 ϩ 0.078P2 ϩ 0.03P). Two positions appeared for
the methoxy group O20᎐C21; the occupation factors of the two
positions converge to 0.78 (A) and 0.22 (B) at the end of the
refinement. Final R indices; R = 0.046, R (all data) = 0.059,
wR2 = 0.12, S = 1.07. Final maximum shift to error = 0.001.
Maximum and minimum heights in final Fourier syn-
thesis = 0.44 and Ϫ0.30 e ÅϪ3. Atomic scattering factors from
ref. 37.†
Theoretical analysis
The molecules and van der Waals complexes under investiga-
tion have been studied at the restricted Hartree-Fock (RHF )
level using the 6-31G(d,p) basis set of Hariharan and Pople.38
Such a basis gives enough flexibility to the wavefunction to
ensure a balanced behaviour of the phosphorus atom. The
geometric structures obtained in this paper result from a full
geometry optimization of all the considered parameters in the
3N Ϫ 6 internal coordinates space, thus relaxing any a priori
symmetry constraint. A conventional gradient technique was
used for the search of the optimal structure. The analytical
computation of the first and second derivatives of the energy
hypersurface guarantees the good behaviour of the extremum
found. All the computations reported in this paper were
obtained using the GAUSSIAN series of programs.39 Due to
the presence of labile intermolecular parameters, the potential
energy surface of the complexes is very flat. This leads to some
looseness of the obtained structures which, nevertheless, have
the properties of a true minimum.
36 G. M. Sheldrick, SHELXL93, Program for the Refinement of
Crystal Structures, University of Göttingen, Germany, 1993.
37 International Tables for X-Ray Crystallography, The Kynoch Press,
Birmingham, 1974, vol. IV.
38 C. Hariharan and J. A. Pople, Theor. Chim. Acta, 1973, 28, 213.
39 J. Frisch, G. W. Trucks, M. Head-Gordon, P. M. W. Gill, M. W.
Wong, J. B. Foresman, B. G. Johnson, H. B. Schlegel, M. A. Robb,
E. S. Replogle, R. Gomperts, J. L. Andres, K. Raghavachari,
J. S. Binkley, C. Gonzalez, R. L. Martin, D. J. Fox, D. J. Defrees,
J. Baker, J. J. P. Stewart and J. A. Pople, GAUSSIAN 92, Revision C,
Gaussian, Inc., Pittsburgh PA, 1992.
† Atomic co-ordinates, thermal parameters and bond lengths and
angles have been deposited at the Cambridge Crystallographic Data
Centre (CCDC). See ‘Instructions for Authors’, J. Chem. Soc., Perkin
Trans 2, 1997, Issue 1. Any request to the CCDC for this material
should quote the full literature citation and the reference number
188/89.
Paper 7/02324C
Received 4th April 1997
Accepted 23rd June 1997
1968
J. Chem. Soc., Perkin Trans. 2, 1997