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Double-Bond Geometry of Norbornene Neutron Diffraction Measurement of a Derivative at 15 K.

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Double-Bond Geometry of Norbornene:
Neutron Diffraction Measurement of a Derivative
at 1 5 K
By Otto Errnu,* Peter Bell, and Sax A . Mason
Norbornene 1 is a prototypal strained olefin and as such
a prominent model system of mechanistic organic chemistry.
Numerous experimental and theoretical studies have shown
that the reactivity of the double bond of 1 is characterized by
a pronounced preference for exo attack. Inter aha, this has
been rationalized in terms of the geometry of the double
bond, which is not completely planar but bent in endo direction by the angle x (endo pyramidalization, symmetric outof-plane bending). The related changes in hybridization and
steric factors favor ex0 attack."]
Despite the multitude of previous work,"] the magnitude
of the endo bending of the double bond of 1, that is, the angle
x, is not reliably known. For example, from theoretical calculations applying quantum-chemical models or empirical
force fields the following x angles have been derived: 1.7"
(MM2 force field),['h] 4.2" (extended Hiickel MO),'"] 4.5"
(CFF force
4.9" (ab initio, STO-3G).['d.g1Further[*] Prof. Dr. 0.Ermer, DipLChem. P. Bell
Institut fur Organische Chemie der UniversitHt
Greinstrasse 4, D-5000 Koln 41 (FRG)
Dr. S. A. Mason
Institut Laue-Langevin
B.P. 156X. F-38042 Grenoble Cedex (France)
more, a number of X-ray analyses have been undertaken,
which in most cases, however, involved unfavorable derivatives. Still worse, the light hydrogen atoms obviously can in
principle not be located reliably by X-ray diffraction.
In order to place the extensive norbornene discussion on a
sound basis, that is, to localize reliably the double-bond H
atoms, we have performed a neutron diffraction measurement at 15 K on the simple derivative 2 (exo-exo-2,3-norborn-5-enedicarboxylic anhydride).[jI Norbornene itself is
not suitable for such purposes: At room temperature the
low-melting (m.p. 46 "C) and rather spherical hydrocarbon
forms soft, deliquescent crystals of unknown structure. The
present structural problem of norbornene is comparable to
that of trans-cyclooctene, which involves a strongly nonplanar double bond and also plays the role of a prominent
reference olefin. Some time ago, we reliably measured the
double-bond geometry of trans-cyclooctene by means of
neutron diffracti~n.[~]
Key results of the neutron diffraction measurement on the
norbornene derivative 2 are collected in Figure 1.Is1 Strictly
Cl4I-Cl51- C(6)-C(IJ
HIS)-C(Sl: CIGI-HIG)
C [ L ) - C ( S l = C161-HIGl
Hl5I-C~51:C~Gl-C11)
C l L I - C ( S l = C(GI...HIS)
C ( l ) . - . C ( 5 ) = CIGI-HIGI
1.096
1.541
1.541
h
U
HILI-CILI
HI11 - C ( I l
- C(51- HI51
- CIGJ - H(G1
0.54
0.7
-172.0
173.2
-172.7
-172.5
23.8
-24.9
Fig. 1. Neutron diffraction results of 2. a) Stereoview, atomic numbering, vibrational ellipsoids (90"t,w<)hability).b) Bond lengths [A], selected bond angles and
torsion angles I"]; estimated standard deviations: CC,CO 0.001, CH 0.002-0.003 A; CCC 0.06-0.07, IICC, HCH 0.2"; CCCC 0.09. HCCC 0.2, HCCH 0.3". c)
Projectionsdown the CC double bond: illustration of the out-of-plane bending (pyramidalization) angles. d) Projections down the C(sp')-C(sp3) single bonds: influence
of the double-bond bending on these partial conformations (ellipsoids 2.5%).
Angew C'hem. Inr. Ed. Engl. 28 / t 9 8 9 ) Nr. 9
8
VCH Verlugsgesellsrhuft mbH, 0-6940 Weinhem, 1989
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speaking, the molecules of 2 possess no symmetry in the
crystal, yet are mirror-symmetric to a very good degree of
approximation. The analytical description of the nonplanar
double-bond deformations requires three independent
geometry
which in the present case are
defined as follows: a) Twisting angle QD = "torsion angle"
between the p orbitals at C5 and C6 = arithmetic mean of
the two double-bond torsion angles C4-C5-C6-C1 and
H5-C5-C6-H6; b) out-of-plane bending (pyramidalization) angles xs and xb at C5 and C6 = deviation from planarity of the two dihedral angles C4-C5-C6/H5-C5-C6
and C1-C6-C5/H6-C6-C5.['01 The twisting angle (P, has
the very small value 0.6(2)", and the double bond is thus
almost untwisted (Q, = 0" for exact mirror symmetry). The
bending angles xs and x6, which are at the focus of this work,
amount to 7.3(2) and 7.5(2)", respectively, and correspond
to endo pyramidalization (Fig. lc). In line with the approximate mirror symmetry, both bending angles are practically
equal, and our neutron diffraction measurement thus
affords a pyramidalization of x = 1/2(xs + x6) = 7.4(2)".
This precise value turns out to be roughly twice as high
as usually estimated for norbornene I (see above).
Like always in such cases, the question arises as to the
influence of crystal packing and substituent effects on x. In
view of the absence of particularly short intermolecular contacts and the closely approximated molecular mirror symmetry, packing effects are likely to be negligible. Likewise, the
rather remote anhydride moiety is expected to exert little
influence on the double-bond bending; comparative quantum-chemical calculations on I and 2 could help settle this
problem
Regarding other structural properties of 2, the reader is
referred to the collection of bond lengths and angles in Figure 1 b. Mention is made only of the bond angle H-C7-H of
109.0(2)", which is somewhat widened relative to the normal
value of about 106-107", since the angle CI-C7-C4 amounts
to only 93.67(6)".[' 'I The bond lengths C1-C2 and C3-C4 are
somewhat lengthened, probably due to the anhydride group.
Suffice it to say that other comparable carboxy derivatives of
norbornane also show this feature, whereas we have found
shortened C1-C2 bonds in ester derivatives of exo-2-norbornanol.
According to the simple empirical force-field models,['h*2a1 the driving force for endo bending of the double
bond of norbornene is to be sought in a decrease of torsional
strain around both C(sp2)-C(sp3) single bonds, since the
endo pyramidalizations lead to a more staggered orientation
of the C(sp2)-H bonds. The two projections of Figure 1d
show quantitatively that the three torsion angles around C4C5 and CI-C6, in which H5 and H6, respectively, participate, each assume more favorable (more staggered) values by
7.1 and 7.2" through the endo pyramidalizations. In the synsesquinorbornene 3, two norbornene systems are fused
sured for a number of derivatives of 3[12]are roughly two
times larger than in norbornene. However, it is to be conceded that this very simple and more qualitative picture most
likely does not convey the whole truth since the quantitative
reproduction of the double-bond bendings in the force-field
calculations is not satisfactory. For example, the amount of
double-bond bending in 3 is probably also influenced by
angle strain factors.['".
For benzonorbornene 4 considerably smaller pyramidalization (at C2 and C3) than in norbornene 1 may be anticipated, since the rotational barriers around C(sp2tC(sp3)
bonds at a benzene ring are substantially lower than around
such bonds at an olefinic double bond. (Compare the sixfold
rotational barrier of the methyl group in toluene of only
0.01 3 kcal m o l ~
with the essentially threefold barrier in
propene, which amounts to 1.98 kcal mol- '.[' 3h1) Furthermore, pyramidalization (out-of-plane bending) at the benzene ring is energetically somewhat more costly relative to
olefinic double bonds.['41 We have prepared the two crystalline benzonorbornene derivatives 5 and 6 and have sub-
L
A=H
5 R = COOH
6 R=COO-D-C~H~-NO~
jetted them to X-ray analysis.[' The resulting average
bendings x (endo direction) at the C(sp2tC(sp2)bond shared
by the norbornene unit and the benzene ring are 2.7(3)" in 5
and 2.0(9)' in 6. These rather similar endo pyramidalizations
are thus indeed much smaller than observed in 2.
Received: April 14, 1989 [Z 3290 IE]
German version: Angew. Chem. fO1 (1989) 1298
CAS Registry numbers:
2, 2746-19-2; 5, 23537-81-7; 6 , 122144-55-2; neutrons, 12586-31-1
[l] a) P. von R. Schleyer, .
I
Am. Chem. SOC.89 (1967) 701; b) H. C. Brown,
P. Geohegan, ibid. 89 (1967) 1522; c) S . Inagaki, H. Fujimoto, K. Fukui,
ibid. 98 (1976) 4054; d) G. Wipff, K . Morokuma, Tetrahedron Lett. 21
(1980) 4445; e) R. Huisgen, P. H. J. Ooms, M. Mingin, N. L. Allinger, .
I
Am. Chem. SOC.102 (1980) 3951 ; f) A. A. Pinkerton, D. Schwarzenbach,
J. H. A. Stibbard, P. A. Carrupt, P. Vogel, ibid. 103 (1981) 2095; g) N. G.
Rondan, M. N. Paddon-Row, P. Caramella, K. N. Houk, ibid. 103 (1981)
2436 (Note that the out-of-plane bending angle chosen in that work differs
from K ; cf. also [12i]); h) U. Burkert, Angew. Chem. 93 (1981) 602; Angew.
Chem. Int. Ed. Engl. 20 (1981) 572; i) J. Spanget-Larsen, R. Gleiter, Tetrahedron Lrtt. 24 (1982) 2435.
(21 a) 0. Ermer. C.-D. Bodecker, H. Preut, Angen. Chem. 96 (1984) 57;
Angew. Chem. l n t . Ed. Engl. 23 (1984) 55. b) In [2a] the X-ray analysis of
7 is described,
a derivative of tetracyclo[6.2.1.13.6.02.7]dodeca-4,9-diene
which consists of t w o norbornene units fused across a single bond and
7
across the double bond such that on endo pyramidalization
the torsional strain is lowered around four C(sp2tC(sp3)
single bonds as compared to two in 1. In the light of the
present norbornene results it is therefore gratifying to note
that the out-of-plane bending angles x of about 16- 18" mea1240
$3 VCH
Verlag~gesellschaftmhH, 0.6940 Wemherm, 1989
which has two parallel double bonds in close proximity. A relatively large
endo pyramidalization of x = 8.1(2.1)0 was found for one of the two double bonds and ascribed to particularly strong nonbonding repulsions between the double bonds due to orbital symmetry. However. comparison
with the reliable neutron diffraction results on 2 now available indicates
that this influence of orbital symmetry is smaller than previously [2a]
believed. More definite conclusions would necessitate diffraction measurement also on the tetracyclododecadiene derivative.
OS70-0833/89/09O9-1240$02.50/0
Angew. Chem. Int.
Ed. Engi. 28 (1989) Nr. 9
[3] A relatively crude X-ray measurement was reported earlier for 2 (photographic intensity data, R = 0.086, hydrogen positions constructed on the
basis of a planar double bond but not refined): G. Filippini, C. M. Gramaccioli, C. Rovere, M. Simonetta. Acra CrystaNogr. 5 2 8 (1972) 2869. In
order to avoid unpleasant disturbances (e.g., disorder), we performed a
renewed X-ray measurement on 2 (four-circle diffractometer) before embarking upon neutron diffraction and obtained a well-refinable “clean”
structure free from any unusual features ( R = 0.038). Altogether we prepared six simple norbornene derivatives and tested their suitability for
neutron diffraction by means of complete X-ray analyses, until eventually
the anhydride 2 was selected.
[4] 0. Ermer, S. A. Mason, Acta Crystulhgr. 838 (1982) 2200.
[5] Thick orthorhombic plates o f 2 [6] were grown from acetone solution; m.p.
142 C. Space group P2,2,2,, Z = 4, u = 7.362(2). h = 7.968(2). c =
12.500(2) A. temperature 15.0(1) K. Neutron intensities of altogether 2991
reflections were measured at this low temperature on the four-circle diffractometer D9 of the Institut Laue-Langevin, Grenoble (focusing Be
monochromator, two-dimensional position-sensitive detector, E. =
0.X429 A, VmdX= 51“, crystal size ca. 3.7 x 3.6 x 1.3 mm3). After averaging.
2687 independent observed reflections with I > 0 remained, which were
fed into the least-squares refinements. Absorption corrections were applied with a correspondingcoefficient p = 1.47 cm-’ adoptingan incoherent scattering cross-section of 35 barns for hydrogen. The final R value
after anisotropic refinement of all atoms was 0.033 ( R , = 0.032, Zw(Afl2
minimized, H’ = l/u(F,,)’. extinction correction, nuclear scattering lengths
taken from a recent compilation of Sears [7]). The refined atomic nuclear
coordinates and anisotropic temperature factor coefficients are given in
Table 1. A list ofobserved and calculated neutron structure amplitudes has
been deposited [8].
Table 1. Fractional atomic nuclear coordinates ( x lo5) and anisotropic temperature factor coefficients (A* x lo4) of 2 at 15 K with estimated standard
deviations in parentheses. Temperature factor expression of the form:
exp[-2rt2(Ullh2aC2 . . . + 2U12hka*b*+ . . . )].
+
I
c1
cz
c3
c4
c5
C6
c7
C8
c9
01
02
03
U22
U11
U33
912
U13
H2
75474(39) -38177(27) 32069(19) 330(11)
64033(39) -34202(78) 57081(17) 324(11)
47726(32) 10106(26) 34000(18) 214(8)
37989(30) -10450(32) 33650(19) 152(7)
149(7)
189(7)
149(61
252(9)
245(8) 77(8) 5 0 ( 8 ) -34(6)
188(7) 11(7)
2(7) 8 7 ( 6 )
251(8) 38(6) -12(7) 20(6)
257(8) -53(7) -37(6) -78(7)
D. Craig. J. Am. Chem. Soc. 73 (1951) 4865.
V. F. Sears in K. Skold, D. L. Price (Eds.): Methods of Experimentui
Physics (Neutron Scattering), Vol. 23, Part A; Academic Press, Orlando,
FL. USA 1986.
Further details of the crystal structure investigations are available on request from the Fachinformationszentrum Karlsruhe, Gesellschaft fur wissenschaftlich-technische Information mbH, D-7514 Eggenstein-Leopoldshafen 2 (FRG), on quoting the depository number CSD-53752, the names
of the authors, and the journal citation.
a) 0. Ermer: Aspekte von Kruftfe/drrchnun.qen, Wolfgang Baur Verlag,
Munchen 1981 ;b) 0. Ermer, S. Lifson, J Am. Chem. Soc. 95 (1973) 4121 ;
c) 0. Ermer, Struct. Bonding (Berlin) 27 (1976) 161; d) 0. Ermer, Z .
Nuturforsch. 5 3 2 (1977) 837.
These two dihedral angles may easily be evaluated as the “improper”
torsion angles C4-C5-C6 ’ . . H5 and C1 . . . C5-C6-H6 (Fig. 1b) [9a-c].
a) W. L. Duax. M. D. Fronckowiak, J. F. Griffin. D. C. Rohrer in J. Jortner, B. Pullman (Eds.): Intramolecular Dynamics, Reidel, Dordrecht, Holland 1982. p. 505; b) 0. Ermer, J. D. Dunifz, I. Bernal, Acra CrystaNoRr.
B2Y (1973) 2278.
a) W. H. Watson, J. Galloy, P. D. Bartlett, A. A.M. Roof, J. Am. Chem.
Soc. 103 (1981) 2022; b) J.-P. Hagenbuch, P. Vogel, A. A. Pinkerton, D.
Schwarzenbach, Helv. Chim. Acta64(1981) 181X;c) R. Gleiter, J. SpangetLarsen. Teirahedron Lett. 24 (1982) 927; J. Spanget-Larsen, R. Gleiter,
Tetruhedron 39 (1983) 3345; d) L. A. Paquette, P. Charumilind, M. C.
Biihm. R. Gleiter, L. S. Bass, J. Clardy, J Am. Chem. Soc. 10s (1983) 31 36;
e) 0. Ermer. C.-D. Bodecker. Helv. Chim. Acra 66 (1983) 943; f~K. N.
Houk. N. G. Rondan. F. K. Brown, W. L. Jorgensen, J. D. Madura, D. C .
Spellmeyer, J Am. Chem. Sor. 105 (1983) 5980; g) F. S . Jorgensen, TerraAnKeM. Chem. I n l . Ed. Enj$ 28 (1989) Nr. 9
9 VCH
I
[H,C(CH,),NLi],, the First Cyclized
Lithium Amide Ladder: Synthesis and Structure
of Hexamethyleneimidolithium **
By Donald Barr, William Clegg, Susan M . Hodgson,
Glenn R. Lamming, Robert E. Mulvey, Andrew J. Scott,
Ronald Snaith,* and Dominic S . Wright
U23
46895(6)
46749 ( 5 )
35239(6)
29962(5)
35509(6)
H1
H5
H6
H7R
H78
hedron Lett. 25 (1983) 5289; h) C. A Johnson, J. Chem. Sot. Chem. Cummun. 1983, 1135; I) see also: W. H. Watson (Ed.): Stereochemistry und
Reaclii4v o/ Systems Containing K Electrons, Verlag Chemie lntl.. Deerfield Beach, FL, USA 1983.
[13] a) H. D. Rudolph, H. Dreizler, A. Jaeschke, P. Wendling, Z. Nururf. A 2 2
(1967) 940; b) D. R. Lide, D. E. Mann, J. Chem. Phys. 27(1957) 868; c) see
also: R. K. Harris, M. Thorley, J. Mu/. Speerrosc. 42 (1972) 407; T. Ogata.
A. P. Cox, D. L. Smith, P. L. Timms, Chem. Phys. Lerr. 26 (1974) 186.
[14] a) H. B. Biirgi, E. Shefter, Tetruhedron 31 (1975) 2976; bj R. A. Kydd,
Speclruchrm. Acta 2 7 A (1971) 2067; c) R. L. Arnett, B. L. Crawford. J.
Chem. Phys. t8(1950) 118; B. L. Crawford, J. E. Lancaster, R. G Inskeep.
ibid. 2t (1953) 678.
I151 5 : inonoclinic crystals, m.p. 113°C (from acetone); space group P2,/i,,
Z = 4, u = 5.896(1), h = 6.624(2), c = 24.688(16) A, /3 = 90.74(3); Q ~
= 1.297 g ~ m - Intensity
~ .
measurements at room temperature o n a fourcircle diffractometer(/.,, = 0.71069 A, VmAx = 27’); direct methods; refinement ( C . 0 anisotropic, H isotropic) including 1529 reflections with
Fo > 3 u(F,,); R = 0.046, R , = 0.047 [S]. 6 : triclinic crystals. m.p. 93°C
(from pentane); space group P i assumed, Z = 2 , u = 8.393(2), h =
9.535(4), c = 10.758(5)A, x = 78.14(3). /3 = 68.02(3). 7 = 71.95(2)’: Q ~
= 1.362 g cm-3; measuring conditions. structure solution, and refinement
as for 5 ; 1716 reflections with Fu 40(F,,); R = 0.080. R , = 0.079 [XI.
Lithium amides (amidolithiums), RR’NLi, are valuable
reagents in both organometallic and organic syntheses.“’ As
such, solutions of these reagents are usually either bought, or
prepared in situ by lithiation of amines, then used forthwith
in subsequent reactions.[’] More recently, however, a selection of these materials has been isolated and they have been
examined in their own right; in particular, structures have
been elucidated (in the crystal by X-ray
and in
solution by molecular mass measurements and NMR spectroscopyr4]), and attempts have been made to rationalize the
occurrence of such structures.[51Here we describe the synthesis, solid-state structure, and solution behavior of hexamethyleneimidolithium, 1. Most importantly, in the crystal 1 is
a hexamer (n = 6), and as such it is the first uncomplexed
lithium amide structure with n > 4 and the first which is not
a simple planar (NLi), ring.161Despite such novel features,
the structure of 1 is consistent with the prior rationalizations
of amidolithium structures: although the hexamer could be
viewed as arising from two vertically stacked trimeric rings,
analysis of N-Li bond lengths within it reveals that it is in
fact best seen as a ladder-type structure with sufficient N-Li
rings (six) that the ladder can turn back on itself and cyclize.
The building blocks for lithium amide structures appear to
be planar (NLi), rings (Scheme 1 a, shown for n = 2). In the
[*] Dr. R. SnaIth, Dr. D. Barr, D. S. Wright
University Chemical Laboratory, Lensfield Road
Cambridge, CB2 1 EW (UK)
Dr. W. Clegg, S. M. Hodgson, G. R. Lamming, A. J. Scott
Department of Chemistry, The University
Newcastle upon Tyne, NEl7RU (UK)
Dr. R. E. Mulvey
Department of Pure and Applied Chemistry, University of Strathclyde
Glasgow, G1 IXL (UK)
[**I This work was supported by the U.K. Science and Engineering Research
Council.
Verla~gesellschaft
mhH, 0-6940 Weinheim. 198Y
0570-0833i89io9~~-124/
$02.S0/0
1241
~
,
~
~
,
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