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Elimination of the Barrier to Cope Rearrangement in Semibullvalene by Li+ Complexation.

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5”’” H,SO, to pH 2. The white solid with a pink luster. which precipitated
immediately &as filtered off at 10 mhar. washed three times with 25 m L portions of ice water. three times with 25 mL portions of ii-hexane, and dried for
3 h at 5 x
mhar. Yield. 0.25 g (1.2 mmol, 57%). M.p. 93 C (decomp.).
‘ H N M R [(D,C,),O]: 6 =7.2-7.6 (m, 10H). 4.35 (s. OHO). IR(film).
Y = 3400cm-’ (hydrogen bonds). 3000cm-’ (C-H), 1650cm-’ ( C = N).
1550, 1450cm-I ( C = C ) . 1300cm-’ ( C = N). Single crystal growth: ucr-Nitrodiphenylmethane. precipitated by acidification of the nitronate. wits dissolved in diethyl ether (20 mL) and dried at 273 K hy stirring for 2 h with
Na,SO,. After filtration. the ether solution was concentrated at 273 K by slow
evaporation until colorless plates crystallized. which have to be stored at 0 C
under argon and aprotic (tit+ . i
1 ppm) mhexane because of their temperatureand moisture-sensitivity.
pm, anglesin ): E,,,,, = - 487.30736a.u.. C-N 129. N-0 129, N - 0136.
126. C-N-0
O ( H ) . . . O 265, 0 - H 97. H . . . O 167, C-N-0
0-N- 0 117. O(H)O 173. Charge distribution: C - 0.12, N +0.44,
0 -0.61. O(H) -0.46, H(0) +0.38. The u/:r-nitrodiphenylmethane dimer
has been calculated from the data by PM3 (J. J. Stewart. J. Comprr. Chem.
1989, /O,209.221 :version MOPAC 6). and the cooperatwe effect has been
estimated by turning one molecular half around one of the O(H)O hydrogen
bonds [9h]; for AHr values. see text.
(91 Cf. a ) H. Bock, T. Vaupel. C. Nither. K . Rupperf. Z. Havlas. Ange~l-.Clzem.
1992. 104. 348: A i i g w . Chm. h.
Ed. EngI. 1992, 31. 299 and references
therein; b) H. Bock, W. Seitz. Z. Havlas. J. W. Bats, ihirl. 1993, 105.410 and
1993. 32.41 1 and references therein.
Received. May 27. 1993 [Z 6110 I€]
German version: A n p w . U w i i i . 1993. 1US. 1826
[ l ] H. Bock. Jrihrh. Dtrch. Ah-or/. A’urrirfor.c/~/7.
LcwiioMiiiu 1993, 3K. 221 -233.
[2] K. Lammertsma. B. V. Prasad ( J . A m . C/iciii. So/:. 1993. 115. 2348) summarize, in addition. the chemical and biological importance of nitronic acids.
both the X-ray and the neutron diffraction structures of solid H,CNO,, and
its microwave gas-phase structure. and the gas-phase detection of nwnitromethane (cf. H. Egsgaard, L. Carlsen, H. Florencio. T. Drewello. H .
Schwarz. BCY. Buii.scwges f / i J .s. C/rtvii. 1989. 93. 76). The experimentally
determined rotational barrier in nitromethane is heloh 25 Jmol-’ (!); for
the harrier between E and Z conformers of uci-nitromethane. 28 kJ m o l ~
are calculated. For a b i n i t i ~calculations cf. also M. L. McKee. J. P k n .
C’/irv?i. 1986. 90, 369 and references therein.
[ 3 ] E. W. Colvin. A. K. Beck, B. Bastani. D. Seebach. Y. Kai. J. D. Dunitz,
Hrh. Ciiiiii. . 4 m 1980. 63. 697. Selected structural data for the diphenyl
derivative (pm und ): C = N 131. N = O 127. N - 0 140, C = N = O 130.
C = N - 0 115. O = N - 0 115. NOSi 139. For comparison. the gas-phase
Structure of ?-nitromethane (F. Shishkov. N. I . Sadova. L V. Vilkev. Y. A.
Panktushev. Zh. Srrrrkr. K k i i i i . / r n g / . ) 1983, 29. 189) is quoted: C N 152.
N = O 123. C N = O 117. C = N = O 125. i;,(ON-CC) 18.
141 R. Dienelt. Master‘s thesis. University of Frankfurt. 1993. oc,i-Nitrodiphenylmethane (“diphenylmethyliiitronic acid”) was first prepared
1893 hq acidification of the aqueous solution of the potassium salt with
diluted sulfuric acid ( M . Konowalow. Bw. D i d . C/itwi. G e . 1896. 29.
2193: cf also A. Hantzsch. 0. W. Schulze ihiri. 1896.29, 863). For a survey
of other isolated rici-nitro compounds cf. D. Dopp. H. Diipp in Houhen!
Wc$ Mi,r/iu&ii r / r v Orpnisdieii C/iw?ic.4th edition. Vol. X. Thieme, Stuttgart, 1990, p. 780ff and references therein.
151 Crystal structure determinations of C, , H , , N O , ) > . M , = 426.4: crystal dimensions 0.41 x 0.66 x 0.51 mm. f = 163 2 5 K and 238 _+ 2 K [values in
b r a c k e t s ] : ~=1140.2(1)[1144.8(1)].h =739.84(3)[743.54(3)],c’=1273.1(1)
[1277.3(1)] pm. /j = 94.969(4) [94.815(4)] . v = 1069.6(6)x 10” [1083.4(6)
x 10”]pm’.Z= 4 . ~ =1.324[1.307]gcm~’.~i(Cu,,)
~ ~ , ~ ~
=154.2pm.monoclinic. spacegroup P2, 17 (no. 14). Enrdf-NoniusCAD4 four-circlediffractometr. 2282 120971 measured reflections within 1 < 0 70 1651. of which
1884 [1704] are independent with I > 0.01. Structure solutions by direct
methods (SHELXS). N = 1884 [i704]. N P = 189 [ I ~ o ] R
. = 0.045 [0.047].
R,, = 0.041. 1:) = 1 uz(&J,residual electron density f0.20:-0.19 [+0.16,
-0.191 e,A-’. G F = 4.639[4.335](/1= 0.00). C.N.Ocentersanisotropica11s; refined. H isotropically refined. The H-bonded dimer is positioned
around a crystallographic symmetry center. The differences observed at the
two temperatures are the change of the dihedral angle of the phenyl rings
relative t o t h e p l a n e c - N O l ( 0 2 ) by0.6 andofthedistancesO-fH)...Oby
7 pm: only the low temperature structure determination is discussed. Further details of the crystal Structure investigation are available from the
Fachinformationszentrum Karlsi-uhe. Gesellschaft fur wissenschaftlichtechnische Information mbH. D-76344 Eggenstein-Leopoldshafen ( F R G )
on quoting the deposit number CSD-57805. the authors and the journal
[h] Cf. :I) S. K . Burley. G A. Persko. Scicncc 1985. 229. 23: C. A. Hunter, J.
Singh, J. M. Thornton. J. M o l . Biol. 1991. ,718, 837: h) G. R. Desiraju. A.
Gavezzotti. Acru C i - i x u h x r . Sccr. B. 1989. 45. 473.
[7] Cf.. for example. A. F. Wells. S/rirc/rrra/ hiorguiiic C/?~w?i.wt.,
Oxford, 1987. p. 367ff and references therein. o r J. Emsley. Chci~i.Soc.
R r r . 1980. 9. 41 and references therein. Hydrogen bonds O ( H ) - . . O
with interoxygen distances between 240 and 260 pm are considered
strong: examples ( N - 0 ... H . 0-N), in Ni(1t)diacetyldioxime. 241 pm:
(RO,P(O.-.H---O),P(OR,).249 pm. In contrast. the 0 .O distance in
the benzoic acid dimer exhibiting a planar eight-membered ring H,C,)CO(O...H..-O,)OC-C,H,areonly263
pmlong(G. Bruno, L. Randaccio,
Arru Cri.stullogr. Sect. B. 1980. 36. 171 I ) .
181 The a b initio calculations for nitromethane have been performed with the
program GAMESS (M. W Schmidt. J. A. Boatz. K. K. Baldridge, S.
Koseki. M S. Gordon, S. T. Elbert. B. Lam. QCPEBullerB? 1987, 7. 115).
All miiiim.i and saddle points are confirmed bq second derivatives and
exhibit none or only one negative eigenvalue of the Hessian matrix. Selected
results for H-bonded, dimeric mi-nitromethane: structure (bond lenpths in
Elimination of the Barrier to Cope Rearrangement
in Semibullvalene by Li’ Complexation**
By Huijun Jiao and Paul von Rag& Schleyer”
Substituents that reduce or eliminate the already quite
small barrier to valence isomerization (Cope rearrangement)
in semibullvalene have been sought in many theoretical“]
and experimental[’I studies. Based on extended Huckel calculations, Hoffmann et al.[’”] predicted that the barrier can be
lowered by n-donor substituents at C1,5 and x-acceptors at
C2,4.6.8. Similar conclusions were reached both by Dewar
et al.”b,cl and by Dannenberg et al.,[ldl who employed
semiempirical methods (MIND0/2, AMI, and MNDO-CI).
The transition state for the rearrangement might even be
stabilized by substituents to such an extent that the
“bishomoaromatic” symmetric structure 1b would become
more stable than the classical structure 1 a !
The isomerization barriers of many substituted semibullvalenes have now been measured using dynamic I3C N M R
spectroscopy,’21 particularly by Quast et al. The lowest measured barrier is approximately 4.5 kcal mol- l for l ,5dimethylsemibullvalene, but the barrier actually increases
with some substituents, for example, to roughly I0 k c a l m o l ~
for 3,7-dicyano-2,6-dimethoxy-l.5-dimethylsemibullvalene.
We now describe a new strategy which might eliminate the
barrier to the Cope rearrangement in semibullvalene. According to ab initio calculations, complexation of a Li’ ion
stabilizes 1 b more effectively than 1 a. The evidence from our
computations establishes that the transition state 1 b does
have bishomoaromatic character.
Geometries were fully optimized first at the RHF/6-31G*
level using the Gaussian 92 program package.’31The stationary points were characterized as minima or saddle points by
calculation of the vibrational frequencies. The geometries
[*IProf. Dr. P. von R Schleyer. DipLChem. H. Jiao
lnstitut fur Orgdnische Chemie der Universitit Erlangen-Nurnberg
Henkestrasse 42, D-91054 Erlangen ( F R G )
Telefax: Int. code + (9131)85-9132
This work was supported by the Fonds der Chemischen Industrie. the
Convex Computer Corporation. and the Deutsche Forschungsgemeinschaft. We also thank Prof. Dr. H. Quast. Wurzhurg(FRG). for his helpful
suggestions and the Shanxi Normal University (People’s Republic of China) for a scholarship (to H. J.).
were then refined at the correlated RMP2(fu11)/6-3 1G * 14]
level. Our final relative energies (Table 1) were computed
at RMP2(fu11)/6-31G* + ZPE (i.e. with zero-point energy
correction, RHF/6-31G*, scaled by 0.89).13] Atomic charges
Tdbk 1 . Calculated total energies [a.u.]. zero-point energies (ZPE) [kcalmol- '1.
the number and values of the imaginary frequencies [cm-'1, and the activation
enthalpy AF/ * [ k c a l m o l ~'I of semibullvalene I a. the transition structure for
the Cope rearrangement 1 b. and their Li'complexes 2 a and 2b. respectively.
79.7'(1. -606)
82.1 '(0)
X1.3,'(1. -427)
- 315.89057
* [b]
worthy. The Wiberg bond orders for 1 a (Table 3) indicate a
largely localized structure, except for the C2-C8 bond.
Transition structure 1 b (C,,, Fig. 1 bottom) has one imaginary frequency ( - 606 cm- '). The calculated activation
enthalpies, 5.2 kcalmolat RMP2(fu11)/6-31G* and
4.1 kcdl mol-' with the RHF/6-31G* zero-point energy correction, agree well with the measured value of
4.8 0.2 kcalmol- .f21 The separation of 2.036 8, and bond
order of 0.368 for C2-C8 and C4-C6 in 1 b show that this
transition structure may indeed be designated as homoconjugdted.['] The bond orders of 1.453 and bond lengths of
1.391 8, for the delocalized, allyl-like bonds in I b are close to
those in the ally1 radical. anion, and cation as well as in
benzene and many homoaromatic systems." 3i
-0.7 [c]
[a] RHF:6-31G* value, scaled by 0.89; in parentheses: number and value
of imaginary frequencies. [b] Relative energies [RMP2(full)j6-31G* ZPE
( R H F 6-31G*)]. [c] See ref. [14].
and bond orders, derived from the natural population analysis
of Reed et al.,"] characterize the ground and transition
states (Tables 2 and 3). The chemical shifts of Li' in 2a and
2 b were computed using the I G L O (Individual Gauge for
Localized Orbitals) methodr6]employing the DZ basis sets[']
and the RMP2(full)/6-3lG* geometries.
Table 1.NPA charges [a] (see text).
- 0.264
- 0.263
- 0.393
- 0.224
0.21 7
+ 0.947
pa] RMP2(full)j6-31G* geometries were used
Fig. 1. The RMP2(fu11)/6-31G* optimized structures for the ground state of
semibullvalene 1 a and the rearrangement transition state I b.
Table 3 Wiberg bond indexes [a] (see text).
Cl -c2
C l -C5
0 973
0 986
The effects of Li' complexation of 1 a and 1 b are remarkable! Although the C,-symmetric structure 2 a (MP2(fu11)/631G*) (Fig. 2 top) is a minimum, the C,, structure 2b (Fig. 2
bottom) is only 0.1 kcalmol-' less stable at this level! The
C2-C8 bond length in 2 a increases to 1.711 (from 1.595 8,
in 1 a); the corresponding bond orders, 0.790 in 2 a and 0.875
in I a, reflect this difference. The changes due to Li complexation in the lengths of the other C - C bonds involved in
the rearrangement of 1 a are smaller, but also point to the
more delocalized character of the electron system of 2a. C3,7
and C2,8 are more negatively charged and C4.6 more positive in 2 a than in 1 a. Li' complexation polarizes 2 a and
results in a partially delocalized structure.
The C - C bond orders in the Li+ complexed transition
structure 2 b are nearly the same as in 1 b. The Li+ ion has a
charge of 0.959 in 2a and 0.947 in 2b. The binding of Li' is
largely electrostatic and involves polarization of the rr-electron systems. The Li complexation energies for 2a and 2 b,
-47.6 and - 52.7 kcalmol- ', respectively, are even somewhat larger than that for benzene ( - 44.8 kcal mol RMPZ(fu11)/6-31G*). (This is due to the shorter Li-C3,7
[a] RMP2(fu11)16-31G* geometries were used.
Possible mechanisms of the prototypical degenerate Cope
rearrangement in 1,5-hexadiene have been investigated experimentally[*. *] and theoretically.". 9 - " ] Because of the
constrictions of the polycyclic structure, the Cope rearrangement in semibullvalene must take place via a boatlike transition state. This is facilitated by the proximity of C4 and C6.
The RMP2(fu11)/6-31 G * optimized structural parameters
for f,semibullvalene 1 a are shown in Figure 1. The optimized bond lengths agree with those obtained by electron
diffraction (values given in parentheses in Fig. 1, top["]).
The elongation of the C2-C8 bond (to ca. 1.60 A) is note-
distances in 2 a and 2 b than the corresponding distance in the
Li'-benzene complex (2.371 A)).
Note that Li+ complexes the transition state l b to a
greater extent than semibullvalene 1 a. This essentially eliminates the activation barrier even before correction for the
zero-point energies. When this ZPE correction is applied, the
computed energy for 2 b at 0 K is 0.7 kcalmol-' lower than
for 2a!r'41 The bishomoaromatic 2 b is indicated to be more
stable than 2 a (Table 1). This result, which has long been
sought by the introduction of substituents, is achieved by
Li+ complexation (at least at the level of theory employed
anion is much less, and the antiaromatic cyclobutadiene
produces a downfield shift (Table 4).
Table 4. Absolute lithium shielding constants (ujand chem~calshifts (6, relative
to LI ' d . 0 ) calculated by the IGLO method (IGLO/DZ!/RMP2(fu11)!6-3lG*)
Point group
L i t -C,H, (cyclobutadiene)
L i t -C,H, (allyl anion)
Lit -C,H, (cu-1.3-butadiene)
Lit - H C = C H
L i t -H,C=CH2
Li' C,HC ( C p - )
L i t C,H, (benzene)
C2 1
95 4
92 3
98 8
96 9
97 2
97 3
101 1
102 3
102 8
106 2
106 2
+ 3.1
- 6.9 [b]
- 7.4
- 10.8
- 10.8 [b]
[a] For additional data see ref. [Sc]. [b] See ref. [15]
The very low activation enthalpy of 4.1 kcalmol-'
[RMP2(fu11)/6-31G* ZPE(RHF/6-31G*)] for the degenerate Cope rearrangement of semibullvalene can be eliminated by Li complexation. The C,, delocalized bishomoaromatic structure 2 b is computed to be slightly more stable
than the unsymmetrical form 2a, but even the latter shows
pronounced delocalization. The Li ' ion stabilizes the more
polarizable C,, transition state to a greater extent than the
C,. We find such electrostatic acceleration of electrocyclic
process to be general, and will report further examples subsequently. The aromaticity of 2 b is reflected by its geometry
and population analysis, as well as the upfield chemical shift
of Li+ due to ring current effects. The partial delocalization
of 2 a also is apparent from the same criteria.
Received: June 2.1993 [Z61 19 I€]
German version: Angcii. Ciiem. 1993, 105. 1830
Fig. 2. The RMP2(fullj,6-31G*optimized structures for the Li' complexes 2a
and 2 b.
The presence of a Lif ion in 2 a and 2 b affords a means
of assessing the "aromaticity" of these species. Normally,
lithium N M R chemical shifts have a small range (6 = ca. i2
relative to the standard) S(Li') used as a reference in calculations). The most conspicuous experimental exceptions
have been noted for lithium cyclopentadienide and its
derivatives, as well as Li ' x-complexed aromatic compounds." The large shieldings observed are attributed to
ring current effects (although this conclusion may be premature). IGLO chemical shift computations provide a convenient means of assessing the influence of various complexed
x systems on Li N M R chemical shifts. It does appear from
the data gathered in Table 4 that b(Li+) can be employed as
an aromaticity probe.
The homoaromaticity of 2 b and the partial homoaromaticity in 2 a are indicated by the upfield chemical
shifts, 6(Lif) = - 10.8 and - 5.7, respectively [a(Li+) = 95.4,
u(Li'[2b] =106.2 and a(Li+[2a]) =101.1, IGLO/DZ//
RMP2(fu11)/6-31G*]. For comparison, the computed Li
NMR shifts are 6 = - 7.4 for the C,, Li' -benzene complex,
-6.9 for cyclopentadienyllithium, and - 10.8 for the biscyclopentadienyllithium anion." In contrast, the influence on
S(Li+) by complexation with ethene, ethyne, and even allyl
VCH Verlugsgr.rell.schu// mbH, 0-69451 Wcwdleirrt, 1993
[ l ] a ) R. Hoffinann. W. D. Stohrer. J A m . Clirm. S o ( . 1971, 93, 6941-6948;
b) M. J. S. Dewar. D. H Lo, ibid 1971.93,7201-7207; c) M. J. S. Dewar,
C. Jie. E,truhtr/ron 1988, 44, 1351 -1358; d) L. S. Miller, K. Grohmdnn.
J. J. Dannenberg.J. A m . Clletn. Soc 1983. fO5.6862-6865; e) K. N. Houk,
Y. Li. S. D. Evdnseck. Angeii'. Chmm. 1992. 104, 711 -739; Angrw. Clmeni.
In/. Ed. Engt. 1992.31.682 -708; f) K . Bergmann. S. Gortler, J. Manz, H.
Quast. J Am. Chein. Soc. 1993, 115. 1490-1495.
121 a ) A. K. Cheng. F. A. L. Anet, J. Mioduski. J. Meinwald. J A m . Clieni.
Soc 1974. 96. 2887-2891; b ) D . Moskau, R Aydin. W. Leber. H.
Gunther. H. Quast. H. D. Martin, K. Hassenriick, L.S. Miller, K .
Grohmann. Chem. Ber. 1989. 122. 925-931; c) H. Quast, J. Christ, E.-M.
Peters. K. Peters. H. G. von Schnering. ihid. 1985. I I K . 1154-1175; d ) H.
Quast, C. A Klaubert, L. M. Jackman, A. 3. Freyer. ibid 1988. /21,10811086; e) H. Quast. Y. Gorlach. E.-M. Peters, K. Peters. H. von Schnenng.
L M. Jackman. G. Ibar, A. J. Freyer, ibrd. 1986. 119, 1801-1835; f ) H.
Quast, A. Mayer, E.-M. Peters. K. Peters, H . G von Schnering. ibid. 1989.
122,1291 1306: g) R. Iyengar, R. Pina. K. Grohmann, J A m . Chem. So?.
1988, 110. 2643-2644
[3] GAUSSIAN 92, Revision B. M. 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. Rcbb, 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, S. A. Pople. Gaussian Inc.. Pittsburgh.
PA, USA, 1992.
[4] W. J. Hehre. L.Radom, P. von R. Schleyer, J. A Pcple, A h Iniirn Mo/ecular
Or-hito/ Tlmmr:,.,Wiley. New York, 1986.
[S] a) A. E. Reed. L. A. Curtiss, F. Weinhold. Clirm. Rrv. 1988,88,899-962:
b) A. E. Reed. P. von R. Schleyer, J A m . Chew. Soc 1990. 112, 14341445.
[6] a) W. Kutzelnigg. h J C ~ I P 1980,
I ~ I . 19, 193; b) M Schindler. W. Kutzelnigg. J. C h i . P b w 1982. 76, 1910: c) Review: W. Kutzelnigg, U. Fleischer. M. Schindler. N M R ' Busic Prmc. Prog. 1990. 23. 165.
0570-0833193:1212-1762 $ IO.O/J+ ,2510
Angew. Clmem. Int. Ed. EngI. 1993. 32. No. 12
[7] S. H iizinaga, Guirs.sron Br/.\i.\ Sim for Moli~cuiur Culculuffons,Elsevier.
Amsterdam. 1984.
[8] a ) R. P. Lutz. Chern. R r y . 1984, 84. 205-247; h) W. von E. Doering. V. G.
Toscano. G . H. Beasley. ?iWulredron 1971. 27, 5299-5306.
191 M . J. S. Dewar. C.J. Jie. J. A m . Chon. Sot.. 1987. 109. 5893-5900.
[lo] M. Dupuis. C. Murray, E. R. Davidson, J. Ant. Clren?. Soc. 1991. 113.
9756- 9756.
11 11 For ex;impie: N . S. Isadcs. Ph?:sicu/Orgunrc C/irni~str.~',
Wiley. New York,
[ I Z ] a ) Y.C. Wang. H. S. Bauer. J. Aii7. Chern. Si~t..1972.94.5651 -5657; h) H.
Quast. J Carlsen. R. Janiak. E.-M. Peters. K . Peters. H. G. von Schnering,
C/lPfJ?.BW. 1992. 125, 955-968.
1131 a ) G. A. Okih. G. Asensio, H. Mayer, P. von R. Schleyer. J Ani, Cliein.
So<. 1978. 100. 4347-4352: b) P. von R. Schleyer. ihid. 1985. 107. 47934794: cj P. \on R. Schleyer, T. W. Bentley. W. Koch. A. J. Kos. H. Schwarz.
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[14] This means that there is only one set of vibrational states for the doublewell potential. and that the lowest state is energetically above the barrier.
We thank a referee for this clarification.
[15] L. A. Paquette. W. Bauer. M. R. Sivik. M. Buhl. M. Feigel, P. von R.
Schleyer. J .4i11. Cirenr. Soi..1990, 112. 8776-8789, and references therein.
tion state geometries of 1,3,5-heptatriene, 1 and 2, respectively (Fig. I), were fully optimized first at the RHF/6-31G*
and then at the correlated RMP2(fu11)/6-31G* levels employing the Gaussian 92 program package.['] RHF/6-31G *
1, c-
A Detailed Theoretical Analysis of the
1,7-Sigmatropic Hydrogen Shift: The Mobius
Character of the Eight-Electron Transition
Structure **
2, c,
By Haijun Jiao and Paul von RaguP Schleyer *
While the [I ,5]sigmatropic hydrogen shift in a polyene is,
according to the Woodward - Hoffmann rules, a thermally
favored suprafacial isomerization involving six electrons, the
eight-electron [1+7]hydrogen shift is an antarafacial sigmatropic rearrangement with a Mobius topology.['' The first
observed [1,7] H shift was the rearrangement of calciferol to
precalciferol.['' Theoretical studies of the [1,7] H shift'' b, 31
have been less detailed than those for the [1,5] H shift.'' b.4,51
The first a b initio calculation of the [I ,7] H shift was carried
out by B. Hess et al.[31with the 3-21G basis set. The transision state was found to have C, symmetry, but the computed
activation energy of 44 kcalmol- ' was much higher than the
experimentally observed values (ca. 15-20 kcalmol- ').I6]
A direct kinetic investigation of the [1,7] H shift in the
parent 1.3.5-heptatriene 1 was reported only recently by
Y. N. Bubnov et aI.I7l As the process is too slow to be measured by N M R line shape analysis, the rates of site exchange
in a deuterium-labeled compound were determined by 'H
N M R spectroscopy over a temperature range of 75- 116°C.
The rate constant was described by the Arrhenius equation:
k , = 1.6 x lo8 expi-(20.8
0.7) kcalmol-'/RTJ s - ' . The
Russian group noted that both the activation energy and the
frequency factor were lower than those measured for [1,5] H
shifts ( E , = 24.3 _+ 0.5 kcalmol-' and A = 1.3 x lo1' s - '
for cyclopentadiene; E, = 35.6 i 0.5 kcalmol-i and A =
2.8 x 10'' for 1,3-pentadiene).'81
We now provide a detailed theoretical analysis of the [1,7]
H shift prototype in 1.33-heptatriene 1. Ground and transi[*I Prof Dr. P. von R. Schleyer. Dipl.-Chem. H. Jiao
Institut fur Organische Chemie
der IJniversitlt Erlangen-Nurnberg
Henkestrasse 42. D-91054 Erlangen ( F R G )
Telefax Int. code + (9131j85-9132
This work was supported by the Fonds der Deutschen Industrie. the
Deutsche Forschungsgemeinschaft, and the Convex Computer Corporation. We also thank Prof. Dr. W. Kutzelnigg. and Dr. U . Fleischer,
Bochum (FRG). and Dr. A. Dorigo, Erlangen. for suggestions and helpful
discussions. H. J. thanks the Shanxi Normal University (People's Republic
of China) for a scholarship.
1111. E d EngI. 1993, 32. N o . 17
Fig. 1. The RMPZ(fu11)/6-31G* optimized ground state structure of (Z.Z)1.3.5-heptatriene (1. top) and the 1.7 H shift transition structure 2 (bottom).
frequency calculations established the nature of each stationary point and provided the thermochemical data, for example, the thermal energies ETh (including zero-point energy,
vibrational, transitional, and rotational energies) and entropies. The final activation enthalpy ( A H ' ) and the activation energy (E, = AH* + RT)"OI were computed at the
RMP2(fu11)/6-31 G * level by addition of AETh (RHF/631G*) scaled by 0.89 at 400 KI"] (Table 1).
Fable I . Experimental [7] and calculated [a] activation parameters for the [1,7] H
shift in (Z,Z)-1,3-5-heptatriene (1) at 400 K. and for the [1.5] H shift in cyclopentadiene (CPD) and 1.3-pentadiene (PD) (measured values in parentheses).
E, [kcal mol-'1 [bl
A S * Id]
A [cl
19.7 (20.8 f 0.7) 97.7 x 10' ( 1 . 6 ~lo8)
-13.4 (-21.6)
1.7 shift in I
1 3 shift in C P D [el 25.2 (24.3 f 0.5) 7.9 x 10" (1.3 x l o L 2 ) -0.2 (-3.3)
1 , 5 s h i f t i n P D [ e ] 35.4(36.3+ 10") 1 . 6 ~ 1 0 '( ~
2 . 8 ~ 1 0 " ) -8.4(-7.1)
[a] Total energies [a.u.l for the ground state (1). -271.76233; the transition state
(2). -271.72672. and for the isomer with two truns double bonds, -271.76775
RMP2(fu11)/6-31GY).These values were adjusted by the RHF/6-31G* zero-point
energies, scaled by 0.89 (88.4 and 86.5 kcalmol- I for 1 and 2. respectively). and by
RT. [c] Frequency factor [ s '1.
thermal energies a t 400 K [lo]. [b] €, = AH *
[d] The computed absolute entropies [calmol ' K '1 are 97.3 for 1 and 83.9 for 2
at 400 K. [el See ref. [5](at the same level of theory) and ref. [XI.
The optimized (RMP2(fu11)/6-31 G*) transition structure 2
(one imaginary frequency, -2200 cm-') is helical (C, symmetry, Fig. 1 bottom).[31The C, axis is defined by the migrating hydrogen atom, H,, and the C 4 carbon atom. The C1C7 distance is 2.600 A, and the W,-C1 and H,-C7 bond
lengths are 1.352 A. In accord with Woodward-Hoffmann
rules for such an allowed process, the transition structure
t> V C H Verl~rgsg~~se//.schuit
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elimination, semibullvalen, rearrangements, cope, complexation, barriers
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