Hyperconjugation and inductive effects, rather than homoaromaticity, are responsible for the stabilization of the title anion in the gas phase; interaction of the double bond with the Li+ gegenion in the endo geometry contributes additionally in solution

Chandrasekhar, Jayaraman

Kaufmann, Elmar

Kos, Alexander J.

Mayr, Herbert

Schleyer, Paul v. R.

English

Open Access LMU

J .  CHEM. SOC., CHEM. COMMUN., 1986 1583 Stabilization of the Alleged 'Bishomoaromatic' Bicyclo[3.2.l]octa-2,6-dienyl Anion by Counterion Interactions and by Hyperconjugation Paul von Rague Schleyer,** Elmar Kaufmann,a Alexander J. Kos,b Herbert Mayr,c and Jayaraman Chandrasekhard a lnstitut fur Organische Chemie der Friedrich-Alexander-Universitat Erlangen-Nurnberg, Henkestrasse 42, 0-8520 Erlangen, Federal Republic of Germany of Germany Molecular Design MDL AG, Wallstrasse 8, CH-4002 Basel, Switzerland lnstitut fur Chemie, Medizinische Universitat zu Lubeck, Ratzeburger Allee 160, 0-2400 Lubeck I, Federal Republic Department of Organic Chemistry, Indian Institute of Science, Bangalore 560 012, India Hyperconjugation and inductive effects, rather than homoaromaticity, are responsible for the stabilization of the title anion in the gas phase; interaction of the double bond with the Li+ gegenion in the endo geometry contributes additionally in solution. Does the bicyclo[3.2.l]octa-2,6-dienyl anion (1) owe its enhanced stability1 [equation (l)] to homoaromaticity?2 While the symmetry in this six-n-system is ideal for interaction between the olefin and ally1 anion moieties,2 the resulting stabilization may be negligible because the two fragments are too far apart (poor orbital overlap).3 The challenge of qualitative1.2 by quantitative theory3 has led to many further investigations and current debate. l b ~ 4 - - 8  The principal new experimental results said to favour homoconjugation are energetic comparisons with model compounds in the gas phaselc and in solution,lb as well as some5a,c of the analyses of n.m.r. data.5 We contest these interpretations by considering other factors which can con- tribute to the stabilization of (1): hyperconjugation and gegenion interactions. The effects of hyperconjugation have A E  = -4.2 kcal/mol (STO-3G)' A E  = +0.6 kcal/mol (MNDO)' A G  = -9.5 2 2.0 kcal/mol (gas phase)*c AG = -12.2 kcal/mol (solution, Cs sa1t)lb not been assessed adequately in the model systems studied in the gas phase,lc while gegenion interactions have not been H H  Li A Hf( M NDO)/ -6.3 kcal/mol 33.6 Li (6) 0.2 21. 2 AHf(MNOO)/ -3.1 kcal /mol 33.6 ( 7 )  0.2 29.0 1584 J. CHEM. SOC., CHEM. COMMUN., 1986 considered in solution phase studies. lb,5 We now provide evidence for the former by means of ab initio calculations on a model system and for the latter through MNDO calculations on the lithium salt of (1). This lithium compound has been characterized recently by an X-ray study,6 but no firm conclusions regarding the electronic structure could be drawn. Analysis of its wave function as well as that of (1) at the ab initio CASSCF level led to the conclusion that homoaromatic- ity is unimportant in this system? However, neither the magnitude of the metal-olefin interaction nor the gegenion effect on the relative acidities were evaluated, e.g. by means of equations (1)-(3). In the present paper we calculate the quantities using the MNDO method,g which has been employed successfully for interpreting and predicting numer- ous organolithium structures.10 Since C(sp2) atoms are more electronegative than C(sp3), the allyl anion system in (1) will be stabilized, relative to (2), by the double bond. Both hyperconjugation'' with the more electron-accepting C( l)-C(7)(sp*) and C(5)-C(6)(sp2) bonds and inductive effects are responsible.3 These influences are demonstrated nicely by model ab initio calculations on the homoallyl anion (3) held in a conformation which precludes homo conjugation but favours negative h ypercon jugation. At the 4-31 +G//3-21G level which provides accurate proton affinities,l2 (3) is calculated to be stabilized relative to the n-butyl anion by 6.6 kcal/mol (1 cal = 4.184 J). The orientations of the carbanion centres in (1) are also favourable for hyperconjugation with the p-C(sp3)-C(sp2) bonds, but this is not true in (4), one of the model systems examined by Lee and Squires;'c (4) can benefit only from the inductive effect of the double bond. Because of unfavourable strain and repul- sive electronic effects, (5)lc may not be satisfactory as a model for (1) either. Another factor which contributes in solution is revealed by MNDO calculations: the more favourable gegenion interac- tions in (1) over those in (2). Bicyclo[3.2.l]octa-2,6-dienyl- lithium is predicted to be most stable in the endo form ( 6 ) ,  since this benefits from JL chelation of Li+ by the double bond. The MNDO structure of ( 6 )  (Figure 1) shows overall agreement with the X-ray structure of ( 6 )  solvated by N,N,N',N'-tetramethylethylenediamine (TMEDA) ,6 and is remarkably close to the CASSCF minimal basis set geometry of the unsolvated compound.7 Although the calculated MNDO bond lengths are somewhat too long, the distortions of the dihedral angles involving the ring planes, commented on by Kohler et a1.,6 are predicted rather well. Evidence for bishomoaromaticity in the X-ray structure can therefore be examined critically using our computational model. Three structural features are of interest: the CC bond lengths in the allyl anion moiety and in the olefinic fragment, and the separation between these units. The X-ray b p d  lengths in the allyl fragment of ( 6 )  are quite normal (1.39 A)13 showing no evidence for homoconjug$on. The slight length- ening of the C(6)-C(7) bond (1.354 A) is also similar to the elongation (0.01--0.02 A) calculated for the C=C bond in ethylene, complexed with Li+ or RLi.14 We assert that tke experimental C(2)-C(7) and C(4)-C(6) separations (2.37 A) are still too large for effective overlap between the interacting fragments. While Lindh et a1.7 concur with us, Brown et a1.8 have argued on the basis of STO-3G calculations that orbital energies and charge distributions of isolated ethylene and allyl anion units, held at such distances, indicate significant interaction, but the effects on the total energies are destabiliz- ing. A more direct way to quantify the electronic effects in these systems is to consider the relative proton affinities and lithium chelation energies of (1) and (2) by means of equations (2) and (3). Figure 1. Computer plot of the MNDO structure of (6), which may be compared with the X-ray structure.6 The inset shows the angles between the projected ring planes. Carbon-lithium distances are underestimated by MNDO, especially when n-donor ligand interac- tions are not included. endo-(6) is nearly 9 kcal/mol more stable than the exo- isomer (7). The energy of equation (2) exceeds that of equation (1) (MNDO) by 6.5 kcal/mol and that of equation (3) by 5.4 kcal/mol. These are measures of the additional Li JC chelation energy and should correct for the underestimation of the energy of equation (1) by MNDO. The MNDO lithium parameterization is known to overestimate the strengths of Li - - - C (and Li - - - H) interactions;10J4 e.g., the MNDO C2H4-LiH complexation energy (23.5 kcal/mol) exceeds the 3-21G (a6 initio) value (12.9 kcal/mo1).14J5 However, the MNDO isobutene-Li+ interaction energy (34 kcal/mol) is not much larger than that determined experimentally in the gas phase (28 kcal/mol).16 The JL chelation energy is expected to attenuate somewhat with the heavier alkali metals, but significant JL interactions, as are found in numerous X-ray structures,l7 still should be present. The calculated stabilization energies provide a rationale for the observed variation of the relative stabilities of (1) and (2) in the gas phase and in solution. While energy differences are generally attenuated in solution, the energy corresponding to the measured pK, difference of the Cs salts of (1) and (2) in cyclohexylamine solventlb exceeds that of the gas phase (ion cyclotron resonance) valuelc by more than 2.7 f 2 kcal/mol (equation 1). Our higher estimates for the greater stabilization of the Li salts, ( 6 )  vs. (7), seem quite reasonable. Lithium gegenion interactions (and those of other alkali metals), in addition to providing a significant energetic effect, will also lead to geometry changes in the carbanions that may influence n.m.r. parameters significantly.5.6.13b Interpretation of these data in terms of bishomoaromatic charge delocaliza- tion therefore is obscured. In conclusion, negative hyperconjugation and inductive effects contribute significantly to the stabilization of (1) in the gas phase; counterion effects, e.g. in ( 6 ) ,  contribute addition- ally in solution.18 Bishomoaromaticity can be, at best, only a minor factor influencing the behaviour of these systems. This work was supported by the Deutsche Forschungs- gemeinschaft and the Fonds der Chemischen Industrie. We thank M. Christl, R. R. Squires, F. H. Kohler, R. Lindh, and J. CHEM. SOC., CHEM. COMMUN., 1986 P. Ahlberg for providing manuscipts prior to publication and discussions. Received, 1st April 1986; Corn. 411 References (a) J. M. Brown and J. L. Occolowitz, Chem. Commun., 1965, 376; J .  Chem. SOC. B ,  1968,411; J .  M. Brown, Chem. Commun., 1967, 638; S. Winstein, M. Ogliaruso, M. Sakai, and J .  M. Nicholson, J. Am. Chem. SOC. ,  1967, 89, 3656; (b) W. N. Washburn, J. Org. Chem., 1983,48,4287; (c) R. E. Lee and R. R. Squires, J. Am. Chem. SOC., 1986, 108, 5078. S .  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1591. Also see refs. 13 and 14.

18 Other possible homoaromatic anionic systems, where counterion effects have not been clarified by structural determinations (X-ray or calculations), are reported by:

For other recent n.m.r. studies, see (b)

in ‘Aromaticity: An International Symposium’

Organometallics, 1983.2, 1344; (c)

Quantum Chemistry Program Exchange 1983,438. A listing of the MNDO parameters has been submitted to the QCPE News Letter.

Stabilization of the alleged bishomoromatic bicyclo[3.2.1]octa-2,6-dienyl anion by counterion interactions and by hyperconjugation

Hyperconjugation and inductive effects, rather than homoaromaticity, are responsible for the stabilization of the title anion in the gas phase; interaction of the double bond with the Li+ gegenion in the endo geometry contributes\ud
additionally in solution

Rague Schleyer, Paul von

Kos, Alexander J

Chandrasekhard, Jayaraman

Name not available

Stabilization of the Alleged 'Bishomoaromatic' Bicyclo[3.2.l]octa-2,6-dienyl Anion by\ud
Counterion Interactions and by Hyperconjugation

Hyperconjugation and inductive effects, rather than homoaromaticity, are responsible for the stabilization of the title anion in the gas phase; interaction of the double bond with the Li+ gegenion in the endo geometry contributes

additionally in solution

Open Access Repository of IISc Research Publications

Stabilization of the Alleged 'Bishomoaromatic' Bicyclo[3.2.l]octa-2,6-dienyl Anion by

Counterion Interactions and by Hyperconjugation

http://epub.ub.uni-muenchen.de/3927/1/054.pdf

Stabilization of the alleged bishomoromatic bicyclo[3.2.1]octa-2,6-dienyl anion by counterion interactions and by hyperconjugation

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