Struct. Chem., Vol. l, pp. 481-489. ISSN 1040-0400

Homoaromaticity and Homoconjugation in the Quinacenes: Biquinacene, Triquinacene, and Hexaquinacene

Donald W. Rogers, * Shiela A. Loggins, Shelley D. Samuel, and Michael A. Finnerty Chemist ry Depar tment , Long Island Universi ty, Brooklyn, NY 11201 USA

Joel F. Liebman Depar tment of Chemist ry and Biochemistry, Univers i ty of Maryland Balt imore Co., Balt imore, MO 2 1 2 2 8 USA

A thermodynamic criterion for aromatic and conju- gative interactions is proposed. Enthalpies of step- wise hydrogenation of, for example, three double bonds are compensated for strain energy changes during hydrogenation. Strain energies are calcu- lated by molecular mechanics. If the compensated values show a monotonic increase from bond 3 to bond 1, the molecule is conjugatively stabilized. If the initial rise is sharp followed by a constant AHh for bonds 2 and I and the molecule is cyclic, sta- bilization is aromatic. If the compensated AHh de- creases, the interaction is destabilizing. By this set of criteria, biquinacene is unstabilized, triquinacene is homoaromatically stabilized, hexaquinacene is homoconjugatively stabilized, and cis, cis, cis-1,4, 7- cyclononatriene is homoaromatically destabilized. New experimental data are presented for the bi- quinacenes (bicycloocta-1,7-diene and its hydrogen- ation products) and the hexaquinacenes.

On the grounds of purely thermochemical measure- ments, conjugative stabilization can be shown to occur in acyclic or in cyclic molecules, whereas aromaticity is distinguished from conjugative stabilization in that it is usually more dramatic in its effect and appears only upon joining a conjugated system end to end in a ring. In this contribution, we shall consider acyclic and cyclic uncharged molecules in the ground state. We shall give examples of stabilization or destablization

*To whom correspondence should be addressed.

Manuscript received 9/13/89; revised 12/4/89; accepted 12/12/89 �9 1990 VCH Publishers, Inc.

brought about by conjugation, homoconjugation, aro- maticity, or homoaromaticity.

A thermochemical definition of aromaticity and conjugative stabilization has the advantage that it is operational. One is not comparing hypothetical species in the manner of those who would seek to learn about zoology by comparing griffins with unicorns. It is im- portant, however, to consider variations in molecular strain enthalpy when comparing the stabilities of mol- ecules according to their enthalpies of hydrogenation [1]. This has been done using strain energies calculated by molecular mechanics.

In this report, we present new thermochemical data on the enthalpies of hydrogenation AHh of the biquinacenes (bicyclooctadienes) and the hexaquina- cenes along with analyses of their molecular mechan- ical strains. Similar molecular mechanical treatment of the cyclopentenes, cycloheptenes, cyclononenes, and triquinacenes in combination with existing thermo- chemical results will lead to a coherent picture of hom- oconjugative and homoaromatic stabilization of ben- zenoid hydrocarbons.

THEORY

"Aromaticity" and "conjugative stabilization" in hydrocarbons are terms used to describe the unusual stability conferred on a molecule by alternant double bonds = C - - C = in a cyclic or an acyclic configura-

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4 8 1 1040-0400/9~$3.50 + .25

482 D. W. Rogers, S. A. Loggms, S. D. Samuel, M. A. Finnerty, and J. F. Liebman

tion. Antiaromaticity is destabilization brought about by a cyclic alternant configuration of double bonds [2]. Conjugative destabilization is known [3]. Homoaro- maticity and homoconjugative stabilization are similar to aromaticity and conjugative stabilization except that alternant double bonds in the hydrocarbon structure are interrupted by an sp 3 carbon: - -C- -C- -C~-- . The existence of homoaromatic and homoconjugative sta- bilization in neutral ground-state molecules is in dis- k pute [4-6].

Unfortunate jargon such as "kinetic stability" notwithstanding, the concept of stability is rightly in the realm of thermodynamics. A substance is stable or unstable according to whether its Gibbs free energy is high or low. The question is, "high or low relative to what?" for in classical thermodynamics, the energy functions have no natural zero point.

If we seek to measure the relative stability of two molecules, A and B, thermochemically, we should measure their free energy of reaction to form the same product or products. Suppose, for the remainder of this section, that we have sound reasons to believe that A is not stabilized. Now, the thermochemical differ- ence of AGrea~tio, between A and B is a direct measure of the stabilization (or destabilization) of B. Hydro- genation of isomeric alkenes to the same alkane per- mits this kind of measurement. Entropy changes being small, one usually takes the relative enthalpy changes upon hydrogenation to the same product as a measure of relative stability of the alkenes.

It is often not feasible to react A and B to give the same product. In this case, one attempts react A and B to give equivalent products. Until recently, this kind of measurement has been bedeviled by competing de- stabilization, due to strain of the reactant, the product, or both [7].

Suppose, for example, PA, the product from A, is not the same as PB, the product from B, and PB is destabilized by strain. The measured enthalpy of the reaction B ~ PB will be diminished by the strain en- thalpy of PB. Without a separate measure of the strain enthalpy of PB, there is no way of apportioning an observed diminution in enthalpy of reaction between effects due to strain in PB and those due to electron delocalization in B, i.e., conjugation or aromaticity.

The most interesting reactions for study by hy- drogen calorimetry involve reactants and products that are both strained, but not necessarily in the same de- gree. Molecular mechanics permits calculation of the strain enthalpy of both reactant and product. If, in a hydrogenation reaction, strain is relaxed, e.g., by di- minishing the rigidity of the reactant through removal of structural constraints imposed by planar double bonds, the measured AHh should be reduced in mag- nitude by the calculated enthalpy of strain relaxation

of obtain AHh compensated for strain. If strain is in- creased during hydrogenation, e.g., by crowding hy- drogens into the product structure, the measured AHh should be increased in magnitude by the calculated increment in strain enthalpy. One need not seek a strain- free hydrogenation enthalpy, one needs only a AHh corrected to the value it would have had if strain had remained constant during reaction (which amounts to the same thing). Variations in AHh that remain after correction can then be discussed without reference to strain.

RES UL TS

Therrnochernistry

The enthalpies of hydrogenation of the biquina- cenes, triquinacenes, and hexaquinacenes measured relative to cyclohexene or 1-hexene as a thermochem- cial standard, are given in Table 1. Cyclopentene is included for comparison. The related series of fused cyclopentenes (Figure 1) was studied to investigate the possibility of homoaromaticity in triquinacene and hexaquinacene [4,8].

The heading "mg/inj" refers to the number of mil-

Table 1. Enthalpies of Hydrogenation of Polycycloalkenes Compound mg/inj AH. (kcal tool-l)a

Cyclopentene b

Dihydrobiquinacene

Tet rahydrot r iquinacene

Te t rahydrohexaquinacene

Biquinacene c

Dihydrot r iquinacene

Dihydrohexaquinacene

Triquinacene

Hexaquinacene

- - - 2 6 . 9 -+ 0.1 - - - 26 .7 +-- 0.1

- - --26.9 -+ 0.1 2.4 -26 .8 -+ 0.2

8.2 -26 .9 + 0.1 8.2 -26 .7 ---+ 0.3

8.4 -26 .9 - 0.3

11.7 - 2 7 . 2 -+ 0.5

12.9 -27 .5 _+ 0.3

11.5 -24 .7 _+ 0.6

10.7 - 2 5 . 6 +_ 0.7

11.3 -24 .2 _+ 0.9

4.2 -54 .1 _+ 0.4

4.2 -54 .3 _+ 0.5

4.2 -54 .1 -+ 0.3 5.7 -54 .5 _+ 0.4 7.2 -55 .0 _+ 0.4

6.8 - 4 6 . 9 -+ 0.5 6.9 - 4 7 . 0 ___ 1.4

5.1 - 4 8 . 0 +_+_ 1.3

2.8 -77 .8 _+ 0.8

5.2 - 7 8 . 0 _+ 0.5

3.8 - 7 1 . 7 _+ 1.6 3.6 - 7 0 , 8 _+ 1.6

2.2 - 7 1 . 0 +- 1.2

a Uncertainties are 95% confidence limits on the thermochemical determinations. bSee also ref. [111. c Hydrogenation of a 2.4:1 mixture of the 2,6- and 2,7-isomers.

Homoaromaticity and Homoconjugation in the Quinacenes 483

Figure 1. (a-c) Bi-, trP, and hexaquinacene; (d) hexaquinacene

project ion showing approach of ~" orbitals to 180 ~ abutment . a b c d

Table 2. Enthalpies of Formation and Hydrogenation and Strain Energy Exper imenta l a Molecular mech. b

Compound z~ Hf ~ Hh 2x Hf ~ Hh E st~

Cyclopentadiene c 32.1 -+ 0.4 32.72 7.41 (Cy5) - 5 0 . 4 -+ 0.2 -50 .99

Cyclopentene d 8.5 + 0,3 8.23 7.12 (H2Cy5) - 2 6 . 8 + 0.2 - 2 6 . 5 0

Cyclopentane - 18.3 -+ 0.2 ~ - 18.27 8.12

(H4Cy5) Cyclohepta- l ,3 ,5- t r iene 43.9 _+ 0.5 46.48 10.09

(Cy7) - 7 2 . 1 + 0.2 - 7 4 . 6 7

Cyclohepta- l ,3 -d iene 22.6 -+ 0.3 22.98 7.99 (H2Cy7) - 5 0 . 8 _+ 0.2 - 5 1 . 1 7

Cycloheptene - 2.2 _+ 0.3 - 2 . 9 6 7.43 (H4Cy7) - 26.0 -+ 0.1 - 25.23

Cycloheptane - 28.2 -+ 0.2 e - 28.19 9.71

(H6Cy7) Bicycloocta-2,6-diene 32.3 -+ 1.2 33.28 15.34

(B) - 56.04

Bicycloocta-2,7-diene 31.8 -+ 1.2 32.82 14.19 (H2B) - 5 4 . 2 _+ 0.5 -55 .58

Bicycloocta-2-ene 4.6 + 0.9 4.60 14.16 (H4B) - 2 6 . 8 _+ 0.3 - 2 7 . 3 6

Bicyclooctane - 2 2 . 2 -+ 0.4 e - 2 2 . 7 6 14.30

(HrB) Cyclo- l ,4 ,7-nonat r iene g 47.2 -+ 0.8 44.60 l l . 3 6

(Cyg) - 7 8 , 9 +_ 0.1 - 7 6 . 5 4

Cyclo- l ,5-nonadiene 16.0 -+ 0.9 17.60 10.95 (HzCy9) - 4 7 . 7 _+ 0.3 - 4 9 . 5 4

Cyc lononene - 7 . 4 _+ 0.8 - 7 . 3 5 14.56 (H4Cy9) - 2 4 . 3 +_ 0.1 - 2 4 . 5 9

Cyclononane - 31.7 -+ 0.7 ~ - 31.94 17.48

(H6Cyg) Tr iquinacene h 53.5 -+ 1.0 60.16 19.31

(T) - 7 7 . 9 _+ 0.7 -82 .23

Dihydrot r iquinacene 30.5 -+ 1.0 32.71 19.14

(HAT) - 5 4 . 8 +- 0.4 - 5 4 . 7 8

Tet rahydrot r iquinacene 3.0 -+ 0.5 5.34 19.82 (H4T) - 2 7 . 4 _+ 0.4 -27 .41

Perhydrot r iquinacene - 2 4 . 5 _+ 0.9 - 2 2 . 0 7 19.90

(H6T) Hexaqu inacene 66.1 + 1.0 75.10 47.71

(Q) - 7 1 . 2 _+ 1.0 - 7 8 . 6 3

Dihydrohexaquinacene 42.2 _+ 0.5 44.37 45.02 (HzQ) - 4 7 . 3 _+ 0.5 - 4 8 . 4 4

Te t rahydrohexaquinacene 19,6 -+ 0.6 17.78 47.00 (H4Q) - 2 4 . 7 _+ 0.6 - 2 2 , 9 2

Perhydrohexaquinacene - 5.14 b - 5.14 51.59

(HrQ)

Results not from this work are selected values from ref. [9]. Enthalpies of hydrogenation are to the alkane, as they actually occur. b MMP2, '77 force field, 1987 modification; self-consistent field calculations were carried out where appropriate. Nonbonded interactions

were included where appropriate (see text and Ref. 13), c This work, Table 1. d Ref. [8].

Ref. [9]. f Calculated from the ratio of isomers, Table 1. g Corrected for solvent effects; see Ref. [10]. h Ref. [4].

484 D. W. Rogers, S. A. Loggins, S. D. Samuel, M. A. Finnerty, and J. F. Liebman

ligrams ofpolycyc loa lkene per 80-p~L injection for each thermochemical run, except for hexaquinacene, for which the injected volume of sample and standard was 120 tzL. Each entry in the table represents 9-12 de- terminations, i.e., 18-24 hydrogenations, one each for the sample and standard. Uncertainties are 95% con- fidence limits for eight degrees of freedom. Uncertain- ties in Table 1 are calculated for the thermochemical part only. Pooled results are given in Table 2 under the heading "Exper imenta l AHh." Uncertainties of pooled data sets include sample handling, dilution, and weighing errors. They are smaller than the average of unpooled uncertainties because the statistical data set is larger, suggesting that weighing and handling errors are small relative to errors due to the thermochemical method.

Hexaquinacene is least soluble of all the com- pounds studied. Solutions were less concentrated than one would have wished. The experimental uncertainty is correspondingly higher. Except for hexaquinacene, the total uncertainty is judged to be less than 1 kcal mo l - l . Duplicate or triplicate experiments were run.

We did not correct any of the values in Tables 1 or 2 for solvent effects because the alkanes are as difficult to obtain and purify as the alkenes. Because comparisons are made among results obtained in the same solvent, none of the conclusions reached should be affected. At most, there would be a systematic er ror of <1 kcal mo1-1 between the experimental AHh and the gas-phase values.

The product of all hydrogenations is the com- pletely saturated polycycloalkane; enthalpies of partial hydrogenation were determined by difference from pooled results of the replicate hydrogenation experi- ments. Experimental uncertainties for the enthalpies of partial hydrogenat ion given in Scheme 1 are cal- culated as the square root of the sum of twice the

variances of the pooled experimental values that de- termine them. Since the results of independent exper- iments are pooled, these uncertainty limits also include error due to weighing and dilution. The uncertainties in Scheme 1 are essentially 95% confidence limits for the stepwise hydrogenations.

Molecular Mechanics

Cyclopentadiene (Cy5) is calculated to be a planar molecule by MMP2. The calculated AH~ is 32.72 kcal mol ~ when ~- overlap between carbons 1 and 4 is not included (experimental value: 32.1) [9] but is 38.70 when ~r overlap is included. This is a reflection of the unfavorable bonding situation that is produced by approaching an antiaromatic pseudo-cyclobutadiene structure. In general, strain energies do not vary much among MM2 and MMP2 calculations with and without overlap. The molecular structure of cyclopentene (H2Cy5) and cyclopentane (H4Cy5) have been de- scribed [10]. Experimental values for gas-phase en- thalpies of hydrogenation in Table 2 are taken from the modern compilation by Pedley, Naylor , and Kirby [9]. They are from the original work by Kist iakowsky [11].

Enthalpies of hydrogenation uncorrected for strain energies lead to the first line in Scheme 1. Values cor- rected for strain energies lead to the first line in Scheme 2. Strain energies for Cy5, H2Cy5, and H4Cy5 are 7.41, 7.12, and 8.12 kcal mo1-1, where SCF zr interactions are included in the calculation for Cy5 but 1-4 overlap is not. If 1-4 overlap is included, the strain energy is increased by only 0.03 kcal tool-1

The molecular mechanics of 1,3,5-cyclohepta- triene and related compounds have been studied [12]. The triene exists in the boat conformation with the sp 3

Cy5 ~ H2Cy5 ~ H4Cy5

-23.6 • 0.3 -26.8 -+ 0.2 Cy7 ~ H2Cy7 ~ H4Cy7 > H6Cy7

-21.3 • 0.3 -24.8 -4- 0.2 -26.0 +_ 0.2 B > HzB > H4B

-27.4 _+ 0.4 -26.8 + 0.3 Cy9 ~ H z C y 9 ~ H4Cy9 > H6N

-31.2 • 0.3 -23.4 • 0.2 -24.3 • 0.1 T ~ H2T > H4T ~ H6T

-23.1 + 0.8 -27.4 _+ 0.6 -27.4 -+ 0.4 Q ~ H2Q > H4Q ~ H6Q

-23.9 + 1.1 -22.6 _+ 0.8 -24.7 • 0.6

Scheme 1. Enthalpies of stepwise hydrogenation.

Cy5 ~ HzCy5 > H4Cy5

-23.3 -27.8 Cy7 > H2Cy7 > H4Cy7 ~ H6Cy7

-19.2 -24.2 -28.3 B ~ H2B ~ H4B

-27.4 -26.9 Cy9 > HzCy9 " > H4Cy9 ~ H6N

-30.8 -27.0 -27.2 T ~ H2T ~ H4T ~ H6T

-22.9 -28,1 -27.3 Q > HzQ , H4Q ~ H6Q

-21,2 -24.4 -29.5

Scheme 2. Enthalpies of hydrogenation compensated for strain.

Homoaromaticity and Homoconjugation in the Quinacenes 485

atom projected sharply upward as the "prow." Mo- lecular mechanics with ~- SCF optimization yields an enthalpy of formation that is more than 5.4 kcal mol higher than the experimental value of 43.24 kcal mol- 1. Inclusion of nonbonded orbital interaction across the homoaromatic interval (carbons 1 and 6) results in cal- culated AHf = 46.46 kcal mol- ' , which reduced the discrepancy between experimental and calculated re- sults but by less than half o f its total amount.

Biquinacene, bicyclo[3.3.0]octa-2,7-diene (B), and its isomer the 2,6-diene are hydrogenated directly to the bicycloalkane H4B through the 2-ene HzB. We are interested in hydrogenation of the 2,7-ene because of its potential for homoconjugative stabilization. The C1--C2 and C8--C1 bond lengths are 1.501 and 1.499

by molecular mechanics [13]. The C2--C3 and C7-- C8 bond lengths are 1.337 and 1.336 A. The angle of planar intersection at the line of fusion of the two cy- clopentene rings (projection of the C2--C1--C8 angle onto the plane perpendicular to the C1--C5 bond) is 118 ~ Perpendiculars to the plane of each of the cis- fused cyclopentene rings projected along the axis of the unhybridized p orbitals of carbon atoms 2 and 8 intersect at an angle of 62 ~ .

The nonbonded separation of s p 2 carbons 2 and 8 is 2.505 A and does not change much upon hydrogen- ation, being 2.522 and 2.580 for the dihydro- and te- t rahydrobiquinacenes HzB and H4B. The C2--C 1--C8 bond angles, upon stepwise hydrogenation, are 113.3, 112.8, and 114.1 ~ and the interplanar angle at the bond of fusion is constant at 118 ~

The molecular mechanics of cis,cis, cis-cyclonona- 1,4,7-triene have been summarized by Burkert and A1- linger [14], and cyclonona-l,5-diene has been studied by Anet and Yivari [15]. The low-energy conformer of the latter is the axial-symmetrical chair form with sym- metry C2. The possible conformational diversity of cy- clononene and cyclononane are daunting but their en- thalpies of formation are well reproduced by our molecular mechanics results in Table 2, leading us to believe that we have found the stable conformer or one that is energetically very close to it.

Triquinacene, tricyclo[5.2.1.04,1~ (T), is hydrogenated through HzT, and H4T to per- hydrotriquinacene H6T. The distance across the hom- oaromatic interval ~---C--C--C-~- in this tris benzen- oid hydrocarbon is 2.49 A, about the same as biquinacene, and increases in two diminishing steps to 2.58.4 for H6T. The angle of planar intersection of the cyclopentene rings is 116.4 ~ leading to an axial inter- section angle for the nonhybridized p orbitals of, e.g., carbon atoms 2 and 9, of 63.6 ~ less than 2 ~ different from the p-orbital intersection angles in biquinacene.

Inclusion of nonbonded overlap yields an enthalpy of formation of 60.16 kcal mol - ' and a strain energy

of 19.31 kcal mol- 1, within 0.01 kcal mol- ' of the strain energy calculated without alternant carbon atom over- lap. The enthalpy of formation with overlap is 1.28 kcal mol - ' less exothermic than the enthalpy of for- mation calculated without overlap, about one-third of the experimental value for the stabilization enthalpy. Molecular mechanics calculations carried out with no SCF ~r optimization, with ~- optimization, and with 7r optimization including nonbonded interactions across the homoaromatic intervals yield 59.83, 61.45, and 60.16 kcal mol ', respectively, for the enthalpies of forma- tion of T.

The benzenoid interval in hexaquinacene is 2.81 A, only 0.32 A larger than in triquinacene. It increases to 2.92, 2.98, and 3.08 A upon stepwise hydrogenation to the low-energy conformer of perhydrohexaquina- cene (C3 symmetry). The larger benzenoid interval in triquinacene is caused by two intervening sp 3 carbon atoms in the interval

C- -C C--C / \ C = C ,

rather than one. The angle of abutment of 7r orbitals is much more favorable than in triquinacene, however, being close to 180 ~ (Figure 1), as opposed to 64 ~ for triquinacene. The benzenoid interval opens much more during hydrogenation of hexaquinacene (0.27 A) than triquinacene (0.09 A).

DISCUSSION

The thermochemical criteria of stabilization or destabilization in conjugated and homoconjugated sys- tems are shown in Figure 2. Enthalpies must first have

c -

O

(-

s

c- (g

o ~

__~ < ~J

( -

UA

/

S /

5

/ / f

/ f

J I

J

I /

/ /

/

2 I Double Bond(s)

Figure 2. Idealized enthaplies of stepwise hydrogenation. Above the horizontal represents destabilization; below it, stabilization. The monotonic sloping curves show conjugation. The broken lines show aromaticity.

486 D. W, Rogers, S. A. Loggms, S. D. Samuel, M. A. Finnerty, and J. F. Liebman

been corrected for change in strain energy from reac- tant to product. The top curve shows a characteristic pattern for conjugative destabilization. Stepwise hy- drogenation of three double bonds produces most heat for the first step, an intermediate amount for the second step, and least heat for the third step. The opposite behavior (bottom curve) indicates conjugative stabili- zation. Aromatic behavior is indicated by a sharp change in •Hh between the first and second steps and no dif- ference between the second and third steps. Aroma- ticity may be stabilizing or destabilizing (antiaroma- ticity).

same for each step and the same as that of hydrogen- ation of cyclopentene. Biquinacene displays no homo- conjugative stabilization. Indeed, there is a small destabilization indicated in the strain-compensated values in Scheme 2. It is likely that this is spurious and is caused by the presence of the 2,6-isomer in the biquinacene sample. There is no benzenoid configu- ration of double bonds, so biquinacene cannot be homoaromatic.

Cyclononatriene

Cyclopentadiene Cyclopentadiene is stabilized by 4.5 kcal tool i of

conjugative stabilization enthalpy relative to strain- corrected cyclopentene (Scheme 2, Figure 3).

Cycloheptatriene Scheme 2 shows that cycloheptadiene is conju-

gatively stabilized by 4.1 kcal mol-~ relative to cyclo- heptene. If conjugative stabilization of cyclohepta- triene is the same relative to cycloheptadiene, as cycloheptadiene is relative to cycloheptene, homoar- omatic stabilization across the C1--C6 interval is 0.9 kcal tool ~.

Biquinacene The enthalpy of hydrogenation for the two-step

process B ~ HzB --> H 4 B (Scheme 1), is almost the

cis, cis, cis-Cyclonona-l,4,7-triene (N) has long been thought of as a "benzenoid" hydrocarbon be- cause of its hexagonal arrangement of double bonds interrupted by sp 3 carbon atoms. On the basis of en- thalpies of hydrogenation and x-ray diffraction data, Turner [7] concluded that the molecule was devoid of homoaromatic stabilization. We think that it exhibits homoaromatic destabilization (Figure 4).

Variations in strain energy of the cyclononenes are considerable along the stepwise hydrogenation path, as can be seen in Table 2. Scheme 2 shows that some, but not all, of the apparent destabilization in Scheme 1 vanishes when strain is taken into account. There remains an apparent destabilization of 30.8 - 27.0 = 3.8 kcal mol-L This may be an artifact of the calcu- lation. Anet and Yivari [15] obtained a total strain en- ergy of 8.87 kcal tool ~ for the low-energy conformer of the 1,5-diene (the axial-symmetrical chair, C2). If this value is used as the strain energy of the 1,5-diene in place of the value 10.95 in Table 2, stepwise hydro- genation enthalpies become -28.1, -27.4, and -27.5,

O E -3o

._o

c

2 r--

o -20 O_

t - *~ 5 2 I c- I I I

Double Bond(s)

Figure 3. Conjugative and homoconjugative stabilization. Open cir- cles; cyclopentadiene; filled circles, cycloheptatriene; open trian- gles, hexaquinacene; filled triangles, hexatriene (middle bond, see text).

O E

o

.o c- O)

t -

O

c~ ~3

t - LU

q

-50

-2s 3

/

Double Bond(s)

Figure 4. Aromatic and homoaromatic stabilization and destabili- zation. Open circles, biquinacene; filled circles, cyclononatriene; open triangles, triquinacene; filled triangles, benzene.

Homoaromaticity and Homoconjugation in the Ouinacenes 487

which values are consistent with a slightly unstabilized triene. It is not permissible to pick strain energies from different force field calculations, however, and we have retained consistency by using the Allinger force field throughout.

Even with Anet's value for the strain energy, small destabilization is indicated. This argues for the signif- icance of the thermochemical stabilization found for triquinacene [4]. It may be that in triquinacene, a small homoconjugative destabilization is overbalanced by a larger homoaromatic stabilization as is true of the per- icyclynes [3]. Relative to nonatriene with Anet's strain energy, the homoaromatic stabilization in triquinacene is 5.2 kcal tool-1. Relative to nonatriene corrected by the Allinger strain energy, the homoaromatic stabili- zation of triquinacene is 7.9 kcal tool- 1.

One would like to know the enthalpy of hydro- genation of cyclonona-l,4-diene, but that experiment has not yet been done.

The Triquinacenes

The low-energy conformer [16] of perhydrotri- quinacene and perhydrohexaquinacene is the C3. Higher energy conformers of all of these compounds exist but are not far in energy from the lowest form, being about 2 kcal tool-~ different for both the perhydrotriquina- cene and perhydrohexaquinacene conformers. Even the barriers between conformers are modest [16], being only about 4.5 kcal mol -~ for the one ethano bridge inve;sion of the C3 (+ + +) conformer of perhydrotri- quinacene or perhydrohexaquinacene to the next higher stable form (+ + - ) . Elsewhere the potential wells are steep and minimization is rapid for the triquinacene series, less so for the hexaquinacene series.

Recently, Dewar and Holder [6] have presented molecular mechanics and other calculations with a dis- cussion of enthalpies of formation of"untwis ted" H2T, H4T, and H6T. Reference to our MMP2 results in Table 2 shows that Dewar's results are not for a hypothetical untwisted species but are values for the actual low- enthalpy conformers. Notwithstanding, one can hy- pothesize a series of reactions of untwisted triquina- cenes that is consistent with the experimental results and shows no homoaromaticity. This amounts to a change in reference state.

The Hexaquinacenes

The enthalpies of hydrogenation AHh of Q, H2Q, and H4Q lead to stepwise enthalpy changes to the final product, perhydrohexaquinacene (Scheme 1), in marked

contrast to results for stepwise hydrogenation of bi- quinacene and triquinacene.

Variation in strain energy during hydrogenation is important in the hexaquinacene series. Strain increases from reactant to product in this series, but by different amounts. The increase in strain energies must augment the observed ~Hh to obtain the strain-corrected value. This yields -75.1, -53.9, and -29.5 as the strain- corrected results for hydrogenation of the hexaquin- acenes to perhydrohexaquinacene. Strain-corrected stepwise enthalpies of hydrogenation (Scheme 2) in- dicate homoconjugative stabilization within the series.

The pattern that emerges is one of no stabilization for the biquinacenes, homoaromatic destabilization in cyclononatriene, homoaromatic stabilization for tri- quinacene and homoconjugative stabilization for the hexaquinacenes. A constant AHh for both biquina- cenes at the same value as cyclopentene shows no stabilization. A sharp rise on the first hydrogenation step, followed by constant values on the second and third steps, shows homoaromatic stabilization for tri- quinacene but no homoconjugation for the series. The classic case of benzene [11]:

+5.6 -26.8 -28.6

is similar, but not identical, in that it shows dramatic stabilization due to aromaticity in the first stepwise hydrogenation (Figure 4), superimposed on a small conjugative stabilization of cyclohexadiene relative to cyclohexene.

Hydrogenation of hexaquinacene shows the steady increase in exothermicity (Figure 3) that one would expect of a system stabilized by homoconjugation. A conjugative model for comparison is hydrogenation of the doubly conjugated middle bond in cis-l,3,5-hexa- triene (AHh = --81.5 kcal tool -~) [17], the singly con- jugated middle bond in cis-l,3-pentadiene (AHh = -54.1 kcal mol-~) [18] and the unconjugated bond in cis-3- hexene (AHh = -29.1 kcal tool- 1) [19]. (The AHh value of cis-l,3-hexadiene is not known.) These entha[py changes become increasingly exothermic in the order -21.1, -23.9, and -29.1 kcal tool -~.

EXPERIMENTAL

The calorimeter used in these studies for deter- mination of the enthalpy of catalytic hydrogenation AHh of polycyclic alkenes has been described [20]. Aliquot portions of a solution of polycycloalkene in an

488 D. W. Rogers, S. A. Loggins, S. D. Samuel, M, A. Finnerty, and d. F. Liebman

inert solvent were injected into the calorimeter in al- ternation with injections of a thermochemical standard, hexene (AHh = -30.2 ___ 0.3 kcal mol -l) [21] or cy- clohexene (AHh = -28.4 -+ 0.2 kcal tool-1) [22]. The known AHh of the standard permits calculation of AHh of the polycycloalkene from the measured ratio of heats produced within the calorimeter. Electronic and pro- cedural details are given elsewhere [19].

Injections were made using a 100/~L syringe (Ham- ilton) with a mechanical stop set at 80/xL. Precalibra- tion of the syringe showed its mean delivery volume to be within 0.2% of the nominal value. Optimum re- producibility was said to be 0.6% by the manufacturer and was verified before proceding with these experi- ments. Solutions of cyclohexene or hexene were made up in cyclohexane or n-hexane so as to be approxi- mately thermochemically equivalent to the polycyclic unknown. Solutions of the unknown were in the same solvent as the standard. The calorimeter fluid was a slurry of 5% Pd on activated charcoal in n-hexane or cyclohexane, according to which solvent was used to make up the sample and unknown solutions. The pur- pose of matching solvents with the hydrogenation product of the standard was to reduce solvent effects to a minimum. We believe that the results produced by this scheme are very close to what would have been produced by gas-phase hydrogenation [20]. Reaction times plus instrumental response times were about 10 s. Total run times were 100 s.

Gas chromatographic analysis [23] of the calorim- eter fluid after nine or more determinations had been run showed a single clean product peak at long reten- tion times. Analysis of samples of calorimeter fluid intentionally contaminated with reactant showed two peaks or a split peak with the satelite at a shorter retention time than the product peak. We estimate that 0.2% of reactant or side-reaction product could have been detected in the calorimeter fluid, had it been pres- ent. Hydrogenation proceeds quantitatively to the polycycloalkane; measurement of/XHh of partial hy- drogenation of one or two double bonds out of three is not experimentally feasible.

Incomplete hydrogenation followed by a slow re- action that consumes the evidence of incomplete re- action is not impossible. One would expect this scen- ario to cause an abnormal reaction curve, which was not observed. In view of the exposed nature of the double bonds (as contrasted to sterically hindered "in- terior" double bonds) we believe that the likelihood of an error from this source is very small.

Some abnormalities, however, were encountered in the study of tetrahydrohexaquinacene. Tetrahy- drohexaquinacene reacts more slowly than hexaquin- acene, dihydrohexaquinacene, or any of the triquina- cenes. Temperature rise was linear, suggesting a zero-

order reaction. Doubling the catalyst charge results in a normal curve, limited by instrument response time, as in all other experiments reported here. Rate is not thought to diminish accuracy. The 95% confidence lim- its reported in Table 1 are double the calculated un- certainty for this compound as a worst-case estimate.

Tetrahydrohexaquinacene solutions in cyclohex- ane were turbid. Upon standing a small amount of flocculant precipitate settled out. A thin coating of a very insoluble glassy material was noted on the interior surface of the volumetric glassware. The value in Table 1 should be regarded as a lower limit, i.e., the hydro- genation reaction is at least as exothermic as indicated in the table.

Substantial amounts of polar impurity were found [24] in the residue from evaporated calorimeter charges collected from the hexaquinacene experiments. These were insoluble in nonpolar solvents. No evidence of insoluble matter was found in any of the experiments except in the case just discussed, which is not germane to the argument concerning either triquinacene or hexaquinacene. We believe that contamination of the calorimeter residue is from another source than the hexaquinacene samples and that it has no influence on the experimental results.

ACKNOWLEDGMENTS

Acknowledgment is made (DWR) to the Donors of The Petroleum Research Fund, administered by the American Chemical Society, for support of this re- search. Long Island University provided released time to carry out this work. JFL wishes to acknowledge support from the Office of Standard Reference Data of the National Institute of Standards and Technology. We wish to thank Prof. Leo Paquette and Drs. Paul Galatsis and Ho-Shin Lin for providing samples of bi- quinacenes, the triquinacenes, and hexaquinacenes and to Prof. James Chickos for analytical work on perhy- drohexaquinacene. We thank a reviewer for the art- work in Figure 1.

REFERENCES AND NOTES

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Homoaromaticity and Homoconjugation in the Quinacenes 489

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