Lloyd N. Ferguson and John c. Nnadi Howard University
Electronic Interactions Between Nonconjugated Groups
Washington, D.C.
O n e of the advantages of the molecular orbital theory over early resonance theory is that the former readily accounts for certain electronic interactions between groups which are not conjugated in the classical sense. Accordingly, resonance theory has been extended to incorporate the concepts of homoconjugation (1) and nonclassical resonance. The purpose of this paper is to discuss some of the different molecular systems in which electronic interactions between classically nonconjugated groups are explicable in terms of molecular orbital theory as well as nonclassical resonance theory.
-
Conjugation
-
-
An isolated C=O bond exhibits an intense a band in the ultraviolet a t -185 mp (e lo4) and a low intensity n s* band a t -280 mp ( E < 50).' r*
--
When the cssbonyl bond is or,@-conjugated with a C=C bond or an aromatic ring, the s r* band moves to 220-260 mfi (e 104) and the n r* band shifts to 300-350 mp ( e 50).
--
or+-Conjugated groups: CHz=CH-C4,
&H,CH=G-~
La, A, r
219 mp = 3600
=
AHa = =
v,=, uc=c
1685 crn-' 1623 om-'
Thus, the conjugated C = O and C=C bonds in the +enone have greater single-bond character than in the separate isolated bonds. Transannular Conjugation
When the C = O and C=C groups are nonconjugated in the classical sense but are suitably oriented, there can be orbital overlap in the usual r fashion (parallel orbitals), called in this paper transannular conjugation, in which the ultraviolet spectral properties are similar to those of the a,@-enoneconjugated system.
Thus, the presence of a strong absorption band in the 21S260 mp region is taken as evidence for this transannular conjugation in the excited state. This band is called a photodesmotic band (from the Greek, meaning "link caused by light") because the transition is believed to be accompanied by a weak bond in the excited state (9).= Several other examples follow (A):
This typical conjugation occurs to a sigoificant extent even in the ground state of the molecules, as reflected by the infrared frequencies of the C-a and C=C bonds. Isolated groups: CHs
\ C=O /
v-o
=
CHs 1720 om-'
CH&HzCH=CHz uc=c
=
1647 cm-I
Taken in part from the M S . Thesis of J. C. Nnadi, Howard University, Washington, D. C. For a. delinition of terminology of the ultraviolet absorption L. N., "Modern Structural Theory of bands, aee FERQUSON, Organic Chemistry," Prentice-Hall, Inc., Englewood Cliffs, New Jersey, 1963, p. 483.
r
= 2214 sh, = 1810,
297 mp 43
A= = 238 mp, a = 2535 v-
=
1704 em-'
*From previously reported UV and IR data, we infer that homoconjugation rather than transannular conjugation occurs in compound 1 (3). Volume 42, Number
7 0, October 7 965
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529
Homoconjugation
When the orbital overlap of the carbonyl carbon and an olefinic carbon is cross-wise, that is, partially sigma in character, there results an enhanced n + s* carbonyl absorption band. For illustration, in 2 the C=C and C=O groups are oriented to allow overlap in sigma fashion as diagrammed in 3.
A number of other examples are listed in Table 1, where comparisons are made with similar nonhomoconjugated compounds. The criterion for homoconjugation, again, is the enhanced n a* absorption for the carbonyl group, i.e., > 50. Enone homoconjugatlon, in contrast to enone conjugation, appears not to affect appreciably the ground state of the molecule. For example, compounds P 6 exhibit enhanced n + T * carbonyl hands, indicating homoconjugation, but their infrared frequencies are in the normal frequency range for an isolated C=O group (6). Compound 5 also gives Schiff bases whose spectral properties are characteristic of isolated carbony1 groups. On this basis, the ultraviolet and in-
-
202, 290 rnr
A,
=
6
= 3000, 110
The interaction mav also involve a carbonyl group an an aromatic ring, e.g. (5): 0 r
A,, = 316 ma e
A, r
= 174
Table 1.
223, 307 mr 2290, 267
A,,, = 300.5 mp, E = 292 ucz0 = 1725, 1740 cm-I
Spectral Data for Some Reloted Nonconjugated and Homoconiugoted pi-Carbonyl Systems.
'Most data in ethanol.
/
= =
= 210, 321 rnp = 10,700, 501
Homoconjugated
530
5
4 A,
Journal of Chemical Education
Nonconjugated
frared spectra of compound 7 indicate that the C=O and C=C bonds are not coplanar for a$-enone conjugation but are angular for homoconjugation. Thus,
they exhibit an enhanced n + a* carbonyl band, and in the infrared, the compounds have strong normal carbonyl absorption at 1700-1705 cm-' and isolated C=C bands a t 1625-1630 ~ m - ' . ~The compounds also give a positive color test with tetranitromethane, usually indicative of an unconjugated olefinic bond. Another system in which homoconjugation occurs is between two aromatic groups separated by a methylene group. This is diagrammed in molecular orbital theory by 8 and in resonance theory by 9.
methane absorbs at slightly longer wavelengths and with much greater intensity than the corresponding toluene or ethylbenzene (7). The effect is magnified by substituting dissimilar electron-attracting groups in the two rings, e.g. 10. Spectral data for some examples are given in Table 2.
It can be seen there that the absorption bands of the p,pl-disubstituted diarylmethanes are of longer wavelengths by 2 4 mr's and of twofold to tenfold greater intensity than those of the correspondingly substituted toluenes, and the effect is greater for the p,p1disubstituted compounds than for the m,mf-isomers. The same type of homoconjugation occurs between the
Spectroscopically, it is observed that the diaryl"ee Leonard and Owens (3). There were also low-intensity coniueated carbonvl maxima at 1670-1675 cm-' in the soectra. .showing that the samples were not oonformationally hamogeneous.
Table 2.
Compound
Triptycene
Top view 01 Triptycene
Spectral Data (in ethanol) for Some Substituted Diphenylmethanes and Related Toluenes. A d m ~
Xmw1
OCH.
p g OIN
265.5
14,890
282
7,200
275
395
276
7,325
OC&
H~CQ
p
Br
cH=
264
7,750
267
10,500
GN
Dc,q O*N
' NNADI, J.
Br
C., MS. Thesis, Howard University, Washington, D. C., 1965. Volume 42, Number 10, October 1965
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531
benzene rings of triptycene. Unlike triphenylmethane, in which the phenyl groups are oriented like a propeller with respect to a single plane, the phenyl rings in triptycene radiate out from an axle with 120' angles between vertical planes of the rings. This orientation allows considerable overlap of the ring-carbon pi orbitals and produces an enhanced absorption over that of triphenylmethane (8): A,
(ma) Triphenylmethane 255 Triptyoene 263
A,
r
1050 1780
(m~) s 262 1100 271 4555
This interaction between classically nonconjugated C=C bonds has been reported for a number of bicycloheptadiene derivatives (11), e.g.
A,
(ma) r 690 271 278 3630
Thus, the maxima of triptycene are a t 8-9 mp longer wavelengths and 2-5 times more intense than those of triphenylmethane. Interaction between classically nonconjugated C=C bonds has also been detected. For illustration, 1,3,6,8-nonatetraene (11) has two isolated butadiene chromophoric groups. If there were no interaction between the two chromophores, A, for 11 should be the
same as for 1,3-nonadiene (12) but with twice the
It must be pointed out that the electronic interactions between nonclassically conjugated pi bonds occnls primarily in the excited state of the various systems. So far, data such as heats of hydrogenation, bond distance and bond angle measurements, or infrared spectral data do not provide evidence for homoconjugation in the ground states of molecules such as bicycloheptadiene, cis,cis,cis-cyclononatriene (15) (Is), or triquinancene (16) (IS). Calculations indicate that there is a small delocalization energy in 15
intensity. Instead, A, for the tetraene is at 10.6 mp longer wavelength and of only 12'3& greater intensity (9). Similarly, 1,3,9, lldodecatetraene (13) &,&,cis-1,4,7-cyclononatriene
Triqninacene
(ca. 3 kcal/mole) (14), and it is quite possible that steric strain obscures such a small homoconjugation effect (16). This appears to be the case for bicycloheptadiene (16') and barrelene (17). The heat of
exhibits homoconjugation of its two hutadiene groups. It exhibits greater absorption than the corresponding 1,3-dodecadiene (14) (9). The cyclic configuration of
these polyenes is supported by the fact that 1,5- and 1,6-dienes preferably undergo cyclic polymerization rather than cross-linked polymerization (10).
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Journal o f Chemical Educafion
barrelene
hydrogenation of the first bond of barrelene (37.6 kcal.) is the largest value known and implies a large strain energy in the ground state (e.g., the value for 1,2-di-t-butylethylene is 36.2 kcal.) which would conceal any homoconjugation energy unless the latter were substantial. It is not surprising to find orbital overlap between two aromatic rings standing closely face to face. For example, the charge-transfer complexes of paracyclophanes (17) are more stable than are the chargetransfer complexes of the respective pdiallcylbenzenes (18) because there can he charge delocalization in 17 from the inner ring to the outer ring by sigma-type overlap of the pi orbitals of the benzene carbon atoms (18).
orbital on CI (26). For illustration, acetolysis of 27 yields 29 and 30. Tosylate 27 reacts 1200times faster thanethyl tosylate, which can be attributed to the formation of the resonance-stabilized intermediate ion 28. In another example, nonclassical resonance can account for the unusual stability of the carbonium ion 31, which can be formed by three different reactions. Chemical properties, too, reveal electronic interactions between nonconjugated groups. One of the earliest examples reported was in the acetolysis of cholesteryl tosylate (1).
21
AcO OAc
The resonance-stabilized ion 20 not only accounts for the enhanced rate of ionization of 19, but also for the retention of configuration in 21 with respect to 19 and in particular, for the formation of 22. Ion 20 is a homolog of the resonance hybrid ally1 ion 23.
Homoconjugation stabilizes the cation to the extent that it is stable in 60% sulfuric acid and its perchlorate salt is stable below -40'. Hence, with a suitable geometry, there can be homoconjugation or nonclassical resonance between groups separated by a saturated group. Such interaction is the basis for neighboring group participation (19, 20), e.g. (21):
I
(CH~-C=CHZ
This analogy led to the designation of ion 24 as a homoallylic ion (1). As in 25, it indicates that there is partial bonding between atoms 1and 3 and a weakening of the double bond between atoms 3 and 4. I n terms of molecular orbital theory, 25 implies that there is a
I (CH3)2F- CH, OAc
Another example is the 2-norbornenyl cation. ExoNorbornenyl bromobenzenesulfonate 32 undergoes acetolysis to give the corresponding em-acetate 34 without any a d o isomer, plus the nortricyclyl acetate 35 (22).
delocalization of the 3,4-s-electron density into an open
-
CHs \C=CH CH'
'cL
CH*
CHZOAC 29
30 C
\c-CH-CH? H ~ 'c&
The issue of a classical versus nonclassical structure of the norbornyl and related cations should be mentioned here. The 2-norbornyl cation 36 can be produced by several reactions, e.g. Volume 42, Number 10, October 1965
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533
rule) I. However, when optically active camphenilone is heated with base, it undergoes racemization (no racemization occurs without the base) (27). This can be interpreted as the abstraction of a proton from Ca to produce the nonclassical anion 39, called a homoenolate ion (27). I t is to be noted that this ion can be regarded as a resonance hybrid 40, which makes the CI-C2and C2-Cebonds equivalent.
and its NMR spectrum in solution has been studied (23). There are two opposing views on the structures of these ions. One school of thought (24) is that they have the nonclassical bridged structure 37, and the other view (25) is that there are rapidly equilibrating classical ions like 36. These two views are currently being vigorously debated in the literature (24, 25). Evidence for the nonclassical structure of the norbornyl cation and its derivatives has been along two lines: (1) unusually fast rates, attributed to the formation of a highly stabilized cation, and ( 2 ) exo substitution, even with norbornyl derivatives containing gemdimethyl groups in the 7 position, attributed to blocking of the a d o side by the nonclassical bond. Proponents of the classical ion structure argue that the bridged ion structure 37 is not necessary and inadequate to explain the data. A third argument is often offered in support of the nonclassical structure, which is that a large exolendo solvolytic rate ratio indicates the bridged structure owing to carbon participation in the exo but not in the endo isomer. I t has been acknowledged, however, that a large ezo/a d o rate ratio is not sufficient evidence upon which to base an assignment of a nonclassical structure to the cation (26). An interesting reaction involving homoconjugation of norbornyl ions (or rapidly equilibrating classical norbornyl ions) is the enolization of the respective ketones. Normally, hydrogens alpha to a carbonyl group are readily removed by base, and the enolate ion which results is a resonance-stabilized anion.
However, camphenilone (38) has no classically enolizable hydrogens [H on C1 is prohibited because a double bond to C1 produces too much strain (Bredt 534
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Journal of Chemical Education
Recapture of a protou at C6 regenerates the original isomer whereas protonation of C1 yields the other enantiomer. Furthermore, when the homoenolate ion is produced by a differentreaction, protonation yields a norbornyl ketone. Thus, hydrolysis of l-acetoxynortricyclene 41 yields an anion 42 which collapses upon protonation to 2-norbornanone 43 (28). By using
a deuterated medium (e.g. tBuOK in tBuOD), it is shown that over 94.5% of the product has an exo D. Hence, if homol~etonizationfollows the reverse path of homoenolization, then the transition state for the latter process involves an exo hydrogen to permit homoconjugation with the carbonyl group from the endo side. Analogous homoenolization-homoketonization reactions have been reported involving transannular y-hydrogen atoms (29). Literature Cited
(1) SIMONETTA, M., AND WINSTEIN,S., J . Am. Chem.Soe., 76, 18 (1954). (2) K o s o w ~ E. ~ ,M.,ET AL.,J . Am. Chem.Soe.,83,2013(1961). ( 3 ) LEONARD, N. J., AND OWENS,F. H.,J . Am. Chem. Soe., 80, 6039 (1958). S., DEVRIES,L., ANDORLIOSKI, R., J.Am.Chem. (4)WINSTEIN, N. J., MILLIC-AN, Soc., 83, 2020 (1961); LEONARD, T. W., AND BROWN, T. L., J . Am. Chem. Soe., 82, 4075 i1960). (5) M;SLO;, K., ET AL., J . Am. Chem. Soe., 84,1455 (1962). (6) MEINWALD, J., ET AL.,J . Am. Chem. Sac., 77, 4401 (1955); BARTLETT, P. D., AND TATE,B. E., J . Am. Chem. Soc., 78, 2473 (1956). (7) STEWART, F. H. C., J . 079. Chem.,27,3374(1962). (8)BARTLETT. P. D.. AND LEWIS.E.S.. J . Am. C h a . Soc.. 72. (9) BUTLER; G.B.,AND RAYMOND, M. S.,presented at the 146th Meeting of the ACS, Denver, Colorado, January, 1964. (10) BUTLER,G. B., J. Polymer Sei., 48, 279 (1960); MARVEL, C . S.,J . Polymer Sci., 48, 101 (1960).
(11) LALONDE, R.T., EMMI,S., ANDFRASER, R. R., J . A m . Chem. Soc., 86, 5548 (1964); WILCOX,C. F., JR., WINSTEIN, S., A N D MCMILLAN, W. G., J. Am. Chem. Soc., 82, 5450 (1960); JONES,E. R. H., MANSFIELD, G. H., AND WHITING, M. C., J . Chem. Sot., 4073 (1956). (12) ROTE,W. R., ET AL.,J. Am. Chem. Soc., 86,3178 (1964). (13) WOODWARD, R. B., FOKUNAGA, T., A N D KELLY,R. C., J. Am. Chem. Soc., 86,3162 (1964). (14) RADLICK, P., AND WINSTEIN, S., J . Am. Chem. Soc., 85,344 (1963); UNTCH,K. G., J. Am. Chem. Soe., 85, 4061 l l-D f i-R,l.~ WINBTEIN, S., AND LOSSING, F. P., J . Am. Chem. Soe., 86, 4485 (1964). \
--
TURNER,R. B., MELDOR,W. R., AND WINKLER,R. E., J . Am. C h . Soe., 79, 4116 (1957). TURNER, R. B., J . Am. Chem. Soc., 86,3586 (1964). CRAM.D. J.. AND BAUER.R. H.. J. Am. Chem. Soe.. 81. , ~~~,
WINSTEIN, S., A N D BUCKLES, R. E., J. Am. Chem. Soe., 64, 2780 (1942).
See CAPON,B., Q U Q T ~ . Rev. (London), 18, 45 (1964) for s. recent review.
(21) HECK,R., A N D WINSTEIN, S., J . Am. Chem. Soe., 79,3432 (1957). (22) WINSTEIN,S., WALBORGKY, H. M., AND SCHREIBER, K., J . Am. Chem. Soc., 72, 5795 (1950). M., SCHLEYER, P. VON R., AND OLAH,G. A,, (23) SAUNDERS, J. Am. Chem. Soe.., 86.5680 11064). , , ,~ (24) See WINSTEIN, S., ET AL., J. Am. Chem. Sac., 87, 376, 378, 379, 381 (1965) for s. recent leading reference; see also ~~
~~
BERSON,J. A., "Molecular Rearrangements," P. nn MAYO,ed., Interscience Publishers, New York, 1963, Pt. 1, Chap. 3. H.C., ET AL.,J . Am. Chem. Soc., 86, 5003(25) See BROWN, 5008 (1964). P. VON R., DONALDSON, M. M., AND WATTS, (26) SCHLEYER, W. E., J . Am. Chem. Soc., 87,375 (1965). (27) NICKON,A., AND LAMBERT, J. L., J . Am. Chem. Soc., 84, 4604 (1962). (28) NICKON, A., ET AL., J. Am. Chem. Soc., 85,3713 (1963). (29) HOWE,R., AND WINSTEIN, S., J . Am. Chem. Soe., 87, 915 (1965); FUKUNAGA, T., J. Am. Chem. Soe., 87, 917 (1965).
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