4485
COMMUNICATIONS TO THE EDITOR
Oct. 20, 1964
relatively small preference for its original AcO- partner. Assuming every ionization results in equivalence of the two ester oxygen atoms, one estimates that only 27Oj, of recombination is with the original AcOpartner. I n Table I1 the behavior of ROAc in acetic acid solvent is directly contrasted with that of the corresponding RC1 by means of the ratio, R, between rate of ionization and rate of chemical reaction incorporating a component of the solvent. For ROAc, this ratio is approximated by [(keq ke)/ke], while for RC1, the polarimetric/titrinietric rate ratio, k,/kt, is a lower limit. On this basis the ratio is greater than 38 with RCl and only 1.38 with ROAc. The latter is also much smaller than the corresponding figure of 2.5 for the fi-nitrobenzoate2d (ROPNB) even in 80% acetone. Thus the case of ROAc in the corresponding acetic acid solvent shows a uniquely small R value. While this was rather,anticipated, it will be necessary to examine other examples before a clear picture emerges regarding the importance of ion-pair return in cases of ionization where the leaving anion is the solvent lyate ion.
+
On the Question of Homoconjugation in 1,4,7-Cyclononatriene
Sir : cis,cis,cis-1,4,7-Cyclononatriene (I), recently reported from three different l a b ~ r a t o r i e s , l - is ~ of interest not only as a synthetic intermediate but from the point of view of homoconjugation. As pointed out previously,' HMO calculations on this hydrocarbon indicate a nonzero delocalization energy. In a recent communicat i ~ n ,Roth ~ and co-workers have referred to these calculations and have reported a heat of hydrogenation and molecular structure parameters from an X-ray diffraction study which give no indication of appreciable homoconjugative stabilization of the cyclononatriene ground state. We are prompted to comment further on the question of homoconjugation in the cyclononatriene ground state and to report the much larger effect in the cyclononatriene positive ion as indicated by the observed ionization potential. 3A
TABLE I1 RATIO( R )BETWEEN IONIZATION A N D CHEMICAL CAPTURERATES FOR p-CHLOROBENZHYDRYL DERIVATIVES (RX) RX
Solvent
T, OC.
Ra0
Rtb
ROPh'B RC1 ROAc
80% acetone AcOH AcOH
100 25 75
1.6 38 1.0
2.5
( k , / k t ) or
(k,/k). * [(keg
1.38
+ k t ) l k t l or [(kea + k ) / k e l .
For the ROAc system in acetic acid solvent, k, and ke are equal within experimental error, so the k,/ke ratio, akin to the k,/kt ratio for RCl, gives no indication of a gap between rate of ionization and rate of chemical capture incorporating a solvent component. It is the occurrence of 180-equilibration in the unexchanged ester which discloses the existence of the small gap. It seems attractive to give a carbonium biacetate description (IIb), rather than a carbonium acetate one (IIa), to the ion pair which results from ionization of ROAc and which gives ROAc back again by ion-pair collapse. The I I b ion pair can be visualized t o arise from an AcOH-promoted ionization of ROAc. The relationships between k,, k,, and ke, then depend on the extent to which I I b loses configuration and/or loses its original acetate component before collapsing to prodU C ~ . On ~ this basis only 27y0 of ion-pair collapse is with retention of configuration, racemization of I I b being 2.6 times as fast as collapse. Assuming equivalence of the two oxygen atoms in the original acetate component of I I b and also equivalence of both acetate groups in TIb, one calculates t h a t exchange of the original acetate component of I I b for a solvent-derived acetate occurs 0.82 times as rapidly as ion-pair collapse. On this basis, 55y0 of ion-pair collapse occurs before the ion pair exchanges and 45y0 after such exchange. (4) More separated ion pair species or dissociated carbonium ions m a y , in principle, occur between formation and collapse of some of the IIb t y p e ion pairs.
CONTRIBUTION No. 1742 OF CHEMISTRY DEPARTMENT UXIVERSITY OF CALIFORNIA Los AXGELES,CALIFORNIA90024 RECEIVED AUGUST5, 1964
ARTHURF. DIAZ S. WINSTEIN
I As reported earlier,' a simple H M O treatment with a = (p24/p12)leads to molecular orbital energy levels [ ( E - a)//3] of + ( a l), A d a 2 - a 1, and + d a 2 - a 1, as compared to A 2 , 1, and + 1 for benzene. These lead to a nonzero delocalization energy ( D E ) , in contrast to the situation in bicycloheptadiene and barrelene where DE is identically zero for symmetry reasons. The extent of homoconjugative stabilization predicted for I can be assessed by substituting a reasonable value5 for a , ca. 0.3, into the abovementioned bonding MO energy levels. The latter then consist of one level a t a f 1.30p and a degenerate pair at a 0.89p. The energy of the six-electron system in I is then 6 a 6.16p, the predicted DE being 0.16@,ca. 8% as large as in benzene. If fl is taken as 20 kcal./mole, D E in I is predicted to be ca. 3 kcal./mole. This is quite small and can easily be obscured by steric factors. In any case, it is too small to be disclosed by heats of hydrogenation because of the well-known difficulties in choosing model compounds for reference. Thus the results of Roth, Turner, et u L . , ~ are not a t all surprising. In their communication, Roth, Turner, et u L . , ~ refer to "more quantitative" MO calculations by Untch2 which suggest a negligible D E for I. Actually, Untch2 reported MO energy levels a t a 1.03p and CY 0.9858 (degenerate pair), leading to a DE 0.0270 as large as in benzene. However, Untch's published results contained an arithmetic error which he has corrected6 to give M O energy levels a t a 1.3Op and a 0.889fl. Thus, Untch6 predicts degenerate pair a t a
+
+
+
+
*
+
+
+
+
+
(1) P. Radlick and S. Winstein, J. 4 m . C h e m . S o c . , 8 6 , 344 (1963). ( 2 ) K. G. Untch, i b i d . , 8 6 , 3 4 5 (1963). (3) W. R . R o t h , A n n . , 6'71, 1 0 (1964). ( 4 ) W . R. R o t h , W . B. Bang, P . Goebel, R . L. Sass. R. B. Tu,-ner, and A . P . Yu, J. A m . C h e m S o c . , 8 6 , 3178 (1961). ( 5 ) (a) M. Simonetta and S. Winstein. J . A m Chem. S o c . , 76, 18 (1954); (b) C. F. Wilcox, S. Winstein, and W. G. .McMillan, ibici., 8 2 , 54.50 (19fi0): (c) R. J . Piccolini and S. Winstein, Telrahedron, Subpl , 2 , 4 2 3 (1963). (6) K G. Untch, J. A m . C'hPm. .Soc.. SI, 4061 (IQfi:+).
4486
COMMUNIC.4TIONS
TO T H E
EDITOR
\'ol. S ( i
a DE 7.8% as large as in benzene, identical with the one potential of bicyclo [2.2.1Iheptadienel? is appreciably mentioned earlier. smaller than that of bicyclo[2.2.1]heptene.'1This The situation as regards homoconjugative stabilizageneral phenomenon and semiempirical methods t u tion can be expected to be markedly different in the calculate the effects are being explored further. parent positive ion from ionization of I. Since ioniza( 1 2 ) S . hreyerson, J . D. SIcCollum, and P N Rylander. J Aril C h v m s a c . , 83, 1401 :1481) tion of an electron is from the a 0.89p M O , one could expect the positive ion to be stabilized by 0.1lp CONTRIBUTION NO 1752 S WINSTEIN DEPARTNEST OF CHEMISTRY more than the ground state. the ionization potential OF CALIFORNIA UNIVERSITY (I.P.) being lowered by 0.118 compared to a correspondLos XNGELES,CALIFORNIA90024 ing monoolefin. Another factor which should lower DIVISIONOF PURECHEMISTRY F P. LOSSISC: the I.P. of I has to do with the neglect of electron SATIONAL RESEARCH COLXCIL OTTA~VA 2 , CASADA repulsion in the simplest HMO treatment. LVith the so-called w-technique,7 some electron repulsion can be RECEIVEDXUGLST 25, 1964 introduced within the framework of the LCAO method. Thus, by setting the Coulomb integral a equal to a Racemization and Oxygen Exchange of Trisubstituted wqP, where q is the charge on the carbon atom, Phosphine Oxides' and using w equal to 1.4, S t r e i t ~ i e s e rand ~~ Ettir~ger~~ Sir: have successfully correlated ionization potentials of many hydrocarbons. For a monoolefin q is 0.5 and The recent study of the racemization and oxygen exthe positive ion has one a-electron, so the w-calculation change of aralkyl sulfoxides by hydrogen chloride under lowers I.P. by 0.5(1.4)@or 0.78. In the positive ion a variety of conditions has prompted a similar study from I, with five a-electrons and q equal to one-sixth, with trisubstituted phosphine oxides. the w-calculation lowers I . P . by five-sixths (1.4) 8 or Phenylmethylpropylphosphine oxide, [ a ] " ~ 1X.5", 1.178, 0.478 more than in the case of the monoolefin. in dry dioxane saturated with hydrogen chloride was From the sum of the 0.478 and 0.118terms and the allowed to stand a t room temperature for 9 days, a t existing correlations between I.P. values and MO which time the specific rotation was f0.3". Periodic calculations,i we could expect the I.P. of I to be ra. 1 measurement of the rotation during the %day period e.v. lower than a corresponding monoolefin. gave the following results: after 20 h r . , f15.8'; 64 h r . , The electron impact I . P . of 1,4,7-cyclononatriene +10.8"; and 7 days +:3.4". The phosphine oxide does indeed turn out to be unusually low. The figure was recovered in 8070 yield and shown to be racemic. obtained, 8.72 e.v., is less than that for benzene8a In another experiment the phosphine oxide, 0.00 148 (9.50), tolueney (9.23), or p-xyleney (8.88). It is also mole, was dissolved in a hydrogen chloride saturated less than that for propylenesby (9.94, 9.84) or 1,2-disolution of 1 ml. of water, containing ca. 1.45 atom yo substituted olefins such as cis-2-butenesc (9.34) or excess l80,and 4 ml. of dioxane, The solution rapidly cyclopentenesC(9.27). In fact, i t is as low as, or lower became yellow and measurement of the rotation was exthan, the values for typical conjugated 1,%dienes such tremely difficult. After 55 days a t room temperature, as c y ~ l o p e n t a d i e n e (9.00, ~ ~ , ~ 8.9), isoprene'O (9.08), and the phosphine oxide was recovered and shown to be 1,3-pentadienesc(8.68). racemic. Oxygen-18 anaIysis showed i t contained 0.87 I n considering the I.P. of I, we must recognize t h a t atom yo excess of 1 8 0 . 3 In a similar experiment triionization potentials of olefins appear to be subject to phenylphosphine oxide containing 0.85 atom o/G excess strain and alkyl-substituent effects. As regards strain, l*O was treated for 80 days with hydrogen chloride disany effect due to slight strain4 in I is probably very solved in water-dioxane ( 1 : 4 ) , The oxide was resmall. Since the olefinic groups in the cyclononatriene covered and shown to contain 0.55 atom yo excess are 1,2-dialkylsubstituted, cis-2-butene represents one of The results of these experiments clearly demonstrate the best available reference compounds for comparison t h a t phosphine oxides are subject to racemization under with I. This choice may well involve an overly generous the conditions specified. The process is certainly much allowance for alkyl substituent effects since the geomslower than t h a t found for the aralkyl sulfoxides. The etry of the crown conformation of I is not especially diminution in rate in the presence of water is in accord favorable to hyperconjugative stabilization of the posiwith the sulfoxide experiments, Cnfortunately, it was tive ion due to C-H electron delocalization. Thus, the not possible to correlate oxygen exchange with rate of ca. 0.6 e.v. (14 kcal./mole) by which the I.P. of I is racemization ; however, it is interesting to note that lower than t h a t of 2-butene may well be a conservative water is not required for racemization. Mislow, et measure of the decrease in ionization potential of the have suggested that sulfoxide racemization occurs by cyclononatriene. formation of R1R2SC124;however, it is not necessary t o The lowering of ionization potential due to proper form R1R2R3PC12to have racemization of the phosphine juxtaposition of nonadjacent olefinic groups may well oxide. Undoubtedly the first step involves protonation be a general phenomenon. For example, the ionization of the oxide oxygen, This should be a rapid and reversible reaction, and also one highly dependent on the ( 7 ) ( a ) A . Streitwieser, J r . , J . A m . Chem. SOL., 82, 4123 ( 1 9 6 0 ) , ( b ) R . Ettinger, Telvahedron. 20, 1579 (1964) basicity of the medium. Addition of chloride ion yields (8) ( a ) I . P. Fisher, T. F . Palmer, and F . P.Lossing, J . A m . Chem. Soc., 11, the pentacovalent compound, Racemization can 8 6 , 2711 ( 1 0 6 4 ) .
+
+
+
(b) J . Collin and F . P. Lossing, i b i d . , 79, 5848 (1957); (c)
J . Collin a n d F. P Lossing, t b L i . , 81, 2064 (1959). ( d ) A. G . Harrison, L. R . Honnen, H. J . Dauben, J r . , and F . P. L o s i n g , i b i d . , 81, 8593 (1960).
(9) See ref ? a for tabulated I . P . values and literature references. ( I O ) J . D . Morrison and A . J. C . Nicholson, J . Chem. P h y s . , 20, 1021 (IRAZ). (11) For example, I.P. is 9.05 for t h e highly strained bicycloheptene ( I . P . Fisher and F P. Lossing, unpublished work).
(1) Research supported by t h e National Science Foundation under N S F GP-202. ( 2 ) K . Mislow, T. Simmons, J . T. hfelillo, and A . L T e r n a y , J . A m C h e n . S a c , 8 6 , 1452 (1964). (3) Result of one analysis only. Because of t h e extreme hygroscopicity of this material t h e value m a p be low. (4) This could be an intermediate or transition s t a t e .
