COMMUNICATIONS TO THE EDITOR
1358
PREDOMINANT IMPORTANCE OF..ENTROPY ON RELATIVE STABILITIES OF BICYCLOOCTANE ISOMERS
Sir: Aluminum bromide converts CsH14 bicyclic hydrocarbons to their three most stable saturated isomers : bicyclo [2.2.2]octane (I), cis-bicyclo [3.3.0]octane (11) and bicyclo [3.2.l]octane (111). Entropy rather than enthalpy differences are shown here to have decisive importance on the relative stabilities of these compounds.
I
I1
Barrett and Linstead' reported that I1 was largely rearranged into 111 with AlC1,; heat of combustion measurements established that trans-bicyclo[3.3.0]octane was G kcal./mole less stable than the cis isomer 11. The same catalyst was said to transform the 2methylnorbornane stereoisomers I V into I11 a t 7 5 " , but no mention was made of the possible presence of I or I1 in the product. In recent years numerous reports of the interconversion of simple monofunctional derivatives of I-IV by carbonium ion processes have a ~ p e a r e d , ~but -~ there have been no quantitative investigations of the relative stabilities of these ring systems. Such information, which we supply in the present report, is needed for a better understanding of rearrangements involving these systems. The rigidity of I and 111 further makes them ideal for testing predictions of quantitative conformational analysis. Recent experiments under thermodynamic conditions have confirmed the generally held belief that derivatives V I are more stable than V; an approximate - A F of isomerization of 0.5 kcal./mole can be estimated from data of Goering and S l ~ a n . Because ~ of heightened ring strain, norbornane derivatives are much less stable than ring expanded products, but it is not clear from the available literature' whether derivatives of I1 or of I11 are the more stable.
Qx
Equilibrium conditions3
-
but trans-bicyclo [3.3.0]octane could not be detected. From the composition data (Table I) the free energies, enthalpies and entropies of isomerization were calculated for each pair of isomers (Table II).7 TABLE I RELATIVEEQUILIBRIUM COSCESTRATIOSSOF BICYCLO~CTASE ISOMERS
-
Temp.,
. 'X VI 2 70%
Compounds 1-117 were equilibrated with aluminum bromide a t various temperatures in the 23-72" range and the product compositions determined by capillary gas chromatography (Table I) .' Moderate quantities of methylnorbornanes and fragmentation products also were present (these will be discussed in the full paper), (1) J . UT.Barrett and R . P. Linstead, J . Chem. Soc., 611 (1936). (2) >B. I.Turova-Polyak, I. E. Sosnina, I. G . Golutvina a n d T. P. Yudkina, Z h . Obshrh. Khim.,29, 1078 (19.59); Chem. Abstr., 64, 1356 (1900). C J ) A good summary of the somewhat conflicting earlier literature on 1-111 derivative interconversions has been given b y H . L. Goering and hl. F. Sloan, J . A ? n . C'hem. Soc., 83, 1397 ( 1 9 0 1 ) , in addition, see ref. 4 . ( 4 ) H . I.. Goering and X I . F. Sloan, ibid., 83, I992 (19iil); H. M.Walborsky, R I . E Baum and A . A . Youssef, ibid.. 83, 988 ( 1 9 ( i l ) ; A . F. Bickel, J . Kniitnerus, E . C. Kooyman and G . C. Vegter, Teliahedioiz, 9, 230 (1900). (.7) Ring expansion of norbornane systems: K . Alder a n d R . Reubke. A??., 91, 1525 (1958): J A . Berson and I ) . Willner, J . A m . Chem. Soc., 84, li7.i (19li2); J A Berson and P. Reynolds-Warnhoff, ibid., 84, G82 (1902); R . K . Sauers and R. J . Tucker, J . Oig. Chem. 28, 870 (19031, and earlier references therein cited. ( G ) Interconversions of bicyclo[3.3.0]octyl and bicyclol3.2 1 joctyl systems: A . C Cope. J >I Grisar . and P E . Peterson, J . A m . Chem. Sac., 82, 42!J!J (IgGOI, and C. S. I'oote, Ph.1). Thesis, Harvard University, IO(j1. ( 7 ) T h e general procedure used has been descrihed in detail; K . R . IJlanchard and P \ o n R . Schleyer. J . Oig. C'hrin., 28, 247 ( 1 Y I i : j ) .
O K .
296.8 317.4 321.8 331,i 345.4
IV
I11
Vol. s5
Percentage composition7 Bicyclo[2.2.2]cis-BicycloBicyclo[3.2.1]octane, I [3.3.0]octane, 11 octane, 111
3.66 3.41 3.36 3.33 3.11
32.95 37.20 37.85 39.78 12.34
63.35 59,35 58.78 56,86 51,50
TABLEI1 THERMODPSAXIC VALUESFOR BICYCLO~CTASE ISOMERIZATIONS Reac- Prod tant uct
I I1
I11
I
I11
I1
1Fzg8 (kcal./mole)
-1.31 f 0.10 - 0 . 3 8 f 0.06 -1.69i0.12
A H (kcal./mole)
+1.94 f 0 . 4 5 -1.89 xt 0 . 2 5 +0,06=!=0.15
A S 2 @ (e.u.)
f10.9 i 1.6 - 5,O & 0 . 8 5.5k2.5
+
For the temperatures investigated, the stability order was TI1 > I1 > I. An analysis of the thermodynamic data (Table 11) is highly interesting. The enthalpy difference between bicyclo [2.2.2]octane (I) and bicyclo [3.2.lloctane (111) is indistinguishable from zero; the considerably greater stability of 111 over I is due entirely to more favorable entropy. Both I and 111 are less strained than cis-bicyclo [3.3.0]octane (11) by a substantial amount, AH = - 1.9 kcal.;mole, but this is counterbalanced by the very high entropy of 11; in fact, above 37S"K. I1 should be the most stable of the three isomers. The major source of strain in bicyclo[2.2.2]octane(I) is torsional; three eclipsed ethane interactions yield an estimate of 2.5 x 3 = 5.4kcal.'mole strain for this molecule. Cyclopentane is known to be strained to the extent of 6.3 kcal.jlmole.8 The five-membered ring in bicyclo [3.2.1]octane (111) is distorted further by valency requirements. It is not hard to understand why the total strain in 111 due to angular and torsional deviations and to non-bonded repulsions should equal the total strain in I . cis-Bicyclo [3.3.0]octane possesses eight cyclopentanoid carbons, each strained roughly to the extent of G.3/3 = 1.2G kcal. The total strain in 11, estimated in this crude way, is 1.20 X 5 = 10.1 kcal./'mole or 1.7 kcal./mole greater than that estimated for I. This difference is in good agreement with the experimentally determined heat of isomerization of I to 11, 1.9 k ~ a l . , / m o l e . ~ The high entropy of I1 must be associated with the extreme flexibility of the cis ring juncture, as noted for other fused-ring bicyclic compounds, l o and with the many possible cyclopentane conformations of equal en erg^.^,^ The entropy differences between the much more rigid isomers I and 111 appear to be due primarily to symmetry considerations.8 If bicyclo [2.2.2]octane (I) possesses D3h symmetry," it then has a symmetry number of six. The entropy of I should therefore be lowered by R In 6, or 3.6 e.u.S Since I11 has a sym(8) E. I.. Eliel, "Steretichemistry of Carbon Compounds," RIcGraw-Hill Book C o . ,I n c . , S e w York, N. Y . . 1962; also see K . S. Pitzer a n d W. E . I h n a t h , J . A m . Chem. Soc., 81, 3213 (1959). ( V ) Computer calculations of the strain present i n bicyclobctane isomers are in progress, J , B. Hendrickson, personal communication. See J . B . Hendrickson, ibid., 83, 4.537 (1961); 84, 3355 (1962). ( I O ) S . L. Allinger, J . O v g . Chem., 21, 915 (1950); N . L. Allingerand J. L. Coke, J . A m . Chem. Soc., 81, 4080 (19.59); 82, 2553 (19GO). (11) For a literature summary and a discussion, see P . von R . Schleyer and R . I). Sicholas, i b i d . , 83, 2700 ( l e l i l ) , especially J . J. MacFarlane a n d 1. G R m s , J . C-hem. Soc., 41ij9 (1960)
May 5, 1963
COMMUNICATIONS TO THE EDITOR
1359
ether. The combined filtrate and wash are rapidly evaporated in vacuo to constant volume a t ca. 5 to 15”. The resulting very pale yellow liquid is subjected to a vacuum of ca. 1 mm. for another 5 min. a t 25” and stored a t -78”; n2’D 1.4468; yield 2.72 g. (95%). Anal. Calcd. for C6H13N02: C, 54.94; H, 9.99; N, 10.68. Found: C, 55.16; H , 9.96; N, 11.02. In principle, nitronic esters in which R # R‘ should exist as cis-trans isomers; such stereoisomerism, however, has not been reported. By the use of nuclear magnetic resonance (n.m.r.) we have now found that such stereoisomers do indeed exist. Thus, the ethyl nitronic ester obtained from 1-nitrobutane has an n.m.r. spectrum in which two “vinyl” hydrogen2 quartets ap(12) Alfred P. Sloan Research Fellow. (13) National Science Foundation Predoctoral Fellow pear. One quartet (at 6.04 6 ) l has an area about (14) A . B. Thesis, Princeton University, 1957. seven times greater than the area of the second quartet FRICKCHEMICAL LABORATORY PAULvos K. S C H L E Y E R ~(at ~ 5.75 6). In the same way, all the nitronic esters PRINCETON CNIVERSITY KENNETHR.BLANCHARD13 in Table I which derive from primary nitro compounds PRINCETOS, SEW JERSEY CHARLESD. W O O D Y ’ ~ prove to be mixtures of the cis-trans isomers in proporRECEIVEDMARCH8, 1963 tions varying from 1:1 to 7 :1. The realization that cis-trans isomerism is exhibited by nitronic esters clarifies some otherwise puzzling THE SYNTHESIS AND CHARACTERIZATION OF NITRONIC ESTERS facts. Thus, the crude ethyl nitronic ester obtained Sir: from p-nitrophenylnitromethane (92% yield) gives good carbon, hydrogen and nitrogen analyses, but it Hitherto there has been no generally useful way of melts over a wide range ( 8 2 - B o ) . Recrystallization synthesizing nitronic esters (I). raises the m.p. to 1OO-10lo, but the analyses remain K 0unchanged. This anomaly is resolved by n.m.r. ; the ‘\ +/ spectrum of the “crude” nitronic ester has two “vinyl” C=K hydrogen singlets (at. 7.22 6 and 6.95 6, in approxi/ \ R‘ OR“ mately a 4 : l ratio) whereas the recrystallized ester I shows only the “vinyl” hydrogen singlet a t 7.22 6. K,R’ = H, alkyl or aryl Recrystallization has simply separated the stereoR ” = alkyl isomers. Of the nitronic esters thus far prepared from secondIndeed, those derived from strictly aliphatic nitro comary nitro compounds, only the ethyl nitronic ester pounds are unknown. derived from %nitrobutane (cf. Table I) is capable of We now report a new and general synthesis of nitronic exhibiting cis-trans isomerism. Here again clear eviesters which involves treating the salts of nitro comdence of stereoisomerism is obtained; 1wo singlets (at pounds with trialkyloxonium fluoroborates’ a t 0” ; the 1.94 6 and 2.00 6) due to two different “vinyl” methyl reaction is rapid and pure nitronic esters are generally groups are found in the n.m.r. spectrum. obtained in yields of excess of 90% (cf. Table I ) . Both stereoisomeric nitronic esters are also produced RR‘CSO2-Sa+ + R3”O+BF4- +I + S a B F 4 + R 2 ” 0 when diazomethane reacts with p-nitrophenylnitromethane and p-br~mophenylnitromethane.~ QuanTABLE I titative yields of the “crude” nitronic esters are obKITRONIC ESTERSPREPARED FROM O X O N I U M tained, but although analytically pure, these products FLUOROBORATES AND NITROSALTS“’* melt over wide temperature ranges. As before, n.m.r. Yield, % studies demonstrate that in each instance mixtures of 90-95 both possible stereoisomers are a t hand; and, as be94 fore, recrystallization serves to separate the stereo79 isomers. 95 Nitronic esters are stable indefinitely a t - 78”, but 95 a t room temperature they decompose relatively 92 rapidly, particularly in the liquid state or in solution. -I 5-80 Furthermore, for a given nitronic ester, the stereo90-95 isomer which is produced in minor proportion proves to be less stable than the isomer produced in major proCHI portion. Thus, the more stable isomer of the methyl a All the nitronic esters mentioned in this Communication nitronic ester of p-nitr~phenylnitromethane,~ in deuexhibit intense absorption in the 6.05 to 6.2-r region. * The teriochloroform solution, has a half-life of 5 days, yields of sodium fluoroborate usually exceed 95%. Prepared a t whereas the less stable isomer has a half-life of only -65 t o -60”. several hours. In general, the ethyl nitronic esters As a typical example: to a stirred, ice-cold, slurry of (2) By “vinyl” hydrogen we refer t o t h e hydrogen atom on t h e unsatu3.00 g. (24.0 mmoles) of the dry, finely powdered, sor a t e d carbon a t o m in t h e compounds listed in Table I. dium salt of l-nitrobutane in 25 ml. of methylene (3) Chemical shifts from t h e internal standard, tetramethylsilane, are reported in 6 values. T h e n.m.r. spectra were obtained with a Varian Aschloride is rapidly added an ice-cold solution of 4.05 g. sociates A-60 spectrometer. (21.8 mmoles) of triethyloxonium fluoroborate in 15 ml. (4) F. Arndt a n d J , D . Rose, J . C h e m . SOC., 1 (1935). This reaction is of methylene chloride. After being stirred under nitroonly applicable t o nitro compounds which are relatively strong acids. Thus, gen for 15 min., the mixture is filtered rapidly and the we have shown t h a t 3-phenyl-1-nitropropane undergoes n o reaction whatsoever on t r e a t m e n t with diazomethane even after 5 days. solid washed with 5 ml. of ice-cold, anhydrous ethyl
metry number of one, there is no symmetry effect of its entropy and ilS1-111 should be f 3 . 6 e.u., a value in approximate agreement with that found experimentally (Table 11). The high symmetry of I is destroyed in its derivatives, such as V, and such derivatives should be favored to a greater extent than the parent hydrocarbon, I, in equilibration processes, e.g., V with VI.3 It is novel to find that the course of some organic reactions can be governed largely by symmetry and entropy factors. Acknowledgment.-The gas chromatography apparatus was purchased with a Grant-In-Aid from the F h l C Corporation.
(1) H. Meerwein, E . Battenberg, H. Gold, E. Pfeil a n d G Willfang, J . i w a k l . Chem., 164, 8 3 (1939).
( 5 ) This is t h e most stable of t h e nitronic esters prepared b y us. I n t h e crystalline s t a t e i t only begins t o show a lowering of t h e m.p. a f t e r 10 days a t room temperature.
