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 the unsatu3.00 g. (24.0 mmoles) of the dry, finely powdered, sorated carbon atom in t h e compounds listed in Table I. dium salt of l-nitrobutane in 25 ml. of methylene (3) Chemical shifts from the 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 and 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 treatment 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 and G Willfang, J . i w a k l . Chem., 164, 83 (1939).
( 5 ) This is the most stable of t h e nitronic esters prepared by us. I n the crystalline state it only begins t o show a lowering of t h e m.p. after 10 days a t room temperature.
13(iO
COMMUNICATIONS TO THE EDITOR
derived from primary, purely aliphatic, nitro compounds have half lives of the order of cu. 1 day for the more stable stereoisomer and CU. 1 hr. for the less stable stereoisomer. Nitronic esters derived from secondary nitro compounds are apparently less stable than those obtained from primary nitro compounds. Thus, both stereoisomers of the ethyl nitronic ester from 2-nitrobutane (they are produced in equal amounts) in carbon tetrachloride a t room temperature have half-lives of only SO min. Least stable of all is the ethyl nitronic ester derived from nitrocyclohexane which cannot be brought to room temperature without decomposing rapidly. Extensions of this new synthesis of nitronic esters are being explored and the chemistry of these corn pounds is under investigation. Acknowledgments.-We take pleasure in acknowledging the support of the U. S. Army Research Office (Durham). We are indebted to Mr. m'illiam E . Baitinger for determining many of the n.m.r. spectra and to Mr. Baitinger and Professor Norbert Muller for much assistance in interpreting these spectra. Satisfactory analyses for the compounds listed in Table I were obtained by Dr. C. S. Yeh, Mrs. T. Eikeri, Mrs. M. Hudgens, Mrs. V. Keblys and Mrs. A. Seo. We deeply appreciate Dr. Yeh's gracious cooperation in making special arrangements for the analyses of these rather unstable compounds. DEPARTMENT O F CHEMISTRY NATHAN KORNBLUM PURDUE UNIVERSITY R . ALANBROWN WESTLAFAYETTE, INDIA~A RECEIVED MARCH21, 1963 THE MECHANISM OF A DIELS-ALDER REACTION'
Sir: Previously, we reported' the isotope effects ~ I / ~ I=I 1.16 and ~ I / ~ I I=I 1.08 in the decomposition of the Diels-Alder adduct from 2-methylfuran and maleic anhydride. It was impossible, from these results, t o differentiate between a stepwise or concerted bond breaking. We now show that bonds a and b break
Monodeuteriomaleic acid was prepared by electrolytic dehalogenation of sodium bromomaleate in D 2 0 (lead cathode, -15') and converted t o the anhydride as reported.' Preparation of the IV-V mixture is similar t o t h a t for I . 2 The IV-V mixture (m.p. 75-76') in 30y0 dioxane70y0isooctane decomposed a t 50' for a specified time, then was cooled in ice and the solvent (and 2-methylfuran) was removed rapidly a t reduced pressure. Maleic anhydride sublimed (oil pump) and the adduct residue was either recrystallized or sublimed under high vacuum. N.m.r. determinations of IV/V before and after a large extent of reaction are recorded in columns 2 and 4 of Table I . TABLEI S.M.R. DETERMINATION OF ISOTOPE EFFECT
I1V t VI"
missinn. ( 2 ) S . Seltzer, 7'rli.ahrdr.oii / , r 1 1 ~ 1 ' 5 ,NO,11, 4Z7 ( l Q ( j 2 ) ( 3 ) See, P R , K B. Woodward and H. Baer. J . A m . Chrrit .Snc., 70, l l l i l (1948). ( 4 ) F. A . 1.. Anet, Cau. J . C ' h r i q 39, 78Y ( l Y ( i 1 ) ; M. M . Anderson and P. h l . Henry, Chein. I n d . (London), 2 0 . 3 (1961). (a) J. Bigeleisen and M . Wolfsberg, in "Advances in Chemical Physics," \'ol. I , I. Prigogine, E d . , Interscience Publishers, Inc., New York, N . Y . , 19.58, 11. 38, if)) A. Streitwieser. J r . , R. H. Jagow, R . C. Fahey a n d S . Suzuki, J. A m . C h m . .Sei.. 80, 2326 (19%); S. Seltzer, ibid., 83, 2623 (1961); 85, 14 lW3)
[IV]f/[V]fa obsd.
If the effect, k1/k11 = 1.16, is mainly due to deuteriation a t R1 ( i . e . bond b breaks in a slow step and a in a prior or latter rapid step) then after large conversion the remaining substrate would be richer in IV than in V. The ratios of final t o initial (IV/V) are given in column 6 of Table I for a hypothetical k v / k ~ v = 1.15 ( i . e . , a two-step reaction), For equal amount of bondbreaking of a and b a t the transition state ( i . e . j onestep concerted reaction), this ratio becomes 1.OO (column 5) because the effect, k v / k ~ v would , equal 1.00, The results shown indicate t h a t reversion conforms closely t o a concerted rupture of a and b. TABLE I1 DETERMISATION OF DEGREEOF REVERSIBILITY DURISC DECOMPOSITION OF 4-METHYL-7-OXA-BICYCLO[2.2.1]-2-HEPTENEEXO-5,6-DICARBOXYLIC ACID ANHYDRIDE
M
( 1 ) W o r k performed under the auspices of the U. S . Atomic Energy Com-
l I \ ' ) ~ ~ , ~ [ V l u " 7Z Reobsd action
[ I V / V ] f [IV/VIo calcd. One Two step stepsb
0.1132 111 1 . 0 0 i 0 . 0 2 87 0 . 9 9 f 0 . 0 4 1 . 0 0 1.31 ,0959 AI 1 . 0 2 f .06 81 1.00 f .04 1 . 0 0 1 . 2 4 ,0796 d l 1 . 0 1 f .02 82 1.00 i .0ic 1.00 1.25 Each ratio is the average of 20 or more integrations. Errors are average deviations. b Calculated by an equation given by Bigeleisen and Wolfsberg.6 Adduct purified by high vacuum sublimation.
Adduct"
simultaneously. Preparation of pure IV or V is precluded by the high 28 min. a t 5 0 ° ) 2 , 3 of this adduct. lability (&I2 Fortunately, however, the n.m.r. spectrum of I shows protons, Ri and R2,t o be represented by an AB quartet with J R , R = ~ 0 (JII~II~ E 7.2 c.P.s., T R , = 6.71, T R ~= 6.96, in CHC13).4 The quartet disappears in 11, is unaltered in 111, and becomes a doublet in a mixture of LV and V. The ratio of the peak areas in the doublet is then equal t o the molar ratio of IV t o V. Determinations of this ratio initially and after a large extent of , decomposition yield5 the isotope effect, k ~ v / k v which determines6 the relative amount of bond breaking of a compared to b in the slow step.
Vol. 85
Maleic a n hydride2,3-C1Pb .M X 10'
% Reaction
Sp. activity Observcd Calcd. % / in remaining %' reformed adduct reformed adduct m+C/mgC adduct
0.0940' 5.78 77.7 8.2 0.490 17.9 ,421 14.6 .0194d 1.25 78.8 1.5 78.6 6.9 .411 19.0 .0743e 3.52 a Adduct obtained by sublimation of the solid residue after evaporation of solvent, except see e. b Initial specific activity = 85.7 mpC/mgC. Vo = 250 ml. (30y0 dioxane in isooctane). VO= 1200 ml. (27% dioxane in isooctane). e Reaction mixture immediately cooled ; solvent lyophillized and adduct sublimed from the solid residue. Vo = 304 ml. (100% dioxane). f Calculate$ from the amount of reverse reaction as
ki A+B+C
(1)
+ + + +
+
2kzx kl knbo S S-k = St kl kzbo s 2k2x kl knbo - S where S = .\/(kl + k2bO)2+ 4klpzao,Az = a0 - x and the other notations have their usual meanlngs. Then calcd. % reformed (Sp. activity in adduct X adduct = (A2 - Al)/Al X 100. 9/4 X 100)/(sp. activity in diluted maleic anhydride at -"/c reaction). From ( 2 ) , In
+
+
The ratio, [IV/V]f, should refer t o unreacted substrate and not t o adduct reformed during decomposition. Reversibility was checked for in two ways: (1) by looking for C14-incorporation in the adduct after partial (7) S . Seltzer, ibid., 83, 1861 (1961)
