June Ti, 1960
XIETHYLENE CROUPS IN CYCLOPENTENE-3,5-DIONE
be, there remains the comforting fact that dimethylnitrosamine also differs from the effective amide catalysts by being devoid of measurable basicity (Table 11, group a). Dipole Moments and Polarizabilities.- The present work has served to emphasize the primacy of linear n-bond moments (as compared to a-bond moments) of polar substances in determining their ability to solvate sodium ions in nonpolar media and, consequently, to catalyze the alkylation reaction. Since the observed dipole moment of an unsaturated polar substance is the algebraic sum of its u- and a-moments, it is not surprising that little connection between dipole moment and catalytic activity can be found. Table IV lists a numTABLE I\’ DIELECTRICCONSTAVTS AI’D
DIPOLE MOMEYTSOF R E -
AGEhTS
Compound
CeHe C2H,Br (CJLO)*CO CHSCIi CeH,CiC’ (CH?)?CO ICH3)?\‘XO CoH,,P\IOJ C*H,OH (CH~)~SCOS(CH~)L HCOAT(CHj)2 CH,COS( CH3)l (CHsI-SO Pyridine S-oxide
€
PI
3 9 3 39 28 21
0 1 0 3 3 2 3 4 1
D
8 9 2 9 8 98 0 7 3 3 3 82 3 87
36 25
37 38 45
4 28
pinching the triatomic molecular orbital in t h e middle and of decreasing delocalization of t h e lone pair of t h e terminal nitrogen. The energy barrier must then be accounted for by overlap of the two lone pair orbitals and the dipole moment by t h e atomic dipole of the central nitrogen combined with t h e dipole of t h e h--O bond. T h e nitroso group thus resembles the nitro group of nitrobenzene more t h a n t h e carbonyl group of a n amide. (37) Compare G. G . Stewart and H. Eyring, J . Chem. E d . , 36, 550 (1RS8j .
[COYTRIl3UTIOX F R O X THE
Cyclopentene-3,5-dione.
DEPARTMENT
2900
ber of reagents and additives used in this work together with their dielectric constants and dipole moments. As the additives are listed in roughly increasing order of activity, it is easy t o see that, while the ineffective compounds may or may not possess strong dipoles, all of the effective additives do, This is compatible with the contention that a certain kind of dipole (due to a linear n-moment) is necessary for catalytic action. It is well known‘@that separation of charge stiffens or reduces the eli ctron polarizability of a bond. Furthermore, a-electrons are usually more polarizable than c-electrons and contribute more to the refractivity of an unsaturated group. That the group refractions (rlwtron polarizabilities) of the PO and SO bonds are much smaller than those for the CO and CIi groups is further indication that, in the former, the a-orbitals are more polarized in the ground state than they are in the latter. The trend discernible in Table I11 also betokens a similar variation of r-orbital polarization, roughly related to catalytic activity, within the series of amides. Judging from these observations, then, it appears that the burden of success in solvation rests with the additive. The importance of inductive polarization brought about by proximity of the positive sodium ion must be relatively small. Again, it is the a-electron density in the ground state of the additive that is critically important.39 Acknowledgments.-The author is indebted t o Mr. Frank Chadde for the ultraviolet spectra, Mr. William Washburn for the infrared spectra, Miss Gwen Prior for the densities and refractive indices and to I I r . David Wimer for the potentiometric titrations. (38) C. K . Ingold, “Structute and Mechanism in Organic Chemistry,” Cornell University Press, Ithaca, N . Y . , 1953, p. 129-130. (39) These views are consistent with Mason’s18 contention t h a t in Nmethyl-2-pyridone, the first electronically excited state is less polarized than t h e ground state.
NORTHCHICAGO, ILL
OF
CHEMISTRY, I O W A STATE L-NIVERSITY]
111. Reactions of the Methylene Group’,?
BY C . H. DEPUYAND P. R.I ~ ~ E L L S RFCEIVED SOVEMBER 2, 1959 Reactions of the active methylene group of c\-clopentene-3,j-dioneare reported It has proved possible t o condense it with benzaldehyde in acid solution, through the intermediacy of P-hydroxy-cyclopentadienone, a s well as in basic solution. By reaction with aryldiazonium salts the arylhydrazones of cyclopentene-3,4,5-trione have been prepared. Some studies on the kinetics of the base-catalpzed polymerization of the dione are recorded and a mechanism for the polymerization is suggested.
Cyclopentene-3,5-dione (I) is a stable, highly reactive compound easily prepared by the oxidation of the commercially available dioL3 Because so little is known about the synthesis and properties of highly unsaturated five-membered ring compounds, and because this enedione contains such a (11 Paper I1 in this series, C. H. DePuy and C. E . Lyons, THIS 83, 631 (1960). (2) Supported by a grant from t h e National Science Foundation. (3) C. €1, DePuy and E;. F. Zaweski, THIS J O U R N A I . , 81, 4920
JOURNAL,
( I 9.59).
variety of reactive centers, we have been exploring the synthetic utility of this compound. Reductions, derivatizations and Diels-Alder reactions have already been reported.‘V3 I n this paper we describe some reactions of the methylene group.
h’ormal alkylation and condensation reactions of cyclopentene-.’3,.S-dione are made synthetically difficult (arid correspondingly interesting froin t h e theoretical standpoint) by two unique aspects of its chemistry. We have already pointed out that the molecule is apparently completely ketonic, the enol form I11 being unfavorable because o f its relationship to cyclopentadienone.l-l I t was anticipated, then, that acid-catalyzed condensations would be especially difficult. On the othcr hand, the enedione is exceptionally unstable in base, polymerizing very rapidly under the Irlildrst conditions (z~idei n f ~ a ) . This fact severc ly limits this route for the preparation of conderisation and alkylation products. As reasonably typical of aldol condensations, the preparation of the benzylidene coni jiountl was extensively investigated (equation 1). If the condensation be attempted in the presence of 0
0
benzaldehyde complex in dry ether.fi I t seems that condensation rcactions of cvclopentene-~,.j-dioiie can be successfully carrictl out, h i t that the conipeting polymerization in base makes the acidcatlyzed reactions most promising. The benzylidene compound displays some intcresting reactions in its own right. It undergoes the Diels-Alder reaction readily, although less so than does the dione itself. Thus, when allowed t c ) rcact with cyclopentadiene in benzene solution a t room t e m p rature, it was gradually convcrted t o the adduct T’, n1.p. 180-1?lo. This same product was obtained when the adduct of cyclo0
‘b pentene-3,3-dione and cycl~pentadiene~ was condensed with benzaldehyde i n alcoholic base. Its structure is also supported by its infrared spectrum, which bears a strong resemblance in the carbonyl region to that of the benzylidene derivative of indane-l,3-dione The pertinent comparisons are recorded in Table I. The ultraviolet spectrum of the ben7ylidene compound is shown in Fig. 1.
aqueous or alcoholic base, no benzylidene conipound is isolated, the only product being polymer from the enedione. Sumerous attempts a t basecatalyzed reactions led to the same results. lye TABLE I next turned our attention to acid-catalyzed conC A R R O \ Y L FREQ‘L‘ENCIES OF CYCLOPEUTEUE-3,j- A Z D densations, for we were especially interested in INDAh’E-1, Q - D I O h E S assessing the ease with which the enol form could Compound C m -1 occur. Surprisingly, only rather mild conditions Cyclopentene-3,5-dione 1T48,1712 were required. Thus, a mixture of the dione and Indane-1,3-dione 1750,1710 excess benzaldehyde in ether containing B F:i, 4-Renzylidene-cyclopeiitene-3,5-dion~ 1748. 1680 when heated at reflux for one hour, readily formed 2-Benzylidene-indane-1,3-dione 1737,1690 the yellow, crystalline benzylidene derivative in about 50% yield. Yields were appreciably smaller The benzylidene compound further resembles (25yc)when equimolar amounts of dione and the corresponding derivative of indan dione in aldehyde were used. Although the BF3-catalyzed condensation un- its ready reversion to benzaldehyde and the dione. doubtedly proceeds by way of an intermediate Lyhen an alcoholic solution is made basic, it imclosely resembling the enol form 111, the presence of mediately develops ultraviolet spectrum characterthe R F 3 and the use of a non-polar solvent compli- istic of an equimolar mixture of benzaldehyde and cate tho picture. Consequently, the condensation polymerized dione. 9 n d if the original alcohol was investigated under conditions in which the solution be treated with 2,4dinitrophenylhydrazine, enol itself is a still more likely intermediate. It benzaldehyde 2,4-DNP is formed in gcod yield. was found that the benzylidene compound was again The compound is, on the other hand, stable to formed (in 42YGyield) when the dione, an excess of acids and does not react with 2 , 4 D S P H in acid benzaldehyde, and a trace of sulfuric pcid were solution. Catalytic hydrogenation of the benzylidene cornheated under reflux in glacial acetic acid. It seeins clear, then, that while the tautomeric hydroxycyclo- pound in ethanol over platinum oxide resembles pentadienone form is not stable enought to be ob- hydrogenation of the dione itself? in that someservable, it is in ready equilibrium with the di0ne.j what more than two molar equivalents of hydrogen iVith the success of the acid-catalyzed condensa- are absorbed and a complex mixture of products is products were not investigated ~ tion, we examined more thoroughly condensations f ~ r r n e d . These catalyzed by bases. It was found t h a t the reaction ,o OH could be carried out by the use of aprotic solvents and weak bases. Thus, a very small amount (5y0 yield) of 4-benzylidene-cyclopentene-3,5-dione was obtained using triethylamine in benzene ‘0 ‘0H for the base and more reasonable yields (20%) (6) s.r. Leonard, J. C . Little and h 1. Kresge, THIS1 O C K h . A I . , were realized using the pre-formed piperidine 7 9 , G47G (19Z7j. (4) A . I. T’ogel. “Elementary Practical Organic Chemistry,” Y o . ‘2. Longmans, Green and Co., London, p. 148. ( 5 ) I t shnuld he recalled t h a t cyclopentane-I ,3-dione is completely enolic; see ref. 3 fur discussion.
( 7 ) I n this regard i t might he p‘inted o u t t h a t the benz]lideiie cnmpound might he considered t < j l i e a “phenylfulvene quinone,” and t h a t the hydrogenation (and perhal)s other reactions) might take place by 1.6-addition.
METHYLENE GROUPSIN C Y C L O P E ~ ~ T ~ . : ~ ' E - ~ , ~ - D T O N E
June 5, 1960
!
2911
7
40 60 80 Time, min. Fig. %.--Variation of pH of aqueous solutions of cyclopentene-3,z-dione with time: (1) 0,008 X; ( 2 ) 0.04 M ; ( 3 ) 0.1
20
200
250
300
350
Wave length, mp Fig. 1.--Ultraviolet absorption spectra in aqueous cj-clopentene-3,5-dione; ( B ) 4-benzylideneethanol for: (-4) cyclopentene-3,5-dione; (C) cyclopentene-3,4,5-trione-phenylhydrazone.
x.
trapped by a more reactive electrophile than a second dione molecule. The ultraviolet and infrared spectra of the products support the hydrazone structure VI1 rather than the tautomeric azo structure VI. These hydrazones are soluble Attempts to carry out condensation reactions in in base, forming highly colored anions, and are basic media with a variety of other compounds, very unreactive in the Diels-Alder reaction. including nitrite esters and acid halides, led only Our interest in the parent cyclopentene-3,4,5to polymeric material. Obviously self-condensa- trione led us to attempt the hydrolysis of these sation is too rapid to allow competition from any phenylhydrazones. Under moderate hydrolysis but the most reactive substrate. It was reasoned conditions with formaldehydelo or levulinic acid" that a diazonium ion might be reactive enough to the hydrazones were recovered unchanged, and compete successfully for the enolate ion. That under the drastic conditions necessary to effect active methylene compounds will react with diazo- any reaction, the hydrolysis products, if any were tized aromatic amines has been known since the formed, were not stable. Other approaches to this time of V. Meyer, who treated nitroethanes and interesting compound are being explored. acetoacetic esterg with benzene diazonium saits. Since the base-catalyzed polymerization interThe initial products of such reactions, mixed azo fered so drastically with many reactions of the compounds, are tautomers of aryl hydrazones, and dione, it seemed pertinent to study it in more dethe tautomerization is generally greatly in favor tail. When a dilute solution of the dione is treated of the latter. with base, a red color rapidly develops. AcidiLVhen an acidic solution of cyclopentene-3,5- fication changes the color to yellow, but no dime dione and benzenediazonium chloride or p-nitro- can be recovered from the solution. Evaporation benzenediazonium sulfate are mixed, no reaction of the solvent leaves a tan solid, very soluble in takes place. Addition of sufficient sodium acetate water and insoluble in organic solvents. If a consolution to neutralize the acid present gives an centrated solution of the dione is treated with base, immediate precipitate of the 4-phenyl- or 4-p- a polymer precipitates which is insoluble in water nitrophenylhydrazone of cyclopentene-3,4,5-trione and organic solvents. When an aqueous solution (171). Thus the enolatc ion has effectively been of the dione is allowed to stand it gradually polymerizes, but it is stable in acid solution. The rate of polymerization has been followed by two methods. As mentioned above, in water solution the pH of the solution falls (Fig. 2 ) , rapidly a t first, then slowly until a constant value is reached and all the dione has polymerized. From the data obtained] it is possible to extrapolate .o .o to zero time and estimate, approximately, the initial rate of reaction (Table 11). These results, although based on an uncertain extrapolation, suggest that the initial step under these conditions is (8) V. hleyer, Ber., 8, 751 (1875). (91 V. Meyer, i b i d . , 10, 2075 (1877).
(10) Cf.A L a p w o r t h , J . Cheni. S U C .91, , 1133 (1907). (11) C . H. D e P u y a n d L1. \V, P u n d e r , IS JOURSAL, 81, 4G29 (1959).
the ionization of the dime to forin the eiiolate anion. Little information can be derived from the later stages of the reaction where, amongst other complications, the acidity of the medium is changing. TABLE I1 ISITIAL
RATESO F
0 104
. (142 .014
0078
P O L Y M E R I Z A ? . l O S O F A i Q L E O U SS O L U T I O X S
2 2 3 3
5 7 0 2
-
7 X 10-j 3x105 1 x 10-6 7 x 10-7
I
8 t
8
The second method of study was to allow the dione to polymerize in buffered solutions (pH 6.79, 6.97, 7.52 and 7.61) and to follow the rate of destruction of the dione by observing the spectrum of samples quenched in aqueous hydrochloric acid. Reliable results were obtained using the method of Dewar and Urch,12 especially after the first third of the reaction. Slight broadening of the band due to the polymer, however, reduced the accuracy of the method. Initially the reaction is first order in dione with rate constants of 5-13 X 10y4 set.-', agreeing well with the values obtained above. The order of the reaction increases with time, its value a t any point being dependent also upon the initial dione concentration and the pH. Coniparison of the spectra of the polymer in acidic and basic aqueous solution with those of cyclopentane- 1,3-dione under the same conditions indicates that the polymer probably contains the same absorbing system, %.e., a-hydroxycyclopentenone. The following path seems to us to be a reasonable one, with the first step being ratedetermining initially, but decreasing in iniportance as the concentration of dione decreases.
J
,/(I
0 0
(-1
Experimental A11 melting points are corrected and were determined using :I Fisher- Johns apparatus. Cltraviolet spectra were recorded on a Cary model 14 recording spectrophotometer and infrared spectra on a Perkin-Elmer model 21 spectrophotometer using S a C l prisms. PH measurements were carried out using a Beckman glass electrode PH meter, model H-2. Analyses are by Drs. Weiler and Strauss, Oxford, England, lis, or by Midwest MicroI;ib, I ~ ~ t l i a i ~ a p oIiitl. .
-
,121 31 J . S I k w i ~ r : i i i < l1) S L'rcli. J . C h i i r ,
.SOL S I 3 (1'157,.
Reaction of Cyclopenten-3,5-dione with Sodium Hydroxide.-The dione (0.5 g., 0.005 mole) in water ( 5 ml.) was treated with 53 ml. of 0.1 N aqueous sodium hydroxide and stirred for 20 minutes. A4fter passage through .imberlitc I . K . 120 ion-exchange resin t o remove sodium ioris, the redbrown solution was evaporated t o dryness at room temperaturc. yielding a tan, highly enolic powder, m . p . > 300", insoluble in organic solvents but readily sduble in aqueous sodium hydroxide and slowly soluble in water. A m l . Calcri. for C ; I I ~ O ~ . ? ~C,~ H55.55; ~ O : H, 4.9;. Found: C, 55.44; H , 4.87. Acidificatiou of the above sodium hydroxide sulutiuii rvitli (J.5 11' aqueous hydrochloric acid yielded a brown co11oid:tl precipitate which rapidly redissolved on basificatioii. .ifter drying a t 50", hoi r, the dark brown powder obtaiued, m . p . 300", diss d only sluggishly in aqueous sodium hydroxide. Anal. Found: C,
