EDWINF. ULLMAN
5386 lCONTRIIlUTION FROM
THE
Vol. 81
LEDERLE LABORATORIES DIVISION, AMERICAS CYASAMID CO.]
Mechanism of Hydrogenation of Unsaturated Cyclopropanes BY EDWINF. ULLMAN RECEIVED FEBRUARY 7, 1959 Saturation of the double bond in vinyl- and alkylidenecyclopropanes by catalytic hydrogenation appears t o be normally accompanied by varying amounts of hydrogenolysis of the cyclopropane ring. A study has been made of the effects of substituents on the course of this reaction and evidence is presented which suggests that hydrogenation of the two groups of compounds may proceed through a common carbanion intermediate formed by an initial transfer of hydride from the catalyst. These studies have confirmed the proposed structure13 I I I b for the pyrolysis product of Feist's ester IJ'c and have led to a new synthesis of alkylidenecyclopropanes.
Among the considerable physical and chemical data which have been presented in support of the view that the combination of a cyclopropane ring with an adjacent unsaturated grouping has some of the properties of a conjugated are numerous examples of catalytic 1,4-addition of hydrogen to vinylcyclopropanes. Recently i t has been pointed out, however, that such conjugate addition reactions probably are less a result of ground state resonance interactions than of the ability of the cyclopropane ring to transmit conjugation in the transition state or in certain reactive intermediates,6 such as, for example, the highly resonance stabilized cyclopropylcarbinyl cation.' The possibility that a related resonance-stabilized intermediate may be operative during the hydrogenation of vinylcyclopropanes is suggested by a number of examples of hydrogenolytic ring cleavage which have been reported to proceed under conditions which do not affect the related saturated cyclopropanes. This idea is further supported by the recent observations that methylenecyclopropane (I) and hypoglycin (11) likewise 2s33b
over, cleavage of the ring in both vinyl- and alkylidenecyclopropanes appears to occur exclusively a t the ring bonds adjacent to the unsaturated linkagel0?l2even though this results from an over-all 1,3- rather than 1,4-addition of hydrogen to the latter compounds. It is thus suggested that the courses of these hydrogenolyses might be interpreted in terms of a hydrogenation intermediate, [ Y ] ,common to both vinyl- and alkylidenecyclopropanes
cI
1
,
undergo hydrogenolysis under conditions which do not affect the saturated derivatives.*eg More(1) (a) E. P. Carr and C. P. Burr, THISJ O U R N A L , 40, 1590 (1918); (b) I. M .Klotz, ibid., 66, 88 (1944); (c) J. D . Roberts and C. Green, i b i d . , 68, 214 (1948); (d) R. H . Eastman. ibid., 76, 4115 (1954); (e) R. H. Eastman and J. C. Selover, rbid., 76, 4114 (1954); ( f ) R. H. Eastman and S. K. Freeman, i b i d . , 77, GO42 (1955); (g) M.T. Rogers, ibid., 69, 2541 (1947): (h) R . P. Mariella and R. R . Raube, ibid., 74, 518. 524 (1952). ( 2 ) K. Van Volkenburgh, K. W. Greenlee, J. hI. Derfer and C. E . Bourd, ibid., 71, 172, 3595 (1949). ( 3 ) (a) R. W.Kierstead, R . P. Linstead and B. C. L. Weedon, J . C h e m . Soc., 3010, 36lG (1952): ( h ) 1794, 180:3 ( 1 9 3 ) ; (c) V . A . Slabey and P. H. Wise, TEXIS J O U R N A L , 74,3887 (1952); ( d ) V. A. Slabey, i b i d . , 74, 4930 (1952). (4) (a) W. A. Bone and W. H. Perkin. J . C h e m . S O L . ,67, 108 (1895); f h ) R . C. Fuson and F. N. Baumgartner, THISJOURNAL, 7 0 , 3255 (1948); (c) C. F. H. Allen a n d R . Buyer, C a n . J . Resrarch, 9, 159 (1953); (d) E . P. Kohler and J. B. Conant, Txrs J O U R N ~ L , 39, 1404 (1917). ( 5 ) (a) J. Nickl, Ber., 9 1 , 553 (1958); (b) B. A . Kazanskii, M . Yu Lukina, A . I. Malyshev, V. T. Aleksanyan and Kh. E. Steriu, l e v e s l . A k a d . N a u k . S S S R , Otdel. Khim. N a a k . , 36 (1956). ( 6 ) For a general discussion of t h e relationship between t h e reactivity and conjugative effects of unsaturated cyclopropanes see (a) E. N . Trachtenberg and G. Odian, Chemistry & I m f u s l r y , 490 (1958); (b) THISJOURNAL, 80, 4018 (1958). (7) J . D. Roberts and R . H. Mazur, ibid., 73,2509, 3542 (1951). (8) J. T. Grdgson. K . 1%'. Greenlee, J . hl. Derfer and C . E. Boord, bid.. 7 5 , 3341 (1453).
C-
c-
c-
I
I
C! R
\CHR I,R = H 11, R = CH?CH(SHd)COOH
I c-
I
HC-
I
R
\
C--
I
K
(9) ( a ) R. S. de Ropp, J. C. Van Meter, E. C. De Renzo, K. W. McKerns. C. Pidacks, P. H. Bell, E . F. Ullman, S. R. Safir, W. J. Fanshawe and S. B. Davis, ibid., 8 0 , 100-1 (1958); (b) S. Wilkinson, Chemistry & Indlrslry, 17 (1958). (10) An apparent exception t o this generalization is t h e reported partial hydrogenation of sabinine (i) t o give ii." T h e structure of t h e latter compound has not been rigorously established. Although the physical constants of its dihydro derivative are in good agreement with those of a n authentic sample of 1,2-dimrthyl-3-isopropylcyclopentane, no comparison with t h e expected product, 1,3-dimethyl-3isopropylcyclopentane, has been made. Moreover, no effort has been made t o establish t h e position of t h e double bond; B. Kazanskii. Ber., 62, 2205 (1929).
(11) F . Richter, W. R'olff and W. Presting, ibid., 64, 871 (1931). (12) Hydrogenolysis of t h e ring bond opposite t h e unsaturated linkage in hypoglycin has also been reported; C. von Holt and W. Leppla. Awgew. Cheer., TO, 2 5 (1958). However, when conditions for the hydrogenation of I or I1 were employed which were demonstrated to have no effect on their cyclic dihydro derivatives, the products detected corresponded only t o cleavage of t h e bonds adjacent t o t h e unsaturated ring carbon.sqsn T h e inability of these compounds t o undergo 1,4-addition of hydrogeu with cleavage of the ring bond opposite the double bond is in accord with t h e expected lack of interaction between these bonds due t o the perpendicular orientation of the ring bund t o the plane of t h e exocyclic =-orbital. T h e absence of conjugative effects in methylenecyclopropanes is also seen in the presence of only end absorption in t h e ultraviolet spectra of hypoglycina' (II), Feist's acid'sb (IVa) and methylenecyclopropane-1,l-dicarboxylic acid (Va) (vrde infra).
Oct. 20, 1959
HYDROGENATION OF UNSATURATED CYCLOPROPAXES
In order to confirm the above hypothesis and to shed more light on the nature of the intermediate [ Y ]hydrogenations , have been carried out on three alkylidenecyclopropanes, and comparisons have been made of the effects of different substitution of vinyl- and alkylidenecyclopropanes on the course of hydrogenation of these compounds. These studies have led to a new synthesis of alkylidenecyclopropanes and a conclusive confirmation of the proposed structure IIIbla for the pyrolysis product of Feist’s ester (IVC).’~-~*
HZC
5387
COOCA VI11
possibility of VI11 being formed could not be excluded, this appeared unlikely in view of the known tendency of isopropylidenemalonic ester c. to alkylate at the a-carbon atom.18 ROOCCH=C CH,=C’ ‘“ I n point of fact, bromination of diethyl isopropyl\ \ IH idenemalonate (IX) with N-bromosuccinimide CHCOOR C proceeded smoothly to give an oil, VII, which decomposed upon attempted distillation. Treatment ‘COOR of the crude bromoester with sodium hydride IVa, R = H IIIa, R = CHI I V b , R = CHa IIIb, R = C2H6 provided only diethyl isopropylidenemalonate (IX) IVC. R = C2H6 in low yield, presumably formed by intermolecular CHz debromination of the bromoester by its anion. /I However, when the crude bromoester was treated CH2=c‘ \ with potassium t-butoxide in t-butyl alcohol, an C-COOR oil was obtained which could be separated by I vapor chromatography into two components, a COOR dehydrobromination product, C10H1404 (Vb), and Va, R = H IX. No evidence for a third volatile fraction V b , R = C,H5 Methylenecyclopropane-1,l-dicarboxylic Acid corresponding to VI11 was found. Alkaline hy(Va).-At the outset of this investigation two drolysis of the ester mixture provided a mixture of unequivocal syntheses of methylenecyclopropanes acids which was separable into Va and isopropyliI and IV had been reported. Since the preparation denemalonic acid by chromatography on silica. Evidence in support of structure V was found in of the parent hydrocarbon, I, by metallic reduction of 2-chloromethylallyl chloride* was inapplicable the presence of a peak a t 11.00 p corresponding to for the synthesis of V, i t was hoped that a con- terminal methylene absorption in the infrared sideration of the mode of formation of Feist’s spectra of both the ester Vb and acid Va. The acid (IVa)l 4 , I 5 would provide a synthetic approach. double bond stretching peak of the starting maFeist’s acid is formed by treatment of ethyl terial a t 6.09 p had disappeared and the ester showed The double bromoisodehydracetate (VI) with aqueous alkali14s15 a single carbonyl band a t 5.75 and can be envisioned to arise by way of the trans- bond was shown to be out of conjugation with the carbonyl groupings by the presence of only end formations ~ Vb in absorption in the ultraviolet ( E Z O O ~ 2200, ethanol). A simple confirmation of the structure c2Hboo* was provided by the nuclear magnetic resonance (n.m.r.) spectra of Va and Vb. The spectrum CH3 0 CH3 of each compound displayed two weakly split &OH peaks of similar intensity (Va, 35 and 163 C.P.S. in DzO; Vb, 49 and 180 C.P.S.in CClJ expected C J - L O O CH CAH for two pairs of similarly oriented protons.20 In contrast, the ring protons in the alternative strucOH-*TVa .ture VI11 would be expected to give a single peak C 6OOH with no splitting. Feist’s acid (IVa) showed simiciao lar n.m.r. absorption in deuterium oxide a t 30 and It therefore seemed likely that formation of the 147 c.p.s.16 with previously unreported splitting of carbanion of the bromoester VI1 might similarly each peak into a triplet ( J = 2.5 c.P.s.). result in direct internal displacement of bromide Hydrogenations of Va were carried out a t ation, a transformation that finds further analogy in mospheric pressure in ethyl acetate with both the intermediate formation of cyclopropanones on platinum and 10% palladium-on-charcoal catalysts. alkaline treatment of cr-haloketones.17 While the A total of 1.86 and 1.58 moles of hydrogen, respec(13) M. G. Ettlinger, THIS JOURNAL, 1 4 , 5805 (1952). tively, was absorbed very rapidly with no further COOR
’r
7
1
I
c2HsooB;r
-
3
/
(14) (a) F. Feist, Ber., 26, 747 (1893); (b) Ann., 436, 125 (1924). (15) F. R . Goss, C. K . Ingold and J. F. Thorpe, J . Chcm. Soc., 123, 327 (1923). (16) (a) A. S. Kende, Chemistry Induslry, 544 (1956); (b) M. G. Ettlinger, ibid.. 166 (1956); (c) A. T. Bottini and J. D. Roberts, J . Org. Chcm., 21, 1169 (1956). (17) (a) R . B. Loftfield, TEIS JOURNAL 73, 632 (1950): (b) 1 3 , 4707 (1951).
(18) A. C. Cope and E. M. Hancock. ibid., 60, 2644 (1938). (19) The corresponding diacid Va showed strongly split carbonyl absorption in the infrared even in dilute solution at 5.69 and 5.99 p. Similar peaks were observed in the spectrum of Z-methylcyclopropane1,l-dicarboxylic acid. (20) Peak positions are measured relative to benzene as an external standard at a radiofrequency of 40 mc.
EDWINF. ULLMAK
5388
uptake on longer stirring. X single recrystallization of the product provided n-propylmalonic acid. However, by chromatography on silica of the crude product from the palladium-catalyzed reduction a second compound was isolated that was indistinguishable from authentic S-methylcyclopropane-1,l-dicarboxylicacid (X, R = H). No CHe
//
CH3CH
\i
C--COOR
Vol. 81
alkaline and acidic conditions, respectively. Equally uninformative, although of particular interest in this study, was Kon’s isolation of adipic acid by saponification of the hydrogenated pyrolysis product. 0-
C,H,OOC~@,,~H~
”7”’
CH30C=CY-C I
’L
C ~ H ~ ’OC H C O O C ~ H ~
‘CHCOOCH,
XIV
XI11
It was found convenient to prepare the pyrolysis COOR product by vapor chromatography of Feist’s ester other products were observed. Hydrogenation (IVb) a t 210’. In this manner the more volatile of the mixture of diethyl isopropylidenemalonate decomposition products were removed simultaand Vb likewise yielded a small amount of X neously and a colorless isomeric oil containing (R = C2H.5) along with n-propyl- and isopropyl- about 6% of the starting material (vide infra) malonic esters. Diethyl ethylmethylmalonate, was obtained free from contamination by side which might have been expected to be formed in products. The spectra of this compound provided analogy to the observed courses of hydrogenation strong evidence in support of structure IIIa. A s of methylenecyclopropane (I) and hypoglycin Ettlinger had previously observed, there was no (11) (vide supra), could not be detected in the acetylenic absorption in the infrared, but a sharp medium intensity peak a t 5.68 p appeared next to reaction mixture. Dimethyl trans-3-Methylenecyclopropane- 1,2-di- the ester carbonyl band a t 5.80 p . The former carboxylate ( I n ) .-Feist’s acid (IVa) was prepared band is apparently characteristic of double bonds of alkylidenecyclopropanes8 and is observed in this according to the published p r o c e d ~ r e l and ~ ~ ~conl~ verted to its dimethyl ester 1%. Both compounds case in spite of the nearby carbonyl absorption were hydrogenated in ethyl acetate solution with because of enhanced intensity due to conjugation 10% palladium-on-charcoal catalyst and found to with the ester grouping. I n contrast, this band absorb 1.12 and 1.25 moles of hydrogen, respec- is too weak to be observed in the spectra of IV or V. tively. Although previous investigators reported The structure was further supported by the presthe isolation of 3-methylcyclopropane-1,2-dicar- ence of a maximum in the ultraviolet spectrum a t boxylic acid as the sole p r o d ~ c t , ~ ~chromatogb , ~ ~ J ~ 203 mp ( E 12,200). The considerable shift from the 5 mp23is predictable raphy of the hydrogenated acid on silica has now calculated maximum a t 222 provided a small yield of ethylsuccinic acid. from the expected abnornially high energy of the Dimethyl ethylsuccinate could also be observed dipolar form XIV, due to the electronegative by vapor chromatography of the hydrogenated character of the cyclopropane ring.24 The n.m.r. ester. No other hydrogenolysis product was de- spectrum of IIIa (CC14 solution) showed a poorly tected. Long treatment of 3-methylcyclopropane- resolved multiplet centered a t 17 c.p.s. correspond1,2-dicarboxylic acid under identical conditions ing to a single olefinic proton, and two strongly split groups of peaks centered a t 169 and 191 was without effect. C.P.S. corresponding to the ring tertiary and secondMethyl 2-Carbomethoxycyclopropylideneacetate (IIIa).-The conclusion by Kon that Feist’s ester ary protons, respectively.?O Contamination of the (IVc), undergoes an unusual rearrangement on sample by roughly 6y0 Feist’s ester (IVb) was estipyrolysis to give X I 2 * went unquestioned for mated by the areas of two weak triplets a t 39 and twenty years until Ettlinger proposed the alterna- 153 C.P.S. that increased in intensity upon addition tive structure IIIbI3 on the basis of the absence of of IVb to the sample. Hydrogenation of the pyrolysis product in ethyl acetylenic absorption in the infrared. The chemical evidence presented by Kon is in accord with acetate with 10yc palladium-on-charcoal catalyst led to the uptake of 1.39 moles of hydrogen. The C2HjOOCCHsCHsC=CCOOC?Hj isolation of the previously unreported saturated XI cyclic acid X\- by hydrolysis of the product left CIH,OOCCH2CH,C=CHCOOCpHj S
*
CH!
OGHj XI1
either structure. Thus, treatment with sodium ethoxide was shown to give the enol ether XI1 which may be envisioned to arise by addition to the unsaturated linkage of either I I I b or XI followed in the former case by .ring scission via a reverse hIichael reaction (XIII, arrows). Similarly, either structure might yield the observed pketoadipic and levulinic acids under aqueous (21) M. G. Ettlinger, S. H. Harper and F. Kennedy, J . Chem. Soc., 922 (1967). (22) G . A. R. Kon and H. R. Nanji. ibid., 2557 (1932).
/
HOOCCH2CH 1yT-
~
‘CHCoOH
no doubt about the cyclic nature of 111. No other products were isolated, but the presence of both adipic and 2- or 3-methylglutaric acids could be demonstrated by paper chromatography. Vapor chromatography of the esters confirmed the presence of these compounds and also permitted de(23) A. T. Nielson, Abstracts, American Chemical Society, 132nd Meeting, New York, N. Y.,14-P. (24) (a) A. D. Walsh, Trans. F a r a d a y S O L ,48, 179 (1949), (b) C. A. Coulson and W. E. Moffit, Phil. M a g , I71 40, 1 (1949).
5389
HYDROGENATION OF UNSATURATED CYCLOPROPANES
Oct. 20, 1939
TABLE I COURSEOF HYDROGENATION OF SOME UNSATURATED CYCLOPROPANES R1
H,
1'y(&-R2
R,
h-R3
,c=c
>I--3
2 k
Molar equiv. H2 absorbed
R: Rz = R3 R, = H R1 = CHZCH(KHz)COOH, RP = R3 Ri Ra = COOCHa, Rz = R3 = H H Ri R3 COOH, Rz = Ra R1 = R3 = COOCH,, RP = R4 = H Ri RS COOH, R3 = R4 = H Ri Ra = COOH, RS = R I H
I8 1198
IIIa" IVa" IVb" Va" Van
(?)dsi
Rq
1. Z',h 1.39b4 1.12"g 1.2Sb4 1 . .58,bfg1,86"."
H
1 ,R2',h
RI = Rz = Ra = R4 = H Rs CHa, Ri = Rz = Rq = €1 ~ 1 x 3 ~ R 3 -- CzHj, Ri Rz Rq = H xx3c R3 = Rq CH3, Ri = RB = H xx13c R3 = CH3, R4 = CzHj, Ri Rz = H XXIP R1 = Rz = COOH, R3 R4 H Ri = COOCzHh, Rn = COCH3, R3 F R + H ~ ~ 1 1 1 3 ~ XXIVb5 Ri RP COOCzH6, R3 = CH3, R4 = H a This work. Ethyl acetate. Methanol. Ethanol. Acetic acid. Platinum. Copper chromite. Raney nickel. S o t determined. xv112~ gd
~ ~ 1 1 1 2 ~ 3 ~
tection of the products of hydrogenation of Feist's methyl ester (IVb) (vide supm) present as an impurity. While the two isomeric methylglutaric esters could not be differentiated by the methods used, i t is probable on the basis of analogy with the above cases that in fact only the 2-methyl derivative was formed. The mechanism of formation of the pyrolysis product is of considerable interest. It seems reasonable that the reaction proceeds through the resonance stabilized intermediate XVI (R = COOCH3) which could collapse in one of two ways to give either starting material or product. According t o calculations by Burr and Dewar the parent system XVI (R = H) should have considerable resonance energy.25 However, a simple valence bond tautomerism cannot be excluded as a possible alternative. 'CH, I
CHz
6/H &
-A. CH
\-
I
CH-
-CH I
t
1
XVI
Discussion Of hydrogenation Of A summary Of the alkylidene- and vinY1cYcloProPanes is given. in I. a Of conditions which permits Only a rough has been correlation of the course of these reactions. I n particular, i t is seen that equivalent substitution in each system has a similar influence on the course of the reaction. Thus, vinylcyclopropane (XVII) (25) J. G. Burr and M. J. S. Dewar, J . Chem. Soc.. 1201 (1954).
'
Bonds cleaved
1, 2 I, 2 1, 2 ( 3 ? ) 1, 2 1, 2 1
k
1.2-1.3'8' 1, 2 1.02,d.i 1.15',{ 1, 2 1.0ld.i .. 1,214i 1,2 1.24d.i 1, 2 1.9b.h 1 1. 9 * , h 1 1 ,8',' 1 Gas phase. 8 10% Palladium-on-charcoal
and the alkyl substituted derivatives I1 and XVIIIXXI in no case are observed to undergo more than about 25% hydrogenolysis, while two carbonyl substituents on the same carbon atom (Va and XXIIXXIV) strongly facilitate ring cleavage in either class of compounds. It is of interest that only one mode of ring scission, that involving cleavage a t the carbonyl-bearing carbon atom, was observed in the latter cases. These data support the original supposition that hydrogenolytic ring cleavage in both alkylidene- and vinylcyclopropanes proceeds through a common intermediate. Conflicting hypotheses on the mechanism of hydrogenation of double bonds have recently been discussed.26-2s Each of these suggestions envisions hydrogenation to proceed from an initial chemisorption of the organic molecule on the catalyst followed by a stepwise attack by hydrogen, but the initial reaction with a hydrogen atom is variously represented as an attack by a proton,28 hydride ionz6 or hydrogen radical?? from the catalyst surface. The above data (Table I) strongly suggest that in the cases studied the halfhydrogenated olefin-metal complex has the properties of a carbanion and thus is formed by a transfer of hydride ion from the catalyst. It is questionable whether i t is permissible to generalize this mechanism beyond these systems, although it is of interest that BrewsterZB has been able to rationalize the steric course of hydrogenation of ketones in a similar manner. (
(26) J. H. Brewster, %IS JOURNAL,76, 6361 (1954). 1 ~W.~ A.~Banner, ~ , C. E. Stehr and J. R. DoAmaraL i b i d . , 80, 4732 (28) F. J. McQuillin, Chemistry & I n d u s t r y , 251 (1957).
EDWIN I?. ULLMAN
5390
Attack by hydride on either vinyl- or alkylidenecyclopropanes can be envisioned to give the common carbanion-metal complex XXV
I
13I
I
R
