2944
JOSEPH
R. LETOAKD MARILYN F. LETO
Experimental Part Materials.-Most of the compounds employed for the spectral measurements were either described elsewhere4s6.' or else available from other investigations in progress.8 We are indebted to Dr. S. B. Soloway of the Shell Development Co. for the exo-exo fused alcohol6 S and to Dr. J. Abell of the California Research Corp. for a sample of cyclododecanol. Also, we are grateful to Professor Prelog of the E.T.H., Zurich, Switzerland, who kindly made a sample of cyclodecanol available to us through Dr. Richard
[COXTRIBUTIOS FROX THE CENTRAL
Vol. 83
Heck, and to Professor Robert L. Scott for some useful comments concerning the theory. Spectral Measurements.-Measurements in the C-H stretching region were carried out on 10% solutions in carbon tetrachloride with a Beckman IR-4 spectrophotometer equipped with lithium fluoride optics. Observed frequencies were corrected by means of a calibration of the instrument with a polystyrene filn1.*3 ( 2 3 ) E. K. Plyler, L R Blaine and 11 Nowak, J . Research S a l . Bur. Slandnviis, 58, 19.5 (1957).
RESEARCH DIVISIOS,h f E R I C A 4
CYANAMID
CO., STAMFORD,
COXS.1
Te trasubstituted Cyclooctatetraenes : Catalytic Cyclotetramerization of Propiolic Acid Esters With Tetrakis-(phosphorus triha1ide)-Nickel(0) Complexes1 BY JOSEPH R. LETOAND MARILYX F. LETO RECEIVED FEBRUARY 10, 1961 Cyclotetramerization of methyl or ethyl propiolate to form positional isomers of tetracarbomethoxy- or tetracarbethoxycyclooctatetraene is catalyzed in hydrocarbon solvents by tetrakis-(phosphorus trihalide-nickel(0) complexes. 1,2,4,6and 1,3,5,7-substitution on the cyclooctatetraene ring was established mainly from proton magnetic resonance spectra. Partial hydrogenation of the 1,2,4,6-isomersyields the cyclooct-7-enes, whereas total hydrogenation of the 1,2,4,6- and 1,3,5,:-isomers yields the cyclooctanes. Saponification of cyclooctatetraene-, cyclooctene- and cyclooctane-carboxyestersyields the corresponding acids. This type of catalysis has been attempted using a wide variety of other unsaturated monomers and other zero valent complexes, but has been found to be extremely limited in scope. The reaction provides, however, a useful route to a rare class of compounds.
Introduction The carbonyls of iron, cobalt and nickel, as well as many of their organic and inorganic derivatives, have proved to be quite active as polymerization catalysts for acetylenic compounds. However, the catalytic activity of a particular metal carbonyl derivative for polymerizing a given monomer is sometimes remarkably specific. This report describes a further example of such specific catalytic activity and the novel products obtained. Reppe2 has discussed the cyclotrimerization of iiionosubstituted acetylenes in the presence of zero valent nickel-carbonyl-phosphine catalysts to give trisubstituted aromatic products. With the catalyst Ni(CO)2jP(CsH5)3]2, for example, propargyl alcohol was cyclized to a mixture of 1,2,4and 1,3,5 - trimethylolbenzene. Similarly, the highly reactive ethyl ester of propiolic acid undergoes aromatization3 to give a mixture of 1,2,4-
1,;3.,5-tricarbethoxybenzene. 111 most cases the unsymmetrical isomer is favored. The nature o f these cyclizations with nickel-carbonyl-phosphine catalysts has been under investigation in this that catalyst 1,aboratory. It has been activation in these trinierizations iiiost probably atid
(1) Presented in part a t the 138th Meeting of the Anierican Chemical Society, New York. N. Y , September. 1960. ., 104 (1016>. 12) J. W. Reppe and W. J Schweckendiek, A > I I I660, ( 3 ) 1.. S . IrIeriwether, E. C. Colthiip. G. W. Kennerly and H . S . l i e u s c h , J . OVR Chein., in press. (4) (a) L. S. Mcriwrethrr a n d SI. I , 1;icue. J . . l m , C h e m .yo< . , 81, 1200 (10391 ; (13) F,. S . h l e r i w c t h e r , E 0 . Colthup, 41.L. Fiene, G. \\', Kcnnerlyand R. S . Reusch, -4bstracls of Papers, 136th Meeting, Anierii'dn Chemical Society, S e w York, N. Y . , Septembcr, 1960, p. 67-P.
proceeds by way of a tricoordinated nickel(0) species, and that the dissociation of carbon monoxide followed by coordination of an acetylene molecule is responsible for catalytic activity. &%ether or not this activity requires the presence Si(CO)s(PR3)2 [Ni(CO)(PR3)2] f CO of CO ligand(s) in the original nickel conlplex is uncertain The CO-free tetrakis-(phosphorus triha1ide)nickel(0) complexes, Ni(PX3)4(X = F, C1, Br), have now been investigated as catalytic initiators for acetylene cyclizations. Preliminary P31nuclear magnetic resonance (n.m.r.) studies of phosphine exchange in these complexes in solution indicate that one or more of the phosphine ligands is subject to a reversible dissociation producing a ligand-
+
S i ( p ~ 3I )J ~ [ x i ( ~ x ~ ) ~(4-n)PXa ]
deficient nickel(0) species containing only phosphines The ability of these tetrakis complexes to catalyze the formation of novel cyclic products has been demonstrated in this work.
Results and Discussion Cyclization Reactions.-We have found that in the presence of each of the three different tetrakis(phosphorus triha1ide)-nickel(0) compounds mentioned above, as well as the tetrakis-(phenyldichlorophosphine) analog, cyclization of ethyl propiolate proceeds in a fashion different from the trimerization reactions, forming a mixture of stable tetramers in addition to the aromatic trimers. These tetramers hdve been identified as 1,2,4,6and 1,3 5,7-tetracarbethoxycyclooctatetr aene (I and 11) The reaction is carried out a t room temperature under a blanket of nitrogen by adding Xi(PCld)4 in cyclohexane or benzene to HCGCCOOCsH6in the same solvent. LTnderthese conditions the temperature rises 50-U0°, and 73y0 of tllc ~
July 5, 1961
TETRASUBSTITUTED CYCLO~CTATETRAENES
2945
TABLE I
SCOPE OF CATALYTIC CYCLIZATION REACTIONS 1. Monomers forming both tetramers and trimers with Ni( PC18)4 2.
Inactive monomers
3. Catalysts forming both tetramers arid trimers (in order of reactivity with ethyl propiolate
4. Catalysts forming only trimers 5 . Inactive catalysts a Forms cyclooctatetraene and benzene derivatives. meric and trimeric hydrocarbons of unknown structure.
b
Forms cyclooctane and cyclohexane derivatives.
monomer is converted in 3 minutes. Processing of the product mixture produces massive yellow crystals of I in moderate yield (28%), and pale yellow needles of I1 in small yield (lye), the principal product (71%) being a mixture of 1,2,4- and 1,3,5tricarbethoxybenzene.
Forms tetra-
reacted to give a 24y0 yield of a mixture of 1,2,4and 1,3,5-triexomethylenecyclohexaneand 1,3,5,7tetraexomethylene cyclooctane. The catalytic formation of these compounds from allene using a nickel-carbonyl-phosphine catalyst has recently been described by Benson and Lindsey.j Butadiene-l,3 also reacts, apparently to give cyclic ROOC ROOC, COOR trimers and tetramers, which are under investiI I.=\ P O R gation and will be the subject of a later report. Co-tetramerization of ethyl propiolate with methyl propiolate was achieved in an equimolar mixture of the two, with the formation of all pos'COOR sible mixed methyl/ethyl esters of the cyclooctaI, R = ClH, 11, R CzH, tetraene and benzene products. The cycloocta111, R = CH3 tetraene derivative with 3 methyl groups and 1 IV, R = 3CH1, 1C2H5(undetd disposition) ethyl group (IV) was isolated and characterized. IX, R = H However, attempts to copolymerize the propiolate With methyl propiolate (33% of the inonoilier is esters with acetylenes which are inactive alone converted in 1 1 minutes, but only one tetramer, were unsuccessful. I n general, the effect of other 111, is obtained. The yield is 8370, the remaining acetylenes seems to be to decompose the catalyst. product ( 17yc being 1,2,4-tricarbomethoxyben-or to coordinate with it in some way which inhibits zene. further reaction. Color changes taking place in Ki(PF3)4causes a 53% conversion of ethyl pro- these mixed systems bear this out. piolate to a mixture of I (30%) and aromatic If the active catalyst for polymerization is intrimers (70%)) while with Ni(PBr3)d and Ni- deed a ligand-deficient nickel species which ac(C6H5PC12)4, only a 1% total conversion of mono- cepts an acetylene to start the cyclization, and if mer is observed, even under fairly strenuous condi- this resulting complex is stable with respect to the tions. However, compound I can be isolated original ligand-deficient species, then the inactive from these reactions, along with the trimers. acetylene would have the effect of removing the In reactions with the propiolate esters, addition active catalyst as i t was formed. Why the proof five moles of PC13 for each mole of Ni(PC1J)4 piolates do not inhibit in this way is unknown. catalyst present caused the total conversion to drop The observed failure of propiolate to compete sucto about 29y0,but did not affect the product dis- cessfully with other acetylenes for the nickel tribution. This fact supports the dissociation species in the attempted copolymerizations is in theory outlined above. With the other catalysts, distinct opposition to their extreme reactivity with added free phosphine did not appear to affect the these species when present alone. reaction in any way. The electron-withdrawing power of the group The molar ratio of monomer to catalyst in all R in HC=CR might be thought an important of the experiments was around 1000. Initial factor in these reactions. However, the carboxyreaction temperatures were not critical, but were ethyl or carboxymethyl group in the active prolimited by thermal decomposition of the catalyst. piolates have Taft D"-values which are intermediate Molecular oxygen was found to inhibit reaction compared with the other R groups in Table I. by decomposing the catalysts irreversibly. The operation of an unusual steric effect with the Scope.-Attempts were made to initiate a propiolates also appears unlikely. Thus it seems cyclotetramerization with Ni(PC18)4 or Ni(PF3)4 unwise a t this time t o speculate further on the in a wide variety of unsaturated monomers. Of apparently unique cyclization reaction of propiothe acetylenic compounds (see Table I) chosen on late esters as compared with other monosubstithe basis of the reactivity of their triple bond in tuted acetylenes. other reactions, only methyl and ethyl propiolate ( 5 ) R.E.Benson and R. V. Lindsey, J r . , J . A i m Chem Sor , 81, 12 17 reacted. Of the ethylenic compounds, allene (1959).
2946
JOSEPH
R. LETOAND MARILYN F. LETO
In an effort to extend the scope of this reaction to other catalysts, four carbonyl-free tetrakis complexes were investigated for reactivity with ethyl propiolate (see Table I). In no case was there any formation of cyclooctatetraene products. The triethyl phosphite and phenyl isonitrile coniplexes caused 100% and 3% conversion,respectively, to the mixture of 1,2,4-and 1,3,5-tricarbcthoxybenzene. The cyclopentadiene and phosphine oxide complexes showed 110 activity whatever. In comparing those catalysts which form tetramers and trimers with those which form only trimers or are inactive, there is no striking difference in the electronic environment of the phosphorus atoms (P3Iresonance),Oin the nickel-ligand force constarits (Raman7ss and far-infrared9), or in the electronic energy levels of the molecule as a whole (ultraviolet).g The unique activity of the complexes possessing a nickel-phosphorus-halogen system might be explained on the basis of a-bonding effects in the ligands. It is known from the strength of their trans effect in planar Pt(II) complexes,’O for example, that the phosphorus halides foriii stronger a-bonds with transition metals than do the phosphites. This fact could increase the catalytic activity of the Ni-P-X complexes in two ways: (1) by decreasing the Ni-P a-bond strength, thus facilitating phosphine dissociation ; or (2) by stabiliLing the resulting ligand-deficient nickel species, thus favoring acetylene conjugation with the nickel catalyst. Some type of specific interaction between ;L halogen atom and the propiolate to promote activity of the latter is also conceivable. However, in the absence of kinetic data on the reaction or more information about the transition state involved it is not possible to judge the importance of the effects mentioned above. Reactions of the Tetracarboxyesters of Cyclo0ctatetraene.-Partial hydrogenation of I and I11 may be carried out a t room temperature and atmospheric pressure over Pd-C catalyst to give the 1,2,4,6-tetracarboxyestersof cyclooct-7-ene in 1007, yields (compounds V and VI). The uptake of 3 inoles of hydrogen is complete in one hour, and hydrogenation may be stopped a t this point. iZ007 YOOR
3
I!
cool?
V,R VI, R
CHI == CzHs
hydrocarbon itself, l1 Alllof the cyclooctenes and cyclooctaiies reported here are high boiling liquids. Hydrogenation of I1 under similar conditions produces the corresponding cyclooctsne in 100% yield in about 10 minutes. In this case, no cyclooctene intermediate could be isolated. In coniparing the hydrogenation of the two isorncrs I and 11, it was found that the relative rate of absorption of the first three moles of hydrogen for the asymmetric isomer I is about twice that for the syirimetric isomer 11, both in cyclohexane. Saponification of either I or I11 with KOH in ethanol yields the same acid upon acidification, cyclooctatetraene-l,2,4,6-tetracarboxylic acid (IX;), in 100% yield. Titration of this high-rnelti!lg crystalline acid shows four inflections, corresponding to the successive plia values for the acid groups shown in Table 11. The other tetraacid coinpounds in Table I1 are formed in nearly quantitative yield by saponification and acidification of the corresponding methyl or ethyl esters V,VI1 and 11. TARLE 11
ps,
VALUES FOR VARIOUS ’rETRA--4ClDS 1 N \L‘\rAT&R Comiiound p K , values (successive)
IS
5 . 9 8 , 4 . 7 2 , 3 . 9 3 , 3 . 4 0(all rtO.06) 6.85, 5 . 3 , 4.35, 3 . 4 5 (all 10.1) 6.35, 5 . 2 , 1 . 4 5 , 3 . 5 ( a l lrt0.l) 4 . 5 (only one infiectiou)
Acid from V Acid from 1‘11 Acid from 11
Structure.--The evidence by which a tetrasubstituted cycloijctatetraerie structure was assigned to these coinpounds follows : (1) Deterinination of the iiiolecular weight and elemental analysis showed coinpounds I, I1 and 111 to be tetramers of the propiolate monomer used. (2) Aromatic and linear structures for these tetramers were ruled out on the basis of their 1i.in.r. and infrared spectra. -411 aromatic foririulations for a tetramer of a propiolate ester, one of which is shown as X, contain both a tetrasubstituted benzene ring and a double bond conjugated to the ring and to a carbonyl group. However the n.m.r. CJOOR
j
