5546
J. N. COKER,A. S. BJORNSON, T. E. LONDERGAN, T. F. MARTENS AND J. R. JOHNSON
[CONTRIBUTION FROM
THE RESEARCH DIVISION, ELECTROCHEMICALS DEPARTMENT, E I DU
Vol. 77
PONTDE NEMOURS A N D Co ,
Ixc.]
The Dealcoholation of a Series of Unsaturated Diethers B Y JAMES N.
COKER,AUGUSTS. BJORNSON, ’rHOMAS E. LONDEKGXN, TEDF.LIARTENS
.\SD JOHN
R.
JOHNSON
RECEIVED APRIL20, 1955 The dealcoholation of a series of 1,5-dialkoxy-2-hexeneshas been investigated in an effort to prepare a l-rilkoxy-2,4-hexadiene. Though the dealcoholation reaction could be accomplished in several ways, a 2,4hexadiene structure was not obtained. Instead, the corresponding 5-alkoxy-1,3-hexadienesresulted in good yield. The infrared absorption characteristics of these compounds are described.
Various 3-methyl-6-chloro-2-hexenes (11), containing a n alkoxyl group in the 5-position and an alkoxyl or acetoxyl group in the 1-position, have been described in an earlier publication.’ These compounds have been examined as possible precursors of the vitamin A side chain. With this objective in mind, the 1,5-dialkoxy compounds have been subjected to a dealcoholation in the presence of p-toluenesulfonic acid. The product desired from this dealcoholation was the corresponding 1-alkoxy-2,4-hexadiene (IIJ), formed by an elimination of the internal alkoxyl group (reaction A), but this product was not obtained. Instead the terminal alkoxyl group was eliminated and 5-alkoxyl-l,3-hexadiene (IV) was isolated as the principal product of the reaction (reaction B).
oxalic acid instead of p-toluenesulfonic acid, the same l,3-diene (IVa) was formed, but the reaction proceeded very slowly. No appreciable reaction was observed when sodium bisulfate was used as the dealcoholation catalyst. Dealcoholation was also accomplished by pyrolysis over alumina a t a temperature of 150-200°. The same 5-ethoxy-1,3hexadiene (IVa) was isolated as the predominant product of this pyrolysis. In order to determine the effect of various alkoxyl groups upon the course of the dealcoholation reaction, other 3-methyl-6-chloro-2-hexene derivatives (IIb-g) have been studied. The basis for this investigation has been the assumption that the formation of the desired 2,4-hexadiene (111) could be accomplished if appropriate dissimilar alkoxyl residues were present in the parent 2CH3 hexene system (11). Presumably, the presence of I CHs CICH?CHCHgC=CHCHzCl either a relatively stable group in the 1-position or I C~CH~CH=CH&=CHCH,OR a relatively labile group in the 5-position of the OR I I11 compound would favor the selective elimination of the group in the 5-position. The latter approach RfOK’ (A)\-ROH has been favored mainly because the presence of a stable ether group in the 1-position might be ex1 pected to lower the biological activity of any vitaCH3 min A structure obtained from the compound.2 1 ClCH2CHCH=C-CH=CHa The dealcoholation of 2-hexenes containing three I CHJ other pairs of alkoxyl groups in the 1- and 5-posiOR I V -R’OH I tions (IIb-d) has been observed to proceed in the +-ClCHzCHCHzC=CHCH20R’ (B) same manner as the reaction observed with the 1methoxy-5-ethoxyadduct (Ha). The failure of these compounds to yield the desired 2,4-hexadiene structure (111) has prompted an investigation of several compounds (He-g) containing either benzyloxyl or phenoxy1 groups. Benzyl ethers in general readily undergo cleavage in the presence of acidic reagents3 The dealcoholation of a 2-hexene containing a 5-benzyloxyl group The dealcoholation reaction was carried out ini- might be expected therefore to yield a 2,4-hexaditially with l-methoxy-3-methyl-5-ethoxy-6-chloroene derivative. The dealcoholation of such a com2-hexene (IIa). This compound could be dealco- pound, 1-methoxy-3-methyl-5-benzyloxy-6-chloroholated in several ways. The most effective method 2-hexene (IIg), did not bear out this contention, employed a catalytic amount of anhydrous p-tolu- however. In fact, the product of the reaction, 3eriesulfonic acid in boiling toluene. The main metliyl-5-benzyloxy-B-chloro-l,3-hexadicne(IVc) product fornied under these conditions was 3- was shown on the basis of spectroscopic data to bc iIicthyl-S-ethoxy-fj-chloro-l,3-hexadiene (IVa). It more stable to the conditions of the dealcoholatioil was observed that the ratc of dealcoholation of the than had been observed previously for 3-methyl-SI -niethoxy-5-ethoxy compound (IIa) w a s not af- ethoxy-6-chloro-1,3-hexadiene (IVa) . fected markedly by varying the amount of the toluThe unexpected stability of 3-niethyl-5-benzylene in the reaction mixture. However, cyclohesanc oxy-ti-chloro-l,3-hexadiene (I17c) to the coiiclitioiis proved to be quite inferior to toluene as a reaction of the dealcoholation reaction has suggested that ;I solvent. ’Ilrhen the reaction was catalyzed with
?
( 1 ) J.
Tms
PI’. Coker, A. S. Rjornson, T. I< 1.onderg.in and J . R . Johnson,
J O U R N A L , 77,
5542 (1955).
( 2 ) 0 . lsler, Section 10, paper 08, X I I t h International Congress of Pure a n d Applied Chemistry, N e w York City, Septernher, 19.51. ( 3 ) n. W. T r o n o w and I,. W. I,adiginn, Rev.. 62, 28-11 (1929).
THEDEALCOHOLATION OF A SERIES OF UNSATURATED DIETHERS
Nov. 5, 1955
5547
1-benzyloxy-2-hexene structure might possess a relatively high stability under such conditions, but this has not been found to be the case. 1-Benzyloxy-3-methyl-5-isopropoxy-6-chloro-2-hexene (IIe) rapidly underwent the elimination of the elements of benzyl alcohol in the presence of p-toluenesulfonic acid to form 3-methyl-5-isopropoxy-6-chloro1,3-hexadiene (IVb). Likewise, an attempt to stabilize the ether linkage in the 1-position of a 2-hexene structure by means of a phenoxy1 group was unsuccessful. While the dealcoholation of l-phenoxy-3-methyl-5-isopropoxy-6-chloro-2-hexene (IIf) appeared to give a mixture of products, the infrared absorption spectrum definitely indicated that a 2,4-diene structure I f’ was not present in this mixture. 0.1 An acetoxyl group in the 5-position of a 2-hexene should be sufficiently labile to permit its removal by pyrolytic means, but our preparative methods did not give access to such a compound (IIj) and this 0 1 2 3 4 5 6 24 condition could not be tested experimentally. On Time (hours). the other hand, the prediction that an acetoxyl group in the 1-position of a 2-hexene structure Fig. 1.--Dealcoholation of l-methoxy-3-rnethyl-5-cthoxy6-chloro-2-hexene. would be removed easily has been confirmed by a study of a l-acetoxy-5-ethoxy-2-hexeneand a 1acetoxy-5-isopropoxy-2-hexene (IIIh and i) . The probably of little significance but the second factor elements of acetic acid were eliminated from these is regarded as the crucial one in determining the compounds by the action of p-toluenesulfonic acid course of reaction. in boiling toluene, and the expected 1,3-hexadienes The observation that the I-phenoxy-5-isopro(IV) were formed. poxy-2-hexene (IIf) did not furnish a 2,4-hexadiene The experimental results establish clearly that (111)suggests that this compound followed a differdealcoholation of the 1,5-dialkoxy-2-hexenes (11) is ent course of reaction. This compound is an allyl a highly selective process in which the 1-alkoxy1 phenyl ether and might be expected to undergo the group is eliminated preferentially, even though a Claisen rearrangement to form an allyl phen01.~ more reactive alkoxy1 group may be present in the Several efforts were made to devise means of con5-position. The reaction is presumed to occur by verting the 3-methyl-5-alkoxy-6-chloro-l,3-hexaa typical carbonium ion mechanism: fixation of a dienes (IVa-c) obtained in this study into useful inproton on the 1-alkoxy1group to form an oxonium termediates for the synthesis of vitamin A, but structure (V), elimination of alcohol a t the l-posi- these have not been successful. I n this connection, tion to form an allylic carbonium ion (VI-VII), particular attention was given to the 5-ethoxyl and stabilization by loss of a proton from the 4- derivative (IVa). This compound has been treated position to yield a 5-alkoxy-1,3-diene (IV). with a variety of acidic reagents in an effort to promote its anionotropic rearrangement into a 2,4CHa I H hexadiene (III).6
-+t
L
ClCHsCHCH2C=CHCHa-OR’ I (+) OR V
--f
CH,
I
ClCHZCHCH=C-CH=CHz
+ R’OH
CICH~CHCH~C=CH-CH~ I (+) OR VI
+
I
ClCHZCHCH=CCH=CH2
I
OR
IL’
+Hi
(CHaC0)sO -\\_L+
catalyst
O C Z H ~IVa CHa
I
0
II
ClCHzCH=CHC=CHCH20CCHs IIIa
4 CH3
I
CHI -1ClCHaCHCH2C-CH=CHz I
I
OR
(+I
\’I1
The selectivity of the reaction may be attributed to the relatively greater proton attraction of the
oxygen of the allylic ether grouping over that of the /3-chloroalkyl ether grouping and the ability of the resonance of the allylic carbonium system in the transition complex to facilitate rupture of the carbon-alkoxy1 bond. The first of these factors is
The catalysts employed were concentrated sulfuric acid, boron trifluoride, zinc chloride, ferric chloride, stannic chloride and p-toluenesulfonic acid. All of these materials were used in the presence of acetic anhydride, a reagent capable of transforming aliphatic ethers into the corresponding acetates.6These reactions did not produce the desired 1-acetoxy-2,4diene but led only to extensive polymerization of the 1,3-diene. (4) D. S. Tarbell, “Organic Reactions,” Vol. 11, John Wiley and Sons, Inc., N e w York, N. Y . , 1944, p. 1. ( 5 ) E. R . H. Jones and D. C. L. Weedon, J . Chem. Soc., 937 (1946). (6) D. Kastner, “Newer Methods of Preparative Organic Chemistry,’’ Interscience Publishers, Inc., New York, N. Y . , 1848, p. 301.
5548
J. N. COKER,A. S. BJORNSON, T. E.LONDKIGAN, ’l’. €7. MAKTENS AND J . R .
Discussion of Infrared Absorption Data7 The complex nature of both the products of the dealcoholation reaction (IV) and the 2-hexene precursors (I and TI) has made the use of infrared absorption spectra invaluable in the assignment of structures to these compounds. With regard to the parent 1,5-dichloro-2-hexenes ( I ) , a consideration of individual absorption spectra clearly brings out characteristics relating a11 of these materials. The absorption spectrum of l,&dicliloro-5-ethoxy-2hexene (Ia without thc 3-methyl group) contains the 10.36 p band associated with a tuum-RCH= CHR structure as well as a broad ether absorption a t 9.1 p . The absorption at 13.4 p is in the proper region for a C-Cl group while absorptions a t 8.0 and 14.7 p are characteristic of the allyl chloride structure, -C(R)=CH-CHpCl. The presence of a inethyl group in the 3-position, or an isopropoxyl instead of an ethoxyl group in the 5-position, introduces only the expected changes in the general absorption pattern. The observed similarities establish, therefore, that all of the prepared isoprenea-chloroether adducts had the same basic structure. Methoxylation and acetoxylation of the 1,B-dichloro-2-hexene coinpounds resulted in the removal of the bands a t 8.0 and 14.7 p which are attributed to C-C1. The acetoxyl group in compound I I h is readily distinguished by absorption inaxima a t 5.8 arid 8.0 p . All of the alkoxylated and acetoxylated adducts (IIa-i) which have been prepared have been found to have infrared absorption spectra similar to that observed for compound IIa. Inasmuch as the structure of the latter was established by ozoncilysis, all of these materials are indicated to be 3niethyl-~i-chloro-2-hexenes (Ha-j), having oxygencontaining groups in the 1- and 5-positions. The infrared spectra of the dienes (IVa-c) prepared by the dealcoholation of the 2-hexenes (11) just described are characterized by an absorption doublet a t 10.1 and 11.1 p . This doublet is ascribed to terminal unsaturation of the type RCH= CH2. Moreover, the resulting conjugated system of double bonds is established by the presence of an absorption band a t t i 2 p and by characteristic ultraviolet absorption inaxima. The strong absorption a t 7.3 p is attributed largely to the presence of a :
