493ti
ILLJ JAM E.
PARHAM A N D
column of alumina (activated a t 200' for 18 h r . ) packed in petroleum ether (b.p. 60-68"). Sulfur (35 mg., 54% yield, m . p . 107-109O, mixed m.p. with authentic (m.p. 115-116") sulfur, 113-116°) was eluted from the column with petroleum ether. Further elution of t h e column with 19-20?; solutions of meth5-lene chloride in petroleum ether ib.p. 60-68") gave the principal fractions (466 mg., m.p. Bt5-1390, 7T':'c recovery) which were shown' to contail1 2l$:& of \'I (18(>:yield), 29Yc of V (255: yicld) aud 43';.;. of startiiig material ( I V , 33Yc recovery). The ratio \Y/Y was esseritially identical t o t h a t found in the previous experiment, and the composition of the mixture was again unchanged bv recrrstallization from Detroleum ether ib.D. 60-68') 185% recovery, m.p; 13$-139', 23L!l \.1/>3;,1 \./44(,'$ 11.) or methanol ( 7 5 5 , recovery, m . p . 135~-145', 23c$ \'1/32C,', T./46(,% 11'). Ethyl p-Methoxyphenacyl Xanthate.--.\ stirred solution of p-niethosyphenacyl chloride (5.6 g., 0.030 mole) in dry acetone (10 ml.) was added dropwise t o a suspension o f piitlissiurn ethyl xanthate (4.83 g . , 0.030 niole) iii dry
EDWINT.HARPER
Vol. 82
acetone (100 ml.). The precipitated potassium chloride was removed b y filtration, the solvent was removed by distillation at reduced pressure and the residue was recrystallized from ethanol. There was obtained 6.08 g. (80% yield) of ethyl p-methoxgphenacyl xanthate (m.p. 60-64'). Further recrystallization of this product from etherpetroleum ether (b.p. 30-60') gave yelltiw transparent plates melting a t 67.5-68.0'. Anal. Calcd. for C12H140:&:C, 53.31; H, 5.22. Found: C , 53.36; H , 5.58. Ethyl p-nitrophenacyl xanthate nas prepared from pnitrophenacyl chloride (6.00 g., 0.030 mole) by a procedure similar t o t h a t described above for ethyl p-methoxyphenacyl xanthate. The crude product (7.52 g., 93% yield, m.p. 81-87') was purified by recrystallization from etherpetroleum ether (b.p. 30-60") and the pure xanthate was obtained as fine white needles melting a t 88-89'. Anal. Calcd. for C11H1104NS2:C, 46.30; H, 3.89. Found: C, 46.80; H , 3.79.
[CONTRIBCTIOS FROM THE S C H O O L O F CHEMISTRY O F T H E UNIVERSITY O F MINSESOTA, LIISNEAPOLIS, M I S N E S O T A ]
Heterocyclic Vinyl Ethers. XIX.
Unsymmetrical 2,4-Diarylthiophenes'
BY \YILLIAM E. PARITAM AND EDWIN T. HARPER? RECEIVEDMARCH *5, 1960 The synthesis of 2-pliei1~l--l-p-1netl1~syp~ie1i~lt11i~~~liene (11) and 2-p-methosgplietiyl-4-phenglthiophene (111) is described by procedures which involve the high temperature sulfuiation of appropriate olefins. A by-product, believed t o be 2-(p-methox).phenyli-thieno ~%,3-b]thianaphthene ( S1, was isolated from the reaction of l-phen~-l-3-p-methoxyphenyl-2butene ( I Y ) and sulfur. The composition of mixtures of the isomeric thiophenes, either alone or containing 2-p-methoxy~ 1 h e i i p l - 5 - ~ h e n y l - l , 4 - ~ l i t l i i (~I~)~, can l i e nbe ~ determiiied conveniently by infrared spectroscopic analysis.
As part of our study3 of the formation of 2,4- ers recently have reported the synthesis of 2-pdiarylthiophenes from 2,%diaryl-l,-l-dithiadienes methoxyphenyl-4-phenylthiophene (111) by the (such as I), it became necessary to prepare 2phenyl-4-p-methoxyphenylthiophene (11) and 2fi-methoxyphenyl-&-phenylthiophene(111) and to C& devise a method for determining the composition of mixtures of I, I1 and 111. IV, R = H, R ' = OCH, V,R=OCHa, R ' = H 200"
I I , R = H , R'=OCHB 111,R z O C H 3 , R ' = H
The synthesis of 2,4diarylthiophenes has been studied in detail,4 but these methods have been generally limited to thiophenes in which the aryl substituents are identical. Schmitt5 and co-work(1) P a r t of this work was supported b y the Office of Ordnance re^ search, U. S. Army, Contract K O . D A - l l - 0 1 2 - 0 r d - 2 6 l l i . ( 2 ) From the P h . D . Thesis of E. T . Harper, Cniversity of hlinnesota, 1959. (3) W. E. P a r h a m and E. T . Harper, T H I S J O U K N A L , 82, 1931 (1900). 14) l a ) I.C d m y a i g u r , ,bid , 6 6 , lib4 (1544). i l l ) I' Ilemrrseman, h g Buu-Hoi, R . Koyer a n d A Chcutin, J ( h P i i i .So< , 2720 (1934). 1.5) (a) J. Schmitt, R . Fallard and M . SuqTict Liiili SLC. ciriin. I ~ r u , i u t , , 2 , 11-17 (1Y5Ci). ( b ) These workers reported their sthrling ketone, #-p-methoxyphenylpropiophenone, to melt a t 90-100" a n d the thiophene (111) t o melt a t 162'. Neither of these melting points are correct. P - p - ~ l e t h o x y p l i e n y l i ~ r o i ~ i o i ~ h e nmelts o n e a t 59-tiO', and the
reaction of V with sulfur, and we have eniployed this procedure (reaction of IV and V, respectively, with sulfur) for the preparation of the isomeric thiophenes I1 and 111. It became evident that the product obtained by Schmitt was not 2-9-methoxyphenyl-4-phenylthiophene (111) as described, but more probably the isomeric thiophene II.5b The olefins, 1-phenyl-3-p-methoxyphenyl-2 - butene (IV) and I-p-methoxyphenyl-3 - phenyl - 2 - butene (V) were synthesized, in 37% and 56% yields, respectively, by Grignard reactions. The dehydration of the alcohols VI1 and IX could logically give olefins with structures corresponding to A, B or C. Structure A (cis-2-butene) was assigned to olefin V (Ar = p-CH30C6H4-, Ar' = C6H5) in the following way. Structure C was eliminated on the basis of the infrared spectrum of thiophene 111 melts a t 115O. I t is reasonable t o assume t h a t Schmitt and co-workers started with t h e isomeric ketone p-phenyl-9-methoxypropioilhenone (m.p, 97') a n d prepared thiophene I1 ( m . p . 163').
Sept. 20, 1960
4937
CNSYXMETRICAL 2,3-9I.\RYLT€IIOPHENES
yet been realized, although a cursory exploration of the conditions revealed the deleterious effect of excess sulfur. These d a t a are siimmnri7ed in Tahlv I1 (Experimental). An interesting by-product was isolated during the preparation of 2-p-methoxyphenyl-4-phenylthiophene (111). The white, crystalline solid, m.p. 1i7.0-179.0°, was assigned the structure 2 (p-methoxypheny1)-thieno [2,3-b]thianaphthene(X) in view of the data that the composition and molecular weight required the molecular formula (217-
0
VI
OH
0 II
1, C,H,MgBr
r
VI11 OH
P XI
V, which did not show absorption characteristic of the terminal methylene groupe6 Comparison of the ultraviolet spectrum of V with those of model compounds (Table I, Experimental) indicated that the olefin was principally, if not exclusively, cis-lp-methoxyphenyl-3-phenyl-2-butene(VA) . The main absorption of V coincided in wave length and intensity with that of ~is-2-phenyl-2-butene,~ and there was no shoulder a t 235 mp characteristic of trans-2-phenyl-2-butene.‘Since the spectra of cis- and trans-2-p-methoxyphenyl-2-butene were not available, the assignment of structure A to olefin IV was made by analogy. This assignment was supported,6 however, by the infrared spectrum of IV, which was quite similar to that of V.
HlzOSz; thus the two most reasonable structural formulas seemed to be X and XI. Assignment of structure X was based on the infrared spectrum of the product which did not show absorption characteristic of a monosubstituted benzene (690 cm.-1).6 The possibility was considered that the by-product X was formed by further direct sulfuration of 2-p-methoxyphenyl-4-phenyl-thiophene(111). However, no crystalline products were isolated when I11 was heated with sulfur a t 200’ for 60 hr. The formation of X, instead of its isomer XI, can be rationalized on the basis of a free radical mechanism similar to the one postulated by Horton8 for the sulfuration of toluene and which involves the intermediate diradical XII.
Er)C=cc=c(;;;, A
ArCH?
B
C
The formation of cis-2-butenes by dehydration of VI1 and IX was not unexpected, since the trans-2butenes (IVB and VB), with the 3-aryl group sandwiched between alkyl groups, should be thermodynamically less stable than the cis isomers (IVA and VA). The larger barrier to coplanarity of the 3-aryl group with the olefinic double bond in the trans isomers undoubtedly decreases their styrene-like conjugation, reducing their stability. T h e greater stability of cis- compared to trans-2phenyl-2-butene may also be inferred from the spectra shown in Table I. The lessened styrenelike conjugation is reflected by the smaller extinction coefficient and shorter wave length of the ultraviolet maximum of the trans isomer. It may be that the cis 2-butene structure of the olefins I V and V, which is sterically similar to the carbon skeleton of thiophene, was partly responsible for the success of the sulfuration reactions. The optimum reaction conditions probably have not (6) L. J. Bellamy, “ T h e Infrared Spectra of Complex Molecules,” 2 n d Ed., John Wiley and Sons, Inc., New York, N. Y., 1958. (7) D. J. Cram, THISJOURNAL, 71, 3883 (1949).
XIIa
CH2S.
The analysis of mixtures of the thiophenes I1 and I11 was carried out by an infrared spectroscopic technique after it was found that the melting points of mixtures of the thiophenes were poor criteria of composition (Fig. 1, Experimental). Mixtures containing 2-p-methoxyphenyl-5-phenyl1,4-dithiadiene (I), as well as the thiophenes 11 and 111, also were analyzed successfully by this method. The application of this analysis is discussed in the preceding paper,3 and details of the method appear in the Experimental section of this report. Experimental l-Phenyl-3-p-methoxyphenyl-2-butene (IV).-(a) A solution containing benzene (100 ml.) and P-phenyl-p-methoxypropiophenone8 (1.1, 13.12 g., 0.0646 mole, m.p. 97-98’) was added to a solution containing ether 130 ml.) and methyl magnesium iodide (0.17 mole) over a 2-hr. period. The mixture was heated a t the reflux temperature overnight, saturated aqueous ammonium chloride 150 nil.) was added and the organic layer was distilled.10 The product (8.43 g., (8) A. ‘W. Horton, J. O i g . Chem., 14, 761 (1949). (9) E. Rothstein, J . Chem. Soc., 1459 (1951). (10) CJ’ the synthesis of 1,3-diphenyl-.?-butanol:R . Stoermer end H. Kootz, B e y . , 61, 2333 (1928).
\VILLWM E.PARILM .\m EUWINT. HARPER
493s
Vol.
ss
TABLE I ULTRAVIOLET ABSORPTIOXS OF 1-ASD MODELCOMPOCSDS Compound
~
Wave lengths
(c)
of maxima---------------
243 (13900) 248 (13700 i b 254 ( 121011)b 2660 (9100)' 274 (3920)c 284 (2250)b a-Meth ylstyrene" 242 (13000) 285 (280) @-Methylstyrene" 244 (13000 1 280(710) cis-2-Phenyl-2-buteneJ 242 (13000 1 trans-2-Phenyl4-butene' 235 (8000) p-Methoxytoluene8 214 ( 5 ) d %78(1600) a Plateau. Shoulder. Inflection. Minimum. R . A. Friedel and M. Orchin, "Ultraviolet Spectra of Aromatic ComDounds." Tohn IViley and Sons. Inc.. New Uork. S.Y . , 1951. j Ref. 7. 0 P. Ramart-Lucas, Bull. SOC. chinz. France, [ 5 ] 9; 934 (is&)
1-
225 (146001"
6 5 5 yield) was collected a t 125-150" (0.25 mm.). T h e infrared spectrum of the product was essentially identical t o that of the undistilled polymeric residue (3.54 g., 27:; yield) and neither contained absorption characteristic6 of th: hydroxyl group. The olefin was redistilled (b.p. 118-121 (0.03 mm.), n Z 6 D 1.5860-1.5893) and a fraction with ? Z * ~ D I .5893 was analyzed. Anal. Calcd. for C1iHl80: C, 85.67; H , 7.61. Found: C, 85.48; H , 7.70. T h e infrared spectrum of 11. ( b . p . 130-133' (0.18 mm.)) was consistent with the absence of appreciable terminal olefin.6 The principal ultraviolet absorption of 11. occurred a t 254 mM (e 17,200). (b) l-Phenyl-3-p-methoxyphenyl-2-butene(IV) was obtained by a procedure similar to that described in (a), above, from the Grignard reagent of P-phenylethyl bromide (19.1 g., 0.103 mole) and p-methoxyacetophone (12.6 g., 0.084 mole). T h e principal fraction of the product (5.65 g., b . p . 140' (0.19 m m . ) , 28Yc yield) was redistilled a t 0.18 mm. and collected a t 120-125' (nZ6ol,5819), 125-130' ( n z 6 D 1.5850) and 130-133" ( n Z 51.5882). ~ The infrared and ultraviolet spectra of the fraction a z 6 1.5882 ~ were essentially identical t o those reported under (a), above; however, qualitative vapor-phase chromatography indicated t h a t this fraction was only about go'/;, pure. This fraction of the product could be used successfully for the next synthetic step. 2-Phenyl-4-p-methoxyphenylthiophene (II).-Ai mixture of 11- (1.87 g . , 7.85 mmoles) and sulfur flowers (0.75 g . , 23 mg. atoms) was heated at 175-185' for 41 hr. The cooled mixture was dissolved in methylene chloride (40-50 ml.) and chromatographed on a 35 X 420 mm. column of alumina (activated a t 170" ( I 5 m m . ) for 3 days) packed in petroleum ether (b.p. 60-68"). Recovered sulfur 10.37 g., 49%) was eluted with petroleum ether (b.p. 60-68'). 2-Phenyl-4-p-methoxyphenylthiophene (11, 0.47 g., 22 yield, m.p. 145-161") was eluted with a solution o f 35 methylene chloride in petroleum ether (b.p. 60-68'). Fractions eluted subsequently may have contained more I1 but were not processed. The product was recrystallized from carbon tetrachloride ( 2 5 ml.), which afforded 0.45 g. (21ycyield) of 11, m . p . 162-163". The melting point was raised t o 161.5-163.5" by further recrystallization of this product from 2-propanol. ilnal. Calcd. for C1;H1,OS: C, 76.65; H , 5.30. Found: C, 76.37; H , 5.35. l-p-Methoxyphenyl-3-pheny1-2-butene(V).-.h ether solution (25 ml.) containing 1-p-methoxyphenyl-3-butanonell (VIII, 9.16 g., 0.0515 mole) was added dropwise (20 minutes) t o a solution (50 ml.) of phenyl Grignard reagent (prepared from bromobenzene, 18 g., 0.11 mole). The resulting solution mas stirred and heated a t the reflux temperature overnight. The crude l-p-methoxyphenyl-3phenyl-3-butanol (IX), obtained from the ether solution after the action of aqueous ammonium chloride, was treated with boiling 5% sulfuric acid ( 5 ml.) for 12 hr. The resulting oil was washed with water, dried and distilled. The crude l-p-methoxyphenyl-3-phenyl-2-butene (V, 9.26 g., --cy l a i c yield, b.p. 124-149' (0.2 mm.), a Z 51.5795-1.5827) ~ thus obtained was refractionated (b.p. 121' (0.03 m m . ) , n Z 5 D1.5842). Anal. Calcd. for C17H180: C , 85.67; H , 7.61. Found: C, 85.61; H, 7.90. (11) Aldrich Chemical C o . ; fractionated before use (b.p. 80-88'
(0.20 mm.), n 3 5 ~1,5178).
The infrared spectrum of V (b.p. 135-138' (0.13 m m . ) ) was consistent6 with the absence of appreciable terminal olefin. The ultraviolet spectrum of V was essentially identical to the spectra of model compounds (Table I ) and was consistent with the proposed czs-%butene structure
(Val). 2-p-Methoxyphenyl-4-phenylthiophene(III).-X mixture of V (3.09 g., 13.0 mmoles) and sulfur (1.26 g., 39.3 mg. atoms) was heated a t 190-205" for 50 hr. The mixture was chromatographed on a 35 X 440 mm. column of alumina (activated at 250' for 18 hr.) packed in petroleum ether (b.p. 60-68'). Recovered sulfur (0.15 g., lac?,, recrystallized from petroleum ether, m.p. and mixed m.p. 111-116') was eluted with petroleum ether (b.p. 60-68'). The thiophene (111, 1.84 g., 53% yield), eluted with a solution of 4Oc/r methylene chloride in petroleum ether (b.p. 60-68'), was recrystallized twice from petroleum ether (b.p. 60-68'), which afforded 1.43 g. (41%) of 111, m.p. 141-144'. The melting point was raised to 144.5-145.5' by further recrystallization from petroleum ether. dnaE. Calcd. for C1,HIIOS: C, 76.65; H , 5.30. Found: C , 76.80; H , 5.23. This sulfuration reaction was carried out a number of times, but t h e results did not point t o an optimum set of conditions. The data are summarized in Table 11. TABLE I1 SYXTHESISOF THIOPHENES BY SULFURATIOS OF OLEFINS ThioTemo., rh ThioOlefin
phene
Sa
OC.
Timeb H?Sc S d
phenee
175-185 41 90 49 22 1190-205 50 Yes 12 53 1. 180-185 18 ? 42 46 1. 190-200 60 Yes 38 36 L7 240-250 4 S o 17
