J. Org. Chem. Hydrogen a t o m were located from differential Fourier syntheses and refmed with isotropic temperature factors. Refinements were accomplished by block-diagonal least-square calculations minimizing wlpd - pc11[2 W = [ a Q 2+ (0.02*FJ21-1] by using programs of UniversaI CrystallographicComputing System 111,UINCS Final R and R, dropped to 0.055 and 0.055, respectively, with maximum shiftlerror ratio of The last differential map showed peaks below 0.21 e A+. Atomic scatteringfactors including anomalous scattering used are those listed in International Tables for X-ray Crystallography.21 All the calculations were carried out on a FACOM M-160F computer. X-ray Photoelectron Spectroscopy. XP spectra were determined by using a JASCO ESCA-1 photoelectron spectrometer, and magnesium K a radiation was used as a source. The samples were ground in a agate mortar and dried under high vacuum for a few hours. The fme powder samples were dusted onto a double (20) Sakurai, T.; Kobayashi, K. Rikagaku Kenkyusho Hokoku 1979, 55, 69. (21) ‘International Tables for X-ray Crystallography”;Kynoch Press: P Johnson, C. K., 1971. ORTEP 11. Report 1974; Vol 4, p 71. O R ~ n, ORNL-3794 (revised), Oak Ridge National Laboratory, TN.
1984,49,3559-3563
3559
sided sticky tape which was mounted on the sample holder. After the sample holder was placed in the sample chamber under high vacuum (1 X 10” Torr) for 2 h, the XPS measurements were performed. To check for radiation damage, C 1s spectra were measured before and after measurements of nitrogen 1s binding energies. Changes in the carbon spectra were not observed. The results of gas analyses by a mass filter (Uthe Technology International) attached to the sample chamber also gave no evidence for the evolution of gaseous decomposition products. All the spectra were run in triplicate and all the peak positions are reported with a precision of *0.20 eV. The C 1s line was taken to be at 284 eV and was used for calibration. Registry No. I, 61157-94-6; 11, 27691-52-7. Supplementary Material Available: Tables I-lV listing final fractional coordinates and anisotropic thermal parameters, all bond lengths, valence angles, torsion angles, and the derivation of atoms from planes for various portions of the molecular framework for N-arylsulfilimine I; Figures 1, 2, and 4 showing the molecular geometry, the molecular packing, and N 1s spectrum of N-arylsulfilimine I (7 pages). Ordering information is given on any current masthead page.
sym -1,2-Diarylethylenes from a-Lithiated Benzylic Sulfones. Catalysis by Elemental Tellurium Lars Engman Department of Organic Chemistry, Royal Institute of Technology, S-10044 Stockholm, Sweden
Received February 8, 1984 The stability of a-lithiated alkyl, allyl, and benzyl phenyl sulfones was studied. a-Lithiated benzyl phenyl sulfones were found to give sym-1,2-diarylethylenesslowly when kept in tetrahydrofuran at ambient temperature for several days. The reaction time was significantlyreduced if a catalytic amount (18-24%) of elemental tellurium was present in the reaction. Other chalcogenides were less effective in this respect. The uncatalyzed reaction produced essentially pure trans olefins whereas the tellurium-catalyzed process afforded substantial amounts of cis isomer (usually 15-35%). Tellurium tetrachloride in chloroform at ambient to reflux temperature was found to be highly effective in promoting cis/trans isomerization of 1,2-diarylethylenes. The involvement of a carbene mechanism or an intermolecular reaction of a-lithiated benzyl phenyl sulfones is considered in a mechanistic discussion.
Introduction The use of sulfones in synthetic organic chemistry is based on the easy formation of a-sulfonyl carbanions and the possibility, after formation of new C-C bonds by alkylation, to remove the SOz moiety by reduction or elimination.’ In a series of papers: Ingold has studied the elimination of sulfinate anions from sulfones and he established the 1,2-elimination as well as the 1,l-elimination reaction. Thus, phenyl @-phenethylsulfone (1)yielded styrene on phS02CH2CH2Fh
phcH2so2Fh
1 z treatment with sodium ethoxide at 200 O C whereas benzyl phenyl sulfone (2), when fused with KOH at 200 O C , afforded stilbene (via phenylcarbene). (1) For recent review in the field of sulfone chemistry see: Field, L. Synthesis 1972,101; Zbid. 1978,713. Magnus, P. D. Tetrahedron 1977, 33, 2019. Reid, D. H., Sr. Reporter “Organic Compounds of Sulphur, Selenium and Tellurium”; The Chemical Society; London, 1970; Vol 1; Zbid. 1973; Vol2; Zbid. 1975; Vol. 3; Hogg, D. R. Sr. Reporter, VoL 4; Zbid. 1977; Vol. 5; Zbid. 1979, Vol. 6; Zbid 1981. Block,E. ‘Reactions of Organosulfur Compounds”, Academic Press, New York, 1978. (2) Fenton, G. W.; Ingold, C. K. J. Chem. SOC.1928,3127; Zbid. 1929, 2338, Zbid. 1930,706. Ingold, C. K.; Jeaeop, J. A. J. Chem. Soc. 1930,708.
0022-3263/84/ 1949-3559$01.50/0
The harsh reaction conditions necessary to effect elimination have of course prevented any synthetic use of these processes. However, it was recently found that the 1,2elimination occurred under considerably milder reaction conditions if the resulting olefin was highly ~onjugated.~ The 1,l-elimination had received little attention up to recently when Julia4 found that a-metalated allylic sulfones were converted by a catalytic amount of Ni(I1) acetylacetonate into symmetrical olefins. Alkyl or benzyl sulfones could also be used,but the best results were obtained by using allylic sulfones (eq 1). NiIIl) RrHSOzPh
M M = Li, Mg R vinyl,alkyl.aryl
RCH=CHR
(1)
(3) See for example: Julia, M.; Amould, D. Bull. SOC. Chim. Fr. 1973, 743; Zbid. 1973, 746. Julia, M.; Badet, B. Zbid. 1976, 1363. Fischli, A.; Mayer, H. Helv. Chim. Acta 1975,58, 1492. Kondo, K.; Tunemoto, D. Tetrahedron Lett. 1975,1007. Olson, G. L.; Cheung, H.-C.; Morgan, K. D.; Neukom, C.; Saucy, G. J.Org. Chem. 1976,41,3287. Manchand, P. S.; Rosenberger, M.; Saucy, G.; Wehrli, P. A.; Wong, H.; Chambers, L.; Ferro, M. P.; Jackson, W. Helv. Chim. Acta 1976, 59, 387. Fischli, A.; Mayer, H.; Simon, W.; Stoller, H.-J. Zbid. 1976, 59, 397. (4) Julia, M.; Verpeaux, J.-N. Tetrahedron Lett. 1982, 23, 2457.
0 1984 American Chemical Society
3560 J. Org. Chem., Vol. 49, No. 19, 1984
Engman
Table I. Synthesis of 1,2-Diarylethylenes, ArCH=CHAr (5) reactn yield, cis/ trans mp (trans catalysis' time: days % ratio isomer), O C Te 90 2080 124 2 86 >97% trans
Ar phenyl (5a)
Te
2-methylphenyl (5b)
95
1585
80
>97% trans
78
2674
76
>97% trans
32
15:85
49
11:89
75
35:65
84
>97% trans
79
26:74
73
3:97
83
37:63
55
>97% trans
77
24:76
70
6:94
88
35:65
48
>97% trans
97
14:86
81
>97% trans
91
694
71
>97% trans
83 5 Te
4-methylphenyl (54
180 10
Te
2-chlorophenyl (5d)
3 3-chlorophenyl (58)
Te 2
Te
4-chlorophenyl (5f)
3 Te
4-bromophenyl (5g)
12
Te
3,5-dimethylphenyl (5h)
2
Te
4-methoxyphenyl (5i)
7 Te
2-naphthyl (53
7 Te
2-biphenylyl (5k)
7
lit. mp, " C 12411
82-8326
178-18OZ7
97-98
96-9728
96
94-9528
177
17711
211-212
2O8-21Om
139
14G-14l3O
215
21531
255
25532
124-125
C
'18-24% mole % of Te was used. bThe reaction time in the Te-catalyzed reactions was always 3 h. CMassspectrum m / e M+ 332.
The present study is concerned with the stability of a-lithiated sulfones. These compounds are generally recognized as relatively stable toward 1,l-elimination. For example, benzyl phenyl sulfone (2) has been lithiated in tetrahydrofuran or ethyl ether at ambient temperature and submitted to further reactions without any reports of a-elimination processes."' However, in a longer perspective (days instead of minutes or hours) we find that a-lithiated benzylic sulfones slowly decompose to give sym-1,2-diarylethylenes. If catalyzed by elemental tellurium, the decomposition process is significantly enhanced and, as shown in the following, synthetically useful for the preparation of olefins.
Results A series of substituted benzyl phenyl sulfones 3 (eq 2) were monolithiated in dry tetrahydrofuran at 0 "C and either stirred under N2at ambient temperature for an extended period (indicated in Table I) or stirred for 3 h at ambient temperature with a catalytic amount of elemental tellurium (18-24%). After quenching with ArCH2S02F'n
ArFHSOzF'n Ll
6
-%
ArCH=CHAr
+
h ~SOZLI
(2)
5
( 5 ) Lehto, E. A.; Shirley, D. A. J. Org. Chem. 1957, 22, 989. (6) Kaiser, E. M.; Solter, L. E.; Schwan, R. A.; Beard, R. D.: Hauser, C . R.J. Am. Chem. SOC.1971,93,4237. (7) Kingsbury, C. A. J. O g . Chem. 1972, 37, 102.
aqueous acetic acid, evaporation of the solvent and purification on silica the isomeric composition of the resulting 1,2-diarylethylenes 5 was determined by using 'H NMR spectroscopy (Table I). The clear yellow to orange colored solutions of the anions 4 gradually became turbid (precipitation of PhS0,Li) in the uncatalyzed reaction and the color slowly faded. The reactions were not quenched until the solutions were colorless or did not fade anymore with time. Similar color and turbidity and changes did in principle occur in the faster tellurium-catalyzed reaction, but these could not readily be seen since the solutions turned almost opaque by the finely divided tellurium. As can be seen from Table I, the yields of 1,2-diarylethylenes 5 are usually good with a small difference in the uncatalyzed vs. the catalyzed reaction. However, a significant difference was observed in the isomeric compositions of the two reaction products. The tellurium-catalyzed process invariably gave a high percentage of cis olefin (6-37 % ) whereas the uncatalyzed reaction was usually trans specific. A control experiment showed that this difference could not be attributed to cis/trans isomerization in the uncatalyzed longtime experiments. The 1,2-diarylethylenes were usually characterized as their trans isomers (Table I). It was found that tellurium tetrachloride was highly effective in promoting cis/trans isomerization at mild reaction conditions (eq 3). Stilbene (5a) (cisltrans = l / J could be completely isomerized to the trans isomer when heated in refluxing
J. Org. Chem., Vol. 49, No. 19, 1984 3561
Diarylethylenes from Benzylic Sulfones ArCH=CHAr cis / t w
P‘
c= c,
CHCl3
A:
Scheme I (3)
H
CHC13 for 1 h with TeC14 (20 mol % of total stilbenes). 4-Chloro and 4-bromostilbene (5f and 5g) could be isomerized if heated to reflux for several hours with a full equivalent of TeCl& 2-Chloro and 3-chlorostilbene (5d and 5e) were unaffected by this treatment and could not be isomerized by using TeCl& All other lI2-diarylethylenes in Table I were easily isomerized with TeC1, at ambient temperature to give the trans isomer. The synthesis of sym-lI2-diarylethylenesfrom a-lithiated benzylic sulfones seems to be a quite general reaction but it also has its limitations. If the benzylic part of the sulfone contained a strongly electron-withdrawing substituent like a 4-nitro or a 2-carbethoxy group (compounds 6 or 7), the reaction failed. C(30Et
z
5
Unfortunately the reaction seems to be restricted to benzylic sulfones. Attempts to synthesize olefiis from the alkyl or allyl phenyl sulfones 8a-c were unsuccessful in longtime experiments as well as in the tellurium-catalyzed process. RSO2Ph
8
o R=hexyl b R=phenacyl c R=E-CH$H=CHPh
Ph TH SO2Ph CH3
9
Substitution in the benzylic position could not be tolerated and the sulfone 9 afforded only trace amounts of an olefinic product. The remarkable catalytic effect of elemental tellurium in the synthesis of 1,2-diarylethylenes also led to an investigation of the activity of other members of group 6A of the Periodic Table (chalcogenides). Elemental selenium as well as elemental sulfur did promote the olefin-forming reaction of a-lithiated benzyl phenyl sulfones. However, to obtain good yields it was necessary to use substantially larger amounts of these chalcogenides (1 equiv). Furthermore, the products had a bad smell and contained selenium or sulfur containing byproducts that could not be readily removed by chromatography. When tellurium was replaced by an equivalent amount of sulfur in the catalytic synthesis of stilbene (5a),the yield was only 21% (90% with Te). The yield of 2,2’-dichlorostilbene (5d) similarly dropped from 32 % to 219’0 when tellurium was replaced by selenium. An attempt to reduce the long reaction times in the uncatalyzed olefin-forming reactions by increasing the temperature met with failure. a-Lithiated 4-methylbenzyl phenyl sulfone yielded only trace amounts of 4,4‘-dimethylstilbene (5c) after 4 h in refluxing tetrahydrofuran.
Discussion The conversion of a-lithiated benzylic sulfones to symlI2-diarylethylenes constitutes a new synthetic method for these compounds (especially stilbenes). The required sulfones are readily available from the reaction of sodium benzene sulfinate with a suitable benzyl bromide,”1° which (8)Suter, C. M. “The Organic Chemistry of Sulfur”;John Wiley & Sons: New York, 1948;p 568. (9)Vennstra, G. E.;Zwaneburg, B. Synthesis 1975,519. (10)Wildeman, J.; van Leusen, A. M. Synthesis 1979,733.
AryS02Fh
ArFHS02Ph
+
Li
TeLi
E
ArCH=CHAr
-PhS02Li
ArCHSO2Fh I
Te?,
-’t
Te
+
-
Li CHAr
1e
~h502 ~i
?e, ArHC - CHAr
11
in turn is easily obtainable by N-bromosuccinimide bromination of the respective methylbenzene. The two most widely applied methods for the synthesis of stilbenes, the Wittig reaction and the decarboxylation of phenylcinnamic acids,11-12 both require a condensation of two distinctly different fragments that have to be separately synthesized. The sulfone-mediated synthesis allows in principle a dimerization of one benzylic fragment and is of course limited to the preparation of symmetrical compounds. The cis-stilbenes obtained in the tellurium-catalyzed reaction can be isomerized to trans-stilbenes by a variety of reagents (light, halogens, acids, bases and elements such as platinum or selenium).13 The use of TeCb for cis/trans isomerizations described in this paper should be a useful addition to the other methods, especially in view of the mild reaction conditions used (many of the other methods were carried out at high temperature). AlbeckI4has recently observed that TeCl, is capable of isomerizing cisstilbene into trans-stilbene in CHzC12or CH3CN at ambient temperature. However, these authors isolated polymeric or oligomeric products after 5-8 h. We never experienced any problems with polymerization during our isomerizations in CHC13. There are several possibilities to explain the formation of sym-1,2-diarylethylenes from a-lithiated benzylic sulfones. In principle, the olefinic compounds might be formed as dimers of arylcarbene, ArCH. However, this pathway seems unlikely since no cyclopropanes 10 could be isolated Fh
19
u
from the uncatalyzed or from the tellurium-catalyzed decomposition of a-lithiated sulfones. Furthermore, the addition of cyclohexene (6 equiv) did not result in the formation of any isolable amounts of compound 11.16 Dodson16 has suggested an intramolecular reaction, proceeding via a dimagnesium derivative 14, to account for the formation of lI3-diphenylpropene (13) from benzyl FhCH2SqCH$H2Ph
PhCH=CHCHzPh
12
I3 /3-phenethyl sulfone (12) on treatment with ethylmagnesium bromide. A similar protocol was applied to account for the ring contraction of cyclic a-sulfonyl car(11)Wheeler, 0.H.; Battle de Pabon, H. N. J. Org. Chem. 1965,30, 1473 and references cited therein. (12)Gorham, J. h o g . Phytochem. 1980,6, 203 and references cited
therein. (13)Maccarone, E.; Mamo, A.; Perrini, G.; Torre, M. J. Chem. SOC., Perkin Trans. 2 1981,324 and references cited therein. (14)Albeck, M.; Tamari, T. J. Organomet. Chem. 1982, 238, 357. (15)Julia‘ did isolate 1,2,3-triphenylcyclopropanein the nickel(I1)catalyzed decomposition of the lithium salt of benzyl phenyl sulfone. When cyclohexene waa present in a large excess, compound 11 was formed as a major product. (16)Dodson, R. M.; Schlangen, P. P.; Mutsch, E. L. J. Chem. SOC., Chem. Commun. 1965,352.
3562
J. Org. Chem., Vol. 49, No. 19, 1984
bani on^"-'^ and the formation of stilbene from a series of benzylic sulfur compounds.20v21 Our observation that a-lithiated benzyl phenyl sulfones yield stilbenes can be explained assuming an intermolecular version of this reaction (proceeding via compound 15). BrMg-C\H R'
i9
BrMg-CHR
14
Ar C;HS02Ph
f
LI -CHAr
E
The catalytic effect of tellurium might be rationalized in the following way: elemental tellurium is known to readily insert into carbon-metal bonds.= Insertion of Te into the carbon-lithium bond of compound 4 would produce a lithium tellurolate 16, that could be expected to be quite nucleophilic. Addition to compound 4 (Scheme I) followed by elimination of PhS02Li would give a labile epitelluride 17 that should readily collapse into stilbene and elemental tellurium. The stereochemistry of the olefinic product would be determined in the formation of compound 17 (the extrusion of tellurium is a syn processB"). If this step is rapid, the observed high percentage of cis olefin in the product is explainable. The mechanistic discussion presented above is of course very speculative, but it hopefully serves to illustrate the complexity of the problem. More experiments have to be made to distinguish between the different mechanisms. Finally, out observation that a-lithiated benzyl phenyl sulfone slowly yields stilbene could possibly explain the formation of small amounts of stilbene in the reaction of phenylmethanesulfonyl halides with phenyllithium. Shirota and co-worker@ did isolate benzyl phenyl sulfone as a byproduct in this reaction.
Experimental Section Melting points were uncorrected. NMR spectra were obtained by using a Bruker WP 200 instrument. They were recorded in CDC13solutions containing Mel% as internal standard and are reported in b units. Mass spectra were recorded with an LKB 9000 instrument. All preparations of 1,2-diarylethyleneswere performed under nitrogen in a three-necked 100-mL flask fitted with a septum, a glass stopper, and a connection to a nitrogen flask via a Firestone valve. Elemental tellurium was finely ground in a mortar and added rapidly to the reaction flask while a brisk stream of nitrogen (17) Dodson, R. M.; Hammen, P. D.;
Jancis, E. H.; Klose, G. J . Org.
Chem. 1971,36, 2698.
(18) Photis, J. M.; Paquette, L. A. J. Am. Chem. SOC.1974,964715. (19) Dodson, R. M.; Zielske, A. G. J. Chem. SOC.,Chem. Commun. 1965, 353. (20) Kaiser, E. M.; Beard, R. D.; Hauser, C. R. J. Organomet. Chem. 1973, 59, 53. (21) Wallace, T. J.; Pobiner, H.; Hofmann, J. E.; Schriesheim, A. J. Chem. SOC.1965, 1271. (22) Engman, L.; Cava, M. P. Organometallics 1982,1, 470 and ref-
erences cited therein. (23) Clive, D. L.; Kiel, W. A.; Menchen, S. M.; Wong, C. K. J. Chem. SOC.,Chem. Commun. 1977, 657.
(24) BPckvall, 3.-E.; Engman, L. Tetrahedron Lett. 1981, 22, 1919. (25) Shirota, Y.; Nagai, T.; Tokura, N. Bull. Chem. SOC.J p 1966,39, 405. Shirota, Y.; Nagai,T.; Tokura, N. Tetrahedron 1967,23,639. (26) Wisliceniua, W.; Wren, W. Ber. Dtsch. Chem. Ges. 1905,38,502. (27) Bulmer, G.; Mann, F. G. J. Chem. SOC.1945, 674. (28) Stanfield, J. A,; Reynolds, L. B., Jr. J. Am. Chem. SOC.1952, 74, 2878. (29) Wislicenius,W.; Elvert, H. Ber. Dtsch. Chem. Ces. 1908,41,4121. (30) Levi, E. J.; Orchin, M. J. Org. Chem. 1966, 31, 4302. (31) Howard, B.; Hilbert, G. E.; Wiebe, R.; Gaddy, V. L. J. Am. Chem. SOC.1932,54,3628. (32) Wood, J. H.; Bacon, J. A.; Meibohm, A. W.; Throckmorton, W. H.; Turner, G. P. J . Am. Chem. SOC.1941,63, 1334.
Engman was passed through the open system to prevent any introduction of air. Tetrahydrofuran was freshly distilled from K/benzophenone and chloroform was repeatedly washed with water to remove ethanol before it was dried over CaC12. Tellurium tetrachloride was sublimed before use and finely crushed with a glass rod. All benzyl phenyl sulfones used were prepared from the respective benzyl bromide and sodium benzene sulfiiate in ethanol, in analogy with literature methods.*VB Phenacyl phenyl sulfone, hexyl phenyl sulfone, and Q-3-pheny1-2-propenyl phenyl sulfone were similarly prepared from the corresponding alkyl or allyl bromide. a-Methylbenzyl phenyl sulfone was obtained from a-lithiated benzyl phenyl sulfone upon treatment with methyl iodide in THF. Typical Procedure. Stilbene (5a) from Benzyl Phenyl Sulfone. To a stirred solution of benzyl phenyl sulfone (1.0 g, 4.3 mmol) in dry tetrahydrofuran (50 mL) a t 0 OC was added butyllithium (2.7 mL, 1.6 M, 4.3 mmol). Freshly crushed and finely ground elemental tellurium (0.10 g, 0.78 mmol) was then added to the orange-yellow solution and stirring continued at ambient temperature for 3 h. During this period the orange-yellow color of the solution was slowly fading away. Protonation of any resulting anion was effected by the addition of aqueous HOAc (2 mL, 50% aqueous). After evaporation of the solvent the product was extracted into methylene chloride and washed once with sodium carbonate (5% aqueous). Drying, evaporation, and chromatography @ioz,CHzClz/lightpetroleum bp 40-60 "C, 3/1) afforded 0.35 g (90%) of stilbene as a cis/trans mixture. The isomeric composition was determined by integration of the 'H NMR spectrum (olefinic protons, b,, 7.11 ppm, beis 6.60 ppm, cis/trans 1/4). In the uncatalyzed reactions the yellow to orange-coloredsolutions of the respective a-lithio sulfones, prepared as described above, were stirred at ambient temperature for a period indicated in Table 1. During this time the color of the solutions gradually faded away and a white precipitate (PhSOzLi) separated. The workup was performed as described above. The yields, melting points, and isomeric compositions of the olefins 5a-k are displayed in Table I. The cis/trans ratios were determined by integration of the 'H NMR spectrum. The chemical shifts of the olefinic protons of the different 1,2-diarylethylenes are reported in the following way (compound, Hciaolefin ppm, Htranaoleru ppm): 5b, 6.72, 7.19; 5c, 6.51, 7.04; 5d, 6.89, 7.52; 5e, 6.57, 7.03; 5f, 6.55, 7.01; 5g, 6.54, 7.02; 5h, 6.47, 7.03; 5i, 6.44, 6.93; 5j, 6.85, 7.41; 5k, 6.37, 7.05. A large volume of CHzCl2was required to successfully isolate the sparingly soluble compound 5j. The crude product was purified by filtration (CH,ClZ)through a short silica column. When elemental tellurium was replaced by sulfur (0.78 mmol) in the catalyzed preparation of stilbene, the isolated yield was 21%. When elemental selenium similarly replaced tellurium in the preparation of 2,2'-dichlorostilbene (sa),the isolated yield was 13%. A cis/trans mixture of stilbene did not show any tendency to isomerize when stirred for five days a t ambient temperature in dry THF containing some elemental tellurium. The presence of 6 equiv of cyclohexene in the catalyzed as well as uncatalyzed decomposition of a-lithiated benzyl phenyl sulfone did not produce any new products. Typical Procedure. TeC14-InducedCis/Trans Isomerization of Stilbene. Stilbene (0.35 g, 2.0 mmol, cis/trans 1/4) was heated to reflux for 1h in dry chloroform (15 mL) containing TeC1, (0.10 g, 0.37 mmol). The cooled solution was then shaken with aqueous Na2CO3(5% aqueous) in a separatory funnel. After separation, drying, and evaporation, 'H NMR analysis of the product (0.33 g) indicated complete isomerization to trans-stilbene, mp 124 OC (lit.11 mp 124 "C). The isomerization of 4,4'-dichlorostilbene and 4,4'-dibromostilbene were carried out with an equivalent amount of TeC1, (TeCl,, cis- + trans-stilbene 1:l)and a reflux period of 4 h was required to complete the isomerization. 2,2'-Dichlorostilbene and 3,3'-dichlorostilbenecould not be isomerized during these reaction conditions. (33) Shriner, R. L.; Struck, H. C.; 1930,52, 2060.
Jorison, W. J. J . Am. Chem. SOC.
J. Org. Chem. 1984,49,3563-3570 2,2’-Dimethyl-, 4,4’-dimethyl-, 3,3’,5,5’-tetramethylstilbeneaa well aa 192-di(2-naphthyl)ethylenewere isomerized to the trans form by stirring at ambient temperature for 1 h with 20 mol % (counted on total.stilbenes) of TeC4. 4,4’-Dimethoxystilbene waa completely isomerized after only 5 min. The isomerizations of very electron-rich stilbenes (4,4’-dimethoxy- and 3,3’,5,5’-tetramethylstilbene) were accompanied by precipitation of small amounts of elemental tellurium.
Acknowledgment. This work was financially supported by the Swedish Natural Science Research Council. Registry No. 3a, 3112-88-7; 3b, 71996-48-0; 30, 19523-24-1; 3d, 91110-67-7; 3e,51229-57-3; 3f, 51229-56-2; 3g, 91110-68-8; 3h,
3563
91110-69-9; 3i, 55539-39-4; 3j, 17164-88-4; 3k, 91110-70-2; 4a, 83600-56-0;4b, 91110-71-3; 4c, 91110-72-4;4d, 91110-73-5; 4e, 91110-74-6; 4f, 91110-75-7; 4g, 91110-76-8; 4h, 91110-77-9; 4i, 91110-78-0; 4j, 91110-79-1; 4k, 91110-80-4; cis-59, 645-49-8; trans-59,103-30-0;cis-56,20657-42-5; trans-56,36888-18-3;cis-%, 2510-76-1; trans-5c, 18869-29-9; cisdd, 20657-43-6; trans-5d, 25144-38-1; cis-be, 20101-53-5; trans-5e, 23958-24-9; cis-5f, 2510-74-9; trans-5f) 1657-56-3; cisdg, 23958-29-4; trans-5g, 18869-30-2; cis-Sh, 20657-30-1; trans-5h, 13863-27-9; cis-5i, 2510-75-0; trans-5i, 15638-14-9; cis-5j, 2633-08-1; trans-5j, 2753-11-9; cis-5k, 91110-81-5; trans-Sk, 91110-82-6; 6,34063-53-1; 7,51229-68-6;8a,16823-63-5;8b, 3406-03-9; 8c, 16212-07-0; Te, 1349480-9; S, 7704349; Se, 7782-49-2; Tech, 10026-07-0;n-BuLi, 109-72-8.
Products, Radical Intermediates, and Hydrogen Atom Production in the Thermal Decomposition of 1,2-Dihydronaphthalene1p2 James A. Franz* and Donald M. Camaioni Pacific Northwest Laboratory, Richland, Washington 99352
Robert R. Beishline* Weber State College, Ogden, Utah 84408
Don K. Dalling University of Utah Research Institute, Salt Lake City, Utah 84108 Received February 17, 1984 The thermal decomposition of 1,2-dihydronaphthalene(DHN) at 300 “C produces tetralin, naphthalene,hydrogen, and five Cm hydrocarbon produds (1-5). The isolation of compounds 1-5 from the thermal decomposition of 1,2-dihydronaphthalene-4-drevealed compounds 1and 2 to be formed by the addition of a-hydronaphthyl(2HN) to DHN followed by intramolecular cyclization and hydrogen abstraction. Compound 3 waa formed by addition of 1-tetralyl radical to DHN followed by hydrogen abstraction, and compounds 4 and 5 were formed by a sequence involving initial addition of 2-tetralyl radical to dihydronaphthalene. The thermal decomposition of DHN at 4W450 OC leads to the formation of a hydrogen atom which participates in subsequent hydrocracking reactions of available substituted aromatic structures.
Introduction The thermal decomposition of 1,2-dihydronaphthalene (DHN) provides an example of the reaction of two closed-shell molecules to produce a pair of free radicals, termed molecule-assisted hom~lysis.~The understanding of this mechanism of atom transfer, as well as related mechanisms of initiation, chain reactions, and hydrogen shuttling pathways is central to the development of a global mechanism of coal liq~efaction.~ In an early study of DHN decompositi~n,~ Gill et al. found the rate of disappearance of DHN to follow second-order kinetics and found that radical inhibitors either did not inhibit the (1) This work was supported in part by the U.S. Department of Energy (U.S.D.O.E.) under Contract DE-ACOB-RLO 1830, at the Pacific Northwest Laboratory, Richland, WA 99352, and under contract DEAC02-79ER10610, U.S.D.O.E., with the Department of Chemistry, Weber State College, Ogden, UT 84408. Support for DKD and for the use of the Varian SC-300 spectrometer under contract DE-AC2279ET14700, U.S.D.O.E., with the University of Utah is gratefully acknowledged. (2) A preliminary account of a portion of this work has appeared F”, James A.; Camaioni, Donald M., Beishline, Robert R.; Dalling, Don Prepr. Fuel Diu., Am. Chem. SOC.1983,28 (S), 150-163. (3) Pryor, W. A. In “Organic Free Radicals”; Pryor, W. A,, Ed. ACS Symp. Ser. 1978,69, 33-62. (4) Stein, S. E. ACS Symp. Ser. 1981,169,97-129. (5) Gill, G. B.; Hawkins, S.; Gore, P. H. J. Chem. SOC.,Chem. Commun. 1974, 18, 742.
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reaction or accelerated it. The temperature dependence of the decomposition over a narrow (15 “C) temperature range gave E, = 37 f 1.2 kcal/mol and log = 9.7. They proposed a concerted scheme for the reaction, but Heesing and Miillers, using stereospecifically deuterium-labeled DHN, demonstrated that the reaction was nonconcerted.6 These workers also noted the presence of a substantial yield (-25%) of Cmproducts and solvent adducts but did not determine the structures of the Cz0 products. Heesing and Mullers determined, by studying recovered DHN, that stereospecificity of labeling in the recovered starting material was not lost in the reaction, and proposed the principal pathway of the of the reaction to be that of eq 1. This bimolecular disproportionation,
(1)
or molecule-assisted homolysis,3 must be effectively irreversible, from the observed preservation of label stereospecificity.6 In the related cyclohexadiene system,Benson (6) Heesing, A.; Miillers, W. Chem. Ber. 1980, 113, 9-18.
0 1984 American Chemical Society
