3146
experimental observation, this being in connection with 1,3-eliminations from 2-norbornyl tosylates for which a truly concerted process may not apply. For only two of the 33 different systems considered in this and earlier papers lq2 are there major discrepancies between the PLM result and experimental observation. In view of this high degree of agreement it is unlikely that the concordance is purely fortuitous. However, the apparent success of the PLM method does not necessarily imply that least motion of atoms is itself the factor which determines stereochemical pathways. It may well transpire that the least motion type of calculation, by virtue of its geometric constraints and
hence its implicit adherence to requirements of local symmetry, is successfully simulating some more important stereoelectronic factors. Acknowledgments. The authors wish to thank many people who have shown interest in this work and who have suggested systems for our consideration. In particular we thank Professors A. Nickon and N. H. Werstiuk for several discussions. The financial support of the National Research Council of Canada is gratefully acknowledged. We also thank the Sir George Williams University Computer Centre and the Institute of Computer Science at the University of Toronto for the use of computer facilities.
Chemistry of cis- and trans-2,3-Di-tert-butylthiiranes (Episulfides). Some Observations on the Consequences of Steric Overcrowding in Small Ring Compounds Peter Raynolds, Steven Zonnebelt, Silvia Bakker, and Richard M. Kellogg* Contribution from the Department of Organic Chemistry, The University, Zernikelaan, Groningen, The Netherlands. Received November 27, 1973 (episulfides)have been investigated. The cis Abstract: Chemical reactions of cis- and trans-2,3-di-tert-butylthiiranes isomer has extra steric strain because of the interference of the bulky tert-butyl groups. One side of the molecule is exposed to attack by various reagents. The trans isomer, although probably less strained, is better shielded from attack on either side of the ring. The sulfoxides from both isomers have been prepared as well as an S-methylsulfonium salt from the cis isomer. Chlorine cleaves a sulfur-carbon bond of the cis isomer but gives a complex reaction with the trans isomer. Somewhat related behavior is seen with tert-butyl hypochlorite. Protonation of both the cis and trans isomers on the sulfur atom occurs in fluorosulfonic acid. In none of the derivatives prepared was there any evidence for ring opening to a presumably less strained 2-thia analog of an allyl cation. The Smethylsulfonium salt underwent ring opening in the expected trans fashion with a variety of nucleophiles (water, methanol, chloride, bromide). No evidence for initial attack on sulfur was forthcoming although precedent exists for this type of reaction. leitmotiv of organic chemistry is the juxtapositioning of tert-butyl groups. Classic examples in which these bulky substituents are positioned on adjacent trivalent carbon atoms are found in o-di-tert-butylbenzene (l),l cis-l,2-di-tert-butylethylene(2), and 1,1,2-tri-
A
1
2
tert-butylethylene (3). The duces undeniable strain (22.3 the para isomer,’ 10 kcal/mol i ~ o m e r apparently ,~ unknown
3
steric crowding introkcal/mol in 1 relative to in 2 relative to the trans in 3). Hope of discov-
(1) (a) For a compilation of references, see E. M. Arnett, J. M. Bollinger, and M. Barber, J . Amer. Chem. Soc., 89, 5889 (1967). (b) For o-di-fert-butyl aromatics, see Ae. de Groot and H. Wynberg, J . Org. Chem., 31, 3954 (1966). (c) For a discussion of much of the thought that led to interest in o-di-tert-butyl compounds, see H. C. Brown, “Boranes in Organic Chemistry,” Cornell University Press, Ithaca, N. Y., 1972. (2) W. H. Puterbaugh and M. S . Newman, J. Amer. Chem. Soc., 81> 1611 (1959). (3) G. J. Abruscato and T. T. Tidwell, J . Amer. Chem. Soc., 92, 4125 (1970). (4) (a) R. B. Turner, D. E. Nettleton, Jr., and M. Perelman, J . Amer. Chem. Soc., 80, 1430 (1958); (b) M. S. Newman, “Steric Effects in Organic Chemistry,” Wiley, New York, N. Y., 1956, p 248; (c) N . L. Allinger and J. T . Sprague, J . Amer. Chem. Soc., 94, 5734 (1972); (d) M. B. Robin, G . N . Taylor, and N. A. Kuebler, J . Org. Chem., 38, 1049 (1973).
Journal of the American Chemical Society 1 96.10
ering the means by which such strain manifests itself in structural features, spectral properties, and chemical reactivity has provided a major impetus for the synthesis and examination of these types of compounds.5)6 Our own interest in this general area was stimulated when the cis- and trans-2,3-di-tert-butylthiiranes (episulfides) 4 and 5 became available to us through the routes in eq 1 and 2.’ tert-Butyl groups and other bulky aliphatic substituents are often useful in stabilizing sensitive three-membered rings but in the examples reported thus fars the tert-butyl groups appear to be in the sterically ( 5 ) The effects of the considerable steric interaction in these types of compounds are often remarkably muted. Available evidence suggests that in 1 there is no significant deviation from planarity of the K SYStem;‘d,6a the same holds true in o-di-tert-butylquinoxaline, the crystal structure of which has been determined.6b Bending from planarity of an aromatic system is accomplished in an [ 8 ] p a r a c y ~ l o p h a n ehowever, ,~~ and calculations indicate that mild deviations from planarity of the In 2 there appears benzene ring require remarkably little to be no appreciable twisting of the carbon-carbon double bond.6e (6) (a) E. H. Wiebenga and E. Bouwhuis, Tetrahedron, 25,453 (1969); (b) G. J. Visser, A. Vos, Ae. de Groot, and H. Wynberg, J . Amer. Chem. Soc., 90, 3253 (1968); (c) M. G. Newton, T. J. Walter, and N. L. Allinger, ibid., 94, 5652 (1972); also C. J. Brown, J . Chem. SOC., 3265 (1953); (d) H. Wynberg, W. Nieuwpoort, and H. T. Jonkman, TetrahedronLett., 4623 (1973); (e) M. B. Robin, G . N . Taylor, N. A. Kuebler, and R . D. Bach, J . O r g . Chem., 38, 1049 (1973); see also N. L. Allinger and J. T . Sprague, J , Amer. Chem. Soc., 94, 5734 (1972), and ref 6a. (7) J. Buter, S . Wassenaar, and R. M. Kellogg, J . Org. Chem., 37,4045 (1972).
May 15, 1974
3147
rine'o leading to cleavage of a carbon-sulfur bond. Chlorinated disulfides are usually obtained as products. A chlorosulfonium ion 8 is presumed to be an intermeCl
,
&*-
A
I
8
diate in these reactions. With chlorine in methylene chloride at -80" 4 gave 9 in 95% yield (eq 3). The more favorable trans arrangement. With both 4 and 5 available (the cis orientation of tert-butyl groups in 4 is to the best of our knowledge unique among threemembered rings) an exceptional opportunity was offered to learn more about the effect on steric strain on three-membered rings using in this case the sulfur atom as a site for chemical manipulation. Results and Discussion A. Oxidation, Chlorination, and Alkylation of 4 and 5. The sulfoxides 6 and 7 were prepared by oxidation -0
-0
6
7
of 4 and 5 with a single equivalent of m-chloroperbenzoic acid. The anti configuration for 6 is anticipated on the basis of attack from the less shielded side of 4 and is consistent with a downfield shift of 0.29 ppm of the methine hydrogens compared to those in 4.9 The methine hydrogens are also shifted 9.93 ppm downfield in the presence of 0.33 equiv of europium(II1) trisdipivaloylmethane as expected for complexation with the oxygen atom from the least hindered side of the molecule. All attempts to form sulfones from 6 or 7 or from 4 or 5 failed. Reagents investigated included m-chloroperbenzoic acid, sodium peroxide in methane sulfonic acid, 30% hydrogen peroxide in acetic acid, 3 % hydrogen peroxide in water, ozone, and aqueous potassium permanganate. Cis isomer 6 was resistant to further reaction, probably because of steric hindrance to attack on the side of the tert-butyl groups. On the other hand, both 5 and 7 were consumed with excess oxidizing agent but no stable products could be isolated. Thiiranes are known to be attacked readily by chlo(8) Representative examples are 2,3-di-tert-butylaziridine [J. C. Sheehan and J. H. Beeson, J . Amer. Chem. SOC.,89, 362 (1967)], 2,3di-tert-butylaziridinones (and other bulky substituents) [F. D. Greene, J. C. Stowell, and W. R . Bergmark, J . Org. Chem., 34, 2254 (1969)], trans-2,3-di-tert-butylcyclopropanone [J. F. Pazos and F. D . Greene, J . Amer. Chem. Soc.,. 89, 1030 (1967)], and di-tert-butyloxadiaziridine [F.D . Greene and S. S. Hecht, J . Org. Chem., 35, 2482 (1970)l. In this case bulky groups are not mandatory for stabilization. 2,3-Di-tertbutylthiadiazirine 1,l-dioxide: J. W. Timberlake and M. L. Hodges, J . Amer. Chem. SOC.,95, 634 (1973). The same effect can be achieved with adamantyl; see, for example, adamantanespiro-2'-(N-l-adamantylaziridone) [E.R. Talaty and A. E. Dupuy, Chem. Commun., 790 (1968)l. (9) (a) Compare with K. Kondo and A. Negishi, Tetrahedron, 27,4821 (1971). Shifts of methine protons syn to an SO group are less trustworthy than those of methyl groups. The europium shift reagent experiment is more definitive. (b) See also B. J. Hutchinson, K. K. Andersen, and A. R. Katritzky, J . Amer. Chem. SOC.,91, 3939 (1969).
g f
CI,-CH,CI,
4-
- 800
(3)
x
9, = SCI 10, = S0,Cl 11, X = S03H
x
product was clearly a single diastereomer. Arguments for the threo configurational assignment for this and related compounds are offered in section C. The presence of the sulfenyl chloride functionality was established by oxidation with m-chloroperbenzoic acid to a sulfonyl chloride 10 (ir 1373 and 1162 cm-I), which was hydrolyzed to the corresponding sulfonic acid 11 (Experimental Section). We are not aware of other examples in which a sulfenyl chloride is isolated on chlorination of a thiirane. Attempts to chlorinate trans isomer 5 led to complex product mixtures from which no identifiable products could be obtained. The chlorination of 4 can be explained by the mechanism of eq 4. As a consequence of this mechanism we
r
CI
i
anticipated that a source of positive chlorine in which the anionic portion has a greater steric bulk than chloride would open the iing less readily, perhaps allowing the isolation of intermediates. tert-Butyl hypochlorite Treatment of 4 with this seemed an ideal candidate. reagent at - 10" led to smooth reaction. Although the hope that stable intermediates might be obtained was not met in fact, the reaction did take a considerably different course from that involved in the reaction with chlorine. Product 12 was isolated in 60% yield along
c1
(IO) N. V. Schwartz, J . Org. Chem., 33,2895 (1968). (11) C . R. Johnson and J. J. Rigau, J . Amer. Chem. Soc., 91, 5398 (1969).
Kellogg, et al. 1 cis- and trans-Di-tert-thiiranes
3148
with a higher molecular weight material that we could not identify. The structural assignment for 12 rests on elemental composition and the pmr spectrum that showed two methine protons with the unique coupling characteristics seen in 9 as well as other ring-opened products (section C ) , a single vinylic proton, and four different tert-butyl groups. The tert-butyl groups on the double bond could conceivably be E rather than Z as indicated. A possible mechanism for the formation of 12 is given in eq 5 ; tert-butoxide is apparently indeed incapable of nucleophilic displacement at a neopentyl carbon and can terminate the reaction only by abstracting a methine proton. In contrast, trans-thiirane 5 was not attacked appreciably by tert-butyl hypochlorite at 0" even on standing 2 weeks. Methylation of 4 took place on treatment with methyl fluorosulfonate at about 0" in methylene chloride leading to sulfonium salt 13, which could be isolated in 62.80
C'H3
enesulfony1)sulfur diimide.I4 Neither 4 nor 5 could be opened with dry hydrogen chloride in methylene chloride although this reaction is characteristic of other thiiranes.'O Some attempts were made to prepare the unusual ring structure 15ls by elimination of water from sulfoxide 6 induced by acetic anhydride (the Pummerer reaction followed by loss of acetic acid was hoped for); 6 was continually recovered unchanged, however. Reactions of 7 were less well investigated owing to a shortage of material. An attempt to prepare 15 by dehydrogenation with diethyl azodicarboxylate was also fruitless. l6 Attempts to prepare a complex from 4 and iron pentacarbonyl or diiron nonacarbonyl in refluxing benzene'? led to extensive color changes but 4 was recovered essentially unchanged. B. Reactions of Sulfonium Salt 13. Sulfonium salts of thiiranes have long been postulated as intermediates in the solvolyses of 0-thio-substituted halides1* and in the addition of sulfenyl halides to 01efins.l~ Helmkamp and coworkers12a made the unanticipated observation that 16, although it ultimately provides 19 on
64.40 H,,
X-
S-CH3
t
X
(X = CI, Br) 61.32
-O~SCGHANOA 16
13
crystalline form. This salt is very sensitive to moisture and also decomposes even on standing a few hours at 20". It is best handled as a solution in methylene chloride with efficient exclusion of moisture. The structure is based chiefly on the nmr data (CDCL) shown and on the reactions described in the following section. The indicated stereochemistry rests on the assumption that substitution will occur from the least hindered side of the molecule. Owing to the instability of 13 an elemental analysis could not be obtained. To the best of our knowledge, this is the second sulfonium salt from a thiirane to be defined in any detail. Helmkamp and coworkers12 have previously described the S-methyl salt of cyclooctene sulfide. Ambient or higher temperatures were required to cause reaction between trans isomer 5 and methyl fluorosulfonate. No characterizable products were isolated. Methylation likely occurs analogously to 4 but the increased steric hindrance to bimolecular substitution forces the use of higher temperatures at which the derived salt is not stable. We mention briefly some attempted but failed reactions. The p-toluenesulfonylsulfinimide 14 could not
a;cHd -0 CH3SX
18
19
treatment with chloride or bromide, reacts initially at sulfur providing an unstable a-sulfurane 1720that decomposes to methanesulfenyl halide and cyclooctene. Subsequent reaction between these two reagents provides ultimately 19 (eq 6). Apparently in this case the rate of nucleophilic attack at sulfur is appreciably greater than s N 2 displacement at carbon. With these results in mind, the chemistry of 13 was investigated. In actual fact, ring opening took place. With relatively soft nucleophiles 13 afforded 20-24, all of which
20,Y=S; 21, Y = S; 22,Y=S 23,Y=S; 24, Y = S;
X=OH X = OCH,
x-c1 X=Br X = NHC(0)CH3
25, 26, 27, 28,
Y =SO,; X =OH Y = SO,; X =OH Y = SO X = OCH, Y = SO,; X =OCH3
are assigned threo structures on the basis of the arguments in the following section. Reaction with water
H& .
14 (not formed)
( 6)
.1
+
- NS02C6H,CH3 I
17
15 (not formed)
be obtained on treatment of 4 with p-toluenesulfonyl azide in methanol with a copper catalyst13 or in water with chloramine-T or by exchange of sulfoxide 6 with N-sulfinyl-p-toluenesulfonamide or N,N'-bis(p-tolu(12) (a) D. J. Pettitt and G. K. Helmkamp, J . Org. Chem., 28, 2932 (1963); 29, 2702 (1964); D. C. Owsley, G. I(. Helmkamp, and S . N. Spurlock, J . Amer. Chem. Soc., 91, 3606 (1969). (b) For less well defined examples, see L . Goodman, A. Benitez, and B. R . Baker, ibid., 80, 1680 (1958). and P. P. Budnikoff and E. A. Schilow, Ber., 55,3848 (1922). (13) H. Kwant and A. A. Kahn, J. Amer. Chem. Soc., 89, 1950 (1967).
Journal of the American Chemical Society
1 96:lO J May
(14) (a) G. Schultz and G. Kresze, Angew. Chem., 75, 1022 (1963); (b) G . Kresze, Tetrahedron Lett., 1671 (1966); (c) D. J. Cram, et al., J . Amer. Chem. Soc., 92,7369 (1970). (15) For the corresponding oxides, see L. A. Carpino, L. V. McAdams, 111, R. H. Rynbrandt, and J. W. Spienak, J. Amer. Chem. Soc., 93,476 (1971), and L. A. Carpino and H.-W. Chen, ibid., 93, 785 (1971). (16) F. Yoneda, K. Suzuki, and Y . Nitta, J . Amer. Chem. Soc., 88, 2328 (1966). _._. \ - --,
(17) R . B. King, Inorg. Chem., 2,236 (1963). (18) A. Streitweiser, Jr., "Solvolytic Displacement Reactions," McGraw-Hill, New York, N. Y.,1962, pp 108-110. (19) (a) W. A. Thaler, W. H. Mueller, and P. E. Butler, J. Amer. Chem. Soc., 90, 2069 (1968), and previous papers from the Esso group. (b) For a review see W. H. Mueller, Angew. Chem., 81, 475 (1969). (20) Nomenclature suggested by B. M. Trost and S . D. Ziman, J . Org. Chem., 38,932 (1973), ref 21.
15, 1974
3149
gave 20 ( 7 3 x yield) and with methanol 21 was obtained ( 8 6 x yield). T o establish that sulfur was bivalent, i.e., that a n-sulfurane analogous to 17 was not the product, oxidation to the respective sulfoxides and sulfones 25-28 was carried out with the required amount of m-chloroperbenzoic acid. * l The hydroxyl groups in 20, 25, and 26 exchanged readily with deuterium oxide. Sulfones 26 and 28 exchanged at the methyl groups in deuteriomethanol-methoxide but the forcing conditions required to obtain exchange at the more hindered methine positions led to some decomposition. With lithium chloride and lithium bromide in acetonitrile, 22 (80z yield) and 23 (40% yield) were obtained together with 24 (after aqueous work-up) in 5 and 59 yields, respectively. Acetonitrile reacted spontaneously with 13 giving, after aqueous work-up, a mixture of 20 and 24. The yield of 23 was raised to 69 % when 13 was allowed to react with lithium bromide suspended in ether. With lithium fluoride in acetonitrile only 24 (90z yield) was obtained. When treated with either lithium iodide or tetrabutylammonium iodide, 13 failed to give an iodide. However, free iodine was formed in these reactions and in the latter small amounts of cis- and trans-di-tert-butylethylenes were noted. Phenol, p-dimethoxybenzene (to see if 13 was capable of accomplishing electrophilic substitution), acetic acid, and n-butylthiol failed to give any identifiable products with 13. The ring opening, at least on the basis of the evidence now available, appears to involve attack of a nucleophile at a (sterically badly hindered) neopentyl carbon rather than by a mechanism analogous to that of eq 6. This conclusion is chiefly based on experiments using 13 and lithium chloride. If a sulfurane 29 were involved (eq 7), then fragmentation would produce 2 and methanesulfenyl chloride. In independent experiments methanesulfenyl chloride and 2 did not react; no 22 could be detected. Moreover, when run in the presence of a large excess of cyclohexene, the reaction of 13 and lithium chloride gave only 22 with no trace of either 2 or 30 (eq 7). Helmkamp and coworkerslZade-
x
13
+
C1-
++
H& '
+&
Hw
+ CHaCI
H
29 (not formed)
2
-CHB
H.&
I
31 (not formed)
n-butyllithium, or diisopropylamide only undefinable products were obtained. C. Structures of the Ring-Opened Products. All the products derived from ring opening of 13 and the chlorination product 9 have in common two distinguishable absorptions for the tert-butyl groups and two separate methine absorptions with the chemical shifts expected for the heteroatom substitution pattern (Table I). These absorptions were sharp or slightly broadTable 1. 6O-MHz Pmr Data" for Ring-Opened Products from 4 Compd
r-Bu
20*
0.96,1.02
2.36,3.42
21b
1.00,1.05
2.30,3.06
22b 23c
1.05,l.lO 1.00,1.13 0.93,l.OO
2.56,4.08 2.56,4.31 2.37,4.10 ( d , J = 10.5Hz)
Ub
Methine
25b
1.01,l.lO
3.04,3.88
26b
1.01,1.30
3.08,3.70
276
1 . 0 2 , l .26
2.93,3.15
28*
1.00, 1.30
3.12(d,J= 2.5Hz) 3 . 3 1 ( d , J = 2.5Hz) 3.22,4.01
9"
1.12d
Other 2.17 (br s, OH) 2.19 ( s , CHIS) 2.12 ( s , CHIS) 3.49 (S, CHIC)) 2.18 (s, CHBS) 2,00(~,CHaS) 1 .99 ( s , CH8CO) 2.20 ( s , CHIS) 6.10 (br, NH) 2.83(brs,OH) 2 . 8 5 (s, CHISO) 3.08 (s, CHIS02) 3.44 (br s, OH) 2.87 (s, CHISO) 3.48 (CHaO) 3.12(CH1S02) 3.60(CHaO)
0 Chemical shifts as 6 values from TMS ; unless otherwise noted r-Bu absorpabsorptions are singlets. * In CDC13. In CC1,. tions overlap in this case.
ened singlets (save in 24 in which the N-H was coupled to a methine proton'; however, the methine protons themselves were not coupled). The only derivative in which any coupling of the methine protons could be discerned was 28 in which a coupling constant of J = 2.5 Hz (60 MHz) was measured. Precursors 21 and 27 failed to show this coupling. These common features point in turn to common stereochemical arrangements with the choice being between threo (32a) or erythro (32b). The bulkiness 03
30
scribed a similar experiment with 16 leading to 18 and 30. We repeated this experiment to test our method and we were able to reproduce the described results fully. Efforts were undertaken to deprotonate 13 to give ylide 31. However, on treatment with methyllithium, (21) A sulfurane can presumably consume no more than 1 equiv of oxidizing agent; see, for example, E. F. Perozzi and J. C. Martin, J . Amer. Chem. SOC.,94, 5519 (1972). No evidence could be obtained for any intermediates that might have characteristics of a u-sulfurane; see, for example, J. C. Martin and R. J. Arhart, ibid., 94, 4997 (1972).
32a
32b
the tert-butyl groups makes it reasonable that the illustrated conformers are most favored. The trans-coplanar arrangement of the methine hydrogens in 32b should lead to appreciable coupling. On the other hand, the gauche-oriented methine protons of 32a would, at best, couple less strongly and in view of the presence of two electronegative substituents, X and Y , coupled with sensitivity of Jvic to small changes in angle near 60°, small or noilexistent coupling is to be exKellogg, et al. / cis- and trans-Di-tert-thiiranes
3150
pected. 2 2 Unfortunately all efforts to obtain examples of 32b by ring opening of trans-5 were unsuccessful. Independent chemical confirmation for the threo assignment was sought. A structural proof was desired in which the elements of X and Y would be eliminated in a trans manner leading to cis-di-tert-butylethylene (2). It was considered essential in any demonstration of stereochemistry by chemical reaction that the sterically least favored product be formed, since in these highly hindered molecules steric factors might change the course of an elimination from the stereochemistry normally encountered in simpler compounds. In our hands a variety of schemes to bring about trans elimination failed. What did succeed was conversion of 9 back to 4 in high yield on treatment with lithium aluminum hydride. A reasonable mechanism is that shown in eq 8. The formation of sterically less favored 4 in-
L
stead of 5 speaks for an intramolecular s N 2 displacement as shown supporting the assigned stereochemistry and also that assigned for 20-28 by spectral analogy. D. Structure of the Three-Membered Rings. Protonated Episulfides. The sulfur atom of not only 4 and 5 but of other thiiranes is subject to attack by good electrophiles. There is reason to wonder about the structure of the intermediates that are formed. Forms 33 and 34 (the sulfur atom possibly undergoing a rapid, Y
!
33
I
34
35
reversible 1,2 shift) have been most commonly considered with the weight of evidence being in favor of 33 at least as extrapolated from the results of the addition of sulfenyl halides to unsymmetrical olefins. l9 A third structural possibility that appears to have received little, if any, consideration is 35. We felt that particularly in the reactions of 4 that 35 (R, = tert-butyl) had a chance of existence. Attack of the electrophile on sulfur followed by or concomitant with outward disrotatory rotation of the tert-butyl groups would relieve the strain arising from steric interaction. Excellent precedent for this type of reaction is found in the solvolyses of cyclopropyl halides23and N-chloroaziridines. 2 4 (22) (a) J. W. Emsley, J. Feeney, and L. H. Sutcliffe, “High Resolution Nuclear Magnetic Resonance Spectroscopy,” Vol. 2, Pergamon Press, London, 1966, pp 678-681. (b) E. D. Becker, “High Resolution NMR,” Academic Press, New York, N. Y., 1969, pp 103-105. (c) cis-Di-rert-butylethylene (2) undergoes trans oxymercuration to give a threo adduct with a 1.6-Hz coupling (60 MHz) between the gaucheoriented methine protons: R . D. Bach and R . F. Richter, J . Org. Chem., 38, 3442 (1973). Rather surprisingly, trans-di-tert-ethylene gives the same oxymercuration product as obtained from 2. It is not at all clear why one isomer reacts cia trans and the other cia syn addition. (23) (a) C. H. DePuy, L. G. Schnack, J. W. Hawser, and W. Wiedemann, J . Amer. Chem. Soc., 87, 4006 (1965); (b) P. v. R. Schleyer, G . W. van Dine, U. Schollkopf, and J. Panst, ibid., 88, 2868 (1966); (c) U. Schollkopf, F. Fellenberger, M. Patsch, P. v. R. Schleyer, T. Su, and G. W. van Dine, TetrahedronLett., 3639 (1967); (d) P.v. R. Schleyer, T. M. Su, M. Saunders, and J. C. Rosenfeld, J. Amer. Chem. Soc., 91, 5174 (1969). (24) (a) P. G . Gassman and D . I
