Concerning Orientations of Certain symElimination Reactions Tse-Lok Ho The NutraSweet Co., 601, Kensington Rd., Mt Prospect, IL 60056 The 8-elimination reactions (I) constitute an important class of oreanic transformations. Thus. the Hofmann elimination, whkh led to the formulation of the ~ o f m a n rule n (2). olaved a orominent role in the structural elucidation of alkaioi& uniil the 1960's when physical methods of analysis s u.~. ~ h n t deeradation ed studies. One particular subclass of 8-elimination involves the pyrolvtic extrusion of a neutral molecule (3).While thermal deiomposition of esters including xanthates (Chugaev reaction) has the longest history, pyrolysis of amine oxides (Cope elimination) (4),sulfoxides (5),selenoxides (6), and telluroxides (7)has gained currency as a valuable synthetic method because of the very mild conditions: reaction temperatures of -150 "C, 100 O C , 25 "C, and 25 OC for the four types of oxides..resoectivelv. . -.are sufficient to effect the eeneration of alkenes. The avrolvtic elimination ~10ceedsvia a nrelic transition state, r&ult&g in a syn-elimination. The interesting aspect of orientation ~ertainsto unsymmetrically substituted oxides, as shown in the followingequations: -
0 ' "0' SOnAr
SOtAr
Of*
ref (11) ( 5 )
SePh
1
-
ref (12) (6)
0-(yNPbSeCN
101
+,SePh
f
ref (13) (7)
~hke ref (6)
ref (9)
ref (10)
ePh
It is generally observed that a 8-hydroxyl, 8-amino, or 8amido group directs the reaction course toward an allylic product. This is contrasting to the formation of vinyl sulfones, sulfoxides, nitriles, nitro, and carbonyl compounds from the corresponding 8-phenylseleno derivatives. No rationale for the chemoselectivity is found in the literature. In our opinion the phenomena can he readily explained on the basis of the polarity alternation rule (16). The eliminations involve proton abstraction from the 8 carbon by the negatively charged oxygen donor, and among the two cis hydrogen atoms the one geminal to the hydroxyl, amino, or amido group has a less tendency to depart as a proton because the hydroxyl oxygen and the aminolamido nitrogen Volume 66 Number 9 September 1989
785
confer a donor character to it by means of an alternating polarity mechanism. On the other hand, the sulfonyl, the suKnyl, the nitro, the cyano, and the carbonyl function, heing acceptom, impart an acceptor character to their respective geminal hydrogen.
Y = OH, NHCOR, NRz
W = SOzAr, CN, NO,, CO
This internretation is consistent with the exnerimental results. It is'interesting to note that conventional wisdom would have oredieted the oonosite direction of elimination in the cases of the hydrod3ated, aminated, or amidated oxides: the electronegative hydroxyl group or amino/amido function pulling electrons away and rendering the methine hydrogen more mobile. Previous work (17) has shown that hydrogen bonding plays an important role in determining the facility of the Cope elimination. The oxides for reactions 1, 2, and 3 can form intramolecular hydrogen bonds, hut the presence of water in the reaction media allows the adootion of other conformations. In fact, the hydrogen-bonded conformations would have favored the elimination in the manner o~oosite to that observed, the donor oxygen atom heing sp&lly locked to the proximity of the hydrogen atom geminal to the functional group.
788
Journal of Chemical Education
In conclusion, we have shown by the examples of B-eliiination the value of the polarity alternation rule in assessing many aspects of organic reactivity. It is speculated that allylic fluorides and vinyl silanes may be synthesizedaccording to the general scheme of addition-oxidation+limination. For other applications of this rule and more hackground material the reader is advised to consult ref 16. Generally, a reaction center is greatly influenced by a nearby polar substituent, which may belong to either the donor or the acceptor family. A donor is a group containing n or r electron pairs (e.g., RO, RzN, RS, halide, C=C, aromatic ring), and an acceptor is agroup that contains empty orhitals or displaceable r electrons (e.g., RCO, COOR, CN, R2S02, NO2, R3Si). 1. BanUl-D. V. Elim'~lianRooeliowElsevilsevilseviNsaYork. 1963. 2. Hdmaaq A. W.LiebigsAnn. Chem L811.78.255:79.11. 3. DePuy, C. H.: King, R W . Chem.Reu. 1960.60,MI. 4. Cope,A.C.;Paeter,T.T.:Towle,P. H. J.Am.Ckm.Sae.1949,71,3929. 5. Kinmbury,C. A.,Cnun,D.S.J.Am.Chem.Sac.1960,82,18l%Ernersoo,D. W.:Craig, R.P.;Pot*, I. W., Jr. J. Org. Chem. 196'7 32 102. 19'73,95,2691; Rsicb, H.J.;Rsieh,I. 6. Shamlasa,K. B.; Lauer.R. F. J. Am. chem.'~&. L.; Ren%a.J. M. J. Am. Chem. Soe. 1973.95,5813:CBue. D. L. J. Chem. Cammun. 13'73.695. 7. Uemura, S.; O h , K.; Pu*uasna.S. T~rmhodmnkft. 198$26,895. 8. Bpieux, J. J.; GO*, J. EW. sac. him.^. ~ 7 1 . 1 ~ ~ . 9. Toshimitau.A.;Aoai,T.;Oaada, H.: Uemura, S.; 0Lano.M. J. Olg C k m . 1981.46, 4727. 10. Rsieh. H.J.;RsngoJ. MJ. Og. Chem. 1975,40,3313. 11. Back,T. G.;Collim,S.J. Org. Chem. 1981,46,3249;Gaouu.,RA.:Kiee, J. L. J. Org. Chem. 1%1.46,1899. 12. Nimlaou, K. C. Tefmhedmn 1981,27,4D37. 1982,871. 13. Tornoda,S.;Talreuehi,Y.: N0murs.Y. C h o n kt(. 11. Hiyama.T.;Tomoda,S.;T&euehi.~.: ~ o m u r aY,. ~ e t m h e d m n k t r1 . 982,w,4733. 15. Sharp1eaa.K. 8.; Y0ung.M. W.: Lauer,R. P. Tetrahedron Lett. 1973,I979 16. Ho, T.-L. Re". Chem.Interm. 1988,9.111. 17. Cram. D. S.:S s h p , M. R.V.: Knoi, G. R J. Am. Chem. Soe. LS62,84,1154.
