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Hydroxide Ion Initiated PTC Reactions. I.
Isomerization of Allylbenzene.

M. Halpern, M. Yonowich-Weiss, Y. Sasson and M. Rabinovitz
Tetrahedron Letters 22, 703-704 (1981)

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Abstract

Allylbenzene (pKa ~34) was isomerized to trans- and cis- β-methyl-styrene under phase transfer catalysis conditions. Several half lives of this reaction were measured under various conditions.

Phase transfer catalysis has been established as a widespread synthetic technique1. Hydroxide ion initiated reactions (alkylations, carbene additions, deuterium exchange, etc.) performed under PTC conditions provide unique advantages in the laboratory and in industry2,3. It is therefore surprising that no systematic study of the various factors involved in such reactions has been published in light of the known anomalous behavior of PTC systems involving the hydroxide ion, e.g. the success of TEBA (triethylbenzylammonium chloride) in such reactions4 vs. the failure of TEBA in SN2 reactions involving softer anions5. In addition, prior to this report, no anionic reaction induced by the hydroxide ion under PTC conditions of a substrate of pKa greater than 23 (fluorene) has been reported6.

We wish to report the successful isomerization of allylbenzene (I) (pKa ~34)7 to trans-(II) and cis-(III) β-methyl-styrene by the hydroxide ion under liquid-liquid PTC conditions, and we wish to present some kinetic data of this reaction.

Conditions and Results of the
Isomerization of Allylbenzene


Quat
Cation
[NaOH]
t½a
Adogen 464b
Cl-
50%
12 min
Adogen 464
Cl-
40%
53 min
THAc
Br-
50%
66 min
TPAd
Br-
50%
82 min

(a) reactions were carried out to
     at least 98% conversion;
(b) tricaprylmethylammonium;
(c) tetrahexylammonium;
(d) tetrapentylammonium.

Allylbenzene (500 µl) was mixed with a solution of 5 mol% PTC in p-xylene (2.00 ml). Aqueous NaOH was introduced into the system preheated to 70C and magnetically stirred in a multireaction Pierce magnetic stirring/heating unit. The conversion of allylbenzene to trans- and cis-β-methyl-styrene was followed by GC analysis. The results are summarized in the Table.

The marked increase of the half life of the reaction by a factor of 4.5 when decreasing the hydroxide ion concentration by 20% indicates that no simple extraction constant governs the behavior of the system. This enhanced reactivity may be due to a combination of two factors:
(1) a large salting out effect of the quat into the organic phase at high aqueous phase ionic strength,
(2) decreased hydration of the attacking hydroxide ion due to the reduced availability of water molecules to accompany the hydroxide ion into the organic phase.

At constant hydroxide ion concentration in systems containing the same counterion, homologous quats (THA and TPA) probably demonstrate behavior consistent with their relative extraction constants.

References

  1. "Compendium of Phase-Transfer Reactions and Related Synthetic Methods", W. Keller, ed. Fluka AG, Q1-9470 Buchs, Switzerland, 1979.
  2. W. Weker, G. Gokel, "Phase Transfer Catalysis in Organic Synthesis", Springer-Verlag, Berlin, 1977.
  3. L. Lindblom, M. Elander, Pharmaceutical Technology, 59 (1980)
  4. M. Makosza, Pure Appl. Chem., 43, 439 (1975)
  5. A. Herriott, D. Picker, J. Amer. Chem. Soc. 97, 2345 (1975)
  6. M. Makosza, Bull. Acad. Polon. Sci., 15, 1656 (1967). DMSO was required for the success of this reaction.
  7. K. Bowden, R. Cook, J. Chem. Soc. Perkin Trans. 2, 1407 (1972)