Tetrahedron Letters No. 39, pp.3669 - 3671, 1971. Pergamon press. printed in Great Britain.

REARRANCgKgNTOFPREBYLCY cDxROPILRAD1cALts

S. C. Chen* (Received in USA 24 April 1971. received in UK for publication 1 September 1971)

Cyclopropyl radicala have been generated from appropriate precursors in both the vapor I

and liquid phases. When cyclopropyl radicals were generated in the vapor phase, p mixture of

unrearranged and rearranged products have resulted. Rut in liquid phase reactions only un-

rearranged products were observed except in the case' of 2,2-diphenylcyclopropyl radical ($,I.

R2 R2

Rl=Ph,R2=H: I,

Rl =.H, R2 =Ph: II,

R1= R2 = H: III,

Ib

'b

IRb

IO

*c III,

Id

=d

In the isomerization of Ib to the ally1 radical Ic, it haa been suggested that the substitution

of two phenyl rings in the 2-position markedly lowers the energy barrier to isomerisation.

However, the substitution may affect the isomeriiation of Ib by releasing steric interaction

of the two phenyl rings2 by leading to a highly resonance stabilized I=, or a combination of

both. In order to clarify these possibilities aa well ae search for an additionalieomeriaa-

tion reaction of a cyclopropyl radical, 2,3-diphenylcyclopropyl radical

*Current Address: Department of Chemistry, Boston University, Boston, Massachusetts O&X5.

3669

3670 Iicl 39

(I+,) and 2-pbenylcyclopropyl radical (IIIb) were investigated.

The precursors for the desired radicals, trans-2,j-diphenylcyclopropanecarbonyl proxide

(IIa) and m-2-phenylcyclopropanecarbonyl peroxide (III,) were synthesized by the method

described by Greene and Kasen3 from trans-2,3-diphenylcyclopropanecarboxylic acid' and trans-

2-phenylcyclopropanecarboxylic acid (Aldrich). The thermal decomposition of IIa in benzene

(0.02 H) at 80" for 24 hrs afforded 3096 yield of 1,3,4,6-tetraphenyl-1,Sheradiene' (IId).

Under the same reaction conditions but in the presence of iodine II, gave no IId. This result

is in accord with the thermal decomposition' of 1,. The formation of IId obviously resulted

from the isomeriaation of IIb to IIc followed by coupling of IIc. However, in the presence of

a good radical trapping reagent such as iodine the iaomeriaation of IIb to IIc was prevented.

The thermal decomposition of IIIa under the same reaction conditions as that of IIa (benzene,

0.02 H) gave only phenylcyclopropane. !Che analogous dimeric product resulting from the pas-

sible intermediate from IIIc was not detected in the reaction mixture. Therefore the isomeri-

zation of III, to IIIc might not be a favorable pathway.

The present result leads one to conclude that the formation of highly resonance stabilized

allylic radicals is the main factor in lowering the activation energy in the isomeriaation.

Unlike the radical Ic, the radical IIc has little apparent steric interaction between the two

phenyl rings* even though both have comparable resonance energies which are lower than that of

IIIc. The fact that the formation of a highly resonance stabilized radical lowers the activa-

tion energy has been seen in the other free radical reactibns and has been well described by

Ruyser.6

Approximate calculations7 of u-electron energy (E,) show that IIIc has an K,, of

9a + 11.9688. Therefore, in order to obtain an isomerisation product in solution, a cyclo-

propyl radical must have substituents which would lead to an allylic radical possessing an E,.,

value larger than ga + 11.9688.

The author kindly expresses his gratitude to Dr. D. B. Ball for reading this M.S.

References

1. H.M. Valborsky and J.C. Chen, J. Am. Chem. .9x., z, 7573 (1970)

2. The steric interaction of Ib can possibly be rationalized by the fact that phenyl NIW absorption of Ia showed a multiplet while Id showed a singlet for the aromatic pMtOns. The MIX- spectrum of II, showed a singlet in the phenyl region.

No. 39 3671

3. F.D. Greene and J. Kazan, J. Org. Chem., 3, 2l.63 (1963).

4. The acid vas prepared by the reaction of tram-stilbene and ethyl diazoacetate In the presence of copper brouze folloved by sapmcation.

5. The spectral data, molecular weight determination and element analysis vere consistent with the proposed structure.

6. E.S. Huyser, &ee &&cdl Chain Reaotioa, p. 75 and 93, wilg titerscience, Nev York. New York (1970).

7. M.J.S. Devar, The IMacular Orbital Theon of Ornanic Chemistry, chapter 6 aad refCrenCC.S therein. IlcGrav-Hill, New York, N.I. (1969).