Acetolysis results for the various (3-methyl-substituted cyclopropyl tosylates are reported. The kinetic results are correlated with a treatment based on electronic and steric effects in ground and transition states, assuming stereospecific ring opening according to the Woodward-Hoffmann-DePuy predictions and considerable progress toward allyl cations in the transition states. The results of the standard mechanistic criteria [m value, (kRa ai J £ « ) , k0TjkBr, and a-methyl/hydrogen rate ratio] for the parent cyclopropyl system indicate that solvolysis is a concerted process. The magnitude of anchimeric assistance in the cyclopropyl system is estimated by two

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The acetolysis rates of a number of alkyl-and polymethyl-substituted cyclopropyl tosylates, bromides, and chlorides have been determined. The kinetic results in all cases are consistent with stereospecific disrotatory ring opening concerted with ionization. For trans-i3-alkyl substitution the observed rates show a regular but rather modest increase in the order CH3 < C2H= < i-C3Hv < tert-C4H9, attributed to inductive effects. The corresponding cis isomers show little variation; we attribute this to a combination of opposing steric and inductive effects. A comparison of -phenyl and -methyl substituent effects with those found in other secondary systems provides an estimate of the anchimeric assistance in cyclopropyl tosylate solvolysis in the range 104 6-107. We conclude that methyl rather than phenyl groups are preferable as probes for detecting nonclassical charge delocalization at positions remote from the site of attachment of the leaving group. The degree of ring opening at the transition state for a given cyclopropyl system does not appear to change significantly with a change in leaving group. The rate enhancements provided by trans-/3 substituents follows the leaving group order Cl > Br > OTs; we attribute this to electronic effects. Ground-state steric effects are a significant factor in the reactivity of cis-j3 substituted cyclopropyl systems. The order of leaving group steric size in these cyclopropyl systems is Br > Cl > OTs, which parallels the van der Waals' radii of the atoms involved (Br > Cl > O). he solvolyses of cyclopropyl derivatives tend strongly to proceed with concerted disrotatory ring opening.1,3-6 At least partial ring opening is simultaneous with departure of the leaving group in almost all cases leading to a transition state in which the positive charge is delocalized over all three-ring carbon atoms; eventually ring-opened allyl products are produced.6 The effect of a change in the leaving group on the reactivity of cyclopropyl derivatives has, however, received little attention.

Sir:In favorable cases, participation by a neighboring cyclopropane ring can rival that by a double bond.' The orientation is very important. Thus, 11, in which edge participation is possible, shows a remarkable rate enhancement over I, which enhancement is completely absent in I1I.lare from the compression in C-C-C angles at carbons 3, 5, and 7 produced on partial flattening at C-4, and from the distortion of angles at C-4 resulting from the inability of this carbon to reach full planarity.A similar situation, though less extreme, is obtained in the bi-and tricyclooctyl'3a systems. Triflate 13 is more reactive than 10, but less reactiveLab by a factor of lo2 than 16. Once again, both trends are well accounted for by calculations12 based only on strain effects (Table I).It is clear from our observations that the proximity of the face of a three-membered ring provides little or no stabilization for a developing positive charge. We infer from this that the approach of an electrophile to the edge of a cyclopropane ring must be a great deal more favorable than approach to the face.Consideration of the Walsh mode1I4 of cyclopropane offers an explanation for this edge-face reactivity difference. When interacting with the face, the empty orbital of an electrophile must mix with the three sp2 orbitals pointed toward the center of the ring. Interaction with an edge occurs between the empty orbital and cyclopropane electrons in more pure p-like orbitals. Even though the electron density in the center of the ring is probably relatively high,I4 the lower energy of sp2 orbitals (ionization potential g 14.7 eVL5) will make them much more reluctant than p orbitals (ionization potential = 11.4 e V b ) to transfer electron density to the incoming electrophile. l 6

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