n participation and secondary deuterium isotope effects in solvolysis of 1-aryl-4-methoxy-1-butyl chlorides. Are there two distinct k/sub Δ/ pathways

A series of 1-aryl-4-methoxy-1-butyl chlorides (3) and their 1-d and 2,2-d2 analogues were prepared and the solvolysis rates measured. All compounds show rate accelerations relative to the corresponding 1-aryl-1-alkyl chlorides. The results indicate that k/sub Δ/ is the main pathway, except possibly for the p-anisyl derivative. α-deuterium isotope effects increase from k/sub H//k/sub D/ = 0.97 to 1.10 with decreasing sigma+ of the phenyl substituent and/or with increasing ionizing power of the solvent. Such results are indicative of a change in mechanism and can best be accommodated by assuming two distinct k/sub Δ/ pathways. The first, operative in cases when an α-deuterium effect of about unity is observed, is the internal direct displacement of the chloride by the methoxy group, yielding an oxonium ion intermediate (5). The other k/sub Δ/ pathway involves a rate-determining ionization into an intimate ion pair (10) which is then converted into 5 in a fast process. With the p-anisyl derivative the 10 to 5 conversion process becomes rate determining, competing with further ionization of 10 into a solvent-separated ion pair (k/sub c/ process). The observed small β-deuterium isotope effects are consistent with such a mechanism except that they appear to be about 3 to 4% too low for a rate-determining ionization to 10. This observation can be explained if it is assumed that there is an interaction between the aliphatic methoxyl and the carbenium ion center in 10, restricting the motions of the side chain and forcing C2-H(D) bonding orbitals into a geometry unfavorable to hyperconjugative electron release. The main defect of the proposed mechanism is a lack of analogy to the corresponding π participation