Impact of high-lying .sigma. orbitals and extensive through-bond interaction on chemical reactivity. 2. Solvolytic studies of hypostrophene derivatives

Abstract

Acetolysis of exo-and endo-tetracyclo[5.3.0.0z~6.03~10]decan-4-yl tosylates (6b and 8b) in buffered HOAc a t 71 "C produced 16b (together with 2% of 6c in the endo case), but a t rates estimated to differ by a factor of 170 (with 6b faster). Comparable solvolysis of the unsaturated exo tosylate 3b (at 25.9 "C) led rapidly to 18b (50%), 19a (5%), and 19b (4,5%j, a product distribution quite unlike that obtained from its endo epimer 5b (18b accompanied by some polymer formation). These findings demonstrate that the frequently observed ability of a proximate double bond to engage in transannular participation has been overridden by lateral u-bond participation. The kinetic and product data suggest that the first cation emanating from 5b is classical, while that from 3b is generated with anchimeric assistance and characterized by u delocalization. Whereas acetolysis (71 "C) of em-cyclopropyl derivative 10b returned only 20a, its endo counterpart 15b gave a complex acetate product mixture a t a rate approximately 440 times slower. The involvement of neighboring-group participation could again be implicated for lob, since interaction 01' the cationic center with the cyclopropane ring is effectively repressed. The rate constants and product distributioiis provide convincing demonstration of the effectiveness with which such anchimeric assistance can accelerate and control the ultimate outcome of cationic rearrangements.Interest in hypostrophene (tetracyclo[5.3.0.0.2~6.03Jo]-deca-4,8-diene (l))l-:] derives chiefly from its structural features, including its Czu symmetry, the overriding by through-bond coupling of direct through-space Z-z interact i~n ,~ and its obvious strain energy. The primary determinants of the observed level ordering in 1 are its exceptionally high-1 -, 2 Y lying lateral u orbitals and their geometric relationship to the T bonds. Such symmetry-enforced indirect interaction between the double bonds should have significant consequences on chemical reactivity. This conclusion has received qualitative support from the response of 1 to attack by electrophilic reagents.lJ Hoffmann, Mollgre, and Heilbronner have previously presented arguments supporting the notion that the exceptional unreactivity of 7-norbornyl cation precursors (2) is due not to the change in angle strain accompanying their ionization,6 but to destabiiization brought on by the inability of proper ribbon orbitals to interact suitably with the carbonium ion center.7 This lack of interaction causes 2 to be less stable and more difficult to generate than it should otherwise be. At issue is whether similar phenomena would be observed in carbonium ions derived from 1 and allied structures.The recent availability of efficient routes to 1 and the knowledge that carbocations are particularly sensitive to prevailing orbital interactions prompted a detailed kinetic study of the solvolytic behavior of a number of hypostrophene derivatives. In particular, a series of e m -and endo-p-toluenesulfonate esters have been prepared in which the transannular...

Regiocontrolled hydrogenolysis of strained .sigma. bonds. Application to the synthesis of syn- and anti-tricyclo[4.2.1.12,5]decane and two elusive cyclopentadiene dimers

“…The corresponding activation parameters are AH* = 26.1 kcal/mol and AS* = -1.98 eu. When this skeletal isomerization was conducted through numerous half-lives, no spectroscopically detectable quantities of cyclopentadiene, endo-dicyclopentadiene, or exo-dicyclopentadiene (23) were detected. Appar-2 o 22 23 ently, the predescribed conditions are not conducive to the generation of biradical 22 which might be expected either to fragment or reclose with formation of 23.…”

“…The observed reaction course delivers 3 in irreversible fashion; although this product is less strained than its precursor, it is clearly not strain-free. The exclusion of more thermodynamically favored products, e.g., 23, is regarded as an exploitable benefit of orbital symmetry.…”