Reactivity in acid-catalyzed carbon-carbon heterolysis

Equilibrium and rate constants have been determined for the acid-catalyzed heterolysis of two alcohols, 9-xanthydrol and p-anisyldiphenylmethanol, and two sulfides, (9-xanthyl) methyl sulfide and (7-tropyl) methyl sulfide. Th ese data together with literature information are compared with rate consta nts for acid-catalyzed C-C heterolysis of several (9-xanthyl) compounds, (7 -tropyl) compounds, a set of 3-arylcyclobutanones, and two 2-arylnitrocyclo propanes, all of which fragment to carbocations plus a carbon-centered nucl eofuge. The fragmentation mechanisms are shown to be Al or Al(ion pair) exc ept for the 2-arylnitrocyclopropanes which cleave in trifluoroacetic acid b y a concerted mechanism. Rate comparisons among several unstrained substrat e sets indicate that O-centered nucleofuges undergo acid-catalyzed heteroly sis ca. 10(3)-10(4) faster than S-centered nucleofuges and ca. 10(9)-10(14) faster than the C-centered nucleofuges used here. Factors assisting C-C he terolysis (and their effectiveness) include the acidity of the medium (stro ng); the basicity and nucleofugality of the nucleofuge (moderate); the stab ility of the electrofugic carbocation (strong); and relief of ring strain ( enormous). Compared with acyclic cleavages, rate accelerations worth ca. 15 kcal/mol (for cyclobutanones) and ca. 27 kcal/mol (for nitrocyclopropanes) are found. These effects are discussed in terms of transition-state struct ure, aided by computational evidence.