Effect of beta-fluoride on dehydrohalogenation reactions

Abstract

A multi-step mechanism with two carbanions accounts for results of alkoxide-promoted dehydrohalogenation reactions: ▪ Chloride leaves from a hydrogen-bonded carbanion, HB, regardless of the nature of RX: CF 2Cl, CHFCl or CH 2Cl. Fluoride leaving from CH 2F can depart from HB, while loss from CF 3 requires formation of the intermediate F, which is not stabilized by contact with either a gegen ion or solvent molecule. Beta-fluoride increases rates of dehydrochlorination: CH 2Cl (1.0), CHFCl (6−25) 1 and CF 3 (175); however, the reverse is noted for loss of HF: CH 2F (1.0), CHF 2 (0.6) and CF 3 (0.05). In all cases observed isotope effects, k H/k D at 25°C, decrease with added beta-fluorines: CH 2Cl (4.24), CHFCl (3.15) and CF 2Cl (3.02); CH 2F (2.16), CHF 2 (1.53) and CF 3 (1.08). The exchange of hydrogen from PhCHClCF 3 occurs ten times faster than loss of HF and results in a k H/k D = 1.05. This is consistent with loss of HF and the exchange reaction requiring the formation of F. Element effects applied to dehydrohalogenation reactions usually refer to bromide vs. chloride. Results for our systems are about half of those reported for PhCH 2CH 2X, k HBr/k HCl ≈ 79: PhCHClCH 2X (35) and PhCHBrCF 2X (39); however, the chloride/fluoride element effect varies widely with PhCHClCH 2F, k HCl/k HF ≈ 68, the same as that reported for PhCH 2CH 2F, but increasing to 1 × 10 3 for PhCHClCHFX 1 and 1.1 × 10 5 for PhCHClCF 3. The measurement of element effects should be ideal for a competition within the same molecule, ▪. Loss of fluoride can not compete with chloride or bromide in the systems sited above. Studies using PhC iHBrCFClBr result in large differences in k HBr/k HCl that depend on solvent, temperature or iH for one diastereomer, but not for the other.