Equilibria and Reaction Rates of Phosphorus Substitution Reactions in 2-R-1,3,2-Disubstituted Phosphorinanes

Equilibrium constants and reaction times were observed for six equilibria involving methanol, ethanol, dimethylamine, diethylamine and pyrrolidine with 2-R-1,3,2-dioxaphosphorinanes wherein R = MeO (1), EtO (2), Me2N (3), Et2N (4) and (CH2)4N (5), or 2-R-1,3-dimethyl-1,3,2-diazaphosphorinanes wherein R equalled the same five groups (6–10, respectively). Reaction times of amine substitutions were shorter for diaza compounds and for those reactions which contained pyrrolidine. Equilibrium constants were dependent not only upon the nucleophilicities of the exchanging groups, but also upon the steric interactions of the 2-substituents with the groups in the 1 and 3 positions. Thus, the values of K's for the equilibria 5 + Me2NH → 3 + (CH2)4NH, and 10 + Me2NH → 8 + (CH2)4NH were experimentally identical, whereas K for 9 + Me2NH → 8 + Et2NH was ca. eight times that for 4 + Me2NH → 3 + Et2NH. The reactions were acid catalyzed, and, for amine substitutions with 8–10, exchange could be rapid on the NMR time scale. Equilibrium positions of amine-containing reactions were [H+] dependent, where coordination of the less basic amine to phosphorus became increasingly favored by the addition of acid. These observations raise questions about a recent mechanistic proposal for catalyzed substitution reactions. Similar substitution reactions, such as Me2NH with the isomeric 4-methyl analogues of 3, provide a procedure for the determination of cis-trans isomer equilibrium distributions.