Diastereoselective formation of 18-membered ring BINOL-hydrogen phosphonate dimers - Quasi-covalent hydrogen bonds?

Diastereoselective syntheses of the unusual dimers, 4-heptyl-2-(2’-hydroxy-binaphthyl)hydrogen phosphonate (5) and the cyclohexyl analogue (7), are achieved by hydrolysis of 4-(3,5-dioxa-4-phosphacyclohepta[2,1-alpha;3,4-alpha’]-dinaphthalene-4-y loxy)heptane (4) and the cyclohexane analogue (6), respectively. Two out of eight possible pairs of monomers units are involved in the stereoselective formation of the dimer 5a of configuration BINOLR-P-S:BINOLR-P-S; this is determined by X-ray crystallographic data, which reveal a centrosymmetric, 18-membered ring structure with C-i symmetry, consisting of two monomers strongly hydrogen-bonded between the oxygen of P=O units and hydroxyl hydrogen atoms. Mass spectrometric, melting point, and thermal decomposition point data, as well as NMR data, support the presence of strong, quasi-covalent hydrogen bonds. Computational analysis suggests that the diastereoselectivity is controlled by molecularly constrained geometry of the monomer. Compound 7, although not characterized crystallo-graphically, appears to be analogous to 5.