BIOORGANOMETALLIC CHEMISTRY .3. ROLE OF THE PHOSPHATE GROUP DURING REACTIONS OF ADENOSINE-MONOPHOSPHATE DERIVATIVES WITH AN (ETA-5-PENTAMETHYLCYCLOPENTADIENYL)RHODIUM AQUA COMPLEX IN THE DIASTEREOSELECTIVE FORMATION OF CYCLIC TRIMERS, [CP-ASTERISK-RH(AMP)]3

The reactions of adenosine 5'-monophosphate (5'-AMP) in water (pH 5-8) with a (eta5-pentamethylcyclopentadienyl)rhodium aqua complex, [Cp*Rh(H2O)2(OTf)2]x (1), showed, by H-1 and P-31 NMR spectroscopy, that the 5'-P(O)2=O group significantly inhibited cyclic trimer formation, [Cp*Rh(mu-eta1(N1):eta2(N6,N7)-5'-AMP)]3, via competition with Nl and NH6 for the Cp*Rh site and, as well, provided other mononuclear and dinuclear Cp*Rh-O-P complexes. In contrast, both the phosphate methyl ester of 5'-AMP and 3'-AMP exclusively formed the cyclic trimer structures [Cp*Rh(mu-eta1(N1):eta2(N6,N7)-methyl-5'-/3'-AMP)]3. The consequence of steric effects, as demonstrated-by the position and substitution of the phosphate group attached to the ribose, on the diastereoselectivity of cyclic trimer formation, as observed by H-1 and P-31 NMR spectroscopy and circular dichroism analysis over time, shows that the phosphate methyl ester of 5'-AMP provides a greater diastereoselectivity (6:1) after 1 week of equilibration compared to 5'-AMP (1.5:1) and 3'-AMP (1.2:1).