Phosphate basicity and nucleophilicity loss upon constraint: The role of the alkoxy oxygens

✍ Scribed by J.G. Verkade

• Publisher: Elsevier Science
• Year: 1974
• Weight: 719 KB
• Volume: 3
• Category: Article
• ISSN: 0006-3061
• DOI: 10.1016/s0006-3061(00)80040-x

✦ Synopsis

Structural evidence is cited which suggests that the POC angle is more flexible than the OPO angle when phosphate ester alkoxy groups are constrained into rings. In hybridization terms, the tendency for the POC angie to decrease from ca. 120" in acyclic systems to probably less than li5" in O=P(O)(OCH&CH is consistent with the notion that an sp" alkoxy oxygen is less capable of pi bonding to phosphorus and consequently the negative charge on the phosphorus and hence also on the phosphoryl oxygen is dccresed. The &Yerence in orbital co&gurations in acyclic phosphates and in monocyclic and bicyclic analogues has an effect which is seen to re-enforce the hybridization argument. These simple considerations are shown to account for the bulk of the evidence demonstrating that phosphate ester nucleophilicity and bssicity decreases on constraiut of these systems. The "hinge effect" described above for oxygen in phosphates also appears to play a role in varying the basicities of phosphites and also possibly in the acidity changes in certain borate derivates on constraint. Nitrogen apparently also displays a %inge effect" in several amino phospbiue systems.