2-Phospha- and 2,3-Oxaphosphabicyclo[2.2.2]octenes - Synthesis and Fragmentation to Low-coordinated Species

Abstract

The 2-phosphabicyclo[2.2.2]octene and 2,3-oxaphosphabicyclo[2.2.2]octene systems undergo fragmentation in inert solvents on heating, or by UV-irradiation at room or low temperature. The fragmentation is of significance because it leads to the extrusion of low-coordinated species Y-P(X)O (Y = RO, R 2 N, Aryl; X = O, S, CH 2 ), which are very effective phosphorylating agents.This review covers different aspects of the 2-phosphabicyclo[2.2.2]octene and 2,3-oxaphosphabicyclo[2.2.2]octene derivatives. The first part is focused on their synthesis and structure. The reactivity of 2,3-oxaphosphabicyclooctenes involving rearrangement and fragmentation with release of the bridging P-O unit including mechanistic studies is described in the next part. The last part of the review is dedicated to the practical aspects of the utilization these compounds as phosphorylating agents.

Carbodiphosphorane-Stabilized Parent Dioxophosphorane: A Valuable Synthetic HO2P Source

“…The most probable mechanism for delivering a RO 2 P unit (e.g., compound 4 , Figure ) for the phosphorylation/phosphonylation of organic substrates was described to occur via the elimination–addition (EA) or the addition–elimination (AE) pathway (Scheme ). ,,, In fact, the concurrent existence of both pathways has also been observed . According to the published experimental data, the nature of the R group played an important role as the presence of amino and alkoxy (i.e., electron-rich and good π-donor) substituents favored the EA pathway, while the existence of both mechanistic pathways was described for the precursors that contained an aryl group.…”

“…21,22,94,95 In fact, the concurrent existence of both pathways has also been observed. 3 According to the published experimental data, the nature of the R group played an important role as the presence of amino and alkoxy (i.e., electron-rich and good π-donor) substituents favored the EA pathway, while the existence of both mechanistic pathways was described for the precursors that contained an aryl group.…”

“…Compounds with the general molecular formula RO 2 P (R = H, alkyl, aryl, alkoxy, amino, halide, etc.) have generated considerable scientific interest due to their potential as phosphorylation/phosphonylation reagents − as well as their participation as intermediates/final products of combustion reactions involving warfare agents and flame retardants. , Dioxyphosphorane ( 1 , Figure ) and cis / trans -alkoxyphosphanone ( 2 , 3 ) are the structural isomers that these molecules are capable of adopting. The former, formally a P­(V) species, is thermodynamically more favorable for all species regardless of the nature of the R substituent − ,− with the exception of the parent compound (i.e., R = H). − In fact, parent dioxophosphorane 1a has been predicted to be more than 40 kJ/mol less stable than the cis-isomer of hydroxyphosphanone ( 2a ), − and it was then not surprising that this least thermodynamically stable isomer (i.e., HPO 2 ) was only recently detected via infrared spectroscopy among several products formed during 193/266 nm laser photolysis of 2a in the Ar matrix at 2.8 K …”

“…In fact, owing to the exceptional electrophilic characteristic of the three-coordinate phosphorus atom, the identity of dioxophosphoranes was normally established by either low-temperature matrix isolation procedures, − , high-temperature gas-phase analysis, intramolecular C–H/C–F bond activations, , or trapping experiments, that is, reactivity studies in the presence of, for example, alcohols, amines, or epoxides . The reactivity of transient dioxophosphoranes with different molecules actually represents a very useful approach for the phosphorylation/phosphonylation of organic molecules potentially yielding compounds, including the biologically important ones, that would be difficult to obtain by other synthetic methods. − For example, fragmentation of compounds that resemble 4 (Figure ) represents arguably the most versatile/valuable approach in generating synthetically usable RO 2 P fragments. ,, On the other hand, base-stabilized compounds such as 9 and 10 ( 8 is in fact produced by the photolysis of 4 at −75 °C in THF), including transition metal-stabilized RPO 2 complexes, are generally not reported to act as a source of the RO 2 P fragment that could be used for further reactivity studies. ,,− A similar conclusion could be reached for neutral but zwitterionic ( 11 , Figure ) − and anionic ( 12 ) − compounds (i.e., the O-deprotonated form of the monobasic tautomer of phosphonous acid) because they could be viewed as parent dioxophosphorane-containing species.…”

“…51−54 For example, fragmentation of compounds that resemble 4 (Figure 1) represents arguably the most versatile/valuable approach in generating synthetically usable RO 2 P fragments. 1,3,22 On the other hand, basestabilized compounds such as 9 and 10 (8 is in fact produced by the photolysis of 4 at −75 °C in THF 13 ), including transition metal-stabilized RPO 2 complexes, are generally not reported to act as a source of the RO 2 P fragment that could be used for further reactivity studies. conclusion could be reached for neutral but zwitterionic (11, Figure 1) 55−59 and anionic (12) 60−63 compounds (i.e., the Odeprotonated form of the monobasic tautomer of phosphonous acid) because they could be viewed as parent dioxophosphorane-containing species.…”