Oxygen atoms phosphonate

Surprisingly carbonyl-substituted carbanions of phosphonates, in which the negative charge is delocalized over two oxygen atoms, are much more nucleophilic than the corresponding phosphoranes. This effea has first been observed by Homer, and has often been utilized in the synthesis of acylated olefins (R.D. Clark, 1975). 

Layered structures are also common in trivalent metal phosphonates [16-18], and practically the sole ones in tetravalent metal phosphonates. Most metal IV phosphonates, such as Zr(03PPh)2 [19, 20], present a structure in which the metal atom is coordinated to six different phosphonate groups through the oxygen atoms. 

Decomposition of the species with loss of the apical phenolate presumably would be more rapid than pseudorotation since any such process brings a negatively charged oxygen atom into an apical position. The stereospecific nature of the hydrolysis may also be viewed as arising from the linear steric requirements of nucleophilic displacements on phosphorus and may not involve the actual intermediacy of a pentacovalent species. A large rate enhancement (107) also has been discovered in the oxime-catalyzed hydrolysis of structurally similar phosphonate diester (57) 37. 

Fig. 14. HOs5C(CO),j(OP(OCHj)2)(P(OCHj)j), 14 (34). The Os,C core is similar to that in Fig. 13. In this case Os(4) and Os(5) occupy apical sites and Os( I), Os(2), and Os(3) are three of the equatorial sites of the incomplete pentagonal bipyramid. The trimethylphosphite ligand is coordinated to Os(3), which also bears two carbonyls and the phosphonate oxygen atom. The hydride is assigned a bridging position on Os(l)-Os(2). The two metal-metal bonds to Os(3) are significantly longer [2.954(20) A] than the remaining five [mean 2.877(9) A]. The mean Os—Cpjrtjjdj distance is 2.07(7) A.

In the example shown the deprotonation of the phosphonate can be achieved under much milder conditions than with the usually employed sodium hydride, which by the way also applies to the standard Horner-Wadsworth-Emmons reagents. Incorporation of the two doubly bound oxygen atoms of the reagent A into sodium chelate B does the trick. Being a cation, B is much more acidic than A and can thus already be deprotonated by an amidine. 

Fig. 3. The catalytic networks of secretory phospholipases A2 comparison with a serine protease. Comparison of the catalytic networks of (A) the class I Naja naja atra venom enzyme (White et al, 1990), (B) the class III bee venom phospholipase As (Scott et at, 1990b), and (C) that of a serine protease Slreptomyces griseus trypsin (Read and James, 1988). The histidine-aspartate couple is highlighted in all cases. W indicates a water molecule the sn-2 phosphonate of the transition-state analog is labeled P2. The oxygen atom, which is thought to serve as the attacking nucleophile during general base-mediated attack on the substrate, is crosshatched.

In this Rh/Ru complex, the phosphonate anions are coordinated to the rhodium through the phosphorous and to the ruthenium through the oxygen atom. The geometries around the metals are the normal pseudooctahedral three legged piano stool commonly found for six-coordinated M(C5Me5) or M(arene) derivatives. 

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