Reactivity tricoordinated

The reactivity of cyclic aminomethylphosphines is of special interest due to the presence of two or more heteroatoms capable of quartemization in a 1,3-position. Besides the ordinary oxidation of tricoordinated phosphorus, mutual transformations and rearrangements are typical. 

Black phosphorus may be either amorphous or crystalline. It exhibits quite low reactivity, both with oxygen and other reagents, compared to white or red phosphorus, and is stable in air. Because it has a puckered shape, the phosphorus remains tricoordinated in black phosphorus and crystallizes in sheets in a way similar to the behavior of graphite. Black phosphorus is generated from white phosphorus either by heating under pressure or in the presence of mercury. It is of relatively little value for the synthesis of organophosphorus compounds. 

Tricoordinate phosphorus >P-X compounds containing a suitable leaving group are indispensable in the synthesis of biophosphates and their structural analogues. In contrast to phosphoryl >P(0)-X and thiophosphoryl >P(S)-X, compounds they are spectacularly more reactive in nucleophilic displacements at the phosphorus centre. Westheimer has compared reactions of P compounds with nucleophiles to enzymatic reactions regarding their reactivity [1]. The essential feature of P compounds is their free electron pair with all the structural, stereochemical and mechanistic consequences that follow. P compounds have the structure of a trigonal pyra-... 

The most direct synthesis is performed by reaction of a dihalo compound with a tricoordinated phosphorus compound including a reactive P—Y bond (reaction 101). This synthesis, first used with secondary phosphines, the corresponding phosphides or diphosphines (Y = H, Na, K, PPh2)2b,38°, has subsequently been developed also with silylphosphines (Y = SiMe3),132,381-383 which are easier to handle (reaction 102). The same kind of cyclization by intramolecular alkylation can be achieved using a functional phosphine prepared in a multi-step synthesis380,384,385 (reaction 103). 

The following example shows that the results of some reactions may be unexpected by allowing P(NMe2)3 to react with p-tert-butylcalix[4]arene, Khasnis et al214 obtained the hexacoordinated compound 236 instead of the expected phosphorane with a PH bond, 238 (Scheme 61). This result is, however, consistent with the particular reactivity of the pyrocatechols in basic media mentioned above (Schemes 54 and 55). Conversely, submitting 238 to heat or to the action of CF3COOH yields nearly quantitatively the P(III) derivative 237 rather than 238. The equilibrium P(III) (237) P(V) (238), if it exists, must be totally shifted to the tricoordinated form. The action of LiBu on 236 does not affect the PH bond but rather results in the deprotonation of the NH bond and the formation of a lithium salt whose structure, as determined by XRD275, places the P atom nearly in the plane of the four O atoms, the H atom of the PH bond within the aromatic cone and the N atom at the apex of the structure. 

To get a broader concept of the nucleophilic reactivity of phosphorus acid derivatives towards organosilanes we studied mechanism of reactions of triorganosilyl halides with esters of tricoordinate and tetracoordinate phosphorus having the general formulae ... 

It seems likely that the magnitude of p represents the "extent of electron demand at phosphorus , in the T. S. or in other words is a measure of "electron transfer" in the T.S., a term (=z) which appears in the semi-empirical equations describing the nucleophilic reactivity of tricoordinated phosphorus. This concept is reinforced by (i) the observation (from the data of Bokonov. and Goetz ) that p-values based on the pKg values of protonated phosphines increase with increasing pKa, i.e. increase with a shift of the equilibrium (eqn. 4) to the left and... 

There is a very extensive heterocyclic chemistry of tricoordinate arsenic wherein arsenic is incorporated in four- to ten-membered ring systems. Although the most prevalent synthetic route involves the reaction of a dihaloarsine with a di-Grignard or dilithium reagent, there are many custom syntheses to specific heterocycles. The synthesis and reactivity of the heterocyclic arsenic chemistry will not be reviewed here. Instead, the reader is referred to the comprehensive treatise by Mann and the primary literature that is cited therein. ... 

There have been several reviews that provide a comprehensive coverage of the synthesis and reactivity of organoarsenic compounds that contain tricoordinate arsenic with an As-N The acyclic aminoarsines,... 

The 14-electron complexes are highly reactive and decompose even when stored in an inert atmosphere. They readily bind an additional small ligand. In particular, they react with dinitrogen under mild conditions. However, the dinitrogen ligand is easily lost and this property has been exploited in the preparation of other tricoordinate complexes (equations 3 5). 

Sulfonium salts are compounds containing a tricoordinate, positively charged sulfur atom. Sulfonium salts (1) are prepared by the reaction of alkyl halides on dialkyl sulfides, the latter functioning as nucleophiles (l).1 The reaction is facilitated by the use of polar solvents like methanol. Methyl iodide is the most reactive alkyl halide, and in the alkyl iodides the reactivity decreases with increasing chain length. 

Another important aspect of these structures is related to the geometries of the dimers or terminal units of oligomers of stannylene acetals. ° The tin atoms adopt a distorted trigonal bipyramidal geometry with the alkyl groups in equatorial orientations. The tricoordinate oxygens are equatorial to one tin atom, but apical to the other. The dicoordinate oxygen atoms are apical (see Scheme 5.1.7) and this may contribute to their reactivity. ... 

In any dimer, it seems likely that the dicoordinate oxygen atoms are much more reactive than the tricoordinate oxygen atoms for three reasons. They are sterically less hindered because they are flanked by one tin atom rather than two, each tin atom bearing two butyl or other alkyl groups that are orthogonal to the Sn—O plane. They are probably more nucleophilic because they are not involved in an additional dative bond to a second tin atom. The dicoordinate tin atoms are apical in the trigonal bipyramidal geometry, which also conveys reactivity. ... 

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