Five-coordinate phosphorus

Note that the first step in Figure 21.6—reaction of the carboxylate with ATP to give an acyl adenylate—is itself a nucleophilic acyl substitution on phosphorus. The carboxylate first adds to a P=0 bond, giving a five-coordinate phosphorus intermediate that expels diphosphate ion as leaving group. 

Another system, which is also allowed by IUPAC and is sometimes very convenient (but which should be avoided when official names are established by IUPAC and cited in Chemical Abstracts), is the so-called a -nomenclature, in which the term phospha indicates replacement of carbon by phosphorus, e.g. phosphacyclohexane = phosphorinane, and phosphabenzene = A3- phosphorin. Five-coordinate phosphorus compounds are called phos-phoranes. However, this name is also used for phosphorus ylides with a four-coordinate phosphorus and a more or less polar R3f—CX moiety or a d -p R3P=CX double bond, e.g. methylenetriphenylphosphorane = triphenylphosphorus methylene ylide or tri-phenylphosphonium methyl ylide, Ph3P=CH2 = Ph3P—CH2. 

The reason these reactions occur can be found in the relative stabilities of the anions involved. This approach could be of general use for the synthesis of inherently chiral calix[4]arenes. The stabilization of the monoanion of a mono-0-alkyl/-acyl calix[4]arene by two intramolecular hydrogen bonds explains the usually easy access to 1,3-derivatives. However, upon further deprotonation the monoanion of a 1,2-O-alkyl (1,2-O-acyl) derivative is stabilized by an intramolecular hydrogen bond (unlike the analogous 1,3-derivative) and rearrangement occurs if there is a reaction pathway available. For the phosphorotropic rearrangement the authors assume a cyclic intermediate with five-coordinated phosphorus, which is not unreasonable although an intermolecular mechanism is not strictly ruled out. 

Cyclophosph(V)azanes with five-coordinate phosphorus are mostly dimeric, four-membered ring derivatives, [X3PNR]2 (6). Several reviews cover the subject. A simple preparation of these compounds involves the reaction of PCI5 with primary amines (or their hydrochlorides), the so-called Kirsanov reaction. Weakly basic amines form monomeric Cl3P=NR, but with more basic amines (e.g. with R = alkyl), the products are the cyclic dimers. Various anilines give either a monomer or a cychc dimer, depending upon their basicity. ... 

The new phosphatrane (118) protonates to form the extremely weak acid (119, pKg ca. 16) with a pentaco-ordinate structure. Oxidation of (118) followed by treatment vdth BF3 again leads to a phosphatrane (121) featuring five-coordinate phosphorus. Apparently the BF3 moiety is capable of polarizing the phosphoryl oxygen of (120) sufficiently to stabilize the chelated pentaco-ordinated structure. Reactions of the cyclophosphoramide Pt(II) complex (122) were also... 

An exhaustive discussion of restrictive situations is beyond the scope of our treatment. Position preference has been examined in pseudorotations of stable five-coordinate phosphorus compounds 17b 23 24). The generalities derived from these studies may possibly be applied to formation of trigonal bipyramids as well. 

The nature of pseudorotation restrictions of stable five-coordinate phosphorus compounds has been studied , 23,24) pSeudorotations which move groups into restricted positions do not occur. Pseudorotations involving axial-equatorial rings occur by interchange of the axial for the equatorial group. However, only those pseudorotations about the leaving group as pivot alter the stereochemical outcome of substitution. 

S. VERKADE Five-Coordinate and Quasi-Five-Coordinate Phosphorus... 

The same trend can be seen in the series of five-coordinate phosphorus compounds depicted in Table 6.5. Decrease of Me>... 

Meanwhile, on more fully documented ground, further discussion has appeared on the conformational equilibria of phosphoranes with S-alkyl-sub-stituted 1,3,2-dioxaphosphorinane rings attached diequatorially to five-coordinated phosphorus. A series of five tetraoxyphosphoranes (85 - 89) were prepared and their conformational properties studied by NMR. It was concluded that although the equilibrium between (A) and (B) is readily perturbed by substitution of Bu for R or R instead of hydrogen or by... 

There is a large body of evidence that describes the behaviour of compounds containing pentacoordinate phosphorus, and these types of species are certainly involved as intermediates in many reactions. Holmes (1980b) has presented a very complete picture of the nature of such species. Thus the corner of the three-dimensional energy diagram contains the five-coordinate phosphorus as a platonic ideal whose existence has been demonstrated as a reality in many instances. Concerted reactions may avoid this corner, but it is certain that stepwise substitution processes involving these species as intermediates do exist (Kluger and Thatcher, 1986). 

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