Hypervalent phosphorus compounds

Investigations of hypervalent phosphorus compounds have tended to concentrate on the more stable bicyclic systems. Elegant studies of spirophosphoranes and a thorough investigation of permutational isomerisations within amidinium fluorphosphates are worth noting. 

Phosphorus also bears a significant (positive) partial charge in neutral compounds. By expanding formally its valence shell, hypervalent bonding structures are formed, which can conveniently be described by the 3c-4e model [34, 35], (Note that there is no need to consider the involvement of rf-orbitals in the bonding for the hypervalent phosphorus compounds [36, 37].)... 

Bonding natnre in hypervalent phosphorus compounds has been of widespread interest in the last years, especially in phosphine and amine oxides [1-11]. In addition, the P-O bond has been of particular interest and it has been discussed in a general review [12]. However, there are still several points of controversy in the P-O bond description. Experimental results [12,13] show that the phosphoryl bond is strong, short and polar. For years, the theoretical description of the P-O bonding was connected to the involvement of virtual d orbitals in the phosphorus atom. Nevertheless, recently evidence against this involvement appeared [2,14-26], as reviewed by Gilheany [13], leading to the exclusion of dorbital participation. 

During last year the majority of researches in this area has been focused on the synthesis and structural determination of novel hypervalent phosphorus compounds as well as on the stereochemistry of pentacoordinated chiral spirophosphoranes. In these studies, Mironov et al. obtained tricyclic pentacoordinated spirophosphoranes containing a phosphorus-carbon bond with high regio- and stereo-selectivity whereas Kawashima presented the synthetic route to perfectly anti-apicophilic carba-phosphatranes. ... 

It is well known that the coordination ability of phosphorus to form hypervalent compound, mainly penta- and hexacoordinated, is the driving force in describing the mechanistic action of phosphoryl transfer enzymes. On the other hand, organophosphorus compounds play also a fundamental role in inorganic, organic and applied chemistry as a key species, reaction intermediates or final products. Therefore, the utility of hypervalent phosphorus compounds in many chemical processess is indisputable and in some cases facilitate the outcome of the reaction to be defined. Recently, some achievements on the role of hypervalent phosphoranes in various chemical processess have been described. 

Numerous stmctural types of hypervalent phosphorus compounds are known (1999MI1). Representative examples of stable hypervalent phos-phoms heterocycles include lO-P-3 compounds 29 (1994CR1215), anionic lO-P-4 species 30 (1983CB3301), pentacovalent lO-P-5 compounds 31... 

In a more recent work, Kornev and coworkers reported the preparation and structural studies of a series of phenylpyrazole-based hypervalent phosphorus compounds 34 and 35 (Figure 8), which represent another example of low-coordinate hypervalent phosphorus heterocycles (2015EJIC2057). According to X-ray structural data, the phosphorus atom in molecule 34 has overall trigonal bipyramidal geometry with chlorine atoms at the apical position, while the N- and C-substituents and the lone electronic pair are occupying the equatorial positions with P—N and P—C bond distances of 1.771 and 1.831 A, respectively. 

Application of hypervalent phosphorus compounds in organometallic catalysis... 

Studies of hypervalent phosphorus compounds in biological phosphoryl transfer reactions include the preparation of novel anti-apicophilic penta-coordinated phosphoranes with frozen stereomutation using bulky biden-tate ligands. Kinetic studies have enabled the activation enthalpy of the stereomutation of an 0-equatorial phosphorane to its O-apical stereoisomer to be calculated. The involvement of hexacoordinated phosphoranes in phosphate transfer reactions has been clarified by theoretical calculations as well as in vitro studies. Applications of hexacoordinated compounds as catalysts have also been described. 

This chapter provides an update on the chemistry of hypervalent phosphorus compounds, bringing in concise form advances in their preparation, application and structural properties, which have been published in 2011. A major portion of this research has been devoted to pentacoordinated compounds whereas relatively little has appeared on hexacoordinate chemistry. 

To explain the existence of a conventional double bond in the phosphate ion requires five electrons to be supplied by the central phosphorus atom, which may enter into MO formation with suitable orbitals from the ligand oxygen atoms. To arrange this requires the use of one of the 3d orbitals of the phosphorus atom. This cannot occur in compounds of elements of the second period, where hypervalence is not observed. The availability of accessible d-orbitals has, in the past, been used as the justification of hypervalence in compounds of the subsequent periods. An alternative method of representing the structure of the PO4 ion is shown in Figure 6.6. 

The bonding in certain tricoordinate phosphorus compounds (ct , 5-P) is also termed hypervalent (DA Scheme 5). In these structures the lone pair of the planar phosphorus is effectively involved in the delocalization (IIB Scheme 5). Bis-meth-ylene-phosphorane (and its N, O, and S analogues) are typical examples for such bonding (Scheme 5). Like in the case of the ylides, no -orbital participation is needed to describe the bonding in these compounds. Rather, the delocalized 3c-4e model can again be used [33], which also accounts for the positive charge at the central phosphorus atom, in accordance with the ylidic resonance structure (IIC Scheme 5). 

A review on the chemistry of thio derivatives of trivalent phosphorus acids which covers the literature to 1986, includes a section on pentaco-ordinate phosphorus compounds derived from addition to a-diketones and unsaturated systems activated to nucleophilic attack by electron withdrawing groups. Chemical bonding in hypervalent molecules has been discussed and qualitative bonding concepts developed to supersede the dsp and d sp models. A review on the mechanism and stereochemistry of the Wittig olefination reaction inevitably includes a discussion of the equilibrium between betaine and 1,2-oxaphosphetane intermediates. A correction has been published to reference 19 of Chapter 2 in SPR14, Vol.21, concerning the Mitsunobu Reaction. ... 

H. H. Karsch, R. Richter, E. Witt, "Novel Sila-Phospha-Heterocycles and Hypervalent Silicon Compounds with Phosphorus Donors", in Organosilicon Chemistry III From Molecules to Materials (Eds. N. Auner, J Weis), VCH, Weinheim, 1997, p. 460. 

Novel Sila-Phospha-Heterocycles and Hypervalent Silicon Compounds with Phosphorus Donors... 

Introduction - The year has again seen a consolidation of knowledge in the area of hypervalent phosphorus chemistry with emphasis on the synthesis and properties of monocyclic and bicyclic phosphoranes. A considerable effort has been made to resolve the mechanism of the Mitsunobu Reaction (see Section 3) and the principles established by studies of pentaco-ordinate phosphorus compounds have again been extended to hypervalent compounds of other elements (e.g. As, Sb, and Si). A comprehensive review of the reactions of phosphorus acids with chlorine includes a number of examples of acyclic, monocyclic and bicyclic phosphoranes prepared by this route together with a useful catalogue of their 31p 5-values. 

Although interest in pn-bonded compounds continues at a high level the pace of advance seems to be slackening, perhaps reflecting that the most interesting chemistry in this area has now been revealed. The year has seen a further consolidation of knowledge in the area of hypervalent phosphorus chemistry and the principles established in this area have expanded further into the chemistry of As, Sb, Bi and Si. Notable contributions have appeared on the importance of the conformational transmissions effect to ligand... 

Introduction - It is gratifying to report that interest in hypervalent phosphorus chemistry has been maintained, especially with regard to structural studies and the synthetic utility of pentaco ordinate phosphorus compounds. A novel departure has appeared in the area of hexaco-ordinate phosphorus with the synthesis of further phosphorus derivatives of the porphyrin ring system containing hypervalent phosphorus coordinated by the tetrapyrrole unit. The chapter will take its usual format, however, and the details of this small nugget will therefore appear in the last section. 

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