Tricoordinate phosphorus

Phosphine-borane 63a (75% ee) was obtained by reduction of compound (Sp)-62a using LDBB at -60°C and nucleophilic substitution with iodomethane in 72 % yield. The observed loss of optical purity may be ascribed to stereomutation of the generated tricoordinated phosphorus species. Recrystallization afforded (S)-63a in > 99% ee. On the other hand, severe racemization was observed using the same method with (Rp)-62b. An alternative strategy consisted of deborana-tion of (Rp)-62b using ZSl-methylpyrrolidine, methylation with methyl triflate. 

Tricoordinate phosphorus is essentially nonplanar in nature and due to the significant s character of the lone pair in a compound such as phosphole, effective overlap with the carbon p orbitals is inhibited and the compound is nonaromatic. Geometry optimizations and aromaticity analyses performed by Glukhovtsev et al. <1996JPC13447> have shown both pentaphosphole 1 and the bicyclic octaphosphane P8 to be both planar and aromatic in character. 

Nyulaszi et al. <1998NJC651> investigated the fused phosphindolizine ring system 2. Their ab initio quantum-chemical calculations have shown this tricoordinated phosphorus to be essentially planar and aromatic in nature. Such planar tricoordinate phosphorus compounds can be used as building blocks for aromatic systems. 

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. 

Acyclic boryloxyalkylphosphines with tricoordinated phosphorus and boron are capable of forming cyclic betaine structures with four-coordinated P and B atoms. The ability to be converted into a more stable four-coordinated state accounts for many chemical transformations of boryloxyalkylphosphines. Diphenylboryloxymethyl(methyl) phenylphosphine (92) readily disproportionates to give 1,3,2,5-dioxabora-taphosphoniarinane (103). [Eq. (60)] (83IZV2541). Similar interaction is... 

For 1,3,2,5-dioxaboraphosphorinanes with a tricoordinated phosphorus atom, equilibration of the stereoisomers has been observed after a few hours at 20°C without any catalyst. In the case of the sulfide and selenide the equilibrium is established in 11-20 h at 80°C in the presence of p-toluenesulfonic acid. The equilibrium composition of 2,5-diphenyl-4,6-... 

Complexes with borane can be obtained due to the presence of the tricoordinated phosphorus atom [Eq. (103)] (90IZV1120). When heated... 

Sometimes more complicated ion exchange reactions are observed, especially when the cation and the anion are able to interact with one another. Then, further transformations of tricoordinated phosphorus derivatives into ammonium 1,3,2,5-dioxaborataphosphorinanes are observed [Eq. (124)]. 

Thus, unlike a-oxyalkylphosphines and their derivatives, which undergo an oxidative rearrangement on heating, P,B-containing heterocycles are transformed into compounds with tricoordinated phosphorus and boron atoms. 

Phosphorus. Substituted phosphorus analogues of pyridine (phosphinines, A3-phosphabenzenes, also called phosphonins or phosphorins) were first prepared by Markl starting from pyrylium cations their chemical properties suggest that their aromaticity is lower than that of pyridine (e.g. phosphinine 186, Scheme 72).230-232 Molecular calculations for other six-membered jr-systems with planar tricoordinate phosphorus, such as phospininines 186 and 187, have evidenced their aromaticity (Scheme 72).156... 

In 1972, Buono et al. observed that tricoordinated phosphorus compounds RP(OMe)2 (R = Me, Ph, CH=CH2, SMe, CN, OPh, NMe2) or Ph2POMe reacted with 1,2-propadienyl ketone 446 to afford 3-alkylidene-A4-l,2-oxaphospholenes 447 [201, 202],... 

Contrary to pyrrole, phospholes are not planar, due to the high inversion barrier of the tricoordinate phosphorus (cf. the 6 kcal/mol inversion barrier of ammonia with the 35 kcal/mol inversion barrier of phosphine). As a consequence, unfortunately phospholes are not aromatic (Mathey, F.). Although the aromaticity of phospholes has been disputed in the past, Mislow considered first that phospholes with pyramidal phosphorus are nonaromatic while with planar tricoordinate phosphorus aromatic phospholes could be obtained. It was just recently found that phosphorus can be flattened or even fully planarized (as discussed comprehensively ), resulting in aromatic systems (see section IV.B.l). 

Structure C is usually written with a pentavalent phosphorus this description, however, should merely denote that the phosphorus lone pair is delocalized and no allenic bond with two perpendicular jr-systems has been formed. H2C=P(H)=CH2, a typical representative of structure C phosphorus, can be described by an allyl-like r-system, ° with the p -type lone pair of a planar tricoordinate phosphorus participating in the 4-electron-3-center jT-bonding, without invoking d-orbital participation. " Thus, the C -type bonding of phosphorus involves tricoordination, with a planar bonding environment, whereby the lone pair is available for jT-bonding. 

The planarity of the tricoordinate phosphorus and thus the aromaticity of 4 can be influenced by substituents. It turns out from ab initio calculations that 7T-acceptor groups either at phosphorus or at the neighboring carbon have a planarizing effect. The barrier to planarity decreases to 1.56 kcal/mol (MP2/ 6-31G(d)) as a result of —BH2 substitution at phosphorus. The bond length alternation in the planar form of this substituted derivative increases in the... 

While 4bp/-type structures can be considered as resonance structures, 4cp/is also an alternative. The importance of structure 4cp/is supported by the fact that the negative charge at the carbon atoms in the 2 and 5 positions is indeed larger than at the 3 and 4 positions. ° The common feature in all 4ap/-, 4bp7-, and 4cp/-type structures is that the lone pair of the planar tricoordinate phosphorus is fully delocalized, resulting in an enhanced bond order about the tricoordinate planar phosphorus. The d-orbital participation at phosphorus is insignificant in 4, either in the planar or nonplanar form. °... 

Incorporation of tricoordinate phosphorus to three delocalized systems resulted in 30, a planar compound (B3LYP/6-31 H-G(d,p)) exhibiting equalized... 

Tris(fluorosulfuroyl)fluoromethane (329) reacted with bis(diethylamido)benzyl phosphite to yield an intermediate (330) which extruded a molecule of SO2 to give as final product the bis(fluorosulfonyl) compound (331). The kinetics and mechanism of the reaction of fluorinated tricoordinate phosphorus compounds (332) and aryl 2,2,2-trifluoroethyl sulfenates (333) have been reviewed. ... 

Abstract This chapter is concerned with preparation and applications of tricoordinate phosphorus compounds in synthesis of biophosphates and their structural analogues and illustrates the recent trends with a series of selected examples. 

In contrast to tetracoordinate phosphorus compounds which are involved in the mechanism of life or are related to biophosphates, tricoordinate phosphorus compounds have not been found in nature. However, their importance in the synthesis of biophosphates is very great. The introduction of P compounds as phosphitylation reagents was a turning point in synthetic biophosphate chemistry. 

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-... 

Example 56 the Isis Pharmaceutical group in their extensive investigations of antisense oligonucleotides as therapeutics has described the synthesis of 3 -C-methylene nucleoside phosphonoamidites for the new backbone modification of oligonucleotides [90]. This paper gives good insight into tricoordinate phosphorus and related H-phosphonate chemistry in the service of nucleotide synthesis. 

---