[organophosphorus chemistry] organophosphorus chemistry volume 38 || pentacoordinated and...

Pentacoordinated and hexacoordinatedcompounds

G.-V. RoschenthalerDOI: 10.1039/b801353p

1. Introduction

The past year has seen a continuing interest in hypervalent phosphoruschemistry. A number of studies were carried out to establish many importantproperties and to understand new mechanisms. The inter-conversion ofpenta- and hexacoordinated states continues to attract great interest due totheir involvement as intermediates (or transition states) in the biologicalphosphoryl reaction. Many of these efforts have been directed towardsthe synthesis of new anions of pentacoordinated phosphorus compoundscontaining fluoro and trifluoromethyl groups.

2. Acyclic phosphoranes

The synthesis of new fluorinated anions of pentacoordinate phosphorus wasbased on the reaction of triphenyl phosphate, Me3SiCF3 with two differentfluorine sources: Me4NF and CsF.1 In the case of [Me4N]F, the compositionof salt (1) was strongly dependent on the stoichiometry of the startingcompounds. The reaction was performed at �40 1C between 1 equiv. of(PhO)3P(O), 1 equiv. of Me3SiCF3 and 4 equiv. of [Me4N]F (Scheme 1).

Interestingly, the salt (1) contains a unique anion that representsthe stable transition state usually postulated in the course of nucleophilicsubstitution at a tetrahedral phosphorus atom. The salt (1) canundergo slow dissociation at temperatures above 0 1C forming stabletetracoordinated (2) and hexacoordinated (3) phosphorus species (Scheme 2).The dissociation of salt (1) was rationalised as proceeding with initialformation of the (trifluoromethyl)phosphonyl difluoride (A) by fluorideelimination; (A) reacts with the anion (1) to form the second intermediate,the dimeric anion (B). The fluoride anion liberated in the first reaction stepthen attacks the hexacoordinate phosphorus atom of the intermediate (B) togive salts (2) and (3) as the final reaction products (Scheme 2).This reaction provides a new method for the preparation of two important

perfluoroalkyl-containing phosphorus anions, that is fluoro(trifluoromethyl)-phosphonate (in salt 2) and l6-pentafluoro(trifluoromethyl)phosphate

Scheme 1

Institute of Inorganic & Physical Chemistry, University of Bremen, Leobener Strasse,28334 Bremen, Germany

318 | Organophosphorus Chem., 2009, 38, 318–331

This journal is �c The Royal Society of Chemistry 2009

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(in salt 3). An equimolar mixture of salts (2) and (3) was also obtained byfluorination of (trifluoromethyl)phosphonyl dichloride with the same sourceof fluorine (Scheme 3). Interestingly, 31P and 19F NMR showed that treatingtriphenyl phosphate with 4 equiv. of [Me4N]F in dme at �40 1C givestetrafluorophosphoranolate (4) as the major product together withdecomposition products (5) and (6) (Scheme 4).

Secondly, in the case of CsF, the Cs analogue of salt (1) was not obtainedusing the same conditions as [Me4N]F. Here, the pentacoordinatedphosphorus anion (difluorobistrifluoromethyl)phosphoranolate (7) wasobserved (Scheme 5). The salt (7) is stable in DME or CH3CN solution,however storing the solid in glassware causes decomposition intobis(trifluoromethyl)phosphinic acid (8) and (trifluoromethyl)phosphonicacid (10) via fluoride (9) (Scheme 6). Furthermore, the Cs analogue (11) was

Scheme 2

Scheme 3

Scheme 4

Scheme 5

Organophosphorus Chem., 2009, 38, 318–331 | 319


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http://dx.doi.org/10.1039/b801353p

obtained by the reaction of (trifluoromethyl)phosphonyl dichloride withCsF (Scheme 7). The salt (11) was detected in the NMR spectra of thereaction mixture. The salts fluoro(trifluoromethyl)phosphonate (12) andl6-pentafluoro(trifluoromethyl)phosphate (13) were separated by fractionalcrystallisation.

3. Bicyclic phosphoranes

In recent years, it has been found that the Martin ligand possessing twotrifluoromethyl groups on a rigid five-membered ring can stabilise manytypes of hypervalent phosphorus compounds both thermodynamically andkinetically. Jiang et al.2 presented the synthesis of new bidentate ligandsbearing two pentafluoroethyl groups that are bulkier than the Martinligands. The alcohol (14) was obtained according to commonly knownCannizzaro-type reaction based on disproportionation involvingintermolecular migration of the pentafluoroethyl group (Scheme 8).

Alcohol (14), after treatment with NaH and n-BuLi, was added to asolution of PCl3 in THF to give P–H spirophosphorane (15) in 50% yieldand O-apical n-butylphosphorane (16) in 6% yield (Scheme 9). Thestructure of spirophosphorane (15) was confirmed by X-ray analysis andas expected this pentacoordinated phosphorane (10-P-5) has a TBP(trigonal-bypiramidal) structure. The C1-P1-C1 angle in (15) (163.31) inthe equatorial plane was larger by 8.71 than that in P–H spirophosphoranes

Scheme 6

Scheme 7

Scheme 8



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bearing Martin ligands. This difference can be explained by the stericrepulsion between the bulky endo-C2F5 groups and the aromatic rings.Furthermore, all O-equatorial phosphoranes (17–19), which were preparedfrom the reaction of P–H phosphoranes with 3 equiv. of RLi,were converted almost quantitatively into the corresponding O-apicalphosphoranes (20–22) by heating a solution (Scheme 10). Similarly,C1-P1-C1 angle of (20–22) was expanded by 7.31 in comparison to theP–H spirophosphoranes possessing Martin ligands due to the stericrepulsion between the endo-C2F5 group and the equatorial aromatic ring.

Further studies by Jiang et al.3 examined the reactivity of the O-equatorialphosphoranes (17 and 18) bearing the bidentate ligand based on decafluoro-3-phenyl-3-pentanol toward different nucleophiles. They found thattreatment of both O-equatorial phosphoranes (17 and 18) with 3 equiv. ofMeLi for 3 h at room temperature led to more stable isomers (20 and 21)(Scheme 11) in contrast to O-apical phosphoranes, where no reactions wereobserved (Scheme 12). The experimental results showed that the steric bulk ofthe pentafluoroethyl group could prevent nucleophiles, like MeLi, fromattacking the d*P�O orbital. Furthermore, deprotonation at the methyl groupof (17) using conventional strong bases (n-BuLi, t-BuLi, LDA, NaHMDS)followed quenching the reaction with D2O was examined (Scheme 13), andgave phosphoranes (17) and (20). Using ‘‘superbase’’ (mixture of t-BuOK andn-BuLi) leads to the generation of the alpha-carbanion (24) (Scheme 14),which readily reacts with several electrophiles to afford new phosphoranes(25–30) (Scheme 15). Reactions with paraformaldehyde gave an isomericpairs of beta-hydroxyethylphosphoranes (31 and 32) (Scheme 16).

Scheme 9

Scheme 10



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The treatment of (31 and 32) with KH afforded a hexacoordinatephosphate (33) bearing an oxaphosphetane ring with 31P NMR d =�97.3 ppm in THF and (34) with 31P NMR: d = �104.4 ppm in THF(Scheme 17). Interestingly, when a solution of (33) was heated at 60 1C, itgradually isomerised to (34).

Scheme 11

Scheme 12

Scheme 13



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Scheme 14

Scheme 15

Scheme 16



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4. Polycyclic phosphoranes

The synthesis of new spirophosphoranes, based on the approach leading tostable bicyclic cage phosphoranes incorporating the phosphorus–carbonbond along with chiral phosphorus and carbon atom was presented byAbdrakhmanova et al.4 It was found that diethyl acetylenedicarboxylatereacts readily with dioxaphosphole (35) to give phosphorane (36). It ispresumed that the process starts probably with nucleophilic attack ofthe phosphorus atom in phosphole (35) on a carbon atom of diethylacetylenedicarboxylate resulting in bipolar ion (A), which then undergoesstabilisation due to the intramolecular attack of the carbanion on theexocyclic carbonyl group to give bipolar ion (B) followed by formation ofa bond between the alkoxide anionic centre and the phosphorus atom(Scheme 18). Furthermore, the reaction of dioxaphosphole (37) with diethylacetylenedicarboxylate also occurs under mild conditions, via the samereaction mechanism. It results in the formation of stable phosphorane (38)(Scheme 19). Both new phosphoranes (36) and (38), containing several chiralcenters, are formed as single diastereoisomers. Presumably, the reactionfollows this pathway because new chiral centers are formed via conforma-tionally rigid cyclic transition states (or intermediates) under strict spatialrequirements for the mutual arrangement of subsituents around newly formschiral centers. Thus, the reaction of s3l3-benzophospholes, with diethylacetylenedicarboxylate can be used for the synthesis of s5l5-phosphoranes.Nemtarev et al.5 has shown that alk-1-ynes can be converted to

derivatives of benzo[e]-1,2-oxaphosphorinines using pentacoordinateand hexacoordinate phosphorus derivatives derived from trichloro-phosphoranes. Thus, 2,2,2-trichlorobenzo[d]-1,3,2-phosphole (39) reacts with

Scheme 17



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hex-1-yne to give benzo[e]-1,2-oxaphosphorinines (40) and (41) that hydro-lyse to acids (42) and (43) which are phosphorus analogues of coumarins ina ratio of 1:1 (Scheme 20).

Furthermore, the reaction with alkylacetylenes with anionic and neutralderivatives of hexacoordinate phosphorus, so called ate-complexes (44) and(45) was demonstrated for the first time (Scheme 21). It was found that the

Scheme 18

Scheme 19

Scheme 20



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reaction of phosphate (44) with alk-1-ynes gives predominantly benzo-phosphorinines (46), in which the chlorine atom is located at the metaposition with respect to the endocyclic oxygen atom of the phosphorinineheterocycle (Scheme 21). Hydrolysis gives acid (47). Chlorophosphorinine(40) is formed as a minor product.

Treatment of oxaphosphorin (43) with a solution of calcium chloridegives complex (48) as shown in Scheme 22.

It has hitherto been known that many organic materials are stabilised byphenol or phosphorus antioxidants. However, many commonly knownphosphorus oxidants suffer from a lack of sufficient oxidation deterioration.Inui et al.6 presented a facile synthetic pathway leading to variety ofpentacoordinated phosphorus compounds which are very useful as stabi-lisers for organic materials. These compounds represented by the formula(49) can be synthesised by reaction between substituted bisphenols withphosphorus trichloride followed by addition of substituted catechols(Scheme 23).The pentacoordinated phosphorus compounds (49a–e) are excellent

stabilisers for various organic materials such as thermoplastic resin (e.g.polyolefin, etc.) and the organic products containing phosphoranes (49a–e)are stable to heat and oxidation during production, processing and use,which results in high quality products.

Scheme 21

Scheme 22



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5. Hexacoordinated phosphoranes

Constant et al.7 reported the synthesis and resolution of the novel nitrogen-containing hexacoordinated phosphate anion (50) denoted TRISPHAT-N,which can interact directly with metal centers and allow the stereocontrol ofmolecular events that previous non-coordinating chiral anions could notachieve. TRISPHAT-N was prepared by the established procedure fromo-chloroanil leading to the desired tri-n-butylammonium salt of racemicphosphate (50) (Scheme 24). The resolution of the anion was achieved bythe addition of N-benzylcinchonidium chloride salt (4Cl); 1.0 equiv.) to aCHCl3 solution of Bu3NH rac-2. The L enantiomer was isolated from themother liquor as (+)-Bu4N L-2 after ion exchange metathesis with Bu4NCland chromatography (SiO2, CH2Cl2). It was established that anion (50) cancontrol effectively the conformation of tropos ligands bound to a metalcenter. Furthermore, the ability of anion (50) to form zwitterionic speciesand thus behave as a chiral ligand was applied to the stereocontrol ofthe absolute P or M geometry of the ligands (51, 52 and 53). TheN-TRISPHAT anion, although binding at a single point, acts as an effectivechiral auxiliary that can control with high selectivity the conformation oftropos ligands and the configuration of stereogenic metal ions (54 and 55)

Scheme 23

Scheme 24



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and (56 and 57). It was found that the chiral C3-symmetric[Mo3S4Cl3(dppe)3]

+ cluster [dppe = 1,2-bis(diphenylphosphinoethane), Por M enantiomers] with incomplete cuboidal structure is shown to beconfigurationally stable at room temperature and configurationally labileat elevated temperature. Enantiopure d- or L-TRISPHAT [(tris(tetracchloro-benzenediolato)phosphate(V)] anions were used both as chiral NMRsolvating and asymmetry-inducing reagents. The enantiomers of thistrinuclear cluster cation can equilibrate at higher temperature (typically72 1C), and in the presence of the hexacoordinated phosphate anion amoderate level of stereocontrol (1:2:1) can be achieved. This resulted in adiastereometric enrichment of the solution in favor of the heterochiral ion



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pairs, e.g., M+ D- or P+ L-. At higher temperature, a partial racemisationof the TRISPHAT anion was observed, and participation at roomtemperature of [rac-Mo3S4Cl3(dppe)3][rac-TRISPHAT] salts from thediastereomeric enriched solution improves the diastereomeric purity ofthe mother liquor to a 4:1 ratio. A low-energy pathway for the inter-conversion of [P-Mo3S4Cl3(dppe)3]

+ and [M-Mo3S4Cl3(dppe)3]+ enantiomers

has been found using combined quantum mechanics and molecularmechanics methodologies. This pathway involved two intermediates withthree transition state regions, which result from the partial decoordinationof the diphosphine coordinated at each center. Such decoordination createsa vacant position on the metal, producing an acidic site that presumablycatalyses the TRISPHAT epimerisation.

6. Hypervalent phosphorus and silicon atoms in single molecules

Several examples of hypervalent compounds containing two or moreidentical hypervalent atoms have been reported. Kano et al.8 for the firsttime presented the synthesis of a compound with both pentacoordinatedphosphorus atom and pentacoordinated silicon. The reaction of a 1:1mixture of phosphoramide (58) and silane (59), both bearing two Martinligands, in THF gave phosphoranylaloxysilicate (60) as a colourless solid invery good yield (88%) (Scheme 25).

Furthermore, the synthesis of other phosphoranylalkoxysilicates withdifferent alkyl chains was prepared. Hydroxyalkylphosphoranes (62–65),which were synthesised from (61) according to the commonly usedprocedure, were firstly deprotonated with KH (1.5 equiv.) in the presenceof 18-crown-6 (1 equiv.) then, successive treatment with silane (59) yieldedthe corresponding phosphoranyloxysilicate (66) and phosphoranylalkoxy-silicates (67), (68), and (60), respectively (Scheme 26).The phosphoranyloxysilicate (66) and phosphoranylalkoxysilicate (60)

were hydrolysed to give corresponding hydroxyphosphorane (62) andhydroxyalkylphosphorane (65), respectively, together with hydroxysilicate(69) (Scheme 27).

7. Biochemistry

The keen interest in pentacoordinated phosphorus compounds withbiologically relevant molecules has continued. Kumar et al.9 reported

Scheme 25



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Scheme 26

Scheme 27

Scheme 28



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chromatography-free synthesis of 2-benzoylated 1,3,5-protected inositol(72) where 4 and 5 position are blocked and utilised by phosphorylation.Thus, the monobenzoylated inositol diol, prepared as shown in Scheme 28,was treated with PCl3 under neat condition to lead to the phosphorochloridite(70) in good yield. Further treatment of (70) with isopropylamine gave thecorresponding phosphoramidite (71). Then, the reaction of (71) witho-chloroanil let to the desired pentacoordinated phosphorane (72) whichis the first example of a pentacoordinated phosphorus compound with aninositol residue (Scheme 28). The molecular structure of (72) had the moreelectronegative oxygen atoms occupying the apical positions while the lesselectronegative nitrogen is equatorial in the trigonal bipyramidal (TBP)structure. The 1,3,2-dioxaphosphorinane ring in compound (72) adopts aboat conformation, which is different from those normally observed for theother analogues where the dioxaphosphoriane rings exhibit either a chairconformation or a boat form different from that observed here.

References

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2 X.-D. Jiang, K. Kakuda, S. Matsukawa, H. Yamamichi, S. Kojima and

Y. Yamamoto, Chem. Asian. J., 2007, 2, 314.3 X. D. Jiang, S. Matsukawa, H. Yamamichi and Y. Yamamoto, Heterocycles,

2007, 73, 805.4 L. M. Abdrakhamanova, V. F. Mironov, T. A. Baronova, M. N. Dimukhametov,

D. B. Krivolapov, I. A. Litvinov, R. Z. Musin and A. I. Konovalov, MendeleevCommun., 2007, 17, 284.

5 A. V. Nemtarev, V. F. Mironov, E. N. Varaksina, Y. V. Nelyubina, M. Y.

Antipin, R. Z. Musin and A. I. Konovalov, Russ. J. Org. Chem., 2007, 43,468–470.

6 N. Inui, T. Kikuchi, K. Fukuda and T. Sanasa, US 5,902,516, 2007.

7 S. Constant, R. Frantz, J. Muller, G. Bernardinelli and J. Lacour,Organometallics,2007, 26, 2141.

8 N. Kano, H. Miyaka and T. Kawashima, Chem. Lett, 2007, 36, 1260.9 K. V. P. Pavan Kumar and K. C. Kumara Swamy, Carbohydrate Research, 2007,

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[organophosphorus chemistry] organophosphorus chemistry volume 38 || pentacoordinated and...

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