2 Pentaco-ordinated and Hexaco-ordinated Compounds

BY C.D. HALL

1 Introduction

Despite the continued interest in phosphorus chemistry as evidenced by an attendance in excess of 400 at the XIIIth International Conference on Phosphorus Chemistry in Jerusalem (1995), the output in the field of hypervalent phosphorus chemistry continues to decline, at least in terms of quantity. The quality however, remains high, as is nowhere more evident than in the volumes (98-100) of Phosphorus, Sulfur and Silicon' dedicated to the outstanding work of Robert Holmes and his group in Massachusetts. The phosphorus community are truly indebted to the contributions from Amherst especially in the area of hypervalent chemistry and it is perhaps appropriate here to recognise the enormous contribution which Professor Holmes has also made as Editor in Chief of Phosphorus, Sulfur and Silicon. The year has also seen a volume of Phosphorus, Sulfur and Silicon dedicated to Professor Reinhardt Schmutzler which begins with a comprehensive review of the types of P-P bond already known together with those envisaged for the future and both categories include numerous hypervalent systems.*

2 Acyclic Phosphoranes

A crystallographic study of pentaco-ordinated antimony and bismuth compounds shows that in contrast to SbPhs and BiPhS (both sqp) the pentamethyl compounds (SbMeS and BiMeS) are trigonal bipyramid~.~ The intermediate, SbPh3Mez is also rbp with, surprisingly, two of the phenyl groups in apical positions.

The established utility of the Mitsunobu reaction to prepare dialkoxy- or diaryloxyphosphoranes (3) has been extended to the synthesis of difluorotriphenyl- phosphorane (4) by the use of H F or HF-pyridine in conjunction with di-isopropyl azodicarboxylate (2).4

The kinetics of the thermal decomposition of 2,2,2-trifluoroethoxyarylphos- phoranes (5) have been studied in some detail.s The major phosphorus-containing product was the corresponding oxide (6) and a rate sequence of n = 3 > 2 > 1 > 0 together with p-values, solvent effects and activation parameters suggested that the rate-limiting step was ionisation of the P-0 bond, possibly to an ion-pair (7) which subsequently decomposed by a number of routes to a variety of products not containing phosphorus. A considerable proportion of the latter was trifluoro- ethanol (8) which suggested the formation of (9) as an unstable intermediate.

62

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2: Pentaco-ordinated and Hexaco-ordinared Compounds 63

0 ti

PhnP(OCH2CF,),, A PhnP(OC H2C F3) 3-n

( 5 ) n = 0-3 (6) Mixture of products

+ [Ph,P(OCH2CF3)4-n - - - - - - - OCH2CF3J - (6) + CF3CH20H + [ :CHCF,]

(7) (8) (9)

Several new difluorophosphoranes (1 2) and bis-phosphoranes (1 4) containing bulky substituents have been prepared by the fluorination of precursor phos- phines (10) or (1 3) with dimethylaminosulfur trifluoride (1 1).6 The compounds were characterised by 'H, 31P and I9F N M R and the tbp geometries within (12a-c) were compared in terms of structure and reactivity with the isoelectronic anionic fluorosilicates.

R3P + Me,NSF3 - R3PF2

a, R = Ph, R2 = @To1

b, R3 = (Mes), c, R = Ph, R2 = (1 -Nap)2

e. R3 = ( ~ T o l ) ~

d, R3 = ( ~ T o I ) ,

f, R = Ph, R2 = (6U')p

The proceedings of the IXth International Symposium on Phosphorus Chem- istry (St.Petersburg, 1994) have been reported in the Russian Journal of General Chemistry (Vo1.64, No.8 pt.2) and contained within this publication is a paper by Mironov and Konovalova on the conversion of carbonyl compounds into gem- dihalides which includes information on the synthesis, thermal stability and hydrolysis of haloalko~yphosphoranes.~ For example, the reaction of phosphite (1 5) with bromal (1 6) gave (1 8) via the intermediate phosphorane (1 7). Similarly, reaction of the bicyclic phosphoranes (1 9ab) with aldehydes (e.g. 16) gave (20ab).

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64 Organophosphorus Chemistry

(19 ab) a, X = CI b. X = Br

(20 ab)

Introduction of strongly electron-withdrawing substituents (F, OCH(CF3)z or OC6FS) into the phosphorane molecule allows preparation of a number of stable acyclic phosphoranes (e.g.22) from bromal or chloral. All the phosphoranes were characterised by multinuclear NMR and were found to be hygroscopic com- pounds which were readily hydrolysed to phosphates. Thermolysis of the 1- haloalkoxyphosphoranes was studied for acyclic and monocyclic compounds and

CI3CCHO [ (CF3)2CH0 px - [ (CF3)zCHO 1 POCHXCCt,

(21) X = CI or Br (22)

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2: Penraco-ordinated and Hexaco-ordinared Compounds 65

S K I 5

in the latter case (e.g.23) the reaction was found to give the vinyl phosphate (24) for X = Y = Br, the spirophosphorane (25) for X = Y = C1 and the phosphate (26) with X = Br and Y = C1.

Gloede has examined the reactions of a series of 2,6-disubstituted aryl- phosphites (27) with chlorine and in some cases (Y=Me, Z = H ) found that chlorophosphonium chlorides (28) were formed whereas in others (Y, 2 = C1 or Br) the pentaco-ordinated structure (29) was the preferred product.* Treatment with SbC15 always gave the tetraco-ordinated structure (30) and similar results were obtained by reaction of the corresponding phenols with PC15. The study was extended to calixarenes (31) which on chlorination with PC15 gave compound (32) containing tetra-, penta-, and hexaco-ordinated phosphorus.

( 3 2

(29) Y,Z = CI or Br S3'P = ca. -61

(28) Y = Me, 2 = H P P = + 5.6

B* u'

HO OH OH OH

3 Cyclic Phosphoranes

3PCIs

(30) S3'P = + 5.6

But Bu' But I But

3.1 Monocyclic. - Autoxidation of the phosphoryl compounds (33a-e) in the presence of KOBu' and oxygen in DMF (or THF) gave pale blue light emission at room temperature which lasted several minutes. Likewise, treatment of the

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66 Organophosphorus Chemistry

brominated phosphonate (34) with alkaline H202 in THF and water gave light emission at 450nm with the final products of the reaction being N-methylacridone (37) and the phosphates (38a-d) or phosphinate (38e) respectively. The chemi- luminescence was cited as evidence for the reactions proceeding through the common intermediates (35) and (36, a pentaco-ordinate phosphadioxetane) with the latter decomposing to N-methylacridone in the excited singlet state which gives rise to the fluores~ence.~

M e N g

(33a-e) a, R = OEt b, R = OMe

d, R = OCHMeCHMeO e, R = Ph

C. R=OPrJ

o* + 6P(O)R;!

0- I

(37) (38a-e) (36a-e)

A new route to 1 ,3,2-h5-oxazaphosphetidines involving the chlorination of N- acyl-aminodihalophosphines has been reported.1° Reaction of (39) with chlorine or sulfuryl chloride gave a mixture of (40) and (41) with the latter predominating and with the equilibrium shifting to produce pure (41) on distillation.

Reaction of the disilylureas (42ab) with p-tolylsulfenyl chloride (43) gave the mixed ureas (44ab) which in turn reacted with phenylphosphonous dichloride to give (46ab) via the intermediates (45ab). ' The compounds were characterised by 'H, 13C and 31P NMR and in one case (46b) a single crystal X-ray analysis

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67 2: Penraco-ordinated and Hexaco-ordinared Compounds

revealed a distorted tbp geometry at phosphorus with the axial P-N bond significantly longer than in similar phosphoranes.

(39a-c) a, R' = Me; R2 = CF3

c. R' = Me; R2 = CCI, b. R' = PI'; R2 = CF3

Me3SiN(CO)NSiMe,

R' R2 I I

(42ab) a, R',R2 = Me b, R'= Me, R2 = Ph

(40a-c) (41a-c)

0

+ p-TolSCI .

CI

(43) (46ab)

r 0 1

In a study of the reaction of phosphorus ylides (47a-d) with dioxetanes (48a-c) the product phosphonium alkoxides (49) were found to be in equilibrium with the 1,4-dioxa-2-h5- phosphorinanes (50) and evidence was cited to indicate that (49) was formed first rather than the reaction proceeding through insertion into the peroxide link to give (50) directly.12 Fluorinated phosphites (51ab) react with the bis-imine (52) by a [l + 41 cycloaddition to give quantitative yields of the h5- spirophosphoranes (53ab). l 3

Ab-initio molecular orbital calculations have been carried out on monocyclic pentaoxaphosphoranes as models for intermediates in the enzymatic hydrolysis of cAMP.14 All the calculations showed the e-e ring location to be higher in energy than the a-e ring arrangement and the computed energies and P-0 bond lengths compared favourably with activation energies for ligand exchange derived from variable temperature NMR dataIs and with the data from X-ray crystal- lographic studies. The computations also supported proposals for in-line enzy- matic hydrolysis of CAMP with the ring situated a-e in a boat conformation within the tbp intermediate and with little likelihood of pseudorotation within the intermediate to place the ring di-equatorial.

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68 Organophosphorus Chemistry

+ Me

R’

R2 Ph3P =(

Me

(47a-d) a. R ’ , R ~ = H (48ax) a, R3,R“ = H b, R ’ = Me; R2 = H C, R ’ ,R2= Me d, R’= Ph; R 2 = Me

b, R 3 = Me; R4 = H c, R3,R4= Me

A -0 Bu”N,,, Me

(51ab) a, R = CH(CF3)2 (52) b, R = CH2CFS

I BU”

(53ab)

3.2 Bi-, Tri- and Polycyclic. - Several bicyclic oxaphosphoranes (54-56) containing a six-membered oxaphosphorinane ring have been synthesised and their structures established by ‘H / 31P NMR in solution and by X-ray crystallography in the solid state.16 All the crystal structures showed tbp geometry with the rings spanning apical-equatorial positions and V.T. NMR over the range from - 10 to 85°C was consistent with ligand exhange in which each of the rings was required to adopt di-equatorial positions in the intermediates for pseudo- rotation. Application of a model treatmentI4 resulted in excellent agreement between calculated and experimental activation energies for ligand exchange and the strain energies for diequatorial placement of the rings (in 54-56) followed the order, dibenzo-fused oxaphosphorinane (54-56) > dioxaphosphorinane (54, 55) > benzo-fused dioxaphosphepane (56).

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2: Pentaco-ordinated and Hexaco-ordinared Compounds 69

The crystal structures of phosphoranes (57) and (58) also showed distorted tbp structures with equatorial 0-P-0 bond angles of 107.1 O and 1 12.7" respectively. The compression of this bond angle in (57) allows the six-membered ring to adopt a chair conformation rather than the half-chair conformation which would be enforced by an equatorial 0 -P -0 bond angle of 120".

During studies of the phosphorylation of proline, the reaction of the hydro- spirophosphorane (59) with the methyl ester of N-(methoxymethyl) proline (60) in the presence of zinc chloride gave the spirophosphorane (61).18 Likewise, in a study of the chemistry of oxathiaphospholenes (62ab), reaction with diketones (63ab) gave the phosphoranes (64ab) which were characterised by elemental analysis, 31P NMR and in the case (64a) by X-ray crystallography which revealed another tbp configuration. l9

CO Me L

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70 Organophosphorus Chem is t ry

(62ab) a, R’ = H; R2 = Me (63ab) a, R3 = Me b, R3 = Ph

(64ab) a, R’ = H; R2,R3 = Me b, R’,R2 = Me b, R’,R2 = Me; R3 = Ph

The P-cyano spirophosphorane (65) reacts with cyclic chlorophosphi tes (66ab) to form (67ab) by exchange of the cyano group together with (69) by ionisation and de-alkylation of (68).20 A similar reaction of (65) with (70) gave (71) and exchange of the cyano group between (65) and P-NMe2 or P-OMe groups was also reported.

CN 0

\ / R’ ‘PCI

Me Me 0

(66ab) a, R = MeCHCH2CH2 b, R = MeCHCHMe

Me

L J

(67ab) (68)

(65) + (Et2N)2PCI L (Et2N)zPCN + (69)

(70) (71 1

In a study of the reactions of various nucleophiles with cyanoacrylates (73) trico-ordinate phosphorus compounds (e.g.72) were found to generate phos- phoranes (75) via the intermediate betaine (74).21 A slight modification of the phosphorus component (72, R=Ph) leads to slow proton transfer within the intermediate (76) to give the imine (77) which dimerises to the diazaphosphetidine (78). Phosphites, as exemplified by (78) however, again gave a spirophosphorane (80) via proton transfer within (79).

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2: Penraco-ordinated and Hexaco-ordinated Compounds 71

(72) R = But or c-C6HII (73)

EtO,C(CN)CHCH2 I Th YH&H(CN)CO,Et

(72) R = Ph + (73)

/ I I L

(76)

[ >P-OCH2CH20H

(78)

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72 Organophosphorus Chemistry

In a continuation of earlier work22, Burgada and Bailly have studied the reactions of a number of spirophosphoranes containing the P-H bond (e.g. 81) with tetra-alkylethenylidene- bis-phosphonates (82ab). The products, which were formed by Michael addition to the activated double bond of (82), were character- ised by multinuclear NMR and in some instances both phosphorus and nitrogen within the nucleophile were alkylated at different rates to form (83) and (84) re~pectively.~~

\ / \ /

(81 1 (82ab) a, R = Me (83) R = Et

0 Y

Houalla's contributions to the chemistry of macrocycles containing bicyclic phosphorane units continues with the preparation of a series of phosphoranes (87-92) from the reaction of (85ab) with several polyethylene glycols ( 8 6 a - ~ ) . ~ ~

Tricyclic phosphoranes (95) and (98) have been prepared by the reaction of (93) with (94) or (96) with (97) but reaction of (99) with (96) gave an acyclic phosphite (100) which reacted with hexafluoroacetone to give the monocyclic phosphorane (101).25

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2: Pentaco-ordinared and Hexaco-ordinated Compounds 73

? 7 . o ~ o ! + o . ; . y "iL-2 + HOCH2+CH20CH2 i C H 2 0 H N-P; h" (85ab) a, n = 2 (86a-c) a, n = 1

b, n = 3 b, n = 2 c, n = 3

2CI4 + 4EI3WCH3CN (TODD) 1

(87) P'P = 4 5 . 2

(ex 85 + 86b)

(88) 63'P = 4 4 . 8

(ex 85 + 86b)

(89) S3'P = 4 5 . 2

(ex 85 + 86c)

(90) P'P = -50

(ex 85 + 86c)

(91) tj3'P = 44.9 (92) m = 3, n = 2 S ~ ' P = 4 5

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74 Organophosphorus Chemistry

OH Me,N

-2 HCI

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2: Pentaco-ordinated and Hexaco-ordinated Compounds 75

The substitution reactions of Li (cyclen) with a variety of electrophilic reagents, RX, have been explored by Lattman in order to determine the factors controlling the formation of the ‘open’, N-R species [R(cyclen)P] as well as the ‘closed’ species [(~yclen)PR].*~ Treatment of Li(cyc1en )P (1 02) with chloromethane gave an oil with 31P signals at -43 and + 103ppm which were assigned to (103) and (104) respectively with (104) as the predominant product. This demonstrates once again that both nitrogen and phosphorus are ‘accessible nucleophilic sites’ under these conditions.

MeCl f?N’Me - /!.-+-Me + /..- \ - - - - - -

Y- Y- P

I N

4 Hexaco-ordinated Phosphorus Compounds

A multi-technique approach involving molecular mechanics, X-ray diffraction and solid-state NMR has been shown to generate detailed information on the geometry of P(V1) compound^.^' In particular, solid-state 31P NMR is shown to be a valuable technique for the elucidation of the structure of P(V1) species when single crystals are not available for X-ray diffraction studies. Analysis of 31P 6 values, anisotropy A6, and span fl gave semi-quantitative information on the distortion of the molecular structure from ideal octahedral geometry and comparison of isotropic chemical shifts in solution and in the solid phase elucidated the changes of molecular structure in both phases.

Zwitterionic compounds containing two phosphorus atoms of opposite charge are quite rare but a new route to such compounds has been designed through reaction of (105) with alkyl isocyanates (106ab) which gives a 7:l mixture of (108ab) and (109ab) via (107ab).28 The compounds were characterised by multi- nuclear NMR (631P, + 32 and -140 with Jpp = 16.8Hz for lO8a) and in one case (108a) an X-ray analysis showed distorted tetrahedral and octahedral geometry about the two phosphorus atoms. Reaction of the product mixtures with Sg gave the known compound (1 10) containing both tetra- and pentaco-ordinated phosphorus.

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76 Organophosphorus Chemistry

+ RNCO

(106ab) a, R = Me; b, R = Et.

(EtZN),CH2

Me\N_4'

CI

(105)

M B N ~ ~ + I .NMe II p-..--N-Me I

(Et2N)2P -CH,-P;I;; (EtzN),PCH, - 0 A N !)@cl ;&CI

CI CI CI CI

CI CI

\

(1 Ofab) (1 10) I #

Cl (1 08ab)

References

Cl ( 109ab)

1. 2. 3. 4. 5. 6.

7.

8.

Phosphorus, Suvur and Silicon, 1995,98 (1 -4). 99 (1 -4) and 100 (1-4). L. Lamande, K. Dillon and R. Wolf, Phosphorus, Sulfur and Silicon, 1995, 103, 1. S. Wallenhauer and K. Seppelt, Inorg. Chem., 1995,34, 116. P. J. Harvey and I . D . Jenkins, Tet. Lett., 1994,35,9775. N. Lowther, P. Crook and C. D. Hall, Phosphorus, Sulfur and Silicon, 1995,102, 195. R. R. Holmes, J. M. Holmes, R. 0. Day, K. C. Kumara Swamy and V. Chandrasekhar, Phosphorus, Su,f.r and Silicon, 1995,103, 153. V. F. Mironov and I . V. Konovalova, Russian J . Gen. Chem. Engl. transl., 1994.64, 1210. J . Gloede. Russian J . Gen. Chem. Engl. transl., 1994,64, 1203.

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2: Penraco-ordinared and Hexaco-ordinated Compounds 77

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