The effect of very bulky groups on the equilibrium of penta- and hexa-coordinated phosphoranes

Article abstract of DOI:10.1080/10426500600683126

The synthesis of dithio-diphenol 4 and sulfonyl-diphenol 5, both with very bulky groups, provided starting materials for reaction sequences that led to the formation of the very stable hexacoordinated phosphorane 2 and sensitive pentacoordinated phosphora

Full text of DOI:10.1080/10426500600683126

Phosphorus, Sulfur, and Silicon, 181:1493–1511, 2006
Copyright © Taylor & Francis Group, LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426500600683126
The Effect of Very Bulky Groups on the Equilibrium
of Penta- and Hexa-coordinated Phosphoranes1
A. Chandrasekaran
Natalya V. Timosheva
Robert R. Holmes
Department of Chemistry, University of Massachusetts, Amherst,
Massachusetts
The synthesis of dithio-diphenol 4 and sulfonyl-diphenol 5, both with very bulky
groups, provided starting materials for reaction sequences that led to the formation
of the very stable hexacoordinated phosphorane 2 and sensitive pentacoordinated
phosphorane 3. Hexacoordination was established in 2 by an intramolecular donor
interaction at the phosphorus center from an oxygen atom of the sulfonyl group
present as part of the eight-membered ring. The solid state structures of 2 and
3 were established by X-ray analysis, as was that of phosphite 1 formed in the
reaction sequence leading to 2. In solution, 2 has two forms existing in a dynamic
equilibrium between a pentacoordinated and the more dominant hexacoordinated
31
19
form as determined by P and F NMR spectroscopy. The high stability of 2 with
respect to hydrolysis and alcoholysis reactions suggests that an associative process
is responsible as the controlling reaction mechanism.
Keywords Cyclic sulfonyl derivatives; hexacoordination; phosphoranes; steric effects;
X-ray crystallography
INTRODUCTION
In recent work, we presented hypervalent phosphorus chemistry and
the importance of penta- and hexacoordinated phosphorus in the phos-
phoryl transfer enzymes in Accounts of Chemical Research articles2 and
in a keynote lecture (at the 7th International Conference on Heteroatom
Chemistry).3 The ease with which phosphorus isomerizes between a
Received February 15, 2006; accepted March 9, 2006.
The research presented here was supported by the donors of the Petroleum Research
Fund, administered by the American Chemical Society, and is gratefully acknowledged.
We thank the X-Ray Structural Characterization Laboratory at the Chemistry Depart-
ment supported by University of Massachusetts and the National Science Foundation
(Grant CHE-9974648).
Address correspondence to Robert R. Holmes, University of Massachusetts, Depart-
ment of Chemistry, Amherst, MA 01003-9336. E-mail: rrh@chem.umass.edu
1493

1494
A. Chandrasekaran et al.
trigonal bipyramid and an octahedral geometry was emphasized. Also,
we had reported a new class of atranes containing phosphorus that equi-
librated between three and six coordinated states in solution. Both co-
ordinate forms were confirmed by crystallographic studies (Scheme 1).4
SCHEME 1
In phosphoranes containing the sulfonyl group, we had reported
the ease with which phosphorus can go from five coordination to six
coordination.5,6 In the latter penta-hexa coordinate equilibrium, we ob-
served only the pentacoordinated phosphorane in the solid state, but
the nature and strength of the hexacoordinated form remained un-
known. To investigate this problem more extensively, we synthesized
new diphenols consisting of very bulky groups. Employing these diphe-
nols in new reaction sequences, we were able to stabilize the phos-
phorane in its hexacoordinated state and obtain its crystal structure.
Herein we report the synthesis of two of these new bulky diphenols and

Penta- and Hexa-coordinated Phosphoranes
1495
their phosphorus derivatives along with their solution NMR behavior.
In addition, details of the crystallographic studies of the phosphorus
derivatives are reported.
EXPERIMENTAL SECTION
2,4-bis(α,α-dimethylbenzyl)phenol (Aldrich) was used as supplied,
˚
whereas trifluoroethanol (Fluka) was dried over molecular sieves (4 A,
flame-dried under vacuum). Sulfur dichloride (80%, Aldrich) was pu-
rified according to literature method.7 N-chlorodiisopropylamine6 was
synthesized according to our earlier methods. Solvents were purified
according to standard procedures.8 All reactions were carried out in an
argon atmosphere. Solution NMR spectra were recorded on a Bruker
Avance-400 (1H and 31P at 400.1 and 162.0 MHz, respectively) or a
Bruker DPX300 FT NMR (19F at 282.4 MHz) spectrometer. Solution
phosphorus NMR spectra were recorded in a sweep-off mode. Chem-
ical shifts are reported in ppm, are downfield positive and relative to
tetramethylsilane for 1H and CFCl3 for 19F NMR and 85% H3PO4 for 31
P
NMR. All were recorded at around 23◦C. Elemental analyses were per-
formed by the University of Massachusetts Microanalysis Laboratory,
Amherst, MA.
Syntheses
O2S[C6H2(CMe2Ph)2O]2P(OCH2CF3) (1)
Diphenol 5 (8.3 g, 11.5 mmol) and phosphorus trichloride (1.00 mL,
11.5 mmol) were stirred in dichloromethane (100 mL). Triethylamine
(5.00 mL, 35.9 mmol) was added dropwise over a period of 20 minutes,
and the solution was stirred further for 2 h. Trifluoroethanol (0.90 mL,
12.3 mmol) was added, and the reaction mixture was stirred for 24 h.
The solvent was removed by distillation, and the residue was extracted
with ether (150 mL) and filtered, and the solvent was removed. The
resultant paste was stirred with heptane (50 mL) to obtain a pow-
dery solid. The solid was filtered, washed with heptane (2 × 20 mL),
and dried. Yield 8.10 g (83%). Single crystals suitable for an X-ray
study were obtained from ether-heptane (1:1) by slow evaporation.
M.p. 165–166◦C. 31P NMR (CH2Cl2): 118.1. 19F NMR(CDCl3): −74.73
3
1
(t, JFCCH = 8.6 Hz). H NMR(CDCl3): 1.37 (s, 6H, CMe2), 1.53 (s, 6H,
3
3
CMe2), 1.72 (s, 12H, CMe2), 3.84 (qd, JFCCH = 8.6, JPOCH = 5.2 Hz,
2H, POCH2CF3), 6.95 (m, 4H, Ph), 7.12 (m, 6H, Ph), 7.19–7.33 (m, 10H,
Ph), 7.51 (d, 2.3 Hz, 2H, aryl-H), 7.87 (d, 2.3 Hz, 2H, aryl-H). Anal.
calcd. for C50H50O5F3PS: C, 70.57; H, 5.92. Found: C, 70.28; H, 6.05.

1496
A. Chandrasekaran et al.
O2S[C6H2(CMe2Ph)2O]2P(OCH2CF3)3 (2)
A solution of 1 (2.00 g, 2.35 mmol), trifluoroethanol (0.35 mL,
4.79 mmol), and N-chlorodiisopropylamine (0.35 mL, 2.38 mmol) in
dichloromethane (50 mL) was stirred for 2 days. The solvent was re-
moved; the residue was extracted with ether (75 mL) and filtered. The
filtrate was concentrated to 20 mL, and hexane was added (80 mL).
This solution on slow evaporation gave a yellow oil. The oil was re-
crystallized from boiling methanol (30 mL). When the solution came
to about 20 mL, it formed a microcrystalline solid, which was sepa-
rated by filtration, washed with methanol (5 mL), and air dried. Yield
1.3 g (53%). Single crystals suitable for an X-ray study were obtained
from a solution of methanol-dichloromethane (1:1). Two types of crys-
tals formed, one quickly that turned to powder on exposure to air (likely
due to a loss of the solvent) and the other that formed that was sta-
ble to air. The stable crystal was used in the X-ray study. M.p. >143–
145◦C. 31P NMR(CH2Cl2): −71.9, −85.5 (in a 2.8:1 ratio). 31P NMR
(toluene): −72.0, −85.1 (in a 5.7:1 ratio). 31P NMR (CDCl3: 295 K):
−71.7, −85.6 (in a 3.2:1 ratio). 31P NMR (CDCl3: 220 K): −71.3, −86.2 (in
a 4.2:1 ratio). 1H NMR(CDCl3: 295 K): 1.36–1.69 (broad singlets, 30H,
CMe2), 3.16–4.30 (broad multiplets, 6H, POCH2CF3), 6.90–7.80 (broad
singlets and multiplets, 24H, Ph/aryl-H). 1H NMR(CDCl3: 220 K):
1.26–1.72 (broad singlets, 30H, CMe2), 3.14–4.30 (broad multiplets,
6H, POCH2CF3), 6.84–7.80 (Doublets and multiplets, 24H, Ph/aryl-
3
H). 19F NMR(CDCl3): Isomer A (minor): −74.56 (t, JFCCH = 8.1 Hz,
3
3
3F), −75.02 (t, JFCCH = 8.8 Hz, 3F), −76.30 (t, JFCCH = 8.4 Hz, 3F);
3
Isomer B (major): −74.32 (t, JFCCH = 8.3 Hz, 3F), −75.42 (br, 6F).
Anal. calcd. for C54 H54O7F9PS: C, 61.83; H, 5.19. Found: C, 61.68; H,
5.24.
[SC6H2(CMe2Ph)2O]2P(OCH2CF3) (3)
Diphenol 4 (6.3 g, 8.71 mmol) and phosphorus trichloride (0.80 mL,
9.17 mmol) were stirred in dichloromethane (120 mL). Triethylamine
(2.50 mL, 18.0 mmol) was added dropwise over a period of 40 min, and
the solution stirred further for 4 h. Triethylamine (1.30 mL, 9.34 mmol)
and trifluoroethanol (0.70 mL, 9.57 mmol) were added, and the re-
action mixture stirred for 24 h. The solvent was removed by distilla-
tion, and the residue was extracted with ether (150 mL) and filtered.
Heptane (20 mL) was added and left under an argon flow. It formed
a dark oil first, which was removed by decanting the solution. The
mother liquor gave a crystalline solid, which was recrystallized once
again from heptane-ether (10:50 mL). The crystalline solid was filtered,

Penta- and Hexa-coordinated Phosphoranes
1497
washed with heptane (2 × 10 mL), and dried. Yield 2.50 g (34%). Single
crystals suitable for an X-ray study were obtained from ether-heptane
(1:1) by slow evaporation. M.p. 149–151◦C. 31P NMR (CH2Cl2) : 18.1.
19FNMR (CDCl3) : −74.95 (t,3 JFCCH = 8.2Hz). 1HNMR (CDCl3): 1.51
(s, 6H, CMe2), 1.59 (s, 6H, CMe2), 1.66 (s, 12H, CMe2), 2.67 (quin-
3
3
tet, JFCCH = JPOCH = 8.4 Hz, 2H, POCH2 CF3), 6.92 (br, 2H, aryl-
H), 7.07–7.12 (m, 8H, aryl-H), 7.17–7.31 (m, 14H, aryl-H). Anal.
calcd. for C50H50O3F3PS2: C, 70.50; H, 5.92. Found: C, 70.43;
H, 6.05.
S2[C6H2(CMe2Ph)2OH]2 (4) and O2S[C6H2(CMe2Ph)2OH]2 (5)
Sulfur dichloride (5.00 mL, 78.1 mmol) was added to a solution of
bis(dimethylbenzyl)phenol (52.0 g, 157 mmol) in hexane (150 mL) and
stirred. A catalytic amount of zinc chloride catalyst (50 mg) was added,
and stirring continued for 2 days. The greenish solution was stirred
with potassium bicarbonate (15 g) for 30 min, and the yellow solution
filtered. The solvent was removed, and the oil was dissolved in hot acetic
acid (50 mL) and left aside. It gave bright yellow crystals of the diphenol
4, which was filtered, washed with acetic acid (2 × 10 mL), and air-dried
for 2 weeks. Yield 6.7 g (5.9%). M.p. 150–155◦C. 1H NMR (CDCl3): 1.57
(s, 12H, CMe2), 1.64 (s, 12H, CMe2), 5.92 (s, 2H, OH), 7.1–7.3 (m, 24H,
Ph/aryl).
To the remaining acetic acid solution, additional acetic acid (70 mL)
and hydrogen peroxide (30%, 30 mL) were added. The solution was
heated for 2 h. The pasty mass was stirred with water (120 mL), and the
solid filtered. The solid was dissolved in dichloromethane-heptane (125–
50 mL), stirred with potassium bicarbonate and anhydrous magnesium
sulfate for 10 min, and filtered. The filtrate on slow evaporation gave
a crystalline solid of diphenol 5. Yield 25.0 g (in two crops) (22%). M.p.
187–189◦C. 1H NMR (CDCl3): 1.54 (s, 12H, CMe2), 1.60 (s, 12H, CMe2),
7.05 (m, 4H, Ph/aryl), 7.1–7.3 (m, 16H, Ph/aryl), 7.38 (m, 4H, Ph/aryl),
8.29 (s, 2H, OH). Anal. calcd. for C48H50O4S: C, 79.74; H, 6.97. Found:
C, 79.54; H, 6.97.
X-Ray Studies
The X-ray crystallographic studies were performed using a Nonius Kap-
paCCD diffractometer and graphite monochromated MoKα radiation
(λ = 0.71073 A). Data were collected at 293 K, θMoKα ≤ 25◦. All of the
˚
data were included in the refinement. Structures were solved by di-
rect methods and difference Fourier techniques and were refined by

1498
A. Chandrasekaran et al.
full-matrix least squares. Refinements were based on F2 and com-
putations were performed on a 2.6 GHz Pentium 4 computer using
SHELXS-86 for a solution9 and SHELXL-97 for refinement.10 All of the
nonhydrogen atoms were refined anisotropically. All hydrogen atoms
were included in the refinement as isotropic scatterers riding in either
ideal positions or with torsional refinement (in the case of methyl hy-
drogen atoms) on the bonded atoms. The final agreement factors are
based on the reflections with I ≥ 2σI.
RESULTS AND DISCUSSION
Crystallographic data are summarized in Table I. The atom-labeling
schemes for 1–3 are given in the ORTEP plots of Figures 1–
3, respectively. These figures were made using the ORTEP-III for
Windows program.11 Thermal ellipsoids are shown at the 50% prob-
ability level. Selected bond parameters are given in Tables II–IV.
TABLE I Crystallographic Data for Compounds 1–3
Compound
1
2
3
Formula
C50H50F3O5PS
850.93
C54H54F9O7PS
1049.00
Triclinic
P1
C50H50F3O3PS2
850.99
Formula weight
Crystal system
Space group
Triclinic
Triclinic
P1
P1
Crystal size, mm
1.00 × 0.50 × 0.30 0.75 × 0.65 × 0.40
0.95 × 0.90 × 0.50
8.6698 (2)
11.4027 (3)
23.0933 (7)
84.806 (1)
86.872 (1)
76.159 (2)
2206.3 (1)
2
˚
a (A)
15.8463 (4)
16.9769 (4)
18.6606 (3)
78.576 (1)
75.008 (1)
71.039 (1)
4550.3 (2)
4
13.2024 (2)
16.6343 (3)
24.1074 (3)
86.7769 (7)
89.4847 (9)
89.1322 (8)
5285.1 (1)
4
˚
b (A)
˚
c (A)
α (◦)
β (◦)
γ (◦)3
˚
V (A )
Z
Dcalc (g/cm3)
µMoKα (cm−1
Total reflns
1.242
1.318
1.281
2.11
7664
)
1.64
1.73
16007
18330
Reflns with I > 2σI
10735
0.0506
0.1109
14212
0.1215
0.3446
6218
0.1209
0.3217
Ra
Rwb
aR = ꢀ||Fo| − |Fc||/ꢀ|Fo|.
1
4 /
2
2
bRw(Fo2) = {ꢀw(Fo − Fc2)2/ꢀwFo
}
.

Penta- and Hexa-coordinated Phosphoranes
1499
FIGURE 1 An ORTEP diagram of 1 (hydrogen atoms are omitted for
clarity).
Syntheses
Two new diphenols were synthesized for use in this study. The thio-
diphenol with very bulky groups could not be purified by crystal-
lization and only produced a pasty mass. However, the minor impu-
rity, dithio-diphenol 4, crystallized readily from the paste when hot
acetic acid was used as a solvent. The remaining paste was oxidized to
sulfonyl-diphenol 5, which was purified and easily crystallized. Phos-
phoranes similar to 2 were usually synthesized by reacting diphenols
with tris(trifluoroethyl)phosphite.5 However, such a reaction proceeded
very slowly with this very bulky diphenol as tested in an NMR tube,

1500
A. Chandrasekaran et al.
FIGURE 2 An ORTEP diagram of 2 (hydrogen atoms are omitted for clarity).
and the major products were phosphates. Hence, a different approach
was employed. The phosphite starting material 1 was made first and
then treated with two moles of trifluoroethanol in the presence of N-
chlorodiisopropylamine in dichloromethane (Scheme 2). This method
provided a higher yield and a lower amount of phosphates. The re-
sulting phosphorane 2 proved to be very stable to moisture such that
it could be crystallized under atmospheric conditions from methanol-
dichloromethane or ethanol-chloroform. Only very slow hydrolysis was
observed.
When this route was used to prepare a similar starting phosphite
from the dithio-diphenol 4, unexpectedly, phosphorane 3 was obtained
as the final product (Scheme 3). This is possibly due to an intramolecu-
lar oxidation of phosphorus by the sulfur–sulfur bond. Though such an

Penta- and Hexa-coordinated Phosphoranes
1501
FIGURE 3 An ORTEP diagram of 3 (hydrogen atoms are omitted for clarity).
intramolecular oxidative addition is not known, an intermolecular ox-
idative addition of S S bonds to phosphites yielding thiophosphoranes
has been reported earlier.12
Structural Studies
Earlier studies have shown that sulfonyl group oxygen atoms can act
as donors to the phosphorus atom depending on the steric bulk of the

1502
A. Chandrasekaran et al.
SCHEME 2
SCHEME 3
eight-membered ring system and the electron-withdrawing ability of
the other substituents present on the phosphorus atom.5,6,13 For ex-
ample, when there were no bulky groups, there was no donor interac-
tion between oxygen and phosphorus as seen in the phosphorane 12
(Scheme 4).6 In phosphorane 12, the eight-membered ring has a twist-
˚
syn conformation with a P O distance of 3.233(5) A. When bulky groups
were introduced closer to the phosphorus, as seen in phosphoranes 9
and 10,5 the eight-membered ring changed to a syn conformation and a
moderate phosphorus–oxygen donor interaction was observed (with the
˚
P O distances being 2.487(3) and 2.546(9) A, respectively). When the
electronegativity of the phosphorus substituents was increased using
fluorine atoms, the interaction became very strong as seen in phospho-
6
˚
rane 11, where the P O distance was 1.936(7) A.
When the bulky groups were retained and the electronegativity of
the substituents was reduced by using alkyl groups, a very interesting

Penta- and Hexa-coordinated Phosphoranes
1503
SCHEME 4
feature was observed. There were isomers at equilibrium with either a
P O interaction or no interaction, leading to the coexistence of penta-
and hexa-coordinated phosphoranes in equilibrium (as shown in phos-
phoranes 6–8, Scheme 5).5,6 However, all three structures provided
only the pentacoordinated phosphorus geometry with the ring occupy-
ing diequatorial positions with an anti conformation. The nature and
strength of the other isomer remained unknown since it could not be
obtained in a crystalline state.
˚
TABLE II Selected Bond Lengths [A] and Angles [deg] for 1
P(1) O(1)
P(1) O(2)
P(1) O(3)
S(1) O(4)
1.6463(18)
1.6489(16)
1.5972(19)
1.4293(16)
1.4354(17)
P(2) O(6)
P(2) O(7)
P(2) O(8)
S(2) O(9)
1.6476(17)
1.6468(17)
1.6021(18)
1.4324(18)
1.4323(18)
S(1) O(5)
S(2) O(10)
O(1) P(1) O(2)
O(1) P(1) O(3)
O(2) P(1) O(3)
O(4) S(1) O(5)
O(4) S(1) C(6)
O(5) S(1) C(6)
O(4) S(1) C(7)
O(5) S(1) C(7)
C(6) S(1) C(7)
C(1) O(1) P(1)
C(12) O(2) P(1)
C(17) O(3) P(1)
100.80(9)
97.45(10)
96.15(9)
O(6) P(2) O(7)
O(6) P(2) O(8)
O(7) P(2) O(8)
O(9) S(2) O(10)
O(9) S(2) C(6A)
O(9) S(2) C(7A)
O(10) S(2) C(6A)
O(10) S(2) C(7A)
C(6A) S(2) C(7A)
C(1A) O(6) P(2)
C(12A) O(7) P(2)
C(17A) O(8) P(2)
101.83(8)
94.70(9)
95.33(9)
118.27(10)
109.51(10)
105.85(10)
109.53(10)
106.02(11)
107.07(10)
121.74(15)
118.42(13)
125.17(17)
118.60(11)
108.70(11)
109.49(11)
106.12(12)
105.46(10)
108.01(11)
127.21(14)
125.63(15)
120.62(15)

1504
A. Chandrasekaran et al.
˚
TABLE III Selected Bond Lengths [A] and Angles [deg] for 2
P(1) O(1)
P(1) O(2)
P(1) O(3)
P(1) O(4)
P(1) O(5)
P(1) O(6)
S(1) O(6)
S(1) O(7)
1.692(4)
1.647(5)
1.619(5)
1.635(5)
1.594(5)
2.658(5)
1.448(5)
1.433(5)
88.4(2)
P(2) O(11)
P(2) O(12)
P(2) O(13)
P(2) O(14)
P(2) O(15)
P(2) O(16)
S(2) O(16)
S(2) O(17)
1.684(4)
1.641(4)
1.614(4)
1.642(4)
1.599(5)
2.753(5)
1.450(5)
1.434(5)
88.9(2)
O(1) P(1) O(2)
O(1) P(1) O(3)
O(1) P(1) O(4)
O(1) P(1) O(5)
O(2) P(1) O(3)
O(2) P(1) O(4)
O(2) P(1) O(5)
O(3) P(1) O(4)
O(3) P(1) O(5)
O(4) P(1) O(5)
O(1) P(1) O(6)
O(2) P(1) O(6)
O(3) P(1) O(6)
O(4) P(1) O(6)
O(5) P(1) O(6)
O(6) S(1) O(7)
O(6) S(1) C(6)
O(6) S(1) C(7)
O(7) S(1) C(6)
O(7) S(1) C(7)
C(7) S(1) C(6)
C(1) O(1) P(1)
C(12) O(2) P(1)
C(49) O(3) P(1)
C(51) O(4) P(1)
C(53) O(5) P(1)
S(1) O(6) P(1)
O(11) P(2) O(12)
O(11) P(2) O(13)
O(11) P(2) O(14)
O(11) P(2) O(15)
O(12) P(2) O(13)
O(12) P(2) O(14)
O(12) P(2) O(15)
O(13) P(2) O(14)
O(13) P(2) O(15)
O(14) P(2) O(15)
O(11) P(2) O(16)
O(12) P(2) O(16)
O(13) P(2) O(16)
O(14) P(2) O(16)
O(15) P(2) O(16)
O(16) S(2) O(17)
O(16) S(2) C(106)
O(16) S(2) C(107)
O(17) S(2) C(106)
O(17) S(2) C(107)
C(106) S(2) C(107)
C(101) O(11) P(2)
C(112) O(12) P(2)
C(149) O(13) P(2)
C(151) O(14) P(2)
C(153) O(15) P(2)
S(2) O(16) P(2)
87.9(2)
170.1(3)
92.1(2)
149.9(3)
88.5(2)
104.1(3)
90.1(2)
105.9(3)
97.8(3)
88.9(2)
74.6(2)
75.5(2)
87.3(2)
171.2(2)
91.5(2)
146.7(2)
89.1(2)
105.5(2)
89.7(2)
107.7(3)
97.3(2)
87.9(2)
73.1(2)
73.7(2)
81.2(2)
83.3(2)
178.3(2)
117.3(3)
110.6(3)
107.8(3)
108.2(3)
108.6(3)
103.4(3)
127.6(4)
130.6(4)
127.8(4)
130.1(5)
124.1(5)
101.9(2)
178.4(2)
116.4(3)
110.1(3)
107.4(3)
108.3(3)
108.1(3)
106.1(3)
128.1(4)
131.6(4)
126.1(4)
129.4(4)
124.1(5)
99.8(2)
˚
TABLE IV Selected Bond Lengths [A] and Angles [deg] for 3
P
P
P
O(1)
O(3)
S(2)
1.694(5)
1.595(6)
2.085(3)
P
P
O(2)
S(1)
1.711(5)
2.078(3)
O(1)
O(2)
O(2)
O(1)
O(3)
P
P
P
P
P
O(2)
O(3)
S(1)
S(2)
S(2)
174.2(3)
O(1)
O(1)
O(3)
O(2)
S(1)
P
P
P
P
P
O(3)
S(1)
S(1)
S(2)
S(2)
90.3(3)
93.0(2)
113.6(3)
92.1(2)
95.5(3)
85.2(2)
84.9(2)
114.5(3)
131.9(1)

Penta- and Hexa-coordinated Phosphoranes
1505
SCHEME 5
In the present study, with the use of bulky groups, the hexacoor-
dinated isomer became easily stabilized and readily crystallized. This
allowed us to study the structure of this missing component of the penta-
hexa coordination equilibrium we reported earlier.5,6 In solution, also,
the hexacoordinated phosphorane was present in much higher propor-
tion than in earlier cases where pentacoordinated forms remained as
the major isomers.5,6
Table V summarizes the structural changes observed in these
sulfonyl-containing phosphoranes. A gradual change from pentacoordi-
nation to hexacoordination is observed at phosphorus as one descends
down the table.
The Mechanism of Hexacoordination
Generally, it is considered that pentacoordination to hexacoordination
occurs through a Square Pyramidal (SP) geometry from a Trigonal
Bipyramidal (TBP) geometry. A careful analysis of the coordination at
these sulfonyl phosphoranes reveals that in addition to having a square
pyramidal distortion on the pathway toward an octahedron, there is
also a change in the ring orientation. Originally, when no coordination
is present, the eight-membered ring occupies a diequatorial orienta-
tion, as seen in phosphoranes 6–8 (Scheme 5).5,6 However, it changes
to an axial-equatorial orientation before distorting toward the SP ge-
ometry. Such axial-equatorial orientation is seen in phosphorane 12,
where there is no bulky group.6 However, when there are bulky groups,
such axial-equatorial orientations are sterically less favored and bulky
groups are even known to promote phenyl or ethyl groups to occupy ax-
ial positions.14 However, in some cases, an axial-equatorial orientation
is observed despite the presence of bulky groups.15

1506
A. Chandrasekaran et al.
TABLE V The Tendency Toward Hexacoordination Influenced by
Steric and Electronic Effects for a Series of Sulfonyl-Containing
Phosphoranes
Trifluoroethoxy/
phenoxy
P
O
TBP→SP
t-Butyl
Phosphorane groups
Donor
interactiona
distance, A
%
˚
12
6
7
8
2
No
Phenoxy
Alkoxy
Alkoxy
Alkoxy
Alkoxy
No (penta)
3.23
—
—
36e
—
—
Yes
Yes
Yes
Yesb
Equilibrium (penta)
Equilibrium (penta)
Equilibrium (penta)
Equilibrium (hexa)
—
—
2.75,
2.66
2.49
2.55
1.94
2.65
60
68
84
87
91e
—
9
Yes
Yes
Yes
Yes
Phenoxy
Phenoxy
Phenoxyc
Phenoxyc
Yes (hexa)
Yes (hexa)
Yes (hexa)
Yes (pseudo-penta)
10
11
13
d
aEquilibrium denotes the existence of an isomeric penta-hexa coordination equil-
ibrium in the solution. The solid state coordination at phosphorus is given in parenthe-
ses as found by X-ray analysis.
bBulkier α,α-dimethylbenzyl group is used here.
cHighly electronegative pentafluorophenoxy groups are used for 11 and 13.
dPhosphite 13 has P O donor interaction13 that is as strong as that found in
phosphoranes.
eCompounds 11 and 12 deviate considerably from the Berry coordinate.
In Figure 6, it can be clearly seen that the eight-membered ring of
2 is in the pseudo axial-equatorial orientation of a distorted TBP ge-
ometry when the weaker P O16 bond is not considered. It is possible
that the diequatorial ring orientation hinders a distortion toward an
SP and, hence, the coordination proceeds through the axial-equatorial
ring orientational change. The SP distortion seems to be only partial,
which allows one to easily see which ligands were in axial and equa-
torial groups in the original TBP intermediate. From the partial TBP
orientation of 2 in Figure 6, it is apparent that the O11 and O14 are
axially oriented, and O12, O13, and O15 are considered to occupy equa-
torial sites. The P O11 bond is longer than the P O12 bond by about
˚
0.044(4) A. In the pentacoordinated phosphorane 12 (Scheme 3), this
6
˚
difference was 0.065(4) A. Similarly, O13 and O15 have the shortest
distances to the phosphorus. The O11-P-O14 angle of 171.2(2) and the
O12-P-O13 angle of 146.7(2) show a slight movement toward the SP
geometry, but it is very far from the SP geometry where these angles
are expected to be similar. Thus, structural changes encountered for
this series of sulfonyl-containing phosphoranes follow the mechanistic
sequence TBP(ee) → TBP(ae) → [TBP(ae)-SP] → Octahedral.

Penta- and Hexa-coordinated Phosphoranes
1507
NMR SPECTROSCOPY
Isomerism and Fluxionality
As described before, there are two isomers present in solution for phos-
phorane 2. The proton NMR was unresolvable even at −60◦C, and,
hence, it was not useful in making structural assignments. However,
the fluorine NMR clearly shows that there are two isomers. The minor
isomer has three sets of fluorine resonances of equal intensity, which
suggests it to be the pentacoordinated form 2a. The major isomer 2b
has two resonances in an intensity ratio of 1:2. The stronger resonance
was very broad, suggesting an additional inequality or fluxionality pos-
sibly caused by the differences seen in Figure 6 and the rotation of the
dimethylbenzyl groups. These differences are associated with the ori-
entation of the three trifluoroethoxy groups. One is trans to the donor
(attached to O15) and the other two are cis to the donor. Among the two
cis-oriented groups, one is in a pseudo-axial site (attached to O14), and
the other is located in a pseudo-equatorial site (attached to O13). They
all can undergo exchange.
The phosphorus NMR for 2 readily shows two isomers as seen before
in phosphoranes 6–8.5,6 However, as mentioned earlier, the hexacoor-
dinated form is very predominant for 2, whereas in 6–8, the penta-
coordinated form was predominant. The isomer ratio of 2 changed on
cooling (3.2:1 at 295 K to 4.2:1 at 225 K) or on changing the solvent
(2.8:1 in dichloromethane to 5.7:1 in toluene). This suggests that there
is an active equilibrium for 2 in a solution similar to that observed for
6–8.5,6
Chemical Shift Contradiction
Generally, when there is a donor interaction due to an increased elec-
tron density at phosphorus, an upfield chemical shift is expected and
most often is observed. However, in this system of phosphoranes, 2
and 6–8, the pentacoordinated forms have values of about −85 ppm,
whereas the hexacoordinated forms have values of about −72 ppm.
This contradiction is readily explained by the fact that the phospho-
rus resides in between two aromatic ring currents in the pentaco-
ordinated form with a diequatorial orientation of substituents. This
causes a shielding at phosphorus. Though a similar shielding effect has
been understood in proton NMR,1b,5,16 it has not been considered for
other nuclei. In the hexacoordinated form, the phosphorus moves out
of this shielding region. This leads to a loss of shielding and results in
a downfield shift, and from this value, the coordination effect moves
the chemical shift upfield. When the coordination is not too strong,

1508
A. Chandrasekaran et al.
the chemical shift we observe is still downfield to that of the penta-
coordinated phosphorane. A similar downfield shift upon coordination
can be seen in the earlier reported phosphite NMR values of 1416 and
1517 although they were not assigned these structures at that time
(Scheme 6).
SCHEME 6
The Structure of Phosphite 1
The phosphorus atom in phosphite 1 has the eight-membered ring in an
anti conformation (Figure 4) and does not have any donor interaction.
Although phosphorane 2 shows donor interaction (Figure 5) compared
to the less bulky tert-butyl systems in 6–8, phosphite 1 lacks this inter-
action despite the increased bulkiness of substituents. However, a phos-
phite with a less bulky tert-butyl system but with a stronger electron-
withdrawing pentaflourophenoxy group13 showed a fairly strong donor
interaction that was nearly as strong as that of phosphorane systems 2
and 6–8. This shows that there is an interplay of steric and electronic
FIGURE 4 An ORTEP diagram of 1 showing the eight-membered ring confor-
mation with the orientation of substituents at phosphorus and at the opposite
sulfur atom.

Penta- and Hexa-coordinated Phosphoranes
1509
FIGURE 5 An ORTEP diagram of 2 showing the eight-membered ring confor-
mation with the orientation of substituents at phosphorus and at the opposite
sulfur atom.
factors in determining whether the donor interaction takes place or
not.
The Structure and NMR of Phosphorane 3
X-ray analysis of phosphorane 3 (Figure 3) reveals that it has two
thiocatechol-moieties with both sulfur atoms occupying equatorial posi-
tions. This leaves the bulky aryloxy groups in the axial positions and the
FIGURE 6 An ORTEP diagram of 2 (molecule 2) showing the pseudo axial-
equatorial orientation of the eight-membered ring along with the substituents
at phosphorus and at the opposite sulfur atom.

1510
A. Chandrasekaran et al.
trifluoroethoxy group at an equatorial site. This is a unique occurrence
in that no previous structure for any thiocatechol-based phosphorane
is known in any form to compare with this structure. Earlier, we have
observed that the bulky tert–butyl-containing aryloxy groups had the
ability to stay in equatorial positions and allow phenyl or ethyl groups
to reside at axial positions in systems with eight-membered rings.14
However, in phosphorane 3, less bulky sulfur atoms remain at the equa-
torial positions, and the more bulky dimethylbenzyl-containing aryloxy
groups position themselves at the axial sites. NMR data does not show
any fluxional behavior for 3 in contrast to that observed for phospho-
rane 2.
The unusual positive value of 31P NMR for phosphorane 3 is very
much the same as found in other systems. Similar thiocatechol deriva-
tives have been shown to have similar 31P chemical shifts in the
range of 16–19 ppm.18 The trifluoroethoxy protons show a high de-
gree of shielding of more than 1 ppm compared to phosphite 1,
most likely due to the aromatic ring currents of the dimethylbenzyl
groups.
The Stability of Phosphoranes
Phosphorane 2 is very stable to moisture, similar to phosphoranes 6–8.
It was crystallized from alcohol under atmospheric conditions over a
few weeks with very little decomposition. The extremely high stability
provides insight into the pathways through which a possible hydrolysis/
alcoholysis might occur. Such nucleophilic reactions can occur by a P O
bond cleavage through a dissociative pathway or by an associative path-
way. If dissociation is the preferred pathway of hydrolysis, the introduc-
tion of bulky groups will destabilize and promote hydrolysis. However, if
the associative pathway is the preferred route, then introducing bulky
groups will stabilize and reduce or stop hydrolysis. So far, among all
phosphoranes we have studied, phosphoranes with bulky groups are
far more stable than those without bulky groups. If a dissociative path-
way was operative, we would have seen only the hydrolyzed products for
phosphorane 2 when kept in moist alcohols. The absence of such decay
and unexpectedly high stability provides insight, suggesting that the
preferred pathway is associative for hydrolysis/alcoholysis reactions of
phosphoranes in this system.2b,19
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Penta- and Hexa-coordinated Phosphoranes
1511
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