Chiral phosphorus(III) triflates

Article abstract of DOI:10.1039/a702419c

A C₂-symmetric bicyclic diazaphospholidine framework supports a phosphorus centre within an inherently chiral environment which displays both solution- and solid-phase behaviour consistent with ambiphilic phosphenium character.

Full text of DOI:10.1039/a702419c

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Chiral phosphorus(iii) triflates†
Victoria A. Jones, Mark Thornton-Pett and Terence P. Kee*
School of Chemistry, University of Leeds, Woodhouse Lane, Leeds, UK LS2 9JT
A C2-symmetric bicyclic diazaphospholidine framework
supports a phosphorus centre within an inherently chiral
environment which displays both solution- and solid-phase
behaviour consistent with ambiphilic phosphenium charac-
ter.
mol dm23, CH2Cl2), reveal a linear dependence upon tem-
perature between 295 K (dP 269.2) and 203 K (dP 274.2) with a
slope of 25.4 3 1022 ppm K21; higher temperatures
presumably favouring uncoordinated phosphenium cation as
suggested by the low-frequency perturbation of dP. This
3
3
behaviour contrasts with the configurationally stable s l
In recent years, chemists have become increasingly interested in
main-group analogues of carbenes,1 principally those of Si,2
Ge,3 Sn,4 As5 and P.6 Indeed of these heterocarbenes, the
phosphorus derivative 3a-Ph which shows linear behaviour but
in the opposite direction with Dd of +4.2 ppm between 203 and
298 K and slope +4.6 3 1022 ppm K21
.
2
3
phosphorus analogues, s l phosphenium cations, have re-
ceived significant attention over the past 20 years and much is
known of their organic and coordination chemistry.7 However,
we report here a class of phosphenium compound which
possesses the significant ‘added value’ of chirality; an effective
Further evidence for a dynamic interaction between phos-
phorus and triflate in 4a[OTf] is revealed upon examination of
the 1H NMR spectrum. Should 4a[OTf] possess a static
covalent or slowly exchanging configuration at phosphorus,
then the methylene hydrogens on each benzyl substituent (Ha,
Hb) should express themselves as two distinct [ABX] patterns
2
3
s l phosphorus atom possessing both electrophilic and
nucleophilic properties contained within chiral,
a
(Scheme 2; A, X = P Y = triflate). Indeed, in the
C2-symmetric, coordination sphere; an environment which, for
the first time, opens up significant possibilities in asymmetric
synthesis, analysis and catalysis.8
configurationally fixed derivative 3a-Ph (Y = Ph; Scheme 2; A,
Y = Ph), two [ABX] patterns are indeed observed centred at d
4.31 (2JHH 14.7, JPH 14.6 Hz) and 3.67 (2JHH 14,9, JPH 12.7
Hz). However, between 300 and 200 K only a single [ABX]
pattern is observable for the methylene hydrogens of 4a[OTf]
centred at d 4.55 (2JHH 14.5, 3JPH 11.3 Hz), behaviour consistent
with local C2 symmetry at the phosphorus atom of 4a[OTf]
caused presumably by rapid exchange of coordinated (Scheme
2, A) and uncoordinated triflate (Scheme 2, B). It is possible to
perturb the interaction between phosphorus and triflate in
solution via 31P NMR titration experiments. Thus, addition of
3
3
The appropriate synthetic procedures are outlined in Scheme
1.9
Various experiments shed light upon the nature of the
interaction between phosphorus and trifluoromethylsulfonate
(triflate) in 4a[OTf]. A single 31P NMR resonance is observed
at d +271.1 (0.54 mol dm23, 300 K, CH2Cl2), well within the
range expected for two-coordinate phosphenium cations (d
110–515)7 along with intriguing, non-linear concentration-
dependent 31P NMR behaviour at room temp.; a 24-fold
increase in concentration from 0.025–0.6 m results in a Dd of
+5.98 ppm. We interpret such behaviour as evidence of a
dynamic interaction between phosphorus and triflate oxygen(s)
at room temperature leading to a more ‘naked’ phosphenium
cation with slightly lower frequency dP at higher dilution.
Consistently, 31P NMR chemical shifts of 4a[OTf] (0.3
10 equiv. of NBun [OTf] to a CH2Cl2 solution of 4a[OTf] (0.02
4
mol dm23, 300 K) results in a small perturbation to dP Dd of
+4.4 ppm.
Presumably, this perturbation reflects a dynamic process such
that higher concentrations of triflate favour increased inter-
actions with phosphorus and consequently, a shift in dP to high
frequency.
19F NMR and solution conductivity studies are also suppor-
tive of this conclusion. Variable-temperature 19F NMR studies
on 4a[OTf] (CD2Cl2, 203–293 K, referenced to C6F6 at d
H
H
R
N
i
2162.9) reveal behaviour which parallels that of ionic NBun -
NH2
N
4
H2N
H
[OTf] more closely than covalent Me3SiOTf (Fig. 1). Me3SiOTf
H
and NBun [OTf] possess solution molar conductivities L (thf,
1
4
296 K, 0.11 mol dm23) of 0.03 and 3.14 W21 cm2 mol21
,
R
iii
ii
H
H
respectively, whilst the corresponding measurements under the
same conditions for 4a[OTf] reveal a molar conductivity L of
0.43 W21 cm2 mol21. It is clear that 4a[OTf], although formally
a weak electrolyte and less conducting than the ionic triflate
H
N
R
R
N
N
N
H
P
X
H
H
NBun [OTf], is a stronger electrolyte than Me3SiOTf by a factor
4
R
3
R
2
of 14; consistent with significant yet dynamic solution inter-
iv
action between phosphorus and trfilate in 4a[OTf].10
A single-crystal X-ray study of racemic 4a[OTf] reveals a
formally two-coordinate trivalent phosphorus centre in which
H
R
[Y] –
N
N
P
H1
H1
+
H
Ha
Ha
Hc
Hb
R
R
–[Y] –
+[Y] –
N
N
+
P
N
N
P
Y
R
4
H1
H1
Ha
Hd
Hb
Hb
Scheme 1 Reagents and conditions: i, ArCHO; Ar = functionalised phenyl
(Ph a, 4-NMe2C6H4 b, 1-C10H7 c, 2-C10H7 d, 4-MeOC6H4 e); ii, LiAlH4,
thf; iii, PX3 (X = Cl, Br, I), NEt3 (2 equiv.); iv, AlCl3 or Me3SiOSO2CF3
(TMSOTf). [Y] = [AlCl4]2, [OTf]2
R
R
A
B
Scheme 2 Relationship between coordinated (A) and uncoordinated (B) in
4[Y]; Y = e.g. [AlCl4]2, [O3SCF3]2, [C5H5N]
Chem. Commun., 1997
1317

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P–B linkage, presumably {N,NA-[trans-1,2-C6H10(NCH2Ph)2]-
PBH3(thf)}[OTf] (dP 96.2, peak separationn 85 Hz resulting
–78
–79
–80
–81
1
from JPB, thf, 0.34 mol dm23) and (ii) pyridine to afford the
adduct {N,NA-[trans-1,2-C6H10(NCH2Ph)2]P-NC5H5}[OTf] (dP
168.3, CH2Cl2, 0.075 mol dm23).12 Variable-temperature 19F
NMR experiments on the latter reveal behaviour commensurate
1
with ionic triflate character (Fig. 1) whilst H NMR spectro-
scopy reveals that pyridine exchange is rapid on the NMR
timescale at 300 K; a single [ABX] pattern being observed for
the benzyl methylene hydrogens. We are currently building
upon these preliminary observations to examine the potential of
chiral phosphenium salts such as 4a[OTf] in asymmetric
synthesis, analysis, coordination chemistry and catalysis.
We thank the EPSRC for financial support, Albright and
Wilson Ltd, for generous gifts of chemicals, Dr Mimi Hii
(Dyson Perrins) for several valuable suggestions and Professeur
Sylvain Juge (Cergy-Pontoise, France) for most enlightening
exchanges of information.
200
220
240
260
280
300
T / K
Fig. 1 Plot of 19F NMR chemical shifts of 4a[OTf] (-), 4a-Py[OTf] (.),
NBun4[OTf] (8) and Me3SiO3SCF3 (/) vs. temperature (CH2Cl2, 0.1
mol dm23
)
Footnotes
† This ChemComm is also available in enhanced multi-media format via the
World Wide Web: http://chemistry.rsc.org/rsc/cccenha.htm. Supplemen-
tary material (including synthetic, analytical and spectroscopic details for
compounds 1–4) is available electronically upon e-mail request to
T.P.Kee@chem.leeds.ac.uk
F(1)
F(3)
C(5)
C(21)
F(2)
O(1)
S
‡ All new compounds returned satisfactory microanalyses.
O(2)
C(6)
C(1)
§ Crystal structure data for 4a[OTf]: C21H24F3N2PS, M
monoclinic, space group P21/a, 13.961(2),
15.742(2) Å, b 114.846(12)°, U =
2251.9(6) Å3, Z
=
472.45,
11.292(2),
4,
C(4)
O(3)
a
=
b =
c
=
=
=
C(2)
N(1)
C(7)
Dc = 1.39 Mg m23, F(000) = 984, m = 2.399 mm21, T = 160 K; crystal
size 0.70 3 0.34 3 0.08 mm. 3472 reflections were measured at 190 K on
a Stoe STADI4 4-circle diffractometer operating in the w–q scan mode
using graphite monochromated Cu-Ka radiation (l = 1.54184 Å). Data
were corrected for absorption empirically using azimuthal y-scans (max.,
min. transmission factors 0.215, 0.559, respectively). The structure was
solved by direct methods13 and was refined by full-matrix least-squares
C(9)
C(3)
C(10)
C(8)
N(2)
C(14)
P
C(20)
C(11)
C(13)
C(19)
C(15)
C(12)
analysis on F2 using all the unique data.14 wR = {S[w(Fo 2 Fc ) ]/
2
2 2
1
2
C(16)
C(17)
2 2
S[w(Fo ) ]} = 0.1598 for all data, conventional R [on F values of 3280
2
reflections with Fo > 2s(Fo2)] = 0.0713, goodness of fit S = 1.183 on all
C(18)
F2 for 275 parameters. All non-hydrogen atoms were refined with
anisotropic displacement parameters. All hydrogen atoms were constrained
to idealised positions using a riding model. Atomic coordinates, bond
lengths and angles, and thermal parameters have been deposited at the
Cambridge Crystallographic Data Centre (CCDC). See Information for
Authors, Issue No. 1. Any request to the CCDC for his material should quote
the full literature citation and the reference number 182/483.
Fig. 2 Molecular structure of 4a[OTf] with atom numbering scheme.
Selected bond lengths (Å) and angles (°): P–N(1) 1.616(3), P–N(2)
1.625(3), N(1)–C(1) 1.465(5), N(1)–C(7) 1.468(5), N(2)–C(2) 1.470(5),
N(2)–C(14) 1.469(5), P–O(1) 2.841(5), P–O(3A) 2.755(5). N(1)–P–N(2)
94.9(2), C(1)–N(1)–C(7) 119.6(3), C(1)–N(1)–P 113.5(3), C(7)–N(1)–P
126.5(3), C(2)–N(2)–C(14) 121.0(3), C(2)–N(2)–P 112.8(3), C(14)–N(2)–
P 121.4(3).
References
both nitrogen atoms possess significant sp2 character as
evidenced by their trigonal-planar environment, the deviations
of N(1) and N(2) from the best-fit planes connecting atoms
P(1)C(1)C(7) and P(1)C(2)C(14) are 20.05 and +0.19 Å,
respectively [sum of angles around N(1) and N(2), 359.6 and
355.2°, respectively (Fig. 2)]. Such trigonal planarity is
expected to facilitate effective delocalisation of formal positive
charge on phosphorus through P–N p-bonding. Arguably the
most interesting features are connected with the phosphorus–
triflate interaction. P(1) has close contacts to one oxygen atom
of each of two triflate groups in the crystal at distances of 2.841
and 2.755 Å; these interactions are well within the van der
Waals distance for a phosphorus–oxygen interaction (ca. 3.35
Å) but are nevertheless > 1 Å longer than expected for a single
P–O covalent bond (ca. 1.63 Å).11 Furthermore, the two O–P
bond vectors are directed along the axes expected for interaction
with opposite lobes of a vacant phosphorus 3p orbital of a
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11 J. E. Huheey, Inorganic Chemistry. Principles of Structure and
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V. Kyrimis, P. K. Bakshi and T. S. Cameron, Inorg. Chem., 1994, 33,
1434 and references therein.
2
3
formally s l phosphenium cation [the angles between the P–O
vectors and normals to the plane N(1)P(1)N(2) are 11.5 and
10.2°]. The conclusion from the solid state is that phosphorus–
triflate interactions are present although likely to be sig-
nificantly weaker than for a formal P–O single covalent bond,12
in agreement with solution data.
13 G. M. Sheldrick, Acta. Crystallogr., Sect. A, 1990, 46, 467.
14 G. M. Sheldrick, SHELXL-93, program for refinement of crystal
structures, University of Gottingen, 1993.
Ambiphilic behaviour may be demonstrated for 4a[OTf] by
its interaction with (i) BH3·thf to afford a species with a direct
Received in Cambridge, UK, 9th April 1997; Com. 7/02419C
1318
Chem. Commun., 1997