Carbodiphosphorane mediated synthesis of a triflyloxyphosphonium dication and its reactivity towards nucleophiles

Article abstract of DOI:10.1039/c7cc00982h

A carbodiphosphorane ((Ph₃P)₂C) mediated synthesis of the first triflyloxyphosphonium dication (1²⁺) bearing two electrophilic sites is presented. Depending on the nucleophile, 1²⁺ reacts selectively either at the sulfur atom of the triflyloxy moiety or at the directly attached phosphorus atom. In substitution reactions at the phosphorus atom the triflyloxy moiety serves as a leaving group and enables the synthesis of rare examples of pseudo-halophosphonium dications.

Full text of DOI:10.1039/c7cc00982h

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Carbodiphosphorane mediated synthesis of a
triflyloxyphosphonium dication and its reactivity
Cite this:DOI: 10.1039/c7cc00982h
towards nucleophiles†
Received 7th February 2017,
Accepted 10th February 2017
S. Yogendra,a F. Hennersdorf,a A. Bauza,b A. Frontera,b R. Fischerc and
´
J. J. Weigand*a
DOI: 10.1039/c7cc00982h
rsc.li/chemcomm
A carbodiphosphorane ((Ph3P)2C) mediated synthesis of the first
triflyloxyphosphonium dication (12+) bearing two electrophilic sites is
presented. Depending on the nucleophile, 12+ reacts selectively either
at the sulfur atom of the triflyloxy moiety or at the directly attached
phosphorus atom. In substitution reactions at the phosphorus atom
the triflyloxy moiety serves as a leaving group and enables the
synthesis of rare examples of pseudo-halophosphonium dications.
Scheme 1 Proposed synthetic route and reactivity of triflyloxyphospho-
nium dications of type B.
Carbodiphosphoranes as four electron donors with s- and
p-symmetry are considered as strong donor ligands.1 Their donor
abilities were recently used to prepare several main group Lewis
acids,2 including the syntheses of phosphenium dications.3 Phos-
phonium cations are gaining strong reputation as very potent Lewis
acids,4 particularly in the field of FLP chemistry,5 as organocatalyst6,7
for molecule transformations8 and as assisting group in boron
based molecular receptors for selective anion complexation.9
Electron deficient phosphonium dications of type A bearing at least
one electron withdrawing substituent X which can also act as a good
leaving group should allow for facile substitution reactions at the
phosphonium moiety, thus, should enable the synthesis of a variety
of functionalized phosphonium cations (Scheme 1). In this respect,
the well-established and conveniently accessible triflyloxy-group
(OTf) comprises two important functions – firstly, it has electron
withdrawing properties and secondly, the possibility to act as a
suitable leaving group via liberation of the weakly coordinating
triflate anion in nucleophilic substitution reactions. Thus,
triflyloxy-substituted pnictogen derivatives such as Ph3Pn(OTf)2
(Pn = As, Sb, and Bi) were synthesized and used in nucleophilic
substitution reactions with a series of Lewis bases.10 The related
P-based [Ph3P-OTf][OTf] salt was suggested to form as an inter-
mediate in the reaction of Ph3P(O) with Tf2O to give the
well-known Hendrickson reagent [(Ph3P)2O][OTf]2;11 however, to
the best of our knowledge, the triflyloxyphosphonium cations of
type [R3P-OTf]+ have never been isolated to date.
Thus, triflyloxyphosphonium dications of type B should readily
undergo substitution reactions at the P(OTf) atom by releasing the
weakly coordinating triflate anion (Scheme 1). In this contribution,
we present a carbodiphosphorane mediated synthesis of the first
triflyloxyphosphonium salt 1[OTf]2 (Scheme 2), which enables the
selective synthesis of a variety of (pseudo)halophosphonium salts.
The implementation of a phosphanyl moiety onto carbo-
diphosphorane (Ph3P)2C (2) was already reported in 1966 as a
result of the reaction of 2 with Ph2PCl.12 The formed salt 3[Cl] is
conveniently converted to the triflate salt via in situ addition of
Me3SiOTf. 3[OTf] is obtained as a colorless and air-stable powder
in very good yields (90%, Scheme 2). The 31P{1H} NMR spectrum
of 3[OTf] displays the expected AX2 spin system with the triplet
resonance at d(PA) = À1.3 ppm and the doublet resonance at
a Department of Chemistry and Food Chemistry, TU Dresden 01062 Dresden,
Germany. E-mail: jan.weigand@tu-dresden.de
b Department of Chemistry, Universitat de les Illes Balears, Palma de Mallorca, Spain
c Department of Inorganic Chemistry, TU Graz, 8010 Graz, Austria
† Electronic supplementary information (ESI) available. CCDC 1517442–1517448,
1518258 and 1518259. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c7cc00982h
Scheme 2 Preparation of 3[OTf], 4[OTf] and 1[OTf]2; (a) +Me3SiOTf,
t
ÀMe3SiCl, C6H5F, rt, 1 h, 90%; (b) +tBuO2H, À BuOH, CH2Cl2, rt, 30 min,
97%; (c) +Tf2O, C6H5F, rt, 12 h, 98%.
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d(PX) = 26.4 ppm (2JPP = 75 Hz).13 The molecular structure of 31P{1H} NMR spectrum of 1[OTf]2 displays an A2X spin system
cation 3+ displays the expected pyramidalized phosphanyl moiety with a pronounced downfield shifted triplet for the PX atom
and the trigonal planar arrangement of the substituents around (d(PX) = 86.0 ppm; 2JPP = 20 Hz) compared to 4[OTf]. The electron
the methanide moiety (angle sum: 360.00(2)1, Fig. 1). The C(1)–P(2) withdrawing properties of the OTf-substituent cause a deshielding
and C(1)–P(3) bonds (1.751(1) Å and 1.747(1) Å) are shortened of the corresponding PX atom and explain this significant shift.
compared to other phosphonium salts (e.g. [Ph4P]+: P–C(average) This effect, however, does not influence the resonance observed
1.80(1) Å) as a result of the negative hyperconjugation from the for the Ph3P-onio substituents (d(PA)) = 25.8 ppm) and, thus,
p-type lone pair of electrons at C(1) into the s*(P(2)/P(3)–CPh
)
only a slight downfield shift of Dd = 3.0 ppm is observed. The
t
orbitals.14 3[OTf] reacts cleanly with BuO2H affording the phos- 19F NMR spectrum reveals two sharp singlets at d = À71.9 ppm
phane oxide triflate salt 4[OTf] (yield: 96%)12 and displays, in and À78.5 ppm for the bonded OTf-group and for the non-
contrast to 3[OTf], an A2X spin system with resonances observed coordinated OTf anions, respectively. No dynamic behavior for
2
at d(PX) = 26.5 ppm and d(PA) = 22.8 ppm. The JPP coupling the OTf-substituent in solution at temperatures from –28 1C to
constant of 18 Hz is much smaller in magnitude compared to that 72 1C is observed as evidenced by variable temperature 31P-/19F
of 3[OTf] (2JPP = 75 Hz) as a result of the coupling between two NMR spectroscopy.17 The molecular structure of 12+ reveals a
tetra-coordinated P atoms.13 The molecular structure of cation 4+ distorted tetrahedral bonding environment at the P(1) atom with a
(Fig. 1) displays the expected distorted tetrahedral bonding pronounced shortening of the C(1)–P(1) bond (1.731(5) Å) and
environment for the P(1) atom with a typical P–O bond length elongation of the C(1)–P(2) (1.789(5) Å) and C(1)–P(3) (1.777(5) Å)
of 1.489(1) Å.15 The reaction of 4[OTf] with one equivalent of triflic bonds, in contrast to those in cations 3+ and 4+ (Fig. 1 and
anhydride (Tf2O) leads to the formation of the highly moisture Table 1). The shortening of the C(1)–P(1) bond is a result of an
sensitive triflyloxyphosphonium triflate salt 1[OTf]2 in excellent increased negative hyperconjugation from the p-type lone pair of
yield (98%, Scheme 2). We propose that the additional p-donor electrons at C(1) into the s*(P(1)–O(1)) orbital. This redistribution
ability of the (Ph3P)2C moiety leads to a sufficient stabilization of of electron densities from the C(1) lone pair in benefit to the P(1)
the cation 12+ by increasing electron density at the P(1) atom, atom may account for the strong stabilization of the electron
which consequently enhances the Lewis basicity of the O(1) atom. deficient triflyloxyphosphonium moiety. The P–O bond in 12+
The synthesis of 1[OTf]2 succeeds also via the direct oxidation of (P(1)–O(1) 1.676(4) Å) is exceptionally long for a typical PV–O bond
3[OTf] with in situ generated PhI(OTf)2,16 which serves as a strong (cf. Hendrickson reagent [(Ph3P)2O]2+: 1.597(3) Å, 1.598(4) Å18) but
oxidant and an OTf source at the same time. Cation 12+ is formed an expected consequence of the above-mentioned donation of
in solution in a spectroscopic yield of 73% according to 31P NMR electron density into the s*(P(1)–O(1)) orbital (vide supra). This
spectroscopy. The formation of significant amounts of side products17 effect is even more pronounced by the highly electron withdrawing
hampers a high yielding isolation of 1[OTf]2 and, thus, makes properties of the triflyl group – SO2CF3 – and leads to an elongated
the two step method, according to Scheme 2, more efficient. The and strongly activated P–O bond. Similarly, the O(1)–S(1) bond
Fig. 1 Molecular structures of 3+, 4+, 12+, 72+, 82+, 92+ and 102+ (hydrogen atoms are omitted for clarity and thermal ellipsoids are displayed at
50% probability).
Table 1 Selected bond length for crystallographically characterized cations 3+, 4+, 12+, 72+, 82+, 92+ and 102+
Bond length in Å
3+
4+
12+
72+
82+
92+
102+
P(1)–C(1)
P(2)–C(1)
P(3)–C(1)
P(1)–X
1.811(1)
1.751(1)
1.747(1)
—
1.803(2)
1.747(2)
1.754(2)
X = O(1)
1.489(1)
1.731(5)
1.789(5)
1.777(5)
X = O(1)
1.676(4)
1.754(1)
1.780(1)
1.775(1)
X = O(1)
1.562(1)
1.721(3)
1.777(3)
1.784(3)
X = F(1)
1.561(2)
1.754(1)
1.788(3)
1.780(1)
X = C(50)
1.788(1)
1.748(5)
1.777(4)
1.778(4)
X = N(1A)
1.663(7)
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Fig. 2 Left: Two views of the space filling representation of 12+; right:
Mulliken charge distribution in 12+
.
(O(1)–S(1) 1.571(4) Å) is also elongated compared to those observed
for non-coordinating triflate anions (S–O(average) 1.43(3) Å). This
bonding environment at P(OTf) in cation 12+ indicates an electron
deficiency at the S(1)- and P(1) atoms, which was probed in reactions
Scheme 3 Reactions of 1[OTf]2 with different nucleophiles; (I) +DMAP,
À4[OTf], yield (5[OTf]): 90%*; (II) +Ph3P(O), À4[OTf], quant. conversion**;
with a variety of nucleophiles. The computed charge distribution in (III) +PhOH, quant. conversion**; (IV) +AgF, ÀAgOTf, 82%*; (V) +Me3SiCN,
cation 12+ confirms P(1) and S(1) as the most electrophilic sites with
almost identical positive charges (P(1): 0.55 e, S(1): 0.59 e, Fig. 2, right).
Thus, a preference for a nucleophilic attack at the P(1) or S(1) atom by
ÀMe3SiOTf, 88%*, (VI) +NH4SCN, ÀNH4OTf, 73%*; (isolated yields are
marked with *; by 31P/19F NMR spectroscopy**).17
charge discrimination is not likely. However, the space-filling repre- Table 2 Reaction energies (in kcal molÀ1) for the two possible nucleo-
sentation of cation 12+ (Fig. 2, left) indicates a significant difference
philic attacks (P(1)- and S(1) atoms) given at the RI-BP86/def2-TZVP level
of theory
between both electrophilic centers with regard to their accessibility.
FÀ
CNÀ
SCNÀ
DMAP
Ph3P(O)
The P(1) atom is sterically shielded by the close proximity of three
phenyl rings which makes it less accessible for a nucleophilic attack.
In contrast, the S(1) atom of the triflyloxy moiety is by far more
exposed, which consequently allows a better interaction with nucleo-
philes. As a consequence, a size discrimination of cation 12+ in the
reaction with nucleophiles can be anticipated. To probe our hypoth-
DE
P(1)
S(1)
À76.5
À64.4
À48.1
À40.6
À40.6
À33.0
—
À21.2
—
À8.3
D(EP–ES)
12.1
7.5
7.6
—
—
esis, 1[OTf]2 was reacted with selected nucleophiles (Scheme 3). Bulky nucleophilic attacks (P(1) and S(1)) at the BP86-D3/def2-TZVP level of
substituents react preferably at the S(1) atom of cation 12+ and, thus, theory in a continuum solvent model (reaction energies are given
1[OTf]2 gives in a quantitative reaction with 4-dimethylaminiopyridine in Table 2).17 The nucleophilic attacks of FÀ, CNÀ and SCNÀ are
(DMAP) the 1-triflyl-4-dimethylaminopyridinium salt 5[OTf] and barrierless processes due to the dicationic nature of the electrophile.
4[OTf] as a result of the nucleophilic substitution at the S(1) atom The substantial difference is that the nucleophilic attack at the P(1)
and transfer of the SO2CF3-group to DMAP (I). Two sets of single atom is more exergonic than the attack at the S(1) atom (Table 2). This
crystals of 5[OTf] suitable for X-ray diffraction were obtained at result further supports that small nucleophiles attack preferentially at
different temperatures (À30 1C and 21 1C) displaying two monoclinic the P(1) atom. For the sterically more demanding nucleophiles DMAP
polymorphs that vary by their packing mode in the crystal structure and Ph3P(O), only the attack at the S(1) atom is possible, although less
(C2/c vs. P21/n; for further details, see the ESI†). Similar to the reaction favorable than the attack of the anions FÀ, CNÀ and SCNÀ at P(1) due
with DMAP, two equivalents of triphenylphosphane oxide (Ph3P(O)) to the stronger electrostatic attraction (see the ESI†). Phosphonium
react with 1[OTf]2 cleanly to give the Hendrickson reagent salts 7–10[OTf]2 could be conveniently isolated in good to very
[(Ph3P)2O][OTf]2 (6[OTf]2) and 4[OTf] (II). Phenol is converted in a good yields (73–88%) and show in the 31P NMR spectra the char-
1 : 1 reaction into phenyltriflate and the hydroxyphosphonium salt acteristic A2X spin systems. The 31P NMR spectrum of 7[OTf]2 displays
7[OTf]2 (III). 7[OTf]2 displays a rare example of a hydroxyphosphonium a triplet at d(PX) = 50.3 ppm and a doublet at d(PA) = 23.9 ppm
dication and can be alternatively prepared directly via the protonation (2JPP = 21.0 Hz). The hydroxy group of 7[OTf]2 is observable in the
of 4[OTf] with HOTf (yield: 83%). Small nucleophiles should interact 1H NMR spectrum, displaying a downfield shifted singlet reso-
with dication 12+ at the P(1) atom preferably via elimination of nance at d(OH) = 13.93 ppm. The molecular structures of 72+–102+
the triflate anion. 1[OTf]2 reacts cleanly with AgF to give the fluoro- were also confirmed by X-ray analysis and are depicted in Fig. 1.
phosphonium salt 8[OTf]2 and AgOTf (IV), with Me3SiCN to the The molecular structure of 72+ reveals a distorted tetrahedral
cyanophosphonium triflate 9[OTf]2 accompanied by the liberation bonding environment at the P(1) atom.
of Me3SiOTf (V), and with NH4SCN to yield the isothiocyanatophos-
Due to protonation the P–O bond length (P(1)–O(1) 1.562(1) Å)
phonium triflate 10[OTf]2 and NH4OTf. We have computed is elongated compared to that of phosphane oxide 4+ but is still
theoretically for FÀ, CNÀ, SCNÀ, DMAP and Ph3P(O) both possible shorter than the P(1)–O(1) bond in 12+. A short O–HÁ ÁÁO hydrogen
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bond is observed between the hydroxy group and one of the SynPhos: 307616). A. B. and A. F. thank DGICYT of Spain (projects
triflate anions (O(2)–H(1)Á Á ÁO(2) 2.549(2) Å, O(2)–H(1)Á ÁÁO(2) CTQ2014-57393-C2-1-P and CONSOLIDER INGENIO CSD2010–
173(3)1).19 The fluorophosphonium salt 8[OTf]2 displays in the 00065, FEDER funds) for funding.
31P{1H} NMR spectrum a distinct dt resonance at d(PX) =
2
85.9 ppm (1JPF = 1012 Hz; JPP = 21 Hz) and a dd resonance at
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