Competing ring cleavage of transient O-protonated oxaphosphirane complexes: 1,3-oxaphospholane and η2-Wittig ylide complex formation

Article abstract of DOI:10.1039/c0cc02436h

O-Protonation and ring cleavage of oxaphosphirane complexes 1a,b enabled the synthesis of novel compounds such as the bicyclic 1,3-oxaphospholane complex 5 and the η²-Wittig ylide complex 7, which demonstrate the emerging chemistry of this new reactive intermediate. Whereas P-O bond cleavage occurred, in the first case, thus revealing the superior ability of the P-bonded Cp* group to stabilize cationic charge, in the second case competing C-O and P-O bond cleavages occurred, thus leading to a mixture of complexes 3, 4 and 7.

Full text of DOI:10.1039/c0cc02436h

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Competing ring cleavage of transient O-protonated oxaphosphirane
complexes: 1,3-oxaphospholane and g2-Wittig ylide complex formationwz
´
Janaina Marinas Perez, Carolin Albrecht, Holger Helten, Gregor Schnakenburg and
Rainer Streubel*
Received 8th July 2010, Accepted 6th August 2010
DOI: 10.1039/c0cc02436h
O-Protonation and ring cleavage of oxaphosphirane complexes
1a,b enabled the synthesis of novel compounds such as the
bicyclic 1,3-oxaphospholane complex 5 and the Z2-Wittig ylide
complex 7, which demonstrate the emerging chemistry of this
new reactive intermediate. Whereas P–O bond cleavage occurred,
in the first case, thus revealing the superior ability of the
P-bonded Cp* group to stabilize cationic charge, in the second
case competing C–O and P–O bond cleavages occurred, thus
leading to a mixture of complexes 3, 4 and 7.
for the newly formed bond one might obtain the first example
of a Wittig ylide complex of type V.
We report studies on the reactivity of transient O-protonated
oxaphosphirane complexes, either leading to the mixture of a
Z2-Wittig ylide and P–F phosphane complexes or a bicyclic
1,3-oxaphospholane complex. The product formation takes
advantage of the high reactivity of two transient species, one
being the O-protonated oxaphosphirane complex and the
other its acyclic isomer, the methylene phosphonium complex.
Reaction of the oxaphosphirane complexes 1a,15 b16 with
HBF4ꢀEt2O in dichloromethane yielded complexes 3, 4, 5 and
7 (Scheme 2). Presumably, O-protonation of 1a,b occurred in
the first step to yield transiently 2a,b, which can undergo either
P–O bond cleavage to give a diastereomeric mixture of fluoro-
phosphane complexes 3 and 4 in the case of complex 2a (i) or
furnish selectively the novel bicyclic 1,3-oxaphospholane
complex 5 (ii) in a multi-step process (from 2b). In contrast,
the formation of the Z2-Wittig ylide complex 7, obtained as a
mixture with 3 and 4 (ratio 3 : 4 : 7 10 : 65 : 25), requires a
preceding C–O bond cleavage step and a haptotropic shift of
the W(CO)5 moiety to give presumably the transient complex
6a (cf. ref. 14), which then adds fluoride (iii) to give complex 7;
complex 6a could not be detected.
Phosphorus ylides I were introduced by Wittig et al.1 into
olefination chemistry and have become an indispensable tool
in academic and industrial synthesis.2,3 The bonding can be
described by the canonical ylide I and ylen I0 structures
and substituent effects have been widely studied including
structures II and III with C-4–6 and P-metallo7–9 substituents
(Scheme 1). Early on, Schmidbaur realized the potential of I as
ligands10 to create complexes IV11–13 thus opening up a rich
new field of coordination chemistry. Surprisingly and to the
best of our knowledge, no Z2-Wittig ylide complexes of type V
are known, so far.
In light of the studies on side-on methylene phosphonium
complexes, recently obtained via C–O bond cleavage of oxa-
phosphirane complexes using very strong acids with weakly
coordinating anions,14 we became interested in studying acids
that might lead to bond formation of the anion to the carbon
or the phosphorus center. If phosphorus would be preferred
The complex 3 showed a 31P NMR doublet at 198.1 ppm
with a 1JW,P value of 296 Hz and a 1JP,F value of 826.0 Hz. A
very broad signal (Wh1/2 = 420 Hz) at 201.0 ppm with
1
approximately the same JP,F coupling value was tentatively
assigned to complex 4, though its weak intensity and the decompo-
sition of the compound during the column chromatography did
Scheme 1 Wittig ylides I, I0, metallo derivatives II, III and Z1 and Z2
complexes IV and V.
Institut fur Anorganische Chemie der Rheinischen
¨
Friedrich-Wilhelms-Universitat Bonn, Gerhard-Domagk-Str. 1,
¨
53121 Bonn, Germany. E-mail: r.streubel@uni-bonn.de;
Fax: +49 228 739616; Tel: +49 228 735345
w The work is dedicated to Prof. H. Karsch on the occasion of his 65th
birthday.
z Electronic supplementary information (ESI) available: Detailed
analytical data of complexes 1, 3, 4, 5 and 7, further explanations on
the computational results. CCDC 776722 (5). For ESI and crystallo-
graphic data in CIF or other electronic format see DOI: 10.1039/
c0cc02436h
Scheme
2
Reaction of oxaphosphirane complexes 1a,b with
HBF4ꢀEt2O.
c
7244 Chem. Commun., 2010, 46, 7244–7246
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not allow further data to be obtained. The Z2-Wittig ylide
complex 7 displayed a 1JW,P value of 144.0 Hz, which is quite
close to the values of recently reported P–OH substituted
methylene phosphonium side-on complexes that displayed
values between 98–118 Hz (cf. ref. 14). Further strong evidence
1
for the constitution of 7 came from the large JP,F value of
1043.0 Hz, indicating the presence of another electronegative
group being the P–OH group, the presence of which
was independently deduced from the IR spectra through its
absorption at 3373 (br) cmꢁ1 17
The P–F functionality would
.
also help to understand the relative increase in the 1JW,P value.
A further strong argument for the side-on coordination of the
W(CO)5 moiety in complex 7, besides the observed phosphorus
tungsten coupling, came from the 13C{1H} NMR spectra,
which revealed the presence of only one resonance for the
pentacarbonyltungsten group, typical for a side-on bonded
W(CO)5 group (cf. ref. 14), at 197.5 ppm having a 2JP,C value
of 10.9 Hz. The only example of a (thermally labile) ylide
tungsten complex of type IV18 did not show any phosphorus–
carbon couplings for the carbonyl groups and, instead,
exhibited the cis-CO and trans-CO resonances in the 13C NMR
spectra as expected. The 31P{1H} NMR spectra of complex 5
showed a doublet at 215.6 ppm (1JW,P = 288.4 Hz) due to
Scheme 3 Computed acid-induced ring opening of complexes 1a,b.
in Table 1. For the C–O bond cleavage (reaction II) both the
free energy of activation and the free energy of reaction are
almost equal for the two differently substituted oxaphosphirane
complexes. However, a remarkable difference is found for the
P–O bond breaking mode (i). The barrier is significantly lower
in the case of R = C5Me5 and the reaction is exergonic for this
derivative. The reason derives from the different extent of
positive charge stabilization in the reactive intermediates 8a
and 8b, respectively. Complex 8a constitutes a phosphenium
complex (see ESIz, Fig. S2), wherein the phosphorus–metal
bonding situation is characterized by cooperative s-donor and
p-acceptor bonds, thus giving rise to a rather short P–W
distance (2.438 A).20 In complex 8b the P–W bond is con-
siderably longer (2.592 A), thus being in the range of W(CO)5
phosphirane complexes. Here, the Cp* ring is Z2-bound to
phosphorus, as it was found also for the non-coordinated
phosphenium ion [P(NHtBu)(C5Me5)]+ by Niecke et al.21 The
positive charge of 8b is transferred to the former Cp* ring,
where it is stabilized through delocalization. Our calculations
further revealed that 8b should have a fluxional structure with
a sequence of degenerate [1.5]-sigmatropic rearrangements
(circumambulatory rearrangement),22 for which the barrier
1
the JP,F coupling (892.6 Hz). The structure of 5 was also
unambiguously confirmed for the solid state by single-crystal
X-ray crystallography (Fig. 1).
The bicyclic structure in complex 5 shows the transforma-
tion of the (former) Cp* group into a partially unsaturated
five-membered ring having a C–C double bond (C(10)–C(11)
1.331(4) A) and a (newly) established C–O bond (O(1)–C(9)
1.461(3) A), which is slightly longer than the O(1)–C(1) bond
with 1.419(3) A; the reason for the latter was not apparent.
In order to find an explanation for the divergent behavior of
the two oxaphosphirane complexes 1a,b with respect to P–O
vs. C–O bond cleavage, both reactions were computed by
+
means of DFT calculations starting from 1a,b–HOEt2
,
where the acid proton, mediated through diethyl ether,19 is
bound to the oxaphosphirane oxygen center via O–H–O
hydrogen bonding (Scheme 3); relative free energies are given
was predicted to be only DGa298 = 31.3 kJ molꢁ1
.
The formation of complexes 3, 4, 5 and 7 can be explained
by assuming O-protonation of 1a and 1b to be the first step,
which induces a kinetically preferred P–O bond cleavage.
However, in the case of 1a the reaction is endergonic and,
hence, complex 8a may partly react back to 1a and via reaction
II to give complex 6a. A transfer of fluoride from tetrafluoro-
borate onto either 7a to give 3 or onto 6a to afford complex 7
occurs. In the case of 1b, the reaction I is irreversible and rapid
fluoride transfer onto complex 8b seems likely to occur, which
Table 1 Calculated thermochemical data for reactions shown in
Scheme 3 (all values in kJ molꢁ; B3LYP/aug-TZVP/ECP-60-MWB(W),
COSMO (CH2Cl2)//RI-BLYP/aug-SV(P)/ECP-60-MWB(W), COSMO
(CH2Cl2))
Fig. 1 Structure of complex 5 (50% probability level; hydrogen
atoms except H1 and H12 are omitted for clarity). Selected bond
lengths [A] and angles [1]: W–P 2.4431(6), P–C(1) 1.843(3), P–C(8)
1.843(3), P–F 1.5975(15), C(1)–O(1) 1.419(3), O(1)–C(9) 1.461(3),
C(8)–C(9) 1.573(4), C(9)–C(10) 1.512(4), C(10)–C(11) 1.331(4),
C(11)–C(12) 1.507(4), C(8)–C12 1.558(4); F–P–W 107.86(6),
C(1)–P–F 97.95(10), C(1)–P–C(8) 91.14(12), P–C(1)–O(1) 114.13(17),
C(1)–O(1)–C(9) 114.12(19), O(1)–C(9)–C(8) 111.2(2), C(9)–C(8)–P
102–92(17), C(12)–C(8)–P 111.72(16).
Reaction
I
a: R = CH(SiMe3)2
b: R = C5Me5
DGa298
+19.3
+2.5
+7.6
ꢁ15.2
+54.9
ꢁ46.5
DRG298
II
DGa298
+51.3
ꢁ40.2
DRG298
c
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then is followed by 1,4-addition of the pending OH group
to the butadiene system of the Cp* substituent to yield
complex 5.
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Complexes 3, 4, 5 and 7: complex 1a or 1b (150 mg,
0.240 mmol) was dissolved in CH2Cl2 (1.5 mL) and HBF4ꢀEt2O
(33 mL, 0.240 mmol) was added at ambient temperature. The
mixture was stirred for 10 min and the solvent was then
removed in vacuo (4 ꢂ 10ꢁ2 mbar). In the case of complex 3,
the residue was subjected to low-temperature chromato-
graphy, whereby the last fraction yielded complex 3 as a white
oil. As complex 7 decomposed during the chromatography it
was characterized from a mixture with 3 and 4. In the case of
complex 5, washing of the residue with n-pentane yielded the
product as a white powder.y
3 (91.4 mg, 59%) (found: C 35.82; H 4.20%. C19H26FO6PSi2W
requires C 35.65, H 4.09%); dC (75.5 MHz, CDCl3; 30 1C) 1.6
(dd, JP,C 1.9, JP,F 1.2, Si(CH3)3), 2.0 (dd, JP,C 1.9, JP,F
3
4
3
4
10 H. Schmidbaur, Angew. Chem., Int. Ed. Engl., 1983, 22, 907–927;
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1.2, Si(CH3)3), 30.0 (mc br, CH((SiCH3)3)2), 78.1 (quasiq,
1JP,C
E
2JF,C 17, PCOH), 126.6 (dd, JP,C 3.8, JF,C = 2.3,
Ph), 127.8 (d, JP,C 1.9, Ph), 128.0 (d, JP,C 2.3 Hz, Ph), 136.7
2
(s; i-Ph), 195.0 (dd, JP,C 7.7, JF,C 3.5, cis-CO), 197.5
3
2
(dd, JP,C 30.1, JF,C 0.9, trans-CO).
3
13 A. Wang, H. Sun and X. Li, Organometallics, 2009, 28, 5285–5288.
14 J. M. Perez, H. Helten, B. Donnadieu, C. A. Reed and R. Streubel,
´
5 (99 mg, 67 %) (found: C 42.55, H 3.51%. C22H22FO6PW
requires C 42.88, H 3.60%); dC (75.5 MHz, CDCl3; 25 1C) 9.5
(s; C-CH3), 11.7 (d, JP,C 7.7, PC-CH3), 12.5 (s, C-CH3), 15.3
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1995, 33, 2427).
2
3
(d, JP,C 8.3, CH-CH3), 21.0 (d, JP,C 2.4, OC-CH3), 48.5
(d, JP,C 20.3, C-CH3), 57.5 (dd, JF,C 16.7, JP,C 8.9, PC-CH3),
85.7 (dd, JF,C 14.9, JP,C 11.9, PC-Ph), 98.6 (d, JP,C 3.6,
PC(H)OC), 128.5 (d, JP,C 1.8, Ph), 128.8–128.9 (m, Ph),
132.1 (d, JP,C 1.2, i-Ph), 132.3 (s, CQC), 138.6 (d, JP,C 4.2;
16 M. Bode, J. M. Perez, G. Schnakenburg, J. Daniels and
´
R. Streubel, Z. Anorg. Allg. Chem., 2009, 635, 1163–1171.
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having P–F and P–OH functions were reported by:
A. Marinnetti and F. Mathey, Phosphorus Sulfur Relat. Elem.,
1984, 19, 311–317.
2
3
CQC), 194.4 (dd, JP,C 7.7, JF,C 3.0, JW,C 125.2, cis-CO)
197.2 (dd, JP,C 31.0, JF,C 1.8, trans-CO).
2
3
7 dC (75.5 MHz; CDCl3, 30 1C) ꢁ0.2 (d, 3JP,C 4.3, Si(CH3)3),
18 F. R. Kreissl, E. O. Fischer, C. G. Kreiter and H. Fischer, Chem.
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0.0 (d, 3JP,C 3.0, Si(CH3)3), 13.0 (d, 1JP,C 17, CH(SiMe3)2), 66
2
(broad; CHPh), 126.8 (s, Ph), 127.8 (d, JP,C 1.9, Ph), 127.3
19 Due to the acid strength of HBF4 it is conceivable to assume that
protonated diethyl ether is the protonating agent; see e.g.,
(a) D. Farca-siu and D. Hancu, J. Chem. Soc., Faraday Trans.,
1997, 93, 2161–2165; I. A. Koppel, P. Burk, I. Koppel, I. Leito,
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5114–5124.
2
(d, JP,C 2.2, Ph), 136.0 (s; i-Ph), 197.5 (d, JP,C 10.9, CO).
Financial support by Thermphos AG Intl. and the COST
action CM0802 ‘‘PhoSciNet’’ is gratefully acknowledged.
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´
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y X-Ray crystallographic analysis: colorless single crystals were
obtained from concentrated n-pentane/CH2Cl2 solutions upon slow
cooling to 4 1C. C22H22FO6PW; crystal size 0.60 ꢂ 0.31 ꢂ 0.27 mm,
monoclinic, P21/c,
a = 10.4284(2) A, b = 6.56720(10) A,
c = 33.0807(7) A, b = 96.3597(9)1, V = 2251.60(7) A3, Z = 4, 2ymax
561, collected (independent) reflections 33 860 (5407), Rint = 0.0727,
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Chem. Soc., 2006, 128, 12169–12173.
m
= , 286 refined parameters, 0 restraints, R1
5.245 mmꢁ1
(for I > 2s(I)) = 0.0223, wR2 (for all data = 0.0589, max./min.
residual electron density) 1.964/ꢁ1.590 e Aꢁ3 23
.
1 G. Wittig and U. Schollkopf, Chem. Ber., 1954, 87, 1318–1330;
¨
G. Wittig and W. Haag, Chem. Ber., 1955, 88, 1654–1666.
23 G. M. Sheldrick, Acta Crystallogr., Sect. A, 2008, 64,
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c
7246 Chem. Commun., 2010, 46, 7244–7246
This journal is The Royal Society of Chemistry 2010