Sulfoxygenation catalysed by oxidovanadium complexes

Article abstract of DOI:10.1002/ejic.200800772

Model complexes of vanadate-dependent peroxidases, viz. oxidovanadium(V) complexes of the general composition [VO(H_n-xL)], have been prepared and characterised. H_nL is an n-basic di- or trichiral aminodiethanol [HOCH(Ph)-CH₂]₂NR, with R = N-methylimidazolyl (H ₂L^A), tris(hydroxymethyl) methyl (H₅L ^B, 2 isomers), 2,3-dihydroxypropyl (H₄L^C, 2 isomers) and 2-hydroxy-3-trityloxypropyl (H₃L^D). These ligands form the complexes [VO(OH)(L^A)], [VO(H₂L ^B)], trigonal-bipyramidal [VO(HL^C)]_tbp and octahedral Λ-[VO-(HL^C)]_oct. The ligands R,R,S-H₄L^C and R,R,R-H₃L^D, and the complexes Λ-[VO(R,R,S-HL^C)]_oct and [VO(R,R,S-HL^C)]_tbp have been characterised by X-ray structure analysis. The complexes [VO(H₂L^B)] and [VO(HL^C)] were immobilised on Merrifield and Barlos resin by anchoring through a free alcoholic group in the ligand side-chain R. {[VO(H ₂O)L^E]}, an oxidovanadium(IV) complex tethered to Merrifield resin, was prepared by treating [VO(acac)₂] with {HL ^E}, the immobilised Schiff base ligand obtained by condensation of salicylaldehyde with resin-anchored lysine. The complexes and in situ systems ([VO(OiPr)₃] + ligand) as well as the immobilised complexes were investigated for their catalytic activity in the oxygenation, by cumyl hydroperoxide, of thioanisol to its sulfoxide. All of the systems were active, with a selectivity (sulfoxide vs. sulfone) of 95-98%. Satisfactory turnover rates and a chiral induction up to 37% ee were observed. Wiley-VCH Verlag GmbH & Co. KGaA, 2008.

Full text of DOI:10.1002/ejic.200800772

FULL PAPER
DOI: 10.1002/ejic.200800772
Sulfoxygenation Catalysed by Oxidovanadium Complexes
Pingsong Wu,[a] Cem Çelik,[a] Gabriella Santoni,[a] Jérome Dallery,[a] and Dieter Rehder*[a]
Keywords: Oxidovanadium complexes / Ethanolamines / Sulfoxygenation / Models of peroxidases / Vanadate-dependent
peroxidase
Model complexes of vanadate-dependent peroxidases, viz.
oxidovanadium(V) complexes of the general composition
[VO(Hn–xL)], have been prepared and characterised. HnL is
an n-basic di- or trichiral aminodiethanol [HOCH(Ph)-
coholic group in the ligand side-chain R. {[VO(H2O)LE]}, an
oxidovanadium(IV) complex tethered to Merrifield resin, was
prepared by treating [VO(acac)2] with {HLE}, the immobilised
Schiff base ligand obtained by condensation of salicylalde-
CH2]2NR, with R = N-methylimidazolyl (H2LA), tris(hydroxy- hyde with resin-anchored lysine. The complexes and in situ
methyl)methyl (H5LB, 2 isomers), 2,3-dihydroxypropyl (H4LC,
2 isomers) and 2-hydroxy-3-trityloxypropyl (H3LD). These li-
gands form the complexes [VO(OH)(LA)], [VO(H2LB)], trigo-
nal-bipyramidal [VO(HLC)]tbp and octahedral Λ-[VO-
(HLC)]oct. The ligands R,R,S-H4LC and R,R,R-H3LD, and the
complexes Λ-[VO(R,R,S-HLC)]oct and [VO(R,R,S-HLC)]tbp
have been characterised by X-ray structure analysis. The
complexes [VO(H2LB)] and [VO(HLC)] were immobilised on
Merrifield and Barlos resin by anchoring through a free al-
systems ([VO(OiPr)3] + ligand) as well as the immobilised
complexes were investigated for their catalytic activity in the
oxygenation, by cumyl hydroperoxide, of thioanisol to its
sulfoxide. All of the systems were active, with a selectivity
(sulfoxide vs. sulfone) of 95–98%. Satisfactory turnover rates
and a chiral induction up to 37% ee were observed.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2008)
sea weed Ascophyllum nodosum are also capable of catalys-
Introduction
ing sulfoxygenations.[4,5] These enzymes contain vanadate
Chiral sulfoxides are important synthons in organic [H2VO4]– in their active centre linked to a histidine of the
chemistry, in particular for enantioselective carbon-carbon protein matrix.[6] Vanadium is in a trigonal-bipyramidal en-
bond formation. The general importance of sulfoxides in vironment, with the histidine Nε and a hydroxo group in
inorganic synthesis and in medication, and approaches to the apical positions. In these enzymes, vanadium(V) does
their stereoselective synthesis have recently been surveyed.[1] not function by switching between different oxidation states
In addition, sulfoxides play a critical role in biological ac- (as commonly observed in oxidation reactions catalysed by
tivity, e.g. in the formation of dimethyl sulfoxide from di- vanadium compounds[7]) but as a Lewis acid through acti-
methyl sulfide as the initiating step in the oxidative conver- vation, by coordination, of peroxide. The genuine reaction
sion of organic sulfur (such as dimethyl sulfide and thio- catalysed by these enzymes is the two-electron oxidation of
phene) to inorganic sulfur compounds. Several enzymes are halide by hydrogen peroxide to, e.g., hypohalous acid
able to catalyse the oxygenation of sulfides to sulfoxides. [Equation (1), upper trace], a reaction which consumes pro-
Examples are ammonia monooxygenase from Nitrosomonas tons.[8] Similarly, sulfides can be oxygenated to sulfones
europae,[2a] bacterial cyclohexane monooxygenases[2b] and [Equation (1), lower trace] and the reaction is usually enan-
dioxygenase from Pseudomonas putida.[3] Haloperoxidases tioselective if prochiral sulfides are employed. Protons are
isolated from a variety of marine macro-algae such as the not consumed in sulfoxygenation but they are, however,
(1)
necessary as cocatalysts. The reaction path has been evalu-
[a] Chemistry Department, University of Hamburg,
ated by DFT calculations, according to which the rate-limit-
20146 Hamburg, Germany
E-mail: rehder@chemie.uni-hamburg.de
ing step in the oxygenation of sulfide to sulfoxide is the
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P. Wu, C. Çelik, G. Santoni, J. Dallery, D. Rehder
FULL PAPER
simultaneous formation of the S–O bond and breaking of
the peroxo O–O bond.[9]
Results and Discussion
Stimulated by the potential of algal peroxidases in sulf-
oxygenation, model reactions were developed in which vana-
dium-based catalysts were employed to (enantioselectively)
Characterisation of Ligands and Complexes
Scheme 1 provides an overview of ligands and complexes
oxidise sulfides,[10–12] disulfides,[12a,13] thioacetals[12a] and which have been characterised and used in homogeneous
thianthrene[14] to the respective sulfoxides, using H2O2, or- catalysis in the present study. The ligands are derived from
ganic peroxides or peroxo acids as oxidants. The catalysts dichiral diethanolamines, with an additional functionalised
typically employed are in situ systems consisting of substituent on the amine nitrogen. They are prepared by
[VO(acac)2] [acac = acetylacetonate(1–)] or [VO(ORЈ)3] (RЈ treating R-styrene oxide with the primary amines H2NR in
= iPr, tBu) as the vanadium component and a chiral tri- a 1:2 molar ratio and subsequent workup by flash-
functional Schiff base ligand, or preformed vanadium (pre)- chromatography on silica gel. A racemic amine was em-
catalysts such as [VO(O2)heida]– [H2heida = N-(2-hydroxy- ployed in the case of R = 1,2-dihydroxypropyl (Ǟ H4LC).
ethyl)iminodiacetic acid].[15]
The main product thus obtained results from the 1,3-cleav-
Our approach to enzyme models has been the design of age of the epoxide (H2LA, H5LB and H4LC) along with
vanadium complexes mimicking the coordination environ- same product from the 1,2-cleavage (H5LBЈ, H4LCЈ) and
ment of vanadate-dependent haloperoxidases with respect minor amounts of secondary amines. H3LD was obtained
to both the nature of the donor set and the coordination by treatment of H4LC with trityl chloride in the presence of
geometry. These structural and functional mimics are repre- N(iPr)2Et (Hünig’s base). The ligands H2LA, H5LB, H5LBЈ,
sented by oxidovanadium(V) complexes formed with chiral H4LC and H4LC’ were identified by their spectroscopic
ligands
derived from
tridentate
diethanolamines characteristics (see Exp. Section). The trichiral potentially
(HOCH(Ph)CH2)2NR, H2L.[16–19] The present work is an pentadentate H4LC and the tetradentate H3LD have been
extension to ligands additionally functionalised in the side- structurally characterised by XRD. For selected structure
chain represented by R, providing a more rigid coordina- parameters see Table 1, for drawings see Figure 1. An inter-
tion sphere for VO3+ and the possibility of anchoring the esting feature in the structure of H4LC is the comparatively
vanadium site to a solid substrate, Merrifield and Barlos short distance between N1 and O3 (the β-OH of the pro-
resins in this case.
panediol moiety) of 2.82 Å which is less than the sum of
Scheme 1.
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Sulfoxygenation Catalysed by Oxidovanadium Complexes
Figure 1. Structure representations of the ligands of H4LC and H3LD.
the van der Waals radii (3.05 Å). This proximity is induced pation of the aromatic imidazole-N and thus an O4N2 donor
1
1
by “proton sharing” between N1 and O3. The N···H···O set is provided by H NMR spectroscopy: the H coordina-
angle is 118.5°.
tion shift for the C–H proton adjacent to the coordinating N
amounts typically to 0.31 ppm.[22] The 51V NMR spectrum
of a solution of [VO(OH)LA] in CD2Cl2 shows two signals at
–495 and –501 ppm (integral intensities 1:0.7) which are
clearly to high field, by 30–60 ppm, of the δ(51V) shift of the
trigonal-bipyramidal oxidovanadium complexes containing
an O4N donor set.[16,18] Such a high-field shift is expected on
going from penta- to hexacoordination.[23] The presence of
two signals reflects the presence of two isomers. Addition of
cumyl hydroperoxide (CmO2H) gives rise to an additional
high-field shift (–536 ppm, one broad resonance) through
the formation of the peroxido complex [VO(O2Cm)-
LA]. Such a high-field shift is generally observed on exchang-
ing OR–, OH– or O2– for peroxide.[22,24] Since peroxide en-
hances binding of imidazole,[22] the coordination skeleton
provided by the tetradentate ligand supposedly remains in-
tact. The FAB mass spectrum of [VO(OH)LA] shows the
mass peak at m/z = 433 with low intensity. The main peak,
at m/z = 416, corresponds to the loss of an OH radical. For
other typical peaks corresponding to the fragmentation of
[VO(LA)] see Scheme 2. The V=O stretch at 924 cm–1 is com-
paratively low [commonly, ν(V=O) values are around
970 cm–1] suggesting involvement of the oxido group in inter-
molecular hydrogen bonding or intramolecular “smearing” of
the proton between OH and the doubly bonded O. A weak
IR band at 506 cm–1 present in the complex but not in the
ligand further supports the formulation [VO(OH)LA] with an
oxido and hydroxido ligand, by analogy with, e.g., [VO(OH)-
oxin] [oxin = 8-oxiquinolinate(1–)][25] and [VO(OH)TpЈ] [TpЈ
= tris(3,5-diisopropyl-1-pyrazolyl)borate(1–)].[26] Presumably,
the OH ligand in our case originates from the hydrolysis of a
[VO(OiPr)LA] intermediate in the presence of residual
amounts of water.
Table 1. Selected bond lengths [Å] and angles [°] for H4LC and
H3LD.[a]
H4LC
H3LD
av. d(N1–C)
C7–O1
C27–O2
C32–O3
C33–O4
1.464
av. d(N1–C)
C17–O1
C27–O2
C32–O3
C33–O4
C34–O4
av. N1
N1–C18–C17
N1–C28–C27
av. C32
O4–C33–C32
C33–O4–C34
(OH)1···O3
(OH)2···O3
(OH)3···O2
(OH)2···N1
1.475
1.426(3)
1.431(3)
1.420(3)
1.426(3)
1.441(3)
1.429(4)
1.423(3)
1.425(3)
1.448(3)
114.9
110.9(3)
110.0(3)
110.8
107.1(2)
119.0(2)
2.778
3.114
2.707
av. N1
113.7
N1–C8–C7
N1–C28–C27
av. C32
113.68(18)
111.03(18)
110.1
O4–C33–C32
110.43(18)
(OH)1···O2
(OH)2···O4
(OH)4···O1
2.850
2.818
2.866
2.721
[a] For one of the two molecules in the asymmetric unit.
As we have shown previously,[16] tridentate aminodiethan-
ols H2L react with [VO(OiPr)3] to form the pentacoordinate,
somewhat distorted trigonal-bipyramidal complex [VO(OiPr)-
L], which converts to [VO(OMe)L] in methanol. Reaction of
ligand H2LA (containing the imidazolyl substituent) with
[VO(OiPr)3] in THF yields an oxidovanadium(V) complex of
the likely composition [VO(OH)LA], with the octahedral co-
ordination environment proposed in Scheme 1. The imidazole
moiety in multidentate ligands can coordinate to or remain
uncoordinated from the oxidovanadium unit. An example for
the former is the complex [VIVO(salim)acac] (salim is the
Schiff base formed between salicylaldehyde and 2-aminoethyl-
imidazole)[20] and an example for the latter is [VVO2(nap-his)]
The complex [VO(H2LB)], with the preliminary formula-
(nap-his is the Schiff base formed from o-hydroxynaphth- tion depicted in Scheme 1, forms on treating [VO(OiPr)3]
aldehyde and histidine).[21] The main evidence for partici- with H5LB in polar solvents. The chemical shift δ(51V) in
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Scheme 2.
freshly prepared solutions in CD3Cl is –411 ppm. After in Figure 2. For selected structural parameters see Table 2.
standing, the solutions turn green due to formation of an In both structures, the ligand is in the R,R,S-configuration
oxidovanadium(IV) complex, possibly of composition (S configuration for C32).
[VO(H3LB)], the EPR parameters of which are AЌ = 68.0,
Aʈ = 172.0ϫ10–4 cm–1; gЌ = 1.988, gʈ = 1.950.
Reaction of H4LC with [VO(OiPr)3] in anhydrous ethanol
under N2 yields a red-brown solution and a light yellow
precipitate. From the solution, yellow-green cubic crystals
of the hexacoordinate complex [VO(HLC)]oct·C2H5OH were
obtained in which all five ligand functions (the nitrogen,
three deprotonated alcoholate groups and the protonated
terminal propanol function) are coordinated to vanadium.
From DMSO solutions of the precipitate, light yellow
needle-like crystals of the pentacoordinate complex [VO-
(HLC)]tbp were isolated. In [VO(HLC)]tbp, the terminal pro-
panol ϪOH remains dangling. Apparently, the polarity of
the solvent determines which of the structural variants is
favoured. The ν(V=O) for both complexes, at 967 and
965 cm–1, respectively, are in the expected range. The 51V
chemical shift for solutions of [VO(HLC)]oct is –411 ppm in
CD3OD and –396 ppm in [D6]DMSO. The respective value
for [VO(HLC)]tbp in [D6]DMSO is –381 ppm. These similar
chemical shifts suggest that the solution structures are
about the same, and the rather low shielding suggests that
the structure corresponds to an approximately ideal trigo-
nal bipyramid in both cases. Trigonal-bipyramidal com-
plexes of composition [VO(OMe)L], where L is the dianion
of a tridentate aminoethanol, give rise to two signals in the
51V NMR spectrum due to the presence of isomers with the
oxido group in the equatorial and axial positions, respec-
tively. An example is the distorted trigonal-bipyramidal (τ
= 0.72) complex derived from H2L = {HOCH(Ph)-
CH2}2NCH2CO(OtBu), with δ(51V) values of –449 and
–470 ppm.[16] In solutions of [VO(HLC)], with the tetraden-
Figure 2. Structure representations of the octahedral and trigonal-
tate ligand (HLC)3– providing a rigid coordination environ-
ment, there is only one species present in which the oxido
ligand is in the axial position, see [VO(HLC)]tbp in
bipyramidal complexes of composition [VO(HLC)].
In the case of [VO(HLC)]oct·C2H5OH, there are two inde-
Scheme 1. [VO(HLC)]oct and [VO(HLC)]tbp have been struc- pendent molecules in the asymmetric unit with very similar
turally characterised. The solid-state structures are shown bonding characteristics. The terminal hydroxy group (O14)
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Sulfoxygenation Catalysed by Oxidovanadium Complexes
Table 2. Selected bond lengths [Å] and angles [°] for [VO(HLC)]oct
·
Immobilisation on Merrifield and Barlos Resin
C2H5OH and [VO(HLC)]tbp
.
Merrifield and Barlos resins, copolymers based on styr-
ene and (ca. 1% of) divinylbenzene contain a small percen-
tage of p-(chloromethyl)phenyl (Merrifield) or 2-chlorotrityl
chloride (Barlos) side chains. The resins, which are widely
used in biochemical contexts, have been employed here to
ascertain the integrity of the vanadium complexes which
would be otherwise jeopardised by the presence of acidic
groups such as in silica gels.[27] Tethering of an oxidovana-
dium complex to such a resin side-chain, subsequently sym-
bolised by {}, was achieved with ligands H5LB, H4LC and
H4LCЈ. Ligand H3LD, with a tertiary butyl group attached
to the primary oxo group of the propanediol side-chain,
models that structural unit of Barlos resin which anchors
to the ligand system and demonstrates that the primary al-
coholic group is the one which primarily reacts with the
resin.
Λ-[VO(HLC)]oct
·
[VO(HLC)]tbp
C2H5OH[a]
V1–O1
V1–N1
V1–O11
V1–O12
V1–O13
V1–O14
O1–V1–N1
O11–V1–O12
O11–V1–O13
O11–V1–O14
O12–V1–O13
1.6065(17)
2.297(2)
V1–O1
V1–N1
V1–O2
V1–O3
V1–O4
V1–O5
O1–V1–N1
O2–V1–O3
O2–V1–O4
O2–V1–O5
O3–V1–O4
1.6074(15)
2.2987(17)
1.8183(16)
1.8024(15)
1.8097(16)
–
179.74(10)
115.38(8)
117.73(8)
–
1.8236(17)
1.8423(18)
1.9324(18)
2.0753(19)
178.15(9)
102.39(8)
89.97(8)
158.38(9)
149.77(8)
116.33(8)
[a] For one of the two molecules in the asymmetric unit.
The procedure with the Merrifield resin was carried out
in two steps: In step (i) the ligand H4LC, after conversion
to (H3LC)–Na+ with NaH dissolved in THF, was anchored
to the resin to yield the resin loaded with the ligand,
{HnLC} (n ≈ 3; 0.27 mmol of ligand per g of resin). In step
(ii) {HnLC} was treated with a THF solution containing
[VO(OiPr)3] and stirred overnight, washed and dried to
yield {[VO(LC)]} (0.22 mmol vanadium per g resin). The
integrity of the complex was confirmed by IR and 51V
NMR spectroscopy. In the corresponding reaction with
H5LB the original red-brown colour (indicative of
{[VVO(HLB)]}) changed to green, suggesting the formation
of {[VIVO(H2LB)]}, Figure 3, i.e., a gradual reduction which
has also been observed with the nonimmobilised complex
(see above). EDX analysis revealed the presence of vana-
dium, Figure 3.
in the propane side-chain gives rise to a comparatively long
d(V–OH) = 2.075(2) Å. Together with the three hydroxido
functions [d(V–O) = 1.823–1.932 Å], this hydroxy group
forms the octahedral plane. The vanadium is only 0.041 Å
displaced from this plane. One of the apical positions is
occupied by the doubly bonded oxygen and the other by
nitrogen at the rather long distance of 2.297(2) Å, a conse-
quence of the trans influence by the oxido ligand. The two
independent molecules are linked by rather short intermo-
lecular hydrogen bonds between (OH)14···O23 (2.449) and
O13···(HO)24. In addition, weak hydrogen bonds are pres-
ent between the two ethanol molecules in the crystal
(2.856 Å), and ethanol and O12 (2.840 Å). There are no
distinct differences in bonding parameters between the free
ligand H4LC and the coordinated ligand (HLC)3–, with the
exception of the angles at C8/C18 and C33 which are nar-
rower in the complex than in the free ligand. The chiral
vanadium centre is in the anticlockwise (Λ) configuration.
The ligand arrangement in [VO(HLC)]tbp is trigonal bi-
pyramidal. The complex thus structurally resembles related
vanadium complexes [VO(OR)L] containing tridentate ami-
nodiethanolato ligands.[16–19] There are, however, also dis-
tinct differences: while, in the complexes [VO(OR)L], the
doubly bonded oxido group is in the equatorial position in
the crystalline solid state, it occupies an axial position in
[VO(HLC)]tbp as a consequence of steric demands imparted
by the tetradentate ligand. This arrangement also implies
an almost ideal trigonal-bipyramidal structure while, in
[VO(OR)L] (H2L = tridentate diethanolamine; R = Me or
iPr), there is always some distortion towards the square
pyramid: τ parameters are commonly around 0.73 (τ = 1
for an ideal trigonal bipyramid, τ = 0 for an ideal square
pyramid). As a consequence of the different geometries,
most of the bond lengths and angles differ largely between
Figure 3. EDX spectrum of [VO(OiPr)H2LB] anchored to Merri-
field resin (inset) showing the bands for C/N, O, Cl (residual
chlorido groups of the resin) and V. The Cu peaks are due to the
carrier material.
With Barlos resin, direct anchoring of the complex
[VO(HLC)]tbp and [VO(HLC)]oct, see Table 2. In [VO- [VO(HLC)] was carried out in CH2Cl2 in the presence of
(HLC)]tbp, the vanadium is 0.3439 Å above the trigonal Hünig’s base to yield {[VO(LC)]} (0.34 mmol V per g resin).
plane formed by the three coordinating ϪO– functions (O2, The corresponding reaction of Barlos resin with [VOH2LB]
O3, O4). The dangling ϪOH is hydrogen bonded to the in DMSO/CH2Cl2 resulted in the formation of
coordinated O2 of a neighbouring molecule and O2···(HO)5 {[VO(HnLB)]} (n ≈ 1; loading: 0.28 mmol V per g resin).
= 2.851 Å.
The stability and integrity of the complex systems formed
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P. Wu, C. Çelik, G. Santoni, J. Dallery, D. Rehder
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Scheme 3.
with [VO(HnLC)] were examined by 51V NMR of mixtures in the presence of NEt3, with Boc-protected lysine (Boc =
of the complex with Hünig’s base, and of the complex with tert-butyloxycarbonyl). After deprotection with trifluoro-
trityl chloride plus Hünig’s base (trityl chloride mimicking acetic acid (TFA), the lysine derivative of the resin was con-
the functional unit of Barlos resin) in the same molar ratio verted into the Schiff base derivative {HLE} by condensa-
as in the immobilisation reaction. The chemical shifts of tion with salicylaldehyde. Further reaction with [VO-
these mixtures were identical within the limits of shift dif- (acac)2] yielded the vanadium complex tethered to the Mer-
ferences imparted by medium effects, Figure 4.
rifield resin, {[VO(H2O)LE]}. Support for complexation
comes from the disappearance, in the IR, of the phenol-OH
band and the appearance of a new V=O stretching band at
985 cm–1. Water as a neutral fifth ligand was added provi-
sionally in order to satisfy the common pentacoordination
found in oxidovanadium(IV) complexes of Schiff bases.[28]
Sulfoxygenation Reactions
Figure 4. Comparison of the 51V NMR spectra of (a) complex
[VO(HLC)], (b) [VO(HLC)] + Hünig’s base, and (c) [VO(HLC)] +
Hünig’s base + trityl chloride. The minor signals at high field are
due to an unidentified secondary product.
The reactions have been carried out with thioanisol
(MeSPh) using cumyl hydroperoxide (CmO2H) as the oxi-
dant, in chlorinated hydrocarbons. The overall reaction,
Equation (2), corresponds to that for the sulfoxygenation of
sulfides with H2O2 catalysed by vanadate-dependent halo-
Generation of an oxidovanadium(IV) complex contain- peroxidases, see Eq. (1) in the Introduction. The precatalyst
ing a Schiff base ligand (HLE) tethered to a Merrifield resin can either be the presynthesised complex itself, or the com-
was achieved as shown in Scheme 3. The resin was treated, plex formed in situ from the precursor [VO(OiPr)3] and the
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Sulfoxygenation Catalysed by Oxidovanadium Complexes
(2)
ligand. The active catalyst is a peroxido complex, contain- troscopy (shift of the signal for the methyl protons from
ing the cumylperoxido ligand, viz. [VO(O2Cm)L], possibly 1.59 ppm for CmO2H to 1.56 ppm for CmOH). In addition,
(as suggested by DFT calculations[16]) in the bent end-on ligand-free peroxovanadates [HVO3(O2)]2– (–630 ppm) and
coordination mode. The formation of the peroxido interme- [HVO2(O2)2]2– (–683 ppm) are observed.
diate is visible by a colour change from yellow to orange
Selected results for the reactions catalysed under homo-
and reflected in the 51V NMR spectrum (see the above dis- geneous conditions by [VO(OH)LA], [VO(H2LB)] and
cussion for [VO(OH)LA]). The substrate MeSPh nucleophil- [VO(HLC)], and under heterogeneous conditions catalysed
ically attacks the activated peroxide to form the sulfoxide by the immobilised systems {[VO(HnLB)]}, {[VO(LC)]} and
and, usually, a small percentage of sulfone as well. The va- {[VO(H2O)LE]} are summarised in Table 3. The time course
nadium complex is partly recovered in the form of for the reaction catalysed by [VO(OH)LA] as followed by
[VO(OCm)L], again evidenced by 51V NMR spectroscopy 1H NMR spectroscopy is represented graphically in Fig-
1
(–480 and –490 ppm in the case of LA) and H NMR spec- ure 5 for 24 °C (left) and 0 °C (right).
Table 3. Collation of data for the catalytically conducted sulfoxygenation of thioanisol (methyl phenyl sulfide).[a]
Complex[b]
Conversion (%)
Time [h]
SO/SO2
ee (%), isomer
[c]
[VO(OH)LA]
76
89
60
0.8
3
3
3
3
98:2
97:3
95:5
95:5
97:3
98:2
87:13
97:3
98:2
98:2
9, R
2, S
37, R
7, S
5, S
6, S
14, S
16, S
19, S
16, S
[VO(H2LB)]
H5LB + VO(OiPr)3
H5LBЈ + VO(OiPr)3
[VO(HLC)][d]
89
100
100
100
100
100
95
H4LC + VO(OiPr)3
{[VO(H2LB)]} (Barlos)
{[VO(HLC)]} (Merrifield)
{[VO(HLCЈ)]}[e] (Barlos)
{[VO(H2O)LE]}[f] (Merrifield)
3
24
0.7
24
6
[a] In CHCl3 at 0 °C, c(sulfide) = 0.1 , sulfide/oxidant/catalyst ratio = 1:1:0.1 (homogeneous) or 1:1:0.01 (heterogeneous systems). [b]
Curly bracket {} represent complexes immobilised by tethering to the Merrifield or Barlos resin. [c] SO = sulfoxide, SO2 = sulfone. [d]
The trigonal-bipyramidal isomer has been employed in these experiments. [e] In CH2Cl2. [f] In toluene at room temp.; 1 mol-% catalyst.
Figure 5. Time course of the oxygenation of thioanisol (S) by cumyl hydroperoxide (CHP), catalysed by [VO(OH)LA] (cat). Conditions:
c(S) = c(CHP) = 0.1 , c(cat) = 0.01 ; solvent CH2Cl2; temperature 24 °C (left) and 0 °C (right). SO = sulfoxide, SO2 = sulfone, OH =
cumyl alcohol.
Eur. J. Inorg. Chem. 2008, 5203–5213
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
5209

P. Wu, C. Çelik, G. Santoni, J. Dallery, D. Rehder
FULL PAPER
As evidenced by 51V NMR spectroscopy in the homo- and enantioselectivities (ee) vary greatly with the systems
geneous systems, the active species is also the peroxo inter- under investigation. Chiral inversion with respect to the cat-
mediate where a mixture of the ligand and the vanadium alysts which have been employed in the R configurations,
precursor [VO(OiPr)3] is employed. Furthermore, most of occurs in most cases. The ee values are notoriously low: the
the catalyst remains intact and the catalytic cycles can be best result, an ee = 37%, was obtained with the in situ sys-
repeated several times without substantial abatement in tem VO(OiPr)3 + H5LB [R = tris(hydroxymethyl)methyl].
conversion rates. Particularly stable are the heterogeneous The selectivities, 95–98% sulfoxide, are satisfactory
catalysts.
throughout. The immobilised systems employed here are
superior to those which have been obtained by anchoring
vanadium complexes to silica gels and mesoporous silica
gels, where (partial) decomposition and thus deactivation
of the complex occurs.[27]
Conclusions
We have prepared chiral diethanolamines of the general
composition {HOCH(Ph)CH2}2NR, where R is a function-
alised (by imidazole or alcoholic groups) side-chain and
employed these ligands in the synthesis of oxidovanadi-
um(V) complexes with the vanadium centre in a trigonal-
bipyramidal or octahedral environment. Interestingly, the
complex [VO(HLC)] (R = 1,2-hydroxypropyl) has been crys-
tallised as a pentacoordinate, trigonal-bipyramidal and a
hexacoordinate octahedral variant. The OH functionalis-
ation in the side-chain R has been exploited for immobilis-
ing the complexes on Merrifield and Barlos resins. These
complexes model the active centre of vanadate-dependent
peroxidases from marine macro-algae and they also model
the sulfoxygenation activity of these enzymes, i.e. the con-
version of prochiral thioanisol to chiral sulfoxide plus some
sulfone, with cumyl hydroperoxide as the oxidant, both in
homogeneous catalysis (with the isolated complexes or the
in situ systems, i.e. vanadium precursor + ligand) and in
heterogeneous catalysis (with the immobilised complexes).
Experimental Section
General and Instrumental Methods: Standard chemicals were ob-
tained from commercial sources and used without further purifica-
tion. Silica gel for chromatography was purchased from Merck
(silica gel 60, 0.040–0.063 mm). Solvents were dried according to
standard methods and distilled under argon and the reactions car-
ried out by employing Schlenk techniques. Elemental analyses:
C,N,H,O rapid analyser (Heraeus and Carlo–Erba); EDX: SEM
Hitachi-S2500 instrument, equipped with a Genesis system with
sapphire Si(Li)-detector (EDAX/AMETEK Inc.); ICP-OES:
SPECTRO CIROS CCD spectrometer. IR spectra: KBr mulls, Per-
kin–Elmer FTIR 1720 FT spectrometer; ATR-IR spectra were re-
corded on a Bruker VERTEX 70 FTIR spectrometer. NMR spec-
tra: Varian Gemini 200 or Bruker Avance 400 with the usual instru-
ment settings. 51V chemical shifts are referenced to VOCl3. FAB
MS: VG-Analytical 70–250S; matrix: m-nitrobenzyl alcohol, xenon
beam.
Single crystal X-ray measurements were carried out at 153(2) K
The selectivity (ratio sulfoxide/sulfone), conversion rates according to the ω-scan method on a SMART Apex CCD (Bruker)
Table 4. Crystal data and details of the structure determination.
H4LC
H3LD
Λ-[VO(R,R,S-HLC)]oct
C2H5OH
·
[VO(R,R,S-HLC)]tbp
Emperical formula
M [gmol–1]
Crystal system
Space group
a [Å]
C19H25NO4
331.40
C38H39NO4
573.70
C21H28NO6V
441.38
tetragonal
P41
11.5705(5)
11.5705(5)
31.755(2)
4251.3(4)
8
C19H22NO5V
395.32
orthorhombic
P212121
orthorhombic
P212121
9.3360(8)
10.7743(10)
17.3992(15)
1750.2(3)
4
1.258
0.088
712
orthorhombic
P212121
7.2471(8)
17.8576(19)
23.712(3)
3068.7(6)
4
1.242
0.080
1224
7.7973(8)
b [Å]
11.9461(12)
18.8807(19)
1758.7(3)
c [Å]
V [Å3]
Z
4
ρ(calcd.) [gcm–3]
µ(Mo-Kα) [mm–1]
F(000)
1.379
0.503
1856
1.493
0.595
824
Crystal size [mm]
θ (min., max.) [°]
Index ranges
0.38 ϫ 0.29 ϫ 0.12 0.50 ϫ 0.10 ϫ 0.07 0.43 ϫ 0.29 ϫ 0.14
0.50 ϫ 0.07 ϫ 0.05
2.02, 27.49
–10 Յ h Յ 10,
–15 Յ k Յ 15,
–24 Յ l Յ 24
21132 / 0.0742
4018 / 1 / 239
0.876
2.22, 27.50
2.82, 27.50
1.87, 27.50
–12 Յ h Յ11,
–13 Յ k Յ 13,
–22 Յ l Յ 22
20789 / 0.0454
2284 / 14 / 237
0.997
–9 Յ h Յ 9,
–23 Յ k Յ 23,
–30 Յ l Յ 30
36989 / 0.1362
3985 / 0 / 391
0.803
–14 Յ h Յ15,
–15 Յ k Յ 15,
–41 Յ l Յ 41
51404 / 0.0783
9699 / 3 / 533
0.754
Total data / R(int.)
Data / restraints / parameters
Gof on F2
R1, wR2 [I Ͼ 2σ(I)]
R1, wR2 (all data)
0.0398, 0.0923
0.0457, 0.0668
0.0404, 0.0486
0.0695, 0.0526
–0.005(12)
0.0368, 0.0623
0.0526, 0.0663
0.01(2)
0.0504, 0.0963
0.1084, 0.0782
[a]
[a]
Flack parameter
Max. / min. residual density [e Å–3]
0.328 / –0.166
0.210 / –0.222
0.603 / –0.343
0.584 / –0.323
[a] The absolute structure could not be determined reliably because of the light atom composition. The configuration indicated for the
ligands in the text is based on that found for the ligands in the complexes.
5210
www.eurjic.org
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2008, 5203–5213

Sulfoxygenation Catalysed by Oxidovanadium Complexes
diffractometer equipped with a graphite monochromator and a
Mo-Kα radiation source. Frames were read with the SAINT pro-
gram, absorption corrections were carried out with SADABS. For
the determination of the space group the program XPREP was
employed, for the solution and refinement of the structures the
program systems SHELXS-97 and SHELXL-97 were utilised. The
following hydrogen atoms were found: H4LC: O3H, and N1H
Scheme 4): δ = 144.89 (4), 140.05 (8), 138.68 (11), 128.86 (2), 127.68
(1), 126.95 (3), 119.66 (9), 71.11 (5), 64.42 (6), 52.29 (7), 33.55
(10) ppm. FAB-MS: m/z = 352.3 [M+ + 1, 88], 244.2 (M+
–
PhCH2OH, 100). C21H25N3O2 (351.45): calcd. C 71.79, H 7.12, N
11.97; found C 71.53, H 7.13, N 11.57. FAB MS: m/z = (assign-
ment): 352 (M +1), 244 (M + 1 – C6H5CH2OH).
N,N-Bis(2-hydroxy-2-phenylethyl)-N-[tris(hydroxymethyl)methyl]-
amine (H5LB), and N-(2-Hydroxy-2-phenylethyl)-N-(2-hydroxy-1-
phenylethyl)-N-[tris(hydroxymethyl)methyl]amine (H5LBЈ): Styrene
oxide (1.43 mL, 12.5 mmol) and tris(hydroxy)methylamine
(758 mg, 6.3 mmol) dissolved in 2-propanol (15 mL) were heated
to reflux for 4 d and the product purified by column chromatog-
raphy on silica gel using hexane/ethyl acetate followed by ethyl ace-
tate/ethanol with gradually increasing polarity. Both products thus
isolated, H5LB (52% yield) and H5LBЈ (22% yield) are colourless,
sticky, gel-like substances. C20H27NO5 (361.44): calcd. C 66.46, H
[H disordered between O3 and N1]; d[O3–H
= 0.840(1),
d(N1–H) = 0.880(1)], O4H, [VO(HLC)]oct·C2H5OH: O14H, O24 H,
[VO(HLC)]tbp: O5H. All other H atoms were calculated in ideal
positions. Crystal data and details of the data collection and refine-
ment are summarised in Table 4.
The sulfoxygenation reactions were carried out in chloroform,
dichloromethane or toluene solution containing equimolar (0.1 )
amounts of thioanisol and cumyl hydroperoxide and 10 (homogen-
eous system) or 1 mol-% (resins) of the catalyst, at room tempera-
ture and/or 0 °C. The progress of the reaction over time was moni-
1
7.53, N 3.88; found for H5LB C 65.82, H 7.49, N 3.41. H NMR
([D6]acetone, cf. Scheme 5)]: R,R-H5LB: δ = 7.36 (m, 10, Ph), 4.94
(ABX) and 2.91 + 2.78 [(ABX), 6 H, 2 and 4], 3.75 (s, 6 H, 3) ppm.
R,R-H5LBЈ: δ = 7.35 (m, 10, Ph), 4.80 (ABX) and 2.91 + 2.78
(ABX) (3 H, 2 and 4), 4.15 (ABX) and 3.58 + 3.44 (ABX) (3 H, 2Ј
and 4Ј), 3.55 (s, 6 H, 3) ppm. 13C NMR ([D6]acetone, assignments
printed in italics, cf. Scheme 5): R,R-H5LB: δ = 72.4 (2), 64.4
[C(CH2OH)3], 63.9 (3), 58.5 (4) ppm. R,R-H5LBЈ: δ = 73.5 (2), 67.1
(2Ј), 64.4 [C(CH2OH)3], 62.6 (4Ј), 62.0 (3), 52.4 (4) ppm.
1
tored by H NMR spectroscopy (methyl protons of the reactants
and reaction products). The enantiomeric excess was determined
1
through the intensity ratio of the H NMR signals of the methyl
protons of the sulfoxide which split into two components by forma-
tion of two diastereomeric adducts (R,R and R,S) on addition of
pirkle alcohol, R-(–)-1-(9-anthranyl)-2,2,2-trifluoroethanol.
CCDC-667532 (for H4LC), -692716 (for H3LD), -669382 (for Λ-
[VO(R,R,S-HLC)]oct·C2H5OH) and -691872 (for [VO(R,R,S-
HLC)]tbp) contain the supplementary crystallographic data for this
paper. These data can be obtained free of charge from The Cam-
bridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
Preparation of Ligands and Complexes
N,N-Bis(2-hydroxyphenylethyl)-N-(1-methyl-1H-imidazol-4-yl)-
methylamine (H2LA): R-Styrene oxide (2.047 mL, 17.76 mmol) dis-
solved in absolute tert-butyl methyl ether (BME) (15 mL) were
treated, under reflux conditions, over 3 h with (1-methyl-1H-imid-
azol-4-yl)methylamine (987 mg, 8.9 mmol) dispensed from a drop-
ping funnel. The dropping funnel was rinsed twice with BME
(25 mL) and the combined solutions heated to reflux until complete
conversion of the epoxide (5 d). The solvent was removed in vacuo,
the orange-yellow residue (3.04 g) dissolved in CH2Cl2/2-propanol
(10:1) and chromatographed on silica gel (450 g, column diameter
8 cm). The fraction containing H2LA was evaporated to dryness
in vacuo at 65 °C to yield 1.29 g (41%) of yellowish, oily H2LA.
C21H25N3O2 (351.45): calcd. C 71.79, H 7.12, N 11.97; found C
71.53, H 7.13, N 11.57. IR (KBr): ν = 3435 (ν, OH), 2932 (and
˜
2886 (ν, CH2)), 2828 (R2N–CH2), 1631 (C=C), 1510 (C=N), 1451
(δ, CH3), 1329 (δ, OH), 1094 (ν, CO) cm–1. 1H NMR ([D6]acetone,
number of protons, assignments printed in italics, cf. Scheme 4), δ
= 7.44 (s, 1 H, j), 7.40 (d, 4 H, c), 7.29 (t, 4 H, b), 7.20 (t, 2 H, a),
6.95 (s, 1 H, h), 4.87 (dd, 3.3 and 9.9, 2 H, d), 3.91 (d, 14.9, 1 H,
f), 3.76 (d, 14.9, 1 H, g), 3.63 (s, 3 H, i), 2.70 (dd, 14.1, 4 H, e and
eЈ) ppm. 13C NMR ([D6]acetone, assignments printed in italics, cf.
Scheme 5.
N,N-Bis(2-hydroxy-2-phenylethyl)-N-(2,3-dihydroxypropyl)amine
(R,R,S-H4LC) and N-(2-Hydroxy-2-phenylethyl)-N-(2-hydroxy-1-
phenylethyl)-N-(2,3-dihydroxypropyl)amine (H4LCЈ): R-Styrene ox-
ide (1.43 mL, 12.5 mmol) and 3-amino-1,2-propanediol (0.487 mL,
6.3 mmol) were dissolved in 2-propanol (10 mL) in a small flask
and heated to reflux for 6 d. By purification using chromatography
on silica gel as described for H5LB, two fractions were isolated. The
first fraction, after evaporation, yielded a colourless white solid of
R,R,S-H4LC (59% of the overall amount of product formed). The
second fraction was a colourless liquid consisting of a mixture of
the isomers H4LCЈ (32%). C19H25NO4 (331.40): calcd. C 68.80, H
1
Scheme 4.
7.54, N 4.22; found for H4LC C 68.24, H 7.58, N 4.10. H NMR
Eur. J. Inorg. Chem. 2008, 5203–5213
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
5211

P. Wu, C. Çelik, G. Santoni, J. Dallery, D. Rehder
FULL PAPER
(CD3OH, number of protons, assignments printed in italics, cf.
analysis: 0.27 mmol of ligand per g of resin. {HnLB} (165 mg ) was
suspended in anhydrous CHCl3 containing [VO(OiPr)3] (9.5 µL)
Scheme 5): R,R,S-H4LC: δ = 7.30 (10, Ph), 4.82 (ABX) and 2.80 +
2.72 (ABX) (6 H, 2 and 4), 3.78 (m, 1 H, 3Ј), 3.53 (m, 2 H, 3), 2.90 (corresponding to a ligand/V ratio of 1:1). The vanadium-loaded,
and 2.73 (m, 6 H, 4 and 5) ppm. R,R,S-H4LCЈ: δ = 7.33 (m, 10 H,
Phc), 4.82 (ABX) and 2.81 + 2.63 (ABX) (3 H, 2 and 4), 3.98 (ABX)
and 4.01 + 3.78 (ABX) (3 H, 2Ј and 4Ј), 3.70 (1 H, 3Ј), 3.39 and 3.47
(m, 2 H, 3), 2.80 and 2.53 (m, 2 H, 5) ppm. 13C NMR (CD3OD,
assignments printed in italics, cf. Scheme 5): H4LC: δ = 70.8 (2), 68.1
(3Ј), 64.01 (3), 63.9 (3), 58.7 (5) ppm. H4LCЈ: δ = 69.1 (2), 68.0 (3Ј),
66.2 (4Ј), 62.1 (3), 60.0 (4), 52.5 (5) ppm.
initially red-brown resin, {[VO(HnLB)]}, changed colour to green
on stirring overnight. It was filtered off, washed with CHCl3, THF
and again with CHCl3, then dried. See Figure 2 in the Discussion
section for the EDX analysis.
To a suspension of [VO(H2LB)] (96.6 mg, 0.23 mmol) in CH2Cl2
(1 mL) was added DMSO (0.4 mL) which resulted in the formation
of a clear solution. To this solution was added Barlos resin
(101.8 mg) and Hünig’s base (0.16 mL, 0.92 mmol). After 24 h of
stirring, the loaded resin, {[VO(HnLB)]}, was filtered, washed six
times with anhydrous CH2Cl2 then dried in vacuo. Vanadium
analysis: found 1.74%, corresponding to a loading of 0.28 mmol
N,N-Bis(2-hydroxy-2-phenylethyl)-N-(2-hydroxy-3-triphenylmeth-
oxypropyl)amine (R,R,R-H4LD): H4LC (100.9 mg, 0.31 mmol) was
dissolved in CH2Cl2 (4 mL) and treated with trityl chloride
(85.3 mg, 0.31 mmol) and Hünig’s base (0.101 mL, 0.58 mmol).
The reaction mixture was stirred for 24 h, the solvent evaporated
and the residue redissolved in anhydrous acetone. Slow evaporation
within a few days afforded needle-shaped colourless crystals in 98%
yield. 1H NMR (CDCl3, number of protons, assignments printed in
italics, cf. Scheme 5): δ = 7.33 (m, 25 H, Ph), 4.70 (ABX) and 2.80
+ 2.76 (ABX) (6 H, 2 and 4), 3.92 (m, 1 H, 3Ј), 3.20 (m, 2 H, 3),
2.60 and 2.49 (m, 2 H, 5) ppm. 13C NMR (CDCl3, assignments
printed in italics, cf. Scheme 5): δ = 87 (CPh3), 71 (2), 70 (3), 68 (5),
63 (4), 58 (3Ј) ppm.
[VO(OH)LA]: H2LA (78.6 mg, 0.224 mmol) dissolved in THF
(3 mL) was treated with [VO(OiPr)3] (54.6 mg, 52.7 µL,
0.224 mmol) and stirred for 12 h. From the yellow-orange solution,
the solvent was removed in vacuo. The residue was redissolved in
[D8]THF and stored at –20 °C for 24 h. The red-brown precipitate
of [VO(OH)(LA)] was recovered and dried. Yield 29.2 mg (31.3%).
of V per g of the resin. IR (KBr): ν = 978 (V=O) cm–1.
˜
Λ-[VO(R,R,S-HLC)]oct, [VO(R,R,S-HLC)]tbp and Immobilisation on
Merrifield and Barlos Resin, {[VO(LC)]}: H4LC (350 mg,
1.06 mmol) was dissolved in anhydrous ethanol (2 mL) and treated
dropwise and with stirring with an ethanol solution (10 mL) con-
taining [VO(OiPr)3] (0.25 mL, 1.16 mmol). The initially red-brown
solution turned orange and a yellow precipitate finally formed
which was filtered off. From the filtrate, kept at –20 °C for a couple
of days, yellow-green cubic crystals of [VO(HLC)]oct·C2H5OH were
obtained. If THF is used instead of ethanol, fine needle-like crys-
tals are obtained. IR (KBr): ν = 967 (V=O) cm–1. 51V NMR ([D6]-
˜
DMSO): δ = –396 ppm.
The precipitate was dissolved in DMSO. From this solution, kept
at room temperature for two weeks, light yellow needles of
[VO(HLC)]tbp could be isolated. IR (KBr): ν = 965 (V=O) cm–1.
˜
IR (KBr): ν = 2926 and 2853 (CH ), 1451 (δ, CH ), 1093 (CO),
˜
2
3
51V NMR ([D6]DMSO): δ = –381 ppm.
924 (V=O), 506 (VOH) cm–1. 51V NMR (CD2Cl2), δ (integral) =
–459.3 (1.0), –501.2 (0.68) ppm. FAB-MS, m/z [assignment (cf. also
Scheme 2), relative intensity]: 433 [M+, 6], 416 (M+ – OH, 94),
207.0 [M+ – 1 – (2 styrenes), 100], 190.0 [(M+ – 1) – 1-N-methylene-
Immobilisation on Merrifield Resin: NaH (65 mg, 1.63 mmol, 60%
in paraffin) was washed several times with anhydrous hexane to
remove the paraffin and dried in vacuo. To this NaH was added
H4LC (530 mg, 1.60 mmol) dissolved in THF (6 mL). The slurry
was stirred for 4 h at room temp. and treated with Merrifield resin
(360 mg, 0.7 mmol Clg–1) previously swelled in THF (6 mL) for 6
h. After stirring overnight, the slurry was washed with water, etha-
nol and THF (three times each) and dried in vacuo. Elemental
analysis: found N 0.37, C 88.51, H 7.65. A loading of 0.27 mmol
HnLC per g of resin was established from the nitrogen analysis. IR
confirmed the integrity of the ligand, e.g. by the presence of the
characteristic ν(CO) at 1065 cm–1. To the immobilised {HnLC}
(253 mg ) was added [VO(OiPr)3] (0.075 mL, 0.32 mmol; 1.2 molar
excess with respect to the ligand) and the mixture stirred overnight
at room temp. The brown coloured, loaded resin was than filtered
off and washed five times with THF, ethanol and finally acetone
then dried in vacuo. Vanadium analysis of {[VO(LC)]}: found
1.12%, corresponding to a loading of 0.22 mmol of V per g of
N-(1-methylimidazolyl)ethaneammonium,
54].
C21H23N3O3V
(433.37): calcd. C 58.20, H 5.58, N 9.70, V 11.76; found C 57.71,
H 5.52, N 9.54, V 12.14.
In the following descriptions in curly brackets { } symbolise the
resin matrix. All preparations directed towards the vanadium com-
plexes were conducted under N2.
[VO(H2LB)]: H5LB (151 mg, 0.42 mmol) dissolved 2-propanol
(2 mL) were treated dropwise whilst stirring with [VO(OiPr)3]
(0.11 mL, 0.42 mmol) dissolved in 2-propanol (4.5 mL) to produce
an orange solution which, over the course of time, became dark
orange with simultaneous formation of a yellow precipitate. The
precipitate was filtered after 10 h, washed with THF and dried in
vacuo. Freshly prepared solutions in CDCl3 show a single reso-
nance in the 51V NMR spectrum at δ = –411 ppm and a ν(V=O)
of 965 cm–1. On storage the solutions turn green, a change which
is accompanied by the disappearance of the NMR signal and ap-
pearance of an EPR signal typical of VO2+ (see Results and Dis-
cussion for data).
Immobilisation on Merrifield and Barlos Resin, {[VO(HnLB)]}: Mer-
rifield resin (0.5 g) was macerated in p-xylene (10 mL) for 24 h at
room temp. and treated with H5LB (152 mg, 0.42 mmol) dissolved
in a mixture of p-xylene (2 mL) and DMF (1 mL). This mixture
was stirred while Hünig’s base (0.14 mL, 0.80 mmol) was added to
the system. The mixture was then heated to reflux, under N2, for
72 h, cooled to room temp. and washed three times with ethanol
and finally with water, carbonate buffer then and ethanol/CH2Cl2
(1:1). The resin loaded with the ligand, {HnLB}, was pumped dry
and kept under N2. Loading rate according to elemental nitrogen
resin. IR (KBr): ν = 975 (br, V=O) cm–1.
˜
Immobilisation on Barlos Resin: Resin (100.5 mg, 0.10 mmol based
on 1.0 mmolg–1) and [VO(HLC)]oct (79.6 mg, 0.20 mmol) were sus-
pended in anhydrous CH2Cl2 (1.0 mL) and treated with Hünig’s
base (0.14 mL, 0.81 mmol) resulting in complete dissolution of the
complex. After 24 h of stirring, the loaded resin was filtered,
washed six times with CH2Cl2 and pumped dry. Vanadium analysis
of {[VO(LC)]}: found 1.74%, corresponding to a loading of
0.34 mmol of V per g of resin.
{[VO(H2O)LE]} ϵ [VO(H2O)LE] Immobilised to Merrifield Resin:
(cf. also Scheme 3 in the Results and Discussion section.) α-Boc-ε-
Z-lysine (2 g, 3.5 mmol) was converted into α-Boc-lysine in meth-
anol/CHCl3, (100 mL, 95:5) in the presence of freshly prepared pal-
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Sulfoxygenation Catalysed by Oxidovanadium Complexes
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ladium black (200 mg) by continuous stirring for 4 h under H2. The
catalyst was filtered, washed with methanol/CH2Cl2 and the filtrate
evaporated to dryness. 700 mg (47% yield) of α-Boc-lysine was thus
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Acknowledgments
This work was supported by the Deutsche Forschungsgemeinschaft
(DFG) (grant RE 431/20). We acknowledge assistance in the XDS
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Received: August 1, 2008
Published Online: October 17, 2008
Eur. J. Inorg. Chem. 2008, 5203–5213
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
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