Inorganica Chimica Acta 376 (2011) 57–63

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Inorganica Chimica Acta

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New chiral molybdenum complex catalyzed sulfide oxidation withhydrogen peroxide

Rajan Deepan Chakravarthy, Kotapati Suresh, Venkatachalam Ramkumar, Dillip Kumar Chand ⇑Department of Chemistry, Indian Institute of Technology Madras, Chennai 600 036, Tamil Nadu, India

a r t i c l e i n f o

Article history:Received 1 February 2011Received in revised form 9 May 2011Accepted 27 May 2011Available online 29 June 2011

Keywords:cis-Dioxomolybdenum(VI)Oxoperoxomolybdenum(VI)Chiral Schiff baseSulfoxidation

0020-1693/$ - see front matter � 2011 Elsevier B.V. Adoi:10.1016/j.ica.2011.05.033

⇑ Corresponding author. Fax: +91 44 2257 4202.E-mail address: dillip@iitm.ac.in (D.K. Chand).

a b s t r a c t

Six new cis-dioxomolybdenum(VI) complexes of chiral Schiff-base ligands, derived from condensation ofvarious amino alcohols and substituted salicylaldehydes, have been prepared and characterized by NMR,IR, ESI-MS and single crystal X-ray diffraction techniques. The geometry around the molybdenum centeris distorted octahedral in which a tridentate Schiff-base ligand with two anionic oxygens and one neutralimine nitrogen occupies meridional position. The octahedral geometry of the cis-dioxomolybdenum cen-ter is additionally completed by a coordinated labile solvent molecule. In some complexes the sixth site isfound to be vacant where the relatively bulky substituents hinder the coordination of the solvent. Thesecomplexes are tested for catalytic enantioselective sulfoxidation reactions using hydrogen peroxide asoxidant at low temperature which shows high selectivity along with good to moderate enantiomericexcess. ESI-MS study of the reaction mixture indicates the formation of oxoperoxoMo(VI) complexes dur-ing catalysis. The steric effect originated from the substituent on chiral ligand on the catalytic reaction isalso discussed. It is found that the substituents at the b position of the amino alcohol seem to greatlyinfluence the enantioselectivity of the oxidation reactions.

� 2011 Elsevier B.V. All rights reserved.

1. Introduction

Use of oxomolybdenum and related compounds in organic syn-thesis is a current trend [1–4]. Molybdenum compounds catalyzedsulfoxidation are interesting because of its mild and selective nat-ure along with sensitive functional group tolerance as reportedfrom our group [5]. Recently, molybdenum complexes containingsimple ONO tridentate Schiff-base ligand has been used as catalystfor sulfoxidation [6]. The application of chiral Mo(VI) complexes ismainly focussed on epoxidation reactions [7–9]. However, in caseof enantioselective sulfoxidation, molybdenum complexes are lessexplored and success is limited [10–12]. The chiral amino alcoholsderived from amino acids are easily available enantiomeric purecompounds which on condensation with salicylaldehyde or itsderivative resulted in ligand having ONO coordination site to themetal center. Several transition metals such as Ti(IV), V(IV), V(V),Fe(III) and Cu(II) with amino alcohol derived chiral Schiff-base tri-dentate ligand are very common and act as catalysts for enantiose-lective sulfoxidation reactions [13]. Although several cis-dioxomolybdenum complexes containing ONO coordinated ligandare well known in the literature [14], the chiral compounds of thisclass are not explored, to the best of our knowledge as catalysts forasymmetric sulfoxidation reactions.

ll rights reserved.

In this paper, we report synthesis and characterization of sixnew cis-dioxomolybdenum(VI)–(ONO) complexes where ONOstands for a chiral Schiff base ligand. The Schiff bases are derivedfrom condensation of chiral b-amino alcohols and salicylaldehydes.Catalytic applications of the complexes in enantioselective sulfox-idation of organic sulfides are studied.

2. Experimental

2.1. Materials and methods

3,5-Di-tert-butyl-salicylaldehyde, salicylaldehyde, aryl methylsulfides and MoO2(acac)2 were purchased from Sigma–Aldrichcompany and used as received. L-phenylglycine and L-phenylala-nine were purchased from Spectrochem, India and used as suchfor preparation of b-amino alcohols. The oxidant 30% aqueousH2O2 was purchased from Ranbaxy Fine Chemical Limited, Indiaand used as received. Reaction temperature was controlled by aJULABO FP50 instrument. The deuterated solvents were obtainedfrom Cambridge Isotope Laboratories. 1H and 13C NMR spectrawere recorded on Bruker Avance-400 instrument. The single crys-tal X-ray analyses were carried out using Bruker X8 Kappa XRDinstrument. IR spectra of the complexes were recorded on Per-kin-Elmer Spectrum 1 FT-IR instrument by KBr pellet method.The mass spectra were recorded on Micromass Q-Tof mass spec-trometer. Autopol IV-Rudolph Research Analytical Polarimeter

58 R.D. Chakravarthy et al. / Inorganica Chimica Acta 376 (2011) 57–63

was used for determining the absolute configuration. The enantio-meric excess values of sulfoxides were determined by Waters HPLCsystem with 2487 detector, 515 pump using CHIRALPAK AS-H,CHIRALCEL OD-H and CHIRALCEL OJ-H columns. The amino alco-hols and the corresponding Schiff-bases were prepared by litera-ture protocols [15–17].

2.2. Preparation of the molybdenum complexes

2.2.1. [MoO2(L1)CH3OH)]�CH3OH, ([1�(CH3OH)]�CH3OH)A mixture of salicylaldehyde (0.610 g, 5 mmol) and (S)-2-ami-

no-3-phenyl-propan-1-ol (0.756 g, 5 mmol) in 20 ml methanolwas heated to reflux for 3 h, which gave the Schiff base L1(H)2.Then MoO2(acac)2 (1.630 g, 5 mmol) was added and refluxed foranother 3 h. A green solid was obtained by concentrating the meth-anolic solution, washed with cold methanol and recrystallizedfrom methanol by slow evaporation. Yield: 51%; 1H NMR (DMSO-d6, 400 MHz): d = 3.03–3.13 (m, 2H, Ph-CH2–), 4.26–4.31 (m, 2H,O-CH2–), 4.44–4.46 (m, 1H, –CH–N@), 6.87–7.47 (9H, aromatic),8.11 (s, 1H, –N@CH–). 13 C NMR (DMSO-d6, 100 MHz): d = 40.00(Ph-CH2 –), 72.44 (O-CH2 –), 75.82 (–C H–N@), 119.47, 119.57,120.46, 126.64, 128.51, 129.87, 134.21, 135.04, 138.13, 160.92,164.16 (–C @N). IR (KBr pellet) m (C@N) 1635; m(MoO2) 913,883 cm�1. MS (ESI): m/z = 384 (98MoO2(L1)+H+).

2.2.2. [MoO2(L2)(CH3OH)]�H2O ([2�(CH3OH)]�H2O)This complex was prepared by the same procedure employed

for making [MoO2(L1)CH3OH)]�CH3OH except that (S)-2-amino-2-phenylethanol was used instead of (S)-2-amino-3-phenyl propan-1-ol. Yield: 60%; 1H NMR (CDCl3, 400 MHz): d = 4.61–4.66 (t, 1H,O-CH2–), 4.93–4.97 (dd, 1H, O-CH2–), 5.18–5.21 (m, 1H, –CH–N@), 6.73–7.45 (9H, aromatic), 8.10 (s, 1H, –N@CH–). 13C NMR(CDCl3, 100 MHz): d = 74.06 (O-CH2–), 79.05 (–CH–N@), 119.59,120.48, 129.33, 129.53, 129.80, 134.15, 135.77, 136.05, 160.66,164.81 (–C@N). IR (KBr pellet) m(C@N) 1626; m(MoO2) 931,912 cm�1. MS (ESI): m/z = 370 (98MoO2(L2)+H+).

2.2.3. [MoO2(L3)] (3)To a methanolic solution of the Schiff base L3(H)2 which was

prepared from salicylaldehyde (0.122 g, 1 mmol) and (S)-2-ami-no-1,1,3-triphenylpropan-1-ol (0.303 g, 1 mmol) by the samemethod as described above, MoO2(acac)2 (0.326 g, 1 mmol) wasadded and refluxed for 3 h. The complex thus formed by concen-trating the resulting solution was washed with methanol anddried. Yield: 84%; 1H NMR (DMSO-d6, 400 MHz): 1H NMR:d = 2.27–2.30 (d, 1H, Ph-CH2–), 3.00–3.03 (1H, Ph-CH2–), 5.27–5.29 (d, 1H, –CH–N@), 6.76–7.67 (aromatic and –N@CH–). 13CNMR (DMSO-d6, 100 MHz): d = 37.74 (Ph-CH2–), 76.81, 88.71,119.02, 119.15, 120.72, 125.85, 126.27, 126.36, 126.75, 127.59,127.98, 128.16, 130.06, 133.87, 134.84, 137.99, 143.97, 145.80,161.61, 164.35 (–C@N). IR (KBr pellet) m(C@N) 1618; m(MoO2)941, 915 cm�1.

2.2.4. [MoO2(L4)(CH3OH)] ([4�(CH3OH)]�H2O)A solution of the Schiff base L4(H)2 was prepared by condensa-

tion of 3,5-di-tert-butyl-2-hydroxybenzaldehyde (0.468 g, 2 mmol)with (S)-2-amino-2-phenylethanol (0.274 g, 2 mmol) in methanolfor 3 h under refluxed condition. MoO2(acac)2 (0.652 g, 2 mmol)was added to the resulting solution and refluxed for another 3 h.The product thus obtained on evaporation of methanol waswashed with cold methanol and dried. This material was recrystal-lized form methanol by slow evaporation method and dried to af-ford the compound. Yield: 67%; 1H NMR (CDCl3, 400 MHz): d = 1.26(s, 9H, t-Bu), 1.38 (s, 9H, t-Bu), 4.63–4.68 (t, 1H, O-CH2–), 4.92–4.96(dd, 1H, O-CH2–), 5.15 (m, 1H, –CH–N@), 7.01–8.19 (7H, aromatic),8.20 (s, 1H, –N@CH–). 13C NMR (CDCl3, 100 MHz): d = 29.90, 31.45,

34.46, 35.48, (four peaks for t-Bu group), 74.24 (O-CH2–), 78.37 (–CH–N@), 119.93, 128.30, 129.35, 129.55, 129.63, 131.70, 136.60,139.84, 142.99, 167.02 (–C@N). IR (KBr pellet) m(C@N) 1633;m(MoO2) 929, 883 cm�1. MS (ESI): m/z = 482 (98MoO2(L4)+H+).

2.2.5. [MoO2(L5)] (5)To a methanolic solution of the Schiff base L5(H)2 which was

prepared from 3,5-di-tert-butyl-2-hydroxybenzaldehyde (0.234 g,1 mmol) and (S)-2-amino-1,1,3-triphenylpropan-1-ol (0.303 g,1 mmol) by the same method as described above, MoO2(acac)2

(0.326 g, 1 mmol) was added and refluxed for 3 h. The resultingclear yellow solution was concentrated, washed with cold metha-nol and dried. The recrystallization for this complex was done inmethanol solvent. Yield: 72%; 1H NMR (CDCl3, 400 MHz): d = 1.21(s, 9H, t-Bu), 1.37 (s, 9H, t-Bu), 2.85–2.90 (dd, 1H, PhCH2), 2.95–3.01 (dd, 1H, PhCH2), 5.02–5.05 (dd, 1H, –CH–N@), 6.65–7.57(18H, (aromatic and –N@CH–) 13C NMR (CDCl3, 100 MHz):d = 29.83, 31.27, 34.34, 35.38, 39.13, 82.96, 90.48, 118.76, 125.88,126.38, 127.07, 127.30, 127.63, 127.72, 128.69, 128.94, 130.47,132.65, 137.08, 140.29, 143.20, 143.52, 144.26, 158.06, 168.16 (–C@N). IR (KBr pellet) m(C@N) 1618; m(MoO2) 944, 916 cm�1. MS(ESI): m/z = 648 (98MoO2(L5)+H+).

2.2.6. [MoO2(L6)] (6)This complex was prepared by the method similar to that of

[MoO2(L5)] except that (S)-2-amino-1,1,2-triphenylethanol wasused instead of (S)-2-amino-1,1,3-triphenylpropan-1-ol (0.289 g,1 mmol). A green solid was obtained by concentrating the metha-nolic solution, washed with cold methanol and dried. Yield: 39%;1H NMR (CDCl3, 400 MHz): d = 1.27 (s, 9H, t-Bu), 1.37 (s, 9H, t-Bu), 6.21 (s, 1H, –CH–N@), 7.03–7.66 (17H, aromatic), 8.74 (s,1H,–N@CH–). 13C NMR (CDCl3, 100 MHz): d = 29.86, 31.30, 34.49,35.47 (four peaks for t-Bu group), 84.67, 91.17, 119.43, 126.38,126.63, 126.90, 127.73, 127.92, 127.99, 128.35, 128.69, 128.72,128.91, 133.67, 139.05, 140.92, 143.38, 143.46, 143.60, 143.68,158.80, 170.80 (–C @N). IR (KBr pellet) m (C@N) 1612; m(MoO2)942, 915 cm�1. MS (ESI): m/z = 634 (98MoO2(L6)+H+).

2.3. Crystal structure determination

Crystals suitable for X-ray measurement were grown frommethanol by slow evaporation method. Crystallographic data forcomplexes 1, 2, 4 and 5 along with other experimental detailsare summarized in Table 1. The structures were solved by directmethods (SHELXS-97 and SHELXL-97) and refined by full-matrix leastsquares methods [18]. Perspective view of the complexes is givenin Figs. 1–4 were drawn with ORTEP-3 for windows.

2.4. Catalytic sulfoxidation

A mixture of molybdenum–Schiff base complex (5 mol%) andphenyl methyl sulfide (0.248 g, 2 mmol) in 5 ml of dichlorometh-ane was stirred at 0 �C. Then 30% hydrogen peroxide (0.226 ml,2 mmol) was added slowly into the reaction mixture. Stirringwas continued for 10–48 h as per requirement. Reaction progresswas monitored by TLC. The resulting solution was diluted withwater and extracted with dichloromethane. The organic layerwas dried over anhydrous sodium sulfate, concentrated and theresulting crude product obtained was purified by column chroma-tography to get the pure sulfoxide. The ee of the sulfoxides weredetermined by HPLC method.

Table 1Crystallographic data and experimental data.

[1�(CH3OH)]�CH3OH [2�(CH3OH)] �H2O [4�(CH3OH)] 5

Empirical formula C18H23MoNO6 C16H19MoNO6 C24H33MoNO5 C36H39MoNO4

Formula weight 445.31 417.26 511.45 645.62Temperature (K) 298(2) 298(2) 298(2) 298(2)Crystal system monoclinic trigonal monoclinic orthorhombicSpace group C2 P31 P2(1) P2(1)2(1)2(1)a (Å) 34.918(7) 15.1894(5) 11.9075(5) 9.9089(5)b (Å) 6.8940(14) 15.1894(5) 5.9916(2) 10.7713(5)c (Å) 15.608(3) 6.4180(2) 17.8879(7) 30.5374(12)a (�) 90 90 90 90b (�) 96.00(3) 90 95.146(2) 90c (�) 90 120 90 90Volume (Å3) 3736.6(13) 1282.36 1271.07(8) 3259.3(3)Z 8 3 2 4qcalc (g cm�3) 1.583 1.621 1.336 1.316Absorption coefficient (mm�1) 0.7356 0.798 0.547 0.440F(0 0 0) 1824 636 532 1344Crystal size (mm3) 0.35 � 0.22 � 0.20 0.35 � 0.22 � 0.20 0.35 � 0.22 � 0.18 0.28 � 0.25 � 0.22Theta range for data collection (�) 1.17–28.29 2.68–28.29 2.73–27.46 1.33–28.27Reflections collected/unique 25 701/8312 7184/3397 9043/4678 24 956/8067Max. and min. Transmission 0.8668 and 0.7828 0.8567 and 0.7676 0.9079 and 0.8315 0.9093 and 0.8866Goodness-of-fit on F2 1.031 0.796 1.150 0.958Final R indices [I > 2r(I)] R1 = 0.0215, wR2 = 0.0517 R1 = 0.0358, wR2 = 0.0525 R1 = 0.0274, wR2 = 0.0691 R1 = 0.0396, wR2 = 0.0755R indices R1 = 0.0268, wR2 = 0.0564 R1 = 0.0886, wR2 = 0.0641 R1 = 0.0346, wR2 = 0.0814 R1 = 0.0686, wR2 = 0.0848Largest difference in peak and hole (e Å�3) 0.432 and �0.493 0.434 and �0.389 0.685 and �0.609 0.220 and �0.381CCDC 768523 775292 768524 768522

Fig. 1. ORTEP diagram of the complex [MoO2(L1)(MeOH)](MeOH) with atomlabeling scheme. Only one of the two similar molecules in the asymmetric unit isshown. Hydrogen atoms and lattice solvent are omitted for clarity.

Fig. 2. ORTEP diagram of the complex [MoO2(L2)(MeOH)](H2O) with atom labelingscheme. Hydrogen atoms and lattice solvent are omitted for clarity.

R.D. Chakravarthy et al. / Inorganica Chimica Acta 376 (2011) 57–63 59

3. Results and discussion

3.1. Preparation and characterization of the complexes

In this study, six new chiral cis-dioxomolybdenum(VI)–(ONO)complexes have been synthesized and assessed as catalysts forenantioselective sulfoxidation. The Schiff base ligands (L1(H)2–L6(H)2) used for the preparation of molybdenum complexes arederived from condensation of various chiral amino alcohols andsubstituted salicylaldehydes. The complexes were prepared bysimple ligand exchange reactions of the Schiff base ligands withMoO2(acac)2 in methanol (Scheme 1). The ligands are designedfor studying the effects of bulky substituents on catalytic asym-metric sulfoxidation.

The complexes were characterized by 1H NMR spectroscopywhere an appreciable change in the chemical shift value of thecomplexes is observed as compared to the corresponding free

ligands. 13C NMR spectra are also in line with requirement of struc-tures. ESI-MS studies were carried out for the complexes in meth-anol solution containing 1% HCOOH. The original complexes areneutral, however the addition of formic acid gives the [M+H]+ frag-ment which were detected. The molybdenum containing fragmentgives isotope pattern corresponding to mononuclear complex. Theisotopic abundance of Mo is 92Mo 14.84%, 94Mo 9.25%, 95Mo15.92%, 96Mo 16.68%, 97Mo 9.55%, 98Mo 24.13%, 100Mo 9.63%. Allcomplexes were also characterized by IR spectroscopy. Two strongbands in the region 883–916 and 913–944 cm�1 which can be as-signed as symmetric and asymmetric stretching frequencies due tocis-dioxo moieties indicating the existence of monomeric com-plexes [19].

Single crystals of the complexes 1, 2, 4 and 5 were successfullygrown by slow evaporation of methanolic solutions of complexes

Fig. 3. ORTEP diagram of the complex [MoO2(L4)(MeOH)] with atom labelingscheme. Hydrogen atoms are omitted for clarity.

Fig. 4. ORTEP diagram of the complex [MoO2(L5)] with atom labeling scheme.Hydrogen atoms are omitted for clarity.

Scheme 1. Synthesis of molybdenu

60 R.D. Chakravarthy et al. / Inorganica Chimica Acta 376 (2011) 57–63

at room temperature which resulted in the formation of[MoO2(L1)(MeOH)](MeOH), [MoO2(L2)(MeOH)](H2O), [MoO2(L4)(-MeOH)] and [MoO2(L5)]. The coordination geometry around theMo center in these molybdenum complexes can be best describedas a distorted octahedron in which the ONO tridentate ligand occu-pies meridional position with two anionic oxygens mutually transand are cis to the oxygens of cis-dioxo group. The octahedral geom-etry of molybdenum center is additionally completed by coordi-nated labile solvent molecule whereas in case of complexeshaving bulky ligand i.e. 5 it remains vacant. Attempts to obtain sin-gle crystal for complexes 3 and 6 were unsuccessful.

The two possible diasteromers of these complexes are exo andendo as shown in (Fig. 5) which is described by the relative positionof the substituent at b position of amino alcohol moiety and axialoxygen atom. In the solid state the molybdenum complexes arefound to exist only as endo isomers as seen from their crystal struc-tures. Variable temperature 1H NMR was recorded for a solution of4 in CDCl3 in the range of +20 to �20 �C. Only one set of peaks wasobserved at room temperature whereas at lower temperaturespeak broadening was noticed. The existence of the two isomerscould not be distinguished by this method. Interconversion of endoand exo isomers for similar molybdenum complexes is proposed byNakajima et al [20,21]. They suggested that the interconversionwill proceed through the partial rotation of the cis-MoO2 moiety.The existence of two isomers and its rapid interconversion in ourcomplexes may lower the enantioselectivity of the sulfoxidationreaction during selective approach of sulfide towards the catalyst.

3.2. Sulfoxidation study

All complexes were tested as catalysts in the asymmetric sul-foxidation reaction. The results are shown in Table 2. The effectof bulky substituents on both amino alcohol and salicylaldehydemoieties has been studied. In case of complex 2, where the aminoalcohol having the phenyl substitution at b position gave goodenantiomeric excess rather than the benzyl substituted complex1. The complexes having more bulky amino alcohol with two phe-nyl groups at a position as in the case of 3, 5 and 6 gave very poorenantiomeric excess. Then the effect of bulky substituent on sali-cylaldehyde has been studied. The salicylaldehyde with two bulkyt-Bu groups in complex 4 provided better enantiomeric excess withhigh yield in comparison with the unsubstituted complex 2. In-crease in enantiomeric excess has also been noticed when the sol-vent was changed from dicholoromethane to chloroform with thecatalyst 4 (see Table 3) at same reaction time although yield waslowered (see Table 2 entry 4 and Table 3 entry 2). All the reactionsare carried out at low temperature. At room temperature the reac-

m(VI)–(ONO) complexes 1–6.

Fig. 5. The endo and exo diastereomers for molybdenum–Schiff base complex [MoO2(L4)] and their interconversion.

Table 2Optimization of reaction conditions for oxidation of thioanisole.a

Mo-Schiff base complex

H2O2, CH2Cl2, 0 oCPh

SCH3 Ph

S(S) CH3 Ph

S(R) CH3

O O

Entry Catalyst Yield (%) ee (%)b

1 1 83 14.6 (S)2 2 74 33.5 (S)3 3 82 6.2 (S)4 4 90 39.6 (S)5 5 77 5.5 (S)6 6 78 2.0 (S)

a Reaction conditions: Molybdenum complexes (5 mol%), sulfide (1 mmol) and H2O2 (1 mmol) in CH2Cl2 at 0 �C for 2 days.b The ee values were determined by HPLC using OD-H column and by comparison with reported specific rotation [22].

Table 3Enantioselective oxidation of various organic sulfides.a

[MoO2(L4)(MeOH)]

H2O2, CHCl3,0 oCArS

CH3 ArS

CH3 ArS

CH3

O O

Entry Ar Time (h) Yield (%) ee (%)b

1 Ph 13 83 55.2 (S)2 Ph 2 days 78 48.1 (S)c

3 p-Cl-C6H4 12 86 43.7 (S)4 p-Br-C6H4 15 75 39.2 (S)5 p-Me-C6H4 10 90 41.3 (S)6 p-MeO-C6H4 19 74 32.6 (S)c

7 p-NO2-C6H4 2 days 55 8.5 (S)

a Reaction conditions: Molybdenum complexes (5 mol%), sulfide (1 mmol) and H2O2 (1 mmol) in CHCl3 at 0 �C.b The ee values were determined by HPLC using OD-H, OJ-H and AS-H columns and by comparison with reported specific rotation [22].c Sufone formation was also observed.

R.D. Chakravarthy et al. / Inorganica Chimica Acta 376 (2011) 57–63 61

tions led to racemization along with the formation of over oxidizedproduct. Similarly, racemization was observed when the reactionsare performed in CH3CN, CH3OH, C2H5OH or THF even at lowertemperature (see Supplementary information).

Using the optimized conditions, the oxidation of various substi-tuted sulfides has been studied and the results are summarized inTable 3. Thioanisole, p-chlorophenyl methyl sulfide, p-bromophenylmethyl sulfide and p-tolyl methyl sulfide were selectively oxidizedto the corresponding sulfoxide with relatively good to moderateenantiomeric excess in high yield. In case of p-methoxyphenylmethyl sulfide the over oxidized product was also observed alongwith unreacted starting material. The reaction achieved a little suc-cess in case of p-nitrophenyl methyl sulfide which took 2 days foraround fifty percent conversion with very low enantiomeric excess.

The reactivity of the molybdenum complex 4 with the hydrogenperoxide has also been tested. To the chloroform solution of molyb-denum catalyst 4 excess of hydrogen peroxide was added in the ra-tio of 1:25. ESI-MS study was carried out for the reaction mixture.Interestingly, the molecular ion peaks for both dioxomolybdenum

and oxoperoxomolybdenum complexes were observed with correctisotopic pattern (Fig. 6). So the oxoperoxo complex of the catalyst 4is involved in the enantioselective sulfoxidation. Unlike olefin epox-idation, the detailed mechanistic study for sulfoxidation is not wellestablished for molybdenum catalyzed reactions. The proposedmechanism of sulfoxidation is shown in Scheme 2. The oxotransfertakes place from the oxoperoxo complex either by direct oxygentransfer or coordination of the sulfide to the metal followed by oxy-gen transfer is proposed. ESI-MS was recorded for the chloroformsolution prepared from the molybdenum catalyst 4, excess ofhydrogen peroxide followed by addition of phenyl methyl sulfox-ide. The molecular ion peak for sulfoxide bound oxoperoxo complexwas observed (see Supplementary information) which supports theproposed mechanism shown in Scheme 2. The dioxo andoxoperoxomolybdenum complexes with sulfoxide coordinated li-gand are well known in the literature [19,23]. Further investigationof the ESI-MS was carried out for the reaction mixture of phenylmethyl sulfide with hydrogen peroxide in presence of catalyst 4after the completion of oxidation reaction. The molecular ion peak

Scheme 2. Proposed pathway for the formation of sulfoxide.

Fig. 6. ESI-MS Spectrum of reaction mixture of [MoO2(L4)(MeOH)] with excess hydrogen peroxide in chloroform solvent shows peaks for both [MoO2(L4)+H]+ and[MoO(O2)(L4)+H]+.

62 R.D. Chakravarthy et al. / Inorganica Chimica Acta 376 (2011) 57–63

for the complex 4 was noticed which showed the recovery of thecomplex during catalytic cycle (see Supplementary information).Based on these experimental data, the reaction pathway for enatio-selective sulfoxidation has been proposed.

4. Conclusion

In summary, we have synthesized six new chiral Mo(VI)–(ONO)complexes and investigated them as catalysts for enantioselective

sulfoxidation. The effects of bulky substituents of ligands on catal-ysis have also been discussed. The reaction provided good to mod-erate enantioselectivity with high yield of sulfoxide. Furtherexperiments are underway to study the mechanism in detail andto modify the chiral ligands to augment catalytic activities.

Acknowledgement

This study was supported by CSIR, India

Appendix A. Supplementary material

CCDC 768522, 768523, 768524 and 775292 for compounds 1, 2,4 and 5, respectively, contains the supplementary crystallographicdata for this paper. These data can be obtained free of charge fromThe Cambridge Crystallographic Data Centre via www.ccdc.cam.a-c.uk/data_request/cif.

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.ica.2011.05.033.

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