Synthesis and characterization of homoleptic titanium bulky alkoxo complexes and their application in 1-octene epoxidation

Journal of Organometallic Chemistry 741-742 (2013) 102e108

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Synthesis and characterization of homoleptic titanium bulky alkoxo complexes and their application in 1-octene epoxidation Yolanda Pérez a, Sandra Bázquez b, Mariano Fajardo a, Pilar de Frutos b, **, Isabel del Hierro a, * a b

Departamento de Química Inorgánica y Analítica. E.S.C.E.T, Universidad Rey Juan Carlos, 28933 Móstoles, Madrid, Spain Repsol, Carretera de Extremadura N-V km 18, 28935 Móstoles, Madrid, Spain

a r t i c l e i n f o

a b s t r a c t

Article history: Received 28 February 2013 Received in revised form 14 May 2013 Accepted 23 May 2013

A new series of titanium alkoxo complexes [Ti(OR)4] (1e8) have been synthesized by the alcoholysis reaction of Ti(OiPr)4 with the corresponding alcohol (ROH ¼ Adamantanol, (1R,2S,5R)-()-menthol, 1,2:5,6-di-O-isopropylidene-a-D-glucofuranose, 1,2:3,4-di-O-isopropylidene-a-D-galactopyranose, (1R)()-myrtenol, 1,2:5,6-Di-O-cyclohexylidene-a-D-glucofuranose, (1S-endo)-()-borneol and ()-sclareol). These homoleptic alkoxo titanium (IV) complexes have been characterized by spectroscopic and electrochemical techniques. In addition, they have been tested in the epoxidation reaction of 1-octene with ethylbenzene hydroperoxide as oxidant; obtaining yields of up to 90% and selectivity values of 50%. The conversion profile of EBHP was found to depend on the amount of titanium and on the chemical environment of titanium sites. Ó 2013 Elsevier B.V. All rights reserved.

Keywords: Alkoxo Homoleptic titanium Epoxidation 1-Octene

1. Introduction The development of solid catalysts and the heterogenization of homogeneous systems on solid supports have taken place as new approaches to prepare epoxidation catalysts [1]. In spite of the spectacular advance of surface science, the exact nature of the processes is not sufficiently understood. Investigations of titanium epoxidation catalysts support the general consensus that the most active and selective sites are isolated, mononuclear, 4-coordinated titanium (IV) centres. The access to the central ion by an alkene and the oxidant must be relatively unhindered, so that, the coordination number can increase from four to six during catalysis [2]. Therefore, the development of molecular systems that serve as models may be an interesting approach. Published work has suggested that molecular [Ti(OSiMe3)4], which shows excellent catalytic properties with various olefins, may act as a homogeneous model catalyst mimicking the highly active isolated tetrahedral Ti sites in silylated titania-silica mixed oxides [3]. In addition, tetrahedral titanium (IV) complexes, prepared by the reaction of titanium tetraisopropoxide or titanium tetrachloride and a silicon precursor, Ph3SiCl or PhSi(CH3)3, show good conversion values and high selectivity in the

* Corresponding author. Tel.: þ34 914887022; fax: þ34 914888143. ** Corresponding author. E-mail address: [email protected] (I.del Hierro). 0022-328X/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jorganchem.2013.05.029

epoxidation reaction of 1-octene using ethylbenzene hydroperoxide [4]. On the contrary, the tetrahedrically coordinated [Ti(OSiPh3)4] and [Ti(OGePh3)4] are inactive in the epoxidation reaction; steric congestion is thought to be responsible for the total catalytic inactivity of these compounds [5]. Consequently, it is pertinent to design novel titanium alkoxo catalysts for such applications, particularly those with readily available ligands that form air-stable complexes and to investigate the effect of the nature of the ligands on activity and selectivity for the epoxidation of 1-octene with EBHP as oxidant. 1-Octene has been selected as the reactant because it is a suitable alkene model for describing reactivity in the epoxidation of propene to propene oxide. 2. Results and discussion 2.1. Synthesis and characterization of homoleptic titanium alkoxo complexes Titanium isopropoxide, Ti(OiPr)4, reacts with 4 equivalents of ROH in dichloromethane at room temperature to give, through a protonolysis reaction the titanium alkoxo complexes [Ti(OR)4] [1e 7] as air stable but moisture sensitive derivatives. Complex 8 was synthesized by the reaction of 2 equivalents of Sclareol (see Scheme 1). After preparation and workup, 1e8 were obtained as colourless oils or white solids and with reduced moisture sensitivity compared to the parent Ti(OiPr)4. The synthesis and structural

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103

Scheme 1.

characterization of compounds 1e4 and 7 have been reported previously [6,7]. Compounds 1e8 have been characterized by FT-IR, 1H, 13C{H}, VT 1H NMR and 1H DOSY NMR spectroscopy. 1H and 13C{H} NMR spectra of the isolated compounds indicate that the desired alcoholysis exchange reaction has taken place cleanly. In the 1H NMR spectra the signals associated with the bulky alkoxo ligands, and the disappearance of the isopropoxo signals, are observed upon substitution. The solution 1H and 13C{H} NMR spectra of Ti(OR)4 are straightforward with a single set of resonances for the corresponding bulky alkoxo ligands. These spectra are consistent with a mononuclear compound or polynuclear species that exhibit rapid dynamic exchange between bridging and terminal alkoxo ligands. 1 H NMR spectra of compounds Ti(OR)4 5e8 have been recorded at room temperature in CDCl3 and different decoupling NMR experiments allowed us to identify the signals and calculate the corresponding coupling constants. Free myrtenol contains an endo double bond in the structure of bicyclo[3.1.1]heptane and has a fairly rigid geometry in “Y form” which makes it a good candidate to be used as a sterically demanding alkoxo ligand in order to provide tetrahedral environment around the titanium atom. In Ti(OMyrten)4 (5) the signal attributed to the methylene group CH2(10), next to the oxygen atom directly bonded to the titanium centre, appears as a singlet at 4.44 ppm, deshielded Dd ¼ 0.24 ppm as a consequence of coordination to the metal atom [8].1H NMR spectrum of Ti(ODCGF)4 (6), with the glucofuranose monoanion bounded as a terminal alkoxo ligand to the titanium centre, shows, as representative signals, a doublet at d 5.85 ppm for the anomeric proton H(1) and a multiplet at d 4.86 ppm attributed to H(3), deshielded Dd ¼ 0.20 ppm as a result of coordination to titanium.

1

H NMR spectrum of Ti(OBorn)4 (7) shows well resolved signals for the borneoxo ligand. H(1) appears as a multiplet at d 4.64 ppm, deshielded by nearly 0.50 ppm with respect to the free ligand. Finally, 1H NMR spectrum of Ti(OSclar)2 (8) shows, as representative signals, from 4.8 to 5.2 ppm the AMX pattern of the terminal double bond, which is not significantly different in comparison with the free ligand. The FT-IR spectra confirm that the alcoholysis reaction has taken place seen by disappearance of the n(OeH) band at 3332 cm1 attributed to uncoordinated hydroxyl groups. Extra information, found in the infrared spectrum of 5 is the disappearance of the weak band at 1652 cm1 in free myrtenol, which may indicate the coordination to the metal centre through the terminal double bond in terpene. Fig. 1 compares the electronic spectra of complexes 2 and 5. The UVevis spectrum of 2 exhibits a strong absorption at lmax ¼ 220 nm for the oxygen to Ti(IV) charge transfer band (LMCT) and a shoulder at 250 nm, much less intense. Furthermore the UVe vis spectrum of 5 shows a very broad band in the region 230e260 nm. Absorption maxima in the range of 210e240 nm are attributed to ligand to metal charge transfer transition for true four-coordinate Ti(IV); strong well defined absorption peaks observed at higher wavenumber are indicative of the presence of titanium species with higher coordination environments [9]. These results are consistent with the presence of polynuclear species and also with the coordination and decoordination of the double bond of terpene in complex 5, in which the alkoxo substituent acts as a chelating ligand by occupying two coordination sites on the metal centre. To elucidate the solution behaviour of this family of compounds, a variable temperature 1H NMR spectroscopic study of compound 7, as representative example, has been carried out. This compound does

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Y. Pérez et al. / Journal of Organometallic Chemistry 741-742 (2013) 102e108 Table 1 Diffusion coefficient of titanium complexes.

Complex Complex

200

25 0

30 0

350

400

2 5

450

500

Wavelenght (nm) Fig. 1. Electronic spectra at UVevis region of the tetrasubstituted titanium complexes Ti(OMent)4 (2) and Ti(OMyrten)4 (5).

not present any fluxional behaviour in the temperature range used in this experiment (r.t to 80  C) (see Supplementary material). In the present study we have carried out diffusion ordered NMR spectroscopy (DOSY) measurements for complexes 1e8 in order to indicate the presence of mono and/or dinuclear titanium complexes in solution, as titanium has a tendency to coordinately saturate its binding sphere unless a large number of sterically hindered ligands are incorporated [10,11]. Diffusion coefficients are related to the speed of molecules in liquid and depend on the size of the dissolved compounds [12]. According to the StokeseEinstein equation the diffusion coefficient for a molecule is inversely proportional to the hydrodynamic radius [13]. We expect that dinuclear titanium complexes should exhibit smaller self diffusion coefficients as compared to mononuclear complexes, with less size and weight. In the DOSY spectra of all the studied complexes proton resonances give a unique 2D peak, a horizontal line in the diffusion dimension which means that all of them belong to a single compound. The signals are, in general, well resolved and can be classified according to their self diffusion coefficients. We have defined a model system to examine the reliability of this technique to distinguish between mono and dinuclear titanium complexes with alkoxo ligands in solution. For this purpose we have studied the parent Ti(OiPr)4, Ti(OMent)4 (2) and the related [{Ti(OiPr)2(OMent)2}2] (MentO ¼ 1R,2S,5R-()-menthoxo), previously synthesized and characterized by our group [14]. The established aggregation state of Ti(OiPr)4 in solution is 1.4, due to the existence of a monomeredimer equilibrium [15], and its diffusion peak is centred at D ¼ 1.99  109 m2/s. For [{Ti(OiPr)2(OMent)2}2] the diffusion peak is centred at a lower value D ¼ 8.91  1010 m2/s, this is as expected, since this compound has been proposed to be a dimer in solution on the basis of variable temperature 1H NMR studies. Homoleptic complex 2 possesses bulky alkoxo ligands which impose a high steric demand; on the basis of other spectroscopic techniques we have previously proposed this compound to be a monomer with a tetrahedral environment around titanium. Nevertheless, the diffusion coefficient value measured, D2 ¼ 7.94  1010, is lower than that of [{Ti(OiPr)2(OMent)2}2], which suggest some degree of oligomerization. The measurements of diffusion coefficients lead to a calculated ratio of “spherical” volumes of dimer/monomer. On the basis of the correlation, given by the StokeseEinstein equation, [D ¼ kT/(6phr)] and the radii/ volume of a sphere (V y r3), the ratio of the diffusion coefficient is D/D2 y 1.12 and hence the ratio of the radii values r2/r y 1.12, which unambiguously demonstrates that 2 is almost it is almost 1.4 times the size of [{Ti(OiPr)2(OMent)2}2] and confirms its dimeric

Complex

Diffusion coefficient D (m2/s)

Ti(OPri)4 Ti(ODICGF)4 [3] Ti(ODCGF)4 [6] Ti(OBorn)4 [7] Ti(OPri)2(OMent)2 Ti(OMent)4 [2] Ti(OSclar)4 [8] Ti(OAdam)4 [1] Ti(OMyrt)4 [5] Ti(ODIGP)4 [4]

1.99 1.58 1.20 1.15 8.91 7.94 6.92 6.76 6.60 5.37

         

109 109 109 109 1010 1010 1010 1010 1010 1010

structure. The existence of a monomeredimer equilibrium favouring polynuclear species at room temperature is proposed. The complexes 1, 2, 5, 7 and 8, with comparable sizes, show diffusion coefficient values in the range 1.12  109e 6.60  1010 m2/s. Complexes 3 and 6, both of them with bulky sugar moieties moves faster than complexes with terpenes. This behaviour can be explained assuming that monomeric structure dominates for these two complexes due to the sterical hindrance imposed by the ligands. Complex 4 is an exception to the above mentioned tendency imposed by the sugar ligands; it shows the least diffusion coefficient which indicates also the existence of polynuclear species. (See Table 1, Fig. 2 and Supplementary material). Nevertheless, the diffusion coefficient is not only a reflection of the molecular association between two components; the molecule conformation affects significantly its hydrodynamic surface and hence its diffusion profile [16]. If complexes 7, 2, 8, 1 and 5, with terpene as alkoxo ligands, are compared, it is not possible to establish a clear correlation between size and motion speed which means that other factors must be taken into account. It is representative that complexes 1 and 5 are slower in spite of its lower molecular weight, this is probably due to their more extended structures since the myrtenoxo and adamantoxo moieties bonded to titanium have a fairly rigid geometry. 2.2. Cyclic voltammetry studies As an alternative means of assessing the electronic effects of ligand binding, voltammetric techniques have been employed to determine redox behaviour of 1e8. These studies have been carried

Fig. 2. DOSY 1H NMR spectra of complex Ti(OMyrten)4 (5) in CDCl3, 400 MHz.

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105

0.10x10 -3 0.08x10 -3 0.05x10 -3

i/A

0.03x10 -3 0 -0.03x10 -3 -0.05x10 -3 -0.08x10 -3 -0.10x10 -3 -0.13x10 -3

-4.00

-3.00

-2.00

-1.00 E/V

0

1.00

Fig. 3. Cyclic voltammogram of compound Ti(ODIGP)4 4 in THF 0.2 M [Bu4N][PF6], at Pt working electrode, vs Fc/Fcþ, scan rate 100 mV/s.

out with a platinum disk electrode in THF solutions with Bu4NPF6 as supporting electrolyte. Potentials were referenced to the Fc/Fcþ couple, used as internal standard. They reveal the presence in the cyclic voltammograms of an irreversible reduction process, associated to Ti(IV)/Ti(III) reduction, at higher negative potential values in comparison to those observed for other tetrasubstituted titanium derivatives (see Supplementary material) [17,18]. As an example, Fig. 3 shows the CV response of complex 4 at 200 mV/s, the cathodic sweep reveals one reduction process associated to Ti(IV)/Ti(III) reduction, at 3.26 V. The lack of an associated reoxidation peak in the reverse scan, as well as, the appearance of an extra peak at 0.68 V shows the instability of the electrogenerated species. If the cathodic scan is limited by reversing the scan at 1.5 V the anodic peak is absent. This means that the reduction step is followed by fast chemical reactions generating electroactive fragments at potential 0.68 V Fig. 4 is also illustrative of this mechanism and shows a set of voltamograms for the alkoxo derivative 6. A reduction peak is observed at 2.85 V; this reduction process is completely irreversible, at higher scan rates an associated reoxidation peak grows in importance. The reduced titanium species decays by a chemical reaction. Differential pulse voltammetry supports the above information; DPV studies show a cathodic peak at high negative potential for all complexes. As an example, Fig. 5 shows the cathodic scan for complexes 6 and 8 with waves at 2.94 and 3.32 V, respectively, shifted towards more negative potential values if compares with CV. Some trends can be establish depending on the alkoxo substituent, for complexes 1 and 7 the reduction peak goes beyond the range imposed by the solvent, the more electron donating substituents make these derivatives more difficult to be reduced. By ordering the complexes from low to high reduction potential values: 2 (3.36 V) < 4 (3.26 V) < 3 (3.23 V) < 8 (3.14 V) < 6 (2.84 V) < 5 (2.52 V) the electrodonating properties of the alkoxo ligands can be inferred. 0.10x10 -4

i/A

-0.10x10 -4 -0.20x10 -4 -0.30x10 -4 -0.40x10 -4 -0.50x10 -4 -0.60x10 -4

-4.0

2.3. Catalytic test The titanium complexes 1e8 have been tested as catalysts or catalyst precursors for the liquid phase epoxidation of 1-octene with ethylbenzene hydroperoxide, EBHP, (35 wt %) in ethylbenzene. Catalytic epoxidation of 1-octene with (EBHP) was performed in a glass batch reactor equipped with a magnetic stirrer, a condenser, and a septum for withdrawing samples. In a typical run, the alkene and a solution of EBHP (7.3 wt %) in ethylbenzene, were mixed in the reactor and heated to 383 K, after which catalyst was added. The reaction time was 3 h. All the catalysts afforded high conversions of ethylbenzene hydroperoxide (in most cases above 90%) (Table 2). This value is titanium content dependent, as can be seen in Fig. 6. The conversion profiles of complexes 2 and 3, reveal the conversion increases over time, slowly over the first hour and then rising sharply for those experiments with the highest amount of catalyst. Epoxide selectivity related to the hydroperoxide conversion varies significantly for the different catalysts. The selectivity of EBHP to epoxide for titanium complexes, at around 50%, appears to be lower than those observed for molybdenum homogeneous catalysts [19]. However, it remains constant along the reaction period, which supports the catalyst stability during the experiments (Fig. 7). The coordination of Ti4þ ions to alkoxo ligand yields Lewis acid sites. Although the electron donor properties of the ligands do not allow us to establish a clear correlation with the conversion and selectivity values obtained, the poorest electron donor ligands in complexes 5, 6 and 8, as observed previously in electrochemical studies, yield the lowest conversion values and slightly higher selectivity values. Previous findings reveal the fact that EBHP preferentially decomposes on hexacoordinated Ti(IV), with the subsequent drop in the selectivity [20] which demonstrates that titanium coordination environment plays an important role in the mechanism of the process. Only 3 and 4, of all the Table 2 Epoxidation of 1-octene.

0

-0.70x10 -4

Fig. 5. Differential pulse voltammograms for Ti(ODCGF)4 6 and Ti(OSclar)2 8 in THF 0.2 M [Bu4N][PF6], at Pt working electrode, vs Fc/Fcþ at 100 mV/s.

-3.5

-3.0

-2.5

-2.0

-1.5

-1.0

-0.5

0

E/V Fig. 4. Cyclic voltammogram of compound Ti(ODCGF)4 6 in THF 0.2 M [Bu4N][PF6], at Pt working electrode, vs Fc/Fcþ, scan rate from 100 to 500 mV/s.

Complex

Catalyst (g)

Conversion of EBHP (%)

Selectivity to epoxide (%)

1 2 3 4 5 6 7 8 Ti(OPri)4 þ PhSiMe3a Ti(OPri)4 þ Ph3SiCla

0.1 0.5 0.5 0.4 0.5 0.5 0.5 0.5 0.3 0.3

100 99 96 86 99 81 99 92 95 87

34 50 47 55 55 61 47 56 77 83

Reaction time ¼ 3 h, Temperature: 110  C, [EBHP] ¼ 7.3%. a Reference [20], Reaction time ¼ 2 h.

106

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100

68 66 64

Complex 2

60

% Selectivity

% EBHP Conversion

80

100 ppm 200 ppm 300 ppm 400 ppm 500 ppm

40 20

62 60 58 56 54

Complex 2 Complex 3

52 50

0

0

50

100

150

200

250

Time (min)

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Fig. 8. EBHP epoxide selectivity plotted for catalysts 2 and 3 (0.25 g of at 110  C) as a function of ethylbenzene hydroperoxide/ethyl benzene ratio.

80 % EBHP Conversion

0.2

EBHP/EB ratio

100

decrease in the organic peroxide efficiency is observed (see Fig. 8), with the selectivity values obtained for both complexes being slightly higher as the amount of oxidant decreases.

Complex 3

60

100 ppm 200 ppm 300 ppm 400 ppm 500 ppm

40

3. Experimental procedure 3.1. General remarks

20 0

0

50

100

150

200

250

Time (min) Fig. 6. Activity results in the epoxidation of 1-octene with EBHP for the catalysts 2 and 3.

complexes, are proposed to be monomeric; nevertheless the results obtained do not evidence any significant difference with the selectivity values obtained for the other complexes studied. The most significant difference arises from the type of ligand used, complexes with sugars present lower conversion values. The effect of the concentration of the oxidant (EBHP) over a range of 7.3 to 2.9 wt % EBHP in EB (ethylbenzene) was also studied, with the organic peroxide concentrations being referred to the whole final reaction mixture tested. As an example, for samples 2 and 3 a small

100 90 80 % Selectivity

48

70 60 50 40

Complex 2 Complex 3 Complex 5

30 20

90

92

94

96

98

% EBHP Conversion Fig. 7. EBHP epoxide selectivity for catalysts 2, 3 and 5.

100

All reactions were performed using standard Schlenk tube and dry box techniques under an atmosphere of dry nitrogen or argon. Solvents were distilled from appropriate drying agents and degassed before used. Adamantanol, (1R,2S,5R)-()-menthol, 1,2:5,6-di-O-isopropylidene-a-D-glucofuranose, 1,2:3,4-di-O-isopropylidene-a-D-galactopyranose, (1R)-()-myrtenol, 1,2:5,6-DiO-cyclohexylidene-a-D-glucofuranose, (1S-endo)-()-borneol and ()-sclareol were purchased from Aldrich and used as received. Ti(OiPr)4 was purchased from Aldrich, distilled and stored under an argon atmosphere prior to use. Ethyl benzenehydroperoxide, EBHP, (35 wt %) in ethylbenzene was kindly provided by Repsol-YPF (Propene oxide/styrene plant, Repsol). 3.2. Characterization Infrared spectra were recorded on a Nicolet-550 FT-IR spectrophotometer in the region 4000 to 400 cm1 as nujol mulls between polyethylene pellets and KBr disks. 1H and 13C{1H}-NMR spectra were recorded on Varian FT-300 and Varian FT-400 spectrometers and chemical shifts were measured relative to residual 1H and 13C resonances in the deuterated solvents. The UVeVis spectroscopic measurements were carried out on a Varian Cary-500 spectrophotometer. The cyclic voltammograms were taken with a potentiostat/galvanostat Autolab PGSTAT302 Metrohm. All experiments were carried out using a conventional three electrode cell. Platinum was used as working and reference electrode. A Pt wire was also used as the auxiliary electrode. Electrochemical data were obtained using 0.2 mol/L solutions of hexafluorophospate tetrabutylammonium in THF as supporting electrolyte. All solutions were deareated by bubbling high purity nitrogen. Ferrocene was employed as an internal standard in THF solution. 3.3. Catalytic test Catalytic epoxidation of 1-octene with ethylbenzene hydroperoxide (EBHP) was performed in a glass batch reactor equipped with

Y. Pérez et al. / Journal of Organometallic Chemistry 741-742 (2013) 102e108

a magnetic stirrer, a condenser, and a septum for withdrawing samples. In a typical run, 45 g of alkene (0.4 mol) and 33 g of a solution of EBHP (35 wt %) in ethylbenzene were mixed in the reactor and heated to 383 K. Titanium catalyst was then added. The concentration of EBHP was measured by standard iodometric titration. The remaining organic compounds were analyzed by gas chromatography with a flame ionization detector (GC-FID) on a Hewlett Packard 6890-plus device equipped with an HP-WAX capillary column. These samples were pre-treated with triphenylphosphine to decompose the EBHP quantitatively to 1-phenylethanol before GC analysis to avoid interferences, since EBHP decomposes without selectively under analysis conditions. The degree of selectivity to epoxide was based on the amount of EBHP consumed. Epoxide selectivity was related to the hydroperoxide converted according to the equation: S (%) ¼ 100  [epoxide]/ ([EBHP]0-[EBHP]), where epoxide represents 1,2-epoxyoctane, the subscript 0 indicates initial values, and all concentrations are expressed on a molar basis. No other by-products, such as diol or ether derivates were detected. 3.4. Synthesis of samples 3.4.1. Synthesis of Ti(OMyrten)4 (5) To a CH2Cl2 (1R)-()-Myrtenol solution (25 mL) (1.5 g, 5.70 mmol) Ti(OiPr)4 (0.43 mL, 1.4 mmol) was added dropwise. The mixture was stirred for 6 h, then, the volatiles were removed in vacuo giving colourless crystals. (1.3 g, 86%). 1H NMR (400 MHz, CDCl3, 25  C): d ¼ 0,81 (s, 24H, CH3), 1.25 (s,24H, CH3) 1,18 (m, 4H, H7ax), 2,06 (m, 4 H, H1), 2.08 (m, 4 H, H5), 2,21-2,36 (m,12 H, H4 and H7eq), 4,44 (broad signal, 8 H, CH2O-), 5,44 (s,4 H, H3). 13C{1H} NMR (400 MHz, CDCl3, 25  C): d ¼ 21.1 (CH3), 26.4 (CH3), 31.6 (C4), 31.3 (C7), 38.2 (C6), 41.2 (C5), 43.4 (C1), 68.2 (C10), 116.1 (C3), 148.1 (C2). I.R. (KBr disk, cm1): 610(br), 706(s), 727(s), 754(s), 850(m), 914(w), 967(s), 1009(s), 1065(s), 1079(s), 1125(w), 1364(m), 1375(m), 1368(m), 1457(m) 2921(m), 2967(m). TiC40O4H60 (MW 652.4): Calc.: C, 73.60; H, 9.26. Found: C, 73.36; H, 9.18%. 3.4.2. Synthesis of Ti(ODCGF)4 (6) The synthesis of 6 was carried out in an identical manner to 5. 1,2:5,6-Di-O-cyclohexylidene-a-D-glucofuranose (4 g, 11 mmol), Ti(OiPr)4 (0.88 mL, 2.75 mmol). Yield 3.6 g, 95%, colourless oil. lH NMR (300 MHz, CDCl3, 25  C): d ¼ 1.2e1.8 (m, 80 H, Cyclohexyl), 3.92 (m, 8 H, C(6)H2), 4.02 (dd, 4 H, C(4)H), 4.28 (m, 4 H, C(5)H), 4.50 (d, 4 H, C(2)H), 4.86 (m, 4 H, C(3)H), 5.85 (d, 4 H, C(1)H). 13C{lH} NMR (300 MHz, CDCl3, 25  C): d ¼ 23.8, 24.0, 24.1, 24.2, 25.1, 25.4, 35.2, 36.0, 36.7 (Cyclohexyl), 67.2 (C6), 72.3 (C4), 82.9 (C5), 85.8 (C2), 86.6 (C3), 105.1 (C1), 109.7 (-C(Cyclohexyl)), 112.5 (-C(Cyclohexyl)). I.R. (Nujol-polyethylene, cm1): 632(br), 742(m), 808(m), 847(m), 925(s), 1016(s), 1109(s), 1162(m), 1231(m), 1264(m), 1365(m), 1448(m), 2861(m), 2932(m). TiC72O24H108 (MW 1405): Calc.: C, 61.53; H, 7.75. Found: C, 61.13; H, 7.45%. 3.4.3. Synthesis of Ti(OBorn)4 (7) The synthesis of 7 was carried out in an identical manner to 5. (1S-endo)-()-borneol (4.54 g, 29 mmol), Ti(OiPr)4 (2.2 mL, 7.35 mmol). Yield 4.58 g, 94%, white solid. lH NMR (400 MHz, CDCl3, 25  C): d ¼ 0.82 (s, 12 H, CH3), 0.83 (s, 12 H, CH3), 0.88 (s, 12 H, CH3), 1.09e1.3 (m, 12 H, H4,H6ax), 1.58 (m, 4 H, H3ax), 1.67 (m, 4 H, H3eq), 2.08 (m, 4 H, H5), 2.27 (m, 4 H, H6eq), 4.64 (m, 4 H, H1). 13C{1H} NMR (300 MHz, CDCl3, 25  C): d ¼ 13.8 (CH3), 18.9 (CH3), 20.5 (CH3), 26.4 (C3), 28.4 (C4), 40.5 (C6), 47.6 (Cbridge), 50.9 (C2), 90.1 (C1). I.R. (Nujolpolyethylene, cm1): 642(br), 703(s), 786(m), 850(s), 889(m), 935(w), 991(m), 1035(s), 1068(s), 1110(s), 1139(s), 1162(s), 1232(m), 1359(m), 1388(m), 1452(m), 2877(s), 2964(s). TiC52O4H11 (MW 848.08): Calc.: C, 72.70; H, 10.37. Found: C, 72.30; H, 10.25%.

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3.4.4. Synthesis of Ti(OSclar)4 (8) The synthesis of 8 was carried out in an identical manner to 5. ()-Sclareol (2.07 g, 6.7 mmol), Ti(OPri)4 (1 mL, 3.3 mmol). Yield 3.9 g, 92%, white solid. 1H NMR (400 MHz, CDCl3, 25  C): d ¼ 0.75 (s, 12 H, CH3), 0.83 (s, 6 H, CH3), 1.13 (s, 6 H, CH3), 1.18 (s, 6 H, CH3), 1.19 (s, 6 H, CH3), carbon skeleton from 1.34 to 1.83 ppm; AMX pattern at 4.88, 5.23 and 5.26 (each 2H, CH ¼ CH2). 13C{1H} NMR (400 MHz, CDCl3, 25  C): d ¼ 18.6 (CH3), 20.7 (CH3), 21.7 (CH3), 24.5 (CH3), 25.6 (CH3), 27.6 (CH3), carbon skeleton from 15.6 to 64.6 ppm, 73.8 ((CH3)eCeOeTi), 75.0 (ring (CH3)eCeOeTi), 111.4 (CH ¼ CH2) and 146.2 (CH ¼ CH2). I.R. (Nujol-polyethylene, cm1): 642(br), 890(s), 908(s), 932(s), 1055(m), 1110(s), 1214(m), 1363(m), 1383(s), 1452(m), 2846(s), 2906(s). TiC40H68O4 Calc.: C, 72.70; H, 10.37. Found: C, 72.36; H, 10.23%. 4. Conclusions In summary, a new family of tetrasubstituted titanium(IV) alkoxo complexes, with readily available bulky ligands derived from natural products, has been prepared and characterized by spectroscopic and electrochemical techniques. On the basis of VT and DOSY 1H NMR spectroscopic studies the existence of a monomere dimer equilibrium has been proposed. The prevalence of monomeric or dimeric structure of the complexes depends on the crowding effect of the alkoxo groups surrounding the titanium centre. These Lewis acid titanium compounds have been studied in the epoxidation reaction of 1-octene with EHBP displaying good activity and constant selectivity along the reaction time which demonstrates the stability of these homoleptic titanium complexes. Acknowledgements We gratefully acknowledge financial support from the Ministerio de Educación y Ciencia, Spain (Projects CTQ2011-24346 and CTQ2012-30762). We are grateful to Repsol Company for technical support in the catalytic studies. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jorganchem.2013.05.029. References [1] (a) H. Wulff, US Pat. 3,642,833; 3,923,843; 4,021,454; 4,367,342; Brit. Pat. 1,249,079, 1971; (b) T. Sato, J. Dakka, R.A. Sheldon, J. Chem. Soc. Chem. Commun. (1994) 1887e1888; (c) J. Tang, J. Liu, J. Yang, Z. Feng, F. Fan, Q. Yang, J. Colloid Interface Sci. 335 (2009) 203e209. [2] (a) M. Taramaso, G. Perego, U. Notari, US Patent 4 410 501, (1983); (b) A. Camblor, A. Corma, J. Pérez-pariente, Zeolites 13 (1993) 82e87; (c) A. Jentys, C. Richard, A. Catlow, Catal. Lett. 22 (1993) 251e257; (d) Y.A. Kalvachev, T. Hayashi, S. Tsubota, M. Haruta, Stud. Surf. Sci. Catal. 110 (1997) 965e972; (e) K.A. Koyano, T. Tatsumi, Chem. Commun. (1996) 145e146; (f) T. Giovenzana, M. Guidotti, E. Lucenti, A.O. Biroli, L. Sordelli, A. Sironi, R. Ugo, Organometallics 29 (2010) 6687e6694. [3] C. Beck, T. Mallat, A. Baiker, New J. Chem. 27 (2003) 1284e1289. [4] G. Blanco-Brieva, J.M. Campos-Martín, M.P. de Frutos, J.L.G. Fierro, Chem. Commun. (2001) 2228e2229. [5] B.F.G. Johnson, M.C. Klunduk, C.M. Martina, G. Sankar, S.T. Teate, J.M. Thomas, J. Organomet. Chem. 596 (2000) 221e225. [6] Y. Pérez, I. Hierro, M. Fajardo, A. Otero, J. Organomet. Chem. 679 (2003) 220e228. [7] Y.S. Matveev, L.L. Frolova, N.A. Kataeva, A.V. Kuchin, Russ. J. Gen. Chem. 77 (2007) 1196e1203. [8] S.-G. Lee, Magn. Reson. Chem. 40 (2002) 311e312. [9] B.A. Borgias, S.R. Cooper, Y.B. Koh, K.N. Raymond, Inorg. Chem. 23 (1984) 1009e1016. [10] T.J. Boyle, D.L. Barnes, J.A. Heppert, L. Morales, F. Takusagawa, J.C. Connolly, Organometallics 11 (1992) 1112e1116.

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