Scandium cyclopentadiene

Chiral titanium- and scandium-based catalysts (61 and 62, Figure 3.11) were used to accelerate the cycloadditions of acyl-l,3-oxazolidin-2-ones 60 (Scheme 3.14) with butadiene, isoprene and cyclopentadiene. The cycloadditions... 

Fukuzawa et al. [99] found analogous scandium(III)triflate/ Pr-PyBOx complex as efficient catalyst for the asymmetric Diels-Alder reaction between cyclopentadiene or acyclic dienes and acyl-l,3-oxazohdin-2-ones with up to 90% ee. They latter described the same reaction in super critical CO2 in the presence of MSdA [ 100] that proceeded more rapidly than in CH2CI2 leading to the expected product with analogous selectivity. 

Scott Oakes et al. (1999a, b) have shown how adoption of SC conditions can lead to a dramatic pressure-dependent enhancement of diastereoselectivity. In the case of sulphoxidation of cysteine derivatives with rert-butyl hydroperoxide, with cationic ion-exchange resin Amberlyst-15 as a catalyst, 95% de was realized at 40 °C and with SC CO2. By contrast, with conventional solvents no distereoselectivity was observed. Another example is the Diels-Alder reaction of acrylates with cyclopentadiene in SC CO2 at 50 °C, with scandium tris (trifluoromethanesulphonate) as a Lewis acid catalyst. The endoiexo ratio of the product was as high as 24 1, while in a solvent like toluene it was only 10 1. 

Although the above demonstrated that product control could be achieved in scC02, the difference in selectivity was relatively small. However, later work using a Lewis acid catalyst, scandium triflate, on the Diels-Alder reaction of n-butyl acrylate and cyclopentadiene (Scheme 7.7) showed that the endo exo ratio was again found to rise to a maximum and then decrease again as the pressure, and hence density, was increased (Figure 7.3) [19]. 

In the Mukaiyama aldol additions of trimethyl-(l-phenyl-propenyloxy)-silane to give benzaldehyde and cinnamaldehyde catalyzed by 7 mol% supported scandium catalyst, a 1 1 mixture of diastereomers was obtained. Again, the dendritic catalyst could be recycled easily without any loss in performance. The scandium cross-linked dendritic material appeared to be an efficient catalyst for the Diels-Alder reaction between methyl vinyl ketone and cyclopentadiene. The Diels-Alder adduct was formed in dichloromethane at 0°C in 79% yield with an endo/exo ratio of 85 15. The material was also used as a Friedel-Crafts acylation catalyst (contain-ing7mol% scandium) for the formation of / -methoxyacetophenone (in a 73% yield) from anisole, acetic acid anhydride, and lithium perchlorate at 50°C in nitromethane. 

In subsequent studies, methyl vinyl ketone (2.0 mmole) was chosen as the dienophile so as to determine the combined effect of the ionic liquid (2 mL) and the Lewis acids (0.2 and 0.5 wt%) upon the yield and selectivity. Without the Lewis acid catalyst, this system demonstrated a 52% conversion of the cyclopentadiene (2.2 mmol) in 1 h with the endojexo selectivity being 85/15. The cerium triflate-catalyzed reaction was quantitative in 5 min and the endo. exo selectivity was very good for this experiment as well (94 6, endo. exo). Also with the scandium or yttrium salts tested, reactions came to completion in a short time with high stereo-selection. Cerium, scandium and yttrium triflates are strong Lewis acids known to be quite effective catalysts in the cycloadditions of cyclopentadiene with acyclic aldehydes, ketones, quinones and cycloalkenones. These compounds are expected to act as strong Lewis acids because of their hard character and the electron-withdrawing triflate group. On the other hand, reaction times of 1 hour were required for... 

Kobayashi reported an asymmetric Diels-Alder reaction catalyzed by a chiral lanthanide(III) complex 24, prepared from ytterbium or scandium triflate [ Yb(OTf)3 or Sc(OTf)3], (Zf)-BINOL and tertiary amine (ex. 1,2,6-trimethylpiperidine) [30], A highly enantioselective and endose-lective Diels-Alder reaction of 3-(2-butenoyl)-l,3-oxazolidin-2-one (23) with cyclopentadiene (Scheme 9.13) takes place in the presence of 24. When chiral Sc catalyst 24a was used, asymmetric amplification was observed with regard to the enantiopurity of (/ )-BINOL and that of the endoadduct [31 ]. On the other hand, in the case of chiral Yb catalyst 24b, NLE was affected by additives, that is, when 3-acetyl-l,3-oxazolidin-2-one was added, almost no deviation was observed from linearity, whereas a negative NLE was observed with the addition of 3-pheny-lacetylacetone. 

Although asymmetric versions of aza Diels-Alder reactions using chiral auxiliaries have been reported, only one example uses a stoichiometric amount of a chiral Lewis acid [44]. The first reported example of a catalytic enantioselective aza Diels-Alder reaction employed a chiral lanthanide catalyst [45]. A chiral ytterbium or scandium catalyst, prepared from Yb(OTf)3 or Sc(OTf)3, (i )-BINOL, and DBU, is effective in the enantioselective aza Diels-Alder reactions. The reaction of A-alkylidene- or N-arylidene-2-hydroxyaniline with cyclopentadiene proceeded in the presence of the chiral catalyst and 2,6-di-rerf-butyl-4-methylpyridine (DTBMP) to afford the corresponding 8-hydroxyquinoline derivatives in good to high yields with good to excellent diastereo- and enantioselectivity (Eq. 15). 

Kobayashi and coworkers have reported that a chiral complex derived from scandium(III)triflate, i -(-i-)-BINOL ((i )-59), and 1,2,6-trimethylpiperidine in the presence of 4 A molecular sieves catalyzes the reaction of unsaturated imides with cyclopentadiene in 96-97% ee (Scheme 49) [130]. 

Further examples also show particularly impressive selectivity. Kiindig has used the cationic iron catalyst (8.83) in the Diels-Alder reaction to give cycloadduct (8.85). The ruthenium (BINAP) catalyst (8.86) effects the Diels-Alder reaction of cyclopentadiene with 2-substituted acrylates with ees up to 99%. Kobayashi has shown that lanthanide and scandium triflate complexes of BINOL with 1,2,6-trimethylpiperidine, formulated as complex (8.87), are effective with oxazohdinone-based substratesand gives good selectivity in the formation of the Diels-Alder product (8.52). 

Supercritical carbon dioxide (SC-CO2) is an environmentally benign solvent that is providing a viable alternative to the traditional organic solvents. A Diels-Alder reaction between n-butyl acrylate and cyclopentadiene was investigated with the Lewis acid catalyst scandium tris (trifluoromethanesuffonate), primarily due to its solubility in SC-CO2 (Eq. 10-12) ... 

Cl2Rh2CgH,g, Rhodium(I), di-/jtetrakis(ethene)di-, 28 86 Cl2Rh2CigH24, Rhodium(I), di-/i-chloro-bis-( ( -l,5-cyclooctadiene)di-, 28 88 CI2RUC7H8, Ruthenium(II), (f/ -bicydo-[2.2.1]hepta-2,S-diene)dichloro-, 26 250 Cl2RuCgH]2> Ruthenium(II), dichloro(t -cycloocta-l,S-diene)-, 26 253 —, Ruthenium(II), di- j-chloro-(j -l,5-cyclooctadiene)-, polymer, 26 69 Cl2Sc2Si4C44Hg4, Scandium, tetrakis[ -l,3-bis(trimethylsily )cyclopentadien-yl]di-/i-chloro-di-, 27 171 Cl2Si2ThC22H42, Thorium(IV), bis(i -l,3-bis(trimethylsilyl)cyclopentadien-yl]dichloro-, 27 173 Cl2Si2UC22H42, Uranium(IV), bis(i -1,3-bis(trimethylsilyl)-cyclopentadieny 1 ]dichloro-,... 

Similarly, the catalyst prepared from jdterbium triflate (Yb(OTf)3) and Bl-NOL acts as efficient enantioselective catalyst in the reaction of the previous oxazolidine derivative and cyclopentadiene. The role of the amine addition is determined (113). The analogous chiral scandium catalysts are prepared from Sc(OTf)3, BINOL, and tertiary amine in the same manner (114). As a major difference between Yb(lll) and Sc(lll) chiral catalysts, their coordination numbers are thought although Sc(III) is known to coordinate up to seven ligands, the specific coordination of Yb(III) allows up to 12 ligands (115). 

Later, several bisimine and diol-based chiral ligands were examined as Sc(OTf)3 complexes in the asymmetric Diels-Alder reaction of cyclopentadiene with 3-acryloyloxazolidin-2-one in the presence of 2,6-lutidine [134]. The scandium-salen complex was revealed to be the most effective catalyst and gave the endo adduct in 81% yield with 85% ee (endo/exo = 89/11). 

A comparative study on the influence of different lanthanide cations and the substituents at the 4 and 5 -positions in the pybox ligands in the Diels-Alder reaction of cyclopentadiene with 3-acryloyl-l,3-oxazolidin-2-one was realized [135], Of the catalysts derived from (4 P,5 P)-4-Me-5-Ph-pybox, the Sc(III)-based one is both strongly endo selective (endo/exo = 97/3) and highly enantioselective (97% ee (endo)). Later, it was found that the scandium-(4 S,5 S)-4-CH20TIPS-5-Ph-pybox complex also gave endo selective (endo/exo = 92/8-96/4) and highly enantioselective (93-99% ee (endo)) for the Diels-Alder reactions of cyclopentadiene with 3-acryloyl-or 3-crotonoyloxazolidinones. 

Polymer-supported scandium-based Lewis acids are also active for the Diels-Alder reaction of 3-acryloyl-l,3-oxazolidin-2-one with cyclopentadiene. This reaction proceeded smoothly in the presence in water (Equation (8.46)) affording the corresponding adduct quantitatively [70]. 

The importance of the Lewis acid cation in this process was shown from the lack of activity of pristine H-USY in the model aza-Diels-Alder of A-benzylidenaniline with cyclopentadiene leading to the tetrahydroquinoline product (Equation (8.48)) [136]. The immobilization of the scandium cation inside the pores of this zeolite with oidy a coordination position occupied by a triflate group changed completely the behavior of this zeolite affording an almost total yield in the condensation product. 

The name of metallocenes is derived from ferrocene, which is a complex compound of iron with dicyclopentadiene. The metallocene complexes are new types of Ziegler-Natta catalysts. The general formula of these catalysts is identified as Cp MeCl, where Cp denotes the cyclopentadiene ring or its derivatives. Me is metal atom, which the most often is zirconium (Zr), hafhium (Hf), titanium (Ti), scandium (Sc), thorium (Th) or another rare earth element. 

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