Scandium-catalyzed reactions

Berardi S, Conte V, Fiorani G et al (2008) Improvement of ferrocene acylation. Conventional vs. microwave heating for scandium-catalyzed reaction in alkylmethylimidazolium-based ionic hquids. J Organomet Chem 693 3015-3020... 

Although the transition states of the chiral lanthanide(III)-catalyzed reactions are rather complicated due to the specific coordination number and stereochemistry of the lanthanide(III), the sense of asymmetric induction in the chiral scandium-catalyzed reactions can be rationalized by assuming an intermediate octahedral Sc(III)-dienophile complex (scheme 14). The axial chirality of (R)-binaphthol is transferred to the amine,... 

Scandium-Catalyzed Reactions in Aqueous Media 621 OSiMeo... 

Scandium-CatalYzed Reactions in Aqueous Media 623 S03Sc(0Tf)2... 

Kobayashi has found that scandium triflate, Sc(OTf)3,36 and lanthanide triflate, Ln(OTf)3, are stable and can be used as Lewis catalysts under aqueous conditions. Many other Lewis acids have also been reported to catalyze Diels-Alder reactions in aqueous media. For example, Engberts reported37 that the cyclization reaction in Eq. 12.7 in an aqueous solution containing 0.010 M Cu(N03)2 is 250,000 times faster than that in acetonitrile and about 1,000 times faster than that in water alone. Other salts, such as Co2+, Ni2+, and Zn2+, also catalyze the reaction, but not as effectively as Cu2+. However, water has no effect on the endo-exo selectivity for the Lewis-acid catalyzed reaction. 

Fig. 13). The cross-linked scandium-modified dendrimer was tested in a number of Lewis acid-catalyzed reactions, including Mukaiyama aldol additions to aldehydes and aldimines, Diels-Alder reactions, and Friedel-Crafts acylations. The dendritic catalyst was recovered by a simple filtration. The Mukaiyama aldol... 

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... 

The synthesis of the 1,4-oxazepine 179 involved the formation of intermediate 178. The latter has been obtained by the three-component reaction of amine 175, aldehyde 176, and benzotriazole, leading to the intermediate 177, which underwent dehydrative cyclization to generate 178. Lewis acid-mediated cyclization with the loss of benzotriazole anion produced the interesting 1,4-oxazepine 179 (Scheme 29) <2001JOC5590>. A similar scandium or copper triflate-catalyzed reaction has been reported <1996SL871>. 

CO2-PEG system is also effective for the scandium-catalyzed aldol reactions, and poly(ethylene glycol) dimethyl ether (PEG(OMe)2, MW = 500) is more effective than PEG (Scheme 3.12) [57]. Emulsions in C02-PEG(0Me)2 medium are observed when the concentration of the additive is 1 g/L. Not only benzal-dehyde but also substituted aromatics, aliphatic, and a, /]-unsaturated aldehydes react smoothly, and various silicon enolates derived from a ketones, esters, and thioesters also react well to afford the corresponding aldol adducts in high yields. 

Kobayashi S, Nagayama S, Busujima T, Polymer scandium-catalyzed three-component reactions leading to diverse amino ketone, amino ester, and amino nitrile derivatives, Tetrahedron Lett., 37 9221-9224, 1996. 

Novel and efficient [2 - - 3] cycloaddition reactions of NADH analogues with Q derivatives rather than the hydride-transfer reactions occur in the presence of scandium triflate, Sc(OTf)3, in MeCN to afford the cycloadducts (142). When 1 -benzyl-4-tert-butyl-1,4-dihydronicotinamide (t-BuBNAH) is used as an NADH analogue in the Sc +-catalyzed reaction with Q, the crystal structure of the cycloadduct was determined successfully, as shown in Fig. 53 (142). 

The benzoylation of both activated and deactivated aromatic compounds with benzoic acids (BACs) is achieved by using bismuth triflate (10% mol) in the presence of TFAA or heptafluorobutyric anhydride (HFBA).27 meffl-Xylene undergoes acetylation with 100% yield by using acetic acid (AAC) in the presence of TFAA under bismuth or scandium triflate catalysis (10% mol). The bismuth-triflate-catalyzed reaction can be extended to different aromatics as well as to aliphatic and aromatic carboxylic acids, giving ketones in nearly quantitative yield. Benzene and chlorobenzene are benzoylated with a BAC/HFBA mixture in the presence of bismuth triflate (10% mol), giving the corresponding BPs in 90%... 

Kitazume and Zulfiqar have investigated the aza-Diels-Alder reaction in 1,8-diazabicyclo[5,4,0]-7-undecenium trifluoromethanesulfonate [EtDBU][OTf] [184] (Fig. 5.2-5). This reaction involved the scandium(iii) trifluoromethanesulfonate catalyzed reaction of an imine (usually generated in situ from an aldehyde and an amine) with a diene. An example of this reaction is given in Scheme 5.2-74. The yields in this reaction were high (80-99%) and it was found that the ionic liquid could be recycled and reused. 

Non-chloroaluminate ILs, which are in general poor nucleophiles, have proven to be attractive alternative media for Lewis acid catalyzed reactions. ILs may have a reaction rate accelerating effect, and they may improve selectivity and facilitate catalyst recovery. This is the case for scandium triflate catalyzed Diels-Alder cycloaddition [8,9], three-component (aldehyde, aniline, triethylphosphite) synthesis of a-aminophosphonates [10], Claisen rearrangement and cyclization reactions [11], or Friedel-Crafts reactions [12, 13]. 

Recently, scandium-catalyzed hydromethylation of propene to iso-butane was achieved, but the reaction was very slow. The large isotope effect (CH4 vs CD4) is consistent with a n-bond metathesis mechanism (Scheme 4) (74). 

Scheme 1 Chiral scandium-catalyzed aldol reactions...

Scandium-catalyzed hydroxymethylation reaction with aqueous formaldehyde in water proceeds without any organic cosolvents. In the presence of Sc(0S03Ci2H25)3, a representative LASC, both achiral and asymmetric hydrox-ymethylations proceed smoothly with high selectivities. Lewis acid-base interactions between the scandium catalyst and formaldehyde were suggested to be cracial on the basis of several experiments. This is the first example of a catalytic hydroxymethylation of silicon enolates with aqueous formaldehyde in water that does not require any... 

The Mannich and related reactions provide one of the most fundamental and useful methods for the synthesis of P-amino ketones or P-amino esters. Three-component Mannich-type reactions of aldehydes, amines, and silyl enol ethers have been developed. With the development of green solvent systems, this reaction was also examined in a fluorous phase using perfluorinated rare earth metal salts including Sc(0S02C8Fi7)3 [5]. A characteristic point of this system is that it can be reused many times without reloading a new catalyst. There are also many reports on other scandium-catalyzed Mannich reactions. For example, Sc(OTf)3 was found to be an efficient catalyst for the three-component Mannich-type reactions of aromatic aldehydes, ketones, and nitriles in the presence of acetyl chloride (Scheme 12.3) [6]. 

In the presence of molecular sieves 5A (MS 5A), scandium perfluoroalkanesul-fonate-catalyzed Diels-Alder reaction of isoprene and methyl vinyl ketone proceeded smoothly in dry dichloromethane (Scheme 12.14) [25]. It was found that water interfered with the reactions, contrary to most rare earth-catalyzed reactions that proceed smoothly in aqueous media. Among the scandium perfluoroalkanesul-fonates tested, Sc(OTf)3, scandium pentafluoroethanesulfonate (Sc(0S02C2Fs)3), and scandium nonafluorobutanesulfonate (Sc(0S02C4H9)3) gave the highest yields and selectivities. 

An alternative method to alkylate pyridine is the use of alkene coupling partners. One example is the scandium-catalyzed transformation in the presence of an excess of norbornene (3 equiv) and a catalytic amount of an activator tris(pentafluorophenyl)borane (eq 34) Both mono- and bis-C2-alkylated products are obtained selectively by adjusting the reaction time. After 27 h at 70 °C in toluene, pyridine is fully converted to the monoalkylated pyridine isolated in 79% yield with no trace of the double addition product. However, if the reaction is allowed to stir for additional 3 h, only the bis-alkylated product is isolated in 99% yield. The scandium catalyst in combination with B(C6p5)3 is thought to promote the C-H metalation ortho to the pyridyl nitrogen, a ratedetermining step as suggested by the representative kinetic isotope effects (fe/ifeD 4.0). 

---