Triflates catalysts

Butyraldehyde undergoes stereoselective crossed aldol addition with diethyl ketone [96-22-0] ia the presence of a staimous triflate catalyst (14) to give a predominantiy erythro product (3). Other stereoselective crossed aldol reactions of //-butyraldehyde have been reported (15). 

Trimethylsilyl iodide can be substituted for the trimethylsilyl triflate catalyst in the reactions of aliphatic aldehydes. TMSI can be generated conveniently in situ either from trimethylsilyl chloride and sodium iodide in acetonitrile314 or from hexamethyldisilane and iodine in dichloromethane334 or pentane.338 It is noted that neither triisopropylsilane nor PMHS is an effective reducing agent for this purpose when used with TMSI under these conditions.314,334... 

In addition to Bronsted acid promoted Fischer-type glycosylations, Lewis acids have been investigated (Scheme 3.4). A variety of Lewis acids promote glycosylation under mild conditions, often in substoichiometric amounts. The earliest examples include ZnCl2 [18] and FeCl3 [19], although these readions were demonstrated only for preparation of trehalose-type disaccharides. Mukaiyama et al. have very recently developed metal triflate catalysts for the dehydrative glycosylation with... 

Given the prevalence of bis(oxazoline)-copper catalysts as chiral Lewis acids, it seems appropriate to comment briefly on catalyst preparations, since differences arise in the nature of the catalyst complex. Triflate-derived catalysts are formed simply by combining the ligand and Cu(OTf)2 in a given solvent and stirring for an appropriate length of time (typically >2 h) to achieve complete dissolution and complexation, Scheme 14. The hydrated version is formed by addition of 2 equiv of water to this catalyst solution, followed by removal of solvent after 15 min of stirring. The hydrated triflate catalyst is bench stable for months. 

It was assumed that the triflate counterions of 269c are fully dissociated in solution, at least in the presence of the dicarbonyl substrate. However, an examination of the influence of the counterion revealed that SbF6 provides a much more active catalyst than the triflate counterpart (199). Whereas the triflate catalyst 269c requires 10 h for the reaction to proceed to completion at -78°C, the SbF6 catalyst 271c induces complete conversion to cycloadduct in 4 h under identical conditions, albeit with slightly eroded diastereoselectivity (96 4 vs 98 2) (200). Enantioselec-tivity is identical for the two catalysts (>98% ee, endo diastereomer). 

Less reactive diene partners could also be successfully used in cycloadditions with acryloylimide in the presence of 271c as catalyst. For example, reaction of cy-clohexadiene and acryloylimide requires only 5 h to proceed to completion in the presence of SbF6 catalyst 271c, as contrasted to 48 h with the triflate catalyst 269c,... 

Allenyltrimethylsilanes add to ethyl glyoxalate in the presence of a chiral pybox scandium triflate catalyst to afford highly enantioenriched homopropargylic alcohols or dihydrofurans, depending on the nature of the silyl substituent (Tables 9.39 and 9.40) [62]. The trimethylsilyl-substituted silanes give rise to the alcohol products whereas the bulkier t-butyldiphenylsilyl (DPS)-substituted silanes yield only the [3 + 2] cycloadducts. A bidentate complex of the glyoxalate with the scandium metal center in which the aldehyde carbonyl adopts an axial orientation accounts for the observed facial preference ofboth additions. 

More recently spinning disc reactors have been used by Wilson et alP0) to carry out catalytic reactions using supported zinc triflate catalyst for the rearrangement of a-pinene oxide to yield campholenic aldehyde. The results of this study, presented in Table 20.1, suggest that by using a supported catalyst on a spinning disc reactor it is possible to... 

Wilson, K., Renson, A. and Clark, J. H. Catalysis Letters 61 (1-2) (1999) 51-55. Novel heterogeneous zinc triflate catalysts for the rearrangement of alpha-pinene oxide. 

The green chemistry advantages of using triflates are not only that it helps avoid the use of hazardous catalysts, but also that the triflates themselves can be recovered and reused rather easily. In one series of reactions involving the reaction between phenethyl alcohol and acetic acid, for example, Barrett s team found that yields of 95-98 percent were possible, with catalyst recovery amounting to no less than 98 percent and, in most cases, nearly 100 percent. The team concluded one of their reports on the use of triflate catalysts by suggesting that the catalysts they used were "readily recyclable and we believe this to be a major step forward in the area of clean technology for aromatic nitration (Francis J. Waller, et ah, "Lanthanide(III)... 

Elsewhere, Faita et al. (438) bound the Evans chiral auxiliary to Wang or Merrifield resin for use as a dipolarophile in cycloadditions with C,N-diphenyl-nitrone. Yields on both resins are significantly reduced in comparison to the solution phase reaction (43-20% compared to 95%) but are unaffected by addition of magnesium perchlorate or scandium triflate catalyst. A one-pot process has been reported by Hinzen and Ley (439) that oxidizes secondary hydroxylamines to the... 

Takacs, J.M., Lawson, E.C., Reno, M.J., Youngman, M.A. and Quincy, D.A. (1997) Enantioselective Diels—Alder reactions Room temperature bis(oxazoline)-zinc, magnesium, and copper triflate catalysts. Tetrahedron Asymmetry, 8, 3073—3078. 

The influence of Lewis acids on the 4 + 2-cycloaddition of (2ft,2/ft)-A,iV/-fumaro-ylbis[fenchane-8,2-sultam] with cyclopentadiene and cyclohexadiene was investigated by IR studies of the sultam compexes with various Lewis acids.101 The first enantios-elective silicon Lewis acid catalyst (91) catalysed the Diels-Alder cycloaddition of methacrolein and cyclopentadiene with 94% ee.102 [A1C13 + 2THF] is a new and efficient catalytic system for the Diels-Alder cycloaddition of a,/9-unsaturated carbonyl compounds with dienes under solvent-free conditions.103 Dendritic copper(II) triflate catalysts with a 2,2 -bipyridine core (92) increased the chemical yields of Diels-Alder adducts.104... 

The activated a-trimethylsilyl group on the pyrimidine moiety reacts with the triflate ion 26 to regenerate the triflate catalyst. Under reversible and thus thermodynamically controlled conditions, the nucleophilic silylated base 23 attacks the carbohydrate cation 25 only from the top (/(-side) to afford exclusively the /(-nucleoside. 

Several catalysts have been found for the ring opening of epoxides. For instance, cyclohexene oxide gave an excellent yield of the trans-fi-amino alcohol when treated with either aromatic or aliphatic amines in the presence of a scandium triflate catalyst.21 Aromatic epoxides react in a regiospeciflc reaction at the benzylic carbon when treated with aromatic amines and scandium triflate but at the least substituted carbon of the epoxide ring when aliphatic amines react. Electronic effects are more important in the reactions of the aromatic epoxides whereas steric factors control the reaction with aliphatic epoxides. 

The use of aromatic imines to produce tetrahydroquinolines has been shown to be promoted by the use of bismuth(m) chloride and triflate catalysts <2000SL1160>. 

C4C,im][PF6] [C4C1im][OTf [RuCp (r 4-sorbic acid)]+ Sorbic acid 16 bar, 20-60 °C MTBE as co-solvent higher rates than [15] in ethylene glycol no catalytic activity in [C4Cim][OTf] due to coordination of the triflate catalyst reused three times detailed kinetic studies product isolated by decantation. 

Yb(OTf)3 is an excellent catalyst for the aldol reactions of silylenol ethers with aldehydes in aqueous solution, working better than in organic solvents like THF and MeCN, though the reactions can also be performed in organic solvents, and, after the reaction has been quenched by the addition of water, the triflate catalyst may be recovered from the aqueous layer. 

Extensions of the triflate catalyst include the use of versions with fluorinated ponytails such as [Sc C(S02C8Fi7)3 3] for use in fluorous phase Diels-Alder reactions (fluorinated solvent.)... 

Allyl groups can be easily attached to azines through the standard activation-nucleophilic attack approach. Quinoline (or isoquinoline) smoothly reacts with chloroformate and allylsilanes (76) in the presence of iodine or triflate catalysts (Scheme 13). Using two equivalents of silane reagent in the isoquinoline reaction, gives adduct 79 [85-87]. 

Copper triflate has been found to promote the [3+2] cycloaddition of iV-tosylaziridines with nitriles <2006TL5399>. Ghorai has found that aziridine 149 and a substituted benzonitrile were dissolved and added to a suspension of copper triflate, resulting in the formation of good yield of the imidazoline 150. The mechanism proposed involves the addition of the nitrogen atom of the nitrile to the presumed copper aziridinium ion formed with the triflate catalyst. Subsequently, the iV-Ts moiety attacks the electrophilic carbon of the nitrilium ion to form the imidazoline. 

Cationic Cyclization. A cationic cyclization of BINOL-derived neryl ether has been accomplished with an organoalu-minum triflate catalyst. Limonene is obtained in 54% yield and... 

Phenols and naphthols also react with unprotected a- and -glycosides, directly, in the presence of trimethylsilyl triflate catalyst under mild conditions (equation 66) . 

Salomon and his coworkers have over the years studied the influence of copper(I) triflate on the cyclization of non-conjugated dienes. In the present example the diene (170) is converted readily on irradiation in the presence of the catalyst and affords the alcohol (171). This is oxidised to the corresponding ketone. The intramolecular cyclization of the diene (172), using a copper triflate catalyst, affords the straight (2-1-2) adduct (173). This cyclization was used as an approach to the synthesis of RobustadiaJ B. However, it was shown that the proposed structure of the natural product was wrong and that the robustadials should have the camphahe moiety in their structure as shown in (174). ... 

In older applications of aluminum species in the Friedel-Crafts reaction ADC 3 (X = C1, Br, I) have been used as catalysts [6], Effects of covalently-attached strongly electron-withdrawing groups on the reactivity of aluminum(III) were investigated. The tris(triflate) catalyst Al(OTf)3 was originally devised by Olah and coworkers (Scheme 6.74) [96]. As expected, tris(trifryl)imide catalyst 102 had more pronounced catalytic efficiency, because of its greater Lewis acidity (Scheme 6.75) [97]. 

A-Glycosylimidazoles (related to nucleosides) are of interest in the preparation of novel chemotherapeutic agents, hence selective A -glycosylation techniques are of interest. The palladium-catalyzed addition of vinyl epoxides and allyl acetates to imidazoles provides one such route to imidazole nucleoside analogues <91JCS(Pi)2603, 9UOC4990>. The silyl-Hilbert-Johnson method is another in which the imidazole is silylated, then treated with a suitable peracetylated carbohydrate derivative in the presence of trimethylsilyl triflate catalyst. With 4-carbamoylimidazolium-5-olate, for example, the major product is the l-glycosyl-5-carbamoyl isomer with only low yields of the... 

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