Catalysts bismuth triflate

The carbonyl group can be deprotected by acid-catalyzed hydrolysis by the general mechanism for acetal hydrolysis (see Part A, Section 7.1). A number of Lewis acids have also been used to remove acetal protective groups. Hydrolysis is promoted by LiBF4 in acetonitrile.249 Bismuth triflate promotes hydrolysis of dimethoxy, diethoxy, and dioxolane acetals.250 The dimethyl and diethyl acetals are cleaved by 0.1-1.0 mol % of catalyst in aqueous THF at room temperature, whereas dioxolanes require reflux. Bismuth nitrate also catalyzes acetal hydrolysis.251... 

We developed a method for the synthesis of a variety of cyclic acetals that utilizes bismuth triflate as a catalyst and does not require the use of a Dean-Stark trap for removal of water [102]. In this method, an aldehyde or ketone is treated with 1,2-bis (trimethylsiloxy)ethane in the presence of bismuth triflate. A comparison study using o-chlorobenzaldehyde showed that with ethylene glycol a low conversion to the dioxolane was observed after 2 h whereas the use of the 1,2-bis(trimethylsiloxy) ethane afforded the corresponding dioxolane in good yields. (Scheme 9). 

Initial investigations in the Mannich-type reaction of silyl enolates with benzal-dehyde and aniline employed a series of bismuth(III) salts (Scheme 9, Table 10). These results were promising because the corresponding (l-amino ketone could be obtained in moderate to good yield with bismuth halides, except bismuth fluoride (Table 10, entries 1 1). Bismuth nitrate smoothly afforded the expected product (Table 10, entry 5). While bismuth acetate gave no conversion, bismuth trifluor-oacetate provided the product in only moderate yield (Table 10, entries 6 and 7). Phenyl bismuth ditriflate and diphenyl bismuth triflate appeared to be more efficient catalysts than all those previously tested (Table 10, entries 8 and 9). Bismuth(III) triflate led to the expected product in a good yield and in a short reaction time, without any difference between the anhydrous and the hydrated form (Table 10, entries 10 and 11). 

Bismuth triflate has been reported by Dubac as an efficient catalyst for the Mukaiyama aldol reaction with silyl enol ethers [27] and was recently used with a chiral ligand, as reported by Kobayashi in an elegant hydroxymethylation reaction... 

Tetrahydropyrans hydroxylated at the 4-position have good synthetic value [113]. Although many synthetic methods have been reported [17-23,114,115], the search for potential alternate approaches and the development of eco-friendly and high-yielding reactions resulted in the development of a method that poses less problems for the environment. Synthesis of tetrahydropyranol derivatives can be achieved through the Prins-type cyclization reaction of homoallylic alcohols with aldehydes using bismuth triflate as catalyst in [bmim]PF6 solvent system [108] (Fig. 22). 

A variety of methods have been developed for the preparation of substituted benzimidazoles. Of these, one of the most traditional methods involves the condensation of an o-phenylenediamine with carboxylic acid or its derivatives. Subsequently, several improved protocols have been developed for the synthesis of benzimidazoles via the condensation of o-phenylenediamines with aldehydes in the presence of acid catalysts under various reaction conditions. However, many of these methods suffer from certain drawbacks, including longer reaction times, unsatisfactory yields, harsh reaction conditions, expensive reagents, tedious work-up procedures, co-occurrence of several side reactions, and poor selectivity. Bismuth triflate provides a handy alternative to the conventional methods. It catalyzes the reaction of mono- and disubstituted aryl 1,2-diamines with aromatic aldehydes bearing either electron-rich or electron-deficient substituents on the aromatic ring in the presence of Bi(OTf)3 (10 mol%) in water, resulting in the formation of benzimidazole [119] (Fig. 29). Furthermore, the reaction also works well with heteroaromatic aldehydes. 

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>. 

The tetrahydropyranylation of alcohols under solvent-free conditions is efficiently catalyzed by bismuth triflate (0.1 mol%). The experimental procedure is simple and works well with a variety of alcohols and phenols. The catalyst is insensitive to air and small amounts of moisture, easy to handle and relatively non-toxic. The deprotection of THE ethers is also catalyzed by bismuth triflate (1.0 mol%). 

Bismuth(III) triflate tetrahydrate can be utilized as a more commercially available catalyst for the Fries rearrangement of PAs to 2-hydrox-yaryl ketones. In the reaction with 1-naphthyl acetate, bismuth triflate (10% mol) gives the best yield in 2-acetyl-l-naphthol, the amount of... 

Diketones condense to furans or pyrroles in the presence of a primary amine and an acidic catalyst. Yadav has carried out numerous such reactions in the ionic liquid [BM1M][BF4], using bismuth triflate as a catalyst [256]. The reaction typically gave 80-90% yield and the ionic hquid/catalyst was easily recycled (Scheme 5.2-112). With the addition of a primary amine to a 1,4-diketone, pyrroles can be formed. 

Epoxides can also be converted to 1,3-dioxolanes by treatment with acetone in the presence of catalytic amounts of bismuth(lll) salts, with yields ranging from 87-99%. For example, the epoxy allyl ether 98 provided the dioxolane 99 in 97% yield using bismuth triflate as the catalyst <01SC3411>. When simple epoxides are treated with Aw(triphenylphosphine)-iminium cobalt tetracarbonyl (PPN-Co(CO)4) under Lewis acid catalysis, a carbonyl insertion reaction provides p-lactones regioselectively in good to high yields. The carbonylation occurs selectively at the unsubstituted position, and the reaction is... 

Water-stable ionic liquids were later used for Friedel-Crafts acylations, using a metal triflate catalyst. Cu(0Tf)2 proved to be the most efficient catalyst for this transformation, and acylation of anisole by benzoyl chloride in [bmim][BF4] gave almost exclusively the para adduct 68 (Scheme 24). This reaction can also be performed in organic solvents, but an accelerated rate is observed in ionic liquids.Catalyst loading can be decreased (up to 1 mol%) using bismuth(m) salts as catalysts. ... 

Mohan has also worked in the multicomponent synthesis of nonchiral homoallyl ethers starting from aldehydes, aUcoxytrimethylsilanes, and trimethylallyl silanes using bismuth triflate (Bi(OTf)3-xHjO) [86] and iron(III) tosylate (Fe(0Ts)3 6H20) [87] as inexpensive and relatively noncorrosive catalysts. 

For a simUar example, using 1 mol% of bismuth triflate (Bi(OTf)j) as catalyst, see T. OUevier, T. Ba, Tetrahedron Lett. 2003,44, 9003-9005. Highly efficient three-component synthesis of protected homoaUyhc amines by bismuth triflate-catalyzed aUylation of aldimines. 

Bismuth triflate was found to be an efficient catalyst in the Mannich-type reaction of silyl enolates with N-alkoxycarbonylamino sulfones. The reaction proceeded smoothly with a low catalyst loading of bismuth triflate (0.5-1.0 mol%) to afford the corresponding protected -aminocarbonyl compounds in very good yield (Equation 7) [29]. 

In continuation of previous studies where Bi(0Tf)3 xH20 was reported to be an efficient catalyst for the direct transformation of internal epoxides to a-diketones in 31-77% yield (Equation 51) [123], a mechanism was proposed with the support of various experiments (Scheme 11.2) [123bj. It is proposed that bismuth triflate acts as a Lewis acid and the transient release of triflic acid is postulated (such ligand exchange has been previously observed) [21]. 

Catalytic asymmetric hydroxymethylation of silicon enolates with an aqueous formaldehyde solution has been developed by Kobayashi et ah using a bismuth triflate associated with a chiral bipyridine in a DME/H2O mixture the reaction proceeded smoothly in the presence of 1 mol% catalyst to afford the hydroxymethylated adducts in high yields and 77-93% ee. Chiral anionic surfactants associated with Ga(OTf)3, Cu(OTf)2, or Sc(OTf)3 catalyzed Mukaiyama aldol reactions in water with moderate to good diastereo-and enantioselectivities. ... 

Bismuth is another example of nonrecommended cation by Table 8.1. It is also one of the most investigated metal triflate catalysts in organic reactions. Ollevier and Lavie-Compin used Bi(OTf)3 as catalyst for the Mannich reaction in water [28], Cyclohexene oxide was treated with amines such as p-methylaniline to isolate the corresponding p-amino alcohols in good yields (Equation (8.10)). Sterically more hindered anilines such as o-methylaniline also led to the alcohols in good yield (SDS, sodium dodecyl sulfate). 

Methoxymethylation of a variety of structurally divergent alcohols and carboxylic acids was also achieved with microencapsulated bismuth triflate into an inert organic polymer matrix (Equation (8.65)) [32], This heterogeneous catalyst affords corresponding ethers, and carboxylic esters in good to excellent yields. 

Metal catalysts such as bismuth triflate (Bi(OTf)3) and iron chloride hexa-hydrate (FeCl3 6H2O) have been shown to catalyse Ritter reaetions by Barrett and Cossy respeetively. Barrett used 20 mol% Bi(OTf)3 in the synthesis of A(-fe/t-butyl amides by coupling a range of alkyl and aryl nitriles with ferf-BuOH or tert-butyl aeetate (terf-BuOAc). Although Cossy used lower catalyst loadings of a cheaper metal, higher temperatures of 150 °C were... 

Bismuth(III) triflate is also a powerful acylation catalyst that catalyzes reactions with acetic anhydride and other less reactive anhydrides such as benzoic and pivalic anhydrides.113 Good results are achieved with tertiary and hindered secondary alcohols, as well as with alcohols containing acid- and base-sensitive functional groups. 

Friedel-Crafts acylation generally involves reaction of an acyl halide and Lewis acid such as A1C13, SbF5, or BF3. Bismuth(III) triflate is also a very active acylation catalyst.46 Acid anhydrides can also be used in some cases. For example, a combination... 

Bismuth tra-tri lluoromcthancsulfonate, Bi(OTf)3, and BiCh were found to be effective catalysts for the Friedel-Crafts acylation of both activated and deactivated benzene derivatives such as fluorobenzene.19 Ga(III) triflate is also effective for Friedel-Crafts alkylation and acylation in alcohols and can tolerate water.20 This catalyst is water-stable... 

Cyclization Reactions Using Bismuth(III) Triflate as a Catalyst. 50... 

Recently, Kobayashi has disclosed significant advances regarding rare-earth and lanthanide triflates as catalysts for Mannich-type reactions [65-68] and there are several reviews available on catalytic Mannich-type reactions [69-73]. High catalytic activity, low toxicity, and low tolerance to moisture and air make lanthanide triflates valuable catalysts. However, the high cost of these catalysts restricts their use. Bismuth compounds are of interest as lower toxicity and cheaper alternatives to such catalysts. 

On the basis of these initial results, various rare earth metal triflates, including Sc(OTf)3, Hf(OTf)4 and Yb(OTf)3 were applied as catalysts [27-29]. Recently Beller and coworkers developed efficient Friedel-Crafts alkylations with catalytic amounts of Rh, W, Pd, Pt and Ir complexes [30] or FeCl3 [31-34] as Lewis acid catalysts. However, in the latter cases high catalyst loadings had to be applied. To overcome these major drawbacks, we decided to develop a Bi(III)-catalyzed Friedel-Crafts alkylation of arenes with benzyl alcohols. Although bismuth-catalyzed Friedel-Crafts acylations were well known at this time, Friedel-Crafts alkylations using benzyl alcohols had not been reported. 

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